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Physics

I INTRODUCTION  Physics, major science dealing with the fundamental constituents of the universe, the forces they exert on one another, and the effects of these forces. Sometimes in modern physics a more sophisticated approach is taken that incorporates elements of the three areas listed above; it relates to symmetry and conservation laws, such as those pertaining to energy, momentum, charge, and parity. SeeAtom; Energy.

II SCOPE OF PHYSICS  
Physics is closely related to the other natural sciences and, in a sense, encompasses them. Chemistry, for example, deals with the interaction of atoms to form molecules; much of modern geology is largely a study of the physics of the Earth and is known as geophysics; and astronomy deals with the physics of the stars and outer space. Even living systems are made up of fundamental particles and, as studied in biophysics and biochemistry, they follow the same types of laws as the simpler particles traditionally studied by a physicist.

The emphasis on the interaction between particles in modern physics, known as the microscopic approach, must often be supplemented by a macroscopic approach that deals with larger elements or systems of particles. This macroscopic approach is indispensable to the application of physics to much of modern technology. Thermodynamics, for example, a branch of physics developed during the 19th century, deals with defining and measuring properties of a system as a whole and is useful in other fields of physics; it also forms the basis of much of chemical and mechanical engineering. Such properties as the temperature, pressure, and volume of a gas have no meaning for an individual atom or molecule; these thermodynamic concepts can only be applied directly to a very large system of such particles. A bridge exists, however, between the microscopic and macroscopic approach; another branch of physics, known as statistical mechanics, indicates how pressure and temperature can be related to the motion of atoms and molecules on a statistical basis

Even into the 19th century a physicist was often also a mathematician, philosopher, chemist, biologist, or engineer. Today the field has grown to such an extent that with few exceptions modern physicists have to limit their attention to one or two branches of the science. Once the fundamental aspects of a new field are discovered and understood, they become of interest to engineers and other applied scientists. The 19th-century discoveries in electricity and magnetism, for example, are now the province of electrical and communication engineers; the properties of matter discovered at the beginning of the 20th century have been applied in electronics; and the discoveries of nuclear physics, most of them not yet 40 years old, have passed into the hands of nuclear engineers for applications to peaceful or military uses.

III EARLY HISTORY OF PHYSICS  
Although ideas about the physical world date from antiquity, physics did not emerge as a well-defined field of study until early in the 19th century.

A Antiquity  
The Chinese, Babylonians, Egyptians, and early Mesoamericans observed the motions of the planets and succeeded in predicting eclipses, but they failed to find an underlying system governing planetary motion. The speculations of Greek philosophers introduced two major rival ideas about the fundamental constituents of the universe: atomism, proposed by Leucippus in the 4th century BC, and the theory of the elements, which had been proposed in the 5th century BC.

Notable progress was made in Alexandria, the scientific centre of Western civilization during the Hellenistic Age. There, the Greek mathematician and inventor Archimedes designed various practical mechanical devices involving levers and screws, and measured the density of solid bodies by submerging them in a liquid. Other important Greek scientists of this time were the astronomer Aristarchus of Samos, who measured the ratio of the distances from the Earth to the Sun and to the Moon; the mathematician, astronomer, and geographer Eratosthenes, who determined the circumference of the Earth and drew up a catalogue of stars; and the astronomer Hipparchus, who discovered the precession of the equinoxes . In the 2nd century AD the astronomer, mathematician, and geographer Ptolemy proposed the system of planetary motion that was named after him, in which the Earth was at the centre and the Sun, Moon, and stars moved around it in circular orbits

B Middle Ages  
Little advance was made in physics, or in any other science, during the Middle Ages. However, many Classical Greek scientific treatises were preserved by such Arab scholars as Averroës and Al-Quarashi (also known as Ibn al-Nafis). The founding of the great medieval universities by monastic orders in Europe, from the 13th century onward, generally failed to advance physics or any experimental investigations. The Italian Scholastic philosopher and theologian St Thomas Aquinas, for instance, attempted to demonstrate that the works of Plato and Aristotle were consistent with the Scriptures. The English Scholastic philosopher and scientist Roger Bacon was one of the few philosophers who advocated the experimental method as the true foundation of scientific knowledge; he also did some work in astronomy, chemistry, optics, and machine design.

C 16th and 17th Centuries  
The advent of modern science followed the Renaissance and was ushered in by highly successful attempts by four outstanding individuals to interpret the behaviour of the heavenly bodies during the 16th and early 17th centuries. The Polish natural philosopher Nicolaus Copernicus propounded the heliocentric system in which the planets move around the Sun. He was convinced, however, that the planetary orbits were circular, and therefore his system required almost as many complicated elaborations as the Ptolemaic system it was intended to replace  The Danish astronomer Tycho Brahe adopted a compromise between the Copernican and Ptolemaic systems; according to him, the planets went around the Sun, while the Sun went around the Earth. Brahe was a great observer, who made a series of remarkably accurate measurements. These provided his assistant, the German astronomer Johannes Kepler, with data to attack the Ptolemaic system and led to the discovery of three laws that conformed with a modified heliocentric theory. Galileo, having heard of the invention of the telescope, constructed one of his own and from 1609 was able to confirm the heliocentric system by observing the phases of the planet Venus. He also discovered the surface irregularities of the Moon, the four brightest satellites of Jupiter, sunspots, and many stars in the Milky Way. Galileo’s interests were not limited to astronomy; by using inclined planes and an improved water clock, he had earlier demonstrated that bodies of different weight fall at the same rate (thus overturning Aristotle’s idea), and that their speed increases uniformly with the time of fall. Galileo’s astronomical discoveries and his work in mechanics foreshadowed the work of the 17th-century English mathematician and physicist Isaac Newton, one of the greatest scientists who ever lived.

IV NEWTON AND MECHANICS  
From about 1665, at the age of 23, Newton developed the principles of mechanics, formulated the law of universal gravitation, separated white light into colours, proposed a theory of the propagation of light, and invented differential and integral calculus. Newton’s contributions covered an enormous range of natural phenomena. He was able to show that Kepler’s laws of planetary motion and Galileo’s discoveries concerning falling bodies follow from Newton’s own second law of motion combined with his law of gravitation. Newton was able to explain the effect of the Moon in producing the tides, and the precession of the equinoxes.

A The Development of Mechanics  The subsequent development of physics owes much to Newton’s laws of motion, notably the second, which states that the force needed to accelerate an object is proportional to its mass times its acceleration. If the force and the initial position and velocity of a body are given, subsequent positions and velocities can be calculated, although the force may vary with time or position (in which case, Newton’s calculus must be applied). This simple law contained another important aspect: each body has an inherent property, its inertial mass, which influences its motion. The greater this mass, the slower the change of velocity when a given force is applied. Even today, the law retains its practical value, as long as the body is not very small, not very massive, and not moving extremely rapidly. Newton’s third law, expressed simply as “for every action there is an equal and opposite reaction”, recognizes, in modern terms, that all forces between particles come in oppositely directed pairs.

B Gravity  
Newton’s more specific contribution to the description of the forces in nature was the discovery of the law of gravity. Today scientists know that in addition to gravity only three other fundamental forces give rise to all observed properties and activities in the universe: electromagnetism; the so-called strong nuclear interaction, which binds together the neutrons and protons within atomic nuclei; and the weak interaction between some of the elementary particles that accounts for the phenomenon of radioactivity. Understanding of the force concept, however, dates from the universal law of gravitation, which recognizes that all material particles, and the bodies that are composed of them, have a property called gravitational mass. This property causes any two particles to exert attractive forces on each other (along the line joining them) that are directly proportional to the product of the masses, and inversely proportional to the square of the distance between the particles. This force of gravity governs the motion of the planets about the Sun and of the objects in the Earth’s own gravitational field, and is also responsible for gravitational collapse, which is believed to underlie many astrophysical phenomena, and to be the final stage in the life cycle of massive stars.

One of the most important observations of physics is that the gravitational mass of a body (which is the source of the gravitational force between it and another particle), is effectively the same as its inertial mass, the property that determines the body’s motion in response to any force exerted on it . This equivalence, now confirmed experimentally to within one part in 1013, holds in the sense of proportionality—that is, when one body has twice the gravitational mass of another, it also has twice the inertial mass. Thus, Galileo’s demonstrations, which preceded Newton’s laws, that bodies fall to the ground with the same acceleration can be explained by the fact that the gravitational mass of a body, which determines the forces exerted on it, and the inertial mass, which determines the response to that force, cancel out.

The full significance of this equivalence between gravitational and inertial masses, however, was not appreciated until Albert Einstein devised the general theory of relativity. Einstein saw that this equivalence led to a further implication: the equivalence of a gravitational field and an accelerated frame of reference

The force of gravity is the weakest of the four forces of nature when elementary particles are considered. The gravitational force between two protons, for example, which are among the heaviest elementary particles, is at any given distance only 10-36 the magnitude of the electrostatic forces between them, and for two such protons in the nucleus of an atom, this force in turn is many times smaller than the strong nuclear interaction. The dominance of gravity on a macroscopic scale is due to two facts: (1) There is only one type of mass, as far as is known, which leads to only one kind of gravitational force, which is attractive. The many elementary particles that make up a large body, such as the Earth, therefore exhibit an additive effect of their gravitational forces, which thus become very large. (2) The gravitational forces act over a large range, and decrease only as the square of the distance between two bodies.

By contrast, the electric charges of elementary particles, which give rise to electrostatic and magnetic forces, are either positive or negative, or absent altogether. Only particles with opposite charges attract one another, and large composite bodies therefore tend to be electrically neutral and inactive.

On the other hand, the nuclear forces, both strong and weak, are extremely short-range and become hardly noticeable at distances greater than 1 million-millionth of a centimetre.

Despite its macroscopic importance, the force of gravity remains so weak that a body must be very massive before its influence is noticed by another. Thus, the law of universal gravitation was deduced from observations of the motions of the planets long before it could be checked experimentally. Not until 1771 did the British physicist and chemist Henry Cavendish confirm it by using large spheres of lead to attract small masses attached to a torsion pendulum, and from these measurements also deduced the mass and density of the Earth.

In the two centuries after Newton, although mechanics was analysed, reformulated, and applied to complex systems, no new physical ideas were added. The Swiss mathematician Leonhard Euler first formulated the equations of motion for rigid bodies, whereas Newton had dealt only with masses concentrated at a point or that were equivalent to point masses, which thus acted like particles. Various mathematical physicists, among them Joseph Louis Lagrange and William Hamilton, extended Newton’s second law with more sophisticated and elegant reformulations. Over the same period, Euler, the Dutch-born scientist Daniel Bernoulli, and other scientists also extended Newtonian mechanics to lay the foundation of fluid mechanics.

C Electricity and Magnetism  
Although the ancient Greeks were aware of the electrostatic properties of amber, and the Chinese as early as 2700 BC made magnets from lodestone, experimentation with and the understanding and use of electric and magnetic phenomena did not occur until the end of the 18th century. In 1785 the French physicist Charles Augustin de Coulomb first confirmed experimentally that electrical charges attract or repel one another according to an inverse square law, similar to that of gravitation. A powerful theory to calculate the effect of any number of static electric charges arbitrarily distributed was subsequently developed by the French mathematician Siméon Denis Poisson and the German mathematician Carl Friedrich Gauss.

A positively charged particle attracts a negatively charged one, and they tend to accelerate towards each other. If the medium through which the particles move offers resistance, they may be reduced to a constant-velocity (rather than accelerated) motion, and the medium will be heated up and may also be otherwise affected. The ability to maintain an electromotive force that could continue to drive electrically charged particles had to await the development of the chemical (cell) battery by the Italian physicist Alessandro Volta in 1800. The classical theory of a simple electric circuit assumes that the two terminals of a cell are maintained positively and negatively charged as a result of its internal properties. When the terminals are connected by a wire, negatively charged particles are simultaneously pushed away from the negative terminal and attracted to the positive one, and in the process heat up the wire that offers resistance to the motion. Upon their arrival at the positive terminal, the particles are forced through the interior of the cell towards the negative terminal, overcoming the opposing forces of Coulomb’s law. The German physicist Georg Simon Ohm first discovered the existence of a simple proportionality constant, known as the resistance of the circuit, relating the current flowing and the electromotive force supplied by a battery. Ohm’s law, which states that the current is proportional to the electromotive force (that is, that the resistance is constant), is not a fundamental and universally applicable law of physics, but rather describes the behaviour of a limited class of solid materials.

The elementary concepts of magnetism, based on the existence of pairs of oppositely charged poles, date from the 17th century. They were developed in the work of Coulomb. The first connection between magnetism and electricity, however, was made through the pioneering experiments of the Danish physicist and chemist Hans Christian Oersted, who in 1819 discovered that a magnetic needle could be deflected by a wire nearby carrying an electric current. Within one week of learning of Oersted’s discovery, the French scientist André Marie Ampère showed experimentally that two current-carrying wires affect each other like poles of magnets. In 1831 the British physicist and chemist Michael Faraday discovered that an electric current could be induced (made to flow) in a loop of wire not connected to a battery, either by moving a magnet nearby or by placing a wire carrying a varying current nearby. The intimate connection between electricity and magnetism, now established, can best be stated in terms of electric or magnetic fields. The strength and direction of a field at any point is a measure of the force that will act on a unit charge or unit current, respectively, placed at that point. Stationary electric charges produce electric fields; currents—that is, moving electric charges—produce magnetic fields. Electric fields are also produced by changing magnetic fields, and vice versa. Electric fields exert forces on charged particles as a function of their charge alone; magnetic fields exert a force on charges in motion.

These qualitative findings were put into a precise mathematical form by the British physicist James Clerk Maxwell, who, in developing the partial differential equations that bear his name, related the space and time changes of electric and magnetic fields at a point to the charge and current densities at that point. In principle, they permit the calculation of the fields everywhere and at any time from a knowledge of the charges and currents. An unexpected result arising from the solution of these equations was the prediction of a new kind of electromagnetic field, one produced by accelerating charges. It propagated through space with the speed of light in the form of an electromagnetic wave, and decreased in strength with the inverse square of the distance from the source. In 1887 the German physicist Heinrich Hertz succeeded in generating such waves by electrical means, thereby laying the foundations for radio, radar, television, and other forms of telecommunications.

The behaviour of electric and magnetic fields in these waves is quite similar to that of a very long taut string, one end of which is rapidly moved up and down in a periodic fashion. Any point along the string will be observed to move up and down, or oscillate, with the same frequency as the source. Points along the string at different distances from the source will reach the maximum vertical displacements at different times. Each point along the string will do what its neighbour did, but a little later, if it is further removed from the vibrating source . The speed with which the disturbance, or “message”, is transmitted along the string is called the wave velocity . This is a function of the string’s mass per unit length and its tension. An instantaneous snapshot of the string (after it had been in motion for a while) would show that points having the same displacement were separated by a distance known as the wavelength, which is equal to the wave velocity divided by the frequency. In the case of the electromagnetic field one can think of the electric field strength as taking the place of the up-and-down motion of each piece of the string, with the magnetic field acting similarly in a direction at right angles to that of the electric field. The electromagnetic wave velocity away from the source is the speed of light.

D Light  
The apparently linear propagation of light has been known since antiquity. The ancient Greeks believed that light consisted of a stream of corpuscles. They were, however, quite confused as to whether these corpuscles originated in the eye or in the object viewed. Any satisfactory theory of light must explain its origin and disappearance and its changes in speed and direction while it passes through various media. Partial answers to these questions were proposed in the 17th century by Newton, who based them on the assumptions of a corpuscular theory, and by the English scientist Robert Hooke and the Dutch astronomer, mathematician, and physicist Christiaan Huygens, who both proposed wave theories. No experiment could be performed that distinguished between the two theories (as the wavelength of light is very small) until the demonstration of interference in the early 19th century by the British physicist and physician Thomas Young. The French physicist Augustin Jean Fresnel decisively favoured the wave theory.

Interference can be demonstrated by placing a thin slit in front of a light source, stationing a double slit further away, and looking at a screen spaced some distance beyond the double slit. Instead of showing a uniformly illuminated image of the slits, the screen will show equally spaced light and dark bands. Further detailed assumptions would have to be added to explain how particles coming from the same source and arriving at the screen via the two slits could produce different light intensities at different points and even cancel each other to yield dark spots. Light waves, however, can quite easily produce such an effect. Assuming, as did Huygens, that each of the double slits acts as a new source, emitting light in all directions, the two wave trains arriving at the screen at the same point will not generally arrive in phase, though they will have left the two slits in phase. (Two vibrations at a given point are said to be in phase when they are at the same stage of the oscillation at each moment—thus their maxima coincide at one moment, their minima at another, and so on.) Depending on the difference in their paths, “positive” displacements of one wave train arriving at the same time as “negative” displacements of the other will tend to cancel out the latter and produce darkness, while the simultaneous arrival of either positive or negative displacements from both sources will lead to reinforcement, or brightness. At each bright spot the light intensity undergoes a time-wise variation as successive in-phase waves go from maximum positive through zero to maximum negative displacement and back. Neither the eye nor any classical instrument, however, can determine this rapid “flicker”, which in the visible-light range has a frequency from 4 × 1014 to 7.5 × 1014 hertz, or cycles per second. Although it cannot be measured directly, the frequency can be inferred from wavelength and velocity measurements. The wavelength can be determined from simple measurements of the distance between the two slits and of the distance between adjacent bright bands on the screen. The wavelength ranges from 4 × 10-5 cm (1.6 × 10-5 in) for violet light to 7.5 × 10-5 cm (3 × 10-5 in) for red light, with intermediate wavelengths for the other colours.

The first measurement of the velocity of light was carried out by the Danish astronomer Olaus Roemer in 1676. He noted an apparent time variation between successive eclipses of Jupiter’s moons, which he ascribed to changes in the distance between Earth and Jupiter, and to the corresponding differences in the time required for the light to reach the Earth. His measurement was in fair agreement with the improved 19th-century observations of the French physicist Armand Hippolyte Louis Fizeau, and with the work of the American physicist Albert Abraham Michelson and his co-workers, which extended into the 20th century. Today the velocity of light is known very accurately as 299,792.46 km/sec (186,282.4 mi/sec) in vacuum. In matter, the speed is less and varies with frequency, a phenomenon known as dispersion.

Maxwell’s work contributed several important results to the understanding of light by showing that it is electromagnetic in origin and that in a light wave electric and magnetic fields oscillate. His work predicted the existence of non-visible light, and today electromagnetic waves or radiations are known to cover the spectrum from gamma rays , with wavelengths of 10-12 cm (4 × 10-13 in) and less, through X-rays, visible light, microwaves, and radio waves, to long waves of hundreds of kilometres and more in length. It also related the velocity of light in vacuum and in media to other observed properties of space and matter on which electrical and magnetic effects depend. Maxwell’s discoveries, however, did not provide any insight into the mysterious medium, corresponding to the string, through which light and electromagnetic waves supposedly had to travel . From their experience with water, sound, and elastic waves, scientists assumed a similar medium to exist, a “luminiferous ether” without mass, which existed everywhere (because light can travel through space). The ether had to act like a solid, because electromagnetic waves were known to be transverse, while gases and liquids can only sustain longitudinal waves, such as sound waves. The search for the ether occupied physicists’ attention for much of the last part of the 19th century.

The problem was further compounded by an extension of a simple problem. A person walking forwards with a speed of 32 km/h (20 mph) in a train travelling at 644 km/h (400 mph) appears to an observer on the ground to move at 676 km/h (420 mph). In relation to the speed of light the question that now arose was: If light travels at about 300,000 km/sec (about 186,000 mi/sec) through the ether, at what velocity should it travel relative to an observer on Earth, since the Earth also moves through the ether? Or, alternatively, what is the Earth’s velocity through the ether, as indicated by its effects on light waves? The famous Michelson-Morley experiment, first performed in 1887 by Michelson and the American chemist Edward Williams Morley using an interferometer, was an attempt to measure this velocity. If the Earth were travelling through a stationary ether, a difference should be apparent in the time taken by light to traverse a given distance, depending on whether it travels in the direction of or perpendicular to the Earth’s motion. The experiment was sensitive enough to detect even a very slight difference by interference; the results were negative. Physics was now in a profound quandary from which it was not rescued until Einstein formulated his theory of relativity in 1905.

E Thermodynamics  A branch of physics that assumed major stature during the 19th century was thermodynamics. It began by disentangling the previously confused concepts of heat and temperature, by arriving at meaningful definitions, and by showing how they could be related to the previously purely mechanical concepts of work and energy.

E1 Heat and Temperature  Different sensations are experienced when hot and cold bodies are touched, leading to the qualitative and subjective concept of temperature. The transfer of energy to a body generally leads to an increase in temperature when no melting or boiling occurs, and in the case of two bodies at different temperatures brought into contact, energy flows from one to the other until their temperatures become the same and thermal equilibrium is reached. Energy that flows from one body to another as a consequence of temperature differences is called heat. To arrive at a scientific measure of temperature, scientists used the observation that the addition or subtraction of heat produced a change in at least one well-defined property of a body. For example, heating a column of liquid maintained at constant pressure increased the length of the column, while heating a gas confined in a container raised its pressure. Temperature, therefore, can invariably be measured by one other physical property, as in the length of the mercury column in an ordinary thermometer, provided the other relevant properties remain unchanged. The mathematical relationship between the relevant physical properties of a body or system and its temperature is known as the equation of state. Thus, for a so-called ideal gas, a simple relationship exists between the pressure, p, volume V, number of moles n, and the absolute temperature T, given by pV = nRT, where R is the same constant for all ideal gases. Boyle’s law, named after the British physicist and chemist Robert Boyle, and Gay-Lussac’s, or Charles’s, law, named after the French physicists and chemists Joseph Louis Gay-Lussac and Jacques Alexandre César Charles, are both contained in this equation of state (seeGases).

Until well into the 19th century, heat was considered to be a massless fluid called caloric, contained in matter and capable of being squeezed out of or into it. Although the so-called caloric theory answered most early questions on thermometry and calorimetry, it failed to provide a sound explanation of many early 19th-century observations. The first true connection between heat and other forms of energy was observed in 1798 by the Anglo-American physicist and statesman Benjamin Thompson, Count von Rumford, who noted that the heat produced in the boring of cannon was roughly proportional to the amount of work done. (In mechanics, work is the product of a force on a body and the distance through which the body moves in the direction of the force during its application.)

E2 The First Law of Thermodynamics  The equivalence of heat and work was explained by the German physicist Hermann Ludwig Ferdinand von Helmholtz and the British mathematician and physicist Lord Kelvin by the middle of the 19th century. This equivalence means that, for example, the same temperature rise can be achieved in a liquid contained in a vessel by heating it or by doing an appropriate amount of work stirring a paddle wheel in the container. The numerical value of this equivalent was first demonstrated by the British physicist James Prescott Joule in experiments carried out between 1840 and 1849.

It was thus recognized that performing work on a system or heating are both means of transferring energy to the system. Therefore, the amount of energy added via heat or work has to increase the internal energy of the system, which in turn determines the temperature. If the internal energy remains unchanged, the amount of work done on a system must equal the heat given up by it. This is the first law of thermodynamics, a statement of the conservation of energy. Not until the activity of molecules in a system was better understood by the development of the kinetic theory could this internal energy be related to the sum of the kinetic energies of all the molecules making up the system.

E3 The Second Law of Thermodynamics  While the first law indicates that energy must be conserved in any interactions between a system and its surroundings, it gives no indication whether all forms of mechanical and thermal energy exchange are possible. That overall changes in energy proceed in one direction was first formulated by the French physicist and military engineer Nicolas Léonard Sadi Carnot, who in 1824 pointed out that a heat engine (a device that can produce work continuously by exchanging heat with its surroundings) requires both a hot body as a source of heat and a cold body to absorb heat that must be discharged. When the engine performs work, heat must be transferred from the hotter to the colder body; to have the reverse take place requires the expenditure of mechanical (or electrical) work. Thus, in a continuously working refrigerator, the absorption of heat from the low temperature source (the cold space) requires the performance of work (usually as electrical power), and the discharge of heat (usually via finned coils in the rear) to the surroundings . These ideas, based on Carnot’s concepts, were eventually formulated rigorously as the second law of thermodynamics by the German mathematical physicist Rudolf Julius Emanuel Clausius and by Lord Kelvin in various alternative, although equivalent, ways. One such formulation is that heat cannot flow from a colder to a hotter body without the expenditure of work.

From the second law, it follows that in an isolated system (one that has no interactions with the surroundings) internal portions at different temperatures will always tend towards a single uniform temperature and thus produce equilibrium. This can also be applied to other internal properties that may be non-uniform initially. If milk is poured into a cup of coffee, for example, the two substances will continue to mix until they are inseparable and can no longer be differentiated. Thus, an initial ordered state, with distinct components, is turned into a mixed or disordered state. These ideas can be expressed by a thermodynamic property called entropy (first formulated by Clausius), which serves as a measure of how close a system is to equilibrium—that is, to perfect internal disorder. The entropy of an isolated system, and of the universe as a whole, can only increase, and when equilibrium is eventually reached, no more internal change of any form is possible. Applied to the universe as a whole, this principle suggests that eventually temperature throughout the cosmos will become uniform, resulting in the so-called heat death of the universe.

However, the entropy can be lowered locally by external action. This applies to machines, such as a refrigerator, in which the entropy of the cold chamber is reduced, and to living organisms. This local increase in order is, however, only possible at the expense of an entropy increase in the surroundings; here more disorder must be created.

This continued increase in entropy is related to the observed non-reversibility of macroscopic processes. If a process were spontaneously reversible—that is, if, after undergoing a process, both it and all the surroundings could be brought back to their initial state—the entropy would remain constant, in violation of the second law. While this is true for macroscopic processes, and therefore corresponds to daily experience, it does not apply to microscopic processes, which are believed to be reversible. Thus, chemical reactions between individual molecules are not governed by the second law, which applies only to macroscopic ensembles.

From the formulation of the second law, thermodynamics went on to other advances and to applications in physics, chemistry, and engineering. Most chemical engineering, all power-plant engineering, air-conditioning technology, and low-temperature physics are just a few of the fields that owe their theoretical basis to thermodynamics and to the subsequent achievements of such scientists as Maxwell, the American physicist Willard Gibbs, the German physical chemist Walther Hermann Nernst, and the Norwegian-born American chemist Lars Onsager.

F Kinetic Theory and Statistical Mechanics  The modern concept of the atom was first proposed by the British chemist and physicist John Dalton in 1808 and was based on his studies showing that chemical elements enter into combinations based on fixed ratios of their weights. The concept of molecules as the smallest particles of a substance that can exist in the free—that is, gaseous—state while still possessing the properties of any larger amount of the substance was first proposed by the Italian physicist and chemist Amedeo Avogadro in 1811. It did not find general acceptance until about 50 years later, when it also came to form the basis of the kinetic theory of gases . As developed by Maxwell, the Austrian physicist Ludwig Boltzmann, and other physicists, it enabled the laws of mechanics and probability to be applied to the behaviour of individual molecules, leading to statistical inferences about the properties of the gas as a whole.

A typical but important problem solved in this manner was the determination of the range of speeds of molecules in the gas, and from this the average kinetic energy of the molecules. The kinetic energy of a body, as a simple consequence of Newton’s second law, is ymv2, where m is the mass of the body and v its velocity. One of the achievements of kinetic theory was to show that temperature, the macroscopic thermodynamic property describing the system as a whole, was directly related to the average kinetic energy of the molecules. Another was the identification of the entropy of a system with the logarithm of the statistical probability of the energy distribution. This led to the demonstration that the state of thermodynamic equilibrium of highest probability is also the state of maximum entropy. Following these successes in the case of gases, kinetic theory and statistical mechanics were subsequently applied to other systems, a process that is still continuing.

G Early Atomic and Molecular Theories  The development of Dalton’s atomic theory and Avogadro’s law had overriding influence on the development of chemistry, in addition to their importance in physics.

G1 Avogadro’s Law  Avogadro’s law, which was easily proved by kinetic theory, indicated that a specified volume of a gas at a given temperature and pressure always contained the same number of molecules, irrespective of the gas selected. This number, however, could not be accurately determined, and physicists therefore had no sound knowledge of molecular or atomic mass and size until the turn of the 20th century. After the discovery of the electron, the American physicist Robert Andrews Millikan carefully determined its charge. This finally permitted accurate determination of Avogadro’s number, which is the number of molecules in an amount of material whose mass in grams is exactly equal to its molecular weight

Besides the mass of an atom, another quantity of interest was its size. Various and only partly successful attempts at finding the size of an atom were made during the latter part of the 19th century; the most successful applied the results of kinetic theory to non-ideal gases—that is, gases whose molecules were not points but had finite volumes. Later experiments involving the scattering of X-rays, alpha particles, and other atomic and subatomic particles by atoms led to more precise measurements of their size; they proved to be between 10-8 and 10-9 cm (4 × 10-9 and 4 × 10-8 in) in diameter. A precise statement about the size of an atom, however, requires some explicit definition of what is meant by size, since most atoms are not exactly spherical and can exist in various states with different distances between the nucleus and the electrons.

G2 Spectroscopy  One of the most important developments leading to the exploration of the interior of the atom, and to the eventual overthrow of the classical theories of physics, was spectroscopy; the other was the discovery of the subatomic particles themselves.

In 1823 the British astronomer and chemist John Herschel suggested that a chemical substance might be identified by examining its spectrum—that is, the pattern of discrete wavelengths in which light from a gaseous substance is emitted. In the years that followed, the spectra of a great many substances were catalogued by two Germans, the chemist Robert Wilhelm Bunsen and the physicist Gustav Robert Kirchhoff. Helium was discovered following the observation of an unexplained line in the Sun’s spectrum by the British astronomer Joseph Norman Lockyer in 1868. From the standpoint of atomic theory, however, the most important contributions were made by the study of the spectra of simple atoms, such as hydrogen, which showed few spectral lines.

Discrete line spectra originate from gaseous substances in which, in terms of modern knowledge, the electrons have been excited by heat or by bombardment with subatomic particles. In contrast, a heated solid has a continuous spectrum over the full visible range and into the infrared and ultraviolet regions. The total amount of energy emitted depends strongly on the temperature, as does the relative intensity of the different wavelength components. As a piece of iron is heated, for example, its radiation is first in the infrared spectrum and cannot be seen; the radiation then extends into the visible spectrum, where the glow shifts from red to white as the peak of its radiant spectrum shifts towards the middle of the visible range. Attempts to explain the radiation characteristics of solids, using the tools of theoretical physics available at the end of the 19th century, led to the prediction that at any given temperature the amount of radiation increased with frequency and without limit. This calculation, in which no error was found, was in disagreement with experiment and also led to an absurd conclusion: that a body at a finite temperature could radiate an infinite amount of energy. This required a new way of thinking about radiation and, indirectly, about the atom.; Ultraviolet Radiation.

H The Breakdown of Classical Physics  By about 1880 physics was serene; most phenomena could be explained by Newtonian mechanics, Maxwell’s electromagnetic theory, thermodynamics, and Boltzmann’s statistical mechanics. It seemed that only a few problems, such as the determination of the properties of the ether and the explanation of the radiation spectra from solids and gases, were unsolved. These unexplained phenomena, however, formed the seeds of revolution, a revolution that was augmented by a series of remarkable discoveries within the last decade of the 19th century: of X-rays by Wilhelm Conrad Roentgen in 1895; of the electron by J. J. Thomson in 1895; of radioactivity by Antoine Henri Becquerel in 1896; and of the photoelectric effect by Heinrich Hertz, Wilhelm Hallwachs, and Philipp Lenard during the period from 1887 to 1899. Coupled with the disturbing results of the Michelson-Morley experiments and the discovery of cathode rays, which are electron streams, the experimental evidence in physics now outstripped all theories available to explain it.

V MODERN PHYSICS  
Two major new developments during the first third of the 20th century, the quantum theory and the theory of relativity, explained these findings, yielded new discoveries, and changed the understanding of physics as it is known today.

A Relativity  
To extend the example of relative velocity introduced with the Michelson-Morley experiment, two situations can be compared. One consists of a person, A, walking forward with a velocity v in a train moving at velocity u. The velocity of A with regard to an observer B stationary on the ground is then simply V = u + v. If, however, the train were at rest in the station and A was moving forward with velocity v while observer B walked the other way with velocity u, the relative speed of A and B would be exactly the same as in the first case. In more general terms, if two frames of reference are moving relative to each other at constant velocity, observations of any phenomena made by observers in either frame will be physically equivalent. As already mentioned, the Michelson-Morley experiment failed to confirm this simple addition of velocities in the case of light beams: two observers, one at rest and the other moving towards a light source with velocity u, observe the same light velocity commonly denoted by the symbol c.

Einstein incorporated the invariance of c into his theory of relativity. He also demanded a very careful rethinking of the concepts of space and time, showing the imperfection of intuitive notions about them. As a consequence of his theory, it is known that two clocks that keep identical time when at rest relative to each other must run at different speeds when they are in relative motion, and two rods that are identical in length when at rest will become different in length when they are in relative motion. Space and time must be closely linked in a four-dimensional continuum where the normal three-space dimensions have an interrelated time dimension.

Two important consequences of Einstein’s relativity theory are the equivalence of mass and energy and the limiting velocity of the speed of light for material objects. Relativistic mechanics describes the motion of objects with velocities that are appreciable fractions of the speed of light, while Newtonian mechanics remains useful for velocities typical of the macroscopic motion of objects on Earth. No material object, however, can have a speed equal to or greater than the speed of light.

Even more important is the relation between the mass m and energy E. They are coupled by the relation E = mc2 and, because c is very large, the energy equivalence of a given mass is enormous. The change of mass giving an energy change is significant in nuclear reactions, as in reactors or nuclear weapons, and in the stars, where a significant loss of mass accompanies the huge energy release.

Einstein’s original theory, formulated in 1905 and known as the special theory of relativity, was limited to frames of reference moving at constant velocity relative to each other. In 1915, he generalized his hypothesis to formulate the general theory of relativity, which applies to systems that accelerate with reference to each other. This extension showed gravitation to be a consequence of the geometry of space-time and predicted the bending of a light ray if it passed close to a massive body, such as a star, an effect first observed in 1919. General relativity, has deep significance for an understanding of the structure of the universe and its evolution.

B Quantum Theory  The puzzle posed by the observed spectra emitted by solid bodies was first explained by the German physicist Max Planck. According to classical physics, all molecules in a solid can vibrate, with the amplitude of the vibrations directly related to the temperature. All vibration frequencies should be possible and the thermal energy of the solid should be continuously convertible into electromagnetic radiation as long as energy is supplied. Planck made a radical assumption by suggesting that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons   Each photon has a characteristic wavelength and an energy E given by E = hf, where f is the frequency of the wave. The wavelength ë is related to the frequency by ëf = c, where c is the speed of light. With the frequency specified in hertz (Hz), or cycles per second, h, now known as Planck’s constant, is extremely small (6.626 × 10-34 joules-second). With his theory, Planck introduced a wave-particle duality into the theory of light, which for nearly a century had been considered to be wave-like only.

C Photoelectricity  If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, now known to be electrons, are ejected from the metal surface. The important aspects of this phenomenon are the following: (1) the energy of each photoelectron depends only on the frequency of the illumination and not on its intensity; (2) the rate of electron emission depends only on the illuminating intensity and not on the frequency (provided that the minimum frequency capable of causing emission is exceeded); and (3) the photoelectrons emerge as soon as the illumination hits the surface. These observations, which could not be explained by Maxwell’s electromagnetic theory of light, led Einstein to assume in 1905 that light can be absorbed only in quanta, or photons, and that the photon completely vanishes in the absorption process, with all of its energy E (=hf) going to one electron in the metal. With this simple assumption Einstein extended Planck’s quantum theory to the absorption of electromagnetic radiation, giving additional importance to the wave-particle duality of light. It was for this work that Einstein was awarded the 1921 Nobel Prize for Physics.

D X-Rays  These very penetrating rays, first discovered by Roentgen, were shown to be electromagnetic radiation of very short wavelength in 1912 by the German physicist Max von Laue and his co-workers. The precise mechanism of X-ray production was shown to be a quantum effect, and in 1914 the British physicist Henry Gwyn-Jeffreys Moseley used his X-ray spectrograms to prove that the number of positive charges in an atom is the same as its atomic number, its position in the periodic table. The photon theory of electromagnetic radiation was further strengthened and developed by the prediction and observation of the so-called Compton effect by the American physicist Arthur Holly Compton in 1923.

E Electron Physics  That electric charges were carried by extremely small particles had already been suspected in the 19th century, and electrochemical experiments that indicated the charge of these elementary particles was a definite, invariant quantity. Experiments on the conduction of electricity through low-pressure gases led to the discovery of two kinds of rays: cathode rays, coming from the negative electrode in a gas discharge tube, and positive or canal rays from the positive electrode. J. J. Thomson’s 1895 experiment measured the ratio of the charge q to the mass m of the cathode-ray particles. Lenard in 1899 confirmed that the ratio of q to m for particles emitted in the photoelectric effect was identical to that of cathode rays. The American inventor Thomas Alva Edison had noted in 1883 that very hot wires emit electricity, an effect known as thermionic emission (now called the Edison effect), and in 1899 Thomson showed that this form of electricity also consisted of particles with the same q to m ratio as the others. About 1911 Millikan finally determined that electric charge always arises in multiples of a basic unit e and measured its value, now known to be 1.602 × 10-19 coulombs. From the measured value of q/m, with q set equal to e, the mass of the carrier, called the electron, could now be determined as 9.109 × 10-31 kg.

Finally, Thomson and others showed that the positive rays also consisted of particles, each carrying a charge e, but of the positive variety. These particles, however, now recognized as positive ions resulting from the removal of an electron from a neutral atom, are much more massive than the electron. The smallest, the hydrogen ion, is a single proton with a mass of 1.673 × 10-27 kg, about 1837 times more massive than the electron . The “quantized” nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles had been identified.

F Atomic Models  
In 1913 the New Zealand-born British physicist Ernest Rutherford, making use of the newly discovered radiations from radioactive nuclei, found Thomson’s earlier model of an atom with uniformly distributed positive and negative charged particles to be untenable. The very fast, positively charged alpha particles that he employed were found to be deflected sharply in their passage through matter. This effect required an atomic model with a heavy positive scattering centre. Rutherford then suggested that the positive charge of an atom was concentrated in a massive stationary nucleus, with the negative electron moving in orbits about it, and held in the atom by the electric attraction between opposite charges. This “solar system” model, however, could not persist, according to Maxwell’s theory, in which the revolving electrons should emit electromagnetic radiation leading to a total collapse of the system in a very short time.

Another sharp break with classical physics was required at this point. It was provided by the Danish physicist Niels Bohr, who suggested that within atoms there were certain specified orbits in which electrons could revolve without emission of electromagnetic radiation. These allowed orbits, or so-called stationary states, are determined by the condition that the angular momentum J of the orbiting electron must be a positive integral multiple of Planck’s constant, divided by 2 ً, that is, J = nh/2ً, where the quantum number n may have any positive integer value. This extended “quantization” to dynamics, fixed the possible orbits, and allowed Bohr to calculate their radii and the corresponding energy levels. In 1913, the year in which Bohr’s first work on this subject appeared, the model was confirmed experimentally by the German-born American physicist James Franck and the German physicist Gustav Hertz.

Bohr developed his model much further. He explained how atoms radiate light and other electromagnetic waves, and also proposed that an electron “lifted” by a sufficient disturbance of the atom from the orbit of smallest radius and least energy (the ground state) into another orbit would soon “fall” back to the ground state. This falling back is accompanied by the emission of a single photon of energy E = hf, where E is the difference in energy between the higher and lower orbits. Each orbit shift emits a characteristic photon of sharply defined frequency and wavelength; thus one photon would be emitted in a direct shift from the n = 3 to the n = 1 orbit, which will be quite different from the two photons emitted in a sequential shift from the n = 3 to n = 2 orbit, and then from there to the n = 1 orbit. This model now allowed Bohr to account with great accuracy for the simplest atomic spectrum, that of hydrogen, which had defied classical physics.

Although Bohr’s model was extended and refined, it could not explain observations for atoms with more than one electron. It could not even account for the intensity of the spectral colours of the simple hydrogen atom. Because it had no more than a limited ability to predict experimental results, it remained unsatisfactory for theoretical physicists.

G Quantum Mechanics  
Within a few years, roughly between 1924 and 1930, an entirely new theoretical approach to dynamics was developed to account for subatomic behaviour. Named quantum mechanics or wave mechanics, it started with the suggestion in 1924 by the French physicist Louis de Broglie that not only electromagnetic radiation but also matter could have wave as well as particle aspects. The wavelength of the so-called matter waves associated with a particle is given by the equation ë = h/mv, where m is the particle mass and v its velocity. Matter waves were conceived of as pilot waves guiding the particle motion, a property that should result in diffraction under suitable conditions. This was confirmed in 1927 by experiments on electron-crystal interactions by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist George Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst Pascual Jordan of Germany and the Austrian physicist Erwin Schrِdinger developed de Broglie’s idea into a mathematical form capable of dealing with a number of physical phenomena and with problems that could not be handled by classical physics. In addition to confirming Bohr’s idea regarding the quantization of energy levels in atoms, quantum mechanics now provides an understanding of the most complex atoms, and has also been a guiding spirit in nuclear physics. Although quantum mechanics is usually needed only on the microscopic level (with Newtonian mechanics still satisfactory for macroscopic systems), certain macroscopic effects, such as the properties of crystalline solids, can be satisfactorily explained only by principles of quantum mechanics.

Going beyond Broglie’s notion of the wave-particle duality of matter, additional important concepts have since been incorporated into the quantum-mechanical picture. These include the discovery that electrons must have some permanent magnetism and, with it, an intrinsic angular momentum, or spin, as a fundamental property. Spin was subsequently found in almost all other elementary particles. In 1925 the Austrian physicist Wolfgang Pauli discovered the exclusion principle, which states that in an atom no two electrons can have precisely the same set of quantum numbers. (Four quantum numbers are needed to specify completely the state of an electron in an atom.) The exclusion principle is vital for an understanding of the structure of the elements and of the periodic table. Heisenberg in 1927 put forward the uncertainty principle, which asserted the existence of a natural limit to the precision with which certain pairs of physical quantities can be known simultaneously.

Finally, a synthesis of quantum mechanics and relativity was made in 1928 by the British mathematical physicist P. A. M. Dirac, leading to the prediction of the existence of the positron and bringing the development of quantum mechanics to a culmination.

Largely as a result of Bohr’s ideas, a statistical approach developed in modern physics. The fully deterministic cause-and-effect relations of Newtonian mechanics were replaced by predictions of future events in terms of statistical probabilities only. The wave properties of matter imply that, in accordance with the uncertainty principle, the motion of a particle can never be predicted with absolute certainty, even if all the forces acting are known. Although this statistical aspect plays no detectable role in macroscopic motions, it is dominant on the molecular, atomic, and subatomic scales.

H Nuclear Physics  
In 1896, Becquerel discovered radioactivity in uranium ore. Within a few years radiation from radioactive materials was found to consist of three types of emissions: alpha rays, later found by Rutherford to be the nuclei of helium atoms; beta rays, shown by Becquerel to be very fast electrons; and gamma rays, identified later as very short-wavelength electromagnetic radiation. In 1898 the French physicists Marie Curie and Pierre Curie separated two highly radioactive elements, radium and polonium, from uranium ore, thus showing that radiations could be identified with particular elements. By 1903 Rutherford and the British physical chemist Frederick Soddy had shown that after the emission of alpha or beta rays the emitting element had changed into a different one.

Radioactive processes were shortly thereafter found to be completely statistical; no method exists that could indicate which atom in a radioactive material will decay at any one time. These developments, in addition to leading to Rutherford’s and Bohr’s model of the atom, also suggested that alpha, beta, and gamma rays could only come from the nuclei of very heavy atoms. In 1919 Rutherford bombarded nitrogen with alpha particles and converted it to hydrogen and oxygen, so producing the first artificial transmutation of elements.

Meanwhile, a knowledge of the nature and abundance of isotopes was growing, largely through the development of the mass spectrometer. A model emerged in which the nucleus contained all the positive charge and almost all the mass of the atom. The nuclear-charge carriers were identified as protons, but the nuclear mass could be accounted for only if some additional uncharged particles were present (except in hydrogen). In 1932 the British physicist James Chadwick discovered the neutron, an electrically neutral particle of mass 1.675 × 10-27 kg, slightly more than that of the proton. Now nuclei could be understood as consisting of protons and neutrons, collectively called nucleons, and the atomic number of the element was simply the number of protons in the nucleus. On the other hand, the isotope number, also called the atomic mass number, was the total number of neutrons and protons present. Thus, all atoms of oxygen (atomic number 8) have eight protons, but the three isotopes of oxygen, O16, O 17, and O18, also contain within their respective nuclei eight, nine, or ten neutrons.

Positive electric charges repel each other, and because atomic nuclei (except for hydrogen) have more than one proton, they would fly apart except for a strong attractive force, called the nuclear force or strong interaction, that binds the nucleons to each other. The energy associated with this strong force is very great, millions of times greater than the energies characteristic of electrons in their orbits, that is, chemical binding energies. An escaping alpha particle (consisting of two protons and two neutrons) will therefore have to overcome this strong interaction force to escape from a radioactive nucleus such as uranium. This apparent paradox was explained by the American physicists Edward U. Condon, George Gamow, and Ronald Wilfred Gurney, who applied quantum mechanics to the problem of alpha emission in 1928 and showed that the statistical nature of nuclear processes allowed alpha particles to “leak” out of radioactive nuclei, even though their average energy was insufficient to overcome the nuclear force. Beta decay was explained as a result of a neutron disruption within the nucleus, the neutron changing into an electron (the beta particle), which is promptly ejected, and a residual proton. The “daughter” nucleus is left with one more proton than its “parent” and thus its atomic number and position in the periodic table are increased by 1. Alpha or beta emission usually leaves the nucleus with excess energy, which it unloads by emitting a gamma-ray photon.

In all these nuclear processes a large amount of energy, given by Einstein’s equation E = mc2, is released. When the process is over, the total mass of the product is less than that of the parent, with the mass difference appearing as energy.

VI DEVELOPMENTS IN PHYSICS SINCE 1930   The rapid expansion of physics in the past few decades was made possible by the fundamental developments of the first third of the 20th century, coupled with recent technological advances, particularly in computer technology, electronics, nuclear-energy applications, and high-energy particle accelerators.

A Accelerators  
Rutherford and other early investigators of nuclear properties were limited to the use of high-energy emissions from naturally radioactive substances to probe the atom. The first artificial high-energy emissions were produced in 1932 by the British physicist John Cockcroft and the Irish physicist Ernest Walton, who used high-voltage generators to accelerate protons to about 700,000 eV and bombarded lithium with them, changing it into helium. One electronvolt is the energy gained by an electron when the accelerating voltage is 1 volt; it is equivalent to about 1.6 × 10-19 joule. Modern accelerators produce energies measured in million electronvolts (usually written mega-electronvolts, or MeV), billion electronvolts (giga-electronvolts, or GeV), or trillion electronvolts (tera-electronvolts, or TeV). Higher-voltage sources were first made possible by the invention, also in 1932, of the Van de Graaff generator by the American physicist Robert J. Van de Graaff.

This was followed almost immediately by the invention of the cyclotron by the American physicists Ernest Orlando Lawrence and Milton Stanley Livingston. The cyclotron uses a magnetic field to bend the paths of charged particles into circles, and during each half-revolution the particles are given a small electric “kick” until they accumulate the high energy desired. Protons could be accelerated to about 10 MeV by a cyclotron, but higher energies had to await the development of the synchrotron after the end of World War II, based on the ideas of the American physicist Edwin Mattison McMillan and the Soviet physicist Vladimir I. Veksler. After World War II, accelerator design made rapid progress, and accelerators of many types were built, producing high-energy beams of electrons, protons, deuterons, heavier ions, and X-rays. For example, the accelerator at the Stanford Linear Accelerator Center (SLAC) in Stanford, California, accelerates electrons down a straight “runway”, 3.2 km (2 mi) long, by the end of which they have attained an energy of more than 20 GeV.

While lower-energy accelerators are used in various applications in industry and laboratories, the most powerful ones are used in studying the structure of elementary particles, the fundamental building blocks of nature. In such studies elementary particles are broken up by hitting them with beams of projectiles, which are usually protons or electrons. The distribution of the fragments yields information on the structure of the elementary particles.

To obtain more detailed information in this manner, the use of more energetic projectiles is necessary. Since the acceleration of a projectile is achieved by “pushing” it from behind, to obtain more energetic projectiles it is necessary to keep pushing for a longer time. Thus, high-energy accelerators are generally larger. The highest beam energy reached at the end of World War II was less than 100 MeV. A bigger accelerator, reaching 3 GeV, was built in the early 1950s at the Brookhaven National Laboratory at Upton, New York. A breakthrough in accelerator design occurred with the introduction of the strong focusing principle in 1952 by the American physicists Ernest D. Courant, Livingston, and Hartland S. Snyder. Today the world’s largest accelerators are built to produce beams of protons beyond 1 TeV. .

B Particle Detectors  
Detection and analysis of elementary particles were first accomplished through the ability of these particles to affect photographic emulsions and to energize fluorescent materials. The paths of ionized particles were first observed by the British physicist C. T. R. Wilson in a cloud chamber, where water droplets condensed on the ions produced by the particles during their passage. Electric or magnetic fields could be used to bend the particle paths, yielding information about their momentum and electric charges. A significant advance on the cloud chamber was the bubble chamber, first constructed by the American physicist Donald Arthur Glaser in 1952. It uses a liquid, usually hydrogen, instead of air, and the ions produced by a fast particle become centres of boiling, leaving an observable bubble track. Because the density of the liquid is much higher than that of air, more interactions take place in a bubble chamber than in a cloud chamber. Furthermore, the bubbles clear out faster than water droplets, allowing more frequent cycling of the bubble chamber. A third development, the spark chamber, evolved in the 1950s. In this device, many parallel plates are kept at a high voltage in a suitable gas atmosphere. An ionizing particle passing between the plates breaks down the gas, forming sparks that delineate its path.

A different type of detector, the discharge counter, was developed early during the 20th century, largely by the German physicist Hans Geiger, and was later improved by the German-American physicist Walther Müller. It is now commonly known as the Geiger-Müller counter, or simply as the Geiger counter, and although small and convenient, it has been largely replaced by faster and more convenient solid-state counting devices, such as the scintillation counter, developed about 1947 by the German-American physicist Hartmut Paul Kallmann and others. It uses the ability of ionized particles to produce a flash of light as they pass through certain organic crystals and liquids.

C Cosmic Rays  
About 1911 the Austrian-American physicist Victor Franz Hess studied cosmic rays. Primary cosmic rays consist of particles originating outside the Earth’s atmosphere. Secondary rays consist of particles and radiation produced by collision of primary cosmic-ray particles with atoms in the atmosphere. Hess found that cosmic rays arrived in a pattern determined by the Earth’s magnetic field. The rays were found to be positively charged and to consist mostly of protons with energies ranging from about 1 GeV to 1011 GeV. Cosmic rays trapped in orbits around the Earth account for the Van Allen radiation belts discovered by the first United States artificial satellite, launched in 1958.

When a very energetic primary proton smashes into the atmosphere and collides with the nitrogen and oxygen nuclei present, it produces large numbers of different secondary particles that spread towards the Earth as a cosmic-ray shower. The origin of the primary cosmic-ray protons is not yet fully understood. Some undoubtedly come from the Sun and the other stars, but it is difficult to account for the highest energies: the likelihood is that weak galactic fields operate over very long periods to accelerate interstellar protons

D Elementary Particles  To the electron, proton, neutron, and photon have been added a number of other fundamental particles. In 1932 the American physicist Carl David Anderson discovered the anti-electron, or positron, predicted in 1928 by Dirac. Anderson found that an energetic cosmic gamma ray could disappear near a heavy nucleus, creating an electron-positron pair out of pure energy. When a positron subsequently meets an electron, they annihilate each other in a burst of photons.

D1 Discovery of the Muon  In 1935 the Japanese physicist Yukawa Hideki developed a theory explaining how a nucleus is held together, despite the mutual repulsion of its protons, by postulating the existence of a particle intermediate in mass between the electron and the proton. In 1936 Anderson and his co-workers discovered a new particle of 207 electron masses in secondary cosmic radiation; now called the muon, it was at first thought to be Yukawa’s nuclear “glue”. Subsequent experiments by the British physicist Cecil Frank Powell and others led to the discovery of a somewhat heavier particle of 270 electron masses, the pi-meson or pion (also obtained from secondary cosmic radiation), which was eventually identified as the missing link in Yukawa’s theory.

Many additional particles have since been found in secondary cosmic radiation and through the use of large accelerators. They include numerous massive particles, classed as hadrons (particles that take part in the strong nuclear interaction, which binds atomic nuclei together), including hyperons and various heavy mesons with masses ranging from about one to three proton masses; and so-called intermediate vector bosons such as the W and Z0 particles, the carriers of the weak nuclear force. They may be electrically neutral, positive, or negative, but never have more than one elementary electric charge e. Enduring from 10-8 to 10-14 sec, they decay into a variety of lighter particles. Each particle has its antiparticle and carries some angular momentum. They all obey certain conservation laws, involving quantum numbers such as baryon number, strangeness, and isotopic spin.

In 1931 Pauli, in order to explain the apparent failure of some conservation laws in certain radioactive processes, proposed the existence of electrically neutral particles of zero or near-zero mass that could carry away energy and momentum. This idea was further developed by the Italian-born American physicist Enrico Fermi, who named the missing particle the neutrino. Uncharged and highly unreactive, it is elusive, easily able to penetrate the entire Earth with only a small likelihood of capture. Nevertheless, it was eventually discovered in a difficult experiment performed by the Americans Frederick Reines and Clyde Lorrain Cowan, Jr. Understanding of the internal structure of protons and neutrons has also been derived from the experiments of the American physicist Robert Hofstadter, using fast electrons from linear accelerators.

In the late 1940s a number of experiments with cosmic rays revealed new types of particles, the existence of which had not been anticipated. They were called strange particles, and their properties were studied intensively in the 1950s. Then, in the 1960s, many new particles were found in experiments with the large accelerators. The electron, proton, neutron, photon, and all the particles discovered since 1932 are collectively called elementary particles. However, the term is actually a misnomer, for most of the particles have been found to have a very complicated internal structure.

Elementary particle physics is concerned with (1) the internal structure of these building blocks and (2) how they interact with one another to form nuclei. The physical principles that explain how atoms and molecules are built from nuclei and electrons are already known. At present, vigorous research is being conducted on both fronts in order to learn the physical principles upon which all matter is built.

The dominant theory of the internal structure of hadrons involves quarks, which are subparticles of fractional charge; a proton, for example, is made up of three quarks. This theory was first proposed in 1964 by the American physicists Murray Gell-Mann and George Zweig. Nucleons consist of triplets of quarks, while mesons consist of pairs of quarks. Isolated quarks cannot be produced by any known process in the modern universe, but they are believed to have existed singly in the extreme conditions found during the very creation of the universe. The theory originally needed three kinds of quarks, but later experiments, especially the discovery of the J/psi particle in 1974 by the American physicists Samuel C. C. Ting and Burton Richter, called for the introduction of three additional kinds.

D2 Unified Field Theories  The most successful theories of interactions between elementary particles, thus far, are called gauge theories. In these, the interaction between two kinds of particles is characterized by symmetry. The symmetry between neutrons and protons, for example, is such that if the identities of the particles are interchanged, nothing changes as far as the strong force is concerned. The first of the gauge theories applied to the electric and magnetic interactions between charged particles. Here, the symmetry consists in the fact that changes in the combination of electric and magnetic potentials have no effect on the results. A powerful gauge theory, which has since been verified, was that proposed independently by the American physicist Steven Weinberg and the Pakistani physicist Abdus Salam in 1967 and 1968. Their model linked intermediate vector bosons with the photon, thus uniting the electromagnetic and weak interactions, although only for leptons (particles that do not “feel” the strong force). Later work by Sheldon Lee Glashow, J. Iliopolis, and L. Maiani showed how the model could be applied to hadrons (the strongly interacting particles) as well.

Gauge theory can in principle be applied to any force field, holding out the possibility that all the interactions, or forces, can be brought together into a single unified field theory. Such efforts inevitably involve the concept of symmetry. Generalized symmetries extend to particle interchanges that vary from point to point in space and time. The difficulty for physicists is that such symmetries, while mathematically elegant, do not extend scientific understanding of the underlying nature of matter. For this reason, many physicists are exploring the possibilities of so-called supersymmetry theories, which would directly relate fermions and bosons. The theory involves further particle “twins” to those now known, differing only in spin. Doubts have been expressed about such efforts, but another approach known as superstring theory is attracting a good deal of interest. In such theories, fundamental particles are considered not as point objects but as “strings” that extend one-dimensionally to lengths of no more than 10-35 metres. Such theories solve a number of problems for the physicists who are working on unified field theories, but they are still only highly theoretical constructs.

E Nuclear Physics  
In 1931 the American physicist Harold Clayton Urey discovered the hydrogen isotope deuterium and made heavy water from it. The deuterium nucleus, or deuteron (one proton plus one neutron), makes an excellent bombarding particle for inducing nuclear reactions. The French physicists Irène and Frédéric Joliot-Curie produced the first artificially radioactive nucleus in 1933-1934, leading to the production of radioisotopes for use in archaeology, biology, medicine, chemistry, and other sciences.

Fermi and many collaborators attempted a series of experiments to produce elements beyond uranium by bombarding uranium with neutrons. They succeeded, and now at least a dozen such transuranic elements have been made. As their work continued, an even more important discovery was made. Irène Joliot-Curie, the German physicists Otto Hahn and Fritz Strassmann, the Austrian physicist Lise Meitner, and the British physicist Otto Robert Frisch found that some uranium nuclei broke into two parts, a phenomenon called nuclear fission. At the same time, a huge amount of energy was released by mass conversion, as well as some neutrons. These results suggested the possibility of a self-sustained chain reaction, and this was achieved by Fermi and his group in 1942, when the first nuclear reactor went into operation. Technological developments followed rapidly; the first atomic bomb was produced in 1945 as a result of a massive programme under the direction of the American physicist J. Robert Oppenheimer, and the first nuclear power reactor for the production of electricity went into operation in Britain in 1956, yielding 78 megawatts.

Further developments were based on the investigation of the energy source of the stars, which the German-American physicist Hans Bethe showed to be a series of nuclear reactions occurring at temperatures of millions of degrees. In these reactions, four hydrogen nuclei are converted into a helium nucleus, with two positrons and massive amounts of energy forming the by-products. This nuclear-fusion process was adopted in modified form, largely based on ideas developed by the Hungarian-American physicist Edward Teller, as the basis of the fusion or hydrogen bomb. First detonated in 1952, it is a weapon much more powerful than the fission bomb, a small fission bomb providing the necessary high triggering temperature.

Much current research is devoted to producing a controlled, rather than an explosive, fusion device, which would be less radioactive than a fission reactor and would provide an almost limitless source of energy. In December 1993 significant progress was made towards this goal when researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 megawatts of power. However, the tokamak consumed more power than it produced during its operation.

F Solid-State Physics  
In solids, the atoms are closely packed, leading to strong interactive forces and numerous interrelated effects that are not observed in gases, where the molecules largely act independently. Interaction effects lead to the mechanical, thermal, electrical, magnetic, and optical properties of solids, which is an area that remains difficult to handle theoretically, although much progress has been made.

A principal characteristic of most solids is their crystalline structure, with the atoms arranged in regular and geometrically repeating arrays (). The specific arrangement of the atoms may arise from a variety of forces. Some solids, such as sodium chloride, or common salt, are held together by ionic bonds originating in the electrical attraction between the ions of which the materials are composed. In others, such as diamond, atoms share electrons, giving rise to covalent bonding. Inert substances, such as neon, exhibit neither of these bonds. Their existence is a result of the so-called van der Waals forces, named after the Dutch physicist Johannes Diderik van der Waals. These forces exist between neutral molecules or atoms as a result of electric polarization. Metals, on the other hand, are bonded by a so-called electron gas, or electrons that are freed from the outer atomic shell and shared by all atoms, and that define most properties of the metal ().

The sharp, discrete energy levels permitted to the electrons in individual atoms become broadened into energy bands when the atoms become closely packed in a solid. The width and separation of these bands define many of the metal’s properties. For example, a so-called forbidden band, in which no electrons may exist, restricts the electron’s motion and results in a good electrical and thermal insulator. The overlapping of energy bands and the associated ease of electron motion results in the metal being a good conductor of electricity and heat. If the forbidden band is narrow, a few fast electrons may be able to jump across, yielding a semiconductor. In this case the energy-band spacing may be greatly affected by minute amounts of impurities, such as arsenic in silicon. The lowering of a high-energy band by the impurity results in a so-called donor of electrons, an n-type semiconductor. The raising of a low-energy band by an impurity such as gallium results in an acceptor, in which the vacancies or “holes” in the electron structure act like mobile positive charges and are characteristic of p-type semiconductors. A number of modern electronic devices, notably the transistor, developed by the American physicists John Bardeen, Walter Houser Brattain, and William Bradford Shockley, are based on these semiconductor properties.

Magnetic properties in a solid arise because the electrons act like tiny magnetic dipoles. Almost all solid properties depend on temperature. Thus, ferromagnetic materials, including iron and nickel, lose their normal strong residual magnetism at a characteristic high temperature, called the Curie temperature. Electrical resistance usually decreases with decreasing temperature, and for certain materials, called superconductors, it becomes extremely low near absolute zero (). These and many other phenomena observed in solids depend on energy quantization and can best be described in terms of effective “particles” with such names as phonons, polarons, and magnons.

G Cryogenics  
At very low temperatures (near absolute zero), many materials exhibit strikingly novel characteristics (). At the beginning of the 20th century the Dutch physicist Heike Kamerlingh Onnes developed techniques for producing these low temperatures and discovered the superconductivity of mercury, which loses all electrical resistance at about 4 kelvins. Many other elements, alloys, and compounds do the same at their characteristic near-zero temperature, with originally magnetic materials becoming magnetic insulators. Since 1986, a number of materials have been made that are superconductive at higher temperatures. The theory of superconductivity, developed largely by John Bardeen and two other American physicists, Leon N. Cooper and John Robert Schrieffer, is extremely complicated, involving the pairing of electrons in the crystal lattice.

Another fascinating discovery was that helium does not freeze but changes at about 2 kelvins from an ordinary liquid, He I, to the superfluid He II, which has no viscosity and has a thermal conductivity about 1,000 times greater than that of silver. Films of He II can creep up the walls of their containing vessels and He II can readily permeate some materials like platinum. No fully satisfactory theory is yet available for this behaviour.

H Plasma Physics  A plasma is any substance (usually a gas) from whose atoms one or more electrons have become detached and that has therefore become ionized. The detached electrons remain, however, in the gas volume, which overall remains electrically neutral. The ionization can be effected by the introduction of large concentrations of energy, such as bombardment with fast external electrons, irradiation with laser light, or heating to very high temperatures. The individually charged plasma particles respond to electric and magnetic fields and can therefore be manipulated and contained.

Plasmas are found in gas-filled light sources such as a neon lamp, in interstellar space where residual hydrogen is ionized by radiation, and in stars whose high interior temperatures produce a high degree of ionization, a process closely connected with the nuclear fusion that supplies the energy of stars. For the hydrogen nuclei to fuse into heavier nuclei, they must be fast enough to overcome their mutual electrical repulsion. This implies a high temperature (millions of degrees). In order to produce a controlled fusion, or thermonuclear reaction, it is necessary to generate and contain plasmas magnetically; this is an important but difficult problem that falls in the field of magnetohydrodynamics.

I Lasers  
An important recent development is that of the laser, an acronym for light amplification by stimulated emission of radiation. In lasers, which may have gases, liquids, or solids as the working substance, a large number of atoms are raised to a high energy level and caused to release this energy simultaneously, producing coherent light in which all waves are in phase. The coherence of the light allows for very high-intensity, sharp-wavelength light beams that remain narrow over tremendous distances.They are far more intense than light from any other source. Continuous lasers can deliver hundreds of watts, and pulsed lasers can produce millions of watts of power for very short periods. Developed during the 1950s and 1960s, largely by the American engineer and inventor Gordon Gould and the American physicists Charles Hard Townes, T. H. Maiman, Arthur Schawlow, and Ali Javan, the laser today has become an extremely powerful tool in research and technology, with applications in communications, medicine, navigation, metallurgy, fusion, and cutting materials.

J Astrophysics and Cosmology  Since World War II astronomers have made many important discoveries, such as quasars, pulsars (), and the cosmic background radiation. These have challenged the ability of current physics to explain them, and have stimulated the development of theory in such areas as gravitation and elementary particle physics. It is now widely accepted that all the matter accessible to people’s observation was originally tightly packed in one location and that between 10 and 20 billion years ago it exploded in one titanic event, the big bang. The explosion has led to a universe that is still expanding. A puzzling aspect of this universe, recently revealed, is that the galaxies are not uniformly distributed. Instead, vast voids are bordered by galactic clusters shaped like filaments. The pattern of these voids and filaments is powerful evidence for the nature of the matter emerging from the big bang. It suggests the strong possiblity that familiar forms of matter were outweighed by exotic dark matter. This is just one of the ways in which the physics of the very large has converged with the physics of the very small.

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