The Physics Book

NUCLEAR AND PARTICLE PHYSICS 249 See also: Generating electricity 148–151 ■ Atomic theory 236–237 ■ Nuclear rays 238–229 ■ The nucleus 240–241 ■ Subatomic particles 242–243 Atoms have a Radioactive materials have central nucleus. unstable nuclei. The released neutrons Splitting the nucleus can cause the nuclei (by bombarding it with of other atoms to split. neutrons) releases neutrons and a huge amount of heat energy. Photographed in 1913 with Otto Hahn in their Berlin laboratory, Lise Meitner later went on to expand on Enrico Fermi’s work and explain nuclear fission. A chain reaction can be initiated. The chain reaction can An uncontrolled reaction nuclei were breaking into fragments, be controlled and releases the massive, in a process Hahn named nuclear the heat used to fission. Crucially, Meitner also used destructive explosion Albert Einstein’s theory of energy– produce electricity. in a nuclear bomb. mass equivalence (summarized in the equation E = mc2) to show that the mass mysteriously lost in the process was being converted into energy. The jigsaw’s pieces finally all fit together. In 1934, Italian physicist Enrico elements were fragments from Powerful forces Fermi bombarded uranium with the original uranium nuclei, but The forces within a nucleus are neutrons (subatomic particles her proposal was dismissed in powerful and precariously balanced. discovered just two years earlier). the scramble for scientists to The nuclear force binds the nucleus Fermi’s experiment seemed to understand Fermi’s experiment. together while positively charged transform uranium into different protons inside the nucleus repel elements—not the new, heavier In 1938, German chemists each other with approximately ones he had expected, but isotopes Otto Hahn and Fritz Strassmann 230N of force. Holding together the (variants with a different neutron expanded on Fermi’s work. They nucleus requires a huge amount of number) of lighter elements. Fermi found that bombarding uranium “binding energy,” which is released had divided the apparently with neutrons produced barium, when the nucleus breaks apart. indivisible atom, although it took apparently losing 100 protons and The loss of equivalent mass is years for the scientific community neutrons in the process. Hahn measurable: for the uranium to grasp the magnitude of what he conveyed the baffling findings to his fission reaction under investigation, had done. German chemist Ida former colleague Lise Meitner, who around one-fifth the mass of a Noddack suggested that the new had fled Nazi Germany for Sweden. proton seemed to vanish in Meitner proposed that the uranium a burst of heat. ❯❯

250 NUCLEAR BOMBS AND POWER No limits exist to the discovery, as part of the Manhattan spiraling out of control, while destructiveness of this Project. He and his colleagues found producing too few would cause weapon … its very existence that slow-moving (thermal) neutrons the reactions to die out. Criticality [is] a danger to humanity. were more likely to be absorbed by requires a careful balance of fuel nuclei and cause fission. U-235 (a mass, fuel density, temperature, Enrico Fermi natural isotope of uranium) was and other variables. identified as an ideal fuel because Meitner and other physicists it releases three thermal neutrons Fermi’s nuclear reactor went recognized that neutrons “boiling every time it fragments. U-235 live in 1942. The Chicago Pile-1 off” from nuclear fission opened up is a rare isotope that makes up was built on a university squash the possibility of chain reactions less than 1 percent of natural court, using almost 5 tons of in which the free nuclei caused uranium, so natural uranium has unenriched uranium, 44 tons of successive fission reactions, to be painstakingly enriched to uranium oxide, and 363 tons of releasing more energy and sustain a chain reaction. graphite bricks. It was crude, neutrons with each reaction. unshielded, and low-power, but it A chain reaction could either The critical factor marked the first time scientists had release a steady flow of energy or The Manhattan Project pursued sustained a fission chain reaction. mount a vast explosion. As nations multiple methods for enriching lurched toward World War II, and uranium and two possible nuclear While nuclear reactors must fearful of how this power might reactor designs: one based at sustain criticality for civilian use (as be used in the wrong hands, Columbia University that used heavy in nuclear power stations), nuclear researchers immediately began water (water containing a hydrogen weapons must surpass criticality to to study how to sustain fission. isotope) to slow down neutrons; and release deadly quantities of binding a second led by Fermi at the energy in a flash. Scientists working University of Chicago that used under Oppenheimer at Los Alamos graphite. The scientists were aiming were responsible for designing for criticality—when the rate at such weapons. One design used which neutrons are produced from implosion, in which explosives fission is equal to the rate at which around a fissile core ignited and neutrons are lost through absorption so produced shock waves. It and leaking. Producing too many compressed the core to a smaller, neutrons would mean the reactions denser volume that passed criticality. An alternative design—a “gun-type” weapon—blasted two The Manhattan Project Large amount Neutrons US president Franklin D. Roosevelt of heat energy released released by the wanted the US and its allies to be when nucleus splits split hit more the first to harness atomic power uranium nuclei, as conflict consumed much of the Uranium-235 triggering further world. Although the US did not nucleus fission reactions enter World War II until Japan’s attack on Pearl Harbor in 1941, the Neutron Unstable nucleus secretive Manhattan Project, which hits atomic splits into two employed many of the greatest nucleus daughter nuclei scientists and mathematicians of The nucleus splits to produce smaller, the 20th century, was established Uranium-235 (U-235) is a purified lighter “daughter” nuclei plus a few in 1939 to develop nuclear weapons isotope of uranium. This isotope is more neutrons, each of which can under the scientific direction of naturally unstable and emits neutrons go on to produce fission in more J. Robert Oppenheimer. and heat. Nuclear fission of U-235 starts uranium nuclei. when a neutron hits a U-235 nucleus. Having fled Fascist Italy in 1938, Fermi moved to the US and restarted work on applications of his

NUCLEAR AND PARTICLE PHYSICS 251 The world’s first atomic bomb development of civilian nuclear Enrico Fermi (code-named Trinity) was detonated at applications. The first nuclear a test site in Alamogordo, New Mexico, reactor for electricity generation Born in Rome in 1901, Enrico on July 16, 1945, producing a massive opened in 1951 in Idaho, and civil Fermi was best known for fireball and mushroom cloud of debris. nuclear power stations multiplied developing early nuclear in the next two decades. Nuclear applications, but was also smaller pieces of fissile material reactors use controlled chain admired as a theoretical together at high speed to create a reactions—sped up or slowed down physicist. Fermi studied at the large mass that exceeded criticality. using control rods that capture free University of Pisa and then left neutrons—to release energy Italy to collaborate with In July 1945, in the New Mexico gradually, boiling water into steam physicists such as Max Born. desert, Manhattan Project scientists to turn electrical generators. The Returning to lecture at the detonated a nuclear weapon for the amount of energy that can be University of Florence in 1924, first time. A massive fireball was extracted from nuclear fuel is he helped to develop Fermi- followed by a radioactive cloud of millions of times that found in Dirac statistics. In 1926, he debris and water vapor. Most similar amounts of fuels like coal, became a professor at the observers were silent. Oppenheimer making it an efficient carbon- University of Rome. Here, he later admitted that the words of the neutral energy source. proposed the theory of the god Vishnu from Hindu scripture weak interaction and leapt to mind: “Now I am become However, in 1986, explosions demonstrated nuclear fission. Death, the destroyer of worlds.” The in a reactor core at Chernobyl in only two nuclear weapons ever Ukraine, then part of the Soviet In 1938, the year Fermi used in armed conflict to date were Union, released radioactive material won the Nobel Prize in dropped on Japan months later: the into the atmosphere, eventually Physics, he fled Fascist Italy to gun-type “Little Boy” on Hiroshima killing thousands of people across escape anti-Jewish laws that on August 6 and the implosion-type Europe. This and other nuclear restricted the rights of his wife “Fat Man” (which used Pu-239, an disasters, together with concerns and colleagues. Despite his isotope of plutonium, as the fissile about how to store long-lived and close involvement with the material) on Nagasaki on August 9. highly radioactive waste, have Manhattan Project, he later undermined nuclear power’s became a vocal critic of Nuclear power for energy environmental credentials. nuclear weapons development. The end of the war signaled a He died in Chicago in 1954. partial shift to nuclear fission for From fission to fusion? peaceful purposes, and the Atomic Some physicists hope that nuclear Key works Energy Commission was created fusion could be the sustainable in the US in 1946 to oversee the energy source of the future. This is 1934 “Artificial Radioactivity the binding of two nuclei to form a Produced by Neutron larger nucleus, releasing excess Bombardment” energy in the form of photons. 1938 “Simple Capture of Scientists have struggled for Neutrons by Uranium” decades to induce fusion—the powerful repulsive force of protons can only be overcome amid extreme heat and density. The most promising method uses a doughnut-shaped device called a tokamak. This generates a powerful magnetic field to hold plasma, matter so hot that electrons are stripped from their atoms, rendering it conductive and easy to manipulate with magnetic fields. ■

252 IN CONTEXT OANWCINRDEAOTWION KEY FIGURE John Cockcroft (1897–1967) PARTICLE ACCELERATORS BEFORE 1919 Ernest Rutherford artificially induces nuclear fission (splitting the nucleus of an atom into two nuclei). 1929 Ukrainian–American physicist George Gamow lays out his theory of quantum tunneling for alpha particles emitted in alpha decay. AFTER 1952 The Cosmotron, the first proton synchrotron, begins operating at Brookhaven National Laboratory in the US. 2009 The Large Hadron Collider (LHC) at CERN in Switzerland becomes fully operational and breaks the record as the highest-energy particle accelerator. I n 1919, Ernest Rutherford’s investigation into the disintegration of nitrogen atoms proved that it is possible to break apart particles bound by the nuclear force—one of the strongest forces in the universe. Soon, physicists were wondering whether they could explore deeper inside the atom by smashing it to pieces and examining the remains. In the late 1920s, former British soldier and engineer John Cockcroft was one of the young physicists assisting Rutherford in such research at Cambridge University’s Cavendish Laboratory. Cockcroft was intrigued by the work of George Gamow, who in 1928

NUCLEAR AND PARTICLE PHYSICS 253 See also: Models of matter 68–71 ■ Quantum applications 226–231 ■ Subatomic particles 242–243 ■ The particle zoo and quarks 256–257 ■ The Higgs Boson 262–263 ■ Mass and energy 284–285 ■ The Big Bang 296–301 Particles can escape Particles with high An accelerated from a nucleus via enough energy can also particle can strike and quantum tunneling. disintegrate a nucleus. penetrate a nucleus. Particle accelerators can be used to throw light on the nature of these particles. described the phenomenon of beams of protons from a canal ray high-speed protons and monitored quantum tunneling. This is the tube (essentially a back-to-front the interactions on a detector, a idea that subatomic particles, such cathode ray tube). When they failed zinc fluoride fluorescent screen. as alpha particles, can escape from to detect the gamma rays noted They expected to see the gamma the nucleus, despite the strong by French scientists engaged in rays that French scientists Irene nuclear force restraining them, similar research, they realized that Joliot-Curie and her husband because they have wavelike their proton energy was too low. Frédéric had reported. Instead, attributes, enabling some to go they inadvertently produced beyond the nuclear force barrier, Ever-increasing energy neutrons (as British physicist escaping its attractive power. The quest for more powerful James Chadwick would later particle accelerators began. In prove). Cockcroft and Walton Reversing the principle 1932, Cockcroft and Walton built then performed the first artificial Gamow visited the Cambridge a new apparatus that was able to disintegration of an atomic nucleus laboratory at Cockcroft’s invitation, accelerate a beam of protons to on a nucleus of lithium, reducing it and the men discussed whether higher energies using lower voltages. to alpha particles. ❯❯ Gamow’s theory could be applied This Cockcroft–Walton accelerator in reverse; was it possible to begins by accelerating charged Particles were coming out of accelerate a proton with enough particles through an initial diode the lithium, hitting the screen, energy to penetrate and burst the (a semiconductor device) to charge and producing scintillations. nucleus of an element? Cockcroft a capacitor (a component that told Rutherford he believed they stores electrical energy) to peak They looked like stars could penetrate a boron nucleus voltage. The voltage is then suddenly appearing using protons accelerated with reversed, boosting the particles and disappearing. 300 kilovolts, and that a nucleus of through the next diode and lithium would potentially require effectively doubling their energy. Ernest Walton less energy. Boron and lithium Through a series of capacitors and have light nuclei, so the energy diodes, charge is stacked up to on splitting the atom barriers to be overcome are lower multiple times what would normally than those of heavier elements. be possible, by applying the maximum voltage. Rutherford gave the go-ahead and, in 1930, joined by Irish Using this pioneering device, physicist Ernest Walton, Cockcroft Cockcroft and Walton bombarded experimented with accelerating lithium and beryllium nuclei with

254 PARTICLE ACCELERATORS Detectors capture radiation and Particle accelerators use electric and magnetic fields particles from to produce a beam of high-energy subatomic particles, the collision such as protons, which are crashed together or fired at a metal target. Electric field strips Beam of Electric field Electromagnet electrons from hydrogen protons accelerates Magnetic field to produce protons protons guides protons PARTICLE DETECTOR Hydrogen Protons RADIATION gas in collide with DETECTOR other subatomic Proton particles PARTICLE PARTICLE PARTICLE GENERATION ACCELERATION GUIDANCE With this historic first, Cockcroft and Most accelerators in particle and synchrotrons. Linacs, such as Walton showed the value of using physics research use oscillating the Stanford Linear Accelerator in particle accelerators (nicknamed electromagnetic fields to accelerate California, accelerate particles in “atom smashers”) to probe the atom charged particles. In electrodynamic a straight line toward a target and discover new particles, offering accelerators, a particle is accelerated at one end. Cyclotrons are a more controlled alternative to toward a plate, and as it passes composed of two hollow D-shaped observing cosmic rays (high-energy through it, the charge on the plate plates and a magnet, which bends particles moving through space). switches, repelling the particle particles into a circular path as toward the next plate. This they spiral outward toward a High-powered accelerators process is repeated with faster target. Synchrotrons accelerate Cockcroft–Walton machines and and faster oscillations to push the charged particles continuously in a all early particle accelerators were particles to speeds comparable to circle, until they reach the required electrostatic devices, using a static that of light. These oscillating fields energies, using many magnets electric field to accelerate particles. are typically produced via one of to guide the particles. These are still widely used today in two mechanisms—either magnetic academic, medical, and industrial induction or radio frequency (RF) Dizzying particle speeds low-energy particle studies, and in waves. Magnetic induction uses a In 1930, Cockcroft and Walton everyday electronic applications, magnetic field to induce movement had recognized that the more they such as microwave ovens. Their of charged particles to create a could accelerate particles, the energy limit, however, precludes circulating electric field. An RF deeper into matter they could see. their use for research in modern cavity is a hollow metallic chamber Physicists can now use synchrotron particle physics. At a certain in which resonating radio waves accelerators that boost particles to energy point, raising voltages to create an electromagnetic field to dizzying speeds, which nudge push particles any further causes boost charged particles as they toward the speed of light. Such the insulators used in construction pass through. speeds create relativistic effects; of the particle accelerators to as a particle’s kinetic energy experience electrical breakdown Modern particle accelerators increases, its mass increases, and start conducting. come in three types: linear requiring larger forces to achieve accelerators (linacs), cyclotrons,

NUCLEAR AND PARTICLE PHYSICS 255 greater acceleration. The largest The ATLAS calorimetry system at consumption of the nearby city and most powerful machines in CERN, seen here during installation, of Geneva. A chain of booster existence are the focus of measures the energy of particles after accelerators (including the Super experiments that can involve a collision by forcing most to stop and Proton Synchrotron) accelerate thousands of scientists from around deposit their energy in the detector. beams of charged particles to ever the world. Fermilab’s Tevatron in higher energies until they finally Illinois, which operated from 16.8 miles (27 km) long, spanning enter the LHC. Here particles 1983 to 2011, used a ring 3.9 miles the border of Switzerland and collide head-on at four collision (6.3 km) long to accelerate protons France 328 ft (100 m) underground, points, with combined energies of and antiprotons to energies up to and capable of accelerating two 13 TeV. A group of detectors records 1 TeV (1012  1 eV), where 1 eV is beams of protons to 99.9999991 their disintegrations. the energy gained by an electron percent the speed of light. accelerated across one volt—1 TeV Recreating a primeval state is roughly the energy of motion of The LHC is a triumph of Among the experiments conducted a flying mosquito. engineering; its groundbreaking at the LHC are attempts to recreate technologies—deployed on an the conditions that existed at the In the late 1980s, at CERN in enormous scale—include 10,000 very genesis of the universe. From Switzerland, scientists armed with superconducting magnets, chilled what is known about its expansion, the Super Proton Synchrotron to temperatures lower than those physicists can work back and competed with Fermilab scientists found in the wilderness of space. predict that the early universe was using the Tevatron in the hunt for During operation, the site draws unimaginably tiny, hot, and dense. the top quark (the heaviest quark). about one third of the total energy Thanks to the sheer power of the Under these conditions, Tevatron, Fermilab scientists elementary particles such as quarks were able to produce and detect and gluons may have existed in a the top quark in 1995, at a mass sort of “soup” (quark–gluon plasma). As space expanded and cooled, of approximately 176 GeV/c² (almost they became tightly bound together, forming composite particles such as heavy as a gold atom). as protons and neutrons. Smashing The Tevatron was knocked off particles at near light speed can for an instant reinvent the universe as its pedestal as the most powerful it was trillionths of a second after particle accelerator in 2009 with the Big Bang. ■ the first fully operational run of CERN’s Large Hadron Collider (LHC). The LHC is a synchrotron, John Cockcroft Born in Yorkshire, England, in Cockcroft’s insistence that filters 1897, John Cockcroft served be installed in the chimneys at in World War I, then studied Windscale Piles in Cumbria electrical engineering. He won limited the fallout in 1957 when a scholarship to Cambridge one of the reactors caught fire. University in 1921, where Ernest Rutherford later supervised his Cockcroft served as president doctorate. The Cockcroft-Walton of the Institute of Physics and of accelerator he built at Cambridge the British Association for the with Ernest Walton won them the Advancement of Science. He 1951 Nobel Prize in Physics. died in Cambridge in 1967. In 1947, as director of the Key works British Atomic Energy Research Establishment, Cockcroft oversaw 1932 Disintegration of Lithium the opening of the first nuclear by Swift Protons reactor in western Europe— 1932–36 “Experiments with GLEEP at Harwell. In 1950, High Velocity Positive Ions I-VI”

256 FTOHRETHHUENQTUARK THE PARTICLE ZOO AND QUARKS IN CONTEXT B y the end of World War II, fundamental quality called physicists had discovered “strangeness” could explain KEY FIGURE the proton, neutron, and these long-observed lifetimes. Murray Gell-Mann electron, and a handful of other Strangeness is conserved in strong (1929–2019) particles. In the following years, and electromagnetic interactions but however, discoveries in cosmic not in weak interactions, so particles BEFORE rays (high-energy particles moving with strangeness can only decay 1947 The subatomic kaon is through space) and particle via the weak interaction. Gell-Mann discovered at the University of accelerators caused the number used strangeness and charge to Manchester, exhibiting a much of particles to balloon into a classify subatomic particles into longer lifetime than predicted. chaotic “particle zoo.” families of mesons (typically lighter) and baryons (typically heavier). 1953 Physicists propose the Physicists were particularly property of “strangeness” to confused by families of particles Quark theory explain the unusual behavior called kaons and lambda baryons, In 1964, Gell-Mann proposed of kaons and other particles. which decay far more slowly than the concept of the quark—a expected. In 1953, American fundamental particle, which could 1961 Gell-Mann proposes the physicist Murray Gell-Mann and explain the properties of the new “Eightfold Way” to organize Japanese physicists Kazuhiko mesons and baryons. According to subatomic particles. Nishijima and Tadao Nakano quark theory, quarks come in six independently proposed that a AFTER 1968 Scattering experiments Exotic new particles with These particles can be reveal pointlike objects inside different properties are arranged according to the proton, proving it is not discovered. a fundamental particle. their properties. 1974 Experiments produce Quarks are a fundamental Their properties depend on building block of matter. constituents called quarks. J/ particles, which contain charm quarks. 1995 The discovery of the top quark completes the quark model.

NUCLEAR AND PARTICLE PHYSICS 257 See also: Models of matter 68–71 ■ Subatomic particles 242–243 ■ Antimatter 246 ■ The strong force 247 ■ Force carriers 258–259 ■ The Higgs boson 262–263 Nucleus Down quark is one Gluon binds of two types of quarks together elementary particle in the nucleus of an atom Up quark Neutron has two Proton has two up Murray Gell-Mann down quarks and quarks and one one up quark down quark Born in New York in 1929 to a family of Jewish immigrants, Protons and neutrons each contain up quarks and down quarks. Murray Gell-Mann began A proton has two up quarks and one down quark, and a neutron has attending college at the age one up quark and two down quarks. The quarks are bound together of just 15. In 1955, he moved by gluons, the carrier particles of the strong force. to the California Institute of Technology (CIT), where he “flavors” with different intrinsic fermions, the other being the taught for almost 40 years. properties. Gell-Mann initially leptons. Like the quarks, leptons His interests expanded far described up, down, and strange come in six flavors, which are beyond physics to include quarks, and charm, top, and related in pairs, or “generations.” literature, history, and natural bottom quarks were added later. These are the electron and electron history, but in 1969 he was Different quarks (and their antimatter neutrino, which are the lightest awarded the Nobel Prize in equivalents, antiquarks) are bound and most stable leptons; the Physics for his work on the by the strong force and are found in muon and muon neutrino; and the theory of elementary particles. composite particles such as protons tau and tau neutrino, which are and neutrons. Evidence for this the heaviest and most unstable Later in his career, Gell- model of the nucleus was found leptons. Unlike quarks, leptons Mann became interested in at the Stanford Linear Accelerator are unaffected by the strong force. the theory of complexity, Laboratory (SLAC) in California, cofounding the Santa Fe when pointlike objects were Quarks and leptons interact Institute for research into this discovered inside protons in 1968. through three of the four area in 1984 and writing a Further SLAC experiments produced fundamental forces: strong, popular book about the theory evidence for the other quarks. electromagnetic, and weak. (The (The Quark and the Jaguar). fourth fundamental force, gravity, At the time of his death in The Standard Model behaves differently.) In the Standard 2019, he held positions at CIT, Quarks play an important role in the Model, these forces are represented the University of Southern Standard Model of particle physics, by their carrier particles—gluons, California, and the University which was developed in the 1970s to photons, and W and Z bosons. of New Mexico. explain the electromagnetic, strong, and weak forces—as well as their The final element of the Key works carrier particles (which are always Standard Model is the Higgs boson, bosons) and the fundamental an elementary particle that gives all 1994 The Quark and the particles of matter (fermions). The particles their mass. The Standard Jaguar: Adventures in the quarks are one of the two groups of Model has brought order to the Simple and the Complex “particle zoo,” although it remains 2012 Mary McFadden: A a work in progress. ■ Lifetime of Design, Collecting, and Adventure

258 IPADALERWNTATIYCICSLAEALSCNTDUOACLNLIOEKATER FORCE CARRIERS IN CONTEXT F rom around 1930, scientists unstable atomic nuclei, and identify began to unpick the process and observe the force carriers that KEY FIGURE of nuclear decay. Beta decay mediate (transmit) the weak Chien-Shiung Wu had puzzled earlier researchers, as interaction in nuclear decay. (1912–1997) energy seemed to disappear, in violation of the law of conservation In 1933, Enrico Fermi proposed BEFORE of energy. Over the next five that beta radiation emerges from 1930 Austrian-born physicist decades, leading physicists would the nucleus as a neutron turns Wolfgang Pauli proposes the discover the bearers of the missing into a proton, emitting an electron existence of a neutrino to energy, detail the changes that and a further neutral particle that explain how energy and other transformed elements within carries away some energy. (Fermi quantities can be conserved called this neutral particle a in beta decay. Conservation of energy appears to be violated in beta decay 1933 Enrico Fermi lays out his (usually the emission of an electron from an atomic nucleus). theory for the weak interaction to explain beta decay. A light, neutral particle—a During beta-minus decay, neutrino or an a neutron turns into a proton, AFTER 1956 American physicists antineutrino carries away an electron, and an Clyde Cowan and Frederick some energy. antineutrino. Reines confirm that both electrons and neutrinos are The weak force involves the The interaction that is emitted during beta decay. exchange of particles responsible for this called force carriers. process is called the weak 1968 The electromagnetic interaction or weak force. and weak forces are unified in “electroweak theory.” 1983 W and Z bosons are discovered at the Super Proton Synchrotron, a particle accelerator machine at CERN.

NUCLEAR AND PARTICLES 259 See also: Quantum field theory 224–225 ■ Nuclear rays 238–239 ■ Antimatter 246 ■ The particle zoo and quarks 256–257 ■ Massive neutrinos 261 ■ The Higgs boson 262–263 neutrino, but it was later identified Gargamelle, seen here in 1970 and Z bosons are very heavy, so as an antineutrino, the antiparticle was designed to detect neutrinos processes occurring through them of the neutrino.) In the other main and antineutrinos. Through its tend to be very slow. type of beta decay—beta-plus portholes, cameras could track decay—a proton becomes a the trail of charged particles. In 1973, the interactions of the neutron, emitting a positron and a bosons were observed at CERN’s neutrino. The force responsible for decayed. She saw that electrons Gargamelle bubble chamber, which such decays was named the weak emitted during beta radiation had a captured tracks providing the interaction—now recognized as one preference for a specific direction of first confirmation of the weak of four fundamental forces of nature. decay (rather than being randomly neutral-current interaction. This emitted in all directions equally), was interpreted as a neutrino The rule-breaking force showing that the weak interaction taking momentum away after The strong and electromagnetic violates conservation of parity. being produced in the exchange interactions conserve parity; their of a Z boson. The W and Z bosons effect on a system is symmetric, The weak interaction involves themselves were first observed in producing a kind of mirror image. the exchange of force-carrier 1983 using the high-energy Super Suspecting this was not true of the particles: W+, W-, and Z0 bosons. Proton Synchrotron (SPS) at CERN. ■ weak interaction, fellow physicists All force-carrying particles are Chen Ning Yang and Tsung-Dao bosons (integer spin), while the Lee asked Chien-Shiung Wu to key building blocks of matter, investigate. In 1956—the same year such as quarks and leptons, are all that Clyde Cowan and Frederick fermions (half-integer spin) and Reines confirmed the existence of obey different principles. Uniquely, neutrinos—Wu got to work at the the weak interaction can also US National Bureau of Standards change the “flavor,” or properties, low-temperature laboratory in of quarks. During beta-minus decay, Washington, D.C. She aligned the a “down” quark changes into an spins (internal angular momentum) “up” quark, transforming a neutron of the nuclei in a sample of into a proton, and emits a virtual cobalt-60, and watched as it W- boson, which decays into an electron and antineutrino. The W Chien-Shiung Wu Born in 1912, in Liuhe, near Yang received the 1957 Nobel Shanghai in China, Chien-Shiung Prize in Physics for discovering Wu developed a passion for parity violation; her contribution physics after reading a biography was not acknowledged. She was of Polish physicist and chemist finally honored in 1978, when Marie Curie. She studied physics she received the Wolf Prize for at National Central University in scientific achievement. Wu died Nanjing, then moved to the US in New York in 1997. in 1936, gaining her PhD at the University of California-Berkeley. Key works Wu joined the Manhattan 1950 “Recent investigation of Project in 1944 to work on the shapes of beta-ray spectra” uranium enrichment. After World 1957 “Experimental test of parity War II, she became a professor at conservation in beta decay” Columbia University and focused 1960 Beta decay on beta decay. Wu’s collaborators Tsung-Dao Lee and Chen Ning

260 NABATSUURREDIS QUANTUM ELECTRODYNAMICS IN CONTEXT T he emergence of quantum that come into existence mechanics, which describes momentarily and affect the motion KEY FIGURES the behavior of objects of “real” particles as they are Shin’ichiroˉ Tomonaga at atomic and subatomic scales, released or absorbed. (1906–1979), Julian forced a transformation of many Schwinger (1918–1994) branches of physics. QED has been used to model phenomena that previously defied BEFORE Paul Dirac proposed a quantum explanation. One example is the 1865 James Clerk Maxwell theory of electromagnetism in 1927, Lamb shift—the difference in lays out the electromagnetic but models describing encounters energy between two energy levels theory of light. between electromagnetic fields and of a hydrogen atom. ■ high-speed particles—which obey 1905 Albert Einstein the laws of special relativity—broke Following his contributions to publishes a paper describing down. This led to an assumption QED, Shin’ichiro¯ Tomonaga received special relativity. that quantum mechanics and the Nobel Prize in Physics, the Japan special relativity were not Academy Prize, and many other awards. 1927 Paul Dirac formulates a compatible. In the 1940s, Shin’ichiro¯ quantum mechanical theory Tomonaga, Richard Feynman, and of charged objects and the Julian Schwinger proved that electromagnetic field. quantum electrodynamics (QED) could be made consistent with AFTER special relativity. In fact, QED was 1965 Tomonaga, Schwinger, the first theory to combine quantum and Richard Feynman share mechanics and special relativity. the Nobel Prize in Physics for their work on quantum In classical electrodynamics, electrodynamics. electrically charged particles exert forces through the fields they 1973 The theory of quantum produce. In QED, however, the chromodynamics is developed, forces between charged particles with color charge as the source arise from the exchange of virtual, of the strong interaction or messenger, photons—particles (strong nuclear force). See also: Force fields and Maxwell’s equations 142–147 ■ Particles and waves 212–215 ■ Quantum field theory 224–225 ■ The particle zoo and quarks 256–257

NUCLEAR AND PARTICLE PHYSICS 261 NOTHEFUETTHMREYINSMOTISSERSYING MASSIVE NEUTRINOS IN CONTEXT S ince the 1920s, physicists Now that neutrino have known that nuclear astrophysics is born, what KEY FIGURE fusion causes the sun and Masatoshi Koshiba (1926–) other stars to shine. They went on should we do next? to predict that this process releases Masatoshi Koshiba BEFORE particles called neutrinos, which 1956 American physicists shower down on Earth from space. this problem. Observations of Clyde Cowan and Frederick atmospheric neutrinos with the Reines publish the results of Neutrinos have been compared detector proved that neutrinos their experiment confirming to ghosts due to the challenge can switch between flavors in the existence of neutrinos. of detecting them. They are flight, a process called neutrino chargeless, almost without mass, oscillation. This means that an 1970 “Homestake” experiments and do not interact through the electron neutrino created in the begin in the US, detecting only strong nuclear or electromagnetic sun may change to a muon or tau one-third of the predicted forces, allowing them to pass neutrino and so elude detectors number of solar neutrinos. through Earth unnoticed. Neutrinos that are sensitive only to electron come in three types, or flavors: neutrinos. The discovery implied 1987 Researchers at locations electron, muon, and tau neutrinos. that neutrinos have mass, in Japan, the US, and Russia challenging the Standard Model, detect a record batch of In 1985, Japanese physicist the theory of fundamental forces 25 neutrinos, originating Masatoshi Koshiba built a neutrino and particles. ■ from a supernova in the detector in a zinc mine. Detectors Large Magellanic Cloud. surrounded a vast water tank, picking up flashes of light as AFTER neutrinos interacted with the 2001 Scientists at Sudbury nuclei of water molecules. Koshiba Neutrino Observatory, Canada, confirmed that there seemed to be find further evidence for far fewer solar neutrinos reaching neutrino oscillation. Earth than predicted. In 1996, he led the construction of an even 2013 Results from the T2K larger detector (Super-Kamiokande), experiment confirm the which allowed his team to solve neutrino oscillation theory. See also: Heisenberg’s uncertainty principle 220–221 ■ Particle accelerators 252–255 ■ The particle zoo and quarks 256–257 ■ Force carriers 258–259

262 IWTEHIHNAKVE IT THE HIGGS BOSON IN CONTEXT T he Standard Model, theory did not match reality. It completed in the early implied that all electroweak force KEY FIGURE 1970s after decades of carriers are massless. While this is Peter Higgs (1929–) research, explained much about true for the photon, W and Z bosons particle physics with a handful of are conspicuously massive. BEFORE fundamental forces and particles. 1959 Sheldon Glashow in Questions remained, however. In 1964, three groups of the US and Abdus Salam in While two of the forces—the physicists—Peter Higgs in the UK; Pakistan propose that the electromagnetic force and Robert Brout and François Englert electromagnetic and weak the weak interaction—could be in Belgium; and Gerald Guralnik, forces merge in intense heat. modeled as merging into a single C. Richard Hagen, and Tom Kibble electroweak force at intense in the US—had proposed that the 1960 Japanese–American temperatures, one aspect of this weak bosons might interact with a physicist Yoichiro Nambu field which gives them mass; this conceives his theory of symmetry breaking, which Electroweak theory predicts that all force carriers (particles that can be applied to the problem excite forces between other particles) have no mass. of W and Z boson mass. Experiments show that W and Z bosons interact AFTER photons have no mass but strongly with the Higgs field. 1983 W and Z bosons are both W and Z bosons (a type of confirmed at the CERN Super force carrier) are massive. Proton Synchroton. The Higgs boson is the The Higgs field gives 1995 Fermilab discovers carrier particle of the particles their mass. the top quark with a mass of Higgs field. 176GeV/c², which matches Higgs field theory predictions. 2012 CERN confirms the discovery of the Higgs boson in its ATLAS and CMS detectors.

NUCLEAR AND PARTICLE PHYSICS 263 See also: Quantum field theory 224–225 ■ Particle accelerators 252–255 ■ The particle zoo and quarks 256–257 ■ Force carriers 258–259 The Higgs field, imagined in this illustration, is a field of energy thought to exist throughout the universe; within it the Higgs boson interacts continuously with other particles. became known as the Higgs field. it, and the search continued, Peter Higgs According to this theory, the Higgs influencing the design of the world’s field began permeating throughout most powerful particle accelerator, Born in 1929 in Newcastle the universe soon after the Big CERN’s Large Hadron Collider upon Tyne, Higgs was the Bang; the more a particle interacts (LHC), which started up in 2008. son of a BBC sound engineer. with it, the more mass it acquires. Frequent house moves Photons do not interact with the As physicists calculated that disrupted the boy’s early field, allowing them to travel at light only one in 10 billion proton-proton education, but at Cotham speed. Meanwhile, W and Z bosons collisions in the LHC would produce Grammar School in Bristol he interact strongly with the field. At a Higgs boson, researchers used was inspired by the work of a ordinary temperatures, this makes CERN’s detectors to sift through the former pupil, the theoretical them heavy and sluggish with short remnants of hundreds of trillions of physicist Paul Dirac. He then ranges. As particles that interact collisions for hints of the particle. went to King’s College with the Higgs field accelerate, they In 2012, CERN announced the London, and was awarded gain mass and therefore require discovery of a boson of mass around his PhD in 1954. After various more energy to push them further, academic posts, Higgs chose preventing them from reaching the 126 GeV/c² (gigaelectron volts to stay at the University of speed of light. Edinburgh. On one of many divided by the speed of light walks in the Highlands, he squared)—almost certainly the began to formulate his theory Higgs boson. CERN physicists were for the origin of mass, which elated, while Higgs was moved to gained him renown, although tears at the validation of his 50-year- he readily acknowledged its old theory. The Standard Model is many other contributors. now complete, but physicists continue to explore the possibility In 2013, Higgs shared the that other types of Higgs bosons Nobel Prize in Physics with exist beyond it. ■ François Englert. He remains Emeritus professor at the Near-mythical importance I never expected this to University of Edinburgh. Scientists realized that the only happen in my lifetime and way to prove the Higgs field theory will be asking my family to Key works was to find an excitation of the field in the form of a heavy particle, put some champagne 1964 “Broken symmetries, and known as the Higgs boson. The in the fridge. the masses of gauge bosons” quest took on a near-mythical Peter Higgs 1966 “Spontaneous symmetry importance, gaining the boson breakdown without massless the nickname of “God particle” bosons” in the 1980s, which Higgs and his colleagues disliked. No particle detector at the time could detect

264 WANHTEIMREATHTAESRAGLOLNTEH?E MATTER–ANTIMATTER ASYMMETRY IN CONTEXT P hysicists have long puzzled preserved if spacetime extends over why the universe is backward beyond the Big Bang KEY FIGURE made up almost entirely into a mirror universe of antimatter. Andrei Sakharov (1921–1989) of matter. P (parity) symmetry, the idea that nature cannot distinguish In 1967, Soviet physicist BEFORE between left and right, suggests Andrei Sakharov proposed instead 1928 Paul Dirac suggests a that the Big Bang should have that an imbalance of matter and new form of matter with produced matter and antimatter antimatter could have evolved if opposite charges: antimatter. in equal amounts. The first clue CP violation occurred in the early came in 1956, when an experiment universe. It has to be sufficient to 1932 In the US, Carl Anderson showed that electrons emanating prompt asymmetry, or else physics discovers “antielectrons” and through beta decay in a weak beyond the Standard Model will calls them positrons. interaction had a preferred direction. be required. Significant CP The following year, to maintain violation has now been shown, 1951 Julian Schwinger symmetry, Soviet physicist Lev supporting Sakharov’s ideas. ■ hints at an early form of Landau proposed CP symmetry— CPT symmetry. combining P symmetry with C Physicists began to think (conservation of charge) symmetry, that they may have been AFTER so that a particle and its oppositely- 1964 American physicists charged antiparticle would behave looking at the wrong James Cronin and Val Fitch like mirror images. symmetry all along. show that weak decay of neutral K-mesons (kaons) Mirror universe or not? Ulrich Nierste violates CP symmetry. In 1964, when experiments showed that neutral K-mesons violate CP German theoretical physicist 2010 Fermilab scientists conservation when they decay, detect a preference for physicists made a last-ditch bid to B-mesons to decay into muons, save symmetry. They incorporated violating CP symmetry. time-reversal symmetry, giving CPT symmetry, which can be 2019 CERN physicists detect asymmetry in D-mesons, the See also: Antimatter 246 ■ Force carriers 258–259 ■ Massive neutrinos 261 lightest particle to contain ■ Mass and energy 284–285 ■ The Big Bang 296–301 charm quarks.

NUCLEAR AND PARTICLE PHYSICS 265 BSOTARRNSAGNEDTDIE NUCLEAR FUSION IN STARS IN CONTEXT T he idea that stars are The JET tokamak reactor in the UK endlessly powered from is the world’s largest, most successful KEY FIGURE the fusion of hydrogen into fusion facility. It is central to European Hans Bethe (1906–2005) helium excited eminent physicists research and the international ITER in the early 20th century. By the project advancing fusion science. BEFORE mid-1930s, they had demonstrated 1920 British physicist Arthur nuclear fusion in the laboratory. electricity has proved more difficult, Eddington suggests that stars Among the leading theorists was due to the immense temperatures primarily get their energy from German-born Hans Bethe, who required (around 40 million Kelvin) hydrogen–helium fusion. realized that the stars (including and the challenge of containing the sun) release energy through such hot materials. The leading 1931 Deuterium, a stable proton–proton chain reactions. candidate for a fusion reactor— isotope of hydrogen, is the tokamak—confines the hot, detected by American chemist Fusion occurs only in extreme charged gas using magnetic fields. Harold C. Urey and associates. environments. Positively charged nuclei strongly repel each other but, The quest continues as fusion is 1934 Australian physicist with sufficient energy, they can be thought safer than fission, with less Mark Oliphant demonstrates pushed close enough together to radioactivity and nuclear waste. ■ deuterium fusion, discovering overcome this repulsion and fuse, tritium, a radioactive isotope forming heavier nuclei. As they of hydrogen, in the process. fuse, binding energy is released. AFTER Toward controlled fusion 1958 The first tokamak (T-1) In 1951, Bethe’s work in the US led begins reactions in the Soviet to the successful testing of the first Union, but loses energy hydrogen bomb, in which fission through radiation. was used to induce nuclear fusion and release a lethal blast of energy. 2006 The ITER agreement is signed in Paris by seven ITER Harnessing such power with members, funding a long-term controlled fusion reactions that international project to develop gradually release energy for nuclear fusion energy. See also: Generating electricity 148–151 ■ The strong force 247 ■ Nuclear bombs and power 248–251 ■ Particle accelerators 252–255

RTHEELAUTNIIVVI our place in the cosmos

ETRYSAEND

268 INTRODUCTION The Greek philosopher Persian astronomer Galileo explains his Albert Einstein’s theory of Aristotle describes a static Abd al-Rahman al-Sufi principle of relativity: the special relativity shows makes the first recorded laws of physics are the and eternal universe observation of Andromeda, same whether a person how space and time where a spherical Earth is describing the galaxy is stationary or moving change, depending on the at a constant velocity. speed of one object relative surrounded by concentric as a “small cloud.” rings of planets and stars. to another. 4TH CENTURY BCE 964 1632 1905 c. 150 CE 1543 1887 Ptolemy creates a mathematical Nicolaus Copernicus In the US, Albert Michelson model of the universe, showing provides a model for a and Edward Morley prove Earth as a stationary sphere heliocentric universe, at the center, with other known in which Earth revolves that light moves at a constant speed regardless bodies orbiting around it. around the sun. of the motion of the observer. A ncient civilizations During the Scientific Revolution, speed—it still travels at the speed questioned what the physicists and astronomers began of light. Nature behaves strangely movement of stars across to study the motion of objects and at close to the speed of light in the night sky meant for humanity’s light through space. Space was order to impose a universal speed existence and place within the imagined to be a rigid grid that limit. Establishing how this works universe. The seeming vastness of spanned the universe, and it was required a drastic rethinking of their planet led most to think that assumed that a length measured both space and time. Earth must be the largest and most on Earth would be the same on any important object in the cosmos, planet, star (including the sun), or Special relativity and that everything else revolved neutron star (a small star created Early in the 20th century, space and around it. Among these geocentric by the collapse of a giant star). time were discovered to be flexible: views, Ptolemy of Alexandria’s Time, too, was thought to be a meter or a second have different model from the 2nd century ce was absolute, with a second on Earth lengths in different places in the so convincing that it dominated equivalent to one anywhere else in universe. German-born physicist astronomy for centuries. the universe. But it soon became Albert Einstein introduced the world clear that this was not the case. to these ideas in his 1905 theory of The advent of the telescope in special relativity. He explained how, 1608 showed that Polish astronomer A ball thrown from a moving from the perspective of an observer Nicolaus Copernicus had been right vehicle will appear to a person on Earth, objects moving through to question Ptolemy’s view. In Italy, standing on the roadside to have space at close to the speed of light Galileo Galilei observed four moons been given a boost of speed from seem to contract, and how time orbiting Jupiter in 1610, and so the vehicle’s movement. Yet if light appears to run more slowly for these provided proof that other bodies is shone from a moving vehicle, the objects. Two years later, in 1907, orbited other worlds. light does not receive a boost in

RELATIVITY AND THE UNIVERSE 269 Einstein publishes his theory British physicist Arthur In the US, Russell Hulse and of general relativity to Eddington photographs Joseph Taylor find indirect describe how time and starlight bent around evidence for gravitational waves when they observe space curve for accelerating the sun in a solar energy being lost from two stars observers or those within eclipse—proof of the a gravitational field. curvature of spacetime. orbiting each other. 1915 1919 1974 1907 1916 1929 2017 Hermann Minkowski Karl Schwarzschild uses Edwin Hubble proves the LIGO scientists announce explains special relativity general relativity to predict the universe is expanding, the first direct detection existence of black holes, the noting that distant galaxies of gravitational waves in terms of four gravity of which is so strong, are moving more rapidly dimensions of space not even light can escape. from the merging of two and time—spacetime. than nearer ones. neutron stars. German mathematician Hermann further, predicting the existence the Milky Way. But American Minkowski suggested that special of extremely massive objects that astronomer Edwin Hubble’s relativity makes more sense if time could curve spacetime so much discovery of another galaxy—the and space are sewn together into a at a single point that nothing, not Andromeda Galaxy—in 1924 led to single fabric of “spacetime.” even light, could move fast enough the realization that the Milky Way to escape. Advances in astronomy is just one of billions of galaxies The theory of special relativity have since provided evidence for that lie well beyond 100,000 light had far-reaching consequences. these “black holes,” and it is now years away. What is more, these The notion that energy and mass thought that the largest stars galaxies are moving apart, leading were simply two forms of the same become black holes when they die. astronomers to believe that the thing, as described by Einstein’s universe began at a single point Beyond the Milky Way 13.8 billion years ago and exploded formula E = mc2, led to the By the early 1920s, astronomers forth in a so-called Big Bang. were able to measure accurately the discovery of the nuclear fusion that distance to stars in the night sky Astrophysicists today have far powers stars, and ultimately to the and the speed at which they are to go in their quest to understand development of the atomic bomb. moving relative to Earth. This the universe. Strange, invisible dark revolutionized how people perceived matter and dark energy make up In 1915, Einstein extended his the universe and our place within 95 percent of the known universe, ideas to include objects moving it. At the start of the 20th century, seeming to exert control without at changing speed and through astronomers believed that showing their presence. Likewise, gravitational fields. The theory of everything existed within 100,000 the workings of a black hole and the general relativity describes how light years of space, that is, within Big Bang remain a mystery. But spacetime can bend, just as a cloth astrophysicists are getting closer. ■ might stretch if a heavy weight is placed upon it. German physicist Karl Schwarzschild went a step

270 BOTHOFEDTIHWEESINHDEIANVGESNLY THE HEAVENS IN CONTEXT S ince time immemorial, Early humans linked objects in humans have gazed at the the night sky with gods and spirits KEY FIGURE night sky in awe, captivated on Earth. They believed heavenly Ptolemy (c. 100–c. 170 ce) by the motions of the sun, moon, bodies had an effect on aspects of and stars. One of the earliest their lives, such as the moon being BEFORE examples of primitive astronomy is linked to the fertility cycle. Some 2137 bce Chinese astronomers Britain’s Stonehenge, a stone circle civilizations, including the Incas in produce the first known record that dates back to about 3000 bce. the 15th century, assigned patterns, of a solar eclipse. While the true purpose of these known as constellations, to stars huge stones is not clear, it is that appeared regularly in the sky. 4th century bce Aristotle believed that at least some were describes Earth as a sphere designed to align with the motion Stonehenge, a prehistoric monument at the center of the universe. of the sun through the sky, and in Wiltshire, southwest England, may possibly also with the motion of the have been built so that ancient people c. 130 bce Hipparchus moon. Many other such monuments could track the motions of the sun compiles his catalog exist around the world. through the sky. of stars. AFTER 1543 Nicolaus Copernicus suggests that the sun, not Earth, is at the center of the universe. 1784 French astronomer Charles Messier creates a database of other star clusters and nebulae in the Milky Way. 1924 American astronomer Edwin Hubble shows that the Milky Way is one of many galaxies in the universe.

RELATIVITY AND THE UNIVERSE 271 See also: The scientific method 20–23 ■ The language of physics 24–31 ■ Models of the universe 272–273 ■ Discovering other galaxies 290–293 Planet Epicycle Orbit The planet makes small circles during its orbit Earth Ptolemy believed that each planet orbits Earth while Ptolemy at the same time moving around a sub-orbit, or epicycle. He thought that this explained the unpredictable Claudius Ptolemaeus, known “retrograde” movements of the stars and planets. as Ptolemy, lived from about 100 ce to 170 ce. Little is known Watching the sky all, he concluded that heavenly about his life, except that he It was not just the motions of stars bodies moved in a range of lived in the city of Alexandria, and planets that so intrigued early complex circular orbits and in the Roman province of astronomers, but also short-lived epicycles (“sub-orbits”), with Egypt, and wrote about a events. Chinese astronomers Earth stationary at the center. variety of subjects, including kept records of Halley’s Comet astronomy, astrology, music, appearing in the night sky as far Earth-centered universe geography, and mathematics. back as 1000 bce, denoting it a Ptolemy based many of his In Geography, he listed the “guest star.” They also recorded calculations on the observations latitudes and longitudes of supernovae, or exploding stars, of Hipparchus, a Greek astronomer many of the places in the most notably one that led to the from the 2nd century bce. In known world, producing a formation of the Crab Nebula in Ptolemy’s best-known work, the map that could be duplicated. 1054 ce. Reports from the time Almagest, which sets out his His first major work on suggest the supernova was bright geocentric theory, he used astronomy was the Almagest, enough to be seen in the daytime Hipparchus’s notes on the motions in which he cataloged 1,022 for about a month. of the moon and sun to calculate stars and 48 constellations, the positions of the moon, sun, and attempted to explain the In the 4th century bce, the Greek planets, and stars at different movement of stars and planets philosopher Aristotle postulated times, and to predict eclipses. in the night sky. His geocentric that Earth was the center of the model of the universe universe, with all other bodies— Ptolemy’s complex model of the persisted for centuries. such as the moon and the planets— universe, known as the Ptolemaic Despite its inaccuracies, revolving around it. In about 150 ce, system, dominated astronomical Ptolemy’s work was hugely Ptolemy, an astronomer from thinking for centuries. It would take influential in understanding Alexandria, furthered Aristotle’s until the 16th century for Polish how things move in space. geocentric (Earth-centered) theory astronomer Nicolaus Copernicus with his attempt to explain in to suggest that the sun, not Earth, Key works mathematical terms the seemingly was at the center of the universe. irregular motions of stars and Although his model was initially c.150 ce Almagest planets in the night sky. Observing derided—and Italian astronomer c.150 ce Geography the back-and-forth movement of Galileo Galilei was put on trial in c.150–170 ce Handy Tables some planets, and the fact that 1633 for supporting it—Copernicus c.150–170 ce Planetary others barely seemed to move at was eventually proved right. ■ Hypotheses

272 EOTAHFRETTHCHEENIUSTNENIVROETRSE MODELS OF THE UNIVERSE IN CONTEXT The irregular movements of T oday, the idea that Earth stars and planets cannot be is flat seems comical. Yet KEY FIGURE explained simply with Earth at early depictions of the Nicolaus Copernicus world, including in ancient Egypt, (1473–1543) the center of the universe. show that this is what people Earth may seem to be believed. After all, Earth seemed to BEFORE stretch into the distance, and from 6th century bce The Greek stationary, but it is in fact its surface it did not appear to philosopher Pythagoras says rotating, which explains why curve. Earth was thought to be a that Earth is round, based on stars appear to cross the sky. circular disk floating on an ocean, the moon also being round. with a dome above—the heavens— Viewed from Earth, the and the underworld below. 3rd century bce Eratosthenes other planets sometimes measures the circumference of It was not until the 6th century Earth with great accuracy. appear to be moving bce that Greek philosophers realized backward, but this is an that Earth was round. Pythagoras 2nd century ce Ptolemy of illusion caused by the fact that first worked out that the moon was Alexandria asserts that Earth Earth itself is moving. spherical, observing that the line is the center of the universe. Copernicus believed the sun is between day and night on the stationary near the center of moon is curved. This led him AFTER the universe, and Earth and to suppose that Earth was also 1609 Johannes Kepler the other planets revolve spherical. Aristotle later added describes the motions of to this by noting Earth’s curved the planets around the sun. around it. shadow during a lunar eclipse, and the changing positions of 1610 Galileo Galilei observes constellations. Eratosthenes went moons orbiting Jupiter. even further. He saw that the sun cast different shadows in the cities 1616 Nicolaus Copernicus’s of Syene and Alexandria, and used work De Revolutionibus this knowledge to work out the Orbium Coelestium (On the circumference of the planet. He Revolutions of the Heavenly calculated it to be between 24,000 Spheres) is banned by the and 29,000 miles (38,000 and Catholic Church. 47,000 km)—not far off its true value of 24,901 miles (40,075 km).

RELATIVITY AND THE UNIVERSE 273 See also: The scientific method 20–23 ■ Laws of motion 40–45 ■ Laws of gravity 46–51 ■ The heavens 270–271 ■ Discovering other galaxies 290–293 ■ The Big Bang 296–301 The passage of time Jupiter is now known to have sun in not quite perfect circles, has revealed to everyone 79 moons, but when Galileo Galilei moving more quickly when they first observed four moons orbiting the were closer to the sun and more the truths that I planet in 1610, he proved Copernicus’s slowly when they were further previously set forth. theory that not everything orbits Earth. away. When Galileo Galilei spotted four moons orbiting Jupiter, this Galileo Galilei Sun-centered universe was proof that other bodies orbited The idea of a universe with the other worlds—and that Earth was Much study also went into working sun at its center remained dormant not the center around which out Earth’s place within the known until the early 16th century, when everything revolved. ■ universe. In the 2nd century ce, Nicolaus Copernicus wrote a short Ptolemy of Alexandria described manuscript, the Commentariolus, Earth as a stationary sphere at and circulated it among friends. In the center of the universe, with it, he proposed a heliocentric (sun- all other known bodies orbiting centered) model, with the sun near around it. Five centuries earlier, the center of the known universe, the Greek astronomer Aristarchus and Earth and other planets orbiting of Samos had suggested that the in a circular motion around it. sun was the center of the universe, He also suggested that the reason but Ptolemy’s geocentric (Earth- why the sun rose and set was due centered) view was the more to the rotation of Earth. widely accepted of the two. The Catholic Church initially accepted Copernicus’s work, but later banned it following Protestant accusations of heresy. Nonetheless, the heliocentric approach caught on. German astronomer Johannes Kepler published his laws of motion in 1609 and 1619, which showed that the planets moved around the Nicolaus Copernicus Nicolaus Copernicus was born Commentariolus), in which on February 19, 1473, in Thorn he proposed that the sun, not (now Torun), Poland. His father, Earth, was near the center of a wealthy merchant, died when the known universe. Although Nicolaus was 10 years old, but he finished a longer work titled the boy received a good education De Revolutionibus Orbium thanks to his uncle. He studied Coelestium (On the Revolutions in Poland and Italy, where he of the Heavenly Spheres) in 1532, developed interests in geography he did not publish it until 1543, and astronomy. two months before he died. Copernicus went on to work Key works for his uncle, who was the bishop of Ermland in northern Poland, 1514 Commentariolus but after the bishop’s death in 1543 On the Revolutions of 1512, he devoted more time to the Heavenly Spheres astronomy. In 1514, he distributed a handwritten pamphlet (the

274 NOOR TTRRUUEE TLIEMNEGSTHS FROM CLASSICAL TO SPECIAL RELATIVITY IN CONTEXT T he ideas of relativity— peculiarities in space and KEY FIGURE time—are widely attributed Hendrik Lorentz (1853–1928) to Albert Einstein in the early 20th century. Yet earlier scientists had BEFORE also wondered whether everything 1632 Galileo Galilei posits that they saw was as it seemed. a person in a windowless room cannot tell whether the room is Galilean relativity Galileo used the example of a ship moving at a constant speed or Back in 1632, in Renaissance Italy, traveling at a constant speed on a flat not moving at all. Galileo Galilei had suggested that sea. A passenger dropping a ball below it was impossible to know whether deck would not be able to tell whether 1687 Isaac Newton devises a room was at rest or moving at a the ship was moving or stationary. his laws of motion, using key constant speed if there were parts of Galileo’s theory. objects moving within it. This idea Lorentz’s work paved the way for is known as Galilean relativity, and Einstein’s theory of special relativity, AFTER others attempted to expand on it in in which he showed that not only 1905 Albert Einstein subsequent years. One approach are the laws of physics the same publishes his theory of was to note that the laws of physics whether a person is moving at a special relativity, showing are the same in all inertial frames of constant speed or not moving, but that the speed of light is reference (those that are moving at that the speed of light is the same always constant. a constant velocity). if it is measured in either scenario. This idea brought a whole new 1915 Einstein publishes his Dutch physicist Hendrik Lorentz understanding of the universe. ■ theory of general relativity, produced a proof for this in 1892. explaining how the gravity His set of equations, known as the of objects bends spacetime. Lorentz transformations, showed how mass, length, and time change 2015 Astronomers in the US as a spatial object approaches the and Europe find gravitational speed of light, and that the speed waves—ripples in spacetime of light is constant in a vacuum. predicted by Einstein a century earlier. See also: Laws of motion 40–45 ■ Laws of gravity 46–51 ■ The speed of light 275 ■ Special relativity 276–279 ■ The equivalence principle 281

RELATIVITY AND THE UNIVERSE 275 MWTHIANESUSTAUEBNSOAUASTGOIETIGHT THE SPEED OF LIGHT IN CONTEXT T he speed of light has long that could split a beam of light vexed humanity. In ancient in two, direct the two parts along KEY FIGURES Greece, the philosopher different paths, and then recombine Albert Michelson Empedocles thought that it must them. Noting the pattern of the (1852–1931), take a while for light from the sun returned light, he calculated the Edward Morley (1838–1923) to reach Earth, whereas Aristotle speed of light to be 299,853 km/s. wondered if it had a speed at all. BEFORE In 1887, Michelson and fellow 4th century bce Aristotle Measuring light speed American Edward Morley set up suggests that light is Isaac Beeckman and Galileo Galilei an experiment to measure Earth’s instantaneous. made the first serious attempts to motion through the “ether” that measure the speed of light, in the light had long been thought to 1629 Dutch scientist Isaac 17th century. Both relied on human travel through. They found no Beeckman tries and fails to sight, and were inconclusive. In evidence of such an ether, but use an explosion and mirrors 1850, Hippolyte Fizeau and Léon recorded increasingly precise to measure the speed of light. Foucault independently produced values for a constant speed of light. ■ the first true measurements, using 1850 French rivals Hippolyte a rotating cog and a rotating mirror Light thinks it travels Fizeau and Léon Foucault each respectively to chop, or interrupt, a faster than anything measure the speed of light. light beam. In Foucault’s case, he calculated the speed of light from but it is wrong. AFTER the angle between the light going Terry Pratchett 1905 Albert Einstein declares to and from the rotating mirror and that the speed of light in a the speed of the mirror’s rotation. British novelist vacuum is always constant. In the early 1880s, American 1920 Interferometry is used to physicist Albert Michelson measure the size of a star other improved Foucault’s technique by than the sun for the first time. reflecting a beam of light off two mirrors over a greater distance. He 1983 An official measurement built an interferometer, a device of the speed of light is defined: 299,792,458 m/s. See also: Focusing light 170–175 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ The Doppler effect and redshift 188–191

276 IN CONTEXT DOSTHOXTFOIESOSPRTADRTAIN? KEY FIGURE Albert Einstein (1879–1955) SPECIAL RELATIVITY BEFORE 1632 Galileo Galilei puts forward his relativity hypothesis. 1687 Isaac Newton sets out his laws of motion. 1861 Scottish physicist James Clerk Maxwell formulates his equations describing electromagnetic waves. AFTER 1907 Hermann Minkowski presents the idea of time as the fourth dimension in space. 1915 Albert Einstein includes gravity and acceleration in his theory of general relativity. 1971 To demonstrate time dilation due to general and special relativity, atomic clocks are flown around the world. R elativity has deep roots. In 1632, Galileo Galilei imagined a traveler inside a windowless cabin on a ship sailing at a constant speed on a perfectly smooth sea. Was there any way for the traveler to determine whether the ship was moving without going on deck? Was there any experiment that, if carried out on a moving ship, would give a different result from the same experiment carried out on land? Galileo concluded that there was not. Provided the ship moved with a constant speed and direction, the results would be the same. No unit of measurement is absolute—all units are defined

RELATIVITY AND THE UNIVERSE 277 See also: From classical to special relativity 274 ■ The speed of light 275 ■ Curving spacetime 280 ■ The equivalence principle 281 ■ Paradoxes of special relativity 282–283 ■ Mass and energy 284–285 An object’s velocity An observer who is that generated the current. Setting is relative to the motion traveling at close out his “Principle of Relativity,” of other objects, but the to the speed of light he declared: “The same laws of speed of light is absolute will still record a light beam electrodynamics and optics will be arriving at the speed of light. valid for all frames of reference for and constant. which the laws of mechanics hold Observers in relative good.” In other words, the laws of The faster a frame of motion to each other physics are the same in all inertial reference travels through experience space and frames of reference. It was similar to the conclusion that Galileo had space relative to you, the time differently. reached in 1632. more slowly you observe object not being acted on by a Light is a constant its time to pass. force. Inertial motion is motion in In James Clerk Maxwell’s equations a straight line at a uniform speed. of 1865 for calculating the variables relative to something else. In order Before Einstein, Newton’s idea of in an electromagnetic field, the to measure anything, be it time, absolute motion—that an object speed of an electromagnetic wave, distance, or mass, there has to be could be said to be moving (or approximately 186,000 miles/s something to measure it against. to be at rest) without reference to (300,000 km/s), is a constant anything else—held sway. Special defined by the properties of the How fast people perceive an relativity would put an end to this. vacuum of space through which it object to be moving depends on moves. Maxwell’s equations hold their speed relative to that object. Principle of Relativity true in any inertial frame. Einstein For example, if a person on a fast- As was well known to scientists of declared that, while some things moving train tosses an apple to the 19th century, an electric current may be relative, the speed of light another passenger, it travels from is generated if a magnet is moved is absolute and constant: it travels one to the other at a few miles per inside a coil of wire, and also if the at a constant velocity, independent hour, but to an observer standing coil is moved and the magnet stays of everything else, including the by the tracks, the apple and the still. Following British physicist motion of the light source, and is passengers all flash by at a hundred Michael Faraday’s discoveries in not measured relative to anything miles per hour. the 1820s and 1830s, scientists else. This is what makes light had assumed that there were two fundamentally different from Frames of reference different explanations for this matter—all speeds below that ❯❯ The idea that motion has no phenomenon—one for the moving meaning without a frame of coil and one for the moving magnet. If you can’t explain it simply, reference is fundamental to Albert Einstein was having none of it. you don’t understand it Einstein’s theory of special relativity. In his 1905 paper “On the well enough. The theory is special in that it Electrodynamics of Moving Albert Einstein concerns itself with the special Bodies,” he said it did not matter case of objects moving at a which was moving—it was their constant velocity relative to one movement relative to each other another. Physicists call this an inertial frame of reference. As pointed out by Isaac Newton, an inertial state is the default for any

278 SPECIAL RELATIVITY of light are relative to the frame of Newton thought that time was According to special relativity, reference of the observer, and absolute, flowing without reference the more quickly a person travels nothing can travel faster than to anything else at the same through space, the more slowly light. The consequence of this is steady pace wherever in the he travels through time. This that two observers moving relative universe it was measured. Ten phenomenon is called time dilation. to each other will always measure seconds for one observer is the Scientists at CERN’s Large Hadron the speed of a beam of light as same as ten seconds for another, Collider, where particles are being the same—even if one is even if one is standing still and smashed together at near light moving in the same direction as the other is rushing by in the speed velocities, have to take the the light beam and one is moving fastest available spacecraft. effects of time dilation into account away from it. In Galilean terms, when interpreting the results this made little sense. Newtonian physics states that of their experiments. velocity equals distance traveled A matter of time divided by the time taken to cover Contracting space The idea that the speed of light is that distance—as an equation: Einstein asked himself a question: constant for all observers is central if I held a mirror while traveling at to special relativity. Everything else v = d/t. So, if the speed of light, v, the speed of light, would I see my derives from this one deceptively reflection? How would light reach simple fact. Einstein saw that there always remains the same, whatever the mirror if the mirror was moving was a fundamental connection the other two values, then it follows at light speed? If the speed of light between time and the velocity of is a constant, then, no matter how light. It was this insight that led that d and t, distance, or space, fast he was moving, the light going him to the completion of his theory. from Einstein to the mirror and Even if an observer is traveling at and time, must change. back again would always be close to the speed of light, she will Time, Einstein argued, passes traveling at 300,000 km/s because still record a light beam arriving at the speed of light does not change. the speed of light. For this to occur, differently in all moving frames said Einstein, time for the observer of reference, which means that In order for light to reach the has to be running more slowly. observers in relative motion (in mirror, not only does time have different moving reference frames) to be slowed down, but also the will have clocks that run at different rates. Time is relative. As Werner Heisenberg said of Einstein’s discovery, “This was a change in the very foundation of physics.” Inside a spaceship moving at near light speed, an astronaut using a Astronaut’s clock to measure the speed of light finds that it travels a relatively short view distance in a short time. For a person watching the light in the spaceship from Earth, the light appears to travel a longer distance in a longer time. But both observers measure light moving at the same speed. Traveling at close to the speed of light Observer’s view To the observer on Earth, To the astronaut, from Earth the beam of light follows a the beam of light much longer, diagonal path follows a vertical The clock in the stationary path from the frame of reference ticks more ceiling to the floor quickly than the clock in the of the spaceship moving frame of reference

RELATIVITY AND THE UNIVERSE 279 Einstein, in the special nearest star would take around Albert Einstein theory of relativity, proved five months in spaceship time. that different observers, However, for an observer back on Albert Einstein was born in in different states of motion, Earth, the trip would appear to take Ulm, Germany, on March 14, more than four years. 1879. Allegedly slow to learn see different realities. to talk, Einstein declared later Leonard Susskind Einstein’s equation that “I very rarely think in One of the most famous equations words.” At age four or five, American physicist in all of physics is derived from Albert was fascinated by the special relativity. Einstein published invisible forces that made a distance traveled by the beam it as a sort of short postscript to his compass needle move; he said of light has to decrease. At special theory. Known sometimes as that it awakened his curiosity approximately 99.5 percent of light the law of mass–energy equivalence, about the world. He began speed the distance is reduced by violin lessons at the age of six, a factor of 10. This shrinkage only E = mc2 effectively says that energy sparking a love of music that takes place in the direction of (E) and mass (m) are two aspects lasted throughout his life. The motion and will only be apparent story that Einstein was bad at to an observer who is at rest with of the same thing. If an object gains math is not true—he was an respect to the moving object. The or loses energy, it loses or gains an able student. crew of a spaceship traveling at equivalent amount of mass in close to the speed of light would accordance with the formula. In 1901, Einstein acquired not perceive any change in their Swiss citizenship and became spaceship’s length; rather, they For example, the faster an a technical assistant in the would see the observer appear object travels, the greater is its Swiss Patent Office where, to contract as they streaked by. kinetic energy, and the greater also in his free time, he produced much of his best work. In One consequence of the is its mass. The speed of light (c) is 1933, he emigrated to the contraction of space is to shorten US to take up the position of the time it would take a spacecraft a big number—squared, it is a very professor of theoretical physics to travel to the stars. Imagine there big number indeed. This means at Princeton, and was made a is a cosmic railroad network with that when even a tiny amount US citizen in 1940. He died on tracks stretching from star to of matter is converted into the April 18, 1955. star. The faster a spacecraft travels, equivalent amount of energy the shorter the track appears to the yield is colossal, but it also Key works become, and therefore the shorter means that there has to be an the distance to be covered to reach immense input of energy to see 1905 “On a Heuristic its destination. At 99.5 percent an appreciable increase in mass. ■ Viewpoint Concerning the of light speed, the journey to the Production and Transformation of Light” Light travels at a constant speed, 1905 “On the Electrodynamics so the speed of light from a car’s of Moving Bodies” headlights will not increase when the vehicle accelerates, nor will it decrease when the car slows down.

280 SAPUANCIEONANODFTIME CURVING SPACETIME IN CONTEXT T he world seems to conform cannot be accurately measured to certain geometric rules. It using straight lines and angles, and KEY FIGURE is possible to calculate the rely on detailed calculations called Hermann Minkowski coordinates of a point, for example, non-Euclidean forms of geometry. (1864–1909) or map out a particular shape. This basic understanding of the world, These can be useful when, for BEFORE using straight lines and angles, is example, calculating distances on 4th century bce Euclid’s work known as Euclidean space, named the surface of Earth. In Euclidean in geometry shows how the after Euclid of Alexandria. space, the distance between two ancient Greeks tried to apply points would be calculated as mathematics to physical space. With the dramatic evolution of though Earth’s surface were flat. physics in the early 20th century, But in non-Euclidean spaces, the 1637 French philosopher René the need for a new way to curvature of the planet has to be Descartes develops Cartesian understand the universe grew. accounted for, using what are coordinates—using algebra to German mathematician Hermann known as geodesics (“great arcs”) calculate positions. Minkowski realized that much of to get a more accurate value. ■ the work of physicists was more 1813 German mathematician easily understood when considered From this moment, space by Carl Friedrich Gauss suggests in four dimensions. In Minkowski itself and time by itself shall the idea of non-Euclidean “spacetime,” three coordinates spaces, which do not conform describe where a point is in space, sink into the background. to Euclidean geometry. and the fourth gives the time at Hermann Minkowski which an event happened there. AFTER 1915 Albert Einstein develops Minkowski noted in 1908 that his theory of general relativity Earth, and the universe, are curved with the help of Hermann and so do not follow straight lines. Minkowski’s work. Similar to how an aircraft follows a curved path over Earth rather than 1919 Arthur Eddington sees a straight one, light itself curves curved spacetime in action by around the universe. This means noting the changed positions that coordinates in spacetime of stars during an eclipse. See also: Measuring distance 18–19 ■ The language of physics 24–31 ■ Measuring time 38–39 ■ Special relativity 276–279

RELATIVITY AND THE UNIVERSE 281 EGAQCRUCAIVEVILATELYREIANSTTIOTNO THE EQUIVALENCE PRINCIPLE IN CONTEXT A lbert Einstein’s theory of In an accelerating spaceship, special relativity describes a dropped ball would behave in exactly KEY FIGURE how objects experience the same way as a ball dropped in Albert Einstein (1879–1955) space and time differently, Earth’s gravitational field. depending on their motion. An BEFORE important implication of special Ball drops c. 1590 Galileo Galilei relativity is that space and time to ground shows that two falling objects are always linked in a four- accelerate at the same speed dimensional continuum called Spaceship regardless of their mass. spacetime. His later theory of general accelerating relativity describes how spacetime in space 1609 German astronomer is warped by massive objects. Mass Johannes Kepler describes and energy are equivalent, and the If a person in a stationary spaceship what would happen if the warping they cause in spacetime on Earth drops an object, the moon’s orbit were stopped creates the effects of gravity. measured mass of the object will and it dropped toward Earth. be the same as if that person is in Einstein based his 1915 general an accelerating spaceship in space. 1687 Isaac Newton’s theory of relativity theory on the equivalence Einstein said it is impossible to tell gravitation includes the idea principle—the idea that inertial whether a person is in a uniform of the equivalence principle. mass and gravitational mass have gravitational field or accelerating the same value. This was first noted through space using this approach. AFTER by Galileo and Isaac Newton in the 1964 Scientists drop test 17th century, and then developed Einstein went on to imagine masses of aluminum and by Einstein in 1907. When a force what a beam of light would look like gold to prove the equivalence is applied to an object, the inertial for the person inside the spaceship. principle on Earth. mass of that object can be worked He concluded that powerful gravity out by measuring its acceleration. and extreme acceleration would 1971 American astronaut The gravitational mass of an object have the same effect—the beam David Scott drops a hammer can be calculated by measuring the of light would curve downward. ■ and feather on the moon to force of gravity. Both calculations show that they fall at the same will produce the same number. rate, as predicted by Galileo centuries earlier. See also: Free falling 32–35 ■ Laws of gravity 46–51 ■ Special relativity 276–279 ■ Curving spacetime 280 ■ Mass and energy 284–285

282 WTTRWHAIYVNEISYLOITNUHGNEGER? PARADOXES OF SPECIAL RELATIVITY IN CONTEXT A lbert Einstein’s theories of work, Langevin suggested relativity have prompted the paradox based on a known KEY FIGURE scientists to explore how consequence of relativity— Paul Langevin (1872–1946) light and other objects behave as time dilation. This means that they reach the extremities of time, any object moving faster than BEFORE space, and motion—for example, an observer will be seen by the 1905 Albert Einstein posits when a moving object approaches observer to experience time more that a moving clock will the speed of light. But they have slowly. The faster the object is experience less time than also resulted in some interesting moving, the more slowly it will a stationary clock. thought experiments that appear, be seen to experience time. at first, to be unsolvable. 1911 Einstein goes on Langevin wondered what might to suggest that a moving One of the best-known thought happen if there were identical twin person will be younger experiments is the “twin paradox.” brothers on Earth, and one of them than a stationary person. This was first put forward by was sent into space on a very fast French physicist Paul Langevin trip to another star and back. The AFTER in 1911. Building on Einstein’s twin on Earth would say that 1971 American physicists the traveler was moving, so the Joseph Hafele and Richard Returning to Earth, traveler would be the younger of Keating prove that Einstein’s having aged by two years, the two when he returned. But the theory of time dilation is [the traveler] will climb out traveler would argue that the twin correct by taking atomic on Earth was moving, and that clocks on board airplanes and of his vehicle and find he was stationary. He would comparing them with atomic our globe aged by at least therefore say that the Earth twin clocks on the ground. would be younger. two hundred years … 1978 The first GPS satellites Paul Langevin There are two solutions to are also found to experience this apparent paradox. The first time dilation. is that the traveler must change his velocity in order to return 2019 NASA launches an home, while the twin on Earth atomic clock designed to stays at a constant velocity—so be used in deep space. the traveler will be younger. The other solution is that the traveler leaves Earth’s reference frame, whereas the twin on Earth

RELATIVITY AND THE UNIVERSE 283 See also: Measuring time 38–39 ■ From classical to special relativity 274 ■ The speed of light 275 ■ Special relativity 276–279 ■ Curving spacetime 280 remains there, meaning that known as length contraction. Paul Langevin the traveler dictates the events However, from the pole vaulter’s that will happen. frame of reference, the barn is Paul Langevin was born in moving toward her at the speed Paris on January 23, 1872. After The barn-pole paradox of light; she is not moving. So the studying science in the French In the “barn-pole paradox,” a pole barn would be contracted, and capital, he moved to England vaulter runs through a barn at the pole would not fit. to study under J.J. Thomson nearly the speed of light, with a at Cambridge University’s pole twice the length of the barn. The solution is that there are Cavendish Laboratory. He While the pole vaulter is inside some inconsistencies that enable then returned to the Sorbonne the barn, both barn doors are the pole to fit. From the observer’s in Paris to get a doctorate in closed simultaneously, and then point of reference, the pole is small physics in 1902. He became opened again. Is it possible for the enough to fit inside before the barn professor of physics at the pole to fit inside the barn while doors open again to allow the Collège de France in 1904, and the doors are closed? pole out. From the pole vaulter’s then director of the School of perspective, the doors do not close Physics and Chemistry in From the reference frame of and open simultaneously, but 1926. He was elected to the someone watching, as something rather one after the other, allowing Academy of Sciences in 1934. approaches the speed of light, it the pole to enter at one end and appears to shorten, in a process escape from the other. ■ Langevin was best known for his work on magnetism, Stationary observer’s Pole vaulter’s frame but also built on the work of frame of reference of reference Pierre Curie at the end of World War I to devise a way to find The pole is 1 submarines using echolocation. twice as long The barn He also helped to spread Albert as the barn contracts Einstein’s theories in France. as it gets In the 1930s, he was arrested 1 closer to by France’s Vichy government the speed for opposing fascism, and spent of light most of World War II under house arrest. Langevin died on 2 2 December 19, 1946, aged 74. The pole One door contracts as closes and Key works it approaches opens after the speed of the other, 1908 “On the Theory of light, so it fits so the barn Brownian Motion” inside the passes over 1911 “The Evolution of Space closed barn the pole and Time” 33 The space around a moving object contracts as the object nears the speed of light. A stationary observer watching the pole reaching the barn would see it contract, allowing it to fit inside. For the pole vaulter, the barn contracts, but as the doors do not open and close simultaneously, it can pass over her.

284 EOAVNFODTLHLUEITFISEOTNARS MASS AND ENERGY IN CONTEXT T he famous equation the energy from a given mass and E = mc2 has taken various work out any changes that have KEY FIGURE forms over the years, and occurred in a nuclear reaction. Arthur Eddington (1882–1944) its impact on physics is hard to Chain reaction overstate. It was devised by Albert Einstein’s equation showed that BEFORE Einstein in 1905 and concerned the mass and energy were linked in 1905 Albert Einstein first mass–energy equivalence—that ways that had not been thought devises the mass–energy possible, and that a small loss of equivalence, with the initial energy (E) is equal to mass (m) mass could be accompanied by a huge release of energy. One of the equation being L/V2. multiplied by the square of the major implications of this was in 1916 Einstein titles an article speed of light (c2). According to with the equation E = mc2. Einstein’s theory of relativity, the equation can be used to calculate AFTER 1939 German-born physicist The core of a star is subjected to huge Hans Bethe produces a pressures and temperatures. detailed analysis of the hydrogen fusion chain that The deuterium atom Under these powers stars such as the sun. fuses with another conditions, hydrogen hydrogen atom to atoms merge together 1942 The world’s first nuclear produce helium. into a deuterium atom. reactor, Chicago Pile-1 (CP-1), is built in Chicago. Mass The lost in the collision gamma rays travel 1945 Trinity, the world’s first is converted into energy, to the surface, where nuclear bomb, is tested by the which is released as they are emitted as US army in New Mexico. gamma rays. visible light. 2008 The world’s largest particle accelerator, CERN’s Large Hadron Collider (LHC) in Switzerland, goes live.

RELATIVITY AND THE UNIVERSE 285 See also: Energy and motion 56–57 ■ Light from the atom 196–199 ■ Nuclear bombs and power 248–251 ■ The speed of light 275 ■ Special relativity 276–279 understanding how stars produce Today, knowledge of how mass can Arthur Eddington energy. Until the pioneering work of be turned into energy has allowed British physicist Arthur Eddington physicists to replicate it in nuclear Arthur Eddington was born in the early 20th century, scientists reactors, which use the process of in 1882 in Westmorland (now were at a loss to explain how stars nuclear fission—the splitting of a Cumbria), UK. He studied in like the sun radiated so brightly. heavy atom into two lighter ones. It Manchester and then at also led to the creation of the atomic Trinity College, Cambridge, When an unstable (radioactive) bomb, in which an unstoppable winning honors for his work. atom is hit by another particle, such chain reaction occurs, releasing From 1906 to 1913, he was as a neutron, it can break apart into a powerful and deadly amount of chief assistant at the Royal new particles. These can hit other energy. Particle accelerators such Observatory of Greenwich. atoms and cause a chain reaction, as the Large Hadron Collider rely in which some of the mass lost in on Einstein’s equation to smash Eddington’s parents were each collision is converted into new particles together and create new Quakers, so he was a pacifist particles, and the rest is released ones. The more energy there is in during World War I. He made as energy. In 1920, Eddington a collision, the greater the mass important contributions to suggested that certain elements of the particles that are created. science from 1914 onward, within stars could be undergoing a and was the first to expound similar process to produce energy. Physicists are now trying to on Einstein’s relativity theory recreate the process of nuclear in the English language. In Nuclear reactions fusion, which would produce more 1919, he traveled to Príncipe, Eddington proposed that hydrogen energy than nuclear fission but off the west coast of Africa, was being turned into helium and requires conditions so extreme that to observe a solar eclipse and other heavier elements by stars— they are difficult to replicate. It is prove the idea of gravitational a process that is now called nuclear hoped that in the coming decades lensing (that large masses fusion—and that this could account nuclear fusion could be a viable bend light, as predicted in for their energy output (the visible source of power on Earth. ■ Einstein’s theory of general light they produce). Using Einstein’s relativity) in the process. He equation, he described how the The sun releases vast amounts of published articles on gravity, intense heat and pressure at the energy by turning hydrogen into helium spacetime, and relativity, and core of a star could enable nuclear in the process of nuclear fusion. It is produced his masterwork on reactions to occur, destroying mass said to have enough hydrogen to keep nuclear fusion in stars in 1926. and releasing energy. it shining for the next 5 billion years. He died in 1944, aged 61. Key works 1923 The Mathematical Theory of Relativity 1926 The Internal Constitution of the Stars

286 IN CONTEXT SSWIPMHAEPCRLEYETIEMNEDS KEY FIGURE Karl Schwarzschild BLACK HOLES AND WORMHOLES (1873–1916) BEFORE 1784 John Michell proposes the idea of “dark stars,” which trap light with their intense gravity. 1796 French scholar Pierre- Simon Laplace predicts the existence of large invisible objects in the universe. AFTER 1931 Indian–American astrophysicist Subrahmanyan Chandrasekhar proposes a mass limit for the formation of a black hole. 1971 The first black hole, Cygnus X-1, is indirectly discovered in the Milky Way. 2019 A global team of astronomers reveals the first image of a black hole. T he idea of black holes and wormholes has been talked about for centuries, but only recently have scientists begun to understand them—and with them, the wider universe. Today, scientists are starting to appreciate just how impressive black holes are, and have even managed to take a picture of one—indisputable proof that they exist. British clergyman John Michell was the first to think about black holes, suggesting in the 1780s that there might be stars with gravity so intense that nothing, not even light, could escape. He called these “dark stars,” and said that while they would be essentially invisible, it

RELATIVITY AND THE UNIVERSE 287 See also: Free falling 32–35 ■ Laws of gravity 46–51 ■ Particles and waves 212–215 ■ From classical to special relativity 274 ■ Special relativity 276–279 ■ Curving spacetime 280 ■ The equivalence principle 281 ■ Gravitational waves 312–315 The gravitational mass of an object warps spacetime. Even so, Schwarzschild’s work If an object is compressed beyond a certain point, it becomes showed that black holes could a black hole. This point is its Schwarzschild radius—the radius mathematically exist, something that many scientists had not of the event horizon. thought possible. Astronomers The gravity of a black hole, or singularity, warps spacetime to today use his equations to estimate the masses of black holes, albeit not such an extent that nothing can escape it—not even light. very precisely, since their rotation and electric charge cannot be No one knows what happens beyond the boundary, accounted for. Back in the early or event horizon, of a black hole. 20th century, Schwarzschild’s findings allowed scientists to start should, in theory, be possible to even light could escape, with its contemplating what might happen see their gravitational effects on edge being what is called the event near or even inside a black hole. other objects. horizon. Similar to Michell’s “dark star” idea, this later became known Mass limit In 1803, however, British as a “black hole.” One important factor was working physician Thomas Young out how black holes formed, demonstrated that light is a wave, Schwarzschild produced an something set forth in 1931 by rather than a particle, prompting equation that allowed him to Subrahmanyan Chandrasekhar. scientists to question whether it calculate the Schwarzschild radius He used Einstein’s theory of special would be affected by gravity at all. (the radius of the event horizon) for relativity to show that there is a Michell’s idea was swept aside and any given mass. If an object is mass limit below which a star at largely forgotten about until 1915, compressed into a volume so dense the end of its life will collapse into when Albert Einstein came up with that it fits within that radius, it will a stable, smaller dense star known his general theory of relativity—and warp spacetime to such an extent as a white dwarf. If the remains of suggested that light itself could be that nothing can escape its the star’s mass are above this limit, bent by the mass of an object. gravitational pull—it will collapse which Chandrasekhar calculated into a singularity, or the center of a as 1.4 times the mass of the sun, No escape black hole. Schwarzschild, however, it will collapse further into either The following year, German did not think that a singularity a neutron star or a black hole. ❯❯ physicist Karl Schwarzschild took could really exist. He saw it as a Einstein’s idea to its extreme. He somewhat theoretical point where The final fate of massive stars suggested that if a mass were some property is infinite, which, is to collapse behind an event condensed enough, it would create in the case of a black hole, is the horizon to form a ‘black hole’ a gravitational field from which not density of the matter. which will contain a singularity. Stephen Hawking

288 BLACK HOLES AND WORMHOLES Black holes Core of star Gravitational Gas, dust, and Matter Matter force disintegrated stars joining spiraling may swirl around accretion inward the black hole in disk an accretion disk Singularity Event horizon Gravity well Singularity When a massive star dies, If the core that remains after the The singularity is now so dense it collapses, unable to resist supernova is still massive (more than that it distorts the spacetime around the crushing force of its own 1.4 times the mass of the sun), it it so that not even light can escape. gravity. This causes a supernova keeps shrinking and collapses under This black hole is pictured in two explosion, and the star’s outer its own weight into a point of infinite dimensions as an infinitely deep parts are blasted into space. density—a singularity. hole called a gravity well. In 1939, American physicists like the one Schwarzschild had occur on a microscopic level, Robert Oppenheimer and Hartland proposed. Three years later, the attempts to draw up larger versions Snyder put forward a more modern- term “black hole” was born during have so far been unsuccessful. looking idea of a black hole. They a talk by American physicist John Nonetheless, the idea of them described how the sort of body Wheeler. He had already come up persists today—and with it, the Schwarzschild had envisaged with a term to describe theoretical very vague possibility that humans would be detectable only by its tunnels in spacetime: “wormholes.” could traverse large distances in gravitational influence. Others had the universe with ease. already considered the peculiar Many physicists were, by now, physics that would take place considering the idea of wormholes. Looking for proof inside a black hole, including The notion of a white hole, in While theories abounded, no one Belgian astronomer Georges which the event horizon would had yet been able to detect a black Lemaître in 1933. His “Alice and not stop light from escaping, hole, let alone a wormhole. But in Bob” thought experiment supposed but rather stop it from entering, 1971, astronomers observed an odd that if Bob were to witness Alice progressed into an idea that black source of X-rays in the Cygnus falling into a black hole, she would holes and white holes could be constellation. They suggested appear to freeze at its edge—the linked. Using exotic matter, which these X-rays, dubbed Cygnus X-1, invisible boundary called the event involves negative energy densities were the result of a bright blue star horizon—but she would experience and negative pressure, it was being torn apart by a large and dark something entirely different as she suggested that information could object. At last, astronomers had fell within its bounds. pass through a wormhole from one witnessed their first black hole, not end to the other, perhaps between by seeing it directly, but by seeing It would take until 1964 for the a black and a white hole, or even its effect on a nearby object. next major development to come, two black holes, across vast when British physicist Roger distances of space and time. From there, black hole theory Penrose suggested that if a star soared in popularity. Physicists imploded with enough force, it In reality, however, the existence produced novel ideas about how would always produce a singularity of wormholes remains questionable. black holes might behave—among Although scientists think they may

RELATIVITY AND THE UNIVERSE 289 them Stephen Hawking, who superheated dust and gas, which Karl Schwarzschild in 1974 came up with the idea that can be seen even at great distances. black holes might emit particles Around the supermassive black hole Born in Frankfurt, Germany, (now known as Hawking radiation). believed to be at the center of the in 1873, Karl Schwarzschild He suggested that the strong Milky Way, astronomers have seen showed an early talent for gravity of a black hole could what they think are stars orbiting science, publishing a paper produce pairs of both particles and it—the very idea suggested by on the orbits of celestial antiparticles, something that is Michell back in the 18th century. objects when he was 16. thought to happen in the quantum He became a professor at world. While one of the particle– The greatest moment in black the University of Göttingen antiparticle pair would be pulled hole theory’s history so far arrived in 1901, and director of the into the black hole, the other would in April 2019, when astronomers Astrophysical Observatory escape, carrying with it information from an international collaboration at Potsdam in 1909. about the event horizon, the called the Event Horizon Telescope strange boundary beyond which (EHT) revealed the first-ever image One of Schwarzschild’s no mass or light can escape. of a black hole. Using multiple main contributions to science telescopes around the globe to came in 1916, when he Supermassive black holes create a virtual super telescope, produced the first solution Black holes of different shapes they were able to photograph the of Einstein’s equations of and sizes were imagined, too. supermassive black hole in a galaxy gravity from the theory of Stellar black holes, such as Cygnus called Messier 87, 53 million light- general relativity. He showed X-1, were thought to contain years away. The image lived up to that objects of a high enough between about 10 and 100 times all predictions, with a ring of light mass would have an escape the mass of the sun squashed into surrounding a dark center. velocity (the velocity needed an area dozens of miles across. But to escape its gravitational it was also thought that black holes When the universe ends in pull) higher than the speed could merge into a supermassive trillions of years, it is thought that of light. This was the first black hole, containing millions or black holes will be the only thing to step in understanding black billions of times the mass of the remain as entropy—the unavailable holes, and ultimately led to sun packed into an area millions energy in the universe—increases. the idea of an event horizon. of miles across. Finally, they, too, will evaporate. ■ Schwarzschild died from an autoimmune disease in 1916, Nearly every galaxy is thought In this illustration, the black hole while serving on the Russian to have a supermassive black hole Cygnus X-1 is pulling material from a front in World War I. at its core, possibly surrounded blue companion star toward it. The by a swirling accretion disk of material forms a red and orange disk that rotates around the black hole. Key work 1916 On the Gravitational Field of a Mass Point after Einstein’s Theory

290 IN CONTEXT UOTHNFEITVHFEREROSKNENTOIEWRN KEY FIGURES Henrietta Swan Leavitt DISCOVERING OTHER GALAXIES (1868–1921), Edwin Hubble (1889–1953) BEFORE 964 Persian astronomer Abd al-Rahman al-Sufi is the first person to observe Andromeda, although he does not realize it is another galaxy. 1610 Galileo Galilei proposes that the Milky Way is made of many stars after observing it through his telescope. AFTER 1953 Galaxies close to Earth are found to be part of a supercluster, called the Virgo Cluster, by French astronomer Gérard de Vaucouleurs. 2016 A study led by American astrophysicist Christopher Conselice reveals that the known universe contains two trillion galaxies. A fter the publication of Copernicus’s heliocentric model of the universe in 1543, which placed the sun rather than Earth at its center, further attempts to understand the size and structure of the universe made little progress. It was nearly 400 years before astrophysicists realized that not only was the sun not the center of the universe, as Copernicus had thought, but it was not even the center of our galaxy, the Milky Way. In the 1920s, the discovery by Edwin Hubble that the Milky Way is just one of many galaxies in the universe marked a significant leap forward in astronomical knowledge.

RELATIVITY AND THE UNIVERSE 291 See also: The Doppler effect and redshift 188–191 ■ Models of the universe 272–273 ■ The static or expanding universe 294–295 Cepheid variables (a type Their pulse period (period Measuring the star’s period of star) can be used to of fluctuation in their helps calculate its true measure distances. brightness. brightness) is related to their true brightness. The Milky Way is Some stars are so far from By measuring how much of not the only galaxy. Earth that they must be the star’s light reaches outside our Milky Way. Earth, its distance from Earth can be calculated. Key to this breakthrough was a Pickering, who ran the observatory, pulsates in a regularly recurring new way of measuring distances had asked Leavitt to measure cycle, or period, dependent upon in space made possible by the work the brightness of the stars on a physical changes within the of American astronomer Henrietta collection of photographic plates star. Compressed by gravity, Swan Leavitt in 1908. showing the Magellanic Clouds, the star becomes smaller, more now known to be small galaxies opaque, and gradually hotter as Determining distance outside the Milky Way. the light energy trapped inside the In the early 1900s, establishing a star starts to build. Eventually, star’s distance from Earth was no In the course of this work, Leavitt the extreme heat causes the gas easy task. Within the Milky Way, discovered a class of very bright in the outer layers to expand distances between objects could stars called Cepheid variables and the star becomes more be measured using parallax, a whose luminosity fluctuates as transparent, allowing the light ❯❯ process that employs basic they pulsate. Each of these stars trigonometry. By measuring the angles of the sight lines to an object Cepheid variable stars pulsate—expand and contract from both sides of Earth’s orbit over a regular cycle—resulting in varying temperature and around the sun, for example, brightness. Astronomers can plot their changing brightness astronomers can calculate the over time on a light curve. angle where the two sight lines converge and thus the distance Hottest state Star expands and contracts Coolest Hottest to that object. However, for (exaggerated here) state state distances greater than 100 light years, and outside the Milky Way, BRIGHTNESS Period of one parallax is too imprecise. Early pulsation 20th-century astronomers needed another method. TIME Light curve of Cepheid variable At the time, Leavitt was working as a “computer,” the name given to data processors at Harvard College Observatory in Cambridge, Massachusetts. Edward Charles

292 DISCOVERING OTHER GALAXIES luminosities had to be due to their intrinsic brightness and not to how far they were from Earth. Since the [Cepheid] variables A new tool An image taken by the Hubble Space are probably at nearly the Leavitt published her findings in Telescope shows RS Puppis, a Cepheid 1912, creating a chart showing the variable with a phenomenon known as same distance from the Earth, periods of 25 Cepheid variables, a light echo—when light from the star their periods are apparently indicating their “true luminosity,” is reflected from a cosmic dust cloud. associated with their actual and how bright they appeared from emission of light. Earth, their “apparent luminosity.” Sciences in the US held a debate Henrietta Swan Leavitt The significance of Leavitt’s on the nature and extent of the discovery was that once the universe. American astronomers energy to pass through. However, distance of one Cepheid variable Heber Curtis and Harlow Shapley as the star expands, it also becomes had been calculated using parallax, argued for and against the cooler, and gravitational forces the distance of Cepheid variables existence of other galaxies in what eventually outweigh the outward of comparable pewriod in stars was known as “the Great Debate” pressure of expansion. At this beyond the limits of parallax could or the “Shapley–Curtis Debate.” point, the star becomes smaller be calculated by comparing the Shapley asserted that the Milky and the process repeats. “true luminosity” established by Way was a single galaxy and that its period with the “apparent the “spiral nebulae” (spiral-shaped Leavitt noticed that the cycles luminosity”—the amount it had concentrations of stars) such as of the stars varied between two dimmed by the time its light Andromeda were gas clouds and sixty days, and the brightest reached Earth. within our own galaxy, which he stars spent longer at their believed was much larger than maximum luminosity. Because Within a year of Leavitt’s most astronomers thought. He all the stars were in the Magellanic discovery, Danish astronomer Ejnar argued that if Andromeda was a Clouds, Leavitt knew that any Hertzsprung created a luminosity separate galaxy, its distance from differences between the stars’ scale of Cepheid variables. These Earth would be too great to be stars were designated as “standard acceptable. He also contended that candles”—benchmarks for working the sun was not at the center of the out the cosmic distances of objects. Milky Way, but on the outer edges Over the next decade, other of the Milky Way. astronomers began using Leavitt’s and Hertzsprung’s work to calculate Curtis argued that spiral the distance to stars. nebulae were separate galaxies. One of his arguments was that The Great Debate exploding stars (known as novae) Using Cepheid variables was not in these nebulae looked similar a perfect method for ascertaining to those in our own galaxy, and distance—it could not account for there were a much larger number light-absorbing cosmic dust, which in some locations—such as distorts apparent luminosity—but it led to a fundamental change in our understanding of the universe. In 1920, the National Academy of Edwin Hubble uses the 100-inch (2.5-meter) Hooker telescope at Mount Wilson Observatory, California. In 1924, Hubble announced that he had used it to see beyond the Milky Way.

RELATIVITY AND THE UNIVERSE 293 Andromeda—than in others. By the death of Miss Leavitt galaxies, each containing millions This suggested that more stars on December 12, 1921, to hundreds of billions of stars. It is existed and hinted at the presence the observatory lost an currently estimated that there are of separate galaxies, which he investigator of the about two trillion galaxies in the called “island universes.” At the highest value. universe, and that the first was same time, Curtis placed the sun Harlow Shapley created just a few hundred million at the center of our own galaxy. years after the Big Bang, which and Shapley’s estimates for the occurred 13.8 billion years ago. The Great Debate is a perceived size of the Milky Way. The age of our own Milky Way is testament to how incomplete Curtis was therefore correct. thought to be almost as ancient, our understanding of the universe Hubble’s calculations proved that at around 13.6 billion years old. was just 100 years ago, as is not only was the Milky Way much Curtis’s incorrect theory about larger than thought, but Andromeda In 1929, Hubble also discovered the sun’s position in the Milky was not a nebula at all, but another that the universe appeared to be Way. (Shapley’s theory that the sun galaxy—the Andromeda Galaxy. It expanding, and that almost all is on the outer edges of the Milky was the first discovery of a known galaxies are moving away from Way is close to being correct.) galaxy outside our own. each other, with increasing velocity as the distances become Hubble breakthrough Expansion and collision greater. One notable exception is In 1924, American astronomer Hubble’s discovery, made possible Andromeda, which is now known Edwin Hubble settled the Shapley- by the work of Leavitt, ultimately to be moving on a collision course Curtis Debate by taking some led to the knowledge that the toward our galaxy—at a rate of measurements using Cepheid Milky Way is just one of many 68 miles (110 km) per second. That variables. During the debate, is relatively slowly compared to its Shapley had surmised that the distance, but it means that in Milky Way was about 300,000 about 4.5 billion years, the Milky light years across, ten times larger Way and Andromeda will collide than Curtis’s estimated 30,000 and form a single galaxy, which light years. Hubble, using Cepheid astrophysicists have nicknamed variables that he had located Milkomeda. This event will not in Andromeda, calculated that be too dramatic, however: even Andromeda was 900,000 light years though the two galaxies will away (now revised to 2.5 million merge, it is unlikely that any light years)—far beyond Curtis two stars will collide. ■ Henrietta Swan Leavitt Born in Lancaster, Massachusetts, group of women known as the in 1868, Leavitt studied at Oberlin “Harvard computers” whose College, Ohio, before attending role was to study photographic the Society for the Collegiate plates of stars. Initially, she was Instruction of Women (now called unpaid, but she later earned Radcliffe College) in Cambridge, 30 cents an hour. As part of her Massachusetts. She developed an work she found 2,400 variable interest in astronomy after taking stars, and in so doing discovered a course in the subject. Around Cepheid variables. Leavitt died the same time, an illness led to of cancer in 1921. hearing loss, which grew worse over the course of her life. Key works After graduating in 1892, 1908 1777 variables in the Leavitt worked at the Harvard Magellanic Clouds Observatory, then led by 1912 Periods of 25 Variable Stars American astronomer Edward in the Small Magellanic Cloud Charles Pickering. She joined a

294 OTHFETHFUETUUNRIVEERSE THE STATIC OR EXPANDING UNIVERSE IN CONTEXT W hen Albert Einstein to the mass and energy moving came up with his theory through it. But when he applied KEY FIGURE of general relativity these equations to the universe, he Alexander Friedmann in 1915, there was one problem. found it should be either expanding (1888–1925) He was adamant that the universe or contracting—neither of which he was static and everlasting, but believed could be true. BEFORE according to his calculations, this 1917 Albert Einstein should mean that the universe Instead, Einstein believed the introduces the cosmological would eventually collapse on itself universe was everlasting, and constant to suggest the due to gravity. To get around this added in his “cosmological universe is static. problem, Einstein proposed the member” (now known as the idea of a cosmological constant, cosmological constant). This AFTER represented by the Greek letter allowed for a universe that could 1929 American astronomer lambda (L). This was a measure overcome the effects of gravity, Edwin Hubble proves the of the “vacuum energy” in space. and, instead of collapsing on itself, universe is expanding—he would remain static. finds that distant galaxies are Einstein’s general theory of moving more rapidly than relativity laid out a set of “field In 1922, Russian mathematician those closer to us. equations” that showed how the Alexander Friedmann came to a curvature of spacetime was related different conclusion. He showed 1931 Einstein accepts that the universe was homogeneous: the theory of an expanding universe. There are three possible futures for the universe. 1998 Scientists from two The universe is The universe The universe independent projects discover static—it will stop is closed and is open and the universe’s expansion will eventually will continue is accelerating. expanding and collapse on itself. expanding will never forever—this is 2013 Dark energy is calculated contract. the current theory. to make up 68 percent of the universe, and is closely linked to the cosmological constant.

RELATIVITY AND THE UNIVERSE 295 See also: From classical to special relativity 274 ■ Curving spacetime 280 ■ Mass and energy 284–285 ■ The Big Bang 296–301 ■ Dark energy 306–307 The shape of the universe If the density of the If the universe is If the universe is less Alexander Friedmann universe is exactly denser than a critical dense than a critical the same as a critical value, it is positively value, it is negatively Alexander Friedmann was value, it is “flat.” In a flat curved or “closed” and is curved or “open” and born in St. Petersburg, Russia, universe, parallel lines finite in mass and extent. therefore infinite. The in 1888. His father was a never meet. The 2-D The 2-D analogy is a 2-D analogy is a saddle- ballet dancer and his mother analogy of this model spherical surface where shaped surface where a pianist. In 1906, Friedmann is a plane (a flat surface). parallel lines converge. parallel lines diverge. was admitted to the St. Petersburg State University it was identical wherever you were Although Einstein initially derided to study mathematics. Here in the universe and whichever way Friedmann’s ideas in 1923, by the he also applied himself to you looked, so the universe could following year he had accepted studying quantum theory not be static. All galaxies were them. However, it would be 1931 and relativity and earned his moving apart from each other, but before he truly accepted that the master’s degree in pure and depending on which galaxy you universe was expanding—two applied mathematics in 1914. were in, all other galaxies would years after American astronomer also be moving away from you. Edwin Hubble had produced In 1920, after serving as an Therefore you might think you evidence that the universe was aviator and instructor in World were at the center of the universe— indeed expanding. Hubble had War I, Friedmann began but any other observer in another noted the stretched light—or research based on Einstein’s galaxy would believe the same redshift—of distant galaxies, general theory of relativity. He to be true of their position. which could be used to measure used Einstein’s field equations the distance of faraway objects. to develop his own idea of a Models for the universe He found that galaxies that were dynamic universe, countering As a result of his work, Friedmann more distant were moving faster Einstein’s belief that the came up with three models for the than closer galaxies, proof that universe was static. In 1925, universe, varying according to the the universe itself was expanding. Friedmann was appointed value of the cosmological constant. director of Main Geophysical One model was that gravity would Following Hubble’s discovery, Observatory in Leningrad, but cause the expansion to slow, the idea of a cosmological constant died later that year, at the age ultimately reversing and ending was deemed to be an error on of just 37, from typhoid fever. in a “Big Crunch.” The second was Einstein’s part. However, in 1998, that the universe would eventually scientists discovered that the Key works become static once expansion universe was expanding at an stopped. A third model was that accelerated rate. The cosmological 1922 “On the Curvature the expansion would continue constant would become crucial to of Space” forever at a faster and faster rate. understanding dark energy and lead 1924 “On the Possibility of a to the term’s reintroduction. ■ World with Constant Negative Curvature of Space”

ETHXEPCLOSOMDICIENGGG, CATRTHEE MAOTMEINOT NOF THE BIG BANG



298 THE BIG BANG IN CONTEXT Lemaître uses mathematics Hubble provides to show that Einstein’s experimental evidence that KEY FIGURE Georges Lemaître theory of general relativity galaxies are moving (1894–1966) means the universe must apart, and that those most BEFORE be expanding. distant are moving 1610 Johannes Kepler the fastest. surmises that the universe is finite, because the night sky is The universe must have Cosmic microwave dark and not lit by an infinite started from a single background radiation number of stars. (CMBR) shows the residual point—Lemaître’s heat left over after the 1687 Isaac Newton’s laws of primeval atom. Big Bang, suggesting it motion explain how things move through the universe. really did occur. 1929 Edwin Hubble discovers F or more than two millennia, universe. Today’s theory about the that all galaxies are moving humanity’s greatest minds origins of the cosmos dates from away from each other. have pondered the origins of the early 1930s, when Belgian the universe and our place within it. astronomer Georges Lemaître first AFTER For centuries, many believed that suggested what is now known as 1998 Leading astronomers some sort of deity created the the Big Bang. announce that the expansion universe, and Earth was at its of the universe is accelerating. center, with all the stars traveling Lemaître’s cosmic egg around it. Few suspected that Earth In 1927, Lemaître had proposed that 2003 NASA’s WMAP probe was not even the center of its own the universe was expanding; four finds evidence for “inflation,” solar system, which itself orbits one years later he developed the idea, or the burst of expansion of hundreds of billions of stars in the explaining that the expansion had just after the Big Bang, by mapping out tiny temperature Born in 1894 in Charleroi, Belgium, developing this idea in 1927. fluctuations across the sky. Georges Lemaître served in World The 1931 theory that Lemaître War I as an artillery officer, then is best known for—the idea that Georges Lemaître entered a seminary and was the universe had expanded from ordained as a priest in 1923. He a single point—was initially studied in the UK at the University dismissed, but was finally of Cambridge’s solar physics proven to be correct shortly laboratory for a year, and in 1924 before his death in 1966. joined the Massachusetts Institute of Technology in the US. By 1928, Key works he was a professor of astrophysics at the Catholic University of 1927 “Discussion on the Leuven, in Belgium. evolution of the universe” 1931 “The primeval atom Lemaître was familiar with the hypothesis” works of Edwin Hubble and others who discussed an expanding universe, and published work