Science: Millimeter Waves

Bit by bit, the physicists have filled the gaps in the electromagnetic spectrum, which runs from X rays at the short end, through light and heat, to miles-long radio waves at the other. One big gap remained between the infra-red (heat) waves and the shortest radio waves (about .8 mm.) that man’s apparatus could generate. Last week Dr. Hans Motz of Stanford University told how the gap has been filled.

Dr. Motz’s “millimeter-wave generator” is made up, first, of a linear accelerator that produces a pulsed beam of electrons about £ inch in diameter. The electrons, whose energy is 2,000,000 electron volts, pass into an “undulator,” a silver wave guide that is held between 16 pointed steel teeth. The teeth set up separate and alternating magnetic fields, and as the electrons pass from field to field, they are made to oscillate, forming the desired waves less than one millimeter long.

Relativity in Action. The teeth are about an inch apart along the undulator, and this seems coarse for an apparatus that yields such tiny waves. But the undulator is “shrunk” by one of the strange effects connected with Einstein’s relativity. According to “the Lorentz contraction,” a stationary object shrinks when it is observed from a moving object. The faster the motion, the more the shrinkage.

Since the electrons in Dr. Motz’s undulator are moving close to the speed of light, the undulator, from their point of view, is only one-seventh as long as it is from the point of view of Dr. Motz. The waves caused by the magnetized teeth shrink in proportion. Other shrinking actions bring them down to .16 mm.

Visible Radio. Dr. Motz can make them even smaller by increasing the speed of the electrons and therefore increasing the Lorentz contraction. Once he hitched his undulator to a large linear accelerator that sent out electrons at 100 million electron volts. From the business end came a beam of blue light. He had actually generated “radio waves” that were short enough to qualify as visible light. This stunt proved that the stubborn gap in the spectrum has been closed, but it is hardly practical. There are better ways of generating the waves of light and heat.

Work on the millimeter-wave generator was financed by the Office of Naval Research, and the Navy has hopes of using the tiny waves for short-range signaling. They fade out quickly in air, so there would be no chance that the enemy might pick them up at a distance. But Dr. Motz is more interested in the scientific uses of his waves. They oscillate so rapidly that they may be able to “see” into atoms, revealing properties that scientists can only guess at now.