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TEXT FLY WITHIN THE BOOK ONLY OU_160736>5 THE EVOLUTION OF PHYSICS THE EVOLUTION OF PHYSICS BY A EINSTEIN AND first L INFELD appeared in THE CAMBRIDGE LIBRARY OF MODERN SCIENCE, a series of new books describing, in language suitable for the general nonspecialist reader, the present position in many branches of modern science The series is edited and is by Dr C P Snow, published by CAMBRIDGE UNIVERSITY PRESS BENTLEY HOUSE, EUSTON ROAD LONDON, N.W I THE EVOLUTION OF PHYSICS BY ALBERT EINSTEIN & LEOPOLD INFELD THE SCIENTIFIC BOOK CLUB in CHARING CROSS ROAD LONDON W.C * QUANTA " 311 from "two different from "two stones" The concepts of the pure numbers 2, 3, 4, freed from the objects from which they arose, are creations of the thinking mind which describe the reality of our world Three trees" is something Again "two trees" is different trees" , The psychological subjective feeling of time enables us to order our impressions, to state that one event precedes another But to connect every instant of time with a number, by the use of a clock, to regard time as a one-dimensional continuum, is already an invention So also are the concepts of Euclidean and non-Euclidean geometry, and our space understood as a threedimensional continuum Physics really began with the invention of mass, force, and an inertial system These concepts are all free inventions They led to the formulation of the For the physicist of the early nineteenth century, the reality of our outer world consisted of particles with simple forces acting mechanical point of view He between them and depending only on the distance tried to retain as long as possible his belief that he would succeed in explaining by all events in nature these fundamental concepts of reality The difficulties connected with the deflection of the magnetic needle, the difficulties connected with the structure of the ether, induced us to create a more subtle reality The important invention of the electromagnetic field ap- A courageous scientific imagination was needed pears to realize fully that not the behaviour of bodies, but THE EVOLUTION OF PHYSICS 312 the behaviour of something between them, that field, may be essential for ordering is, the and understanding events Later developments both destroyed old concepts ones Absolute time and the inertial and created new co-ordinate system were abandoned by the relativity theory The background for all events was no longer the one-dimensional time and the three-dimensional space continuum, but the four-dimensional time-space continuum, another free invention, with new transfor- mation properties The inertial co-ordinate system was no longer needed Every co-ordinate system is equally suited for the description of events in nature The quantum theory again created new and essential features of our reality Discontinuity replaced conInstead of laws governing individuals, probability laws appeared The reality created by modern physics is, indeed, far tinuity removed from the reality of the early days But the aim of every physical theory still remains the same With the help of physical theories we try to find our way through the maze of observed facts, to order and understand the world of our sense impressions We facts to follow logically from our want the observed concept of reality Without the belief that it is possible to grasp the reality with our theoretical constructions, without the belief in the inner harmony of our world, there could be no science This belief is remain the fundamental motive tion Throughout all our and always will for all scientific crea- efforts, in every dramatic QUANTA 313 struggle between old and new views, we recognize the eternal longing for understanding, the ever-firm belief in the harmony of our world, continually strengthened by the increasing WE obstacles to comprehension SUMMARIZE: Again the rich variety phenomena forces us to has a granular structure; the elementary quanta of facts in invent the new physical realm of atomic concepts Matter it is composed of elementary particles, of matter Thus, the electric charge has a granular structure and most important from the point of so has energy Photons are the view of the quantum theory energy quanta electrons a of which light is composed wave or a shower of photons? Is a beam of shower of elementary particles or a wave? These Is light a fundamental questions are forced upon physics by experiment In seeking to answer them we have to abandon the description of atomic events as happenings in space retreat still further from and time, the old mechanical view we have to Quantum physics formulates laws governing crowds and not individuals Not properties but probabilities are described, not laws disclosing the future of systems are formulated, but laws governing the changes in time of the probabilities and relating to great congregations of individuals INDEX INDEX Absolute motion, 180, 224 Diffraction (continued) of light, 1 Aristotle, of X-rays, 286 Dipole Black, 39, 40, 51 Bohr, 283, 302 : electric, Born, 302 Brown, 63, 64 Brownian movement, 6367 Dispersion, 102, 117 Dynamic picture of motion, 216 Caloric, 43 Change in velocity, 10, 18, 2324, 28 Classical transformation, 171 Electric potential, 80-82 substances, 74 Electromagnetic : one-dimensional, 210 two-dimensional, 1 three-dimensional, 212 four-dimensional, 219 Co-ordinate of a point, 68 Co-ordinate system, 162, 163 79, 86 Crucial experiments, 44-45 Crystal, 285 163 Current, 88 induced, 88 G.S., de Broglie, 287, 290, 302 Democritus, 56 : 151 field, theory of light, 157 wave, 154 Electronic wave, 292 Electrons, 268 Electroscope, 71 Copernicus, 161, 223, 224 Corpuscles of light, 99, 100, 275 Coulomb, : charge, 80-82 current, 88 Conductors, 74 Constant of the motion, 49 Continuum 84 magnetic, 85 Dirac, 302 Elementary magnetic dipoles, 85 particles, 206 quantum, 264 Elements of a battery, 88 : Energy : 50 282 mechanical, 51 potential, 49, 50 kinetic, 49, level, Ether, 112, 115, 120, 123-126, Diffraction of electronic wave, 293 172, : 184 317 175, 176, 179, 80- INDEX 3i8 Law of gravitation, 30 of inertia, 8, 160 of motion, 31 Faraday, 129, 142 Field, 131 representation, 131 static, 141 structure of, 149, 152 Fizeau, 96 Frame of reference, 63 Fresnel, 118 Force, u, 19, 24, Leibnitz, 25 Light, bending in gravitational field, 234, 252 homogeneous, 103 quanta, 275 28 substance, 102-104 white, 101 Lorentz transformation, lines, 130 matter, 56 198- 202 Galilean relativity principle, 165 Galileo, 5, 7, 8, 9, 39, 56, 94, 95,96 Galvani, 88 Generalization, 20 General relativity, 36, 224 Gravitational mass, 36, 227, 230 Heat, 38, 41, 42 capacity, 41 energy, 50-55 specific, 41 substance, 42, 43 Heisenberg, 302 Helmholtz, 58, 59 Hertz, 129, 156 Huygens, no, in Induced current, 142 Inertial mass, 36, 227, 230 system, 166, 220, 221 system, local, 228, 229 Insulators, 71, 74 Mass, 34 energy, 208 of one electron, 270 of one hydrogen atom, 266 of one hydrogen molecule, 67, 265 Matter = energy, 54 field, 256 Maxwell, 129 Maxwell's equations, 148, 149, 150, 153 Mayer, 51 Mechanical equivalent of heat, 54 Mechanical view, 59, 87, 92, 120, 124, 157 Mercury, 253 Metric properties, 246 Michelson, 97, 183 Molecules, 59 number of, 66 Morley, 183 Invariant, 170 Newton, Joule, 51, 52, 53 Kinetic theory, 59-67 5, 8, 9, 25, 79, 92, 100, 101 Nodes, 288 Nuclear physics, 272 INDEX 319 Nucleus, 271 Spectroscope, 281 Oersted, go, 91 Spectrum, visible, 102, 284 Static picture of motion, 216 298 Synchronized clocks, 190, 191 Statistics, Photoelectric effect, 273 Photons, 275 ultraviolet, 284 Temperature, 38-40 Planck, 275 Test body, 130 Pole, magnetic, 83 Thomson, J J., 268 Tourmaline crystal, 121 Principia, 1 Probability, 298 Transformation laws, 169 wave, 304 Ptolemy, 223, 224 Two New Sciences , 10, 94 Uniform motion, Radioactive disintegration, 299 matter, 206 Radium, 206 Rate of exchange, 52 Rectilinear motion, 12 propagation of light, 97, 99, 120 Vectors, 12-19 of electromagnetic Velocity wave, 155 of light, 97 vector, 21 Reflection of light, 99 Volta, 88, 89 Voltaic battery, 88 Refraction, 98, 99, 115, 116 Relative uniform motion, 180 Wave, 104 Relativity, 186 general, 36, 224 06, 117 longitudinal, 108, 121 length, no special, 224 Rest mass, 205 Roemer, 96 Rowland, 92 plane, Rumford, transverse, 108, 121 45, 47, 51 Rutherford, 272 spherical, 109 standing, 288 theory of light, velocity, 106 Weightless Schrodinger, 287, 302 substances, 79 Sodium, 103 Solenoid, 136 X-rays, 284-286 Special relativity, 224 Spectral lines, 280 no Young, 118 43, CAMBRIDGE PRINTED BY W LEWIS, M.A AT THE UNIVERSITY PRESS :