Physics

INTRODUCTION

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.

MODERN PHYSICS

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 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.

B. QUANTUM THEORY

The quandary 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 postulating that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons. Quantum Theory. Each photon has a characteristic wavelength in the spectrum and an energy E given by E = hf, where f is the frequency of the wave. The wavelength λ related to the frequency by λf = 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^-27 erg-sec). With his theory, Planck again introduced a partial duality into the theory of light, which for nearly a century had been considered to be wavelike only.

C. PHOTOELECTRICITY

If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, later identified as 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 to cause 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 in 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 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. 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, 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 (see Ion; Ionization). The “quantized” nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles 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, 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.

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 behavior. Named quantum mechanics or wave mechanics, it started with the suggestion in 1923 by the French physicist Louis Victor, Prince de Broglie, that not only electromagnetic radiation but matter could also have wave as well as particle aspects. The wavelength of the so-called matter waves associated with a particle is given by the equation λ = 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 the experiments on electron-crystal interactions by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist Sir George Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst Pascual Jordan of Germany and the Austrian physicist Erwin Schrödinger developed 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 postulate 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, also exist that can only be satisfactorily explained 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 expounded 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 forth 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 Paul Adrien Maurice 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 different and statistical approach developed in modern physics. The fully deterministic cause-effect relations produced by Newtonian mechanics were supplanted by predictions of future events in terms of statistical probability only. Thus, the wave property of matter also implies that, in accordance with the uncertainty principle, the motion of the particles can never be predicted with absolute certainty even if the forces are known completely. Although this statistical aspect plays no detectable role in macroscopic motions, it is dominant on the molecular, atomic, and subatomic scale.

H. NUCLEAR PHYSICS

The understanding of atomic structure was also facilitated by Becquerel's discovery in 1896 of radioactivity in uranium ore (see 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, 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 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.

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 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.


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