Symmetry

Six short years ago mathematical builders of the universe got along with two fundamental particles, the proton and the electron. The proton was the nucleus of the simplest atom, hydrogen. It had a charge of positive electricity and its mass was .0000000000000000000000166 gram. The electron, which in the hydrogen atom throbbed alone around the nuclear proton, had a negative charge matching the proton's positive charge and its mass was 1,847 times less than that of the other particle. The more complex atoms of other elements were constructed from various combinations of electrons and protons.

Then, in quick succession, the neutron was discovered by Chadwick in England and the positron by Anderson in California. The neutron was about as heavy as the proton but had no electric charge. The positron had the same mass as the electron but an opposite charge. Physicists then saw that if they could add to their collection a particle heavy like the proton but negatively charged, and a particle light like the electron but not charged at all like the neutron, they would have a neat array of pairs and triplets, as follows :

MASS CHARGE Plus Minus No Charge Heavy Proton (Negative Neutron proton?) Light Positron Electron (Neutrino?)

Actually, although all attempts to pin it down in the laboratory have failed, the little neutrino has already made itself useful. When particles are knocked out of a nucleus, there is a mysterious disappearance of both energy of motion and energy of "spin" (angular momentum). It is therefore assumed that the lost energies ride away on the elusive neutrino.

There is also a demand for the negative proton--which also fails to leave a recognizable track in ionization chambers. Neutrons have been supposed to consist of a positive proton and a negative electron jammed together, canceling the electric charges. But to explain why some neutrons had positive "spins" and some negative, Tolansky of England two years ago suggested that the negative-spinners were composed of negative protons combined with positrons. The existence of positive-negative electron mates, said he, "suggests, on grounds of symmetry, that a negative proton might be expected to exist." Anderson had also declared himself for this particle; Gamow of Russia thought it might help to explain artificial radioactivity. Max Born, distinguished German exile now in England, guessed that in some distant regions of the universe the rule of the atom might be reversed--negative protons at the core and positive electricity outside.

Last week it appeared in the British journal Nature that H. J. Bhabha of Copenhagen's Institute of Theoretical Physics, using the results of Italian cosmic raymen, had spotted what might be negative protons in primary rays from far beyond the Milky Way. When the recorders were tilted 30DEG westward from the vertical, the number of incoming rays was appreciably diminished. This indicated the presence of negative particles, slanted in the other direction by Earth's magnetic field. These bullets were so powerful that 16 cm. of lead slowed them hardly more than 4 cm. It followed, according to accepted theory, that they could not be electrons, for electrons would have been more than a thousand times more strongly absorbed by the greater thickness of lead. They must therefore be either negative protons or some wholly unsuspected negatively charged particle, and since there was already a theoretical demand for negative protons, Dr. Bhabha preferred to set them down as such. U. S. physicists rubbed their chins, decided to wait a while before committing themselves.

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