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Physics. Period I (1900-1945)






The decisive events of the first period have been the conception of the Theory of Relativity and that of Quantum Mechanics. Rarely in the history of science have two complexes of ideas so fundamentally influ­enced natural science in general.

There are important differences between the two achievements. Relativity theory should be regarded as the crowning of classical physics of the eighteenth and nineteenth centuries. The special theory of relativity brought about a unification of mechanics and electromagnetism. These two fields were inconsistent with each other, when dealing with fast-moving electrically charged objects. Of course, relativity created new notions, such as the relativity of simultaneity, the famous mass-energy relation, the idea that gravity can be described as a curvature of space. But, altogether, the theory of relativity uses the concepts of classical phys­ics, such as position, velocity, energy, momentum, etc. Therefore it must be regarded as a conservative theory, establishing a logically coherent system within the edifice of classical physics.

Quantum mechanics was truly revolutionary. It is based on the recog­nition that the classical concepts do not fit the atomic and molecular world: a new way to deal with that world was created. Limits were set to the applicability of classical concepts by Heisenberg's uncertainty relations. They say 'down to here and no further can you apply classical concepts'. This is why it would have been better to call them 'Limiting Relations'. It would also have been advantageous to call relativity theory 'Absolute Theory', since it describes the laws of Nature independently of the systems of reference. Much philosophical abuse would have been avoided.

It took a quarter of a century to develop non-relativistic Quantum Mechanics. Once conceived, an explosive development occurred. With­in a few years most atomic and molecular phenomena could be under­stood, at least in principle. It is appropriate to quote a slightly altered version of a statement by Churchill praising the Royal Air Force: 'Never have so few done so much in so short a time'.

A few years later, the combination of relativity and quantum me­chanics yielded new unexpected results. P.A.M. Dirac conceived his relativistic wave equation which contained the electron spin and the fine structure of spectral lines as a natural consequence. The application of quantum mechanics to the electromagnetic field gave rise to Quantum Electrodynamics with quite a number of surprising consequences, some of them positive, others negative.

The positive ones included Dirac's prediction of the existence of an antiparticle to the electron, the positron, which was found afterwards in 1932 by C.D. Anderson and S.H. Nedermeyer. Most surprising were the predictions of the creation of particle - antiparticle pairs by radiation or other forms of energy and the annihilation of such pairs with the emis­sion of light or other energy carriers. Another prediction was the exist­ence of an electric polarization of the vacuum in strong fields. All these new processes were found experimentally later on.

The negative ones are consequences of the infinite number of degrees of freedom in the radiation field. Infinities appeared in the coupling of an electron with its field and in the vacuum polarization when the contri­bution of high-frequency fields is included. These infinities cast a shad­ow on quantum electrodynamics until 1946 when a way out was found by the so-called renormalization method.

Parallel to the events in physics during Period I, chemistry, biology, and geology also developed at a rapid pace. The quantum mechanical explanation of the chemical bond gave rise to quantum chemistry that allowed a much deeper understanding of the structure and properties of molecules and of chemical reactions. Biochemistry became a growing branch of chemistry. Genetics was established as a branch of biology, recognizing the chromosomes as carriers of genes, the elements of inher­itance. Proteins were identified as essential components of living sys­tems. The knowledge of enzymes, hormones, and vitamins vastly in­creased during that period. Embryology began to investigate the early development of living systems: how the cellular environment regulates the genetic program. Darwin's idea of evolution was considered in greater detail, recognizing the lack of inheritance of acquired properties. A kind of revolution was also started in geology by A. Wegener's concept of plate tectonics and continental drift. W. Elsasser's suggestion of eddy currents in the liquid-iron core of the Earth as the source of the Earth's magnetism was published at the end of Period I, and led to the solution o! a hitherto unexplained phenomenon.

The year 1932 was a miracle year in physics. The neutron was discovered by J. Chadwick, the positron was found by Anderson and Neddermeyer, a theory of radioactive decay was formulated by E. Fermi in analogy with quantum electrodynamics, and heavy water was discovered by H. Urey. The discovery of the neutron initiated nuclear physics; theatomic nucleus was regarded as a system of strongly interacting protons and neutrons. This interaction is a consequence of a new kind of force, the 'nuclear force', besides the electromagnetic and gravity forces, and the 'weak force' that Fermi introduced in his theory of radioactivity. Nuclear physics in the 1930's was a repeat performance of atomic quantum mechanics albeit on a much higher energy level, about a million times the energies in atoms, and based on a different interaction. It led to an understanding of the principles of nuclear spectroscopy and of nuclear reactions. Artificial radioactivity, and later nuclear fission and fusion were discovered with fateful consequences of their military applications. One of the most important insights of nuclear physics in Period I was the explanation of he sources of solar and stellar energy by fusion reactions in the interior of stars.

What is most striking was the small number of experimental and theoretical physicists who dealt with the new developments. The yearly Copenhagen Conferences, devoted to the latest progress in quantum mechanics and relativity, were attended by not more than fifty or sixty people. There was no division into specialities. Atomic and molecular physics, nuclear physics, condensed matter, astronomy, and cosmology were discussed and followed up by all participants. In general, every­body present was interested in all subjects and their problems. Quantum mechanics was regarded as an esoteric field; practical applications were barely mentioned.

Most characteristic of pre-World-War II science were small groups и ml low costs of research, primarily funded by universities or by foundations and rarely by government sources. Foundations had a great influence on science. Some of the impressive developments of the thir­ties in biology can be traced to the decision of the Rockefeller Foun­dation under Warren Weaver to support biology more than other sciences.

 

Ответьте на вопросы по тексту:

1. What is art?

2. What is the difference between an artist and an artisan?

3. How many systems of classification exist?

4. What was so revolutionary about the Quantum Mechanics?

5. What other sciences developed rapidly during the first period?

6. What was the beginning of nuclear physics?

7. Why is the Theory of Relativity the integral part of classical physics?


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