Theoretical Physics Conference, 1935

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REPORT ON THEORETICAL PHYSICS CONFERENCE, APRIL 19, 20, 21, 1935

The first annual conference on theoretical physics, called on April 19, 20, 21, 1935, by George Washington University together with the Carnegie Institution of Washington, was devoted to the discussion of the latest problems of nuclear physics.

The first discussion (afternoon of April 19) was started by Gamow, who called the attention of participants to the Bohr explanation of the non-applicability of Dirac’s theory to a proton, from this point of view, due to the failure of Dirac's theory in the case of a proton one must not only expect its magnetic moment to be different from 1/1840 of Bohr’s magneton but also one may not use the ordinary value of gyromagnetic ratio for the orbital motion of a proton within the nucleus.

In this connection Bethe told about an as yet unpublished theory of Heisenberg and Wiezecker according to which the large value of magnetic momentum of a proton, as observed by Stern, can be explained as due to the virtual disintegration of a free proton into a neutron and a positive electron. The electron existing for a very short time "in status nascendi" contributes to the total magnetism momentum and this makes it as observed about 2.5 times larger than would be expected from Dirac's theory.

The second question raised by Gamow was in connection with the probability of gamma-radiation by radioactive nuclei. Discussing the experimental evidence on this question, Gamow came to the conclusion that the probability of any nuclear radiative transition is given by quadrupole (and not by dipole) formulae. The discussion centered on various attempts to understand why in the case of nuclei the probability of dipole-transitions is reduced to such a large extent. Although one could indicate a number of factors reducing the dipole momentum of the nucleus, the complete answer to this question was not given.

The second meeting (evening of April 19) was begun with a discussion of the laws of force between heavy elementary particles and their bearing on nuclear models. The conclusion was reached that perhaps one should not consider the motion of each nuclear particle in the average field of the others (as done in Heisenberg's theory), in which case the construction of a proper nuclear model should be very difficult. Breit reported on his calculations concerning the effective cross-sections for the processes of artificial nuclear transformations by collision which are in satisfactory agreement with experimental evidence. He also gave the theoretical probability for the radiative capture of protons by carbon-nuclei. This is about ten thousand larger than the experimental evidence obtained by Tuve and Hafstad. According to Breit, this discrepancy may be reduced by better calculations.

Finally Bethe reported about his explanation of the Fermi-effect with slow-neutrons. Due to the very large wave-length of the corresponding de-Broglle wave the probability of capture of a neutron by a nucleus must increase with decreasing velocities.

The third meeting (afternoon of April 20) was started with Beck's discussion concerning modern theories of beta-decay. A very interesting discussion of this question was given by Uhlenbeck who has shown that the shape of continuous beta-spectra can be easily obtained from quite general statistical considerations and energy-partition. The question of the beta-activity of potassium was brought up and the discussion continued in the direction of a possible explanation of the long decay-period observed in this case. At the fourth meeting (evening of April 20) a very stimulating discussion led by Bethe was devoted to the question of high-energy radiations (cosmic-ray particles) and the nature of the non-ionizing links of the cosmic-ray showers. On Sunday afternoon (April 21), after a two-hour discussion of various problems over a most excellent dinner, the members of the conference spent the remainder of the day discussing, in small groups, nuclear problems that had arisen during the conference and inspecting the nuclear-disintegrations apparatus of the Department of Terrestrial Magnetism.

G. Gamow, Professor of Theoretical Physics Dr. J. A. Fleming, Director, Department of Terrestrial Magnetism Washington, D. C.

George Washington University Washington, D. C., May 2, 1935

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Author or Source: President's Papers/RG0002; Office of Public Relations
Document Location: University Archives
Date Added to Encyclopedia: January 9, 2007
Prepared by: Lyle Slovick, Assistant University Archivist

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