The Cockcroft-Walton experiment is often cited as the first experimental proof of Einstein’s mass-energy equivalence. Actually there were hints of its correctness a dozen years earlier. (Einstein himself, we can be confident, had no doubts about it.) By 1920 there was evidence that the proton—the nucleus of the most common isotope of hydrogen—was just a tad “overweight.” With the masses of the most common isotopes of carbon, nitrogen, and oxygen pegged at 12, 14, and 16 units, and other known isotopes also following very closely a whole-number rule, the mass of the lightest isotope of hydrogen was not exactly 1, it was about 1.01.{33} This slight oddity was just enough to make Arthur Eddington in England suggest that energy would be released if four hydrogen nuclei fused to make a helium nucleus (with electrons participating to preserve charge conservation) and that such fusion might be the source of the Sun’s (and other stars’) energy.{34}
So nuclear fusion entered the consciousness of physicists nearly twenty years before nuclear fission did. And—unlike with the startling discovery of fission—there was nothing particularly surprising about the idea of fusion. Physicists (and chemists, and astronomers) assumed that nuclei were composed of smaller entities (initially supposed to be protons and electrons) and that these entities were held together by a “binding energy,” which, in accordance with E = mc2, would make the nuclear mass less than the sum of the masses of its constituents. So, just as it would take energy to pry apart a nucleus into its component parts, energy would be released if these parts came together to form a nucleus. Moreover, since the earliest days of radioactivity, it was clear that on a per-atom basis, these nuclear energies would be much greater than chemical energies.
In the years following Eddington’s imaginative leap, other physicists explored the possibilities of fusion as the source of stellar energy. Following the discovery of the neutron in 1932 and the refinement of mass spectroscopy in the 1930s, it became possible to predict with some accuracy just how much energy would be released in a variety of possible fusion reactions. Finally, in 1939, just on the heels of the discovery of fission, Hans Bethe, a brilliant émigré physicist from Germany, then at Cornell University, put it all together and suggested two principal fusion cycles that might power stars, one involving principally hydrogen and helium, the other involving also carbon, nitrogen, and oxygen as intermediaries.{35} Astrophysicists continue to believe that Bethe got it right, and that his fusion cycles are the main sources of stellar energy. (Bethe was awarded the Nobel Prize in Physics in 1967.)
It’s an oddity of history that in the very year that fusion was established as a reality in the cosmos and fission as a reality here on Earth, Hitler launched an attack on Poland, and World War II was under way. Quite naturally, physicists asked themselves: Can fission and/or fusion be harnessed to produce practical power for humankind? Can one or both be exploited to make powerful weapons of warfare? Needless to say, the emphasis at the time was on the latter question.[32] As it turned out, controlled fission (a nuclear reactor) and explosive fission (an A bomb) were both achieved within half a dozen years. Explosive fusion (the H bomb) came seven years after the fission bomb. Controlled fusion for power production remains a yet-to-be-achieved goal.[33]
Chapter 6
Some Physics
Here is some physics related to thermonuclear weapons, for those who want to read it.
Fission and Fusion
Both fission and fusion release nuclear energy, which, as the previous chapter has made plain, is vastly greater per atom or per unit of mass than the energy of chemical change. Despite the huge difference in scale, there is one thing that nuclear energy and chemical energy do have in common—they can be released either explosively or gradually. For chemical energy, think dynamite or gunpowder vs. a candle flame. For fission, think Hiroshima vs. that relatively benign power reactor up the river. For fusion, think H bomb vs. the yet-to-be realized fusion reactor that will use deuterium from the ocean to produce electricity. (As for the Sun: In one sense it releases energy gradually, over billions of years; but in another sense, it is a nonstop nuclear explosion, rather like the carolinium imagined by H. G. Wells.)
Humankind discovered chemical energy long ago, first fire, then gunpowder—the gradual before the explosive. For fission energy, the gradual and the explosive were more nearly simultaneous. Fission reactors—the gradual—actually came first, by a few years, but these early reactors produced no usable power. They served to establish the principle of large-scale fission energy and to produce plutonium for weapons. Then, a dozen years after the explosive release of fission energy, came power-producing reactors. In the long span of history, gradual fission energy and explosive fission energy came at pretty much the same time. For fusion energy, the explosive came first. More than half a century after the first fusion explosion (that is, the H bomb), the gradual release of fusion energy remains a hope, not a reality.
Nuclear energy is all about E = mc2. It is also all about the forces that exist within the nucleus. Nuclei are composed of neutrons and protons (collectively called nucleons).[34] Two kinds of force are at work. The nuclear force, or strong force, acts to attract neutrons to each other, protons to each other, and protons to neutrons. In short, it attracts every particle within the nucleus to every other one. The electric force acts to repel the positively charged protons from one another and has no effect on the electrically neutral neutrons. Besides the forces, there is a physical principle also at work: It is called the Pauli exclusion principle. I won’t go into this principle except to say that its effect is to favor an equal number of protons and neutrons within the nucleus. Because the strong force is not all that strong, the Pauli principle also works to prevent the existence of a simple nucleus of just two neutrons or just two protons.
If there were no electrical repulsion between protons, there would be no limit to how many nucleons could join together to form a nucleus. There would be a carbon nucleus with six protons and six neutrons (as there in fact is in the real world), a uranium-184 nucleus with 92 protons and 92 neutrons, a nucleus with 300 protons and 300 neutrons, and so on—plenty of room for carolinium and no end of other imaginatively named elements. Visualize a graph in which proton number is plotted vertically and neutron number horizontally. As shown in Figure 1, the nuclei with equal proton and neutron number would lie along a straight line inclined at 45 degrees and extending on without limit. This is the so-called line of stability. (There would be some stable nuclei with proton and neutron numbers not exactly equal, but we don’t need to be concerned with them. They would just change the line of stability into a band of stability.)
32
There remains debate about what aspect of fission energy German scientists chose to emphasize—controlled energy or explosive energy. Every scientist or historian has his or her own opinion. Mine is that Heisenberg’s “Uranium Club” would have vigorously pursued the explosive option if its members foresaw the possibility of success before the war ended. Instead they assumed that the war would end well before a nuclear bomb could be achieved, and accordingly focused on building a reactor, with no sustained high-priority push toward a bomb.
33
There is a joke among fusion scientists that fusion power is a decade away and always will be.