The Physics Book

Electron orbit track came with an explanation for SOUND AND LIGHT 199 a series of lines in the spectrum Nucleus of super-hot stars, known as the Niels Bohr Pickering series. Bohr correctly Photon explained that these lines were Born in Copenhagen, with a long associated with electrons jumping Denmark, in 1885, Bohr wavelength between orbits in ionized helium studied physics at the city’s as electron (He+)—atoms of helium that had university. His PhD thesis, falls just already been stripped of one of completed in 1911, was a one orbit their two electrons. groundbreaking investigation of the distribution of electrons Photon with a short Roots of the laser in metals. A visit to British wavelength as electron The following decades showed laboratories in the same year falls two orbits that Bohr’s model was an inspired Bohr to formulate the oversimplification of the strange model of atomic structure, for The wavelength of light that an quantum processes going on inside which he was awarded the atom emits depends on how much real atoms, but it nevertheless Nobel Prize in Physics in 1922. energy an electron loses in falling from marked a huge step forward in By this time he was director one orbit to another. The further it falls, scientific knowledge. The Bohr of the new Danish Institute the shorter the wavelength of the light model not only explained the link for Theoretical Physics. and the higher its energy. between atomic structure and spectral lines for the first time, In 1940, Denmark was (and entire empty orbits) further but also paved the way for new occupied by the Nazis. Three out were available for electrons that technologies that would take years later, Bohr, whose were closer in to “jump” into if they advantage of this emission, such mother was Jewish, fled to the received a boost of energy—from as the laser—an intense beam US, where he contributed to a ray of incoming light, for example. of photons created by triggering the Manhattan Project to build If a gap remained in the closer a cascade of emission events an atomic bomb. Returning orbit, the transition would usually within an energized material. The to Denmark in 1945, he helped be brief, and the “excited” electron behavior upon which the laser establish the International would almost immediately fall is based was predicted by Einstein Atomic Energy Agency. back into the lower energy state. as early as 1917, and proven to exist Bohr died at his home in In this case, it would emit a small in 1928 by Rudolf W. Ladenburg, Copenhagen in 1962. burst of light whose frequency, but a working laser beam was not wavelength, and color were achieved in practice until 1960. ■ Key works determined by the equation We must be clear that 1913 “On the Constitution ∆E = hn (called Planck’s equation, when it comes to atoms, of Atoms and Molecules” after Max Planck), where ∆E is the language can be used only 1924 “The Quantum Theory of Radiation” energy difference in the transition, as in poetry. 1939 “The Mechanism of n (the Greek letter nu) is the Niels Bohr Nuclear Fission” frequency of the emitted light, and h is the Planck constant, relating the frequency and energy of electromagnetic waves. Bohr convincingly applied this new model to the simplest atom, hydrogen, and showed how it could produce the familiar lines of the Balmer series. However, the most convincing evidence that confirmed he was on the right

200 SWEIETIHNGSOUND PIEZOELECTRICITY AND ULTRASOUND IN CONTEXT Crystals are piezoelectric if heating them and/or distorting their structure can create an electric current. KEY FIGURES Pierre Curie (1859–1906), High-frequency electric Ultrasound echoes Jacques Curie (1855–1941), currents can cause bouncing off an object are Paul Langevin (1872–1946) piezoelectric crystals to affected by its shape, BEFORE produce high-pitched composition, and distance. 1794 Italian biologist Lazzaro Spallanzani reveals how bats ultrasound. navigate by listening to the echoes of their own calls. When the crystals compress, These echoes cause they produce electric signals the piezoelectric crystals AFTER 1941 In Austria, Karl Dussik that can be transformed to compress. is the first person to apply into images. ultrasound imaging to the human body. U sing echolocation—the about icebergs and other large detection of hidden objects submerged or semi-submerged 1949 John Wild, a physician by perceiving the sound objects. Richardson noted that in the US, pioneers the use of waves reflected from those objects a ship emitting sound waves ultrasound as a diagnostic tool. as echoes—was first proposed in with higher frequencies and 1912 by British physicist Lewis shorter wavelengths—later called 1966 At the University of Fry Richardson. Shortly after the ultrasound—would be able to Washington, Donald Baker RMS Titanic disaster of that year, detect submerged objects with and colleagues develop the Richardson applied for a patent greater accuracy than normal first pulsed-wave ultrasound on a method for warning ships sound waves allowed. He to take account of the Doppler effect—when the source of the sound waves is moving— to measure the movement of body fluids.

SOUND AND LIGHT 201 See also: Electric potential 128–129 ■ Music 164–167 ■ The Doppler effect and redshift 188–191 ■ Quantum applications 226–231 The absorption of Langevin, a former pupil of Pierre Pierre Curie sound by water is less Curie, continued to investigate. With the outbreak of World War I The son of a doctor, Curie than that by air. and the appearance of a new form was born in Paris in 1859 and Lewis Fry Richardson of warfare, the German U-boat educated by his father. After (submarine), Langevin realized that studying mathematics at the envisaged a mechanical means of piezoelectricity could be used to University of Paris, he worked producing such waves and showed produce and detect ultrasound. as a laboratory instructor in that they would carry further in the university’s science faculty. water than in air. New technology The experiments he conducted Pulses of high-frequency sound in the lab with his brother Curie’s crystals waves could be driven out into the Jacques led to the discovery Richardson’s invention never water using powerful oscillating of piezoelectricity and his became a reality. The RMS Titanic currents running through stacks invention of an “electrometer” disaster faded from the spotlight of crystals sandwiched between to measure the weak electrical and the need became less urgent. metal sheets, in a device called a currents involved. Furthermore, a more practical transducer. The reverse principle means of creating and detecting could convert the compression For his doctorate, Curie ultrasound had already been caused by returning echoes into researched the relationship discovered through the work of an electric signal. Langevin’s between temperature and French brothers Jacques and application of the Curie brothers’ magnetism. Through this, he Pierre Curie. discovery forms the foundation began working with Polish of sonar and other echolocation physicist Maria Skłodowska, In around 1880, while systems that are still used today. whom he married in 1895. investigating the way certain They spent the rest of their crystals can create an electric At first, the interpretation of lives researching radioactivity, current when heated, they found echolocation data entailed turning leading to a Nobel prize in that using pressure to deform the the returning pulse back into sound 1903. Pierre died in a traffic crystal structure also created an through a speaker, but in the early accident in 1906. electric potential difference—called 20th century electronic displays piezoelectricity. A year later, they were developed. These eventually Key work confirmed a prediction made by evolved into the sonar display French physicist Gabriel Lippmann systems used in navigation, 1880 “Développement, par that passing an electric current defense, and medical ultrasound. ■ pression, de l’électricité through a crystal would cause polaire dans les cristaux a reverse effect—physical In piezoelectric crystals such as hémièdres à faces inclinée” deformation of the crystal. amethyst, the structure of the cells is (“Development of electricity not symmetrical. When pressure is through pressure on the The practical applications applied, the structure deforms and the inclined faces of hemihedral for piezoelectricity were not atoms move, creating a small voltage. crystals”) immediately realized, but Paul

202 FEALCLUHACORTGUEATING SEEING BEYOND LIGHT IN CONTEXT P hysicists’ discovery of Earth’s atmosphere blocks or electromagnetic radiations swamps many of these radiations. KEY FIGURE beyond visible light in the In the case of radio waves from Jocelyn Bell Burnell (1943–) 19th and early 20th centuries planets, stars, and other celestial introduced new ways of observing phenomena (which can penetrate BEFORE nature and the universe. They had the atmosphere), the main problem 1800 In the UK, William to overcome many challenges in was their huge wavelengths, Herschel accidentally their quest, particularly since which made it difficult to locate discovers the existence their source. of infrared radiation. Invisible radio waves from space penetrate The first steps in radio 1887 Heinrich Hertz Earth’s atmosphere. astronomy were made in 1931, when successfully generates radio American physicist Karl Jansky waves for the first time. Radio telescopes can erected a large sensitive antenna gather radio waves on a turntable. By measuring how AFTER to identify their signals changed through the day as 1967 US military Vela Earth rotated, revealing different satellites, designed to detect approximate location. parts of the sky above the horizon, nuclear tests, record the first he was able to prove that the center gamma-ray bursts from violent This helps astronomers of the Milky Way was a strong events in the distant universe. locate distant stars source of radio waves. 1983 NASA, the UK, and the and galaxies. Mapping radio waves Netherlands launch the first In the 1950s, the first giant “dish” infrared space telescope, IRAS telescope was built in the UK by (Infrared Astronomical Satellite). Bernard Lovell at Jodrell Bank in Cheshire. With a diameter of 250 ft 2019 Using aperture synthesis (76.2 m), the telescope was able to to construct a collaboration of produce blurry images of the radio telescopes around the world, sky, pinning down the location and astronomers observe radiation shape of individual radio sources for around a supermassive black the first time. hole in a distant galaxy. Significant improvements in the resolving power of radio telescopes were achieved in the 1960s, with

SOUND AND LIGHT 203 See also: Electromagnetic waves 192–195 ■ Black holes and wormholes 286–289 ■ Dark matter 302–305 ■ Dark energy 306–307 ■ Gravitational waves 312–315 Radio maps of the sky show the source of waves arriving from deep space. The most intense radio emissions (red) come from the central regions of our Milky Way galaxy. the work of Martin Ryle, Antony to discover interplanetary This ultimately proved to be a Hewish, and graduate student scintillation (IPS)—predicted pulsar—the first known example Jocelyn Bell Burnell at the variations in radio signals from of a rapidly rotating neutron star, University of Cambridge. Ryle distant sources as they interact an object whose existence had first invented a technique called with the sun’s magnetic field and been proposed in the 1930s. aperture synthesis, in which a solar wind. grid, or “array,” of individual radio Bell Burnell’s discovery of antennae function as a single giant Bell Burnell’s project involved pulsars was the first of many telescope. The principle behind it monitoring the IPS telescope’s breakthroughs for radio astronomy. involved measuring the varying measurements and looking for the As technology improved, aperture amplitude (strength) of incoming occasional variations that were synthesis would be used to radio waves received by the different predicted. Among her findings, make increasingly detailed images antennae at the same moment. however, was a much shorter and of the radio sky, helping to reveal This method, and some basic more predictable radio signal, lasting the structure of the Milky Way, assumptions about the shape of the 1/25th of a second and repeating distant colliding galaxies, and the radio waves, allowed the direction every 1.3 seconds, keeping pace material surrounding supermassive of their source to be calculated with the movement of the stars. black holes. ■ with much greater precision. Using Ryle’s discovery, Hewish and Bell Burnell set out to build a radio telescope with several thousand antennae, ultimately covering 4 acres (1.6 ha) of ground. Hewish hoped to use this telescope Jocelyn Bell Burnell Born in Belfast, Northern Ireland, gone on to a highly successful in 1943, Jocelyn Bell became career in astronomy and interested in astronomy after been awarded several other visiting the Armagh Planetarium prizes, including the 2018 as a young girl. She went on to Special Breakthrough Prize in graduate in physics from the Fundamental Physics, for both University of Glasgow in 1965, her scientific research and her and moved to the University of work promoting the role of Cambridge to study for a PhD women and minorities in science under Antony Hewish. It was here and technology. that she discovered the first pulsar. Key work Despite being listed as the second author on the paper that 1968 “Observation of a Rapidly announced the discovery, Bell Pulsating Radio Source” Burnell was overlooked when (Nature paper, with Antony her colleagues were awarded the Hewish and others) Nobel Prize in 1974. She has since

WTHOERLUDAN our uncertain universe

TUM

206 INTRODUCTION Max Planck describes Niels Bohr develops American Arthur Austrian Wolfgang the radiation emitted the first quantum Compton discovers the Pauli discovers the by a blackbody as model of the atom. exclusion principle. quantum nature of discrete packages, X-rays, confirming the or quanta. existence of photons. 1900 1913 1923 1925 1905 1922 1924 Albert Einstein explains the Germans Otto Stern and Frenchman Louis de Broglie states photoelectric effect by Walter Gerlach discover that all matter has wavelike properties, suggesting that assuming light interacts with quantum spin—the electrons as discrete lumps, quantization of angular particles such as electrons exhibit a wave–particle duality. now known as photons. momentum. O ur world is deterministic, world that exists on the smallest and suggested that light must be following laws that set scales. Our door to this new realm made from discrete lumps. Einstein out definitively how a was unlocked by finding out that had to invoke this seemingly system will evolve. Usually through light behaves in unfamiliar ways. strange conclusion to describe trial (such as playing sports and the observed phenomenon of the predicting a ball’s trajectory) and Waves or particles? photoelectric effect. error (getting hit by a few balls), we Since the 16th century, a debate innately learn these deterministic had raged as to the nature of light. There was now evidence for laws for specific everyday situations. One camp vowed that light was both sides of the particle–wave made up of tiny particles, an idea debate. Something very fishy was Physicists design experiments championed by English physicist happening—welcome to the world to uncover these laws and so Isaac Newton, but others viewed of quanta, objects that behave enable us to predict how our world light as a wave phenomenon. In as both particles and waves, or things within it will change over 1803, British physicist Thomas depending upon the situation. time. These experiments have led Young’s double-slit experiment Most quanta are elementary, or to the deterministic physics we seemed to provide definitive fundamental, subatomic particles, have talked about in the book up evidence for wavelike light as it not composed of other particles. until this point. However, it was was seen to exhibit interference, When looking at them, they unnerving at the start of the 20th a behavior that could not be appear particle-like, but between century to discover that at its heart explained by particles. In the early observations they behave as if they this is not how nature behaves. years of the 20th century, German are waves. The wavelike behavior is The deterministic world we see physicists Max Planck and Albert not like that of water. If two waves each day is just a blurred picture, Einstein revisited Newton’s ideas combine to make a larger wave, this an average, of a far more unsettling does not increase the energy of a

THE QUANTUM WORLD 207 Werner Heisenberg discovers Austrian Erwin American Richard The IBM Q System the uncertainty principle Schrödinger proposes Feynman proposes One quantum after developing a matrix the idea of quantum computer is mechanics interpretation of his famous “cat in launched. a box” thought computing. quantum theory. experiment. 1927 1935 1981 2019 1926 1927 1964 1985 German Max Born Niels Bohr champions John Bell proposes his Briton David Deutsch gives the probability the Copenhagen theorem to quantitatively publishes a paper interpretation of interpretation of the quantum state probe the mechanism setting out his ideas quantum mechanics. of a particle. behind quantum for a universal entanglement. quantum computer. quantum as it would a water wave. Niels Bohr championed the accurately you measure one Instead, it boosts the probability of Copenhagen interpretation of property, the less accurately a wave being seen in that particular quantum physics. This series you will know the other. location. When we view a quantum, of ideas maintains that the wave it cannot be in all locations at the function represents probabilities Even more bizarre is the same time; instead, it decides upon for final outcomes and it collapses phenomenon of quantum a single location dependent upon the into just one possible outcome entanglement, which allows one probabilities outlined by its wave. when measured by an observer. particle to affect another in an It has been followed up since with entirely different place. When two This was a new, probabilistic a collection of more complicated particles are entangled, even if way of behaving, one where we interpretations. And still the separated by a great distance, they could know all about a quantum debate rages on. are effectively a single system. In at one point in time but never be 1964, Northern Irish physicist John able to predict definitively where it Nothing is certain Stewart Bell presented evidence would be later. These quanta do not That is not where the strangeness that quantum entanglement really behave like sports balls—the flight ends. You can never truly know existed, and French physicist Alain of which is predictable—there is everything about a quantum. Aspect demonstrated this “action always a probability that wherever German physicist Werner at a distance” in 1981. we stand we might get hit. Heisenberg’s uncertainty principle explains that it is impossible to Today, we are learning how The mechanism for how quantum know certain pairs of properties, to utilize these strange quantum objects transition between such as momentum and position, behaviors to do some fantastic wavelike behavior and particle-like with exacting precision. The more things. New technology using measurements has been fiercely quantum principles is set to change debated. In 1927, Danish physicist the world of the near future. ■

208 IN CONTEXT IIDTSNHISDSECIPESOANTNCERTERIIBNGUUYTOOEUFDSLLIYGHT KEY FIGURE Max Planck (1858–1947) ENERGY QUANTA BEFORE 1839 French physicist Edmond Becquerel makes the first observation of the photoelectric effect. 1899 British physicist J.J. Thomson confirms that ultraviolet light can generate electrons from a metal plate. AFTER 1923 American physicist Arthur Compton succeeds in scattering X-rays off electrons, demonstrating that they act like particles. 1929 American chemist Gilbert Lewis coins the name “photons” for light quanta. 1954 American scientists working at Bell Laboratories invent the first practical solar cell. O n October 19, 1900, Max Planck gave a lecture to the German Physical Society (Deutsche Physikalische Gesellschaft) in Berlin. Although it would be a few years before the full implications of his pronouncements became apparent, they marked the beginning of a new age for physics: the era of the quantum. Planck submitted a solution to a problem that had been vexing physicists up until then. The problem involved blackbodies— objects that are perfect absorbers and emitters of all frequencies of electromagnetic radiation. A blackbody is so called because all the radiation striking its surface is

THE QUANTUM WORLD 209 See also: Thermal radiation 112–117 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Electromagnetic waves 192–195 ■ Light from the atom 196–199 ■ Particles and waves 212–215 We are used to thinking In the quantum Electrons absorb or emit of energy as being emitted world, things are very energy quanta across a continuous range. different. Here, energy comes in discrete units, discontinuously. or quanta. absorbed; no radiation is reflected a quantum of violet light has twice Planck’s solution worked—the and the energy it emits depends the frequency, and therefore results of experiments were in solely on its temperature. Perfect twice the energy, of a quantum accord with the predictions made blackbodies don’t exist in nature. of red light. This proportionality by his theory. Yet Planck was not explains why a blackbody does entirely happy and resisted the idea A theory formulated by British not give off energy equally across for years that his quanta had any physicists James Jeans and John the electromagnetic spectrum. basis in reality, viewing them Strutt, Lord Rayleigh, accurately instead as more of a mathematical explained the behavior of Planck’s constant “fix” for a difficult problem. He blackbodies at low frequencies, Planck denoted the constant of could give no good reason why but predicted that, as there was quanta should be true and by his effectively no upper limit to the proportionality as h—now known as own admission had introduced higher frequencies that could be them as “an act of desperation,” but generated, the amount of energy Planck’s constant—and related the it led to the quantum revolution radiated from a blackbody should energy of a quantum to its frequency that transformed physics. continue to increase infinitely. This was dubbed the “ultraviolet by the simple formula E = hv, where The photoelectric effect catastrophe” as it concerned short- E equals energy and v equals When Albert Einstein heard about wavelength radiation beyond the Planck’s theory, he commented, ultraviolet. Everyday observation frequency. The energy of a quantum “It was as if the ground had been ❯❯ shows the prediction to be wrong. can be calculated by multiplying its If it was correct, bakers would be frequency by Planck’s constant, exposed to lethal doses of radiation which is 6.62607015  10–34 J s every time they opened their ovens. (joule-seconds). But in the late 19th century, no one could explain why it was wrong. The photoelectric effect is the emission of electrons by some metals when they are hit by a beam of light. The higher Planck made the radical the frequency of the light, the higher-energy photons it has assumption that the vibrating and the higher the energy of the electrons emitted. atoms in a blackbody emit energy in discrete packets, which he called Low-energy Higher- Very quanta. The size of these quanta photon of energy high-energy is proportional to the frequency red light photon of ultraviolet of vibration. Although there is in No electrons green light photon theory an infinite number of higher emitted Low-energy High-energy frequencies, it takes increasingly from metal electron electron large amounts of energy to release surface quanta at those levels. For example,

210 ENERGY QUANTA The photoelectric effect in space Electric charges can build up on When a spacecraft is exposed to current to flow from one side the exterior of spacecraft such as the prolonged sunlight on one side, of the spacecraft to the other. In SpaceX Dragon unless systems high-energy ultraviolet photons, the early 1970s, before measures are put in place to drain them. or light quanta, striking its were taken to counter this metallic surface cause a steady phenomenon, it disrupted the stream of electrons to be ejected. delicate circuitry of several The loss of electrons causes the Earth-orbiting satellites, even spacecraft to develop a positive causing the complete loss of a charge on the sunlit side, while military satellite in 1973. The the other, shaded, side has a rate of electron loss depends relative negative charge. on the surface material of the spacecraft, the angle at which Without conductors to prevent the sun’s rays strike it, and the charge from building up, the amount of solar activity, difference in charge across the including sunspots. surface will cause an electric pulled out from under us.” In 1904, intensity of the light, but on its sense until the revelations of Einstein wrote to a friend that he frequency. Shifting the beam Planck and Einstein. In March had discovered “in a most simple to higher frequencies, from blue to 1905, Einstein published a paper, way the relation between the size violet and beyond, produces higher- which took Planck’s quanta and of elementary quanta … and the energy electrons; a low-frequency married them to the photoelectric wavelengths of radiation.” This red light beam, even if it is effect, in the monthly Annals of relationship was the answer to a blindingly bright, produces no Physics journal. This paper would curious aspect of radiation that had electrons. It is as if fast-moving eventually win him the Nobel Prize previously defied explanation. In ripples can readily move the sand in 1921. Einstein was particularly 1887, German physicist Heinrich on a beach but a slow-moving interested in the differences Hertz had discovered that certain wave, no matter how big, leaves between particle theories and wave types of metal would emit electrons it untouched. In addition, if the theories. He compared the formulae when a beam of light was directed electrons are going to jump at all, that describe the way the particles at them. This “photoelectric effect” they jump right away: no buildup in a gas behave as it is compressed is similar to the phenomenon of energy is involved. This made no or allowed to expand with those harnessed for use in fiber-optic describing the similar changes as communications (although optical There is no physical analogy radiation spreads through space. fibers are made of semiconductor we can make to understand He found that both obey the same materials rather than metals). what goes on inside atoms. rules, and the mathematics Atoms behave like atoms, underpinning both phenomena is A problem with electrons the same. This gave Einstein a way Physicists at first assumed that nothing else. to calculate the energy of a light the electric field (a region of space John Gribbin quantum of a particular frequency; where electric charge is present) his results agreed with Planck’s. part of the electromagnetic wave British science writer provides the energy that electrons and astrophysicist From here, Einstein went on need to break free. If that were the to show how the photoelectric case, then the brighter the light, effect could be explained by the the more high-energy the emitted existence of light quanta. As electrons should be. That was Planck had established, the energy found not to be the case, however. of a quantum was determined by The energy of the released its frequency. If a single quantum electrons depends not on the transfers its energy to an electron, then the higher the energy of the

THE QUANTUM WORLD 211 Atomic theory and latter’s predictions. Yet Millikan Max Planck quantum mechanics still spoke of Einstein’s “bold, not to demonstrated that say reckless, hypothesis.” It wasn’t Born in Kiel, Germany, in 1858, everything, even space until the experiments conducted Max Planck studied physics at and time, exists in by American physicist Arthur the University of Munich, discrete bits—quanta. Compton in 1923 that the quantum graduating at 17 and gaining Victor J. Stenger theory finally began to gain his doctorate four years later. acceptance. Compton observed the Developing a keen interest in American particle physicist scattering of X-rays from electrons thermodynamics, in 1900 he and provided plausible evidence produced what is now known quantum, the higher the energy of that in scattering experiments, as Planck’s radiation formula, the emitted electron. High-energy light behaves as a stream of introducing the idea of quanta blue photons, as light quanta were particles and cannot be explained of energy. This marked the later named, have the heft to punch purely as a wave phenomenon. In beginning of quantum theory, out electrons; red photons simply his paper, published in the Physical one of the cornerstones of didn’t. Increasing the intensity of Review journal, he explained that 20th-century physics, the light produces greater numbers “this remarkable agreement although its far-reaching of electrons, but it does not create between our formulas and the consequences weren’t more energetic ones. experiments can leave but little understood for several years. doubt that the scattering of X-rays Further experiments is a quantum phenomenon.” In 1918, Planck received Whereas Planck had viewed the the Nobel Prize for Physics for quantum as little more than a Einstein’s explanation of the his achievement. After Adolf mathematical device, Einstein photoelectric effect could be Hitler came to power in 1933, was now suggesting that it was an verified by experiment—light, Planck pleaded in vain with actual physical reality. That didn’t it seemed, acted as if it were a the dictator to abandon his go down well with many other stream of particles. However, light racial policies. He died in physicists who were reluctant to also acted like a wave in familiar Göttingen, Germany, in 1947. give up the idea that light was a and well-understood phenomena wave, and not a stream of particles. such as reflection, refraction, Key works In 1913, even Planck commented diffraction, and interference. So, about Einstein, “That sometimes for physicists, the question still 1900 “On an Improvement … he may have gone overboard in remained: what was light? Was it a of Wien’s Equation for his speculations should not be held wave or was it a particle? Could it the Spectrum” against him.” possibly be both? ■ 1903 Treatise on Thermodynamics Skeptical American physicist Every rascal 1920 The Origin and Robert Millikan performed thinks he knows Development of the experiments on the photoelectric [what light quanta Quantum Theory effect that were aimed at proving that Einstein’s assertion was are], but he is wrong, but they actually produced deluding himself. results entirely in line with the Albert Einstein

212 IN CONTEXT EBTTAVHHENEHEAYRTYATVHSYDEIOEONLEUGNINKHOEATVE KEY FIGURE Louis de Broglie (1892–1987) PARTICLES AND WAVES BEFORE 1670 Isaac Newton develops his corpuscle (particle) theory of light. 1803 Thomas Young performs his double-slit experiment, demonstrating that light behaves like a wave. 1897 British physicist J.J. Thomson announces that electricity is made up of a stream of charged particles, now called “electrons.” AFTER 1926 Austrian physicist Erwin Schrödinger publishes his wave equation. 1927 Danish physicist Niels Bohr develops the Copenhagen interpretation, stating that a particle exists in all possible states until it is observed. T he nature of light lies at the heart of quantum physics. People have tried for centuries to explain what it is. The ancient Greek thinker Aristotle thought of light as a wave traveling through an invisible ether that filled space. Others thought it was a stream of particles that were too small and fast moving to be perceived individually. In 55 bce, the Roman philosopher Lucretius wrote: “The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air.” However, the particle theory did not find much favor and so, for the

THE QUANTUM WORLD 213 See also: Thermal radiation 112–117 ■ Force fields and Maxwell’s equations 142–147 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Electromagnetic waves 192–195 ■ Energy quanta 208–211 ■ Quantum numbers 216–217 Physics isn’t just about so that the light coming through peak, while two troughs make writing equations on a board the hole in the window blind a deeper trough. If a trough and a would pass through the pinholes peak coincide, then they cancel and sitting in front of a and onto a screen. If Newton was each other out. Unfortunately, computer. Science is about right, and light was a stream of Young’s findings were not well particles, then two points of light received as they disagreed with exploring new worlds. would be visible on the screen the great Isaac Newton’s view Suchitra Sebastian where the particles traveled that light was carried by a through the pinholes. But that stream of particles. Indian physicist is not what Young saw. Particles of light next 2,000 years or more, it was Rather than two discrete In the 1860s, Scottish scientist generally accepted that light points of light, he saw a series of James Clerk Maxwell declared traveled in waves. curved, colored bands separated that light was an electromagnetic by dark lines, exactly as would wave. An electromagnetic wave Isaac Newton was fascinated be expected if light were a wave. is made up of two adjoined waves by light and carried out many Young himself had investigated traveling in the same direction but experiments. He demonstrated, interference patterns in waves at right angles to each other. One for example, that white light two years earlier. He had of these waves is an oscillating could be split into a spectrum described how the peak of one magnetic field, the other an of colors by passing it through wave meeting the peak of another oscillating electric field. The two ❯❯ a prism. He observed that light add together to make a higher travels in straight lines and that shadows have sharp edges. It All matter and energy has both wavelike seemed obvious to him that and particlelike characteristics. This is called light was a stream of particles, and not a wave. wave–particle duality. The double-slit experiment In some experiments, a In other experiments, a The formidably talented British beam of light will act like stream of electrons will act scientist Thomas Young theorized that if the wavelength of light was a stream of discrete like a wave. sufficiently short, then it would particles, called quanta. appear to travel in straight lines as if it were a stream of particles. In The experiments that we perform 1803, he put his theory to the test. determine what we see. First, he made a small hole in Neither energy nor matter can a window blind to provide a point ever be seen as both a wave and a source of illumination. Next, he took a piece of board and made particle at the same time. two pinholes in it, placed close together. He positioned his board

214 PARTICLES AND WAVES assuming that light was indeed Wavelength of Velocity at which composed of photons, or discrete particle particle is moving energy quanta. As far as Einstein We have two contradictory was concerned, light quanta were l = h/mv pictures of reality; separately a physical reality, but he struggled neither of them fully explains without success for the rest of his Planck’s Mass of the phenomena of light, but life to resolve the apparent paradox constant particle that light was also demonstrably together they do. wavelike. In an experiment De Broglie’s 1924 equation is Albert Einstein carried out in 1922, American used to calculate the wavelength of a physicist Arthur Compton particle by dividing Planck’s constant fields keep in step with each other succeeded in scattering X-rays by the particle’s momentum—its mass as the wave travels along. When, off electrons. The small change in multiplied by its velocity. in 1900, Max Planck solved the the frequency of the X-rays that blackbody radiation problem by resulted became known as the De Broglie considered Einstein’s assuming that electromagnetic Compton effect and showed that energy was emitted in quanta, he both the X-rays and the electrons famous E = mc2, which relates didn’t actually believe they were were acting like particles when real. The evidence that light was they collided. mass to energy, and the fact that a wave was too overwhelming for Einstein and Planck had related Planck to accept that it was Symmetry of nature energy to the frequency of waves. actually composed of particles. In 1924, Louis de Broglie put By combining the two, de Broglie forward a theory in his doctoral suggested that mass should have In 1905, Albert Einstein thesis that all matter and a wavelike form as well and came showed how the photoelectric energy—not just light—has the up with the concept of a matter effect—which could not be characteristics of both particles wave: any moving object has an explained by the wave theory and waves. De Broglie believed associated wave. The kinetic of light—could be explained by intuitively in the symmetry of energy of the particle is nature and Einstein’s quantum proportional to its frequency theory of light. He asked, if a wave and the speed of the particle can behave like a particle then is inversely proportional to its why can’t a particle such as an wavelength—with faster particles electron also behave like a wave? having shorter wavelengths. Einstein supported de Broglie’s idea as it seemed a natural continuation of his own theories. De Broglie’s claim that electrons could behave like waves was experimentally verified in 1927 when British physicist George Thomson and American physicist Clinton Davisson both demonstrated that This image shows the X-ray diffraction pattern for platinum. X-rays are wavelike forms of electromagnetic energy carried by particles which transmit light called “photons.” The diffraction experiment here shows X-rays behaving like rays, but a different experiment might show them behaving like particles.

THE QUANTUM WORLD 215 a narrow electron beam directed Electrons When particles, such through a thin crystal of nickel as electrons or atoms, formed a diffraction pattern as it are passed through passed through the crystal lattice. a dual-slit apparatus, interference patterns How can it be like that? of light and dark bands Thomas Young had demonstrated are produced, just as that light was a wave by showing happens with waves. how it formed interference patterns. This shows that In the early 1960s, American particles have wavelike physicist Richard Feynman properties and exhibit described a thought experiment wavelike behavior. in which he imagined what would happen if one photon or electron at Screen with Optical Front view of a time was sent toward twin slits two slits screen optical screen that could be opened or closed. The expected result would be that the “know” where to go to build up the Nobody knows how it can be like photons would travel as particles, interference pattern. It is as if each that.” Feynman’s predicted findings arrive as particles, and be detected particle travels as a wave, passes have since been confirmed by on the screen as individual dots. through both slits simultaneously, other scientists. Rather than interference patterns, and creates interference with itself. there should be two bright areas But how does a single particle What is apparent is that both when both slits were open, or just traveling through the left-hand slit the wave theory and the particle one if one of the slits was closed. know whether the right-hand slit is theory of light are correct. Light However, Feynman predicted an open or closed? acts as a wave when it is traveling alternative outcome, that the pattern through space, but as a particle on the screen builds up, particle by Feynman advised against when it is being measured. There particle, into interference patterns even attempting to answer these is no single model that can describe when both slits are open—but not questions. In 1964 he wrote: “Do not light in all its aspects. It is easy if one of the slits is closed. keep saying to yourself, if you can enough to say that light has “wave– possibly avoid it ‘But how can it be particle duality” and leave it at that, Even if subsequent photons are like that?’ because you will go down but what that statement actually fired off after the earlier ones have the drain into a blind alley from means is something that no one hit the screen, they still somehow which nobody has yet escaped. can answer satisfactorily. ■ Louis de Broglie Born in 1892 in Dieppe, France, wave mechanics. De Broglie Louis de Broglie obtained a degree taught theoretical physics at the in history in 1910 and a science Institut Henri Poincaré in Paris degree in 1913. He was conscripted until he retired. In 1929, he was during World War I, and when the awarded the Nobel Prize for war ended he resumed his physics Physics for his discovery of the studies. In 1924, at the Faculty of wave nature of electrons. He Sciences at Paris University, de died in 1987. Broglie delivered his doctoral thesis, “Recherches sur la théorie Key works des quanta” (“Researches on the quantum theory”). This thesis, 1924 “Recherches sur la théorie which was first received with des quanta” (“Researches on astonishment, but later confirmed the quantum theory”), Annals by the discovery of electron of Physics diffraction in 1927, served as the 1926 Ondes et Mouvements basis for developing the theory of (Waves and Motions)

216 OAFNREWEALIDITEYA QUANTUM NUMBERS IN CONTEXT I n 1802, British chemist saying that light could behave and physicist William Hyde like a stream of packets of energy, KEY FIGURE Wollaston noticed that the called quanta. In 1913, Danish Wolfgang Pauli (1900–1958) spectrum of sunlight was overlaid physicist Niels Bohr proposed a by a number of fine dark lines. model of the atom that accounted BEFORE German lens-maker Joseph von for both quanta and the spectra of 1672 Isaac Newton splits Fraunhofer first examined these elements. In Bohr’s atom, electrons white light into a spectrum. lines in detail in 1814, listing more traveled around the core nucleus than 500. In the 1850s, German in fixed, or quantized orbits. Light 1802 William Hyde Wollaston physicist Gustav Kirchhoff and quanta (later called photons) sees dark lines in the solar German chemist Robert Bunsen striking the atom could be spectrum. found that each element produces absorbed by electrons, which then its own unique set of lines, but did moved to higher orbits (further 1913 Niels Bohr puts forward not know what caused them. from the nucleus). A sufficiently his shell model of the atom. energetic photon may eject an Quantum leaps electron from its orbit altogether. AFTER In 1905, Albert Einstein had Conversely, when an electron gives 1927 Niels Bohr proposes the explained the photoelectric effect, up its “extra” energy as a photon Copenhagen interpretation, by which light can cause electrons of light, it falls back down to its stating that a particle exists to be emitted from some metals, original energy level, closer to the in all possible states until it is observed. Fraunhofer lines are fine dark lines that overlay the 680 spectrum of visible light. Each element produces its own 1928 Indian astronomer unique set of Fraunhofer lines (designated by one or more Subrahmanyan Chandrasekhar letters), determined by the quantum numbers of its electrons. calculates that a large enough The most prominent of these lines are shown here. star could collapse to form a black hole at the end of its life. Wavelength (nm) 400 440 480 520 560 600 640 1932 British physicist James Chadwick discovers the neutron. F (Hydrogen) D (Sodium) C (Hydrogen) G’ (Hydrogen) b (Magnesium)

THE QUANTUM WORLD 217 See also: Magnetic monopoles 159 ■ Electromagnetic waves 192–195 ■ Light from the atom 196–199 ■ Energy quanta 208–211 ■ Subatomic particles 242–243 The laws of quantum physics Each electron around an atom prevent identical particles has a unique code made up of from occupying the same four quantum numbers. space at the same time. Two electrons with the same The quantum numbers define Wolfgang Pauli quantum numbers would characteristics of the have to occupy different electron—energy, spin, The son of a chemist, energy levels in an atom. angular momentum, Wolfgang Pauli was born in and magnetism. 1900 in Vienna, Austria. When he was a schoolboy, he is said atom’s nucleus. These steps up and level and the number of electrons to have smuggled Albert down are called “quantum leaps.” that each energy level, or shell, Einstein’s papers on special Their size is unique to each atom. could hold? The reason he came relativity into school to read up with was that each electron at his desk. As a student at Atoms give off light at specific had a unique code, described by the University of Munich in wavelengths, and so each element its four quantum numbers—energy, Germany, Pauli published his produces a characteristic set of spin, angular momentum, and first paper on relativity in spectral lines. Bohr proposed that magnetism. The exclusion principle 1921. This was praised by these lines were related to the stated that no two electrons in an Einstein himself. energy levels of the electron orbits, atom could share the same four and that they were produced by quantum numbers. No two After graduating, Pauli an electron absorbing or emitting identical particles could occupy the assisted physicist Max Born a photon at the frequency that same state at the same time. Two at the University of Göttingen, corresponds to the spectral line. electrons could occupy the same Germany. The threat of Nazi shell but only if they had opposite persecution led him to move The energy level that an electron spins, for example. ■ to Princeton, New Jersey, in can have in an atom is denoted by 1940 and he later became an the primary, or principal, quantum Physics is puzzle solving, but American citizen. He was of puzzles created by nature, awarded the Nobel Prize in number, n, in which n = 1 equals 1945 for his discovery of the the lowest possible orbit, n = 2 the not by the mind of man. exclusion principle. Pauli Maria Goeppert Mayer moved to Zurich, Switzerland, next highest, and so on. Using this in 1946 and worked as a scheme, Bohr was able to describe German–American physicist professor at the city’s the energy level in the simplest Eidgenössische Technische atom, hydrogen, in which a single Hochschule until his death electron orbits a single proton. in 1958. Later models that incorporated the wavelike properties of electrons Key works were able to describe larger atoms. 1926 Quantum Theory Pauli exclusion principle 1933 “Principles of Wave In 1925, Wolfgang Pauli was trying Mechanics” to explain the structure of atoms. What decided an electron’s energy

218 WALALVEISS MATRICES AND WAVES IN CONTEXT B y the 1920s, scientists were light absorbed and emitted by atoms starting to challenge the when an electron jumps from one KEY FIGURE model of the atom that orbit to another. He used these Erwin Schrödinger Danish physicist Niels Bohr had observations to produce tables of (1887–1961) proposed in 1913. Experiments had numbers to represent the positions begun to show not only that light and momentum of electrons and BEFORE could behave like a stream of worked out rules to calculate the 1897 J.J. Thomson discovers particles, but also that electrons values of these properties. the electron and suggests that could act like waves. it is the carrier of electricity. Matrix mechanics German physicist Werner In 1925, Heisenberg shared his 1924 Louis de Broglie Heisenberg tried to develop a calculations with German–Jewish proposes that particles system of quantum mechanics physicist Max Born, who realized have wavelike properties. that relied only on what could be that this type of table was known observed. It was impossible to see as a matrix. Together with Born’s AFTER an electron orbiting an atom directly, student, Pascual Jordan, Born and 1927 Werner Heisenberg but it was possible to observe the Heisenberg worked out a new publishes his uncertainty theory of matrix mechanics that principle. Why do all experiments that could be used to link the energies involve, say, the position of a of electrons to the lines that had 1927 Niels Bohr endorses the been observed in the visible Copenhagen interpretation, particle make the particle light spectrum. claiming that observation suddenly be somewhere determines the quantum rather than everywhere? One of the interesting things state of a particle. about matrix mechanics is that No one knows. the order in which calculations are 1935 Erwin Schrödinger sets Christophe Galfard made is important. Calculating the out the scenario of a cat that is momentum and then the position of simultaneously alive and dead French physicist a particle will give a different result to illustrate a problem with the from calculating the position first Copenhagen interpretation. and then the momentum. It was this difference that would lead Heisenberg to his uncertainty principle, which stated that in quantum mechanics the velocity

THE QUANTUM WORLD 219 See also: Electromagnetic waves 192–195 ■ Light from the atom 196–199 ■ Energy quanta 208–211 ■ Heisenberg’s uncertainty principle 220–221 ■ Antimatter 246 A particle that is behaving We can think of the like a wave has no fixed wave like a “probability position in space. graph” mapping out the chances of finding the particle in a particular place. Schrödinger’s A wave equation is Erwin Schrödinger equation is a wave used to determine the equation that gives us all shape of the probability wave, Erwin Schrödinger was born possible locations of in 1887 in Vienna, Austria, and a particle, such as an or wave function, studied theoretical physics. electron or photon, at a of the particle. After serving in World War I, he held posts at universities given time. in Zurich, Switzerland, and Berlin, Germany. In 1933, as of an object and its position cannot mechanics as Newton’s laws of the Nazis came to power in both be measured exactly at the motion are for events on a large Germany, he moved to Oxford same time. scale. Schrödinger tested his in the UK. equation on the hydrogen atom and In 1926, Austrian physicist found that it predicted its properties That same year, he shared Erwin Schrödinger devised an with great accuracy. the Nobel Prize with British equation that determines how theoretical physicist Paul probability waves, or wave functions In 1928, British physicist Dirac for “the discovery of (mathematical descriptions of a Paul Dirac married Schrödinger’s new productive forms of quantum system) are shaped and equation to Einstein’s special atomic theory.” In 1939, he how they evolve. The Schrödinger relativity, which had demonstrated became director of theoretical equation is as important to the the link between mass and energy, physics at the Institute of subatomic world of quantum encapsulated in the famous Advanced Studies in Dublin, Ireland. He retired to Vienna Can nature possibly be so E = mc2 equation. Dirac’s equation in 1956, and died in 1961. absurd as it seemed to us in Fascinated by philosophy, these atomic experiments? was consistent with both special Schrödinger remains best relativity and quantum mechanics in known for his 1935 thought Werner Heisenberg its description of electrons and other experiment, Schrödinger’s particles. He proposed that electrons cat, which examined the should be viewed as arising from an idea that a quantum system electron field, just as photons arose could exist in two different from the electromagnetic field. states simultaneously. A combination of Heisenberg’s Key works matrices and Schrödinger’s and Dirac’s equations laid the basis 1926 “An undulatory theory for two fundamentals of quantum of the mechanics of atoms and mechanics, the uncertainty molecules,” Physical Review principle, and the Copenhagen 1935 “The Present Situation interpretation. ■ in Quantum Mechanics”

220 TAHLIEVCEAATNIDS DBEOATDH HEISENBERG’S UNCERTAINTY PRINCIPLE IN CONTEXT According to quantum theory, until a particle is observed and measured, it simultaneously exists in all the possible locations KEY FIGURE Werner Heisenberg and states that it could be in. This is called superposition. (1901–1976) All the possible states a Erwin Schrödinger compared BEFORE particle might be in are this to a cat that is 1905 Einstein proposes that light is made up of discrete described by its simultaneously alive packets called photons. wave function. and dead. 1913 Niels Bohr sets out The particle’s properties The collapse of a model of the atom with have no definite the wave function fixes the electrons around a nucleus. properties of the particle. value until we measure 1924 French physicist Louis them—when we do so, the de Broglie suggests that wave function collapses. particles of matter can also be considered as waves. I n classical physics, it was interacting with it. To determine generally accepted that the the position of an electron, we AFTER accuracy of any measurement bounce a photon off it. The accuracy 1935 Einstein, American– was only limited by the precision of the measurement is determined Russian physicist Boris of the instruments used. In 1927, by the wavelength of the photon; Podolsky, and Israeli physicist Werner Heisenberg showed that the higher the frequency of the Nathan Rosen publish the this just wasn’t so. photon, the more accurate the “EPR paradox,” challenging the position of the electron. Copenhagen interpretation. Heisenberg asked himself what it actually meant to define Max Planck had shown that the 1957 American physicist the position of a particle. We can energy of a photon is related to its Hugh Everett puts forward only know where something is by his “many worlds theory” frequency by the formula E = hn, to explain the Copenhagen interpretation.

THE QUANTUM WORLD 221 See also: Energy and motion 56–57 ■ Lumpy and wavelike light 176–179 ■ Light from the atom 196–199 ■ Energy quanta 208–211 ■ Particles and waves 212–215 ■ Matrices and waves 218–219 Werner Heisenberg Werner Heisenberg was born in project. Whether this failed due 1901 in Würzburg, Germany. He to a lack of resources or because went to work with Niels Bohr at Heisenberg wanted it to fail is Copenhagen University in 1924. unclear. He was eventually Heisenberg’s name will always be taken prisoner by American associated with his uncertainty troops and sent to the UK. principle, published in 1927, and After the war, he served as he was awarded the Nobel Prize the director of the Max Planck for Physics in 1932 for the creation Institute until he resigned of quantum mechanics. in 1970. He died in 1976. In 1941, during World War II, Key works Heisenberg was appointed director of the Kaiser Wilhelm 1925 “On Quantum Mechanics” Institute for Physics (later 1927 “On the Perceptual renamed the Max Planck Institute) Content of Quantum Theoretical in Munich, and was put in charge Kinematics and Mechanics” of Nazi Germany’s atomic bomb where E equals energy, n equals about the universe. The properties poison that will be released when frequency, and h is Planck’s of a quantum particle have no a quantum event occurs. According definite value until a measurement to the Copenhagen interpretation, constant. The higher the frequency is made. It is impossible to design the cat is in the superposition state of the photon, the more energy it an experiment that would allow us of being both alive and dead until carries and the more it will knock to see an electron as wave and someone looks in the box, which the electron off course. We know particle at the same time, for Schrödinger thought was where the electron is at that example. The wave and particle ridiculous. Bohr retorted that there moment, but we cannot know nature of matter are two sides of was no reason why the rules of where it is going. If it were possible the same coin, Bohr suggested. The classical physics should also apply to measure the electron’s momentum Copenhagen interpretation opened to the quantum realm—this was with absolute precision, its location a sharp divide between classical just the way things were. ■ would become completely uncertain, physics and quantum physics over and vice versa. whether or not physical systems The atoms or elementary have definite properties prior to particles themselves are Heisenberg showed that the being measured. not real; they form a world uncertainty in the momentum of potentialities or possibilities multiplied by the uncertainty in Schrödinger’s cat the position can never be smaller According to the Copenhagen rather than one of than a fraction of Planck’s constant. interpretation, any quantum state things or facts. The uncertainty principle is a can be seen as the sum of two or fundamental property of the more distinct states, known as Werner Heisenberg universe that puts a limit on what superposition, until it is observed, we can know simultaneously. at which point it becomes either one or the other. Copenhagen interpretation What came to be known as the Erwin Schrödinger asked: “Copenhagen interpretation” of when is the switch made from quantum physics was championed superposition into one definite by Niels Bohr. It accepted that, as reality? He described the scenario Heisenberg had shown, there are of a cat in a box, along with a some things we simply cannot know

222 SATPOAODKIYSTAACNTCIOEN QUANTUM ENTANGLEMENT IN CONTEXT O ne of the main tenets of of momentum laws tell us that the quantum mechanics is the momentum of one particle is equal KEY FIGURE idea of uncertainty—we and opposite to that of the other. John Stewart Bell cannot measure all the features According to the Copenhagen (1928–1990) of a system simultaneously, no interpretation, neither particle matter how perfect the experiment. will have a definite state until it BEFORE The Copenhagen interpretation of is measured, but measuring the 1905 Albert Einstein publishes quantum physics championed by momentum of one will determine his theory of special relativity, Niels Bohr effectively says that the the state and momentum of the which is based in part on the very act of measurement selects the other, regardless of the distance idea that nothing can travel characteristics that are observed. between the particles. faster than the speed of light. Another peculiar property of This is known as “non-local 1926 Erwin Schrödinger quantum mechanics is called behavior,” though Albert Einstein publishes his wave equation. “entanglement.” If two electrons, called it “spooky action at a for example, are ejected from a distance.” In 1935, Einstein attacked 1927 Niels Bohr champions quantum system, then conservation entanglement, claiming that there the Copenhagen interpretation of the way in which quantum When any two subatomic particles, such as electrons, systems interact with the interact with each other, their states become large-scale world. interdependent—they are entangled. AFTER The particles remain connected even when 1981 Richard Feynman physically separated by an enormous distance proposes superposition and the entanglement of particles as (for example, in different galaxies). basis for a quantum computer. Measuring the properties As a result, manipulating 1995 Austrian quantum of one particle gives one particle physicist Anton Zeilinger demonstrates wave/particle information about the instantaneously alters switching in an experiment properties of the other. its partner. using entangled photons.

THE QUANTUM WORLD 223 See also: Energy quanta 208–211 ■ Heisenberg’s uncertainty principle 220–221 ■ Quantum applications 226–231 ■ Special relativity 276–279 Particle A and particle B have interacted with Pair of entangled particles each other and become entangled. They will remain sent in different directions entangled even if they are sent in different directions. Particles entangled even when separated Particle A Particle B John Stewart Bell are “hidden variables“ at work that correlate with that of particle B if John Stewart Bell was born make it unnecessary. He argued normal probability (as opposed to in 1928 in Belfast, Northern that for one particle to affect the quantum entanglement) is at work. Ireland. After graduating from other, a faster-than-light signal The statistical distribution of his Queen’s University Belfast, between them (which was forbidden results proved mathematically that he went on to gain a PhD in by Einstein’s theory of special Einstein’s “hidden variables” nuclear physics and quantum relativity) would be required. idea isn’t true, and that there is field theory at the University instantaneous connection between of Birmingham. He then Bell’s theorem entangled particles. Physicist Fritjof worked at the Atomic Research In 1964, Northern Irish physicist Capra maintains that Bell’s theorem Establishment in Harwell, John Stewart Bell proposed an shows that the universe is UK, and later at the European experiment that could test whether “fundamentally interconnected.” Organization for Nuclear or not entangled particles actually Research (CERN) in Geneva, did communicate with each other Experiments such as the one Switzerland. There, he worked faster than light. He imagined the carried out by French physicist on theoretical particle science case of a pair of entangled electrons, Alain Aspect in the early 1980s and accelerator design. After one with spin up and the other (which used entangled photon a year spent at Stanford, with spin down. According to pairs generated by laser) Wisconsin-Madison, and quantum theory, the two electrons demonstrate convincingly that Brandeis universities in are in a superposition of states until “action at a distance” is real—the the US, Bell published his they are measured—either one quantum realm is not bound by breakthrough paper in 1964, of them could be spin up or spin rules of locality. When two particles which proposed a way to down. However, as soon as one is are entangled, they are effectively distinguish between quantum measured, we know with certainty a single system that has a single theory and Einstein’s notion that the other has to be its opposite. quantum function. ■ of local reality. He was elected Bell derived formulas, called the Bell a member of the American inequalities, which determine how Academy of Arts and Sciences often the spin of particle A should in 1987. Bell’s early death in 1990 at the age of 62 meant This conceptual computer artwork he did not live to see his ideas shows a pair of particles that have tested experimentally. become entangled: manipulating one will result in manipulating the other, Key work regardless of the distance between them. Entanglement has applications 1964 “On the Einstein- in the new technologies of quantum Podolsky-Rosen Paradox,” computing and quantum cryptography. Physics

224 OTHFEPHJEYWSIECLS QUANTUM FIELD THEORY IN CONTEXT A field maps out the O ne of quantum mechanics’ strength of a force across biggest shortcomings is KEY FIGURE that it fails to take Richard Feynman space and time. Einstein’s relativity theories into (1918–1988) Quantum field theory says account. One of the first to try that forces are transmitted by reconciling these cornerstones BEFORE of modern physics was British 1873 James Clerk Maxwell force-carrier particles. physicist Paul Dirac. Published publishes his equations The force carriers of the in 1928, the Dirac equation describing the properties electromagnetic force viewed electrons as excitations of of the electromagnetic field. an electron field, in the same way are photons. that photons could be seen as 1905 Einstein proposes that The ways in which photon excitations of the electromagnetic light, as well as acting like exchange interactions take field. The equation became one a wave, can be imagined place can be visualized using of the foundations of quantum as a stream of particles field theory. called quanta. a Feynman diagram. The idea of fields carrying AFTER forces across a distance is well 1968 Theoretical physicists established in physics. A field can Sheldon Glashow, Abdus be thought of as anything that has Salam, and Steven Weinberg values that vary across space and present their theory of the time. For example, the pattern electroweak force, uniting made by iron filings scattered the electromagnetic and around a bar magnet maps out the weak nuclear forces. lines of force in a magnetic field. 1968 Experiments at the In the 1920s, quantum field Stanford Linear Accelerator theory proposed a different Laboratory, CA, discover approach, which suggested that evidence of quarks, the forces were carried by means building blocks of of quantum particles, such as subatomic particles. photons (particles of light that are the carrier particles of electromagnetism). Other particles that were discovered subsequently,

THE QUANTUM WORLD 225 See also: Force fields and Maxwell’s equations 142–147 ■ Quantum applications 226–231 ■ The particle zoo and quarks 256–257 ■ Force carriers 258–259 ■ Quantum electrodynamics 260 ■ The Higgs boson 262–263 Tomonaga in Japan. It proposes Departing that charged particles, such as electrons electrons, interact with each other by emitting and absorbing photons. e- Exchange e- This can be visualized by means of Feynman diagrams, developed by of photon Richard Feynman (see right). What we observe is not TIME nature itself, but nature QED is one of the most exposed to our method astonishingly accurate theories e- Approaching e- ever formulated. Its prediction for electrons of questioning. the strength of the magnetic field Werner Heisenberg associated with an electron S PA C E is so close to the value produced, This Feynman diagram represents such as the quark, the gluon, and that if the distance from London electromagnetic repulsion. Two the Higgs boson (an elementary to Timbuktu was measured to electrons (e-) approach, exchange particle which gives particles their the same precision, it would be a photon, then move apart. mass) are believed to have their accurate to within a hair’s breadth. own associated fields. interaction, or strong nuclear force, The Standard Model which binds together the protons QED QED was a stepping stone toward and neutrons in the nucleus of an Quantum electrodynamics, or QED, building quantum field theories for atom. The electroweak theory is the quantum field theory that the other fundamental forces of proposes that the electromagnetic deals with electromagnetic force. nature. The Standard Model and weak interactions can be The theory of QED was fully (and combines two theories of particle considered two facets of a single, independently) developed by physics into a single framework, “electroweak,” interaction. The Richard Feynman and Julian which describes three of the four Standard Model also classifies Schwinger in the US and Shin’ichiro¯ known fundamental forces (the all known elementary particles. electromagnetic, weak, and strong Reconciling the gravitational force interactions), but not gravity. The with the Standard Model remains strongest of these is the strong one of physics’ biggest challenges. ■ Richard Feynman Born in 1918, Richard Feynman His autobiography, Surely You’re grew up in New York City. Joking, Mr. Feynman!, is one of Fascinated by math from an the best-selling books written early age, he won a scholarship by a scientist. One of his last to the Massachusetts Institute major achievements was of Technology and achieved uncovering the cause of NASA’s a perfect score in his PhD 1986 Challenger space shuttle application to Princeton. In 1942, disaster. He died in 1988. he joined the Manhattan Project to develop the first atomic bomb. Key works After the war, Feynman 1967 The Character of worked at Cornell University, Physical Law where he developed the QED 1985 QED: The Strange Theory theory, for which he was jointly of Light and Matter awarded the Nobel Prize in 1965. 1985 Surely You’re Joking, He moved to the California Mr. Feynman! Institute of Technology in 1960.

UBCEONTLWILEVAEBNEOPRRARSAATLEILOSENL QUANTUM APPLICATIONS



228 QUANTUM APPLICATIONS IN CONTEXT low temperatures. When the The Meissner effect is the temperature of the mercury reached expulsion of a magnetic field from a KEY FIGURE 268.95°C, its electrical resistance superconducting material, causing David Deutsch (1953–) disappeared. This meant that, a magnet to levitate above it. theoretically, an electrical current BEFORE could flow through a loop of super- of these junctions, known as 1911 Heike Kamerlingh Onnes cooled mercury forever. “Josephson junctions,” the current discovers superconductivity in vibrates at a very high frequency. super-cooled mercury. American physicists John Frequencies can be measured Bardeen, Leon Cooper, and John with much greater precision than 1981 Richard Feynman puts Schrieffer came up with an voltages, and Josephson junctions forward the idea of a quantum explanation for this strange have been used in devices such computer. phenomenon in 1957. At very low as SQUIDS (superconducting temperatures, electrons form quantum interference devices) to 1981 French physicist Alain so-called “Cooper pairs.” Whereas examine the tiny magnetic fields Aspect proves that quantum a single electron has to obey the produced by the human brain. entanglement, the linking of Pauli exclusion principle, which They also have potential for use paired particles, takes place. forbids two electrons from sharing in ultrafast computers. the same quantum state, the 1988 American physicist Cooper pairs form a “condensate.” Without superconductivity, the Mark Reed coins the term This means that the pairs act as if powerful magnetic forces harnessed “quantum dots.” they were a single body, with no in magnetic resonance imaging resistance from the conducting (MRI) scanners would not be AFTER material, rather than a collection possible. Superconductors also act 2009 Researchers at Yale of electrons flowing through the to expel magnetic fields. This is the University use a 2-qubit conductor. Bardeen, Cooper, and Meissner effect, a phenomenon (quantum bit) superconducting Schrieffer won the 1972 Nobel Prize that has allowed the building of chip to create the first solid- in Physics for this discovery. magnetically-levitated trains. state quantum processor. In 1962, Welsh physicist Brian Superfluids 2019 The IBM Q System One Josephson predicted that Cooper Kamerlingh Onnes was also the first with 20 qubits is launched. pairs should be able to tunnel person to liquefy helium. In 1938, through an insulating barrier Russian physicist Pyotr Kapitsa and T he grand realm of quantum between two superconductors. British physicists John Allen and physics seems a long way If a voltage is applied across one Don Misener discovered that below from the common-sense everyday world, but it has given Technology at the forefront rise to a surprising number of of human endeavor is technological advances that play a hard. But that is what vital role in our lives. Computers and makes it worth it. semiconductors, communications Michelle Yvonne networks and the internet, GPS and Simmons MRI scanners—all depend on the quantum world. Professor of quantum physics Superconductors In 1911, Dutch physicist Heike Kamerlingh Onnes made a remarkable discovery when he was experimenting with mercury at very

THE QUANTUM WORLD 229 See also: Fluids 76–79 ■ Electric current and resistance 130–133 ■ Electronics 152–155 ■ Particles and waves 212–215 ■ Heisenberg’s uncertainty principle 220–221 ■ Quantum entanglement 222–223 ■ Gravitational waves 312–315 A blue laser beam excites Quantum clocks absolute zero temperatures strontium atoms in the 3-D quantum by slowing them with lasers. gas atomic clock developed Reliable timekeeping is crucial in by physicists at JILA in synchronizing the activities that Today, the most accurate Boulder, Colorado. our technological world depends atomic clocks measure time on. The most accurate clocks in using the transitions between the world today are atomic clocks, spin states in the nucleus of which use atoms “jumping” back an atom. Until recently, they and forth between energy states used atoms of the cesium-133 as their “pendulum.” isotope. These clocks can achieve an accuracy equivalent The first accurate atomic clock to gaining or losing a second was built in 1955 by physicist over 300 million years and Louis Essen at the UK’s National are crucial to GPS navigation Physical Laboratory. The late and telecommunications. The 1990s saw major advances in most recent atomic clock uses atomic clock technology, including strontium atoms and is believed the cooling of atoms to near to be even more accurate. 270.98°C, liquid helium completely in the 1920s by German physicist semiconductors. This can cause lost its viscosity, seemingly flowing Friedrich Hund and others, this problems, as chips become smaller without any apparent friction and phenomenon allows particles to and the insulating layers between possessing a thermal conductivity pass through barriers that would components become too thin to that far outstripped that of the best traditionally be impassable. This stop the electrons at all—effectively metal conductors. When helium oddity arises from considering making it impossible to turn the is in its superfluid state, it does electrons, for example, as waves of device off. not behave like it does at higher probability rather than particles temperatures. It flows over the rim existing at a particular point. In Quantum imaging of a container and leaks through transistors, for example, quantum An electron microscope depends the smallest of holes. When set tunneling allows electrons to pass on the wave/particle duality of rotating, it does not stop spinning. across a junction between electrons. It works in a similar ❯❯ The Infrared Astronomical Satellite (IRAS) launched in 1983 was cooled In classical physics, an object—for example, a rolling ball— Quantum by superfluid helium. cannot move across a barrier without gaining enough energy particle to get over it. A quantum particle, however, has wavelike passes A fluid becomes a superfluid properties, and we can never know exactly how much energy through when its atoms begin to occupy the it has. It may have enough energy to pass through a barrier. barrier same quantum states; in essence, Rolling ball they lose their individual identities and become a single entity. Superfluids are the only quantum phenomenon that can be observed by the naked eye. Tunneling and transistors Classical Quantum Some of the devices we use daily, physics tunneling such as the touchscreen technology of mobile phones, would not work if not for the strange phenomenon of quantum tunneling. First explored

230 QUANTUM APPLICATIONS Magnetic resonance imaging (MRI) changing the way they are an individual atom. The quantum scanners produce detailed images, magnetized. When the radio waves dot can absorb and emit energy such as this image of the human brain, are switched off, the protons as electrons move between levels. using a magnetic field and radio waves. return to the previous spin state, The frequency of the light emitted emitting a signal which is recorded depends on the spacing between way to a light microscope, but electronically and turned into the levels, which is determined instead of using lenses to focus images of the body tissues they by the size of the dot—larger dots light, it employs magnets to focus form a part of. glow at the red end of the spectrum beams of electrons. As the stream and smaller dots at the blue. The of electrons passes through the Quantum dots luminosity of the dots can be tuned magnetic “lens,” they behave like Quantum dots are nanoparticles, precisely by varying their size. particles as the magnet bends their typically consisting of just a path, focusing them on the sample few dozen atoms, made of Quantum computing to be examined. Now they act like semiconducting materials. They Quantum dot technology is waves, diffracting around the object were first created by Russian likely to be used in constructing and on to a fluorescent screen where physicist Alexey Ekimov and quantum computers. Computers an image is formed. The wavelength American physicist Louis Brus, depend on binary bits of of an electron is much smaller than who were working independently information, corresponding to the that of light, allowing the image in the early 1980s. The electrons on (1) and off (0) positions of their resolution of objects millions of within semiconductors are locked electronic switches. Spin is a times smaller than can be seen inside the crystal lattice that makes quantum property that seems to with a light microscope. up the material, but can be freed if turn up quite often in quantum they are excited by photons. When technology. It is the spin of the Magnetic resonance imaging the electrons are free, the electrical electron that gives some materials (MRI) was developed in the 1970s by resistance of the semiconductor their magnetic properties. Using researchers such as Paul Lauterbur falls rapidly, allowing current to lasers, it is possible to get electrons in the US and Peter Mansfield in flow more easily. into a superposition state where the UK. Inside an MRI scanner, the they have both up and down spin at patient is surrounded by a magnetic Quantum dot technology can the same time. These superposition field many thousands of times more be used in television and computer electrons can theoretically be used powerful than that of Earth, screens to produce finely detailed as qubits (quantum bits) which produced by a superconducting images. Quantum dots can be effectively can be “on,” “off,” and electromagnet. This magnetic field precisely controlled to do all kinds something in between, all at the affects the spin of the protons that of useful things. The electrons in same time. Other particles, such as make up the hydrogen atoms in the a quantum dot each occupy a polarized photons, can also be used water molecules that constitute different quantum state, so the dot as qubits. It was Richard Feynman so much of a human body, has discrete energy levels just like magnetizing them in a particular In display technology, way. Radio waves are then used Blue light quantum dots change their size to alter the spin of the protons, from LED and shape to emit light of specific colors when they are stimulated Surface molecules by blue light from LEDs. stabilize quantum dot and increase Core absorbs its efficiency blue light and emits red light Red light from quantum dot

THE QUANTUM WORLD 231 Normal computers use on/off switches to store binary information (1s and 0s). Calculations must be gone Qubits in quantum through step by step. computers can be “on” and “off” at the same time. Entanglement between qubits enables David Deutsch quantum computers to perform many calculations Born in Haifa, Israel, in simultaneously. 1953, David Deutsch is one of the pioneers of quantum who first suggested, in 1981, that become a reality, the problem computing. He studied physics enormous computing power would of decoherence will have to be in the UK, at Cambridge and be unleashed if the superposition solved. The smallest disturbance Oxford. He then spent several state could be exploited. Potentially, will collapse, or decohere, the years working in the US at the qubits can be used to encode and superposition state. Quantum University of Texas at Austin, process vastly more information computing could avoid this by before returning to Oxford than the simple binary computer bit. making use of the phenomenon University. Deutsch is a of quantum entanglement, which founding member of the Centre In 1985, British physicist David is what Einstein called “spooky for Quantum Computation at Deutsch began to set out ideas on action at a distance.” This allows Oxford University. how such a quantum computer one particle to affect another could actually work. The field of somewhere else and could allow In 1985, Deutsch wrote computer science is in large part the value of the qubits to be a groundbreaking paper, built on the idea of the “universal determined indirectly. ■ “Quantum Theory, the computer,” first suggested by Church–Turing Principle British mathematician Alan Turing It’s as if you were trying and the Universal Quantum in the 1930s. Deutsch pointed out to do a complex jigsaw puzzle Computer,” which set out his that Turing’s concept was limited in the dark with your hands ideas for a universal quantum by its reliance on classical physics computer. His discovery of the and would therefore represent only tied behind your back. first quantum algorithms, his a subset of possible computers. Brian Clegg theory of quantum logic gates, Deutsch proposed a universal and his ideas on quantum computer based on quantum British science writer, computational networks are physics and began rewriting on quantum computing among the most important Turing’s work in quantum terms. advances in the field. The power of qubits is such Key works that just 10 are sufficient to allow the simultaneous processing of 1985 “Quantum Theory, 1,023 numbers; with 40 qubits, the Church–Turing Principle the possible number of parallel and the Universal Quantum computations will exceed 1 trillion. Computer,” Proceedings of Before quantum computers the Royal Society 1997 The Fabric of Reality 2011 The Beginning of Infinity

PNAURCTLIECALRE inside the atom

PAHNYDSICS

234 INTRODUCTION Henri Becquerel The experiments of Ernest Rutherford studies James Chadwick discovers that German physicist Hans the hydrogen nucleus, a discovers the some atoms emit radiation Geiger and British positively charged electrically neutral spontaneously. physicist Ernest Marsden particle found in every neutron particles atomic nucleus. He will lead to the discovery of that exist inside the atomic nucleus. later dub it a proton. atomic nuclei. 1896 1911 1919 1932 1897 1912 1928 J.J. Thomson discovers Austrian physicist Victor British physicist Paul Dirac the first subatomic Hess discovers the existence proposes a mirror world of antimatter and a particle: the electron. of cosmic rays by measuring ionization rates positively charged electron, from an atmospheric balloon. later named the positron. T he idea of atoms as tiny particles more than a thousand of subatomic particles he named particles of matter dates times lighter than a hydrogen by the first three letters of the back to the ancient world, atom; these subatomic particles Greek alphabet. their seemingly indivisible nature were later named electrons. enshrined in their Greek name Rutherford and other physicists atomos (“uncuttable”). British Exploring the nucleus used alpha particles as tiny physicist John Dalton, who Becquerel’s student Marie Curie projectiles, firing them into atoms proposed his atomic theory in proposed that such rays came from to search for smaller structures. 1803, remained convinced of their within the atom, rather than being Most passed through the atoms, indestructible nature—as did most the result of chemical reactions— but a small fraction bounced almost 19th-century scientists. However, an indication that atoms contained entirely backward in the direction in the late 1890s, some researchers smaller particles. In 1899, New they were fired from. The only began to challenge this view. Zealand-born physicist Ernest possible explanation appeared to Rutherford confirmed that there be that a densely packed region of In 1896, French physicist Henri are different types of radiation. positive charge within the atom was Becquerel discovered by chance, He named two—alpha rays, which repelling them. Danish physicist when experimenting with X-rays, he later recognized were positively Niels Bohr worked with Rutherford that the uranium salt coating charged helium atoms, and beta to produce, in 1913, a new model of his photographic plate emitted rays, negatively charged electrons. an atom with electrically positive radiation spontaneously. A year In 1900, French scientist Paul nucleus surrounded by light later, British physicist J.J. Thomson Villard discovered a high-energy electrons, orbiting like planets. deduced that the rays he produced light, which Rutherford called in a cathode ray experiment were gamma rays to complete the trio Further research led physicists made up of negatively charged to suggest that other particles must exist to make up the mass

NUCLEAR AND PARTICLE PHYSICS 235 American physicist Carl American physicist The UA1 and UA2 D. Anderson discovers the Sheldon Glashow and experiments at CERN muon, a heavier cousin of the electron and the first in Pakistani physicist in Switzerland the second generation of Abdus Salam propose uncover the weak fundamental particles. that electromagnetic force carriers, the and weak forces merge W and Z bosons. at high temperatures. 1936 1959 1983 1935 1956 1964 2012 Hideki Yukawa predicts American physicists Murray Gell-Mann CERN announces the the existence of mesons— Frederick Reines and Clyde first uses the word discovery of the Higgs particles exchanged between quark to denote the boson, the final piece of protons and neutrons in the Cowan discover the the Standard Model nucleus to provide a strong neutrino—26 years after its smallest type of theory of particle physics. elementary particle. binding nuclear force. prediction by Austrian physicist Wolfgang Pauli. of the nucleus: Rutherford’s 1919 reality as the first commercial The neutrino, first proposed in discovery of protons provided the nuclear power reactors opened in 1930 to explain missing energy positive electric charge, while the US and UK in the 1950s. from beta radiation, was detected electrically neutral neutrons were in 1956. Heavier versions of the identified by British physicist Meanwhile, particle research electron and quarks were detected; James Chadwick in 1932. continued, and ever more powerful from their interactions, physicists particle accelerators uncovered a began to piece together a picture More particles revealed host of new particles, including of how these particles exchanged Among the next conundrums to kaons and baryons, which decayed forces and changed from one type solve was why positively charged more slowly than expected. Various to another. The go-betweens that protons within a nucleus did not scientists, including American made everything happen—the push the nucleus apart. Japanese physicist Murray Gell-Mann, force-carrying bosons—were also physicist Hideki Yukawa supplied an dubbed this quality of long decay discovered, and the Higgs boson answer in 1935, proposing that an “strangeness” and used it to completed the picture in 2012. ultra short-range force (the strong classify the subatomic particles nuclear force), carried by a particle that exhibit it into familiar groups, However, the Standard Model— called a meson, held them together. according to their properties. the theory explaining the four Gell-Mann later coined the name basic forces, force carriers, and When two nuclear bombs were quark for the constituents that the fundamental particles of dropped on Japan to end World War determined such properties, and matter—has known limitations. II in 1945, the lethal force of nuclear established different “flavors” of Modern particle physics is starting power became patently clear. Yet quarks—initially up, down, and to push these limits in pursuit of its use in peacetime to produce strange quarks. Charm, top, and dark matter, dark energy, and a clue domestic energy also became a bottom quarks came later. as to the origin of matter itself. ■

236 IDMNIAFVTIINSTIIETBRELLEISY NOT ATOMIC THEORY IN CONTEXT T he concept of atoms dates four or five elements (usually earth, back to the ancient world. air, fire, and water) and categorized KEY FIGURE The Greek philosophers elements as oxygen, hydrogen, John Dalton (1766–1844) Democritus and Leucippus, for carbon, and others, but they instance, proposed that eternal had yet to discover what made BEFORE “atoms” (from atomos, meaning each unique. c. 400 bce The ancient Greek uncuttable) make up all matter. philosophers Leucippus and These ideas were revived in Europe Dalton’s theory Democritus theorize that in the 17th and 18th centuries as An explanation arrived with atomic everything is composed of scientists experimented with theory, which was developed in “uncuttable” atoms. combining elements to create the early 19th century by British other materials. They shifted scientist John Dalton. Dalton 1794 French chemist away from historic models of proposed that if the same pair of Joseph Proust finds that elements always join in Atoms make up all Each element is made up the same proportions to matter. of one type of atom. form compounds. Ratios depend on the Different elements join to AFTER relative masses of the form compounds in simple 1811 Italian chemist Amedeo Avogadro proposes gases are atoms making up mass ratios. composed of molecules of two each element. or more atoms, drawing a distinction between atoms Each element has its own unique atomic mass. and molecules. 1897 British physicist J.J. Thomson discovers the electron. 1905 Albert Einstein uses mathematics to provide evidence for Dalton’s theory.

NUCLEAR AND PARTICLE PHYSICS 237 See also: Models of matter 68–71 ■ Light from the atom 196–199 ■ Particles and waves 212–215 ■ The nucleus 240–241 ■ Subatomic particles 242–243 ■ Antimatter 246 ■ Nuclear bombs and power 248–251 elements could be combined in John Dalton used wooden balls in water molecules couldn’t be seen, different ways to form different public demonstrations of his theory Einstein proposed that occasionally compounds, the ratio of these of atoms. He imagined atoms as hard, a small group would mostly move elements’ masses could be consistent spheres. in the same direction and that this represented with whole numbers. was enough to “push” a grain of He noticed, for instance, that the or destroy a particle of hydrogen.” pollen. Einstein’s mathematical mass of oxygen in pure water However, matter could be changed description of Brownian motion was nearly eight times the mass by separating particles that are made it possible to calculate the of hydrogen, so whatever made combined and joining them to other size of an atom or molecule based up oxygen must weigh more than particles to form new compounds. on how fast the pollen grain moved. whatever made up hydrogen. (It was later shown that one oxygen Dalton’s model made accurate, While later scientific advances atom weighs 16 times that of a verifiable predictions and marked revealed that there was more inside hydrogen atom—since a water the first time that scientific the atom than Dalton could ever molecule contains one oxygen atom experimentation had been used have imagined, his atomic theory and two hydrogen atoms, this fits to demonstrate atomic theory. set the foundation for chemistry with Dalton’s discovery.) and many areas of physics. ■ Mathematical evidence Dalton concluded that each Evidence to support Dalton’s theory element was composed of its own followed in 1905 when a young unique particles: atoms. These Albert Einstein published a paper could be joined to or broken apart explaining the erratic jiggling from other atoms, but they could of pollen particles in water—the not be divided, created, or so-called Brownian motion destroyed. “No new creation or described in 1827 by Scottish destruction of matter is within botanist Robert Brown. the reach of chemical agency,” he wrote. “We might as well attempt Einstein used mathematics to introduce a new planet into the to describe how pollen was solar system, or to annihilate one bombarded by individual water already in existence, as to create molecules. Although the overall random movement of these many John Dalton John Dalton was born in 1766 in meteorologist. He became a England’s Lake District to a poor high-profile scientist, giving Quaker family. He began working lectures at the Royal Institution to support himself at the age of 10 in London. After his death in and taught himself science and 1944, he was granted a public mathematics. He took up teaching funeral with full honors. at Manchester’s New College in 1793 and went on to propose a Key works theory (later disproved) on the cause of color blindness, a 1806 Experimental Enquiry into condition he shared with his the Proportion of the Several brother. In 1800, he became Gases or Elastic Fluids, secretary of the Manchester Constituting the Atmosphere Literary and Philosophical Society 1808 and 1810 A New System and wrote influential essays of Chemical Philosophy, 1 and 2 describing his experiments with gases. He was also an avid

238 OTARFVAEMNRASITTFTAOEBRRLMEATION NUCLEAR RAYS IN CONTEXT U ntil the end of the 19th coated with a uranium salt century, scientists believed (potassium uranyl-sulfate) in KEY FIGURE that matter emits radiation a drawer. Despite remaining in Marie Curie (1867–1934) (such as visible and ultraviolet light) darkness, strong outlines of the only when stimulated, such as by sample developed on the plate. BEFORE heating. This changed in 1896 Becquerel concluded that the 1857 Frenchman Abel Niepce when French physicist Henri uranium salt was emitting de Saint-Victor observes that Becquerel conducted an experiment radiation on its own. certain salts can expose a into a newly discovered type of photographic plate in the dark. radiation—X-rays. He expected Radioactive insights to find that uranium salts emit Becquerel’s doctoral student Marie 1896 Henri Becquerel radiation as a result of absorbing Curie threw herself into studying discovers that uranium sunlight. However, overcast this phenomenon (which she later salts can emit radiation weather in Paris forced him to put termed radioactivity) with her without having been the experiment on hold, and he left husband Pierre. In 1898, they exposed to sunlight. a wrapped photographic plate extracted two new radioactive 1897 J.J. Thomson, a Uranium salts emit Levels of British physicist, discovers radiation without radioactivity are the electron. absorbing sunlight. affected by the mass of uranium present, not AFTER chemical reactions. 1898 Marie and Pierre Curie discover radioactive polonium Atoms can decay. Radiation is and radium. emitted from within 1900 Paul Villard discovers atoms. gamma radiation. 1907 Ernest Rutherford identifies alpha radiation as an ionized helium atom.

NUCLEAR AND PARTICLE PHYSICS 239 See also: Particles and waves 212–215 ■ The nucleus 240–241 ■ Subatomic particles 242–243 ■ Nuclear bombs and power 248–251 There are many different types of radiation, each with unique properties. Ernest Rutherford identified alpha and beta radiation; Paul Villard discovered gamma radiation. Alpha particle Paper stops Plastic or sheet Thick lead stops alpha particles metal stops gamma rays beta particles Beta Marie Curie particle Marie Curie (née Skłodowska) Gamma ray was born in Warsaw, Poland, in 1867, to a struggling family elements, polonium and radium, Alpha emissions are positively of teachers. She traveled to from uranium ore. Marie noticed charged helium atoms and are France in 1891 and enrolled at that the level of electrical activity unable to penetrate more than the University of Paris, where in the air surrounding uranium ore several inches of air; beta she met her future collaborator was associated only with the mass emissions are streams of negatively and husband, Pierre Curie. of radioactive substance present. charged electrons that can be The Curies shared the Nobel She postulated that radiation was blocked by an aluminum sheet. Prize in Physics with Henri not caused by chemical reactions Gamma radiation (discovered by Becquerel in 1903. In 1911, but came from within atoms—a French chemist Paul Villard in Marie was awarded the bold theory when science still held 1900 as a high-frequency ray) is Nobel Prize in Chemistry. that atoms could not be divided. electrically neutral. It can be blocked with several inches of lead. Later in life, Curie led the The radiation produced by Radium Institute in Paris and uranium was found to be a result Changing elements developed and organized of the “decay” of individual atoms. Rutherford and his collaborator mobile X-ray units that were There is no way to predict when an Frederick Soddy found that alpha used to treat over a million individual atom decays. Instead, and beta radiation were linked soldiers in World War I. She physicists measure the time it with subatomic changes: elements sat on the League of Nations’ takes for half of the atoms in a transmute (change from one committee on academic sample to decay—this is the half- element to another) via alpha decay. cooperation with Albert life of that element and can be Thorium, for example, changed to Einstein. She died in 1934 from anything from an instant to billions radium. They published their law complications likely to have of years. This concept of half-life of radioactive change in 1903. been caused by a lifetime’s was proposed by New Zealand–born exposure to radioactivity. physicist Ernest Rutherford. This series of rapid discoveries overturned the age-old concept of Key works In 1899, Rutherford confirmed the indivisible atom, leading suspicions raised by Becquerel scientists to probe inside it and 1898 On a new radioactive and Curie that there are different establish new fields of physics and substance contained categories of radiation. He named world-changing technologies. ■ in pitchblende and described two: alpha and beta. 1898 Rays emitted by compounds of uranium and of thorium 1903 Research on radioactive substances

240 OCTHFOENMSATTITTUERTION THE NUCLEUS IN CONTEXT T he discoveries of the Rutherford began to dismantle electron and of radiation the plum pudding model through KEY FIGURE emanating from within a series of tests conducted at Ernest Rutherford atoms at the end of the 19th his Manchester laboratory with (1871–1937) century raised the need for a more Ernest Marsden and Hans Geiger. sophisticated atomic model than It became known as the gold BEFORE had previously been described. foil experiment. 1803 John Dalton proposes the atomic theory, stating that all In 1904, British physicist J.J. Flashing particles matter is composed of atoms. Thomson, who had discovered Rutherford and his colleagues the electron in 1897, proposed the observed the behavior of alpha 1903 William Crookes, a “plum pudding” model of the atom. radiation targeted at a thin sheet British chemist and physicist, In this model, negatively charged of gold foil, just 1,000 or so atoms invents the spinthariscope for electrons are scattered throughout thick. They fired a narrow beam detecting ionizing radiation. a much larger, positively charged of alpha particles at the foil from atom, like dried fruit in a Christmas a radioactive source that was 1904 J.J. Thomson proposes dessert. Four years later, New enclosed by a lead shield. The the “plum pudding” model Zealand–born physicist Ernest gold foil was surrounded by a of the atom. zinc sulfide–coated screen that When we have found how emitted a small flash of light AFTER the nucleus of atoms is (scintillation) when struck by 1913 Danish physicist Niels built up, we shall have the alpha particles. Using a Bohr develops a new model of found the greatest secret microscope, the physicists could the atom, in which electrons of all—except life. watch as alpha particles struck the travel in orbits around the Ernest Rutherford screen. (The set-up was similar to central nucleus and can a spinthariscope, which had been move between orbits. developed by William Crookes in 1903 for detecting radiation). 1919 Ernest Rutherford reports that the hydrogen Geiger and Marsden noticed nucleus (proton) is present that most of the alpha particles in other nuclei. shot straight through the gold foil, implying that—in contradiction to Thomson’s model—most of an atom was empty space. A small fraction

NUCLEAR AND PARTICLE PHYSICS 241 See also: Models of matter 68–71 ■ Atomic theory 236–237 ■ Nuclear rays 238–239 ■ Subatomic particles 242–243 ■ Nuclear bombs and power 248–251 of the alpha particles was deflected Rutherford’s model had some Ernest Rutherford at large angles with some even similarities to the Saturnian model bouncing back toward the source. of the atom proposed by Japanese Ernest Rutherford was born in Approximately one in every 8,000 physicist Hantaro Nagaoka in 1904. Brightwater, New Zealand. In alpha particles were deflected by Nagaoka had described electrons 1895, he won a grant that an average angle of 90 degrees. revolving around a positively allowed him to travel to the Rutherford was stunned by what charged center, rather like Saturn’s University of Cambridge, UK, he and his colleagues observed—it icy rings. and work with J.J. Thomson seemed these particles were being at its Cavendish Laboratory. electrically repelled by a small, Science is an iterative process, While there, he found a way heavy, positive charge inside however, and in 1913, Rutherford to transmit and receive radio the atoms. played a major role in replacing his waves over long distances, own model of the atom with a new independently of Italian A new model of the atom one that incorporated quantum inventor Guglielmo Marconi. The results of the gold foil mechanics in its description of the experiment formed the basis for behavior of electrons. This was the Rutherford became a Rutherford’s new model of the Bohr model, which was developed professor at McGill University atom, which he published in 1911. with Niels Bohr and had electrons in Canada in 1898 but returned According to this model, the vast orbiting in different “shells.” to the UK in 1907—this time to majority of an atom’s mass is Regardless, Rutherford’s discovery Manchester, where he concentrated in a positively of the atomic nucleus is widely conducted his most famous charged core called the nucleus. considered one of the most research. He was awarded This nucleus is orbited by electrons significant discoveries in physics, the Nobel Prize in Physics in that are electrically bound to it. laying the foundation for nuclear 1908. Later in life, he became and particle physics. ■ director of the Cavendish Laboratory and President of The gold foil experiment the Royal Society. Following his death, he was awarded A small proportion of Nucleus the honor of a burial at particles are deflected Westminster Abbey. Scattered alpha when they hit the particles dense nucleus Key works Source of Gold Gold 1903 “Radioactive Change” particles foil atom 1911 “The Scattering of Alpha and Beta Particles by Matter Most particles and the Structure of the Atom” pass straight 1920 “Nuclear Constitution through of Atoms” Beam of A zinc sulfide–coated alpha particles screen emits flashes of light when hit by alpha particles

242 WTAHRHEEICBBHRUIIACLTTKOSUMPOSF SUBATOMIC PARTICLES IN CONTEXT F or millennia, atoms were of negatively charged particles thought to be unbreakable more than 1,000 times lighter than KEY FIGURES units. A succession of a hydrogen atom. He concluded Ernest Rutherford discoveries by three generations that these particles were a (1871–1937), James of Cambridge-based physicists universal component of atoms, Chadwick (1891–1974) dismantled this idea, revealing naming them “corpuscles” (later smaller particles within the atom. called electrons). BEFORE 1838 British chemist Richard The first subatomic particle Delving into the atom Laming suggests that was discovered in 1897 by British Thomson incorporated electrons subatomic particles exist. physicist J.J. Thomson while he was into his 1904 “plum pudding” model experimenting with cathode rays. of the atom. However, in 1911 a new 1891 Irish physicist George J. These rays are produced by the model was proposed by Thomson’s Stoney names the fundamental negative electrode (cathode) of an student, Ernest Rutherford. This unit of charge “electron.” electrically charged vacuum tube had a dense, positively charged and are attracted to a positive nucleus orbited by electrons. 1897 J.J. Thomson provides electrode (anode). The cathode rays Further refinements made by evidence for the electron. caused the glass at the far end of the Niels Bohr, along with Rutherford, tube to glow, and Thomson deduced produced the Bohr model in 1913. AFTER that the rays were composed 1934 Italian physicist Enrico In 1919, Rutherford discovered Fermi bombards uranium with … the radiation consists that when nitrogen and other neutrons, producing a new, of neutrons: particles of elements were struck with alpha lighter element. mass 1, and charge 0. radiation, hydrogen nuclei were emitted. He concluded that the 1935 James Chadwick is James Chadwick hydrogen nucleus—the lightest awarded the Nobel Prize for nucleus—is a constituent of all other his discovery of the neutron. nuclei and named it the proton. 1938 Austrian-born physicist Physicists struggled to explain Lise Meitner takes Fermi’s the properties of atoms with only discovery further, describing protons and electrons, because nuclear fission. these particles accounted for only half the measured mass of atoms. In 1920, Rutherford suggested that a neutral particle composed

NUCLEAR AND PARTICLE PHYSICS 243 See also: Models of matter 68–71 ■ Electric charge 124–127 ■ Light from the atom 196–199 ■ Particles and waves 212–215 ■ The nucleus 240–241 ■ Nuclear bombs and power 248–251 ■ The particle zoo and quarks 256–257 Models of the atom Cloudlike Electron Electron Electron Electron body Neutron Proton Nucleus Nucleus Shell Thomson’s plum pudding In Rutherford’s model The Niels Bohr model In 1932, James Chadwick model (1904) has electrons (1911), electrons whizz (1913) has electrons orbiting discovered that the nucleus dotted randomly in a around a dense, positively the nucleus in inner and of an atom is made up of positively charged atom. charged nucleus. outer shells. protons and neutrons. of a bound-together proton and discovered type of neutral radiation the experiments with more accurate electron—which he called a (thought to be a form of gamma measurements. He showed that this neutron—may exist inside nuclei. radiation), which emerged when radiation had about the same mass While this offered a simple alpha radiation struck light elements as protons and concluded that this explanation for how electrons such as beryllium. The Joliot-Curies new radiation was composed of radiated from nuclei, it violated found this radiation carried enough neutrons—neutral particles principles of quantum mechanics: energy to eject high-energy protons contained in the nucleus. This there was simply not enough energy from compounds rich in hydrogen. discovery completed the atomic to trap electrons inside nuclei. But neither Rutherford nor James model in a way that made sense. It Chadwick—Rutherford’s former also set in motion the development In Paris in 1932, Irène Joliot- student—believed that these were of new medical technologies and Curie and her husband Frédéric gamma rays. Chadwick repeated the dawn of the nuclear age. ■ experimented with a newly James Chadwick When Cheshire-born James Physics for the discovery of the Chadwick won a scholarship neutron. During World War II, to the University of Manchester, Chadwick headed the British he was accidentally enrolled in team working on the Manhattan physics instead of mathematics. Project to develop nuclear He studied under Ernest weapons for the Allies. He later Rutherford and wrote his first served as British scientific paper on how to measure gamma adviser to the UN Atomic radiation. In 1913, Chadwick Energy Commission. traveled to Berlin to study under Hans Geiger but was imprisoned Key works in the Ruhleben detention camp during World War I. 1932 “Possible Existence of a Neutron” After Chadwick’s release, he 1932 “The existence of a joined Rutherford at the University neutron” of Cambridge in 1919. In 1935, he was awarded the Nobel Prize in

244 LOIFTTCLLEOUWDISPS PARTICLES IN THE CLOUD CHAMBER IN CONTEXT S ubatomic particles are expanding moist air in a sealed ghostly objects, usually made chamber, making it supersaturated. KEY FIGURE visible only through their He realized that when ions (charged Charles T.R. Wilson interactions. The invention of the atoms) collide with water molecules, (1869–1959) cloud chamber, however, allowed they knock away electrons to create physicists to witness the movements a path of ions around which mist BEFORE of these particles for the first time forms, and this leaves a visible trail 1894 Charles T.R. Wilson and determine their properties. in the chamber. By 1910, Wilson creates clouds in chambers had perfected his cloud chamber while studying meteorological Interested in designing a tool and he demonstrated it to scientists phenomena at the Ben Nevis to study cloud formation, Scottish in 1911. Combined with magnets observatory, Scotland. physicist and meteorologist Charles and electric fields, the apparatus T.R. Wilson experimented with enabled physicists to calculate 1910 Wilson realizes a cloud properties such as mass and chamber can be used to study Wilson’s cloud chamber, on display electrical charge from the cloudy subatomic particles emitted at the Cavendish Laboratory museum, trails left by particles. By 1923, he by radioactive sources. Cambridge, produced tracks inside “as added stereoscopic photography to fine as little hairs.” AFTER 1912 Victor Hess proposes that high-energy ionizing radiation enters the atmosphere from space as “cosmic rays.” 1936 American physicist Alexander Langsdorf modifies the cloud chamber by adding dry ice. 1952 The bubble chamber supersedes the cloud chamber as a basic tool in particle physics.

NUCLEAR AND PARTICLE PHYSICS 245 See also: Electric charge 124–127 ■ Nuclear rays 238–239 ■ Antimatter 246 ■ The particle zoo and quarks 256–257 ■ Force carriers 258–259 Ionizing radiation knocks away negatively charged electrons from water molecules. Water molecules In supersaturated vapor, ions are ionized (have a are the center of water droplet formation. positive charge). These cloud trails Ionizing radiation creates Charles T.R. Wilson show the path of cloud trails in chambers of subatomic particles. Born into a family of farmers supersaturated vapor. in Midlothian, Scotland, Charles T.R. Wilson moved to record them. While Wilson observed an equal but opposite charge. He Manchester after his father’s radiation from radioactive sources, eventually concluded the trails death. He had planned to cloud chambers can also be used belonged to an anti-electron (the study medicine but won a to detect cosmic rays (radiation positron). Four years later, Anderson scholarship to the University from beyond the solar system and discovered another particle—the of Cambridge and switched to Milky Way). muon. Its trails showed it was natural sciences. He began to more than 200 times heavier than an study clouds and worked for The concept of cosmic rays electron but had the same charge. some time at the Ben Nevis was confirmed in 1911–1912 when This hinted at the possibility of meteorological observatory, physicist Victor Hess measured multiple “generations” of particles which inspired him to develop ionization rates from an atmospheric that are linked by similar properties. the cloud chamber. In 1927, he balloon. During risky ascents to shared the Nobel Prize in 17,380 ft (5,300 m), day and night, Discovery of the weak force Physics with Arthur Compton he found increased levels of In 1950, physicists from Melbourne for its invention. ionization and concluded: “radiation University found a neutral particle of very high penetrating power among cosmic rays, which decayed Despite Wilson’s huge enters from above into our into a proton and other products. contribution to particle atmosphere.” These cosmic rays They named it the lambda baryon physics, he remained more (mostly composed of protons and (L). There are several baryons— interested in meteorology. He alpha particles) collide with the composite particles each affected by invented a method to protect nuclei of atoms and create cascades the strong force that acts within the British barrage balloons from of secondary particles when they nucleus. Physicists predicted that lightning strikes during World impact Earth’s atmosphere. the lambda baryon should decay in War II and proposed a theory 10–23 seconds, but it survived much of thunderstorm electricity. The anti-electron longer. This finding led them to While investigating cosmic rays in conclude that another fundamental Key works 1932, American physicist Carl D. force, which worked at a short range, Anderson identified something that was involved. It was named the 1901 On the Ionization of seemed to be an exact reflection of weak interaction, or weak force. ■ Atmospheric Air an electron—of equal mass with 1911 On a Method of Making Visible the Paths of Ionizing Particles Through a Gas 1956 A Theory of Thundercloud Electricity

246 OCPAPNOESXIPTLEOSDE ANTIMATTER IN CONTEXT I n the 1920s, British physicist bend in different directions based Paul Dirac proposed a mirror on their charge. He discovered a KEY FIGURE world of antimatter. In a 1928 particle with a curving trail like Paul Dirac (1902–1984) paper, he demonstrated it is equally that of an electron but pointing in valid for electrons to have positive the opposite direction. BEFORE or negative energy states. An 1898 German-born British electron with negative energy Anderson’s discovery was physicist Arthur Schuster would behave the opposite way to followed by other antimatter speculates on the existence an ordinary electron. For instance, particles and atoms. It is now of antimatter. it would be repelled by a proton known that all particles have an rather than attracted to it, so it equivalent antimatter particle, but 1928 Paul Dirac proposes that has positive charge. the question of why ordinary matter electrons could have both a is dominant in the universe positive and negative charge. Dirac ruled out the possibility remains unresolved. ■ that this particle was a proton, 1931 Dirac publishes a paper given that a proton’s mass is far I think that the discovery that predicts the positively greater than that of an electron. of antimatter was perhaps charged “anti-electron.” Instead, he proposed a new particle the biggest jump of all the with the same mass as an electron AFTER but a positive charge. He called it big jumps in physics 1933 Dirac proposes the an “anti-electron.” On meeting, an in our century. antiproton—the antimatter electron and anti-electron would equivalent to the proton. annihilate each other, producing a Werner Heisenberg mass of energy. 1955 Research at the University of California, Carl D. Anderson is credited Berkeley confirms the with confirming the existence of antiproton exists. this anti-electron (which he renamed the positron) in 1932. Anderson 1965 CERN physicists allowed cosmic rays to pass through describe the production of a cloud chamber under the influence bound antimatter in the form of a magnetic field, causing them to of antideuterium. See also: Subatomic particles 242–243 ■ Particles in the cloud chamber 244–245 ■ The particle zoo and quarks 256–257 ■ Matter–antimatter asymmetry 264

NUCLEAR AND PARTICLE PHYSICS 247 IANTOSMEAICRCGHLUOEF THE STRONG FORCE IN CONTEXT T he discovery of subatomic Proton particles at the beginning KEY FIGURE of the 20th century raised The strong Hideki Yukawa (1907–1981) as many questions as it answered. force binds One such question was how are protons and BEFORE positively charged protons bound neutrons in 1935 Yukawa predicts together in a nucleus despite their the nucleus the existence of a new force natural electric repulsion? within the atomic nucleus. Neutron In 1935, Japanese physicist 1936 Carl D. Anderson Hideki Yukawa provided an answer Neutrons and protons (nucleons) discovers the muon, thought when he predicted an ultra short- are bound together inside the nucleus for a while to be the carrier of range force that acts within the by the strong force carried by mesons. this new force. atomic nucleus to bind its Within protons and neutrons, smaller components (protons and neutrons). particles (quarks) are bound by gluons. AFTER He said this force is carried by a 1948 A team of physicists particle he called a meson. In fact is in fact responsible for the vast at University of California, there are many mesons. The first energy unleashed in nuclear Berkeley, produce pions to be discovered was the pion (or weapons and nuclear reactors artificially by firing alpha pi-meson), which physicists from as atoms are split. Mesons are particles at carbon atoms. the UK and Brazil found in 1947 by the carriers of this force between studying cosmic rays raining down nucleons. The strong force between 1964 American physicist on the Andes. Their experiments quarks (which combine in threes to Murray Gell-Mann predicts confirmed that the pion was make up a proton) is carried by the existence of quarks, involved in the strong interactions gluons (elementary particles, or which interact via the Yukawa had described. bosons), named for their ability strong force. to “glue together” quarks with By far the strongest of different “colors” (a property 1979 The gluon is discovered the four fundamental forces (the unrelated to normal colors) to at the PETRA (Positron- others being electromagnetic, form “colorless” particles such Electron Tandem Ring gravitational, and the weak force), as protons and pions. ■ Accelerator) particle this newly discovered strong force accelerator in Germany. See also: The nucleus 240–241 ■ Subatomic particles 242–243 ■ The particle zoo and quarks 256–257 ■ Force carriers 258–259

248 IN CONTEXT DOARMFEOEANUDENFRTUGSLY KEY FIGURES Enrico Fermi (1901–1954), NUCLEAR BOMBS AND POWER Lise Meitner (1878–1968) BEFORE 1898 Marie Curie discovers how radioactivity emanates from materials like uranium. 1911 Ernest Rutherford proposes that the atom has a dense nucleus at its core. 1919 Rutherford shows that one element can be changed to another by bombarding it with alpha particles. 1932 James Chadwick discovers the neutron. AFTER 1945 The first atomic bomb is tested in New Mexico. A-bombs are dropped on Hiroshima and Nagasaki. 1951 The first nuclear reactor for electricity generation opens. 1986 The Chernobyl disaster highlights nuclear power risks. A t the turn of the 20th century, physicists were unknowingly laying the groundwork for scientists to understand and eventually seize the immense power trapped inside the atom. By 1911, Ernest Rutherford had proposed a model of the atom that placed a dense nucleus at its core. In Paris, Marie Curie and her collaborators, including husband Pierre, had discovered and described how radioactivity emanated from within the atom of natural materials such as uranium.