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200 INTRODUCTION Max Planck describes J. J. Thomson is awarded Thomas Hunt Morgan Werner Heisenberg sets discrete packets, or the Nobel Prize in Physics introduces the out his uncertainty principle. quanta, of energy. for his discovery of chromosome theory the electron. of inheritance. 1900 1906 1915 1927 1905 1912 1926 1928 Albert Einstein produces Alfred Wegener Erwin Schrödinger Paul Dirac introduces his paper on special proposes a theory of unleashes wave quantum relativity. continental drift. mechanics. electrodynamics. W hile the 19th century “black box,” which had stubbornly aspects of the same phenomenon, had seen a fundamental resisted classical equations, by capable of being converted from change in the way imagining that electromagnetism one to the other, and his equation scientists view life processes, traveled not in continuous describing their relation—E = mc2 the first half of the 20th would waves, but in discrete packets, —hinted at an enormous potential prove even more of a shock. The or “quanta.” Five years later, energy locked inside atoms. old certainties of classical physics, Albert Einstein, a clerk working largely unchanged since Isaac at the Swiss Patent Office, Wave–particle duality Newton, were about to be thrown produced his paper on special Worse was to follow for the old away, and nothing short of a new relativity, asserting that the picture of the universe. At way to view space, time, and speed of light is constant and Cambridge, English physicist matter was to replace it. By 1930, independent of the movement of J. J. Thomson discovered the the old idea of a predictable source or observer. After working electron, showing that it has a universe had been shattered. through the implications of general negative charge and is at least relativity, Einstein had found by a thousand times smaller and A new physics 1916 that notions of an absolute lighter than any atom. Studying Physicists were finding that the time and space independent of the the properties of the electron equations of classical mechanics observer had gone, to be replaced was to produce new puzzles. were producing some nonsensical by a single space-time, which was Not only did light have particle-like results. It was clear that something warped by the presence of mass properties, but particles had was fundamentally wrong. In 1900, to produce gravity. Einstein had wavelike properties, too. Austrian Max Planck solved the puzzle of the further demonstrated that matter Erwin Schrödinger drew up a spectrum of radiation emitted by a and energy should be considered series of equations that described

A PARADIGM SHIFT 201 Edwin Hubble finds Georges Lemaître Konrad Lorenz Linus Pauling writes that the universe suggests that the explains the basis of The Nature of the universe began as a is expanding. primeval atom. animal instinct. Chemical Bond, which uses the ideas of quantum physics to explain chemistry. 1929 1931 1935 1939 1930 1934 1936 1942 Subrahmanyan Fritz Zwicky Alan Turing J. Robert Oppenheimer Chandrasekhar proposes the existence describes the Universal takes on the describes black holes. of dark matter. Turing Machine, Manhattan Project a programmable to develop the atomic bomb. computer. the probability of finding a particle Linus Pauling took this new picture universe was suddenly enormously in a particular place and state. of an atom and used the ideas of bigger than anyone had thought. His German colleague Werner quantum physics to explain how Hubble further found that the Heisenberg showed that there atoms bonded to each another. In universe was expanding in all was an inherent uncertainty to the process, he showed how the directions. Belgian priest and the values of place and momentum, discipline of chemistry was, in physicist Georges Lemaître which was initially thought to be a reality, a subsection of physics. By proposed that the universe had problem of measurement, but later the 1930s, physicists were working expanded from a “primeval atom.” found to be fundamental to the on ways to unlock the energy in This was to become the Big Bang structure of the universe. A strange the atom, and in the US, J. Robert theory. A further puzzle was picture was emerging of a warped, Oppenheimer led the Manhattan uncovered when astronomer Fritz relative space-time with particles Project, which was to produce the Zwicky coined the term “dark of matter smeared across it in the first nuclear weapons. matter” to explain why the Coma form of probability waves. galaxy cluster appeared to contain The universe expands 400 times as much mass (as seen Splitting the atom Up to the 1920s, nebulae were from its gravity) as he could explain New Zealander Ernest Rutherford thought to be clouds of gas or dust from the observable stars. Not only first showed that an atom is made within our own galaxy, the Milky was matter not quite what it had mostly of space, with a small, Way, which comprised the entire been thought to be, but much of it dense nucleus and electrons in known universe. Then American was not even directly detectable. orbit around it. He explained astronomer Edwin Hubble It was clear that there were certain forms of radioactivity as the discovered that these nebulae still major holes in scientific splitting of this nucleus. Chemist were in fact distant galaxies. The understanding. ■

202 IN CONTEXT OUANTA ARE BRANCH DISCRETE Physics PACKETS OF ENERGY BEFORE 1860 The distribution of MAX PLANCK (1858–1947) so-called black-body radiation fails to match predictions made by theoretical models. 1870s Austrian physicist Ludwig Boltzmann’s analysis of entropy (disorder) introduces a probabilistic interpretation of quantum mechanics. AFTER 1905 Albert Einstein proposes that the quantum is a real entity, using Planck’s concept of quantized light to introduce the idea of the photon. 1924 Louis de Broglie proves that matter behaves both as a particle and as a wave. 1926 Erwin Schrödinger formulates an equation for the wave behavior of particles. I n December 1900, the German theoretical physicist Max Planck presented a paper setting out his method for resolving a long-standing theoretical conflict. In doing so, he made one of the most important conceptual leaps in the history of physics. Planck’s paper marked the turning point between the classical mechanics of Newton and quantum mechanics. The certainty and precision of Newtonian mechanics was to give way to an uncertain, probabilistic description of the universe. Quantum theory has its roots in the study of thermal radiation, the phenomenon that explains why we feel heat from a fire, even when the

A PARADIGM SHIFT 203 See also: Ludwig Boltzmann 139 ■ Albert Einstein 214–21 ■ Erwin Schrödinger 226–33 Classical mechanics treats radiation as if it were emitted across a black-body object whose emitted continuous range. spectrum is almost entirely a result of its own temperature. Studying But nonsense results are reached the distribution of a black body’s for the distribution of black-body radiation, light would show that emission of radiation depended only on a body’s assuming a continuous range. temperature, and not its physical shape or chemical composition. Kirchhoff’s hypothesis kick-started a new experimental program designed to find a theoretical framework that would describe black-body radiation. The problem is solved by treating radiation as if it were Entropy and black bodies produced in discrete “quanta.” Planck arrived at his new quantum theory through the failure of Radiation is not continuous, classical physics to explain the but is emitted in discrete quanta of energy. experimental results of black-body radiation distribution. Much of air in between it and us is cold. reflects radiation, and it is this Planck’s work focussed on the Every object absorbs and emits reflected light that gives objects second law of thermodynamics, electromagnetic radiation. If its color even when they do not glow. which he had identified as an temperature rises, the wavelength “absolute.” This law states that of the radiation it emits decreases In 1860, German physicist isolated systems move over time while its frequency increases. For Gustav Kirchhoff thought of an toward a state of thermodynamic example, a lump of coal at room idealized concept he called a equilibrium (meaning that all temperature emits energy below “perfect black body.” This is a parts of the system are at the same the frequency of visible light, in the theoretical surface that, when at temperature). Planck attempted to ❯❯ infrared spectrum. We cannot see thermal equilibrium (not heating the emission, so the coal appears up or cooling down), absorbs every A new scientific truth black. Once we set the coal alight, frequency of electromagnetic does not triumph by however, it emits higher-frequency radiation that falls on it, and does convincing its opponents and radiation, glowing a dull red as the not itself reflect any radiation. making them see the light, but emissions break into the visible The spectrum of thermal radiation rather because…a new spectrum, then white-hot and coming off this body is “pure,” generation grows up that is finally a brilliant blue. Extremely since it is not mixed with any hot objects, such as stars, radiate reflections—it will only be familiar with it. even shorter-wavelength ultraviolet the result of the body’s own Max Planck light and X-rays, which again we temperature. Kirchhoff believed cannot see. Meanwhile, in addition that such “black-body radiation” is to producing radiation, a body also fundamental in nature—the Sun, for example, comes close to being a

204 MAX PLANCK explain the thermal radiation Planck reasoned that this principle pattern of a black body by figuring should be evident in any theoretical out the entropy of the system. black-body model. Entropy is a measure of disorder, though more strictly it is defined The Wien–Planck Law No real-world object is a perfect as a count of the number of ways By the 1890s, experiments in black body, but the Sun, black velvet, a system can organize itself. The Berlin came close to Kirchhoff’s and surfaces coated with lampblack, higher the entropy of a system, perfect black-body, using so-called such as coal tar, come close. the more ways the system has cavity radiation. A small hole in a of organizing and producing the box kept at a constant temperature the data—it could accurately same overall pattern. For instance, is a good approximation of a black describe the short-wavelength imagine a room where all the body, as any radiation entering the (high-frequency) spectrum of molecules of air start off bunched box gets trapped inside, and the thermal emission from objects, but up in the top corner. There are far body’s emissions are purely a result not the long-wavelength (low- more ways for the molecules to of its temperature. frequency emissions). This is the organize themselves so that there point at which Planck broke with is roughly the same number of The experimental results proved his conservatism and resorted to them in each cubic centimeter of bothersome for Planck’s colleague Ludwig Boltzmann’s probabilistic the room than there are for them all Wilhelm Wien, since the low- approach to arrive at a new to remain in the top corner. Over frequency emissions recorded did expression for his radiation law. time, they distribute themselves not fit his equations for radiation at equally throughout the room as all. Something had gone wrong. In Boltzmann had formulated a the entropy of the system rises. 1899, Planck arrived at a revised new way to look at entropy by A cornerstone of the second law equation—the Wien–Planck law— regarding a system as a large of thermodynamics is that entropy that attempted a better description collection of independent atoms works in one direction only. of the spectrum of thermal and molecules. While the second En route to thermal equilibrium, radiation from a black body. the entropy of a system always Science cannot solve the increases or remains constant. Ultraviolet catastrophe ultimate mystery of nature. A further challenge came a year And that is because, in the A cavity with a small hole will later, when British physicists last analysis, we ourselves are trap most of the radiation that enters Lord Rayleigh and Sir James Jeans a part of the mystery that we through the hole, making it a good showed how classical physics approximation of an ideal black body. predicts an absurd distribution are trying to solve. of energy in black-body emission. Max Planck The Rayleigh–Jeans Law predicted that, as the frequency of the radiation increased, the power it emitted would grow exponentially. This “ultraviolet catastrophe” was so radically at odds with experimental findings that the classical theory must have been seriously awry. If it were correct, a lethal dose of ultraviolet radiation would be emitted whenever a light bulb was turned on. Planck was not too troubled by the Rayleigh–Jeans Law. He was more concerned about the Wien– Planck Law, which, even in its revised form, was not matching

A PARADIGM SHIFT 205 law of thermodynamics remained Radiation intensity The ultraviolet catastrophe was a valid, Boltzmann’s reading gave Visible light nonsense result predicted by classical it a probabilistic, rather than an physics (shown here as the Raleigh– absolute, truth. Thus, we observe Jeans Law) in which black-body radiation entropy simply because it is increased exponentially as its wavelength overwhelmingly more likely than shortened. By quantizing radiation, the alternative. A plate breaks but Planck produced a formula that fit the does not remake itself, but there experimental data. is no absolute law preventing a plate from putting itself back Planck Rayleigh– together—it is just exceedingly Radiation Jeans Law unlikely to happen. Formula Quantum of action 1000 2000 3000 Planck used Boltzmann’s statistical interpretation of entropy to arrive at Wavelength of radiation (nm) a new expression for the radiation law. Imagining thermal radiation as Introducing “quanta” of energy to come to terms with the being produced by individual reduced the number of states of consequences of his own work. “oscillators,” he needed to count the energy available to the system, While he was never in any doubt ways in which a given energy could and in doing this (although it about the revolutionary impact be distributed between them. wasn’t his goal), Planck solved the of what he had done, he was— ultraviolet catastrophe. He thought according to historian James To do this, he divided the total of his quanta as a mathematical Franck—“a revolutionary against energy into a finite number of necessity—as a “trick”—rather his own will.” He found the discrete energy chunks—a process than something that was real. But consequences of his equations called “quantization.” Planck was a when Albert Einstein used the not to his taste since they often gifted cellist and pianist and might concept to explain the photoelectric gave descriptions of physical reality have imagined these “quanta” in effect in 1905, he insisted that that clashed with our everyday the same way that a fixed number quanta were a real property of light. experience of the world. But for of harmonics is available to the better or worse, after Max Planck, vibrating string of an instrument. As with many of the pioneers the world of physics has never The resulting equation was simple, of quantum mechanics, Planck been the same. ■ and it fit the experimental data. spent the rest of his life struggling Max Planck Born in Kiel in northern Germany giving birth to their children. in 1858, Planck was an able pupil During World War II, an Allied at school and graduated early, bomb destroyed his house in at 17. He chose to study physics at Berlin and his papers, and in the the University of Munich, where closing stages of the war, his he soon became a pioneer of remaining son was caught up in quantum physics. He received the plot to assassinate Hitler the Nobel Prize in Physics in 1918 and was executed. Planck for his discovery of energy quanta, himself died soon after the war. although he never was able to satisfactorily describe the Key works phenomena as a physical reality. 1900 Entropy and Temperature Planck’s personal life was of Radiant Heat beset by tragedy. His first wife 1901 On the Law of Distribution died in 1909, and his eldest of Energy in the Normal son was killed in World War I. Spectrum Both of his twin daughters died

NOW I KNOW WHAT THE ATOM LOOKS LIKE ERNEST RUTHERFORD (1871–1937)



208 ERNEST RUTHERFORD IN CONTEXT T he discovery at the turn theory based on his “law of multiple of the 20th century that proportions,” which explained BRANCH the basic constituent of how elements (simple, uncombined Physics matter—the atom—could be substances) always combine in broken into smaller fragments simple, whole-number ratios. Dalton BEFORE was a watershed moment for saw that this meant that a chemical c.400 BCE Greek philosopher physics. This astonishing reaction between two substances Democritus envisages atoms breakthrough revolutionized ideas is no more than the fusing of as solid, indestructible about how matter is constructed individual small components, building blocks of matter. and the forces that hold it and repeated countless times. This the universe together. It revealed was the first modern atomic theory. 1805 John Dalton’s atomic an entirely new world at the theory of matter marries subatomic level—one that required A stable science chemical processes to physical a new physics to describe its A self-congratulatory mood was reality and allows him to interactions—and a slew of detectable in physics at the end of calculate atomic weights. tiny particles that filled this the 19th century. Certain eminent infinitesimally small domain. physicists made grandstanding, 1896 Nuclear radiation is declarations to the effect that the discovered by Henri Becquerel, Atomic theories have a long subject was all but finished—that and is used to reveal the history. The Greek philosopher the principal discoveries had all internal structure of the atom. Democritus developed the ideas of been made and the program going earlier thinkers that everything is forward was one of improving the AFTER composed of atoms. The Greek accuracy of known quantities “to 1938 Otto Hahn, Fritz word átomos, which is credited to the sixth decimal place.” However, Strassman, and Lise Meitner Democritus, means indivisible and many research physicists of the split the atomic nucleus. referred to the basic units of matter. time knew better. It was already Democritus thought that the clear that they were facing an 2014 Firing increasingly materials must reflect the atoms entirely new and strange set of energetic particles at the they are made of—so atoms of iron phenomena that defied explanation. nucleus continues to reveal are solid and strong, while those of a slew of new subatomic water are smooth and slippery. In 1896, Henri Becquerel, particles and antiparticles. following a lead from Wilhelm At the turn of the 19th century, Röntgen’s discovery of mysterious English natural philosopher John “X-rays” the previous year, had Dalton proposed a new atomic Alpha particles fired This means that Electrons are into atoms sometimes travel an atom must have found to have specific straight through, sometimes are a small, dense orbits around deflected, and sometimes central nucleus. the nucleus. bounce back. Now I know So, the atom is made of what the atom a small, massive nucleus looks like. with electrons orbiting it in shells.

A PARADIGM SHIFT 209 See also: John Dalton 112–13 ■ August Kekulé 160–65 ■ Wilhelm Röntgen 186–87 ■ Marie Curie 190–95 ■ Max Planck 202–05 ■ Albert Einstein 214–21 ■ Linus Pauling 254–59 ■ Murray Gell-Mann 302–07 found an unexplained radiation. he demonstrated that he could J. J. Thomson is pictured here at What were these new radiations break lumps out of atoms. While work in his Cambridge laboratory. and where were they coming from? investigating the “rays” emanating Thomson’s “plum pudding” model Becquerel correctly surmised from high-voltage cathodes of the atom was the first to include that this radiation was emanating (negatively charged electrodes), he the newly discovered electron. from within uranium salts. found that this particular kind of When Pierre and Marie Curie radiation was made of individual theoretically. If an atom contains studied the decay of radium, “corpuscles,” since it created charged particles, why shouldn’t they discovered a constant and momentary, pointlike sparkles of the opposing particles have equal seemingly inexhaustible source of light on hitting a phosphorescent mass? Previous atomic theories energy inside radioactive elements. screen; it was negatively charged, held that atoms were solid lumps. If this were the case, it would since a beam could be deflected As befit their status as the most break several fundamental laws of by an electric field; and it was basic constituent of matter, they physics. Whatever these radiations exceedingly light, weighing less were entire, whole, and perfect. were, it was clear that there were than a thousandth of the lightest But when viewed in the light of large gaps in current models. atom, hydrogen. Moreover, the Thomson’s discovery, they clearly weight of the corpuscle was the were divisible. Put together, Discovery of the electron same, no matter which element these new radiations raised The following year, the British was used as a source. Thomson the suspicion that science had physicist Joseph John (J. J.) had discovered the electron. These failed to understand the vital Thomson caused a sensation when results were totally unanticipated components of matter and energy. ❯❯

210 ERNEST RUTHERFORD The plum-pudding model All science is either physics Rutherford had demonstrated that Thomson’s discovery of the or stamp collecting. radioactivity involved one element electron earned him the Nobel Ernest Rutherford spontaneously changing into Prize for Physics in 1906. He was another. Their work was to suggest enough of a theoretician, however, very different visualization of the new ways to probe the inside of to see that a radical new model internal structure of the basic unit the atom and see what was there. of the atom was needed to of all elements. adequately incorporate his Radioactivity findings. His answer, produced At the Physical Laboratories Although radioactivity was first in 1904, was the “plum-pudding” at the University of Manchester, encountered by Becquerel and model. Atoms have no overall Ernest Rutherford devised and the Curies, it was Rutherford who electric charge and, since the directed an experiment to test identified and named the three mass of this new electron was Thomson’s plum-pudding model. different types of what we would small, Thomson postulated that a This charismatic New Zealander now call nuclear radiation. These larger positively charged sphere was a gifted experimentalist with are slow-moving, heavy, positively contained most of the atom’s mass, a keen sense of which details to charged “alpha” particles; fast- and the electrons were embedded pursue. Rutherford had received the moving, negatively charged “beta” in it like plums in the dough of 1908 Nobel Prize in Physics for his particles; and highly energetic but a Christmas pudding. With no “Theory of Atomic Disintegration.” uncharged “gamma” radiation evidence to suggest otherwise, (p.194). Rutherford classified these it was sensible to assume that The theory proposed that different forms of radiation by the point charges, like the plums the radiations emanating from their penetrating power, from the in a pudding, were arbitrarily radioactive elements were the least-penetrating alpha particles, distributed across the atom. result of their atoms breaking apart. which are blocked by thin paper, With the chemist Frederick Soddy, to gamma rays that require a Rutherford revolution thickness of lead to be stopped. However, the positively charged He was the first to use alpha parts of the atom steadfastly particles to explore the atomic refused to reveal themselves, and realm. He was also the first to the hunt was on to locate the outline the notion of radioactive missing member of the atomic half-life and discover that “alpha pair. The quest resulted in a particles” were helium nuclei— discovery that would produce a atoms stripped of their electrons. Ernest Rutherford Brought up in rural New Zealand, Rutherford was an accomplished Ernest Rutherford was working in administrator, too, and during the fields when the letter from his lifetime he headed up the J. J. Thomson arrived informing three top physics research him of a scholarship to Cambridge laboratories. In 1907, he took University. In 1895, he was made a the chair in physics at the research fellow at the Cavendish University of Manchester Laboratories, where he conducted where he discovered the atomic experiments alongside Thomson nucleus. In 1919, he returned to that led to the discovery of the the Cavendish as director. electron. In 1898, at 27 years old, Rutherford took up a professorial Key works post at McGill University in Montreal, Canada. It was there 1902 The Cause and Nature that he carried out the work on of Radioactivity, I & II radioactivity that won him the 1909 The Nature of the α Particle 1908 Nobel Prize in Physics. from Radioactive Substances

A PARADIGM SHIFT 211 Scattered Thin gold microscopes and counting the tiny particles foil flashes of light on the scintillation screens. Then, acting on a hunch, Beam of Rutherford instructed them to particles position screens that would catch any high-angle deflections as Source of Circular scintillation well as at the expected low-angle particles screen scintillations. With these new screens in place, they discovered Geiger and Marsden aimed alpha particles from that some of the alpha particles a radioactive source at an incredibly thin gold leaf. were being deflected by more than The scintillation screen could be spun around to 90º, and others were rebounding off detect particles rebounding at any given angle. the foil back the way they came. Rutherford described the result as The gold foil experiment through the foil. Most of the like firing a 15-inch shell at tissue In 1909, Rutherford set out to probe particles would be deflected only paper and having it bounce back. the structure of matter using alpha slightly by interaction with the particles. The previous year, along gold atoms and would be scattered The nuclear atom with the German Hans Geiger, across shallow angles. Halting heavy alpha particles in he had developed zinc sulphide their tracks or deflecting them by “scintillation screens,” which Geiger and Marsden spent high angles was possible only if enabled individual collisions of long hours sat in the darkened the positive charge and mass of alpha particles to be counted as laboratory, peering down an atom were concentrated in brief bright flashes, or scintillations. small volume. In light of these With the help of undergraduate It was quite the most results, in 1911, Rutherford student Ernest Marsden, Geiger incredible event that has ever published his conception of would use these screens to happened to me in my life. It the structure of the atom. The determine whether matter was was almost as incredible as if “Rutherford Model” is a solar infinitely divisible or whether you fired a 15-inch shell at a system in miniature, with electrons atoms contained fundamental orbiting a small, dense, positively building blocks. piece of tissue paper and it charged nucleus. The model’s came back and hit you. major innovation was the They fired a beam of alpha Ernest Rutherford infinitesimally small nucleus, particles from a radium source at which forced the uncomfortable an extremely thin strip of gold leaf, conclusion that the atom is not at just a thousand or so atoms thick. all solid. Matter at an atomic scale If, as the plum-pudding model held, is mostly space, governed by gold atoms consisted of a diffuse energy and force. This was a cloud of positive charge with definitive break from the atomic pointlike negative charges, then the theories of the previous century. massive, positively charged alpha particles would plough straight While Thomson’s “plum- pudding” atom had been an instant hit, Rutherford’s model was largely ignored by the scientific community. Its failings were all too plain to see. It was well established that accelerating electric charges emit energy as electromagnetic radiation. Thus, as electrons swoop around the nucleus—experiencing circular acceleration that keeps ❯❯

212 ERNEST RUTHERFORD them in their orbits—they now call photons. Bohr sought to If your experiment needs ought to be continually emitting explain the precise pattern of statistics, you ought to have electromagnetic radiation. Steadily absorption and emission of light done a better experiment. losing energy as they orbited, the from atoms. He suggested that electrons would spiral inexorably each electron is confined to fixed Ernest Rutherford into the nucleus. According to orbits within atomic “shells,” and Rutherford’s model, atoms ought to that the energy levels of the orbits falling out of orbit into the nucleus be unstable, but clearly they are not. are “quantized”—that is, they can was, for electrons, impossible. only take certain specific values. Bohr’s was a purely theoretical A quantum atom model of the atom. However, it Danish physicist Niels Bohr saved In this orbital model, the agreed with experiment and solved the Rutherford model of the atom energy of any individual electron many associated problems in an from languishing in obscurity is closely related to its proximity to elegant stroke. The way in which by applying new ideas about the atom’s nucleus. The closer an electrons would have to fill up quantization to matter. The electron is to the nucleus, the less empty shells in a strict order, quantum revolution had begun energy it has, but it can be excited getting progressively farther from in 1900 when Max Planck had into higher energy levels by the nucleus, matched the march proposed the quantization of absorbing electromagnetic of the properties of the elements radiation, but the field was still radiation of a certain wavelength. seen across the periodic table as in its infancy in 1913—it would Upon absorbing light, an electron atomic number increases. Even have to wait until the 1920s for a leaps to a “higher,” or outer, orbit. formalized mathematical framework Upon attaining this higher state, of quantum mechanics. At the the electron will promptly drop time Bohr was working on back into the lower-energy orbit, this problem, quantum theory releasing a quantum of energy that essentially consisted of no more precisely matches the energy gap than Einstein’s notion that light between the two orbitals. comes in tiny “quanta” (discrete packets of energy) that we Bohr offered no explanation for what this meant or what it might look like—he simply stated that The plum-pudding model of the atom Electron Proton Neutron with the electrons spread across a diffuse nucleus was replaced by Rutherford’s model with electrons in orbit around a small, dense nucleus. Bohr refined Rutherford’s model by adding quantized orbits for the electrons. Here, a carbon atom is illustrated. 6 protons + 6 neutrons Plum-pudding model Rutherford model Bohr model

A PARADIGM SHIFT 213 more convincing was the way in James Chadwick discovered the neutron by bombarding which the theoretical energy levels beryllium with alpha particles from radioactive polonium. of the shells neatly fit actual The alpha particles knocked neutrons out of the beryllium. “spectral series”—the frequencies Then the neutrons dislodged protons from a layer of paraffin, of light absorbed and emitted by and these protons were detected by an ionization chamber. different atoms. A long sought after way to marry electromagnetism Alpha Neutrons and matter had been realized. particles Going inside the nucleus Protons Once this picture of the nuclear atom had been accepted, the next Polonium Beryllium Paraffin Ionization stage was to ask what lay inside chamber the nucleus. In experiments reported in 1919, Rutherford found positive charges crammed into a it feels no repulsion as it passes that his beams of alpha particles tiny nucleus. Like charges repel through matter. However, with could generate hydrogen nuclei each other, so he theorized that mass slightly greater than a proton, from many different elements. there must be another particle it can easily knock protons out Hydrogen had long been that somehow dissipates the of the nucleus, something that recognized as the simplest of all charge or binds the jostling protons otherwise only extremely energetic the elements and thought of as tightly together. There was also electromagnetic radiation can do. a building block for all other extra mass in elements heavier elements, so Rutherford proposed than hydrogen, which could be Electron clouds that the hydrogen nucleus was in accounted for by a third, neutral but The discovery of the neutron fact its own fundamental particle, equally massive subatomic particle. completed the picture of the the proton. atom as a massive nucleus However, the neutron proved with electrons in orbit around The next development in atomic difficult to detect and it took nearly it. New discoveries in quantum structure was James Chadwick’s a decade of searching to find it. physics would refine our view of 1932 discovery of the neutron, in Chadwick was working at the electrons in orbit around a nucleus. which Rutherford once again had Cavendish Laboratory under the Modern models of the atom feature a hand. Rutherford had postulated supervision of Rutherford. Guided “clouds” of electrons, which the existence of the neutron in 1920 by his mentor, he was studying a represent only those areas in as a way to compensate for the new kind of radiation that had been which we are most likely to find repulsive effect of many point-sized found by the German physicists an electron, according to its Walther Bothe and Herbert Becker quantum wavefunction (p.256). The difficulties disappear when they bombarded beryllium if it be assumed that the with alpha particles. The picture has been further radiation consists of particles complicated by the discovery of mass 1 and charge 0, Chadwick duplicated the that neutrons and protons are Germans’ results and realized not fundamental particles, but or neutrons. that this penetrating radiation was are made of arrangements of James Chadwick the neutron Rutherford had been smaller particles called quarks. looking for. A neutral particle, Questions about the true structure such as the neutron, is much of the atom are still actively more penetrating than a charged being researched. ■ particle, such as a proton, because

GRAVITY IS A DISTORTION IN THE SPACE-TIME CONTINUUM ALBERT EINSTEIN (1879–1955)



216 ALBERT EINSTEIN If the speed of light through a vacuum is unchanging… IN CONTEXT And the laws of physics appear the same BRANCH to all observers… Physics Then there can be no absolute BEFORE time or space. 17th century Newtonian physics provides a description Observers in relative motion to each other of gravity and motion, which experience space and time differently. is still adequate for most everyday calculations. Special relativity shows that there is no absolute simultaneity. 1900 Max Planck first argues that light can be considered to consist of individual packets, or “quanta,” of energy. AFTER 1917 Einstein uses general relativity to produce a model of the universe. Assuming that the universe is static, he introduces a factor called the cosmological constant to prevent its theoretical collapse. 1971 Time dilation due to general relativity is demonstrated by flying atomic clocks around the world in jet aircraft. I n the year 1905, the German of the nature of light and energy. of general relativity that presented scientific journal Annalen der A second was an elegant proof a new and deeper understanding Physik published four papers that a long-observed physical of gravity, space, and time. by a single author—a little-known effect called Brownian motion 26-year-old physicist named Albert could demonstrate the existence Quantizing light Einstein, then working at the of atoms. A third showed the The first of Einstein’s 1905 papers Swiss patent office. Together, these presence of an ultimate speed limit addressed a long-standing problem papers would lay the foundations to the universe, and considered the with the photoelectric effect. This for much of modern physics. strange effects thereof, known as phenomenon had been discovered special relativity, while the fourth by German physicist Heinrich Einstein resolved some forever changed our understanding Hertz in 1887. It involves metal fundamental problems that of the nature of matter, showing electrodes producing a flow had appeared in the scientific that it was interchangeable with of electricity (that is, emitting understanding of the physical energy. A decade later, Einstein electrons) when illuminated by world toward the end of the followed up the implications of certain wavelengths of radiation— 19th century. One of the papers of these latter papers with a theory typically ultraviolet light. The 1905 transformed understanding

A PARADIGM SHIFT 217 See also: Christiaan Huygens 50–51 ■ Isaac Newton 62–69 ■ James Clerk Maxwell 180–85 ■ Max Planck 202–05 ■ Erwin Schrödinger 226–33 ■ Edwin Hubble 236–41 ■ Georges Lemaître 242–45 principle behind the emission Light photons Electrons ejected is fairly easily described in from the surface modern terms (energy supplied by Electrons are the radiation is absorbed by the ejected from the Sodium outermost electrons in the metal’s surface of sodium surface atoms, allowing them to only by certain that visible light was just one break free). The puzzle was that the wavelengths of manifestation of a wider spectrum same materials stubbornly refused light. Einstein of electromagnetic waves, all of to emit electrons when illuminated showed that this which must move through the by longer wavelengths, no matter phenomenon can universe at a single speed. how intense the light source. be explained if light travels as individual Since light was understood This was a problem for the quanta, or photons. to be a transverse wave, it was classical understanding of light, No matter how many assumed that it propagated which assumed that intensity, of them there are, if through a medium, just as water above all, governed the amount of the photons are of the waves travel on the surface of energy being delivered by a light wrong wavelength, a pond. The properties of this beam. Einstein’s paper, however, they will not eject hypothetical substance, known as seized on the idea of “quantized electrons. the “luminiferous ether,” would light” recently developed by Max give rise to the observed properties Planck. Einstein showed that if the bombard the surface (that is, how of electromagnetic waves, and beam of light is split into individual intense the light source is)—if none since they could not alter from “light quanta” (what we would of them carries sufficient energy, place to place, they would provide today call photons), then the energy the electrons will not break free. an absolute standard of rest. carried by each quantum depends only on its wavelength—the shorter Einstein’s idea was rejected by One expected consequence of the wavelength, the higher the leading figures of the day, including the fixed ether was that the speed energy. If the photoelectric effect Planck, but his theory was shown of light from distant objects should relies on interaction between an to be correct by experiments vary depending on the relative electron and a single photon, then it conducted by American physicist motion of source and observer. does not matter how many photons Robert Millikan in 1919. For example, the speed of light from a distant star should vary ❯❯ The grand aim of all science Special relativity is to cover the greatest Einstein’s greatest legacy was born in the third and fourth 1905 papers, number of empirical facts by which also involved an important logical deduction from the reconceptualization of the true smallest number of nature of light. Since the late hypotheses or axioms. 19th century, physicists had Albert Einstein faced a crisis in their attempts to understand the speed of light. Its approximate value had been known and calculated with increasing accuracy since the 17th century, while James Clerk Maxwell’s equations had demonstrated

218 ALBERT EINSTEIN significantly depending on whether Mass and energy are both packets of electromagnetic energy, it was observed from one side of but different manifestations able to travel through the vacuum Earth’s orbit, as our planet moved of space with particle-like away from it at 20 miles/s (30 km/s), of the same thing. properties while still maintaining or on the opposite side, when the Albert Einstein their wavelike characteristics. observer was moving toward it at a similar speed. head on. Special relativity, as Accepting these two his theory became known, was postulates, Einstein considered Measuring Earth’s motion developed from an acceptance of the consequences for the rest through the ether became an two simple postulates—that light of physics, and mechanics in obsession for late 19th-century moves through a vacuum with a particular. In order for the laws of physicists. Such a measurement fixed speed that is independent physics to behave in the same way was the only way of confirming of the motion of the source, and in all inertial reference frames, they the existence of this mysterious that the laws of physics should would necessarily appear to be substance, and yet the proof appear the same to observers in different when looking from one remained elusive. However precise all “inertial” frames of reference— frame to another. Only relative the measuring equipment, light that is, those not subject to external motion mattered, and when the always seemed to move at the forces such as acceleration. relative motion between two same speed. In 1887, US physicists Einstein was undoubtedly helped separate frames of reference Albert Michelson and Edward in accepting the first bold postulate approached the speed of light Morley devised an ingenious by his previous acceptance of (“relativistic” speeds) strange experiment to measure the the quantum nature of light— things began to happen. so-called ether wind with high conceptually, light quanta are often precision, but once again found imagined as tiny self-contained The Lorentz factor no evidence for its existence. The Although Einstein’s paper made negative result of the Michelson- no formal references to other Morley experiment shook the belief scientific publications, it did in the ether’s existence, and similar mention the work of a handful results from attempts to repeat it of other contemporary scientists, over the following decades only for Einstein was certainly not the intensified the sense of crisis. only person working toward an unorthodox solution to the ether Einstein’s third 1905 paper, On crisis. Perhaps the most significant the Electrodynamics of Moving of these was Dutch physicist Bodies, confronted the problem Albert Einstein Born in the southern German city He continued to explore the of Ulm in 1879, Einstein had a implications of his earlier work, somewhat bumpy secondary contributing to innovations in education, eventually training at quantum theory. In 1933, fearing Zurich Polytechnic to become a the rise of the Nazi party, mathematics teacher. After failing Einstein elected not to return to find teaching work, he took a to Germany from a foreign tour, job at the Swiss Patent Office in settling eventually at Princeton Bern, where he had plenty of University in the United States. spare time to develop the papers published in 1905. He attributed Key works his success in this work to the fact that he had never lost his childlike 1905 On a Heuristic Viewpoint sense of wonder. Concerning the Production and Transformation of Light Following the demonstration 1915 The Field Equations of general relativity, Einstein of Gravitation was propelled to global stardom.

A PARADIGM SHIFT 219 In Einstein’s thought experiment, B appears to run more slowly as for a stationary observer at point M, measured from the observer’s two lightning flashes at A and B occur reference frame. simultaneously. However, to an observer at point M1 on a train moving at high Illustrating relativity speed away from A and toward B, the Einstein illustrated special flash at B occurs before the flash at A. relativity by asking us to consider two frames of reference in motion Near the speed of light relative to each other: a moving train and the embankment next M1 to it. Two flashes of lightning, at points A and B, appear to occur A simultaneously to an observer standing on the embankment at M a midpoint between them, M. An observer on the train is at a Hendrik Lorentz, whose “Lorentz Lorentz’s work had been coolly position M1 in a separate frame factor” lay at the heart of Einstein’s received, largely because it of reference. When the flashes description of physics close to the could not be incorporated into occur, M1 may be passing right by speed of light. It is defined standard ether theories. Einstein M. However, by the time the light approached the problem from the has reached the observer on the mathematically as: 1 other direction, showing that train, the train has moved toward the Lorentz factor arose as an point B and away from point A. √ 1 –– v2 / c2 inevitable consequence of the As Einstein puts it, the observer is principle of special relativity and “riding ahead of the beam of light Lorentz developed this equation to reexamining the true meaning coming from A.” The observer on describe the changes in time and of measured time and distance the train concludes that lightning length measurements required in intervals. An important result of strike B occurred before strike A. order to reconcile the Maxwell this was the realization that events Einstein now insists that: “Unless equations of electromagnetism that appeared simultaneous for an we are told the reference-body to with the principle of relativity. observer in one reference frame which the statement of time refers, It was crucial to Einstein since it were not necessarily so for there is no meaning in a statement provided a term for transforming someone in a different reference of the time of an event.” Both time results as seen by one observer to frame (a phenomenon known as and position are relative concepts. show what they look like to another the relativity of simultaneity). observer who is in motion relative Einstein also showed how from Mass-energy equivalence to the first observer. In the term the point of view of a distant The last of Einstein’s 1905 papers quoted above, v is the speed of one observer, the length of moving was called Does the Inertia of a observer compared to the other, objects in their direction of travel Body Depend on its Energy and c is the speed of light. In most became compressed as they Content? Its three brief pages situations, v will be very small approached the speed of light, in expanded on an idea touched on in compared to c, so v2/c2 will be close accordance with a simple equation the previous paper—that the mass to zero, and the Lorentz factor governed by the Lorentz factor. of a body is a measure of its energy. close to 1, meaning that it makes Even more strangely, time itself Here, Einstein demonstrated that almost no difference to calculations. if a body radiates away a certain amount of energy (E) in the form of electromagnetic radiation, its mass will diminish by an amount equivalent to E/c2. This equation is easily rewritten to show that the energy of a stationary particle ❯❯

220 ALBERT EINSTEIN within a particular reference frame Over the next few years, many a person standing in a sealed is given by the equation E = mc2. scientists reached the conclusion elevator in empty space. The This principle of “mass-energy that special relativity offered a elevator is being accelerated in one equivalence” was to become a better description of the universe direction by a rocket. The person keystone of 20th-century science, than the discredited ether theory, feels a force pushing up from the with applications that range from and devised experiments that floor, and pushes back against the cosmology to nuclear physics. demonstrated relativistic effects floor with equal and opposite force in action. Meanwhile, Einstein following Newton’s Third Law. Gravitation fields was already moving on to a new Einstein realized that the person in Although Einstein’s papers in challenge, extending the principles the elevator would feel exactly as that annus mirabilis seemed too that he had now established in they would if they were standing obscure at first to make much order to consider “noninertial” still in a gravitational field. impression beyond the rarefied situations—those involving world of physics, it propelled him acceleration and deceleration. In an elevator undergoing to fame within that community. constant acceleration, a beam of As early as 1907, Einstein had light fired at an angle perpendicular Our experience of hit upon the idea that a situation to the acceleration would be gravity is equivalent to of “free fall” under the influence deflected onto a curved path, and of gravity is equal to an inertial Einstein reasoned that the same that of being inside a situation—the equivalence would occur in a gravitational field. constantly accelerating principle. In 1911, he realized that It was this effect of gravity on a stationary frame of reference light—known as gravitational frame of reference. influenced by a gravitational field lensing—that would first is equivalent to one undergoing demonstrate general relativity. constant acceleration. Einstein illustrated this idea by imagining Einstein considered what this said about the nature of gravity. The acceleration can Real light Real location Apparent be explained by a trajectory of the star location of distortion in the the star space-time manifold. If objects with mass Apparent light distort space-time, trajectory this explains their Sun gravitational attraction. General relativity According to general relativity, Observer explains gravity as mass creates a “gravitational well” in a distortion in the space-time. The idea can be visualized space-time manifold. by representing three-dimensional space as a two-dimensional plane. The gravitational well of a massive object such as the Sun causes light to be deflected onto a curved path, altering the apparent position of distant stars to an observer—an effect called gravitational lensing.

A PARADIGM SHIFT 221 Arthur Eddington’s photographs of a solar eclipse in 1919 provided the first evidence for general relativity. Stars around the Sun appeared out of place, just as Einstein had predicted. In particular, he predicted that relativistic effects such as time dilation should occur in strong gravitational fields. The closer a clock is to a source of gravitation, the more slowly it will tick. This effect remained purely theoretical for many years, but has now been confirmed using atomic clocks. Space-time manifold to solve a long-standing mystery— a few months after the end of the Meanwhile, also in 1907, Einstein’s the way in which Mercury’s closest war, Eddington led an expedition former tutor Hermann Minkowski approach to the Sun (aphelion) to the island of Príncipe, off the had hit upon another important precesses, or rotates, around the west coast of Africa, in order to part of the puzzle. Considering the Sun much more quickly than test the theory of general relativity effective trade-offs between predicted by Newtonian physics. and its prediction of gravitational the dimensions of space and time General relativity solved the puzzle. lensing in the most spectacular involved in special relativity, he circumstances. Einstein had developed the idea of combining Gravitational lensing predicted as early as 1911 that a the three dimensions of space Einstein published at a time when total solar eclipse would allow the with one of time in a space-time much of the world was swept up in effects of gravitational lensing to be manifold. In Minkowski’s World War I, and English-speaking seen, in the form of apparently out- interpretation, relativistic effects scientists had other things on their of-place stars around the eclipsed could be described in geometrical minds. General relativity was a disk (a result of their light being terms by considering distortions in complex theory and might have deflected as it passed through the the way that observers in relative languished in obscurity for many warped space-time around the motion observe the manifold in a years had it not been for the Sun). Eddington’s expedition different frame of reference. interest of Arthur Eddington, a delivered both impressive images conscientious objector to the war, of the solar eclipse and convincing In 1915, Einstein published his and, as it happened, Secretary of proof of Einstein’s theory. When complete theory of general relativity. the Royal Astronomical Society. published the following year, they In its finished form, it was nothing proved to be a worldwide sensation, less than a new description of the Eddington became aware of propelling Einstein to global fame nature of space, time, matter, and Einstein’s work thanks to letters and ensuring that our ideas about gravity. Adopting Minkowski’s from Dutch physicist Willem the nature of the universe would ideas, Einstein saw the “stuff of the de Sitter, and soon became its never be the same again. ■ universe” as a space-time manifold chief advocate in Britain. In 1919, that could be distorted thanks to relativistic motion, but could also be warped by the presence of large masses such as stars and planets in a way that was experienced as gravity. The equations that described the link between mass, distortion, and gravity were fiendishly complex, but Einstein used an approximation

222 EARTH’S DRIFTING CONTINENTS ARE GIANT PIECES IN AN EVER-CHANGING JIGSAW ALFRED WEGENER (1880–1930) IN CONTEXT I n 1912, German meteorologist eastern coasts of the Americas are Alfred Wegener combined roughly parallel with the western BRANCH several strands of evidence to coasts of Europe and Africa. This Earth science put forward a theory of continental led scientists to speculate that these drift, which suggested that Earth’s landmasses were once connected, BEFORE continents were once connected but challenging conventional notions of 1858 Antonio Snider-Pellegrini moved apart over millions of years. a solid, unchanging planet. makes a map of the Americas Scientists only accepted his theory connected to Europe and once they had figured out what In 1858, Paris-based geographer Africa, to account for identical made such vast landmasses move. Antonio Snider-Pellegrini showed fossils found on opposite sides that similar plant fossils had been of the Atlantic Ocean. Looking at the first maps of the found on either side of the Atlantic, New World and Africa, Francis dating back to the Carboniferous 1872 French geographer Bacon had noted, in 1620, that the period, 359–299 million years ago. Élisée Reclus proposes that motion of the continents South America’s Similar plant and Matching rock caused the formation of the east coast fits animal fossils formations oceans and mountain ranges. are found in Africa’s west coast are found in South 1885 Eduard Suess suggests like two giant America and South America the southern continents were jigsaw pieces. in Africa. and in Africa. once linked by land bridges. The continents must once have formed a single landmass. AFTER 1944 British geographer Earth’s drifting continents are giant pieces Arthur Holmes proposes in an ever-changing jigsaw. convection currents in Earth’s mantle as the mechanism that moves the crust at the surface. 1960 American geologist Harry Hess proposes that seafloor spreading pushes the continents apart.

A PARADIGM SHIFT 223 See also: Francis Bacon 45 ■ Nicholas Steno 55 ■ James Hutton 96–101 ■ Louis Agassiz 128–29 ■ Charles Darwin 142–49 He made maps showing how the ago, and pointed to Africa’s Great destruction of ocean crust that American and African continents Rift Valley as evidence of ongoing leads to the displacement of may once have fit together, and continental breakup. continents. This theory not only attributed their separation to the vindicated Wegener but is now biblical Flood. When fossils of Search for a mechanism the bedrock of modern geology. ■ Glossopteris ferns were found in Wegener’s theory was criticized by South America, India, and Africa, geophysicists for not explaining Pangaea, 200 million years ago Austrian geologist Eduard Suess how continents move. In the argued that they must have evolved 1950s, however, new geophysical 75 million years ago on a single landmass. He suggested techniques revealed a wealth of that the southern continents were new data. Studies of Earth’s past Present day once linked by land bridges across magnetic field indicated that the Wegener’s supercontinent is just the sea, forming a supercontinent ancient continents lay in a different one in a long series. Geologists think that he called Gondwanaland. position relative to the poles. Sonar the continents may be converging mapping of the seabed revealed again, to form another supercontinent Wegener found more examples signs of more recent ocean-floor 250 million years from now. of similar organisms separated by formation. This was found to occur oceans, but also similar mountain at mid-ocean ridges, as molten rock ranges and glacial deposits. Instead erupts through cracks in Earth’s of earlier ideas that portions of a crust and spreads away from the supercontinent had sunk beneath ridges as new rock erupts. the waves, he thought perhaps it had split apart. Between 1912 and In 1960, Harry Hess realized 1929, he expanded on this theory. that seafloor spreading provided His supercontinent—Pangaea— the mechanism for continental drift, connected Suess’s Gondwanaland and presented his theory of plate to the northern continents of North tectonics. Earth’s crust is made up America and Eurasia. Wegener of giant plates that continually shift dated the fragmentation of this as convection currents in the single landmass to the end of the mantle below bring new rock to the Mesozoic era, 150 million years surface, and it is the formation and Alfred Wegener Born in Berlin, Alfred Lothar produced revised and expanded Wegener obtained a doctorate in editions in 1920, 1922, and 1929, astronomy from the University of but was frustrated by the lack of Berlin in 1904, but soon became recognition for his work. more interested in earth science. Between 1906 and 1930, he made In 1930, Wegener led a fourth four trips to Greenland as part of expedition to Greenland, hoping his pioneering meteorological to collect evidence in support of studies of Arctic air masses. He the drift theory. On November 1, used weather balloons to track air his 50th birthday, he set out circulation and took samples from across the ice to get badly deep within the ice for evidence of needed supplies, but he died past climates. before reaching the main camp. In between these expeditions, Key work Wegener developed his theory of continental drift in 1912, and 1915 The Origin of Continents published it in a book in 1915. He and Oceans

224 CHROMOSOMES PLAY A ROLE IN HEREDITY THOMAS HUNT MORGAN (1866–1945) IN CONTEXT When cells divide, their chromosomes split and replicate in ways that parallel the emergence BRANCH Biology of inherited characteristics. BEFORE This suggests that genes controlling these 1866 Gregor Mendel describes characteristics occur on the chromosomes. laws of inheritance, concluding that inherited characteristics Some characteristics depend Chromosomes are controlled by discrete on the sex of the organism, play a role particles, later called genes. so must be controlled by sex- in heredity. 1900 Dutch botanist Hugo de determining chromosomes. Vries reaffirms Mendel’s laws. D uring the 19th century, roles of genes and chromosomes 1902 Theodor Boveri and biologists observing cells in inheritance, explaining evolution Walter Sutton independently divide under a microscope at a molecular level. conclude that chromosomes noticed the appearance of pairs of are involved in inheritance. tiny threads in every cell’s nucleus. Particles of inheritance These threads could be stained by By the early 20th century, scientists AFTER dyes for observation, and came to had traced the chromosome’s 1913 Morgan’s student Alfred be called chromosomes, meaning precise movements at cell division, Sturtevant constructs the first “colored bodies.” The biologists and noticed that the number of genetic “map,” of the fruit fly. soon began to wonder whether chromosomes varied between chromosomes had something to species, but that the number in the 1930 Barbara McClintock do with heredity. body cells of the same species were discovers that genes can shift usually the same. In 1902, German positions on chromosomes. In 1910, experiments conducted biologist Theodor Boveri, having by American geneticist Thomas studied the fertilization of a sea 1953 James Watson and Hunt Morgan would confirm the Francis Crick’s double-helix model of DNA explains how genetic information is passed on during reproduction.

A PARADIGM SHIFT 225 See also: Gregor Mendel 166–71 ■ Barbara McClintock 271 ■ James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19 urchin, concluded that an organism’s white-eyed, and always male. The Thomas Hunt Morgan chromosomes had to be present in “white gene” must be linked to sex. a full set for an embryo to develop When other traits linked to sex Born in Kentucky, US, Thomas properly. Later that same year, an appeared, Morgan concluded that Hunt Morgan trained as a American student named Walter all these traits must be inherited zoologist before going Sutton concluded from his work on jointly and the genes responsible on to study the development grasshoppers that chromosomes for them must all be carried on of embryos. After moving to might even mirror the theoretical the chromosome that determines Columbia University in New “particles of inheritance” proposed sex. The females had a pair of X York in 1904, he began to in 1866 by Gregor Mendel. chromosomes, while males had focus on the mechanism of an X and a Y. During reproduction, inheritance. Initially sceptical Mendel had done exhaustive the offspring inherits an X from the of Mendel’s conclusions, and experiments in the breeding of pea mother, and an X or a Y from the even of Darwin’s, he focused plants and, in 1866, suggested that father. The “white gene” is carried his efforts on the breeding of their inherited characteristics were by the X. The Y chromosome has fruit flies to test his ideas determined by discrete particles. no corresponding gene. about genetics. His success Four decades later, to test the with fruit flies would lead link between chromosomes and Further work led Morgan to many researchers to use them Mendel’s theory, Morgan embarked the notion that specific genes in genetics experiments. on research that would combine were not only located on specific breeding experiments with modern chromosomes, but occupied Morgan’s observation of microscopy, in what came to be particular positions on them. This stable, inherited mutations in known as the “Fly Room” at opened up the idea that scientists fruit flies eventually led him to Columbia University, New York. could “map” an organism’s genes. ■ realize that Darwin was right, and in 1915, he published a From peas to fruit flies First Generation (F1) work explaining how heredity Fruit flies (Drosophila) are gnat- functioned according to sized insects that can be bred in Male Female Mendel’s laws. Morgan small glass bottles and can produce continued his research at the next generation—with a great Second Generation (F2) the California Institute of many offspring—in just 10 days. Technology (Caltech) and, This made the fruit fly ideal for Crossbreeding fruit flies over two in 1933, he was awarded the studying inheritance. Morgan’s generations shows how the white-eyed Nobel Prize in Genetics. team isolated and crossbred flies trait is passed only to some males, with particular characteristics, and through the sex chromosomes. Key works then analyzed the proportions of variations in the offspring—just as 1910 Sex-limited Inheritance Mendel had done with his peas. in Drosophila 1915 The Mechanism of Morgan finally corroborated Mendelian Heredity Mendel’s results after he spotted 1926 The Theory of the Gene a male with eyes that were white rather than the normal red. Mating a white-eyed male with a red-eyed female produced only red-eyed offspring, which suggested that red was a dominant trait and white was recessive. When those offspring were crossbred, one in four of the next generation was

PARTICLES HAVE WAVELIKE PROPERTIES ERWIN SCHRÖDINGER (1887–1961)



228 ERWIN SCHRÖDINGER IN CONTEXT E rwin Schrödinger was a key wavelengths of light emitted when figure in the advancement certain elements were heated. By BRANCH of quantum physics—the modeling the structure of the atom Physics science that explains the tiniest with electrons orbiting in discrete levels of subatomic matter. His star “shells” whose distance from the BEFORE contribution was a famous equation nucleus determined their energy, 1900 A crisis in the that showed how particles moved Bohr could explain the emission understanding of light in waves. It formed the basis of spectra (distribution of light inspires Max Planck to find today’s quantum mechanics and wavelengths) of atoms in terms a theoretical solution that revolutionized the way we perceive of photons of energy given off as involves treating light as the world. But this revolution did electrons jumped between orbits. quantized packets of energy. not happen suddenly. The process However, Bohr’s model lacked a of discovery was a long one, with theoretical explanation, and could 1905 Albert Einstein many pioneers along the way. only predict the emissions from demonstrates the reality hydrogen, the simplest atom. of Planck’s quantized light Quantum theory was originally through his explanation of limited to the understanding of Wavelike atoms? the photoelectric effect. light. In 1900, as part of an attempt Einstein’s idea had breathed new to solve a troubling problem in life into the old theory of light as 1913 Niels Bohr’s model of theoretical physics known as streams of particles, even though the atom uses the idea that the “ultraviolet catastrophe,” the light had also been proved, through electrons shifting between German physicist Max Planck Thomas Young’s double-slit energy levels within an atom proposed treating light as though experiment, to behave as a wave. emit or absorb individual it came in discrete packets, or The puzzle of how light could quanta of light (photons). quanta, of energy. Albert Einstein possibly be both particle and wave then took the next step and argued received a new twist in 1924 from AFTER that light quanta were indeed a real 1930s Schrödinger’s work, physical phenomenon. 1927 saw a gathering of greats at the along with that of Paul Dirac Solvay Conference of physics in Brussels. and Werner Heisenberg, forms Danish physicist Niels Bohr Among others are: 1. Schrödinger, the foundation of modern knew that Einstein’s idea was 2. Pauli, 3. Heisenberg, 4. Dirac, particle physics. saying something fundamental 5. de Broglie, 6. Born, 7. Bohr, 8. Planck, about the nature of light and 9. Curie, 10. Lorentz, 11. Einstein. atoms, and in 1913 used it to solve an old problem—the precise 1 4 23 56 8 9 10 11 7

A PARADIGM SHIFT 229 See also: Thomas Young 110–11 ■ Albert Einstein 214–21 ■ Werner Heisenberg 234–35 ■ Paul Dirac 246–47 Richard Feynman 272–73 ■ Hugh Everett III 284–85 If you fire electrons one-by-one This means that they are through two slits onto a screen, they behaving like waves. will build up an interference pattern on the screen. Particles have A wave function provides a way wavelike properties. of calculating the probability of detecting an electron at a particular point in space-time. a French PhD student, Louis de h divided by its wavelength. also relatively easy to test. By Broglie, whose suggestion led the However, since he was dealing 1927, scientists in two separate quantum revolution into a dramatic with particles whose energy and laboratories had conducted new phase. Not only did de Broglie mass might be affected by motion experiments to show that electrons demonstrate with a simple at speeds close to that of light, de diffracted and interfered with each equation how, in the subatomic Broglie incorporated the Lorentz other in exactly the same way as world, particles could equally be factor (p.219) into his equation. photons of light. De Broglie’s waves, he also showed how any This produced a more sophisticated hypothesis was proved. object, of whatever mass, could version that took into account behave as a wave to some extent. the effects of relativity. Growing significance In other words, if light waves had In the meantime, a number of particle-like properties, then particles De Broglie’s idea was radical theoretical physicists were of matter—such as electrons— and daring, but it soon had sufficiently intrigued by de Broglie’s must have wavelike properties. influential supporters, including hypothesis to investigate it further. Einstein. The hypothesis was In particular, they wanted to know Planck had calculated the how the properties of such matter energy of a light photon with the Two seemingly incompatible waves could give rise to the pattern simple equation E = hv, where E conceptions can each represent of specific energy levels among the is the energy of the electromagnetic electron orbitals of the hydrogen quanta, v is the wavelength of an aspect of the truth. atom proposed by Bohr’s model of the radiation involved, and h is Louis de Broglie the atom. De Broglie himself had a constant, today known as suggested that the pattern arose the Planck constant. De Broglie because the circumference of each showed that a light photon also has orbital must accommodate a whole momentum, something normally number of wavelengths of the only associated with particles matter wave. Since the electron’s with mass and given by multiplying energy level depends on its the particle’s mass with its speed. distance from the atom’s positively De Broglie showed that a light charged nucleus, this meant that ❯❯ photon had a momentum of

230 ERWIN SCHRÖDINGER only certain distances, and certain A classic illustration of wave-particle Interference energy levels, would be stable. duality involves firing electrons from a pattern However, de Broglie’s solution relied “gun” through a barrier with two slits in on treating the matter wave as a it. If electrons are allowed to build up over one-dimensional wave trapped in time, an interference pattern forms, just orbit around the nucleus—a full as it would for light waves. description would need to describe the wave in three dimensions. The wave equation Narrow slits into the lowest possible energy In 1925, three German physicists, state, Pauli developed the Werner Heisenberg, Max Born, and Electrons exclusion principle. Reasoning Pascual Jordan, tried to explain the that a particle’s overall quantum quantum jumps that occurred in Gun state could be defined by a certain Bohr’s model of the atom with a number of properties, each with a method called matrix mechanics, to interpret it as the density of fixed number of possible discrete in which the properties of an atom electric charge, but this was not values, his principle stated were treated as a mathematical entirely successful. It was Max that it was impossible for two system that could change over Born who eventually suggested particles within the same system time. However, the method could what it really was—it was a to have the same quantum not explain what was actually probability amplitude. In other state simultaneously. happening inside the atom, and words, it expressed the likelihood its obscure mathematical language of a measurement finding the In order to explain the pattern did not make it very popular. electron in that particular place. of electron shells that was apparent Unlike matrix mechanics, the from the periodic table, Pauli A year later, an Austrian Schrödinger wave equation or calculated that electrons must be physicist working in Zurich, Erwin “wave function” was embraced by described by four distinct quantum Schrödinger, hit upon a better physicists, although it threw open numbers. Three of these—the approach. He took de Broglie’s a whole range of wider questions principal, azimuthal, and magnetic wave-particle duality a step further about its proper interpretation. quantum numbers—define the and began to consider whether electron’s precise place within there was a mathematical equation Pauli’s exclusion principle the available orbital shells and of wave motion that would describe Another important piece of the subshells, with the values of the how a subatomic particle might puzzle fell into place in 1925 move. To formulate his wave courtesy of another Austrian, equation, he began with the laws Wolfgang Pauli. In order to describe governing energy and momentum why the electrons within an atom in ordinary mechanics, then did not all automatically fall directly amended them to include the Planck constant and de Broglie’s law connecting the momentum of a particle to its wavelength. When he applied the resulting equation to the hydrogen atom, it predicted exactly the specific energy levels for the atom that had been observed in experiments. The equation was a success. But one awkward issue remained, because no one, not even Schrödinger, knew exactly what the wave equation really described. Schrödinger tried

latter pair limited by the value of Another major success for A PARADIGM SHIFT 231 the principal number. The fourth Schrödinger’s approach was number, with two possible values, that it offered an explanation for Erwin Schrödinger was needed to explain why two radioactive alpha decay—in which electrons can exist in each subshell a fully formed alpha particle Born in Vienna, Austria, in with slightly different energy levels. (consisting of two protons and two 1887, Erwin Schrödinger Together, the numbers neatly neutrons) escapes from an atomic studied physics at the explained the existence of atomic nucleus. According to classical University of Vienna, attaining orbitals that accept 2, 6, 10, and physics, in order to remain intact, an assistant’s post there 14 electrons respectively. the nucleus had to be surrounded before serving in World War I. by a potential well steep enough to After the war, he moved first Today, the fourth quantum prevent particles escaping from it. to Germany, and then to number is known as spin; it is (A potential well is a region in the University of Zurich, a particle’s intrinsic angular space where the potential energy Switzerland, where he did momentum (which is created by is lower than its surroundings, his most important work, its rotation as it orbits), and has meaning that it traps particles.) immersing himself in the positive or negative values that If the well was not sufficiently steep, emerging field of quantum are either whole- or half-integer the nucleus would disintegrate physics. In 1927, he returned numbers. A few years later, Pauli completely. How, then, could the to Germany, and succeeded would demonstrate that values of intermittent emissions seen in Max Planck at the Humboldt spin split all particles into two alpha decay happen while allowing University of Berlin. major groups—fermions such as the remaining nucleus to survive electrons (with half-integer spins), intact? The wave equations Schrödinger was a vocal which obey a set of rules known as overcame the problem because opponent of the Nazis, and left Fermi–Dirac statistics (pp.246–47), they allowed the energy of the Germany for a post at Oxford and bosons such as photons (with alpha particle within the nucleus University in 1934. It was zero or whole-number spin), which to vary. Most of the time, its energy there that he learned he had obey different rules known as would be low enough to keep it been awarded the 1933 Nobel Bose–Einstein statistics. Only trapped, but occasionally it would Prize in Physics, with Paul fermions obey the exclusion rise high enough to overcome the ❯❯ Dirac, for the quantum wave principle, and this has important equation. By 1936, he was implications for the understanding Schrödinger’s equation, in its most back in Austria, but had to flee of everything from collapsing stars general form, shows the development again following Germany’s to the elementary particles that of a quantum system over time. It annexation of the country. He make up the universe. requires the use of complex numbers. settled in Ireland for the rest of his career before retiring to Schrödinger’s success ih⎯ —∂∂t Ψ = Η Ψ Austria in the 1950s. Combined with Pauli’s exclusion principle, Schrödinger’s wave Key works equation allowed a new and deeper understanding of the orbitals, 1920 Color Measurement shells, and subshells within an 1926 Quantization as an atom. Rather than imagining them Eigenvalue Problem as classical orbits—well-defined paths on which the electrons circle the nucleus—the wave equation shows that they are actually clouds of probability—doughnut-shaped and lobe-shaped regions in which a particular electron with certain quantum numbers is likely to be found (p.256).

232 ERWIN SCHRÖDINGER wall and escape (an effect now known as quantum tunneling). The probability predictions of the wave equation matched the unpredictable nature of the radioactive decay. Uncertainty principle particle can never be “localized” to a Dane Niels Bohr (left) collaborated The great debate that shaped the point in space and at the same time with Werner Heisenberg, to formulate development of quantum physics have a defined wavelength. The the Copenhagen interpretation of during the middle years of the 20th more accurately a particle’s position Schrödinger’s wave function. century (and remains essentially was pinned down, for example, unresolved today) surrounded what the harder its momentum was to apparatus (subject to the classical the wave function actually meant measure. Thus, particles defined by laws of physics) that causes the for reality. In an echo of the Planck/ a quantum wave function existed wave function to “collapse” and Einstein debate two decades in a general state of uncertainty. a definite outcome to arise. This previously, de Broglie saw his and interpretation is perhaps the most Schrödinger’s equations as mere The road to Copenhagen widely (though not universally) mathematical tools for describing Measuring the properties of a accepted, and appears to be movement: for de Broglie, the quantum system always revealed borne out by experiments such electron was still essentially a the particle to be in one location, as electron diffraction and the particle—just one that had a wave rather than in its wavelike smear. double-slit experiment for light property governing its motion and On the scale of classical physics waves. It is possible to devise location. For Schrödinger, however, and everyday life, most situations an experiment that reveals the the wave equation was far more involved definite measurements wavelike aspects of light or fundamental—it described the and definite outcomes, rather than electrons, but impossible to record way in which the properties of the myriad overlapping possibilities. the properties of individual electron were physically “smeared The challenge of reconciling particles in the same apparatus. out” across space. Opposition to quantum uncertainty with reality Schrödinger’s approach inspired is called the measurement problem, However, while the Copenhagen Werner Heisenberg to develop and various approaches to it interpretation seems reasonable another of the century’s great have been put forward, known when dealing with small-scale ideas—the uncertainty principle as interpretations. systems such as particles, its (pp.234–35). This was a realization implication that nothing is that the wave function meant that a The most famous of these is determined until it is measured the Copenhagen interpretation, troubled many physicists. Einstein God knows I am no friend devised by Niels Bohr and Werner famously commented that “God of probability theory, I have Heisenberg in 1927. This states does not throw dice,” while hated it from the first moment simply that it is the very interaction Schrödinger devised a thought between the quantum system and experiment to illustrate what he when our dear friend a large-scale, external observer or viewed as a ridiculous situation. Max Born gave it birth. Erwin Schrödinger

A PARADIGM SHIFT 233 Schrödinger’s cat universe splits into mutually an attempt at a strictly causal, Taken to its logical conclusion, the unobservable alternate histories rather than probabilistic, Copenhagen interpretation resulted for each of the possible outcomes. explanation, and postulates the in a seemingly absurd paradox. In other words, Schrödinger’s cat existence of a hidden “implicate” Schrödinger imagined a cat sealed would both live and die. order to the universe. The in a box that contains a vial of transactional approach involves poison linked to a radioactive The “Consistent Histories” waves traveling both forward source. If the source decays and approach addresses the problem and backward in time. emits a particle of radiation, a in a rather less radical way, using mechanism will release a hammer complex mathematics to generalize Perhaps the most intriguing that breaks the vial of poison. the Copenhagen interpretation. possibility of all, however, is one According to the Copenhagen This avoids the issues around the that verges on the theological. interpretation, the radioactive collapse of the wave function, but Working in the 1930s, Hungarian- source remains in its wave function instead allows probabilities to be born mathematician John von form (as a so-called superposition assigned to various scenarios, or Neumann concluded that the of two possible outcomes) until it “histories,” on both a quantum measurement problem implied is observed. But if that is the case, and classical scale. The approach that the entire universe is subject the same has to be said of the cat. accepts that only one of these to an all-encompassing wave histories eventually conforms equation known as the universal New interpretations to reality, but does not allow wave function, and that it is Dissatisfaction with apparent prediction of which outcome constantly collapsing as we paradoxes such as Schrödinger’s that will be—instead it simply measure its various aspects. cat has spurred scientists to describes how quantum physics Von Neumann’s colleague and develop various alternative can give rise to the universe we see countryman Eugene Wigner took interpretations of quantum without wave function collapse. the theory and expanded it to mechanics. One of the best known suggest that it was not simply is the “Many Worlds Interpretation” The ensemble, or statistical, interaction with large-scale put forward in 1956 by American approach is a minimalist systems (as in the Copenhagen physicist Hugh Everett III. This mathematical interpretation that interpretation) that caused the resolved the paradox by suggesting was favored by Einstein. The wave function to collapse—it that during any quantum event, the de Broglie–Bohm theory, which was the presence of intelligent developed from de Broglie’s initial consciousness itself. ■ reaction to the wave equation, is Radioactive Geiger material counter Hammer Poison If the source decays, it releases We must measure the poison and the cat dies. system to find out A cat inside a sealed box remains whether the source has alive as long as a radioactive source decayed. Until then, we in the box does not decay. must think of the cat as both dead and alive. Schrödinger’s thought experiment produces a situation in which, according to a strict reading of the Copenhagen interpretation, a cat is both alive and dead at the same time.

234 UNCERTAINTY IS INEVITABLE WERNER HEISENBERG (1901–1976) IN CONTEXT F ollowing Louis de Broglie’s Working with Danish physicist suggestion in 1924 that Niels Bohr, Heisenberg built on BRANCH on the smallest scales of Schrödinger’s work to develop the Physics matter, subatomic particles could “Copenhagen interpretation” of display wavelike properties the way that quantum systems, BEFORE (pp.226–33), a number of physicists governed by the laws of probability, 1913 Niels Bohr uses the turned their attention to the interact with the large-scale world. concept of quantized light to question of understanding how One key element of this is the explain the specific energy the complex properties of an atom “uncertainty principle,” which levels associated with could arise from the interaction limits the accuracy to which we electrons inside atoms. of “matter waves” associated can determine properties in with its constituent particles. quantum systems. 1924 Louis de Broglie In 1925, German scientists proposes that just as light can Werner Heisenberg, Max Born, The uncertainty principle arose exhibit particle-like properties and Pascual Jordan used “matrix as a mathematical consequence of so, on the smallest scale, mechanics” to model the hydrogen matrix mechanics. Heisenberg particles might sometimes atom’s development over time. This realized that his mathematical show wavelike behavior. approach was later supplanted by method would not allow certain Erwin Schrödinger’s wave function. pairs of properties to be determined AFTER simultaneously with absolute 1927 Heisenberg and Bohr put forward the highly influential Classical picture Energy Quantum tuneling Copenhagen interpretation Electron barrier is explained by of the way that quantum-level Heisenberg’s principle. events affect the large-scale Quantum picture There is a nonzero (macroscopic) world. chance that an electron can pass through 1929 Heisenberg and a barrier even if it Wolfgang Pauli work on the appears to have too development of quantum field little energy to do so. theory, whose foundations have been laid by Paul Dirac. Electron wave

A PARADIGM SHIFT 235 See also: Albert Einstein 214–21 ■ Erwin Schrödinger 226–33 ■ Paul Dirac 246–47 ■ Richard Feynman 272–73 ■ Hugh Everett III 284–85 Subatomic particles have This means that you cannot wavelike qualities. accurately measure both a particle’s position and its momentum. Uncertainty is This uncertainty is a Werner Heisenberg inevitable. property inherent to the universe. Born in the southern German town of Würzburg in 1901, precision. For example, the far stranger—it turns out that Werner Heisenberg studied more accurately one measures uncertainty is an inherent feature mathematics and physics at a particle’s position, the less of quantum systems. the universities of Munich and accurately one can determine Göttingen, where he studied its momentum, and vice versa. A helpful way of thinking under Max Born and met his Heisenberg found that for these about the issue is to consider the future collaborator Niels Bohr two properties in particular, the matter waves associated with for the first time. relationship could be written as: the particles: in this situation, the particle’s momentum affects its He is best known for his ������x������p ≥ h⎯/2 overall energy and therefore its work on the Copenhagen where ������x is the uncertainty of wavelength—but the more tightly interpretation and the position, ������p the uncertainty of we pin down the particle’s position, uncertainty principle, but momentum, and h is a modified the less information we have about Heisenberg also made version of Planck’s constant (p.202). its wave function, and therefore important contributions to about its wavelength. Conversely, quantum field theory and An uncertain universe accurately measuring the developed his own theory The uncertainty principle is often wavelength requires us to consider of antimatter. Awarded the described as a consequence of a broader region of space, and Nobel Prize in Physics in quantum-scale measurements—for therefore sacrifices information 1932, he became one of its example, it is sometimes said that about the particle’s precise youngest recipients, and his determining a subatomic particle’s location. Such ideas might seem stature enabled him to speak position involves the application of strangely at odds with those we out against the Nazis after a force of some sort that means its experience in the large-scale world, they seized power the kinetic energy and momentum are but they have nevertheless been following year. However, less well defined. This explanation, proved real by many experiments, he chose to stay in Germany put forward at first by Heisenberg and form an important foundation and led the country’s nuclear himself, led various scientists of modern physics. The uncertainty energy program during including Einstein to spend time principle explains seemingly World War II. devising thought experiments that strange real-life phenomena such might obtain a simultaneous and as quantum tunneling, in which Key works accurate measurement of position a particle can “tunnel” through a and momentum by some form of barrier even if its energy suggests 1927 Quantum Theoretical “trickery.” However, the truth is that it should not be able to. ■ Re-interpretation of Kinematic and Mechanical Relations 1930 The Physical Principles of the Quantum Theory 1958 Physics and Philosophy

THE UNIVERSE IS BIG… AND GETTING BIGGER EDWIN HUBBLE (1889–1953)



238 EDWIN HUBBLE B y the early 20th century, amorphous cloud of light, including ideas about the scale of objects that were later found to be IN CONTEXT the universe divided galaxies beyond the Milky Way. astronomers into two schools of BRANCH thought—those who believed that As telescopes improved Cosmology the Milky Way galaxy was, generally dramatically during the 19th speaking, its entire extent, and century, some of the objects BEFORE those who thought that the Milky catalogued as nebulae began to 1543 Nicolaus Copernicus Way could be just one galaxy reveal distinctive spiral features. concludes that Earth is not among countless others. Edwin At the same time, the development the center of the universe. Hubble was to solve the puzzle, and of spectroscopy (the study of the show that the universe is much interaction between matter and 17th century The changing larger than anyone imagined. radiated energy) suggested that view of stars offered by Earth’s these spirals were in fact made orbit around the Sun gives rise Key to the debate was the up of countless individual stars, to the parallax method for nature of “spiral nebulae.” Today, blending seamlessly together. measuring stellar distances. a nebula is the term used for an interstellar cloud of dust and gas, The distribution of these 19th century Improvements but at the time of this debate, it nebulae was interesting too— to telescopes pave the way for was the name used for any unlike other objects that clustered the study of starlight and the together in the plane of the Milky rise of astrophysics. There is a simple relation Way, they were more common in between the brightness of the the dark skies away from the plane. AFTER As a result, some astronomers 1927 Georges Lemaître variables and their periods. adopted an idea from the German proposes that the universe Henrietta Leavitt philosopher Immanuel Kant, who can be traced back to a single in 1755 suggested that nebulae point of origin. were “island universes”—systems similar to the Milky Way but vastly 1990s Astronomers discover more distant, and only visible that the expansion of the where the distribution of material universe is accelerating, in our galaxy permits clear views driven by a force known into what we now call intergalactic as dark energy. space. Those who continued to believe that the universe was far Edwin Hubble Born in Marshfield, Missouri, in “extragalactic nebulae” in 1889, Edwin Powell Hubble had 1924–25, and his proof of cosmic a fiercely competitive nature that expansion in 1929. In later manifested itself in his youth as a years, he campaigned for gifted athlete. Despite his interest astronomy to be recognized by in astronomy, he followed his the Nobel Prize Committee. The father’s wishes and studied law, rules were only changed after but at 25 years old, after his his death in 1953 and so he was father’s death, he resolved to never awarded the prize himself. follow his early passion. His studies were interrupted by Key works service in World War I, but after his return to the United States 1925 Cepheid Variables in he began to work at the Mount Spiral Nebulae Wilson Observatory. There he 1929 A Relation Between did his most important work, Distance and Radial Velocity publishing his study on among Extra-galactic Nebulae

A PARADIGM SHIFT 239 See also: Nicolaus Copernicus 34–39 ■ Christian Doppler 127 ■ Georges Lemaître 242–45 A Cepheid variable If the Cepheid variable is a star whose brightness is millions of light years we can know for certain, away, it must be which means that in a galaxy far we can calculate outside our own. how far away it is. Henrietta Leavitt received little The light of every distant The light coming recognition in her lifetime, but her galaxy is redshifted, and from other galaxies may discoveries relating to Cepheid variable the farther the galaxy, the be blueshifted (moving stars were the key that allowed greater the redshift. toward us) or redshifted astronomers to measure the distance from Earth to faraway galaxies. (moving away from us). more limited in extent argued that the spirals might be suns or solar systems in the process of formation, in orbit around the Milky Way. Stars with a pulse The universe is big… The answers to this long-standing and getting bigger. puzzle came in several stages, but perhaps the most important was the of the clouds, she found, contained were an indication of differences in establishment of an accurate means huge numbers of variable stars, and their “absolute magnitude” (actual of measuring the distance to stars. by comparing them across many brightness). Publishing her first The breakthrough came with the different plates, she not only saw results in 1908, Leavitt noted in work of Henrietta Swan Leavitt, one that their light was varying in a passing that some stars seemed to of the team of female astronomers regular cycle, she could also figure show a relationship between their at Harvard University who were out the period of the cycle. variability period and their absolute analyzing the properties of starlight. magnitude, but it took another four By concentrating on these small, years for her to figure out what this Leavitt was intrigued by the faint, isolated star clouds, Leavitt relationship was. It turned out that, behavior of variable stars. These could safely assume that the stars for a certain type of variable star were stars whose brightness within them were all at more or known as a Cepheid variable, stars appeared to fluctuate, or pulse, less the same distance from Earth. with greater luminosity have longer because they periodically Though she could not know the variability periods. expanded and contracted as they distance itself, this was still neared the end of their lives. She enough to assume that differences Leavitt’s “period-luminosity” began to study photographic plates in the “apparent magnitude” law would prove the key to of the Magellanic Clouds, two small (observed brightness) of the stars unlocking the scale of the ❯❯ patches of light visible from the southern sky that look like isolated “clumps” of the Milky Way. Each

240 EDWIN HUBBLE We are reaching the universe once and for all. The “island universe” supporters into space, farther and Respected Princeton astronomer were represented by Heber D. Curtis farther, until, with the Harlow Shapley spoke for the “small of the University of Pittsburgh’s faintest nebulae that can universe” side. He had been the Allegheny Observatory. He based be detected…we arrive first to use Leavitt’s work on his arguments on comparisons Cepheids to measure the distance between the rates of bright “nova” at the frontier of the to globular clusters (dense star explosions in distant spirals and in known universe. clusters in orbit around the Milky our own Milky Way. Novae are very Edwin Hubble Way), and discovered that they bright star explosions that can were typically several thousand serve as distance indicators. universe—if you could figure out light years away. In 1918, he had the star’s absolute magnitude used RR Lyrae stars (fainter stars Curtis also cited the evidence from its variability period, then that behave like Cepheids) to of another, crucial factor—the high the star’s distance from Earth estimate the size of the Milky Way redshift exhibited by many spiral could be calculated from its and show that the Sun was nebulae. This phenomenon had apparent magnitude. The first nowhere near its center. His been discovered by Vesto Slipher step in figuring this out was arguments appealed to public of the Flagstaff Observatory, to calibrate the scale, which scepticism toward notions of an Arizona, in 1912—apparent through was done in 1913 by Swedish enormous universe with many distinctive shifts in the pattern of a astronomer Ejnar Hertzsprung. galaxies, but also cited specific nebula’s spectral lines toward the He figured out the distances to evidence (later to be proved red end of the spectrum. Slipher, 13 relatively nearby Cepheids inaccurate), such as reports that Curtis, and many others believed using the parallax method (p.39). over many years some astronomers that they were caused by the Cepheids were immensely had actually observed the spiral bright—thousands of times more nebulae rotating. For this to be By measuring the light from Cepheid luminous than our Sun (in modern true without parts of the nebula variable stars in the Andromeda nebula, terminology they are “yellow exceeding the speed of light, Hubble established that Andromeda supergiants”). In theory, then, they they must be relatively small. was 2.5 million light years way— were an ideal “standard candle”— and was a galaxy in its own right. stars whose brightness could be used to measure huge cosmic distances. But despite the best efforts of astronomers, Cepheids within the spiral nebulae remained stubbornly elusive. The Great Debate In 1920, the Smithsonian Museum in Washington DC hosted a debate between the two rival cosmological schools, hoping to settle the issue of the scale of

A PARADIGM SHIFT 241 Doppler effect (a change in the In 1842, Christian wavelength of light due to relative Doppler (p.127) motion between source and showed that if a light observer), and therefore indicated source is moving that the nebulae were moving away toward us or away from us at very high speeds—far from us, the light too fast for the Milky Way’s gravity waves arrive at to keep hold of them. different rates. If the light source is Measuring the universe correctly called spiral galaxies. moving toward us, By 1922–23, Edwin Hubble and As if this revolution in the way we see a bluer color Milton Humason of California’s we see the universe were not as waves bunch Mount Wilson Observatory were in enough, Hubble then went on to together at the blue a position to end the mystery once look at how galaxy distances end of the light and for all. Using the observatory’s related to the redshifts already spectrum; if it is new 100 in (2.5 m) Hooker Telescope discovered by Slipher—and here moving away, we (the largest in the world at that he found a remarkable relationship. see a redder color. time), they set out to identify By plotting the distances for more Hubble guessed that Cepheid variables shining within than 40 galaxies against their sodium light was the spiral nebulae, and this redshifts, he showed a roughly the same color in time they were successful in linear pattern: the farther away a far galaxies as it is finding Cepheids in many of the galaxy is, on average, the greater on Earth, but the largest and brightest nebulae. its redshift and therefore the faster Doppler effect meant it is receding from Earth. Hubble that it would be Hubble then plotted their immediately realized that this blueshifted or periods of variability and therefore could not be because our galaxy is redshifted if moving their absolute magnitude. From uniquely unpopular, but must be toward or away this, a simple comparison to a from us. star’s apparent magnitude revealed its distance, producing figures that the result of a general cosmic were typically millions of light expansion—in other words, space years. This proved conclusively itself is expanding and carrying that the spiral nebulae were really every single galaxy with it. The huge, independent star systems, far wider the separation between beyond the Milky Way and rivaling two galaxies, the faster the space it in size. Spiral nebulae are now between them will expand. The rate of expansion of space soon Equipped with his five became known as the “Hubble senses, man explores the Constant.” It was conclusively universe around him and calls measured in 2001 by the space telescope bearing Hubble’s name. the adventure science. Edwin Hubble Long before then, Hubble’s discovery of the expanding universe had given rise to one of the most famous ideas in the history of science—the Big Bang theory (pp.242–45). ■

242 IN CONTEXT THE RADIUS BRANCH OF SPACE Astronomy BEGAN AT ZERO BEFORE 1912 US astronomer GEORGES LEMAÎTRE (1894–1966) Vesto Slipher discovers the high redshifts of spiral nebulae, suggesting they are moving away from Earth at high speeds. 1923 Edwin Hubble confirms that the spiral nebulae are distant, independent galaxies. AFTER 1980 US physicist Alan Guth proposes a brief period of dramatic inflation in the early universe to produce the conditions we see today. 1992 The COBE (Cosmic Background Explorer) satellite detects tiny ripples in the cosmic microwave background radiation (CMBR)—hints of the first structure that emerged in the early universe. T he idea that the universe began with a Big Bang, expanding from a tiny, superdense, and extremely hot point in space, is the basis of modern cosmology, and one that is often said to have originated with Edwin Hubble’s 1929 discovery of cosmic expansion. But the precursors of the theory predate Hubble’s breakthrough by several years, and first sprang from interpretations of Albert Einstein’s theory of general relativity as it applied to the universe as a whole. When formulating his theory, Einstein drew on the available evidence of the time to assume that the universe was static—

A PARADIGM SHIFT 243 See also: Isaac Newton 62–69 ■ Albert Einstein 214–21 ■ Edwin Hubble 236–41 ■ Fred Hoyle 270 Since the Big Bang 13.8 billion years ago, the expansion of the universe has been through different phases. There was an initial period of rapid expansion known as inflation. After that, expansion slowed, then started to speed up once more. Present Time Accelerating Georges Lemaître (~15 billion years) expansion Born in Charleroi, Belgium, in Inflation Slowing expansion 1894, Lemaître studied civil Big Bang engineering at the Catholic University of Louvain and Expanding universe served in World War I before returning to academia, where neither expanding nor contracting. Famously, Einstein later called the he studied physics and General relativity indicated that constant his greatest mistake, mathematics as well as the universe should collapse under but even at the time he proposed theology. From 1923, he its own gravity, so Einstein fudged it there were some who found it studied astronomy in Britain his own equations by adding a unsatisfactory. The Dutch physicist and the United States. On his term known as the cosmological Willem de Sitter and Russian return to Louvain in 1925 as constant. Einstein’s constant mathematician Alexander a lecturer, Lemaître began mathematically counteracted the Friedmann independently to develop his theory of an gravitational contraction to produce suggested a solution to general expanding universe as an the presumed static universe. relativity in which the universe was explanation for the redshifts expanding, and, in 1927, Belgian of the extragalactic nebulae. astronomer and priest Georges Lemaître reached the same First published in 1927, in conclusion, two years ahead of a little-read Belgian journal, Hubble’s observational proof. Lemaître’s ideas took off after he published an English The first stages of the Beginning in fire translation with Arthur expansion consisted of a In an address to the British Eddington. He lived until rapid expansion determined Association in 1931, Lemaître 1966—long enough to see by the mass of the initial atom, took the idea of cosmic expansion proof that his ideas were almost equal to the present to its logical conclusion, suggesting correct with the discovery that the universe had sprung from of the cosmic microwave mass of the universe. a single point that he called the background radiation (CMBR). Georges Lemaître “primeval atom.” The response to this radical idea was mixed. Key works The astronomical establishment 1927 A Homogeneous Universe of the time was attached to the of Constant Mass and idea of an eternal universe ❯❯ Growing Radius Accounting for the Radial Velocity of Extragalactic Nebulae 1931 The Evolution of the Universe: Discussion

244 GEORGES LEMAÎTRE General relativity leads Hubble demonstrates away from a steady state universe. Lemaître to predict that cosmic expansion. This was a 1948 paper written by the universe is expanding. Ralph Alpher and George Gamow of the Johns Hopkins University Lemaître theorizes that the universe began with a in the US. It was called The Origin “primeval atom,” a theory later dubbed the “Big Bang.” of Chemical Elements, and described in detail how subatomic The discovery of cosmic microwave background radiation particles and lightweight chemical (CMBR) confirms the Big Bang theory. elements could have been produced from the raw energy of the Big The radius of space began at zero. Bang, in accordance with Einstein’s equation E = mc2. But without end or beginning, and the In 1949, Hoyle scornfully referred this theory, later known as Big prospect of a distinct point of origin to the rival theory as a “Big Bang.” Bang nucleosynthesis, explained a (especially when proposed by a The name stuck. process that could form only the Catholic priest) was seen as four lightest elements—hydrogen, introducing an unnecessary Making the elements helium, lithium, and beryllium. religious element into cosmology. By the time Hoyle had inadvertently Only later was it discovered that named the theory, a persuasive the heavier elements of the However, Hubble’s observations piece of evidence in favor of universe are the product of stellar were undeniable, and some kind of Lemaître’s hypothesis had been nucleosynthesis (a process that model was needed to explain the published, tipping the balance takes place inside stars). Ironically, expanding universe. Numerous the evidence showing how stellar theories were put forward in the nucleosynthesis worked was to be 1930s, but by the late 1940s, just developed by Fred Hoyle. two remained in play—Lemaître’s primeval atom, and the rival Nevertheless, there was still “steady state” model, in which no direct observational evidence matter was continuously created to determine the truth of either as the universe expanded. British the Big Bang or a steady state astronomer Fred Hoyle was the universe. Early attempts to test champion of the steady state idea. the theories were made in the 1950s using a basic radio telescope known as the Cambridge Interferometer. These tests relied on a simple principle: if the steady state theory was true, then the Tiny variations have been found in the cosmic microwave background radiation—the different colors in this image show temperature differences of less than 400 millionths of a Kelvin.

A PARADIGM SHIFT 245 Arno Penzias and Robert Wilson detected the background radiation by accident. At first, they thought the interference had been caused by bird droppings on their radio antenna. universe must be essentially and his colleagues at Princeton minute variations in the signal that uniform in both time and space; University set out to build a radio allow us to study conditions in the but if it originated 10–20 billion telescope that could detect this faint universe back to 380,000 years years ago, as the Big Bang theory signal, which they thought would after the Big Bang. suggested, and evolved throughout take the form of low-energy radio its history, then distant reaches of waves. However, they were Later developments the universe, whose radiation had ultimately beaten to the prize by Despite being proved correct in taken billions of years to reach Arno Penzias and Robert Wilson, principle, the Big Bang theory has Earth, should appear substantially two engineers working at the undergone many transformations different. (This cosmic time nearby Bell Telephone Laboratories. since the 1960s to match it to machine effect, whereby we see Penzias and Wilson had built a radio our growing understanding of more distant celestial objects as telescope for satellite communication, the universe. Among the most they were in the distant past, is but found themselves plagued by an significant are the introduction known as “lookback time.”) By unwanted background signal that of dark matter and dark energy measuring the number of distant they could not eliminate. Coming to the story, and the addition of a galaxies emitting radiation above from all over the sky, it corresponded violent growth spurt in the instant a certain brightness, it should be to microwave emission from a body after creation, known as Inflation. possible to distinguish between at a temperature of 3.5K—just 6°F The events that triggered the Big the two scenarios. (3.5°C) above absolute zero. When Bang remain beyond our reach but Bell Labs contacted Dicke to ask measurements of the rate of cosmic The first of the Cambridge for help with their problem, Dicke expansion, aided by instruments experiments delivered a result that realized that they had found the such as the Hubble Space Telescope, seemed to support the Big Bang. remnants of the Big Bang—now now allow us to pin down the epoch However, problems were discovered known as the cosmic microwave of cosmic creation with great with the radio detectors, so the background radiation (CMBR). accuracy—the universe came into results had to be disregarded. existence 13.798 billion years ago, Later results proved more equivocal. The discovery that the CMBR give or take 0.037 billion years. permeates the universe—a Various theories exist about the Traces of the Big Bang phenomenon for which the steady future of the universe, but many Fortunately, the question soon state theory had no explanation— think that it is set to continue resolved itself by other means. decided the case in favor of the Big expanding until it reaches a state As early as 1948, Alpher and his Bang. Subsequent measurements of thermodynamic equilibrium, or colleague Robert Herman had have shown that the CMBR’s true “heat death,” in which matter has predicted that the Big Bang would average temperature is about disintegrated into cold subatomic have left a residual heating effect 2.73K, and high-precision satellite particles, in around 10100 years’ time.■ throughout the universe. According measurements have revealed to the theory, when the universe was about 380,000 years old, it had cooled enough to become transparent, allowing light photons to travel freely through space for the first time. The photons that existed at this time had been propagating through space ever since, growing longer and redder as space expanded. In 1964, Robert Dicke

246 EVERY PARTICLE OF MATTER HAS AN ANTIMATTER COUNTERPART PAUL DIRAC (1902–1984) IN CONTEXT Dirac corrects E nglish physicist Paul Dirac Schrödinger’s wave contributed a huge amount BRANCH equation to take into account to the theoretical framework Physics relativistic effects. of quantum physics in the 1920s, but is probably best known today BEFORE Dirac’s new equation for predicting the existence of 1925 Werner Heisenberg, predicts the existence antiparticles through mathematics. Max Born, and Pascual Jordan develop matrix mechanics of antimatter. Dirac was a postgraduate to describe the wavelike student at Cambridge University behavior of particles. Antimatter is when he read Werner Heisenberg’s subsequently discovered, groundbreaking paper on matrix 1926 Erwin Schrödinger confirming Dirac’s prediction. mechanics, which described how develops a wave function particles jump from one quantum describing the change in Every particle state to another. Dirac was one an electron over time. of matter has of the few people capable of grasping an antimatter the paper’s difficult mathematics, AFTER counterpart. and noticed parallels between 1932 The existence of the Heisenberg’s equations and parts positron, the antiparticle to of the classical (pre-quantum) the electron, is confirmed by theory of particle motion known Carl Anderson. as Hamiltonian mechanics. This allowed Dirac to develop a method 1940s Richard Feynman, by which classical systems could Sin-Itiro Tomonaga, and be understood on a quantum level. Julian Schwinger develop quantum electrodynamics— One early result of this work a mathematical way to was a derivation of the idea of describe the interaction quantum spin. Dirac formulated a between light and matter, set of rules now known as “Fermi- which fully unites quantum Dirac statistics” (since they were theory with special relativity. also independently found by Enrico Fermi). Dirac named particles such as electrons that have a half-integer spin value “fermions,” after Fermi. The rules describe how large

A PARADIGM SHIFT 247 See also: James Clerk Maxwell 180–85 ■ Albert Einstein 214–21 ■ Erwin Schrödinger 226–33 ■ Werner Heisenberg 234–35 ■ Richard Feynman 272–73 numbers of fermions interact with Positron Photon When a particle one another. In 1926, Dirac’s PhD Electron Annihilation and its antiparticle supervisor Ralph Fowler used his come together, statistics to calculate the behavior Photon they annihilate. of a collapsing stellar core and Their mass turns explain the origin of superdense into photons of white dwarf stars. electromagnetic energy in accord Quantum field theory with the equation While much of schoolbook physics E = mc2. focuses on the properties and dynamics of individual particles the electron (that is, one that could Carl Anderson in 1932, detected and bodies under the influence of take into account the effects of first in cosmic rays (high-energy forces, a deeper understanding particles moving close to the speed particles showered into Earth’s can be gained by developing field of light, and therefore model the atmosphere from deep space), theories. These describe the way quantum world more accurately and then in certain types of that forces make their influence felt than Schrödinger’s nonrelativistic radioactive decay. Since then, across space. The importance of equation). The so-called Dirac antimatter has become a subject fields as independent entities was equation also predicted the for intense physical research, first recognized in the mid-19th existence of particles with identical and also beloved of science-fiction century by James Clerk Maxwell properties to particles of matter but writers (particularly for its while he was developing his with opposite electric charge. They habit of “annihilating” with a burst theory of electromagnetic radiation. were dubbed “antimatter” (a term of energy on contact with normal Einstein’s general relativity is that had been bandied around in matter). Perhaps more importantly, another example of a field theory. wilder speculations since the late however, Dirac’s quantum 19th century). field theory laid the foundations Dirac’s new interpretation of for the theory of quantum the quantum world was a quantum The antielectron particle, electrodynamics brought to fruition field theory. In 1928, it allowed him or positron, was experimentally by a later generation of physicists. ■ to produce a relativistic version of confirmed by US physicist Schrödinger’s wave equation for Paul Dirac Paul Dirac was a mathematical and Copenhagen before genius who made several key returning to Cambridge, having contributions to quantum been appointed the Lucasian physics, sharing the Nobel Chair in Mathematics. Much of Prize in Physics with Erwin his later career was focused on Schrödinger in 1933. Born in quantum electrodynamics. He Bristol, England, to a Swiss father also pursued the idea of unifying and an English mother, he earned quantum theory with general degrees in electrical engineering relativity, but this endeavor met and mathematics at the city’s with limited success. university, before continuing his studies at Cambridge, where he Key works pursued his fascination with general relativity and quantum 1930 Principles of Quantum theory. After his groundbreaking Mechanics advances of the mid-1920s, he 1966 Lectures on Quantum continued his work at Göttingen Field Theory

248 THERE IS AN UPPER LIMIT BEYOND WHICH A COLLAPSING STELLAR CORE BECOMES UNSTABLE SUBRAHMANYAN CHANDRASEKHAR (1910–1995) IN CONTEXT T he development of known as a singularity—forming a quantum physics in the black hole. This “Chandrasekhar BRANCH 1920s had implications for Limit” for a collapsing stellar core is Astrophysics astronomy, where it was applied to now known to be 1.44 solar masses the understanding of superdense (or 1.44 times the mass of the Sun). BEFORE stars known as white dwarfs. However, there is a middle stage 19th century White dwarf These are the burned-out cores of between white dwarf and black stars are discovered when sunlike stars that have exhausted hole—a city-sized neutron star astronomers identify a star their nuclear fuel and collapsed, stabilized by another quantum that has far more mass than under their own gravity, to objects effect called “neutron degeneracy its tiny size would suggest. about the size of Earth. In 1926, pressure.” Black holes are created physicists Ralph Fowler and Paul only when the neutron star’s core AFTER Dirac explained that collapse stops exceeds an upper limit somewhere 1934 Fritz Zwicky and Walter at this size due to the “degenerate between 1.5 and 3 solar masses. ■ Baade propose that explosions electron pressure” that arises known as supernovae mark whenever electrons are packed The black holes of nature are the deaths of massive stars, together so tightly that the Pauli the most perfect macroscopic and the collapse of their cores exclusion principle (p.230)—that no form neutron stars. two particles can occupy the same objects in the Universe. quantum state—comes into play. Subrahmanyan 1967 British astronomers Chandrasekhar Jocelyn Bell and Anthony Forming a black hole Hewish detect rapidly pulsing In 1930, Indian astrophysicist radio signals from an object Subrahmanyan Chandrasekhar now known as a “pulsar”—a figured out that there was an upper rapidly rotating neutron star. limit to the mass of a stellar core beyond which gravity would 1971 X-ray emissions from a overcome the degenerate electron source known as Cygnus X-1 pressure. The stellar core would are found to originate from hot collapse to a single point in space material spiraling into what is probably a black hole—the first See also: John Michell 88–89 ■ Albert Einstein 214–21 ■ Paul Dirac 246–47 ■ such object to be confirmed. Fritz Zwicky 250–51 ■ Stephen Hawking 314