The Mysterious Rays

A whole galaxy of cosmic ray experts gathered last week at the California Institute of Technology, Pasadena, to honor Nobel Prizeman Dr. Robert A. Millikan, 80, principal discoverer and namer of cosmic rays. Dr. Millikan, who was spry enough last summer to travel by station wagon from Texas to Canada observing cosmic rays, described the gathering as "a testimonial to my longevity and a kind of celebration of my passage through the portal leading into second childhood."

Dr. Millikan still sticks to his original long-held theory that cosmic rays come from the "annihilation of matter" (atoms turning into energy) somewhere out in space. He has elaborate evidence to prove his theory. The gathered cosmic raymen did not try to argue with their dean; neither did they agree with him. Cosmic rays are a baffling and complex phenomenon. In spite of years of concentrated work, the experts do not even agree about what they are, much less about what causes them.

Big Smash. One thing is fairly certain: the "primary rays" are particles of some sort (coming from somewhere) that smack into the earth's atmosphere with enormous speed and energy. They hit and apparently crack up atmospheric atoms, scattering the fragments violently, like bullets smashing into a bag of marbles. The atomic debris cascades toward the earth, some of the pieces with enough energy to crack atoms on their own. Occasionally one of these "extensive showers" covers an area 1,000 meters wide. The total energy in a shower runs up to 10 18 (one billion billion) electron-volts.

The experts are not sure what particle (or particles) causes this great commotion. Some think the primary rays are electrons which must be moving (to have so much energy) at almost the speed of light. But Dr. Bruno Rossi of M.I.T. presented a paper at last week's meeting claiming that there are practically no electrons or photons (high energy electromagnetic waves) among the primary radiations. They are observed in the showers along with other particles, but Rossi believes that they are formed when the primary ray (whatever it is) hits the atmosphere.

An average of over 100 cosmic particles of some sort pass through every human head every minute, according to Millikan, but they cannot be felt. Elaborate special apparatus is needed to observe them. Some scientists work with stacks of Geiger tubes, which register each particle that passes through them. Others use special photographic plates, where certain particles leave microscopic tracks of silver in the sensitive emulsion. The best instrument, and the hardest to use, is the Wilson cloud chamber, where the particles make visible tracks of white condensed moisture.

Tunnel from the Sun. There may be many different primary rays. Some, thinks Dr. S. E. Forbush of the Carnegie Institution, may come from the sun. There are plenty of high-speed particles in the sun, but ordinarily they cannot escape into space because of the sun's powerful magnetic field. But sunspots, which are whirling solar hurricanes, have magnetic fields of their own. Sometimes these cancel the sun's general field, making a narrow, nonmagnetic tunnel through which the particles escape. Dr. Forbush claimed to have observed sudden increases of cosmic radiation when such a tunnel happened to point toward the earth.

Not all cosmic raymen made their reports at Pasadena. Drs. E. ]. Iofgren, E. P. Ney and F. Oppenheimer of the University of Minnesota told a conference at Madison, Wis. how they sent automatic Wilson cloud chambers up to 94,000 feet (almost 18 miles) on special sounding balloons of thin plastic. The apparatus was enclosed in a 30-inch aluminum sphere. While passing through the cold middle-altitude atmosphere, the sphere's interior had to be heated by a thermostatically controlled battery. When it reached the thin upper atmosphere, the strong rays of the sun, absorbed by a black patch on the top of the sphere, supplied heat enough. An automatic camera took pictures of the particle tracks in the cloud chamber.

The whole apparatus, released automatically, returned to earth on a parachute. When the photographs were developed, some showed cloud tracks apparently made by particles with mass 180 (almost as heavy as mercury). They had 160 billion electron-volts of energy, and therefore must have been traveling at 85% of the speed of light. If these observations are correct, the primary rays may be good-sized atomic nuclei speeded up by some process that is still unknown.

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