What Is Light?

The question has been debated ever since Newton. But physicists are still in the dark about whether light is 1) a wave, 2) a particle or 3) a combination of both. Sometimes it behaves like one, sometimes like the other. Last fortnight a physicist advanced a brand-new theory: that light is some kind of electrodynamic force which travels not in waves or straight lines but like a corkscrew.

Author of this idea was unorthodox Physicist Felix Ehrenhaft, whom most of his colleagues consider a champion leaper-to-a-conclusion. Last year Dr. Ehrenhaft started a sharp argument among physicists by announcing that magnetism has currents which flow like electricity (TIME, May 22). At a Manhattan meeting of the American Physical Society last week, he told how he had projected a very fine light beam vertically in a glass tube, then dropped into the beam microscopic particles of matter (e.g., chromium). When the particles were smaller than the light's wave length, they fell straight down. But bigger particles, instead of falling straight, as they would have if affected only by gravity, fell in a corkscrew spiral, with regularly spaced turns.

Ehrenhaft concluded that the energy which made the particles rotate could have come only from the light. He pointed out that physicists had previously observed that a beam of light creates electric and magnetic fields, exerts pressure against matter, can be rotated by a magnetic field (Faraday) or by certain substances which radiate energy affecting light.*

"Clearly," said he, "we here have an entirely new phenomenon--the circular movement of matter caused by light. It reveals that just as matter rotates light, so light rotates matter. . . . Light can exert a force on a small particle comparable to the force of gravity. It can exert a push or a pull."

Pondering this revolutionary theory, and well aware that Dr. Ehrenhaft is a cantankerous man in an argument, his fellow physicists kept skeptically mum.

* Standard device for demonstrating light's pressure is the radiometer: a glass box containing a set of vanes which have one side polished, the other painted black. When light strikes the vanes, it makes them whirl, because of the difference in the amount of light energy absorbed by the dark and polished sides.

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