FOR four years the hydrogen bomb grew in secret and silence, stirring like a quickened fetus in the guarded laboratories. Few qualified physicists, U.S. or foreign, cared to talk about it. They knew that their science would soon give monstrous birth, but they had been warned to keep quiet. When the pictures of the bomb's fury hit the public last week, not many laymen remembered that the scientists long ago predicted what was likely to happen (TIME, Feb. 13, 1950).
"Fusion" of light elements, on which the hydrogen bomb depends, is the senior source of nuclear energy. More than 20 years ago, at Cambridge University, Physicists John D. Cockcroft and Ernest T. S. Walton shot hydrogen nuclei (protons) from a primitive high-voltage machine at a lithium target. A few of the protons hit lithium nuclei. The product of each such reaction: two atoms of helium and 17.3 million electron-volts of energy.
That experiment in 1932 was man's first taste of nuclear energy, but it was like the quick-fading taste of a single grain of sugar. Since most of the protons missed their targets, the hydrogen-lithium reaction gave a net loss of energy, and no one knew how to improve its efficiency. Other reactions of light elements yielded theoretical energy too, but all of them were overshadowed by the wartime development of atom-splitting uranium fission.
The scientists, however, did not forget fusion. Graven on their minds was a curious set of facts: when the elements are arranged in series according to their atomic weights, the atoms of those in the center of the series are lighter than they "should be." So when an atom of uranium (the heaviest natural element) splits into two fragments and a few loose neutrons, all the pieces, added together, weigh less than the original uranium atom. By Einstein's famous equation (E = Mc2), this loss of weight shows up as the energy that powers uranium bombs.
OMINOUS'PROSPECT
A similar thing happens at the light end of the series. If light atoms, e.g., hydrogen, are packed together into a larger atom, it weighs less than the pieces that form it. Here again, the loss of weight shows up as energy. A little figuring told the physicists that a given amount of a light element, forced to fuse, would yield more energy than the same amount of uranium. Besides, light elements are plentiful, while uranium is scarce.
This was an exciting and ominous prospect, but the trouble with fusion reactions is that they are not self-starting; uranium fission is. When a sufficient amount (critical mass) of U-235 is assembled, a single, slow-moving neutron can start an atom-splitting chain reaction in it and make the whole chunck explode. Light elements are not so accommodating. Their atoms must be slammed together violently to make them group into larger atoms and yield energy.
Except for such demonstrations as the 1932 Cockcroft-Walton experiment, the only way to get a fusion reaction is to raise the temperature. The hotter a material gets, the faster its atoms move. If it gets hot enough, they may hit one another so hard that they combine into larger atoms, yielding the energy of fusion.