["NERISSA ESCANLAR\/EARTH-LIFE SCIENCE INSTITUTE BIG CRUNCH, a\u00a0false vacuum state, then it could spontaneously BIG FREEZE transition to a true vacuum. That would rewrite all OR BIG RIP? the\u00a0laws of physics and constants of nature as we know\u00a0them. Before the discovery of dark energy, the future of\u00a0the universe depended on geometry. Either the Physicists call this process vacuum decay. In the cosmos was closed and would collapse in on itself universe after vacuum decay, the new laws of physics in a \u201cbig crunch\u201d or it was open and would expand would make it impossible for, say, molecules to exist, forever. Now, however, cosmology\u2019s standard because the way that atoms interact with one another model assumes that we live in a flat universe would be different. Space itself would be unstable, and that,\u00a0thanks to dark energy, will expand eternally. eventually everything would collapse into a black hole. If dark energy is nothing more exotic than What could make this happen? a\u00a0cosmological constant, meaning it doesn\u2019t fluctuate over time, then the heat death of the Imagine balancing a glass right on the edge of the universe (or the \u201cbig freeze\u201d) will be the outcome. table.\u00a0It\u2019s fine right now, but it would prefer to be on But it isn\u2019t the only possibility. An alternative is the\u00a0floor because that\u2019s the lowest energy state, and the\u00a0\u201cbig rip\u201d. Here, the dark energy keeps getting something could happen at any time that could push stronger and the expansion of the universe keeps it\u00a0over. Similarly, it\u2019s possible that our universe prefers accelerating. This is, in some ways, the most a\u00a0different value of the Higgs field and the slightest exciting option on the table. Even gravitationally touch could knock it over. Like the glass, it would be bound objects like galaxies can eventually get more stable, but it would be broken. pulled apart, as dark energy overpowers the gravity holding them together. There are two ways for this to happen. One would be\u00a0that something disturbs the Higgs field. That would have to be an extremely high-energy event, much higher energy than we can even imagine. When the LHC first started up, there was some worry that its collisions could create a high enough energy to disturb the Higgs field, but they are nowhere near powerful enough to do that. What is the other way? > The other idea is that the transition could happen spontaneously through a phenomenon known as quantum tunnelling. If you have a particle on one side\u00a0of a wall, quantum mechanics says it\u2019s possible Chapter 3 | Dark matter and dark energy | 49","for the particle to spontaneously appear on the other are they sad about it, or have they come to terms side. In theory, if you put a glass on the edge of the with the fact that we won\u2019t go on forever? table, all its constituent particles could align and allow it to just spontaneously quantum tunnel to the floor. A few people said that it was really sad. One person It\u2019s extremely unlikely to happen, but we can\u2019t rule it out. said that when she gives lectures about the heat death, people sometimes cry. If something like this happened to our universe, a\u00a0bubble of the new vacuum would spontaneously I haven\u2019t really decided how I feel about it yet. form within it: a region where we can\u2019t exist, because I\u2019m\u00a0still\u00a0kind of trying to wrap my head around it in our molecules would fall apart, and space itself some meaningful way. I am somebody who is not at all collapses. And it would expand at roughly the speed comfortable with the idea that I will die some day, for of\u00a0light. It would plough through the universe and example. Intellectually, I know that that\u2019s true, but it\u2019s destroy everything within it. If it got you, you wouldn\u2019t also terrifying. So the idea that the whole universe will see anything or feel anything: you\u2019re just done. It\u2019s die some day, that everything I love and care about will this\u00a0very dramatic way to destroy the universe. be over, is hard to wrap my head around. Should we be worried about vacuum decay? Does thinking about things on this massive scale help you put daily troubles in perspective? There are several reasons not to worry \u2013 for one, the false vacuum is predicted to stay stable for way longer There\u2019s something about studying the forces of than the current age of the universe \u2013 but physicists nature that changes how you view everyday life. are\u00a0paying a lot of attention to it now because our It doesn\u2019t so much make everything insignificant, experiments do suggest that it\u2019s possible. but it makes clear how little control we really have. What comes after the end of the universe? Is it We live in a society with the illusion of control, just\u00a0nothingness? and there\u2019s a sense of security in how much we\u2019ve altered our surroundings and built a world that suits I define the end of the universe as the end of our us. But when you get to the bigger picture, we\u2019re observable universe \u2013 the volume of space that we can this tiny little speck of dust adrift in the cosmos with interact with, that has any impact on us or that we have no say over what happens to our cosmic environment any impact on. If everything in that region is destroyed, or the universe as a whole, however much we I rate that as the end of the universe. It doesn\u2019t mean eventually come to understand it. that there couldn\u2019t be more space beyond that where more things continue, or another universe after ours, Studying these kinds of things, it\u2019s not like it\u2019s but for us, the end of the observable universe is the end. reassuring at all, but it chips away at the illusion of control in a way that lets you step back a little bit. How do you feel about the end of the universe? Sometimes things are just going to happen and the universe doesn\u2019t care about any of it. When I was putting together my book, I interviewed a bunch of other cosmologists and astronomers All we can do is make the best out of what we about how they feel about the end of the universe: have.\u00a0There\u2019s some amount of comfort in the fact that\u00a0we\u2019re all in this together, at the mercy of some of\u00a0these bigger forces, and that\u2019s OK.\u00a0\u00a0\u275a 50 | New Scientist Essential Guide | Einstein\u2019s Universe","D\u0122\u0137\u0137\u0003\u0176\u011e\u0101\u0003\u00f0\u0137\u00d6\u00f3\u0134\u0003\u011e\u014b\u0137\u0101\u016d\u0003\u0122\u0142\u0003\u0198\u014b\u017d\u0169\u0003\u017d\u0142\u00f9\u0101\u0169\u016d\u0176\u00d6\u0142\u00f9\u0122\u0142\u0117\u0003\u014b\u0115\u0003\u0176\u011e\u0101\u0003\u00f3\u014b\u016d\u013f\u014b\u016d\u0003 \u0003\u0192\u0122\u0176\u011e\u0003\u00d6\u0142\u0003\u014b\u017d\u0176\u031f\u014b\u0115\u031f\u0176\u011e\u0122\u016d\u0003\u0192\u014b\u0169\u0137\u00f9\u0003\u0137\u0101\u00d6\u0169\u0142\u0122\u0142\u0117\u0003\u0101\u0197\u0165\u0101\u0169\u0122\u0101\u0142\u00f3\u0101\u030d\u030d\u030d \f\u0101\u00fd\u0111\u010d\u0472\u0102\u0111\u010e\u010c\u0472\u0113\u0107\u0101\u0472\u0101\u0117\u010f\u0101\u0111\u0113\u0112\u0472\u00fd\u0113\u0472\u0118\u010e\u0114\u0111\u0472\u010e\u0116\u010d\u0472\u0472 \u010f\u00fd\u00ff\u0101\u0472\u0116\u0108\u0113\u0107\u0472\u000e\u0101\u0116\u0472\u0013\u00ff\u0108\u0101\u010d\u0113\u0108\u0112\u0113\u0472\u0001\u00ff\u00fd\u0100\u0101\u010c\u0118\u03ee\u0112\u0472 \u0108\u010c\u010c\u0101\u0111\u0112\u0108\u0115\u0101\u0472\u0108\u010d\u0113\u0101\u0111\u00fd\u00ff\u0113\u0108\u0115\u0101\u0472\u00ff\u010e\u0114\u0111\u0112\u0101\u0112\u03fb\u0472\u0472 \u0004\u0108\u0112\u00ff\u010e\u0115\u0101\u0111\u0472\u0113\u0107\u0101\u0472\u0002\u0101\u0118\u010e\u010d\u0100\u0472\u0005\u0108\u010d\u0112\u0113\u0101\u0108\u010d\u0472\u00ff\u010e\u0114\u0111\u0112\u0101\u0472\u00fd\u0113\u0472 \u010d\u0101\u0116\u0112\u00ff\u0108\u0101\u010d\u0113\u0108\u0112\u0113\u03fb\u00ff\u010e\u010c\u03fc\u00fe\u0101\u0118\u010e\u010d\u0100\u03cd\u0101\u0108\u010d\u0112\u0113\u0101\u0108\u010d","CHAPTER 4 52 | New Scientist Essential Guide | Einstein\u2019s Universe","Few predictions of Einstein\u2019s relativity capture the imagination like black holes. The existence of these monsters, whose gravity is so great not even light can escape them, was long disputed. It was only starting in the 1960s that we became convinced of their reality \u2013 but even now, we aren\u2019t sure what we have seen. In general relativity, black holes represent \u201csingularities\u201d where unruly infinities enter the mathematics, meaning all bets are off. What the true nature of astronomical black holes is, and the\u00a0startling challenges that the theoretical objects pose to\u00a0any\u00a0consistent view of how the cosmos works, are unresolved\u00a0questions that take us to the cutting edge of\u00a0cosmological research. Chapter 4 | Black holes | 53","THE REALITY OF BLACK HOLES Over the past few decades, a battery of T WAS while serving in the German army on evidence has left us in little doubt that the\u00a0Russian front in the winter of 1915 to 1916 black holes exist \u2013 whatever they are. that physicist Karl Schwarzschild sent Albert Einstein some papers. He had solved Einstein\u2019s PREVIOUS PAGE: JUST_SUPER\/ISTOCK equations of general relativity for the first time PHILIPP TUR\/ISTOCK and shown what happens to space-time inside and outside a massive object \u2013 in this case, a perfectly spherical, non-spinning star. Einstein was thrilled. He wasn\u2019t so thrilled with a prediction that\u00a0eventually emerged from Schwarzschild\u2019s work. Make a\u00a0star compact enough and it could develop a gravitational pull so great, and warp space-time so much, that even light wouldn\u2019t get away. In fact, he wasn\u2019t the first person to toy with the idea\u00a0of an object so massive that not even light can escape the pull of its gravity \u2013 that was English geologist John Michell back in 1783. General relativity gave the idea new impetus. Within a year of his exchange with Einstein, Schwarzchild was dead, so it was left to others to work through the details of the curious compact objects he had envisaged, the surfaces of which became\u00a0known as \u201cSchwarzchild singularities\u201d. Chief among them was a young Indian physicist named Subrahmanyan Chandrasekhar. Whiling away\u00a0an 18-day voyage aboard a steamer to take up a scholarship at the University of Cambridge, he worked on the properties of highly compact white dwarf stars. He found that if they had more than 1.4 times the sun\u2019s mass, they would implode under their own gravity, becoming so dense that they would form Schwarzschild singularities. This didn\u2019t go down well. At a meeting of the Royal Astronomical Society in 1935, Arthur Eddington\u00a0\u2013 the > 54 | New Scientist Essential Guide | Einstein\u2019s Universe","Chapter 4 | Black holes | 55","eminent astrophysicist who had verified general THE FIRST EHT COLLABORATION relativity\u2019s predictions during a solar eclipse in 1919\u00a0\u2013 BLACK HOLE declared that \u201cthere should be a law of nature to IMAGE prevent a star from behaving in this absurd way\u201d. In\u00a01939, Einstein himself published a paper to In April 2019, the Event Horizon Telescope explain\u00a0why Schwarzschild singularities couldn\u2019t collaboration unveiled the first direct image exist\u00a0outside the minds of theorists. of\u00a0a\u00a0black hole (see above), the product of a network of telescopes around the globe wired The impasse remained until the 1960s, when up\u00a0to turn Earth into an enormous radio dish. physicists such as Roger Penrose proved that black holes\u00a0\u2013 a term coined at about this time, probably This image shows the large black hole in by\u00a0astrophysicist John Wheeler\u00a0\u2013 were a seemingly the\u00a0centre of another galaxy called Messier 87, inevitable consequence of the collapse of massive which is 55\u00a0million light years away. The black stars.\u00a0At a black hole, physical quantities such as the hole is about 7\u00a0billion times the mass of the sun curvature of space-time would become infinite, and and some 100\u00a0billion kilometres wide \u2013 about the\u00a0equations of general relativity would break down. 22\u00a0times the average distance between Neptune and the sun. Not only that, but a black hole\u2019s interior would be permanently hidden behind its \u201cevent horizon\u201d, the Nothing we can see in the image is coming out surface of no return for light. Nothing happening in of the black hole. The black hole\u2019s event horizon the\u00a0interior could influence events outside, because is in the black area \u2013 the shadow of the black hole no\u00a0matter or energy could escape. \u201cThe first major against the bright, luminous material circling it paradigm shift was the understanding that these and eventually falling in. The asymmetry in the solutions [of general relativity] are meaningful, and image is caused by the black hole\u2019s rotation\u00a0\u2013 the that there is a notion called a horizon, and that it is a light that is coming towards us appears brighter, causal barrier separating the inside from the outside,\u201d and the light moving away doesn\u2019t seem as bright. says theorist Don Marolf at the University of California,\u00a0Santa Barbara. The process of collapse would also destroy every characteristic of the original star except its mass, spin and electric charge: everything else is radiated away as gravitational waves. The resulting black hole is said to \u201chave no hair\u201d \u2013 to bear no trace of its former existence. The notion of such stellar-mass black holes seemed no more than a mathematical trick until 1968, with the\u00a0discovery of pulsars. Pulsars are rapidly spinning neutron stars that contain about as much matter as our\u00a0sun in a volume about that of a large mountain on\u00a0Earth. That puts them very close to the critical density at which gravity would overwhelm them and they would collapse into black holes. A neutron star could gain enough extra mass to do the trick, either by accumulating matter from interstellar space or by its gravitational pull stripping gas from a companion star. This made the possibility of astronomical black holes respectable, and although they give out no light, there are several ways astronomers can search for them. 56 | New Scientist Essential Guide | Einstein\u2019s Universe","ESSAY Their existence can be inferred by their powerful THE GREAT gravitational pull on nearby stars. In some cases, BLACK HOLE stars\u00a0are found to be orbiting an invisible partner, PARADOX and\u00a0if\u00a0calculations show that the partner has more than\u00a0a certain mass, it is probably a black hole. Black holes devour everything that comes too close. Explaining what happens to that stuff A black hole\u2019s intense gravity also tends to attract has\u00a0exposed a fundamental gap in our theories gas\u00a0and dust. Friction heats up this material, causing it of the cosmos, says Paul Davies. to release vast amounts of radiation, which telescopes can detect. Light from stars that lie behind a black hole BLACK hole\u2019s defining feature is as seen from Earth should be deflected by its gravity. its\u00a0event horizon, the boundary This process is called gravitational lensing, and the inside\u00a0which gravity is so strong measurements of the deflection of light can again that\u00a0light can\u2019t escape. As nothing be\u00a0used to infer the existence of the black hole. can\u00a0go faster than light, this means anything crossing the event In 1970, astronomers observing a compact object in horizon\u00a0is\u00a0irreversibly lost to the constellation Cygnus saw jets of X-rays consistent the\u00a0outside universe. with theoretical predictions of radiation streaming from hot matter spiralling towards an event horizon. At least, that is the case on a simple Black holes were soon invoked to explain another reading of general relativity. This puzzling discovery of the 1960s: quasars. These are also says that hidden from view in the heart of a black the\u00a0energetic cores of some galaxies, which produce hole is a \u201csingularity\u201d, an infinitely warped edge or enormous amounts of energy from a region of space boundary of space-time where the laws of physics no bigger across than our solar system. break down. Any matter that hits a singularity \u2013 and, crucially, any information encoded in that matter, The kind of black hole involved would contain maybe for\u00a0example how the molecules in a cloud of gas a few hundred million times our sun\u2019s mass, far more are\u00a0distributed \u2013 must disappear from space-time. than a stellar-mass black hole. Such supermassive black This is a challenge to traditional views in physics holes could arise simply because too many stars got too about time and irreversibility. Compare the fate of an close together in the core of a galaxy. We now think encyclopedia thrown into a black hole with one put most galaxies have one at their heart. into an incinerator. With the incinerator, if you knew the precise state of every molecule and every photon That includes our Milky Way, the existence of radiated as heat, you could, in principle, \u201crun the > whose\u00a0central black hole, Sagittarius A*, was proved by\u00a0Reinhard Genzel and Andrea Ghez by following the motions of nearby stars over decades \u2013 a feat for which they received a share of the 2020 Nobel prize in physics. \u2193- There\u2019s an interview with Andrea Ghez- on page 64 later in this chapter- More recently, observations of gravitational waves \u2013 ripples in space-time given out when two black holes collide and merge \u2013 have provided more evidence of black holes\u2019 existence. Finally, in 2019, a black hole was directly pictured for the first time (see \u201cThe first black hole image\u201d, left). That they exist is now hardly in doubt\u00a0\u2013 and that\u2019s where the true problems start.\u00a0\u00a0\u275a Chapter 4 | Black holes | 57","Stephen Hawking\u2019s work hugely increased the mystery of black holes KEVIN DIETSCH\/UPI\/ALAMY STOCK PHOTO movie backwards\u201d and recover the information contained in the encyclopedia. Not so with a black PROFILE hole.\u00a0The information loss appears to be absolute PAUL and\u00a0objective: there is no rewind button. DAVIES The puzzlement really turned up a notch, however, Paul Davies is a cosmologist in\u00a0early 1974, when Stephen Hawking delivered and physicist at Arizona a\u00a0famous lecture at what is now the Rutherford State\u00a0University in Tempe. Appleton\u00a0Laboratory near Oxford, UK. I was there. His\u00a0research interests span Hawking announced that black holes aren\u2019t totally quantum gravity and black black, but glow faintly because of the effects of holes, the nature of time, quantum particles that pop up out of the vacuum the\u00a0origins of life and the near\u00a0its event horizon and are radiated away. The evolution of cancer. He is the process of emitting \u201cHawking radiation\u201d slowly author of more than 30 books sucks\u00a0energy from the black hole, so it gradually shrinks over an immense length of time. This was a sensational claim. The Hawking effect was\u00a0puzzling on several levels, but one question stood\u00a0out: if a black hole goes on shrinking, does it\u00a0eventually totally disappear \u2013 and if so, what happens\u00a0to all the stuff that fell into it? Hawking derived his result by appealing to quantum\u00a0mechanics. Its laws are time-symmetric, so\u00a0in\u00a0theory you should be able to gather all the information encoded in the Hawking radiation and work backwards to the starting state, just as with an incinerated encyclopedia. But Hawking\u2019s calculations showed that the radiation produced by a black hole is\u00a0precisely \u201cthermal\u201d\u00a0\u2013 entirely random\u00a0\u2013 containing no information whatsoever about what fell into the hole originally. This is the basis of what is known as the black hole\u00a0information paradox. The laws of quantum mechanics say that information can\u2019t be destroyed. General relativity, by introducing black holes, apparently says it must be. I first discussed this clash with Hawking in a\u00a0hotel\u00a0room in Boston in the 1970s, where we had both\u00a0travelled for a conference. At that time, Hawking, who was steeped in the general theory of\u00a0relativity and its predictions about black hole singularities, thought the paradox indicated that quantum mechanics must break down in black holes. He published a paper claiming as much, containing the\u00a0memorable aphorism \u2013 echoing Einstein\u2019s criticism\u00a0of quantum 58 | New Scientist Essential Guide | Einstein\u2019s Universe","theory that \u201cGod does not play dice\u201d \u2013 that \u201cnot phenomenon, because it forms the basis for the design only does God play dice, but\u2026 he sometimes throws of quantum computers. Applied to Hawking radiation, the dice where they cannot be seen\u201d. pairs of entangled particles are created near a black hole, with one escaping and the other falling down Over the subsequent decades, however, many the\u00a0hole. Their entanglement implies a subtle residual physicists have come to believe that quantum connection reaching across the event horizon. mechanics is sacrosanct, and that the lost information must somehow be returned to the outside universe. In thermodynamics, physicists quantify lost or That is especially true among string theorists, whose hidden information in terms of entropy, a general efforts to construct a quantum theory of gravity are measure of disorder. When information goes down, rooted in the standard rules of quantum mechanics. entropy goes up, and vice versa. Every time a pair of After wobbling for years, Hawking finally concurred. photons is produced and one slips over the event What went into the hole, he declared, must come out\u00a0\u2013 horizon, \u201centanglement entropy\u201d increases. When in one form or another. But how? the\u00a0Hawking effect starts out, the entanglement entropy is zero, but it rises steadily as more and more In the absence of a satisfactory theory of quantum particles get created and separated by the horizon. gravity, Hawking\u2019s original calculation was, crucially, \u201csemiclassical\u201d. It applied quantum mechanics to fields Page realised that this inexorable rise must have a such as electromagnetism around the black hole, but limit. As originally suggested by Jacob Bekenstein in not to the black hole\u2019s own gravitational field. There is 1972, and confirmed by Hawking a couple of years later, general agreement that such an approximation will a black hole possesses a total entropy proportional to break down, and quantum gravity effects must kick in, its surface area. As a black hole evaporates, its surface at the Planck scale of about 10-\u00b3\u00b3 centimetres. This is area shrinks, and so does its total entropy. Thus, the a\u00a0number calculated by combining Planck\u2019s constant, entanglement entropy rises and the total entropy which sets the strength of quantum effects, and falls\u00a0until, about halfway through the evaporation Newton\u2019s gravitational constant, which determines process, they become equal. the\u00a0strength of gravitation. The hope was that when a\u00a0black hole shrank to such a size, new effects would At that point, something changes. Entanglement emerge to solve the paradox. entropy can no longer go up, but falls with the total entropy as the hole continues to shrink. This loss \u2192- of\u00a0entanglement entropy implies the appearance Chapter 6 has more on quantum gravity- of\u00a0information. But where? As departures from randomness in the Hawking radiation; that is to But as Don Page, a former postdoctoral collaborator say,\u00a0correlations between particles within it. These of\u00a0Hawking\u2019s, pointed out in a major twist back in correlations grow over time as the black hole shrinks 1992,\u00a0we can\u2019t sweep the problem under a Planck-scale towards its eventual demise. According to Page\u2019s carpet. That is because of entanglement, the quantum analysis, the original entanglement between pairs phenomenon described by Einstein as \u201cspooky of\u00a0outgoing and ingoing particles reappears as action\u00a0at a distance\u201d. It says that if a pair of particles \u2013 entanglement between outgoing particles \u2013 for example photons of light \u2013 is created from the specifically, between particles emitted at earlier quantum vacuum, and the particles fly off in times\u00a0and those emitted at later times. Entanglement opposite\u00a0directions, they remain intimately linked in space becomes entanglement in time. in\u00a0their properties. Independent measurements performed simultaneously on the two particles Significantly, the turnover point occurs when the will\u00a0uncover that link. black hole is still a macroscopic, possibly huge object, very far from the Planck size at which quantum gravity Entanglement is a much-studied quantum can\u2019t be ignored. The build-up of correlations in the outgoing Hawking flux would seem to be a neat way > Chapter 4 | Black holes | 59","out of the information paradox. Information If that were the case, you could fall through a in\u00a0does\u00a0indeed equal information out, but it is black\u00a0hole and come out in a completely different concealed\u00a0by being spread over time. If this is space. Then there needn\u2019t be an information paradox. correct,\u00a0the reversibility of the laws of physics is The information about the infalling matter could preserved by the black hole evaporation process. simply traverse the wormhole and continue to exist in\u00a0the other region of space-time. As long as we That is all well and good, but to buy this argument, humans are restricted to \u201cour\u201d space-time region, you must conclude that there is something missing information is lost, but taking a God\u2019s-eye view, from Hawking\u2019s original calculations, which say there information would be conserved. is\u00a0no entanglement or information in the radiation from the black hole. And there is no agreement on The possibility that wormholes might connect the where the flaw might lie. Attempts to provide an interior of black holes with another area of our own answer so far have either appealed to idealised special space-time outside the hole, allowing information to cases or descended into speculative mathematical sneakily leak back out, is the basis of renewed claims backwaters with only a tenuous link to reality. They recently that the black hole information paradox is provide at best circumstantial (and entirely theoretical) close to resolution. But these calculations, as is so often evidence that the information about the material that the case, rely on highly idealised analogues of real black went into the black hole reappears in some guise in holes and involve layers of simplifying assumptions, the\u00a0Hawking radiation. so\u00a0it isn\u2019t clear how relevant they really are. One such idea is that the entanglement between There is a more general concern, too, about the pairs of particles produced near a black hole\u2019s event uncritical application of quantum mechanics to the horizon somehow gets erased before one falls down black hole evaporation process. Calculations tend to the hole. This entanglement destruction would release assume that the black hole and its products form an a vast amount of energy, resulting in an intensely isolated system, which is obviously unrealistic. Quite destructive, incinerating surface known as a firewall apart from the disturbing effects of the rest of the encircling the event horizon. This firewall should universe, there is a fundamental question concerning produce conspicuous effects outside the black hole, but what we mean by information. For information to be it contradicts a fundamental tenet of general relativity extracted from a quantum system, a measurement has that the event horizon has no special local properties: to be performed by an external system. The very act of it\u00a0just marks the boundary where the strength of the measurement breaks the time symmetry of quantum black hole\u2019s gravitational field becomes great enough mechanics in a process sometimes described as the that light can\u2019t escape. The firewall prediction also collapse of the wave function. So if \u201cinformation\u201d is comes from the questionable practice of considering treated as something that could actually be gleaned particles as little packets of localised energy. Direct from a measurement performed on Hawking radiation, calculations of the quantum energy density round the rewind button is destroyed as soon as that a\u00a0black hole, first done in the 1970s, show it to be measurement is made. smoothed out and continuous at the event horizon. The black hole information paradox is an Some theoretical physicists retain the belief that inconvenient truth at the heart of physics, yet it has only\u00a0a fully worked-out theory of quantum gravity spurred a rich variety of theoretical investigations will\u00a0produce a resolution to the paradox. Such a theory that\u00a0have pushed the frontiers of the subject in will probably include not just intense space-warping important new directions. When Hawking announced effects, but a feature known as topology change. Way his black hole evaporation result, it established a back in the 1950s, John Wheeler pointed out that, on link\u00a0between quantum mechanics, gravitation and the Planck scale, quantum vacuum fluctuations would thermodynamics. This is surely an important clue, be so powerful that they would bend space-time into and\u00a0suggests that the resolution of the paradox \u2013 a\u00a0sort of foamy structure \u2013 a frenetically shifting which\u00a0is undoubtedly out there \u2013 lies in a revolution landscape of wormholes and bridges connecting that unites our understanding of all three of those different regions. Wheeler thought that, in place of elements. Almost half a century on, however, we are a\u00a0point-like singularity at a black hole\u2019s centre, there still waiting for that revolution. It might take another should be a foamy blob. Topology change might also Stephen Hawking to start it.\u00a0\u00a0\u275a create a type of tunnel or wormhole linking the interior of a black hole with another universe or a distant region \u2193- of our own universe, an idea first suggested by Wheeler Turn to page 66 for physicist Carlo Rovelli\u2019s- and championed by several others. answer to the black hole paradox- 60 | New Scientist Essential Guide | Einstein\u2019s Universe","WHAT HAPPENS IF YOU FALL\u00a0INTO A BLACK HOLE? VECTARAY\/DRGROUNDS\/ISTOCK No one knows for certain what \u201cBy the time you are able to see If you fell in feet first, your goes\u00a0on beyond a black hole\u2019s the event horizon,\u201d says Natarajan, ankles\u00a0would stretch away from event\u00a0horizon \u2013 but we can sketch \u201cyou can see starlight bend around your knees before your neck out some of what would happen to it.\u201d Curved streaks of light would elongated into a strand of linguine, you if you were to stray too close. wrap around the black hole, but as but the difference in time would you accelerated through the point be\u00a0small enough that you probably For a start, a black hole\u2019s of\u00a0no return, the intensity of the wouldn\u2019t notice. \u201cIt would happen gravitational pull is so massive that gravitational field would change the in\u00a0the blink of an eye, which may time itself would start to warp. You nature of the light you could see. If not\u00a0be the best expression since wouldn\u2019t feel anything different as your neck muscles had the strength your eyes are going to pop out,\u201d you fell in, but to anyone watching, to let you take one last look over your says\u00a0Natarajan. you would appear to slow down. shoulder, all the starlight behind you \u201cThat\u2019s when the universe starts to would appear to come together to After this, your fate is a matter go bizarre on you,\u201d says Priyamvada form a single reddish dot. of\u00a0pure conjecture. Some physicists Natarajan at Yale University. have suggested that you could Meanwhile, the quiet of outer cling\u00a0on inside a black hole as it Circling down the drain of this space would turn to total darkness evaporates via Hawking radiation, cosmic plughole, all the photons and you would feel that you were before getting belched out in its being pulled alongside you would falling downhill, says Natarajan\u00a0\u2013 dying breath. create a stream of blinding light \u201cexcept downhill is everywhere\u201d. orbiting a hole of total blackness. Others say that by the time Two freaky effects would colour the It wouldn\u2019t be a pleasant the\u00a0black hole got small enough final approach: a looming darkness experience. The gravity inside to\u00a0vomit out its last meal, there would wash over your eyes as the a\u00a0black hole increases so quickly wouldn\u2019t be anything left to expel. black hole seemed to grow in size that\u00a0it wouldn\u2019t just crush you, but Or\u00a0perhaps you might escape via a much more quickly than you would pull apart every bit of your body white hole, or a wormhole passage expect, and the surrounding stars at\u00a0different speeds, resulting in to another part of the universe. would start to distort and bend. \u201cspaghettification\u201d. Perhaps it\u2019s better just not to test it. Chapter 4 | Black holes | 61","The old picture of supermassive black hole formation has the cosmic giants feeding constantly on surrounding gas over hundreds of millions of years SUPERNOVA ACCRETION The outer layers of the star are The black hole grows as blasted off, while the core collapses surrounding material is sucked in into a black hole over hundreds of million years Hundreds of SEED BLACK HOLE solar masses Tens of FIRST MASSIVE HYDROGEN solar masses AND HELIUM STARS Millions to billions of solar masses WHAT MADE BLACK HOLES SO BIG? Supermassive black holes have all the space-warping strangeness of their smaller kin, but they hold another level of mystery. Nobody can explain the genesis of the giants\u00a0\u2013 unless, perhaps, it points back to an origin right at the beginning. HATEVER made sweeps away any incoming gas, cutting off the supply. supermassive black holes, Theory, backed by the behaviour of black holes nearby, it\u00a0made an army of them. says that a black hole can double in size once every Observations of stars 30\u00a0million years at most. whirled around by powerful gravity suggest that there is The problem lies in the fact that we have now a\u00a0huge black hole at the heart seen\u00a0several large supermassive black holes way of almost all large galaxies, out\u00a0in\u00a0the distant cosmos within the first billion including our own. The years\u00a0after\u00a0the big bang \u2013 and those black holes Milky Way\u2019s is 5\u00a0million simply\u00a0shouldn\u2019t have been able to become so times the mass of the sun. The one imaged at the centre big in\u00a0the time\u00a0available to them. of the giant elliptical galaxy Messier 87 in 2019 has more than 6\u00a0million solar masses. Its event horizon One possibility is that supermassive black holes is\u00a0nearly five times as wide as the orbit of Neptune. began not with single stars, but many. \u201cWe know The conventional tale of supermassive black holes that\u00a0early in the history of the universe, stars tended starts with a smaller seed maybe a few hundred times to\u00a0form in bursts \u2013 regions that were spectacularly the mass of the sun. As this seed gorges on gas, it sinks active,\u201d says Fred Rasio at Northwestern University in towards the centre of its galaxy, eventually becoming Evanston, Illinois. Rasio\u2019s educated guess is that they the powerful heart of a quasar. But theory suggests a might well collapse to form a black hole of perhaps a limit to this process. The more gas a black hole gulps, few thousand solar masses, a weightier starting seed. the more light and other radiation that shines out. The catch is that we can\u2019t find similar middleweight Eventually, the flood of light grows so fierce that it black holes in star clusters today. Another possibility is that a bigger seed might come\u00a0not from a collapsing star, but from the collapsing 62 | New Scientist Essential Guide | Einstein\u2019s Universe","Quasar observations suggest supermassive black holes already existed in the early universe \u2013 meaning we need to explain how their seeds got big so quick CLUSTER COLLAPSE SEEDS ACCRETION Many stars collide and collapse ~1000 into one black hole solar masses Millions to billions of solar masses DARK STARS Up to 100,000 solar masses Early stars powered by dark matter could have collapsed into bigger black holes Up to 100,000 ~1 million solar masses solar masses Millions of solar masses BIG BANG COLLAPSE GALAXY COLLAPSE Overdense regions of space-time collapsed into black holes in the first few seconds An entire galaxy core collapses in BIG BANG on itself at once heart of a galaxy. This was originally suggested as universe, it\u2019s easy to get to a billion,\u201d says Bernard Carr an\u00a0outside possibility in 1978 by Martin Rees at the at Queen Mary University of London. University of Cambridge, but it turns out it isn\u2019t easy to\u00a0cram so much matter into a galaxy\u2019s heart. A more Primordial black holes are popular among theorists exotic idea is that early \u201cdark stars\u201d powered by dark for other reasons, too. For one thing, they might make matter might have collapsed into bigger black holes. up the universe\u2019s missing dark matter. The easiest way to catch them red-handed would probably be by \u2190- spotting gravitational waves given out as they merge. Page 39 has more on dark matter- We could alternatively look for radiation emitted by matter falling into them, or use gravitational lensing, But perhaps the most radical suggestion is that a\u00a0phenomenon whereby massive objects stretch and giant\u00a0black holes were forged directly in the fires of distort the light passing near them. the\u00a0big bang. In some models of the process of cosmic inflation, a period of faster-than-light expansion most Seeing one for certain would mean peering back cosmologists think happened in the universe\u2019s first billions of years into the first few hundred million instant, tiny fluctuations in the universe\u2019s density years\u00a0of the universe, as systems with non-primordial would have ballooned into small areas of extreme black holes wouldn\u2019t have been possible then. Beyond density that could have pulled in beams of ambient finding a small, evaporating black hole or spotting one light before collapsing into black holes. Such in the early universe, though, there are very few ways to \u201cprimordial\u201d black holes might solve the problem prove that an observed black hole is primordial. Carr, of\u00a0the over-supermassive black holes. \u201cIf you start who has devoted his career to these black holes, puts out\u00a0with a million-solar-mass black hole in the early the probability that they are real at between 20 and 50\u00a0per cent \u2013 and so the mystery of how supermassive black holes got so vast so fast continues.\u00a0\u00a0\u275a Chapter 4 | Black holes | 63","INTERVIEW HOW I PROVED SUPERMASSIVE BLACK\u00a0HOLES ARE REAL Twenty years ago, Andrea Ghez set out to show that there is a black hole at the centre of our galaxy by watching stars orbit it \u2013 and succeeded in what many thought an impossible task. PROFILE Why did you start studying supermassive black holes? ANDREA GHEZ I think it was the early moon landings that first got me\u00a0interested in astrophysics and thinking about Andrea Ghez is an the\u00a0scale of the universe. What was bothering me was astronomer at the boundaries \u2013 the beginning and end of time and the University of California, boundaries of space. Black holes really capture a lot Los Angeles. Together with of\u00a0those problems with space and time, especially her collaborator Reinhard with\u00a0how general relativity and quantum mechanics Genzel, she won a share of come together, so I think that\u2019s originally what got the 2020 Nobel prize in me\u00a0interested in black holes. They really represent physics for her studies of the\u00a0boundary of our understanding of how the Sagittarius A*, the Milky universe works. Way\u2019s central black hole These monsters live at the centres of galaxies. What is that environment like? In our galaxy, as you go towards the centre, things become much more extreme in almost any way you\u00a0can describe. The density of stars increases, the\u00a0speeds of stars increase and the strength of other characteristics, like their magnetic fields, increases. I\u00a0like to think of it like an urban centre, and we\u2019re out here in the suburbs where everything\u2019s a little slower and calmer. The centre of the galaxy takes everything to the extreme, basically. Is that what makes it so hard to study the area at the centre of\u00a0our galaxy? The centre of our galaxy has the advantage of being really close compared with the black holes in other 64 | New Scientist Essential Guide | Einstein\u2019s Universe","ELENA ZHUKOVA\/UNIVERSITY OF CALIFORNIA\/UPI\/ALAMY STOCK PHOTO galaxies, so we have some advantages in terms of absolutely\u00a0my favourite star in the universe. sorting out what\u2019s going on there. The disadvantage But we are measuring thousands of stars, and is\u00a0that we\u2019re looking through the plane of our own galaxy to perceive what\u2019s at the centre. they\u2019re\u00a0all important, they just have different roles to\u00a0play. Behind those measurements of S0-2, you In addition to having a lot of stars in it, our galaxy need\u00a0stars that tell you how to line up these images also has a lot of dust. That dust makes it difficult for across all your observations. So many stars are light that\u2019s emitted from the centre of the galaxy to playing\u00a0what I would call supporting roles, but reach us. If we were to try to look at the wavelengths they\u00a0are\u00a0still absolutely essential. that our eyes detect, we would perceive very little, because only one out of every 10 billion of these What is so special about S0-2? kinds\u00a0of photons makes it to us. It has a really short orbit, and what I mean by \u201creally So it is less that there is so much going on there, more that short\u201d is shorter than a human lifetime, or maybe a there is stuff in between us and there? career. It only takes about 16 years for S0-2 to complete an orbit of Sagittarius A*. To put this in context, the sun Yes, although it is true that in the centre of the galaxy takes 200\u00a0million years to go around the centre of the the crowding of stars becomes an issue as well. Of galaxy. You are not going to wait for that to happen or course, that just gets more and more problematic as try to see the curvature of that orbit. It\u2019s the orbits of the\u00a0galaxy centre becomes further and further away. S0-2 and a few other stars like it that give us evidence So\u00a0our own galaxy is still our best hope for making any that there must be a compact, massive object \u2013 a black detailed measurements. But there are also technical hole \u2013 there. challenges to that. When observing from ground telescopes, the atmosphere blurs the images. The What was it like when you finally got that proof that there was, atmosphere is great for us. But it is a total headache in\u00a0terms of astronomical imaging. in fact, a supermassive black hole at the centre of our galaxy? It seems like a lot of our best knowledge about Sagittarius A* Oh my goodness, this has been such an exciting project comes from just a few stars, including your work. Why is that? to do because every stage of making progress towards It is true that today there is one star that is, so to\u00a0speak,\u00a0the star of the show, called S0-2. It is the answer to the question \u201cis there a supermassive black hole?\u201d has been so much fun and so exciting. There\u2019s nothing like doing a project where people don\u2019t\u00a0think it\u2019s going to work. > Chapter 4 | Black holes | 65","ESSAY If black holes existed in reverse, they could give us our first glimpse of the quantum source of space-time, says Carlo Rovelli. Speaking of exciting, how has it been since you won the BLACK HOLE\u2026 Nobel\u00a0prize in physics? WHITE HOLE? It has been surreal. It\u2019s surreal to get the Nobel prize, PROFILE period. It\u2019s something that I never anticipated. To get it CARLO in the middle of the [covid-19] pandemic adds another ROVELLI element of surrealism to already surreal times. It was really lovely to have good news to share with friends Carlo Rovelli is a physicist and family and colleagues during these hard times. at Aix-Marseille University in France and author of All of a sudden, there are a lot of opportunities books including Seven and\u00a0invitations to do things, and it forces you to Brief Lessons on Physics, think:\u00a0what\u00a0are you going to do now? What are your Reality Is Not What It Seems responsibilities that are associated with receiving a and The Order of Time prize like this? What are the opportunities that you want to pursue? I really feel strongly about both taking some of the responsibilities of being a spokesperson for science, but also continuing to pursue the cool questions at the centre of the galaxy. A lot of women in the sciences, particularly in physics, can feel\u00a0unwelcome. How can we make the field more accessible and welcoming to everyone? I think the best thing that you can do is do good science, to show that women can be just as effective at being a\u00a0scientist as anyone else can. The more women that succeed at the very top, the more I think it helps the field change just through demonstration. And that demonstration is partially for your peers, but probably more importantly, it shows the next generation the possibilities. In my book, the best way you can change the field is by having the people who are in the minority succeeding. Thinking about black holes, what is the next big question we need to answer? There are lots. We still don\u2019t understand what a black hole is \u2013 that is certainly a big question. How do we make quantum mechanics come together with general relativity to explain these objects? I think that is an enormous question that really drives so much of our work. We\u2019re still nowhere near answering it.\u00a0\u00a0\u275a 66 | New Scientist Essential Guide | Einstein\u2019s Universe","ENERAL relativity predicts that infalling into\u00a0a separate and new region of space-time, where matter ends up concentrating onto a not just matter but also the entire space-time is single central point of infinite density, bouncing out. This is what we call a white hole. called a singularity. This is a sort of end to reality, a point where time itself A ball that bounces up follows a trajectory that looks stops\u00a0and everything vanishes into like a movie of its fall projected backwards. A white hole nothingness. But this prediction isn\u2019t is like a movie of a black hole projected backwards. From reliable, because the centre of the hole the outside, it isn\u2019t much different: it has mass just like is outside the domain of Einstein\u2019s a black hole, so things are attracted by it and can orbit great theory. Here, gravity is so strong around it. But whereas a black hole is surrounded by a that quantum effects can\u2019t be neglected. To understand horizon through which it is possible to enter but not to what happens, we need a quantum theory of gravity. exit, a white hole is surrounded by a horizon through which it is possible to exit but not to enter. \u2192- Chapter 6 has more on quantum gravity- The theoretical possibility of white holes is predicted by general relativity. They are exact solutions of the Quantum theory has a habit of solving problems equations of the theory. But they have long been of\u00a0this\u00a0kind. At the beginning of the 20th century, viewed as mathematical niceties not representing classical\u00a0theory predicted that the energy of an anything real, precisely as were black holes in the past, electron\u00a0orbiting the atomic nucleus would spiral because it was hard to see how they could originate. down\u00a0infinitely. Quantum theory clarified why this doesn\u2019t happen: it is forbidden by the discreteness As early as the 1930s, however, Irish physicist John of\u00a0energy. The energy of the electron can change only Lighton Synge saw that a minimal adjustment in the by specific amounts, and it has a finite bottom level. solution of the equations of general relativity might allow the possibility that the geometry of the interior Quantum effects can similarly prevent infinite of a black hole could continue into a white hole. density from forming at the centre of a black hole. Quantum mechanics can permit such adjustment. In\u00a0this case, it is the discreteness of space-time itself, predicted by quantum theories of gravity such as loop Where would the daughter white hole be located? quantum gravity (which I work on) that does the trick. Would it be far away, connected by a wormhole, or There are no infinitely small points where density can in\u00a0a\u00a0different universe? No, we don\u2019t need outlandish become infinite. Space is composed of individual units, speculations. It will be found at the same place where or quanta, which are small but finite. Falling matter can the black hole was, only in its future. Because of the squeeze into a super-dense state, called a Planck star, peculiar elasticity of space-time as understood with but no more. And then? Then matter can do what Einstein\u2019s theory, \u201cthe other side of the centre\u201d can matter commonly does at the end of a fall: bounce. simply be in the future of the hole. This is hard to visualise, but the result is simple. In the first part of It can\u2019t bounce up within the black hole, where its\u00a0life, the hole is black and matter falls in \u2013 but during things\u00a0can only move downward. But here is the the second, after the quantum transition, it is white magic.\u00a0Quantum gravity allows the entire space-time and matter bounces out. geometry of the black hole to bounce \u2013 that is, to continue across the central point of the black hole For this to happen, there should be a moment in\u00a0which the horizon switches from being that of a black hole to being that of a white hole. Here again, it is\u00a0quantum theory that allows this to happen, thanks > Chapter 4 | Black holes | 67","to a well-known phenomenon known as quantum it is lost if time comes to an end inside a black hole. tunnelling. This is a brief violation of the standard, The\u00a0solution is simple: if anything ends up bouncing classical equations of physics that can happen, with out, information is recovered. low\u00a0probability, even where one wouldn\u2019t expect strong quantum phenomena. Tunnelling is what gives rise, \u2191- for\u00a0instance, to nuclear radioactivity. A particle trapped See Paul Davies\u2019s essay on page 57 for the- inside the atomic nucleus wouldn\u2019t be able to escape lowdown on the information paradox- according to classical mechanics, but quantum theory allows it to \u201ctunnel below\u201d the potential wall that traps All this gives an appealing scenario for the full life it, and thus radiate outside the nucleus. evolution of a black hole. In the interior of the hole, there is no singularity, no place where space-time Tunnelling takes time. Radioactive substances ends,\u00a0and seen from the exterior a black hole isn\u2019t remain quasi-stable for millennia. Similarly, black eternal. Rather, at some time, the black hole turns holes \u00a0have long lifetimes. If we were to buy the classical white\u00a0and whatever fell inside it escapes. theory, a black hole would be eternal. But nothing is eternal. Stephen Hawking showed that quantum The scenario is theoretically beautiful. Does it imply theory implies that black holes slowly evaporate and that the sky is truly full of white holes? And if so, can we shrink. As they shrink, the probability that they tunnel see them? into a white hole increases. At some point, it happens. And again, the important thing is the geometry of The answer depends on things we don\u2019t yet fully space-time itself \u2013 this is the thing that is tunnelling. understand. Most of the black holes we see in the sky Instead of evolving according to the equations of are formed by the collapse of a star. These are too classical general relativity, it suddenly tunnels from young and big to have already tunnelled into white a\u00a0black to a white hole horizon. holes \u2013 big holes live longer. But it is possible that smaller black holes formed in the fierce environment There is a puzzling aspect to this picture. We see of the early universe, shortly after the big bang. These black\u00a0holes that are millions of years old, therefore primordial black holes might have already tunnelled a\u00a0very long time is needed for a large black hole to into white holes, or may be tunnelling today. But we tunnel into a white hole. But matter falling into the aren\u2019t sure about their number, and this makes hole reaches the centre rapidly, in a matter of seconds. predictions about current white holes uncertain. It would be equally quick to bounce out again. How can\u00a0matter then find itself exiting a white hole so A further source of uncertainty is the lifetime of a soon,\u00a0when forming a white hole takes so long? black hole. Detailed calculations have been attempted using loop quantum gravity, but they depend on The answer is enchanting. Time is incredibly approximations and aren\u2019t yet conclusive. Still, we flexible\u00a0in general relativity. We know it passes more have\u00a0a pretty firm bound between a maximum \u201clong\u201d slowly at sea level \u2013 closer to Earth\u2019s centre \u2013 than on lifetime, limited by Hawking\u2019s evaporation time, and the\u00a0mountains. Approaching a massive star or a black a\u00a0minimum \u201cshort\u201d lifetime required by the onset of hole, it slows down even more. And this solves the quantum phenomena. This allows us to draw some puzzle: a very short time inside the hole can match preliminary conclusions. a\u00a0very long exterior time. Seen from the outside, the internal evolution of the hole appears like a bounce, If the lifetime is long, only small primordial holes but in super slow motion. The holes we see in the sky have already turned white. This would mean most may simply be objects that collapse and bounce back of\u00a0the white holes currently in the sky should be of out, perceived by us from the exterior in exaggerated minimal size. The minimal white hole size is Planckian, slow motion. namely around a microgram, or the weight of half an\u00a0inch of a single strand of human hair. This is an A bonus from this scenario is that it solves the intriguing possibility because white holes of this size famous black hole information paradox \u2013 we expect can be relatively stable, and they could be a component information never to be completely lost in nature, but of the mysterious dark matter that astronomers have 68 | New Scientist Essential Guide | Einstein\u2019s Universe","If we could spot a black hole ESA\/V. BECKMANN (NASA-GSFC) observed by radio telescopes. Again, we might have turning white, it would be a glimpse already seen white holes. of quantum gravity in action We can\u2019t confirm that these signals are indeed from (indirectly) detected in the sky. Most other hypotheses white holes with just a few detections \u2013 other sources on the nature of dark matter demand modifications are possible. But there is a signature we will look for of\u00a0well-established laws of physics. For instance, across a large sample: a flattened redshift. Signals they\u00a0rely on theories predicting new entities called emitted by distant and therefore younger white holes super-symmetric particles. But failures to detect such should produce shorter wavelengths than nearby, particles have raised questions about these theories. older \u00a0ones. This is something we might be able to spot\u00a0in high-energy cosmic rays or fast radio bursts \u2190- once we have enough data. If we do, we will have some Turn back to page 39 for more on dark matter- evidence that white holes exist. Finding evidence of white holes in the sky would be a beautiful step ahead The possibility that dark matter is instead composed in our understanding of the universe. They could of\u00a0small black holes doesn\u2019t require anything more represent the first direct observation of quantum than established physics, namely general relativity gravity at work, and so open a window on the greatest and\u00a0quantum theory, and isn\u2019t ruled out by any problem in fundamental physics, the problem of observation. If this is correct, we have already understanding the quantum aspects of space-time. observed\u00a0white holes: they are the dark matter! I close by mentioning one last very speculative Alternatively, if the black hole lifetime is short, idea.\u00a0Our universe might not have been born at the primordial black holes tunnelling today should have big\u00a0bang: it may have bounced out from a previous the mass of a small planet and could be exploding collapsing phase. This possibility is allowed by loop violently, transforming most of their mass into emitted quantum gravity and other theoretical frameworks. radiation. This event should send us extremely The quantum mechanism of the cosmic bounce energetic cosmic rays and short, violent signals in the is\u00a0similar to the black-to-white-hole bounce. The microwave or radio band. The latter are particularly Planckian white holes in today\u2019s dark matter could intriguing because similar signals have already been have\u00a0formed before the bounce. If so, the geometry detected: the mysterious fast radio bursts recently of\u00a0space-time at the bounce wasn\u2019t homogeneous as\u00a0conventional cosmology suggests, but rather very\u00a0crumpled, because each white hole is like a\u00a0long\u00a0spike out in the geometry of space-time. \u2192- Turn to page 88 for more on bouncing universes- This fact could be relevant for the mystery of the arrow of time \u2013 the question of why time goes in only one direction. The arrow of time might not be caused by the initial state of the universe being \u201cspecial\u201d (that is, low entropy), as is commonly believed. Instead, it may be a perspectival phenomenon related to the very \u201cspecial\u201d location of us observers: we are outside all the holes. White holes are a plausible \u2013 albeit almost completely\u00a0unexplored \u2013 possibility. We are yet to\u00a0identify one, but then we didn\u2019t recognise black holes\u00a0for long time either.\u00a0\u00a0\u275a Chapter 4 | Black holes | 69","CHAPTER 5 70 | New Scientist Essential Guide | Einstein\u2019s Universe","Albert Einstein was a prolific predictor of new phenomena \u2013 and often had difficulty coming to terms with the results. So it was with black holes, and so it was with gravitational waves. These minuscule ripples in the fabric of space-time are given out when massive objects move in the cosmos, for example merging black holes. For a long time, they remained the only experimental prediction of general relativity not to be verified. That all changed with a stunning detection in 2015, almost exactly 100 years after Einstein first presented general relativity to the world. It opened up a new era in astronomy. Chapter 5 | Gravitational waves | 71","THE LONG ROAD TO DETECTION Gravitational waves eluded detection for ATTER causes space-time to a\u00a0century after Einstein predicted them. curve,\u00a0says general relativity. That had two main causes \u2013 scepticism When a large mass accelerates, on\u00a0the part of theorists, and the sheer scale that curvature should change. of\u00a0the challenge faced by experimentalists. The\u00a0result is ripples in space-time that spread out at the speed of PREVIOUS PAGE: JUST_SUPER\/ISTOCK light, just as electromagnetic SPANTELDOTRU\/ISTOCK waves generated by accelerating electric charges spread. Gravitational waves are actually\u00a0a subtle consequence of the special theory of\u00a0relativity derived by Einstein in 1905, where\u00a0they serve to prevent gravitational influences propagating across space-time instantaneously, faster\u00a0than the speed of light. The analogy with electromagnetic waves, also bound\u00a0to the speed of light, isn\u2019t perfect: rather than propagating through space-time as electromagnetic waves do, gravitational waves are contractions and expansions of space-time itself. Because gravity is much weaker than electromagnetism, they are also minuscule by comparison. All of that makes calculations with gravitational waves rather hairy. Einstein realised straight away that\u00a0the equations of general relativity had wave-like solutions, and in 1918 he derived a formula that allowed him to estimate how much energy these waves should carry. But general relativity\u2019s core equations are so intractable that controversy persisted for decades over whether the formula was even theoretically sound. Almost two decades later, together with a young colleague, Nathan Rosen, Einstein titled another paper\u00a0sent to Physical Review \u201cDo gravitational waves exist?\u201d, answering the question with a cautious \u201cno\u201d. > 72 | New Scientist Essential Guide | Einstein\u2019s Universe","Chapter 5 | Gravitational waves | 73","The paper never appeared: the journal\u2019s editor sent it on \u2193- to an anonymous referee. Offended, Einstein withdrew The next section has more on the LIGO detector- it. He didn\u2019t take kindly to being peer-reviewed. This made the announcement on 11 February 2016 But that gave him time to realise the mistake in his by\u00a0researchers at LIGO that they had finally spotted a calculations and reverse his prediction before the paper gravitational wave all the more sensational. The signal was eventually published in 1937 in the Journal of the was actually picked up by LIGO\u2019s two observatories in Franklin Institute. The existence of ripples in the fabric Hanford in Washington and Livingston in Louisiana of reality, extending every which way throughout the on\u00a014\u00a0September 2015. The five-month delay was cosmos, finally had Einstein\u2019s personal seal of approval. down\u00a0to painstaking analysis and cross-checking to\u00a0ensure that the signal couldn\u2019t have been from It was only after Einstein\u2019s death that gravitational any\u00a0other source. waves became widely accepted, however. Experimentalists duly built detectors \u2013 initially large, It wasn\u2019t. It fit perfectly with the sort of signal suspended cylinders that might be nudged by a passing we\u00a0would expect if two black holes, each about wave. In the late 1960s, US physicist Joseph Weber was 30\u00a0times\u00a0the mass of the sun, collided. The details of the first to claim a sighting. More than a dozen such the\u00a0signal suggest that they circled each other closer claims followed, but none stood up to scrutiny. and closer until they finally merged into one. But even before we had detected them directly, we This immediately resolved one open question already had overwhelming indirect proof that they for\u00a0astronomers. Before the signal came in, the very exist. In 1974, astronomers Russell Hulse and Joseph existence of such black hole binaries was contested. Taylor discovered a binary pulsar \u2013 an orbiting pair Because they are dark, black holes of these masses of\u00a0neutron stars beaming out radio waves at precise are\u00a0almost impossible to spot unless something intervals \u2013 and started tracking its rotation rate. By bright\u00a0\u2013 like a star \u2013 orbits them. the\u00a0early 1990s, they had shown that the stars were losing energy at precisely the rate Einstein predicted \u2190- they would if they were emitting gravitational waves. Turn back to chapter 4 for more on black holes- A\u00a0handful of similar binary systems studied since then\u00a0have confirmed this view. Today, LIGO \u2013 now working in consort with a third gravitational wave detector, Virgo, located in Italy \u2013 There is one big reason why a direct detection took has\u00a0many tens of confirmed detections under its belt. so\u00a0long. \u201cSpace-time is really, really stiff,\u201d says Vernon These include gravitational waves from the death Sandberg at the Laser Interferometer Gravitational- spiral\u00a0of two neutron stars. Unlike black holes, which Wave Observatory (LIGO) collaboration that eventually hide their mass behind an event horizon even as they made the breakthrough. It is something like a thousand crash, colliding neutron stars spew hot, bright matter billion billion times stiffer than diamond, so even across space, which could help us explore other mighty cosmic events generate pitifully weak, mysteries. Studying these explosions may explain murmuring waves in it. A gravitational wave from the short gamma-ray bursts, mysterious and incredibly death tango of two neutron stars that each had a mass bright electromagnetic phenomena. They might also of 1.4\u00a0suns \u2013 the benchmark used by gravitational-wave help explain where much of the universe\u2019s heavy astronomers \u2013 would squeeze or stretch the 4.3\u00a0light elements, like uranium, thorium and gold, are forged. > years between us and the star system Alpha Centauri by\u00a0less than half the width of a human hair. 74 | New Scientist Essential Guide | Einstein\u2019s Universe","THE GRAVITATIONAL WAVE\u00a0BACKGROUND Everything in the universe is the signals from many pulsars it\u00a0will be a useful tool for constantly being stretched and across the sky simultaneously. understanding the most massive squeezed by gravitational waves. objects in the universe. \u201cThis will That suggests a way we can make \u201cThese pulsars are spinning tell\u00a0us more about black holes in detections of a background of with\u00a0millisecond periods and we the\u00a0universe, and especially the waves\u00a0permeating the universe are\u00a0able to detect changes in the supermassive black holes in galactic using normal radio telescopes, time of arrival [of signals]\u2026 at the centres,\u201d says Nelson Christensen rather than making single detections hundreds of nanosecond level,\u201d at\u00a0the Observatory of Nice in France. with ultra-sensitive detectors on says\u00a0Joe Simon at the University \u201cThis NANOGrav signal is likely from Earth or in orbit. of\u00a0Colorado Boulder. [black hole] binaries with billions of solar masses.\u201d As these enormous That is the guiding idea behind Early in 2021, Simon and other pairs of black holes merge, they the\u00a0North American Nanohertz NANOGrav researchers presented emit thrums of gravitational waves Observatory for Gravitational data gathered on 45 pulsars over powerful enough to persist Waves\u00a0(NANOGrav) consortium, the\u00a0course of 13 years, and found throughout space-time. which uses a so-called pulsar a\u00a0gravitational wave signal that was timing\u00a0array to attempt to build a identical across multiple pulsars. This effort will build a bridge sort of map of gravitational waves. This strange, low-frequency hum between the gravitational waves Pulsars are neutron stars that rotate could be the first evidence of the we\u00a0have already spotted coming extremely rapidly and regularly, gravitational wave background. from smaller black holes with the sending out beams of light that act To\u00a0prove that this signal is from Laser Interferometer Gravitational- as \u201cticks\u201d in extraordinarily precise the\u00a0gravitational wave background, Wave Observatory (LIGO) and cosmic clocks. however, they would need to Virgo\u00a0detectors, and those from see\u00a0a\u00a0distinctive pattern in the supermassive black holes, says When a gravitational wave gravitational waves affecting Christensen. Such a bridge will passes\u00a0through the same region each\u00a0pulsar. help\u00a0us understand how different of\u00a0space-time that those beams of types of black holes form, how light are travelling through, it makes Gathering the additional data galaxies evolve with the black holes the light appear to take slightly more necessary to find that pattern within them and maybe even the or less time to reach us, causing should\u00a0only take about a year, larger, mysterious forces at work the\u00a0\u201cticks\u201d from a pulsar to seem says\u00a0Simon, although analysing in\u00a0our universe like dark matter irregular. Pulsar timing arrays it\u00a0may take longer. and\u00a0dark energy. require radio telescopes to observe If the signal is in fact the gravitational wave background, Chapter 5 | Gravitational waves | 75","Such single event detections are just the start, experiment, LISA Pathfinder, was in orbit from 2015 however. Put several together and we should be able to\u00a02017 and was deemed a success. to\u00a0get new insights into the history and composition of\u00a0the universe as a whole, says Avi Loeb at Harvard Further ahead, we might see more sensitive University. The signals from several black hole mergers, gravitational wave detectors, working at shorter for example, can be combined to help understand the wavelengths than LIGO. These may allow us to sense nature of dark energy, which is causing the universe\u2019s primordial gravitational waves from the very young expansion to accelerate. universe. These waves should have been produced in the period of inflation \u2013 the tremendous growth spurt From the \u201cshape\u201d of the signal \u2013 how the waves\u2019 in the first instants after the big bang. Unlike photons frequency and volume rise and fall \u2013 we can discern and other electromagnetic radiation, they would have the\u00a0sizes of the black holes involved, and determine travelled freely through the newborn universe. At the how loud the event was at its source. Comparing how moment, we can only see as far back as 380,000 years powerful it really was to the faint vibrations that LIGO after the big bang, when the universe became detected tells us how far away it occurred. Combined transparent to light and the cosmic microwave with observations from standard telescopes, this can background was emitted. show us how space has expanded during the time that the waves took to reach us, providing a measure of \u2190- dark\u00a0energy\u2019s effect on space. Chapter 2 has more on the cosmic- microwave background and inflation- \u2190- Turn back to page 43 for more on dark energy- Gravitational waves may even point the way towards a\u00a0unified theory of the universe. Theory suggests This measure should be stronger and more reliable that\u00a0at some point in the universe\u2019s history, all four than anything we have used so far. \u201cIf you have tens fundamental forces were united into a single force. of\u00a0[detections], it will be a new branch in cosmology,\u201d As\u00a0the universe expanded and cooled, the forces says Loeb. split\u00a0off from one another in a series of as-yet poorly understood events. Another suggestion is that jitters Other researchers hope to use gravitational wave in\u00a0gravitational wave signals could reveal the existence signals to put Einstein\u2019s general theory of relativity of gravitons, the hypothetical quantum particles of to\u00a0even more stringent tests. One way is through the gravity, and so point to a theory beyond general equivalence principle, an assumption that gravity affects relativity that unites all four forces on the basis all masses in the same way. \u201cIn the age of GPS and space of\u00a0quantum theory. travel, where even minute deviations from the assumed theory of gravity would have major consequences, it is \u2192- of enormous importance,\u201d says Xue-Feng Wu at Purple Chapter 6 has more on quantum gravity- Mountain Observatory in Nanjing, China. Now, Maulik Parikh at Arizona State University and Erminia Calabrese, an astronomer at Cardiff his\u00a0colleagues have calculated that the existence of University in the UK, sees gravitational waves as a way gravitons could create jitters in gravitational wave to check whether gravity behaves as relativity predicts signals. They found that these could theoretically be it should over large distances. \u201cIf their strength fell off detected with current gravitational wave observatories. with distance in a surprising way, we could detect this with\u00a0the upcoming LIGO data,\u201d she says. All of that goes some way to explaining the buzz surrounding gravitational waves. \u201cThere\u2019s going to New detectors are already in the pipeline. be\u00a0a\u00a0revolution\u201d, in the words of LIGO team member The\u00a0European Space Agency is working on Erik Katsavounidis at the Massachusetts Institute the\u00a0Laser\u00a0Interferometer Space Antenna (LISA), of\u00a0Technology, in response to the first discovery.\u00a0\u00a0\u275a a\u00a0huge,\u00a0space-based detector due to blast off in the mid-2030s. A preparatory proof-of-principle 76 | New Scientist Essential Guide | Einstein\u2019s Universe","PICTURE ESSAY ENRICO SACCHETTI HOW WE FOUND GRAVITATIONAL WAVES It took decades to prove that gravitational The students on my course were fascinated by the waves are real, as the people who made it their life\u2019s work recall idea\u00a0that gravitational waves might exist. I didn\u2019t know In 1969, Rainer Weiss was a young professor at the much about them at all, and for the life of me I couldn\u2019t Massachusetts Institute of Technology (MIT). At the time, gravitational waves were a theoretical curiosity: Einstein understand how a bar interacts with a gravitational wave. himself took years to be convinced by his own prediction that moving cosmic bodies would send out ripples I kept thinking, well, there\u2019s one way I can explain through space-time. Then physicist Joseph Weber claimed to have recorded one on a xylophone-like instrument he how gravitational waves interact with matter. I said, called a resonant bar detector. Weiss takes up the story. suppose you take a light \u2013 I was thinking of just light bulbs because, in those days, lasers weren\u2019t yet really there \u2013 and sent a light pulse between two masses. Then you do the same when there is a gravitational wave. Lo and behold, you see that the time it takes light\u00a0to\u00a0go from one mass to the other changes because\u00a0of\u00a0the wave. If the wave is getting bigger, it\u00a0causes the time to grow a little bit. If the wave is trying\u00a0to contract, it reduces it a little bit. So you > Chapter 5 | Gravitational waves | 77","can\u00a0see this oscillation in time on the clock. Previous page: The concrete housing I was hiding in this little office in Building 20 of\u00a0LIGO\u2019s two beam tubes stretch off die-straight into the distance at\u00a0MIT,\u00a0and for about three months I thought about how you might do this. First, I thought you couldn\u2019t Above: Within the beam tubes, lasers get\u00a0clocks good enough. But we did some experiments ping back and forth hundreds of times, and I learned you could do unbelievably exquisite reflected by mirrors measurements with lasers. The NSF had lost confidence and were basically giving I wrote this up, but I didn\u2019t publish it. The people at up on it. There was a lot of resistance. It represented the MIT wanted to know where the hell I\u2019d been spending extreme of what you might call a high-risk, high-pay- my time, so I put it into the quarterly progress report off project. So we revamped it entirely. Over a period for my lab. I came to the conclusion that, if you built of\u00a0six months, we made it look like a new project. this thing big enough, you could probably detect And\u00a0it\u00a0was hard, because if you had asked me then, gravitational waves. could we\u00a0build what we now know we need to detect gravitational waves, the answer would have been no. Based on Weiss\u2019s idea, the US National Science Foundation (NSF) eventually began funding the So the idea, and the words that I used, was that with development of what became the Laser Interferometer the\u00a0initial version of LIGO, it would be possible to detect Gravitational-Wave Observatory (LIGO) in 1979. But gravitational waves. And then we would evolve into a progress was slow, and when physicist Barry Barish detector, which we called Advanced LIGO, where it would at\u00a0the California Institute of Technology took the lead be probable. But in all honesty, we had nothing more on\u00a0the project in 1994, questions were being asked. 78 | New Scientist Essential Guide | Einstein\u2019s Universe","than ideas on how to do the Advanced LIGO part. To The \u201cvertex\u201d where the two laser me, the miracle in this whole thing is that we somehow beams are recombined. The got financial support for 22 years until we succeeded. interference pattern they create changes when a gravitational LIGO\u2019s detectors occupy two sites in Livingston, wave disturbs either one of them Louisiana, and Hanford, Washington. Since joining the\u00a0project in 2000, Hanford\u2019s lead detection scientist waves hitting against the continental shelf. Michael Landry has been ensuring that the instrument is as sensitive as possible to the minuscule signals it was If you get storms off the coast of Alaska or the Gulf of built to detect. Mexico, ground motions increase. We have to suppress Space is a stiff medium, so it doesn\u2019t want to vibrate. The detector has to register changes that are about a that motion by registering it with seismometers and thousandth the size of a proton. If you were trying to measure the distance between here and our nearest feeding it into seismic suppression systems \u2013 kind star, Proxima Centauri, it would be like watching that\u00a0distance change by the width of a human hair. of\u00a0the way noise-cancelling headphones work by It is an ongoing battle to suppress noise in the sampling the ambient noise and then playing it instrument. There are ground noises like earthquakes, but a less obvious example is Earth\u2019s ringing \u2013 at with\u00a0the right phase to cancel it out at your ear. low\u00a0frequencies, it rings like a bell because of ocean There are also a lot of internal noises that we have to\u00a0suppress. Things like electronic noise, or quantum noise in the laser. All of that means the LIGO detectors are absolutely the most quiet and the most sensitive detectors ever built. > Chapter 5 | Gravitational waves | 79","In September 2015, just a few days after Advanced LIGO Far left: LIGO\u2019s cylindrical mirrors are finally came on stream, Michael Landry received notice polished to nanometre precision to reflect of an unexplained signal. At first, he was convinced it the laser beams without absorption and was\u00a0an \u201cinjection\u201d \u2013 an artificial pulse used occasionally ensure no surface roughness affects the to test the instrument. detector\u2019s timing On the morning of 14\u00a0September, I opened my Near left: The ultra-pure silica mirrors are computer and saw an email indicating this event suspended on glass fibres. Their suspension seen\u00a0in the data literally tens of minutes earlier. cage adjusts for the changing motions of I\u00a0thought, it\u2019s probably an injection. It was so early in the\u00a0ground that might otherwise disturb the\u00a0observation phase. It wasn\u2019t until later, in the lab, the\u00a0measurement that we determined there was no such injection. We did\u00a0a whole lot more investigation and it took months Right: The LIGO Livingston control room to validate it. But it was immediately obvious that if in\u00a0Louisiana, where a distinctive \u201cchirp\u201c this\u00a0thing wasn\u2019t an injection, it was the best damn was\u00a0recorded at 09:51 UTC on 14 September thing we had ever seen. 2015. It lasted just a fraction of a second, but was registered almost simultaneously 3000 The gravitational wave apparently came from the kilometres away in Hanford, Washington \u2013 collision of two black holes, formed from collapsed stars, the expected signal of a gravitational wave 1.3 billion light years away. For Rainer Weiss, 83 at the travelling at light speed time and a professor emeritus at MIT, it was a long- sought vindication. For Nergis Mavalvala, an MIT physicist who had worked on LIGO for 25 years, this detection was just the beginning. The discovery itself was spectacular. To me, it was the thing that I would have wanted most, to see the collision I don\u2019t think this was some master plan from nature, of two black holes. If you want to ask, what was the reason as\u00a0in \u201clet\u2019s be nice to these people here on this little for building this thing in the first place, it\u2019s to check up on Earth place\u201d. Black holes collide all the time. What was whether Einstein\u2019s theory works in strong gravitational lucky was that we happened to have a detector with fields. That was the one place where general relativity sufficient sensitivity to see this one at the moment hadn\u2019t been tested. And here, suddenly, we have in our it\u00a0went up. If we continue, we will see more. hands a thing that says Einstein\u2019s field equations, the whole thing, is absolutely right. One of the amazing things about general relativity is\u00a0you solve the equations\u00a0\u2013 although that\u2019s taken decades\u00a0\u2013 and create templates for how signals should look. Nature was kind in that the very first signal we saw was so clear. Many people expected we would see really weak signals, barely poking above the noise, and that there would be a lot of discussion about whether it\u00a0was a detection or not. None of that happened. That\u2019s\u00a0where we lucked out. The discovery drives us harder because we know there is stuff out there waiting to be observed. You might imagine we would think, \u201cOK, now we\u2019ve seen it\u00a0we can pack up and go home\u201d. But in fact it\u2019s just the opposite. We\u2019ve seen the very first gravitational wave, but we have so much more to discover. We have a lot to learn about black holes, and then there are neutron stars. Personally, my hope is that we\u00a0will see something that really has us scratching our\u00a0heads. Maybe we will have discovered some new object that I can\u2019t begin to describe or name.\u00a0\u00a0\u275a The pictures of LIGO accompanying this essay are part of\u00a0a\u00a0specially commissioned set by Enrico Sacchetti 80 | New Scientist Essential Guide | Einstein\u2019s Universe","HOW DOES LIGO WORK? Each of LIGO\u2019s detectors, at two sites then shorten as a gravitational wave for 100 reflections, making in Washington and Louisiana, consists passes, while the other shortens each\u00a04-kilometre arm effectively of two 4-kilometre-long arms. They and\u00a0lengthens. A standard-issue 400\u00a0kilometres long. For that to work, are identical in almost every respect. gravitational wave is expected to the reflecting mirrors had be held The arms each contain a steel pipe expand and contract LIGO\u2019s arms unerringly still, hung on wires of housing a vacuum through which one by\u00a0just 10-19 metres, a distance one fused silica glass barely a millimetre half of a split laser beam pings back ten-thousandth the width of a proton. thick to isolate them from motions of and forth, bouncing off a mirror at the ground as far as possible. the\u00a0far end. Back at the main building, Hence why LIGO has two twin this reflected laser beam is reunited detectors 3000 kilometres apart. Even with such precautions, with its twin that has zipped up and See the same signature in one and the\u00a0signals LIGO detects last just down the other arm. 10 milliseconds later in the other seconds. The gravitational waveform (the\u00a0time it would take a wave to produced by black holes as they Any change in the length of travel between the two) and then spiral towards each other and finally either\u00a0arm caused by a passing you\u00a0would be talking. merge lasts for many millions, gravitational wave distorting perhaps even billions of years. space-time would alter the patterns Or perhaps not: to make that That\u00a0creates a pattern in which both of constructive and destructive delicate a measurement, every frequency and amplitude increase interference that form when the two possible disturbance, every tiniest as the black holes approach each light waves recombine. Gravitational reverberation that might put things other, orbiting ever faster \u2013 but it is waves squeeze space in one out of kilter must be suppressed. only in the final second that the signal direction while stretching it in the To\u00a0increase the detector\u2019s sensitivity, reaches high enough frequencies other, making the LIGO detector light is made to bounce back and and high enough amplitudes for with\u00a0its arms splayed at right angles forth many times along the LIGO to detect the vibration, above doubly sensitive: one arm of the interferometer arms. In the upgraded the general background noise from interferometer should lengthen and \u201cAdvanced LIGO\u201d configuration that other non-cosmic sources. made the first detection, the aim was Chapter 5 | Gravitational waves | 81","CHAPTER 6 82 | New Scientist Essential Guide | Einstein\u2019s Universe","Albert Einstein\u2019s relativity has proved a peerless guide to the universe \u2013 as far it goes. But it has limitations and deficiencies that leave physicists and cosmologists aspiring for more. That becomes most obvious in general relativity\u2019s failure to\u00a0mesh\u00a0with quantum theory, the theory that describes the other\u00a0three fundamental forces of nature besides gravity. In environments where both theories become relevant \u2013 at the very\u00a0moment of the big bang, at the event horizon of a black hole\u00a0\u2013 we are struggling for understanding. The dream is a quantum theory of gravity, or some other theory that unifies our understanding of space, time and everything in it.\u00a0But that dream seems as far removed from reality as ever. Chapter 6 | Beyond relativity | 83","THEORIES OF RATHER glib distinction is often EVERYTHING made\u00a0between the two pillars of modern physics. Quantum mechanics Since the times of the ancient Greeks, is the physics of the very small, while we\u00a0have had the idea that, if we dig down, general relativity is the physics of the we can uncover the ultimate building very large. That\u2019s not quite accurate \u2013 blocks\u00a0of reality and the universal rules for example, quantum-mechanical that\u00a0govern its behaviour. Today, things effects have been observed spanning don\u2019t\u00a0look quite that simple, not least hundreds of kilometres. And at some because of how general relativity fails scale, surely these two supremely to\u00a0mesh with the theory that governs accurate theories must come together. the\u00a0rest of physics: quantum theory. Yet wherever they do cross paths, the two theories fail\u00a0to play nicely together \u2013 such as around black PREVIOUS PAGE: JUST-SUPER\/ISTOCK holes.\u00a0Even things as basic as space and time highlight KASEZO\/ISTOCK how badly these two theories get along. Relativity\u2019s space-time is a smooth four-dimensional sheet; the quantum field theories that underlie the standard model suggest that space is pixelated into units with sizes of about 10-\u00b3\u2075 metres, and don\u2019t even treat time as\u00a0a real and observable thing. Efforts to establish a quantum theory of gravity have\u00a0stumped many physicists over the past century. Asked to choose between the two theories, most would place their money on quantum theory being \u201cright\u201d, because its mathematics is such a successful prism through which to view the world. Others, from Einstein onwards, have taken issue with quantum theory\u2019s seeming \u201cirreality\u201d and spooky, counter-intuitive correlations between apparently unrelated objects. If\u00a0we can\u2019t find a convincing physical reason why these correlations are just so, they argue, perhaps quantum theory is just an approximation to something better. Einstein was just one of those burned by the search, becoming extremely unproductive in his later years as\u00a0he sought a theory of everything. To understand the difficulties involved, we must start with a fundamental tenet of quantum physics. Heisenberg\u2019s uncertainty principle embodies the fuzziness of the quantum world. It allows particles, such as electrons or photons of light, the equivalent of an interest-free loan: they may borrow energy from empty space and use it to make mass, according to Einstein\u2019s E = mc\u00b2. This mass takes the form of short-lived \u201cvirtual\u201d particles. The > 84 | New Scientist Essential Guide | Einstein\u2019s Universe","Chapter 6 | Beyond relativity | 85","Gravitons are conjectured Particles such as electrons can interact by producing quantum particles of gravity \u2013 and exchanging massless photons in countless ways, but theories incorporating them often resulting in infinities in the calculations tend to be unruly ELECTRON only caveat is that they must pay this energy back \u2013 joke\u00a0on us: our scheme to eliminate one sort the particles must disappear once again \u2013 before of infinity creates another. anyone asks any questions. The more energy they borrow, the quicker this must happen. Attempts to get round this fundamental roadblock have led us to destinations such as string theory, Given such freedom, one can imagine an electron, which\u00a0assumes that all particles are manifestations photon or any other particle going to town, taking of\u00a0more fundamental vibrating strings. When we start out\u00a0many zero-interest loans in succession. As a result, summing over all possible histories of these \u201cfluffier\u201d calculating even a prosaic quantum process \u2013 an objects, the hard infinities produced by virtual particles electron travelling from left to right, say \u2013becomes drop away almost by magic. Another commonly enormously complex. In the words of physicist considered idea is loop quantum gravity, which Richard\u00a0Feynman, we must \u201csum over all possible suggests that space-time itself is chopped up into histories\u201d, taking into account the infinite variety of discrete blocks. This pixelation imposes an upper ways virtual particles can be produced (see diagram). limit\u00a0on the amount of energy any particle can borrow,\u00a0again rendering calculations finite. The history of applying quantum theory to nature\u2019s forces is a history of getting to grips with these unruly Despite their seemingly radical assumptions, infinities. One huge success story is electroweak theory, these\u00a0two candidate unified theories are in many the theory that combines the electromagnetic and ways\u00a0the most conservative extensions of current weak nuclear forces to explain how electrons and models: both attempt to preserve as much of the photons work. Its predictions, of everything from theoretical underpinnings of quantum mechanics particle masses to their decay rates, are accurate up and\u00a0general relativity as possible (see \u201cSix routes to\u00a010 decimal places. The eventually successful variant, to\u00a0a\u00a0theory of everything\u201d, right). a\u00a0bedrock of today\u2019s \u201cstandard model\u201d of particle physics, tamed the mathematics using a bunch of What about more esoteric ideas, such as changing undiscovered massive particles, the W, Z and Higgs the rules of the existing game? For instance, if general bosons \u2013 theoretical predictions that have all since relativity were to treat space and time separately again, been discovered to be components of reality. rather than lumping them into one combined space- time, that might provide some wiggle room. But But follow this same principle, and postulate that relativity and quantum mechanics both tally so gravity is made of quantum particles called gravitons, well\u00a0with reality in their respective spheres that and you hit a problem. Eliminating the infinities that it\u00a0is\u00a0devilishly difficult to formulate such tweaks. emerge requires inventing a second particle with a mass 10 billion billion times that of a proton. As ever, That failing means the idea of a theory of everything the larger the amount of energy borrowed, the more has quietly disappeared, says Renate Loll at Radboud quickly it must be paid back, so these fixer particles University in Nijmegen in the Netherlands. \u201cFor a are\u00a0very short-lived. This means they can\u2019t get very while, you would see it in papers, in the heyday of string far,\u00a0and so occupy only a minute amount of space. theory, but it has gone totally out of fashion.\u201d Chris Isham at Imperial College London goes further. A But general relativity says that mass bends theory of everything is \u201cpsychologically compelling\u201d, space-time. Concentrate enough mass into a small he says, but there is no reason to think one exists \u2013 area\u00a0and a\u00a0black hole will form, a point of infinite or\u00a0that we can find it. That we have got so far with curvature in space-time. And this is exactly the mathematics is a remarkable fact, but it doesn\u2019t mean guise\u00a0our new particle takes. Nature plays a cruel we can go all the way. Others, however, beg to differ.\u00a0\u00a0\u275a 86 | New Scientist Essential Guide | Einstein\u2019s Universe","The situation is saved by the existence of heavier \\\" Performing the same trick with graviton interactions particles \u2013 the W, Z and Higgs bosons that aren\u2019t so requires a particle so massive that it acts like a easily produced, cancelling out the infinities black hole \u2013 and all calculations are off again GRAVITON SIX ROUTES TO A THEORY OF EVERYTHING STRING THEORY gravity is their only real rival as space and time exist, then try to String theory predicts that space a\u00a0theory of everything. The basic build up the rest of the universe. has hidden extra dimensions, idea is that space isn\u2019t continuous, Quantum graphity \u2013 the brainchild invoking symmetries embedded as\u00a0we usually think, but is instead of\u00a0Fotini Markopoulou at the in\u00a0these 10 (mainly tiny) dimensions broken up into tiny chunks 10\u00af35 Perimeter Institute for Theoretical to \u201cfold\u201d energy into geometric metres across. These are then Physics in Waterloo, Canada, shapes that look like certain connected by links to make the and\u00a0colleagues \u2013 tries to do away fundamental particles, or mimic space we experience. When these with them. the\u00a0way that space curves in the links are tangled up into braids presence of mass. Different string and\u00a0knots, they produce elementary When the universe formed in vibrations produce particles with particles. As with string theory, the\u00a0big bang, says Markopoulou, different characteristics: electrons, a\u00a0true experimental test is still there was no such thing as space quarks, the Higgs boson, the some\u00a0way off. as\u00a0we know it. Instead, there was hypothetical \u201cgraviton\u201d. an\u00a0abstract network of \u201cnodes\u201d CAUSAL DYNAMICAL of\u00a0space, in which each node was M-THEORY TRIANGULATION (CDT) connected to every other. Very soon A more general 11-dimensional Just as loop quantum gravity breaks afterwards, this network collapsed theory that ties together variants up space into tiny \u201cbuilding blocks\u201d, and some of the nodes broke away of\u00a0string theory. A puzzling feature CDT assumes that space-time is from each other, forming the large is\u00a0that it predicts many (possibly split into tiny building blocks \u2013 this universe we see today. infinitely many) ways of curling time, four-dimensional chunks up\u00a0these dimensions, leading to a called pentachorons. I N F O R M AT I O N multiverse of different universes. THEORY Some may look like ours, with three The pentachorons can then The usual focus on general relativity generations of quarks and leptons be\u00a0glued together to produce a and quantum theory ignores the and four forces; many will not. But large-scale universe \u2013 which turns part\u00a0that the third great pillar of from a theoretical point of view, out to have three space dimensions physics, thermodynamics, might they\u00a0all seem plausible. and one time dimension, just as the play. The laws of thermodynamics real one does. CDT as it currently rule everything, including the flow of LOOP QUANTUM GRAVITY stands, however, can\u2019t explain information. A better understanding It hasn\u2019t had quite the same the\u00a0existence of matter. of how they do this might provide exposure as string theory or clues as to why things are as M-theory, but so far loop quantum QUANTUM GRAPHITY they\u00a0are \u2013 and narrow down the All the theories above assume that parameters of any ultimate theory. Chapter 6 | Beyond relativity | 87","BEYOND THE CYCLIC UNIVERSES BIG BANG WAS the big bang perhaps not truly a beginning? That What exactly happened at that point is\u00a0one hypothesis cosmologists have put forward in 13.8\u00a0billion years ago when our universe recent years to explain away the inflationary big bang\u2019s sprang into existence? This isn\u2019t something problems. Rather than marking a \u201csingularity\u201d at the our current theories can explain fully absolute beginning of space and time, it might just convincingly. The inflationary big bang have been a recent event in a much longer history. remains the most widely supported hypothesis, but when you add quantum \u2190- theory\u00a0into the mix, other possibilities Turn back to page 30 for the basics on the- become\u00a0available. inflationary big bang- COLLIDING BRANES The idea was first put forward in 2001 by Paul Steinhardt, who had become dismayed by the If our universe is a 4D \u201cbrane\u201d floating in a higher-dimensional consequences of the inflationary theory he had space, it might collide with other branes, acquiring energy that we helped\u00a0develop two decades earlier, together with interpret as the big bang his\u00a0colleagues Justin Khoury, Burt Ovrut and Neil Turok. The inspiration for this idea came from VISIBLE string\u00a0theory, which predicts the existence of extra BRANE dimensions beyond the four of space and time we see. UNKNOWN OUR This leads to the possibility that our 4D cosmos BRANE EXPANDING is\u00a0situated on a \u201cbrane\u201d, a lower-dimensional object UNIVERSE floating in a higher-dimensional space. In this picture, COLLISION ours isn\u2019t necessarily the only brane: two branes BIG BANG floating a microscopic distance away from each other can form the boundaries to a five-dimensional space in INTERPRETATION between, like two slices of bread bounding a sandwich. OF THE BIG BANG: Steinhardt and his colleagues\u2019 idea was that every New injection of energy into few\u00a0trillion years or so, neighbouring branes float together and touch, resulting in an explosive exchange a preexisting cosmos of energy as the fifth dimension briefly disappears into a singularity, reappearing as the branes move apart again (see diagram \u201cColliding branes\u201d, left). Our 4D brane receives a huge injection of energy in this collision, something we interpret as a big bang. It doesn\u2019t represent the absolute beginning of our space and time, however, but a reinvigoration of a cosmos with an eternal existence \u2013 a \u201cbig bounce\u201d, if you like. Such \u201cekpyrotic\u201d universes do much of what the inflationary big bang was invented to do. \u201cYou can solve\u00a0problems that would have been intractable without inflation given only 14 billion years,\u201d says Steinhardt. The branes are essentially flat to begin with, so the flatness problem disappears. When the branes clash, they hit at almost the same time everywhere, so the energy that will form matter and radiation pours 88 | New Scientist Essential Guide | Einstein\u2019s Universe","BOUNCE BACK 10-44 seconds: SPACE-TIME Loop quantum cosmology predicts that the universe BIG BOUNCE did not arise from nothing in a big bang. Instead it grew from the collapse of a preexisting universe that bounced back from oblivion PREEXISTING UNIVERSE Collapse due to gravity SPACE-TIME IS CLASSICAL SPACE-TIME 1017 seconds: IS CLASSICAL TODAY 10-38 seconds: 1016 seconds: INFLATION BEGINS FIRST GALAXIES 10-45 seconds: 1013 seconds: SUPERINFLATION ERA COSMIC MICROWAVE BACKGROUND RADIATION in\u00a0almost uniformly, creating a nearly homogeneous quantum fluctuations remain small at all times. cosmos. Small quantum energy fluctuations are all That\u00a0means the outcome of the bouncing scenario that\u00a0are needed to give enough density variation to is\u00a0definite, unlike in the messy multiverse scenario, eventually seed galaxies. And because the idea needs which is produced by wild quantum fluctuations no multiverse, the \u201cmeasure problem\u201d that makes during inflation. making cosmological predictions impossible disappears with it. In 2018, cosmologist Anna Ijjas at the Max Planck Institute for Gravitational Physics in Potsdam, Since then, many other cyclic universe models Germany, published the first theoretical account that\u00a0propose some form of cosmos existed before of\u00a0how such a bounce could happen, producing the\u00a0big bang have sprung up. If nothing else, they the\u00a0kind of smooth distribution of energy and flat, introduced some competition into the market. untwisted geometry of space-time that we observe. \u201cIt\u00a0shows that you\u2019re not stuck with inflation \u2013 other\u00a0ideas are possible,\u201d says Steinhardt. A natural extension of the concept is that we could be\u00a0living in a cyclic universe with bounces occurring Cyclic scenarios depend on the existence of a long every 100\u00a0billion years or so. It is even possible to phase of ultra-slow contraction before the bounce. imagine a cyclic universe with no beginning or end. Just\u00a0as inflation required a special form of energy, the Each period of ultra-slow contraction would erase inflaton field, to drive rapid expansion, ultra-slow any\u00a0fine details of the previous cycles and bring the contraction requires a special form of energy that universe to the bounce point with the same conditions exerts extraordinarily high pressure. as it had the cycle before. The high pressure slows contraction by resisting As a result, all the features of the universe would compression and, at the same time, tends to smooth be\u00a0the same on average during each cycle, including out any irregularities in the distribution of energy and the\u00a0temperature, the concentration of dark matter, in the fabric of space-time. Unlike an inflationary ordinary matter and dark energy and the number phase, however, a slowly contracting phase doesn\u2019t of\u00a0observable stars and galaxies. require special starting conditions. It can be triggered in various ways, for example by decaying dark energy. In other words, if you had lived on a planet like Earth What\u2019s more, in a slowly contracting, cold universe, in the cycle before our own, you would observe roughly the same basic properties of the universe as we do. > Chapter 6 | Beyond relativity | 89","NO BOUNDARIES CHANGING PHASE By adding the possible quantum histories of the universe together, Did the big bang create space and time, or could they have existed we can work back from today\u2019s cosmos to the universe\u2019s origin already in a bizarre, unfamiliar form? Water provides clues UNIVERSE STEAM TODAY LIQUID WATER BIG BANG CONDENSATION INTERPRETATION OF THE BIG BANG: When steam turns into liquid water, the molecules get closer together and Timeless point at the beginning of many possible quantum are less able to move randomly histories where classical physics breaks down NO-BOUNDARY UNIVERSES Others have produced cyclic models from different starting points, for example loop quantum gravity, a Models of the big bang that involve a singularity in rival to string theory for the title of a viable theory of our\u00a0space-time, including the inflationary big bang, everything. If \u201cloop quantum cosmology\u201d turns out neatly excuse us from explaining what happened at the to\u00a0be right, our universe emerged from a pre-existing universe\u2019s beginning: the singularity is a place where universe that had been expanding before contracting the universe falls off the cliff of existence and the laws due to gravity. As all the matter squeezed into a of physics break down. A criticism of cyclic models is microscopic volume, this universe approached the that they don\u2019t explain how extra dimensions and pre- so-called Planck density, 5.1 \u00d7 1096 kilograms per cubic existing universes survive their momentary lapse into metre. At this stage, it stopped contracting and a singularity. \u201cTo me, it doesn\u2019t seem to work,\u201d says rebounded, giving us our universe (see diagram Thomas Hertog at the Catholic University of Leuven \u201cBounce back\u201d, previous page). (KU Leuven) in Belgium, who worked on the idea for a\u00a0couple of years. \u201cThe calculations suggest that the That is because an extraordinary repulsive force transition through the singularity is very unlikely.\u201d develops in the fabric of space-time at densities equivalent to compressing a trillion solar masses The many clashes between branes that the model down\u00a0to the size of a proton. At this point, the implies just compound the problem, says Sean Carroll at quanta\u00a0of\u00a0space-time can\u2019t be squeezed any further. the California Institute of Technology. \u201cIf you follow the The compressed space-time reacts by exerting an cyclic universe backward in time, the conditions that outward\u00a0force strong enough to repulse gravity. This you need become more and more special, or unlikely.\u201d momentary act of repulsion causes the universe to The way to solve that problem is to postulate some kind rebound. From then on, the universe keeps expanding of beginning that provided a special set of conditions \u2013 because of the inertia of the big bounce. Nothing can but that seems to defeat the object of the theory. slow it down \u2013 except gravity. Carroll thinks Steinhardt might have had the right idea Such models lead to a dramatic prediction: in seeking an answer to the big-bang conundrum in the the\u00a0current phase of the universe in which its unification of general relativity and quantum mechanics, expansion rate is slowly accelerating will come to he says. But the answer could lie in using those ideas an\u00a0end and the universe will enter a new contracting not to replace inflation, but to make it better. phase. It will then head towards a new bounce and a\u00a0new phase of expansion. If that is true, the dark \u201cWe use a half-assed version of quantum mechanics energy\u00a0that is driving the current accelerated when we do cosmology,\u201d he says. Inflation is a theory expansion must decay away, which may be about space-time and gravity, so it is anchored in detectable\u00a0in future experiments. general relativity. It incorporates a few aspects of 90 | New Scientist Essential Guide | Einstein\u2019s Universe","SPACE-TIME TIME CONDENSATION BIG BANG Space-time may also be built of atom-like parts. The big bang could have been when they \\\"condensed\\\", allowing familiar features like geometry and time to emerge quantum physics, such as the uncertainty fluctuations historical universes \u2013 pops into existence from that push the inflaton off the mountain ledge, but as nothing\u00a0with all its laws of physics intact. Because we\u00a0lack a sure-fire way of connecting relativity and of\u00a0this lack of a clear beginning, Hawking and quantum theory, it remains a \u201csemiclassical\u201d theory. Hartle\u00a0called it the no-boundary proposal. \u201cMaybe that\u2019s not good enough,\u201d says Carroll. Following the rules of quantum mechanics, But how can we put the quantum into cosmology? they\u00a0added up all the possible histories that began in a Just after inflation had burst on to the scene in the universe with no boundary and ended in the universe we 1980s, the late Stephen Hawking and his collaborator see today (see diagram \u201cNo boundaries\u201d, far top left). The James Hartle at the University of California, Santa idea is a kind of multiverse in reverse: a single universe Barbara, made a stab at it. In quantum physics, when a with multiple histories. The resulting wave function gets particle travels from A to B, it doesn\u2019t take a single path rid of inflation\u2019s measure problem, as it encodes a unique but can pass along two or more paths simultaneously, set of probabilities for anything we might observe. And interfering with itself at the other end as if it were because the admissibility of the histories is determined a\u00a0wave. To find out which path we are most likely by what we see in the cosmos today, problems such as to\u00a0observe, we must add together the quantum- the flatness of space-time or the homogeneity of the mechanical \u201cwave functions\u201d encoding each possible cosmic microwave background cease to be problems: path, working out how their individual peaks and instead, they are the inputs to the theory. troughs cancel and amplify. Encoded within this total wave function is everything we need to know about the The no-boundary universe had its attractions for quantum particle at B, including the probabilities for many physicists. \u201cIf it turns out the universe has to the outcomes of any measurement we choose to make. have an origin, I find this initial state to be a very plausible one,\u201d says Alan Guth, the originator of Hawking and Hartle argued that a similar approach inflation. And as far as his theory is concerned, the could be applied to the universe as a whole. Point B no-boundary proposal turned up a pleasant surprise. is\u00a0the universe we see today. Looking back towards As Hartle, Hawking and Hertog showed in 2008, its\u00a0origin, we can trace many valid histories of its although the theory wasn\u2019t dreamed up with inflation expansion back towards a point \u2013 point A \u2013 where in mind, it crops up naturally along many paths the semiclassical physics breaks down and quantum universe could have taken to get here. \u201cYou can space\u00a0and time become so gnarled that the two cease calculate the probability that inflation occurred, and to\u00a0be clearly distinguished. This point is no longer a it\u00a0turns out that probability is very high,\u201d says Hertog. beginning of time in a singularity as in the standard inflationary cosmology, but a timeless point where The idea also fits neatly with one of the most the\u00a0universe \u2013 or rather a superposition of all possible profound ideas to come out of string theory in recent years: the holographic principle. This states that the > Chapter 6 | Beyond relativity | 91","physics of a 4D universe such as ours, including gravity, There is more to Hu\u2019s analogy than meets the eye. is mathematically equivalent to the physics on its 3D In\u00a0the past few years, physicists have made models of boundary without gravity. The implication is that the warped regions of space-time from fluids, and found world we see around us is nothing but a holographic that the two are eerily similar. Think of water in its projection of information from the edge of reality. It three familiar phases: ice, liquid water and steam. All sounds implausible, but the principle pops up not just are made of water molecules, but how those molecules in string theory, but in almost any approach to unifying interact varies. In steam, they whizz around, doing their relativity and quantum theory dreamed up so far. own thing. If they hit a cold window pane, however, they begin huddling together, condensing from gas Although the no-boundary proposal says that the to\u00a0liquid. Hu thinks space-time can undergo similar universe has no boundary in the far past, it does give phase changes. Without something like condensation, a\u00a0boundary in the infinitely far future. By calculating the atoms of space would exist as some nebulous the physics on this boundary, Hertog extracted the netherworld bereft of time and geometry. probabilities of all the possible universes that can emerge as its holographic projections. Remarkably, Subsequent theoretical work has shown how such the\u00a0probabilities for things like the homogeneity of the atoms could indeed undergo an orderly phase change cosmic background or the amount of dark energy are into something resembling space, with basic features the same as those that you get from the no-boundary we take for granted, like geometry (see diagram wave function. This supplies a direct connection \u201cChanging phase\u201d, page 90). In 2017, further between string theory, the most popular route towards calculations showed that what emerged looked like a theory of everything, and the no-boundary proposal, the\u00a0expanding space-time of our universe. And there which produces inflation naturally. Of course, that is far was a surprise: the space-time fluid didn\u2019t like being from a concrete proof that it is a true picture of how the funnelled into a singularity at the moment of the big universe began \u2013 or didn\u2019t. bang. Instead, it wanted to bounce back outwards. THE BIG BOIL But Daniele Oriti, a theorist at the Max Planck Institute for Gravitational Physics in Potsdam, Imagine tipping a bucket of water over your head. Germany, who was involved in that work, believes The\u00a0water is made of molecules that are ultimately that\u00a0once these models are examined more closely, governed by quantum theory, but you needn\u2019t be aware there is every chance that the big event at the start of the details to know you will be soaked. You could of\u00a0their universe will be neither a bang nor a bounce. even work out precisely how the water would cascade using the science of hydrodynamics, which existed In such extreme conditions, he says, space-time long before quantum theory. If hydrodynamics allows could well have changed from one phase to another, us to describe fluids without fussing over the fine meaning it didn\u2019t have a definite beginning at all. details of molecules, might it be possible to create What\u00a0we think of as the big bang was just the moment space-time from atoms of space, without first of condensation. The big condensation, you might perfecting a description of those atoms? say.\u00a0Or, if we are still in rewind, the big boil. That was an idea that came to Bei Lok Hu, a theorist So what is the space-time netherworld on the other at\u00a0the University of Maryland in Baltimore, as a way side of the big boil like? Here, language fails, because of\u00a0getting round the problem that no one knows what every question \u2013 what, where, how \u2013 presupposes happens to space-time when, as we wind the story of concepts that simply wouldn\u2019t have existed. \u201cYou the\u00a0expanding universe backwards, the entire universe have\u00a0to think about these atoms of space without shrinks to a scale where quantum effects can\u2019t be ignored. them\u00a0existing somewhere in space, or evolving somewhere in time,\u201d says Oriti. \u201cThe very notion of\u00a0time and space has to be constructed out of them.\u201d\u00a0\u00a0\u275a 92 | New Scientist Essential Guide | Einstein\u2019s Universe","ESSAY UNPICKING ET\u2019S say you want to meet a friend for THE FABRIC OF THE coffee. You have to tell them where you UNIVERSE are going to be \u2013 your location in space \u2013 Space-time is the fundamental fabric of the universe according to general relativity \u2013 but you also need to let them know when. but\u00a0it might have a deeper origin in quantum theory, says Sean Carroll. Both bits of information are necessary PROFILE because we live in a four-dimensional SEAN CARROLL continuum: three-dimensional space Sean Carroll is a physicist and everything within it, from steaming at\u00a0the California Institute of\u00a0Technology, and author coffee machines to stars exploding in of\u00a0books including Something Deeply Hidden: faraway galaxies, all happening at Quantum worlds and the emergence of spacetime different moments of one-dimensional time. \u201cSpace-time\u201d is simply the physical universe inside which we and everything else exists. And yet, even after\u00a0millennia living in it, we still don\u2019t know what space-time actually is. Physicists have strived to work it\u00a0out for more than a century. In recent years, many of\u00a0us have been trying to figure out what might be the\u00a0threads from which the fabric of reality is woven. We\u00a0have ideas, each with its own selling points and shortcomings. But for my money, the most exciting one is the most surprising. It is the idea that space-time emerges from a weird property of the quantum world that means particles and fields, those fundamental constituents of nature, can be connected even if they are at opposite ends of the universe. If that is correct, we might finally have found a bridge between the two irreconcilable totems of physics, placing us on the threshold of a theory of\u00a0quantum gravity. We would also have the most startling demonstration yet that the world we see isn\u2019t\u00a0the world as it is \u2013 that there is always \u201csomething deeply hidden\u201d, as Albert Einstein put it \u2013 and that the only way to understand the fundamental nature of reality is by confronting quantum mechanics head on. Space-time is a relatively new notion. Isaac Newton had no need for it. For him, space and time were individually real and absolute. Only when Einstein formulated his special theory of relativity in 1905 did\u00a0the two start to come together. He showed that different observers will generally divide space-time into \u201cspace\u201d and \u201ctime\u201d in different, incompatible ways; what is \u201cspace\u201d and what is \u201ctime\u201d are relative to\u00a0how an observer is moving. > Chapter 6 | Beyond relativity | 93","Various thinkers had previously speculated that the produces a quantum version of gravity, but not two should be rolled together. In Edgar Allan Poe\u2019s 1848 one\u00a0that connects with our world in an obvious way. prose poem Eureka, for instance, he wrote that \u201cspace Nor does it resolve those fundamental conceptual and duration are one\u201d. But it wasn\u2019t until 1908 that problems. String theory\u2019s leading rival, loop quantum mathematician Hermann Minkowski unified them gravity, is an attempt to directly quantise general in\u00a0a\u00a0scientific way. He dramatically proclaimed: relativity. Loop proponents typically take the \u201cHenceforth, space for itself, and time for itself, shall conceptual challenges of quantum gravity more completely reduce to a mere shadow, and only some seriously than their stringy colleagues, but the sort of union of the two shall preserve independence.\u201d challenges remain. Einstein was unimpressed, grumbling about This has led some physicists to take a step back \u201csuperfluous learnedness\u201d. But he eventually came and\u00a0ask the question in a different way. The standard round to the idea, putting the geometry of space-time approach to developing a quantum description of firmly at the heart of his general relativity. It said that some phenomenon, like the electromagnetic field space-time isn\u2019t merely a static background in which or\u00a0a\u00a0collection of atoms, is to start with a classical things happen. It is a dynamic entity, warping and description and then \u201cquantise\u201d it. That approach stretching under the influence of mass and energy. has\u00a0failed again and again when it comes to gravity The\u00a0curvature of space-time manifests itself to us and\u00a0space-time. It also isn\u2019t how nature works. as\u00a0the\u00a0force of gravity. The\u00a0real\u00a0world doesn\u2019t start classically and then somehow\u00a0quantise. It is quantum from the start, Still, it would seem weird to ask what space-time and\u00a0the classical world emerges as an approximation. is\u00a0\u201cmade of\u201d in classical physics. In general relativity, space-time changes over time in response to other So maybe we shouldn\u2019t be trying to quantise gravity stuff. But it is still a background, and a fundamental at all. Perhaps we should instead formulate a quantum constituent of nature. It isn\u2019t made of anything; theory from the start, and then show how classical it\u00a0just\u00a0\u201cis\u201d. space-time emerges from that. It is a new approach that\u00a0has dramatic consequences for how we think The problems with that view started with the about what space-time itself is made of. discovery of quantum mechanics, the rules that govern\u00a0the behaviour of subatomic particles and fields. To make progress in this direction, it is helpful to Scientists haven\u2019t been able to construct a quantum- start with our current best physical theory, which is mechanical theory of gravity as they have for the quantum field theory. According to this theory, the three\u00a0other fundamental forces of nature. Part of the fundamental ingredients of the world are fields, such issue is technical: when we try to make classical general as\u00a0the electric and magnetic fields. Even particles like relativity into a quantum-mechanical theory using electrons and quarks are simply vibrations in fields standard techniques, our equations blow up and we that\u00a0stretch through space. get\u00a0nonsensical answers. But part of it is conceptual. Classically, we can specify the value of a field in Quantum mechanics tells us that systems exist in an\u00a0approximate fashion by dividing space into tiny superpositions of different measurable quantities regions, then listing the value of the field in each like\u00a0position and velocity. There is no such thing as region. Once we graduate to quantum field theory, \u201cthe\u00a0position\u201d of a quantum particle; there are many an\u00a0extra feature comes into the game: the values of possible positions, which take on definite values only the\u00a0field in different regions can be entangled with when we observe them. How in the world can space- each\u00a0other. Due to quantum uncertainty, we don\u2019t time exist in a superposition of different possibilities? know exactly what answer we will get if we measure That would make it impossible to say for sure that the\u00a0field at some location, but entanglement means a\u00a0certain event happened at a definite location in the\u00a0answer we get at one point will affect what we space\u00a0and time. would measure at any other point. Physicists of different persuasions have taken In the vacuum state of an ordinary quantum field different approaches to constructing a solution in the theory \u2013 empty space, no particles flying around \u2013 form of a theory of quantum gravity. The most popular the\u00a0entanglement between fields in different regions is string theory, which replaces particles with loops and is\u00a0directly tied to the distance between them, and segments of vibrating string. String theory successfully therefore to the geometry of space-time. Nearby 94 | New Scientist Essential Guide | Einstein\u2019s Universe","Space-time might be woven from quantum entanglement regions are highly entangled with each other, In a model universe, the equations describing gravity in a while\u00a0faraway regions share little entanglement. volume of space are equivalent to those describing the surface of that volume, which don\u2019t include gravity. This suggests This suggests an intriguing way to reverse our space on the inside somehow emerges from the properties of normal way of thinking and so find space-time the outside, namely entanglement within\u00a0quantum theory. Let us imagine starting with\u00a0just a quantum state, no preexisting notion SURFACE: of\u00a0space-time. Now we can try to work backwards, ENTANGLED to\u00a0extract space-time from entanglement. FIELDS\/PARTICLES INTERIOR: If in ordinary physics the entanglement between EMPTY SPACE two\u00a0regions goes down as the regions get further apart, let us imagine defining the distance as (inversely) Sure enough, when you reduce the entanglement related to the entanglement. In that case, having a connecting two regions of the outside surface, the quantum state automatically gives us the \u201cdistance\u201d space inside pulls apart as if pulling at two ends of between any two parts of it, and therefore defines a piece of chewing gum a\u00a0geometry on this emergent space. If you eliminate all entanglement, the space inside So far, so good. But a quantum state exists at each splits in two, suggesting that entanglement is the moment of time, so it can at best define the geometry thread that binds space-time of space at that moment. We want to extend this to four-dimensional space-time. Thankfully, here we can\u00a0borrow a trick from physicist Ted Jacobson at the University of Maryland in Baltimore, who, in 1995, showed how we could derive Einstein\u2019s equation for general relativity from simple assumptions about the\u00a0relationship between entropy and geometry. Entropy, a measure of disorder, is directly related to entanglement: the more entangled a region is with the rest of the world, the more entropy it contains. Einstein said that it is adding matter or energy to a region that causes space-time to curve. Jacobson showed that increasing the entanglement of a region can have the\u00a0same effect, if we insist that the amount of entropy must be proportional to the area bounding that region. That is automatically true in empty space, but Jacobson suggested that it remains true even when space isn\u2019t empty. You can try to add more entanglement, but space-time will bend to compensate, so that entropy always remains proportional to area. So Einstein says that energy causes curvature, while Jacobson says entanglement does. But Jacobson also argued that it is really the same thing: whenever you add entanglement, energy necessarily follows. From this logic, he was able to derive that the curvature of space-time in his approach obeyed the same equation that Einstein first wrote down for general relativity. Gravity, it appears, can arise from entanglement, rather\u00a0than directly from mass and energy. This remarkable result was the beginning of what is now\u00a0called \u201cthermodynamic\u201d or \u201centropic\u201d gravity. But it doesn\u2019t quite get us to where we need to be. > Chapter 6 | Beyond relativity | 95","In deriving his alternative picture of where gravity quantum-gravity side of this correspondence is directly comes from, Jacobson assumed a classical space-time tied to quantum entanglement on the field-theory side. and imagined that there were quantum fields living As we decrease entanglement in the field theory, space- within it. Ideally, we would like to keep everything time on the quantum-gravity side grows apart (see quantum from the start and derive the existence of diagram, previous page). space-time itself. This is something I recently attempted with my collaborators, ChunJun (Charles) Maldacena and Leonard Susskind at Stanford Cao and Spiros Michalakis at the California Institute University in California have taken this connection of\u00a0Technology. Rather than starting with vibrating to\u00a0extremes with a bold idea they dubbed \u201cER = EPR.\u201d quantum fields living in space-time, we started with ER\u00a0stands for Albert Einstein and Nathan Rosen, completely abstract quantum \u201cdegrees of freedom\u201c. who\u00a0wrote a paper in 1935 proposing the existence of\u00a0wormholes, or shortcuts through space-time. This is just some quantity that can take on EPR,\u00a0meanwhile, stands for Einstein, Boris Podolsky different\u00a0values, independently of other quantities. and Rosen, who collaborated on another paper In\u00a0Newtonian mechanics, the degrees of freedom are emphasising the role of entanglement in quantum positions and velocities of particles; in field theory, theory. The ER = EPR conjecture therefore posits that they are the values and rates of change of the fields. whenever you have two entangled particles, there is In\u00a0our approach, the degrees of freedom don\u2019t have a\u00a0tiny wormhole connecting them. any\u00a0direct physical interpretation. They are the basic stuff of reality, the essence out of which everything Don\u2019t take this too literally. The wormholes that else\u00a0is made \u2013 a kind of \u201cquantumness\u201d that preexists purportedly connect pairs of particles would be everything. Most importantly, these quantum degrees microscopically small and impossible for anything of freedom are entangled with each other. to\u00a0pass through. It is only when massive amounts of entanglement become involved that we begin to see With that in mind, we flip around Jacobson\u2019s idea. a\u00a0macroscopic distortion in the fabric of space. Now we can define the area surrounding a region as the\u00a0entanglement of its degrees of freedom with the Moreover, our universe has a positive vacuum outside world. And sure enough, the corresponding energy, not a negative one, so the implications of geometry obeys Einstein\u2019s equation of general the\u00a0equivalence revealed in Maldacena\u2019s negative- relativity. Gravity, in other words, can emerge vacuum-energy thought experiment don\u2019t translate directly\u00a0from the quantum essence of reality, directly to an actionable strategy for dealing with without\u00a0quantising any assumed classical stuff. quantum gravity in the real world. They do, however, serve as another strong hint that quantum That might sound like a conclusion, but it is more entanglement is at the heart of it all. like\u00a0a promising beginning. Many assumptions went into our derivation, and whether these assumptions All of these ideas are, at present, somewhere hold true in nature remains to be seen. Most between\u00a0promising conjectures and optimistic importantly, our derivation of Einstein\u2019s equation dreams.\u00a0We don\u2019t know the best way to think about from\u00a0entanglement only works when gravity is these supposed fundamental degrees of freedom that weak\u00a0and space-time is nearly flat. Once gravity entangle together to make space-time, nor do we know becomes strong and space-time is curved, as in how they interact with each other in any detailed way. the\u00a0big\u00a0bang or near a black hole, radically new We can\u2019t yet derive the emergence of quantum fields phenomena\u00a0become important. living within space-time, obeying the rules of relativity. And we certainly can\u2019t answer important questions The most dramatic of these is the holographic like\u00a0why the energy of empty space is so small, or principle, the idea that the degrees of freedom why\u00a0space has four macroscopic dimensions. describing a black hole can be thought of as living on its\u00a0edge, the event horizon, rather than the interior. Even so, imagining that space-time emerges from Juan Maldacena at the Institute for Advanced Study in quantum entanglement is a promising way to think Princeton, New Jersey, used the holographic principle about the basic nature of reality. It may be that it was a to show an equivalence between two very different mistake to start with general relativity and try to theories: quantum field theory without gravity in quantise it; maybe space-time was lurking within four-dimensional space-time, and quantum gravity quantum mechanics all along. with a negative vacuum energy in five dimensions. And even if formulating a complete theory of Subsequent work by Mark Van Raamsdonk at the quantum gravity isn\u2019t your thing, thinking about University of British Columbia in Canada and others space-time this way should at least put a new slant on has shown that the space-time geometry on the the familiar four-dimensional continuum in which we live, rushing around in space to be on time for coffee.\u00a0\u00a0\u275a 96 | New Scientist Essential Guide | Einstein\u2019s Universe","ESSENTIAL GUIDES NEW SCIENTIST ESSENTIAL GUIDES DELIVERED DIRECT TO YOUR DOOR &EWIH\u0002SR\u0002XLI\u0002FIWX\u0002GSZIVEKI\u0002JVSQ\u00022I[\u00027GMIRXMWX\u000f\u0002XLI\u0002)WWIRXMEP\u0002+YMHIW\u0002EVI\u0002 GSQTVILIRWMZI\u000f\u0002RIIH\u0010XS\u0010ORS[\u0002GSQTIRHMYQW\u0002GSZIVMRK\u0002XLI\u0002QSWX\u0002I\\\\GMXMRK\u0002 XLIQIW\u0002MR\u0002WGMIRGI\u0002ERH\u0002XIGLRSPSK]\u0002XSHE]\u0012\u0002\u0002 +IX\u0002XLI\u0002WIVMIW\u000f\u0002MRGPYHMRK\u0002XLI\u0002VIGIRXP]\u0002TYFPMWLIH\u0002MWWYI\u0002SR\u0002RYXVMXMSR\u0002\t\u0002HMIX\u000f\u0002[MXL\u0002ER\u0002 )WWIRXMEP\u0002+YMHIW\u0002WYFWGVMTXMSR\u0012\u0002-X\u0002QIERW\u0002]SY\u0002HSR X\u0002LEZI\u0002XS\u0002WIEVGL\u0002JSV\u0002MWWYIW\u0002 MR\u0002XLI\u0002WLSTW\u0002r\u0002[I\u0002GER\u0002HIPMZIV\u0002XLIQ\u0002HMVIGX\u0002XS\u0002]SYV\u0002HSSV\u0012 FOR MORE INFORMATION ON FUTURE ISSUES AND SUBSCRIPTION OFFERS, VISIT: NEWSCIENTIST.COM\/ESSENTIALGUIDE","ESSENTIAL GUIDE\u211610 EINSTEIN\u2019S UNIVERSE HOW DID THE UNIVERSE BEGIN, AND HOW IS IT CHANGING? WHAT IS IT MADE OF? DO BLACK HOLES TRULY EXIST? ALBERT EINSTEIN\u2019S SPACE-AND-TIME-WARPING THEORIES OF RELATIVITY HAVE REVOLUTIONISED OUR VIEW OF THE COSMOS OVER THE PAST CENTURY. FIND OUT HOW IN THIS TENTH NEW SCIENTIST ESSENTIAL GUIDE, WITH TOPICS INCLUDING: \u2776 How relativity works \u2777 The big bang universe \u2778 Dark matter and dark energy \u2779 Black holes and gravitational waves \u277a The hunt for quantum gravity \u00a39.99 10 9 772634 015019"]
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