Ray Retraction

In Chicago last week Arthur Holly Compton, one of the world's most famed cosmic ray experts, startled a gathering of some 100 distinguished physicists by retracting a theory which he had espoused two and a half years ago, and by putting forth a new one in its place. Cosmic rays are electrified particles which constantly bombard Earth from every direction. It is estimated that about 30 shoot through every human body every second. They have energies higher than any particles ever propelled by man-made machines-- energies, in some cases, measured in hundreds of billions of electron-volts.*

For several years, while cosmic ray research was emerging from its infancy, University of Chicago's Compton engaged in a polite but nonetheless spirited controversy with California Institute of Technology's Robert Andrews Millikan. Compton contended that the rays were mostly electric particles, Millikan that they were mostly photons (electrically inert bundles of radiation). In January 1936, Compton presented a thoroughgoing resume of his researches up to that time which neutral observers considered a "cosmic clearance"--i.e., a victory for Compton (TIME, Jan. 13, 1936). By that time most cosmic ray workers were speaking in terms of particles, and photons were dropping rapidly out of the picture. Among the mass of evidence for particles, one key point is that the fact that they respond--as electrically inert photons could not--to Earth's magnetic field, showing variations by latitude.

Dr. Compton, like practically all of his colleagues, still believes the rays to be particles. The retraction he made last week concerned their place of origin. He once believed they came from the remotest depths of space beyond the Milky Way, which is the huge galaxy of stars to which the sun and its planets inconspicuously belong. The disc-shaped Milky Way appears to be slowly rotating like an enormous wheel. Therefore, if the rays come from outside the galaxy, whichever side of Earth happens to be facing the direction of rotation should receive a few more rays than the back of the planet, just as a child riding a carousel in the rain should be struck by more drops in front than in back. This should result in a small daily variation in cosmic ray incidence at a given point on Earth, as the earth's own daily rotation swings that point from front to back of the galactic movement. Some observers claimed that such a variation, of 1% or less, was actually recorded in their meters.

Since then Dr. Compton, who has seven observation stations in his far-flung cosmic ray empire, has checked up on the variation. Another cosmic ray bigwig, Professor Manuel Sandoval Vallarta of Massachusetts Institute of Technology, calculated what it should be theoretically, after discounting the effect of the magnetic field of the sun. On making this correction, the observed variation was practically nil. Hence, Dr. Compton now prefers to believe that the cosmic rays come from within, not without, the Milky Way--that they whiz around inside it like rats in a trap, prisoned there by the gigantic magnetic field of the galaxy, while electrically neutral starlight escapes to the outside.

Dr. Compton prefers this explanation for another reason. If the cosmic particles were thought to come from outside the Milky Way, it must be presumed that all space is filled with them, that they represent a vaster total of energy than star light -- in fact, the greater part of the energy of the universe. If they are of Milky Way origin, however, their intensity is reasonably set down as an effect of "local" concentration.

This theory -- which Dr. Compton admitted last week is only tentative -- nevertheless bumps into the views of Belgium's Abbe Georges Lemaitre, proponent of the "Exploding Universe." Abbe Lemaitre believes the cosmic rays are fragments of a universal explosion which took place bil lions of years ago, and therefore that the rays should fill all space more or less uniformly. This is only one of several hypotheses advanced to account for the rays' origin. Dr. Millikan used to believe they were liberated in interstellar space during the coalescence of light elements into heavier ones. Dr. Fritz Zwicky of Caltech believes that cosmic rays may be the products of individual stellar explosions which occur all the time in some region or other of the universe. Hannes Alfven of Sweden's Upsala University holds that the rays are free-moving particles in space accelerated to cosmic ray energies by the magnetic fields of spinning double stars -- somewhat as the man-made machine called the cyclotron speeds up atom-smashing particles to high velocities by means of repeated electrical pushes.

*An electron-volt is the energy acquired by an electron to which a force of one volt is applied.

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