Physics, major science, dealing with the fundamental constituents
of the universe, the forces they exert on one another, and the results
produced by these forces. Sometimes in modern physics a more sophisticated
approach is taken that incorporates elements of the three areas listed
above; it relates to the laws of symmetry and conservation, such as those
pertaining to energy, momentum, charge, and parity.
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 the elucidation and measurement of properties of a system as a whole and remains 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.
Physics emerged as a separate science only in the early 19th century; until that time a physicist was often also a mathematician, philosopher, chemist, biologist, engineer, or even primarily a political leader or artist. 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 the domain of 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.
The 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. Little was added
by the Greek civilization, partly because the uncritical acceptance of
the ideas of the major philosophers Plato and Aristotle discouraged experimentation.
Some progress was made, however, notably in Alexandria, the scientific center of Greek civilization. There, the Greek mathematician and inventor Archimedes designed various practical mechanical devices, such as levers and screws, and measured the density of solid .
Little advance was made in physics, or in any other science, during
the Middle Ages, other than the preservation of the classical Greek treatises,
for which the Arab scholars such as Averroës and Al-Quarashi, the
latter also known as Ibn al-Nafis, deserve much credit. The founding of
the great medieval universities by monastic orders in Europe, starting
in the 13th century, generally failed to advance physics or any experimental
investigations. The Italian Scholastic philosopher and theologian Saint
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 and who also did some work in astronomy, chemistry, optics, and
The advent of modern science followed the Renaissance and was ushered
in by the highly successful attempt by four outstanding individuals to
interpret the behavior of the heavenly bodies during the 16th and early
17th centuries. The Polish natural philosopher Nicolaus Copernicus propounded
the heliocentric system that 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, believing
in the Ptolemaic system, tried to confirm it by a series of remarkably
accurate measurements. These provided his assistant, the German astronomer
Johannes Kepler, with the data to overthrow the Ptolemaic system and led
to the enunciation 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, starting in 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 dictums), 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 Sir Isaac Newton, one of the greatest
scientists who ever lived.
Starting about 1665, at the age of 23, Newton enunciated the principles
of mechanics, formulated the law of universal gravitation, separated white
light into colors, proposed a theory for the propagation of light, and
invented differential and integral calculus. Newton's contributions covered
an enormous range of natural phenomena: He was thus able to show that not
only Kepler's laws of planetary motion but also Galileo's discoveries of
falling bodies follow a combination of his own second law of motion and
the law of gravitation, and to predict the appearance of comets, explain
the effect of the moon in producing the tides, and explain the precession
of the equinoxes.
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 will be proportional to its mass times the acceleration. If the force and the initial position and velocity of a body are given, subsequent positions and velocities can be computed, although the force may vary with time or position; in the latter 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 impressed. Even today, the law retains its practical utility, 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 more sophisticated modern terms, that all forces between particles come in oppositely directed pairs, although not necessarily along the line joining the particles.
Although the ancient Greeks were aware of the electrostatic properties
of amber, and the Chinese as early as 2700 BC made crude 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 particle, tending to accelerate one toward the other. If the medium through which the particle moves offers resistance to that motion, this 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 battery by the Italian physicist Alessandro Volta in 1800. The classical theory of a simple electric circuit assumes that the two terminals of a battery are maintained positively and negatively charged as a result of its internal properties. When the terminals are connected by a wire, negatively charged particles will be 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 battery will force the particles toward 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 between the current flowing and the electromotive force supplied by a battery, known as the resistance of the circuit. Ohm's law, which states that the resistance is equal to the electromotive force, or voltage, divided by the current, is not a fundamental and universally applicable law of physics, but rather describes the behavior of a limited class of solid materials.
The historical concepts of magnetism, based on the existence of pairs of oppositely charged poles, had started in the 17th century and owe much to 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 after learning of Oersted's discovery, the French scientist André Marie Ampère showed experimentally that two current-carrying wires would 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 wire without connection to a battery, either by moving a magnet or by placing another current-carrying wire with an unsteady—that is, rising and falling—current nearby. The intimate connection between electricity and magnetism, now established, can best be stated in terms of electric or magnetic fields, or forces that will act at a particular point 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 will exert an additional force only if the charges are in motion.
These qualitative findings were finally 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 with the charge and current densities at that point. In principle, they permit the calculation of the fields everywhere and 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 that was produced by accelerating charges, that was propagated through space with the speed of light in the form of an electromagnetic wave, and that decreased with the inverse square of the distance from the source. In 1887 the German physicist Heinrich Rudolf Hertz succeeded in actually generating such waves by electrical means, thereby laying the foundations for radio, radar, television, and other forms of telecommunications.
The behavior 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 period or 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, or at a different phase. Each point along the string will do what its neighbor did, but a little later, if it is further removed from the vibrating source (Oscillation). The speed with which the disturbance, or the message to oscillate, is transmitted along the string is called the wave velocity (Wave Motion). This is a function of the medium, its mass, and the tension in the case of a string. An instantaneous snapshot of the string (after it has been in motion for a while) would show equispaced points having the same displacement and motion, 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 at a direction at right angles to that of the electric field. The electromagnetic-wave velocity away from the source is the speed of light.
The apparent linear propagation of light was known since antiquity,
and 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 proposed a wave theory. No experiment could be performed that distinguished
between the two theories 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 favored the wave
Interference can be demonstrated by placing a thin slit in front of a light source, stationing a double slit farther away, and looking at a screen spaced some distance behind the double slit. Instead of showing a uniformly illuminated image of the slits, the screen will show equispaced light and dark bands. Particles coming from the same source and arriving at the screen via the two slits could not produce different light intensities at different points and could certainly not cancel each other to yield dark spots. Light waves, however, can 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. Depending on the difference in their paths, "positive" displacements arriving at the same time as "negative" displacements of the other will tend to cancel out and produce darkness, while the simultaneous arrival of either positive or negative displacements from both sources will lead to reinforcement or brightness. Each apparent bright spot undergoes a timewise 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 Hz, 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 a simple measurement of the distance between the two slits, and the distance between adjacent bright bands on the screen; it 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 colors.
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 the intervening change in the distance between Earth and Jupiter, and to the corresponding difference 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 coworkers, which extended into the 20th century. Today the velocity of light is known very accurately as 299,292.6 km (185,971.8 mi sec) in vacuum. In matter, the velocity is less and varies with frequency, giving rise to a phenomenon known as dispersion. also Optics; Spectrum; Vacuum.
Maxwell's work contributed several important results to the understanding of light by showing that it was electromagnetic in origin and that electric and magnetic fields oscillated in a light wave. His work predicted the existence of nonvisible light, and today electromagnetic waves or radiations are known to cover the spectrum from gamma rays (Radioactivity), with wavelengths of 10-12 cm (4 × 10-11 in), through X rays, visible light, microwaves, and radio waves, to long waves of hundreds of kilometers in length (. X Ray). It also related the velocity of light in vacuum and through 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 had to travel (. the Electricity and Magnetism section above). Based on the experience with water, sound, and elastic waves, scientists assumed a similar medium to exist, a "luminiferous ether" without mass, which was all-pervasive (because light could obviously travel through a massless vacuum), and had to act like a solid (because electromagnetic waves were known to be transverse and the oscillations took place in a plane perpendicular to the direction of propagation, and gases and liquids could only sustain longitudinal waves, such as sound waves). The search for this mysterious ether occupied phycisists' 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 forward with a speed of 3.2 km/h (2 mph) in a train traveling at 64.4 km/h (40 mph) appears to move at 67.6 km/h (42 mph), to an observer on the ground. In terms of the velocity 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 while the earth also moves through the ether? Or, alternately, what is the earth's velocity through the ether? 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 traveling 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.
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 William Thomson, 1st Baron Kelvin, by the middle of the 19th century. Equivalence means that doing work on a system can produce exactly the same effect as adding heat; thus the same temperature rise can be achieved in a gas contained in a vessel by adding heat or by doing an appropriate amount of work through a paddle wheel sticking into the container where the paddle is actuated by falling weights. The numerical value of this equivalent was first demonstrated by the British physicist James Prescott Joule in several heating and paddle-wheel experiments between 1840 and 1849.
That performing work or adding heat to a system were both means of transferring energy to it was thus recognized. Therefore, the amount of energy added by heat or work had to increase the internal energy of the system, which in turn determined 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 action 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.
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 while only 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 inverse 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 addition of work (usually as electrical power), and the discharge of heat (usually via finned coils in the rear) to the surroundings (. Refrigeration). 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 alternate, 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 adjust to a single uniform temperature and thus produce equilibrium. This can also be applied to other internal properties that may be different 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 separate or ordered state is turned into a mixed or disordered state. These ideas can be expressed by a thermodynamic property, called the 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 all temperature in space becomes uniform, resulting in the so-called heat death of the universe.
Locally, the entropy can be lowered by external action. This applies to machines, such as a refrigerator, where the entropy in the cold chamber is being 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 nonreversibility 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 promulgation of the second law, thermodynamics went on to other advances and applications in physics, chemistry, and engineering. Most chemical engineering, all power-plant engineering, and air-conditioning 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.
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 backward
with velocity u, the relative speed between 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
the concept of adding velocities, and two observers, one at rest and the
other moving toward a light source with velocity u, both observe the same
light velocity V, 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 (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 must be augmented by 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 that applied 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 light in its passage close to a massive body like a star, an effect first observed in 1919. General relativity, although less firmly established than the special theory, has deep significance for an understanding of the structure of the universe and its evolution. . also Cosmology.
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 Theodor Felix von Laue and his coworkers. 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 atomic number of an element, and hence the number of positive charges in an atom, is the same as its position in the periodic table (. Periodic Law). 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.
That electric charges were carried by extremely small particles had already been suspected in the 19th century and, as indicated by electrochemical experiments, 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. Sir Joseph John 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 photoelectric particles was identical to that of cathode rays. The American inventor Thomas Alva Edison had noted in 1883 that very hot wires emit electricity, called 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 the value of e, now known to be 1.602 × 10-19 coulombs. From the measured value of q to m ratio, with q set equal to e, the mass of the carrier, called electron, could now be determined as 9.110 × 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 (. Ion; Ionization). The "quantized" nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles identified.
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, massive, positively charged
alpha particles he employed were found to deflect sharply in their passage
through matter. This effect required an atomic model with a heavy positive
scattering center. 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 positioned by the electric attraction
between opposite charges. This solar-system-like atomic model, however,
could not persist according to Maxwell's theory, where the revolving electrons
should emit electromagnetic radiation and force 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 Henrik David Bohr, who postulated the existence within atoms of certain specified orbits in which electrons could revolve without electromagnetic radiation emission. 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 multiple integral of Planck's constant, divided by 2 p, that is, J = nh/2p, 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. Also in 1913 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 colors of the simple hydrogen atom. Because it had no more than a limited ability to predict experimental results, it remained unsatisfactory for theoretical physicists.
The understanding of atomic structure was also facilitated by Becquerel's
discovery in 1896 of radioactivity in uranium ore (. Uranium). Within a
few years radioactive radiation 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 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 the emission
of alpha or beta rays resulted in the transmutation of the emitting element
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, thus 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 spectrograph. 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 except for hydrogen, the nuclear mass could be accounted for only if some additional uncharged particles were present. In 1932 the British physicist Sir 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 sum of the neutrons and protons present. Thus, all atoms of oxygen (atomic no. 8) have eight protons, but the three isotopes of oxygen, O16, O17, 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 or chemical binding energies. An escaping alpha particle (consisting of two protons and two neutrons), therefore, will 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 proton left behind leaves the "daughter" nucleus with one more proton than its "parent" and thus increases the atomic number and the position in the periodic table. 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 E = mc2 equation, is released. After the process is over, the total mass of the product is less than that of the parent, with the mass difference appearing as energy. . Nuclear Energy.
Developments in Physics Since 1930
The rapid expansion of physics in the last few decades was made possible by the fundamental developments during the first third of the century, coupled with recent technological advances, particularly in computer technology, electronics, nuclear-energy applications, and high-energy particle 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 Sir John Douglas Cockcroft
and the Irish physicist Ernest Thomas Sinton Walton, who used high-voltage
generators to accelerate protons to about 700,000 eV and to bombard lithium
with them, transmuting it into helium. One electron volt is the energy
gained by an electron when the accelerating voltage is 1 V; it is equivalent
to about 1.6 × 10-19 joule (J). Modern accelerators produce energies
measured in million electron volts (usually written mega-electron volts,
or MeV), billion electron volts (giga-electron volts, or GeV), or trillion
electron volts (tera-electron volts, 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 trajectories of charged particles into circles, and during each half-revolution the particles are given a small electric "kick" until they accumulate the high energy level 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 (1939-1945), 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, at the end of which they attain 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 that 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 in size. 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 have been or are being built to produce beams of protons beyond 1 TeV. Two are located at the Fermi National Accelerator Laboratory, near Batavia, Illinois, and at the European Laboratory for Particle Physics, known as CERN, in Geneva, Switzerland. . Particle Accelerators.
About 1911 the Austrian-American physicist Victor Franz Hess discovered
that cosmic radiation, consisting of rays originating outside the earth's
atmosphere, arrived in a pattern determined by the earth's magnetic field
(. Cosmic Rays). The rays were found to be positively charged and to consist
mostly of protons with energies ranging from about 1 GeV to 1011 GeV (compared
to about 30 GeV for the fastest particles produced by artificial accelerators).
Cosmic rays trapped into orbits around the earth account for the Van Allen
radiation belts discovered during an artificial-satellite flight in 1959
(. Radiation Belts).
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 toward the earth as a cosmic-ray shower. The origin of the cosmic-ray protons is not yet fully understood; some undoubtedly come from the sun and the other stars. Except for the slowest rays, however, no mechanism can be found to account for their high energies and the likelihood is that weak galactic fields operate over very long periods to accelerate interstellar protons (. Galaxy; Milky Way).
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 coworkers discovered
a new particle of 207 electron masses in secondary cosmic radiation; now
called the mu-meson or muon, it was 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" interaction, which binds atomic nuclei together), including hyperons and various heavy mesons with masses ranging from about one to three proton masses; and 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, postulated the existence of electrically neutral particles of zero-rest mass that nevertheless could carry 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 tiny, 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. But the term is actually a misnomer, for most of the particles, such as the proton, have been found to have 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.
One popular theory about the internal structure of elementary particles is that they are made of so-called quarks (. Quark), 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. Despite the theory's ability to explain a number of phenomena, no quarks have yet been found, and current theory suggests that quarks may never be released as separate entities except under such extreme conditions as those found during the very creation of the universe. The theory postulated 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.
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 and 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 transuranium 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 program 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 England in 1956, yielding 78 million watts. . Nuclear Weapons.
Further developments were based on the investigation of the energy source of the stars, which the German-American physicist Hans Albrecht 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 toward this goal when researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 million watts of power. However, the tokamak consumed more power than it produced during its operation.
At very low temperatures (near absolute zero), many materials exhibit
strikingly different characteristics (. Cryogenics). 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: It loses all electrical resistance at about 4 K. Many other
elements, alloys, and compounds do the same at their characteristic near-zero
temperature, with originally magnetic materials becoming magnetic insulators.
The theory of superconductivity, developed largely by the American physicists
John Bardeen, 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 K from an ordinary liquid, He I, to the superfluid He II, which has no viscosity and has a thermal conductivity about 1000 times greater than 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 behavior.
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 where all waves are in phase.
Similar techniques are used for producing microwave emissions by the use
of masers. 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 of power, 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 Leonard 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 material cutting.
The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including quasars, which are billions of light-years away, and has led to a better understanding of the structure of the universe. Radio astronomy has yielded other important discoveries, such as pulsars and the interstellar background radiation, which probably dates from the origin of the universe. The evolutionary history of the stars is now well understood in terms of nuclear reactions. As a result of recent observations and theoretical calculations, the belief is now widely held that all matter was originally in one dense location and that between 10 and 20 billion years ago it exploded in one titanic event often called the big bang. The aftereffects of the explosion have led to a universe that appears to be 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 lends itself to nonlinear mathematical analysis of the sort used in chaos theory. . also Inflationary Theory.