Isaac Asimov

“ Since it has a unit positive charge, it can be accelerated just as an isolated proton can be.
Suppose, then, that a deuteron is accelerated to a high energy and is aimed right at a positively charged nucleus. That nucleus repels the deuteron, and it particularly repels the proton part. The nuclear interaction that holds together a single proton and a single neutron is comparatively weak as nuclear interactions go, and the repulsion of the nucleus that the deuteron is approaching may force the proton out of the deuteron altogether. ”

“ It could be shown by Einstein’s reasoning that no object possessing mass could move faster than the speed of light. What’s more, as an object moved faster and faster, its length in the direction of motion (as measured by a stationary 72 observer) grew shorter and shorter, while its mass grew greater and greater. At 260,000 kilometers per second, its length in the direction of movement was only half what it was at rest, and its mass was twice what it was. As the speed of light was approached, its length would approach zero in the direction of motion, while its mass would approach the infinite. ”

“ The alpha particle is a helium nucleus made up, by this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha particle, its mass number should decline by 4 and its atomic number by 4 - 2, or 2. And, indeed, when uranium-238 (atomic number 92) gives off an alpha particle, it becomes thorium-234 (atomic number 90) .
Suppose a beta particle is emitted. A beta particle is an electron and if a nucleus loses an electron, its mass number is almost unchanged. (An electron is so light that in comparison with the nucleus, we can ignore its mass.) ”

“ In Rutherford’s first bombardment of nitrogen, it was calculated that only 1 alpha particle out of 300,000 managed to strike a nitrogen nucleus.
The result of these considerations is clear. There is energy to be gained out of nuclear reactions, but there is also energy that must be expended to cause these nuclear reactions. In the case of nuclear bombardment by subatomic particles (the only way, apparently, in which nuclear reactions can be brought about) , the energy expended seems to be many times the energy to be extracted. ”

“ By the new theory, can we suppose that it is neutrons rather than electrons that somehow hold the protons together against their mutual repulsion, and that more and more neutrons are required to do this as the nucleus grows more massive? At first the number of neutrons required is roughly equal to the number of protons. The helium-4 nucleus contains 2 protons and 2 neutrons, the carbon-12 nucleus contains 6 protons and 6 neutrons, the oxygen-16 nucleus contains 8 protons and 8 neutrons, and so on. ”

“ And yet, by the proton-electron theory of nuclear structure, the nitrogen-14 nucleus, with a mass number of 14 and an atomic number of 7, had to be made up of 14 protons and 7 electrons for a total of 21 particles altogether—an odd number.
The nuclear spin of nitrogen-14 indicated “even number” and the proton-electron theory indicated “odd number”. One or the other had to be wrong, but which? The nuclear spin was a matter of actual measurement, which could be repeated over and over and on which all agreed. The proton-electron theory was only a theory. ”

“ In that case, protons in virtual contact, as within the nucleus, would attract each other, but if the distance between them was increased sufficiently to place one outside the nucleus, the nuclear interaction would decrease in intensity to less than the electromagnetic repulsion. The proton would now be repelled by the positive charge of the nucleus and would go flying away. That is why atomic nuclei have to be so small; it is only when they are so tiny that the nuclear interaction can hold them together. ”

“ To shift the protons within the nucleus (“nuclear reactions”) and make nuclear energy available was a harder problem by far. The particles involved were much more massive than electrons and correspondingly harder to move. What’s more, they were buried deep within the atom. No temperatures available to the physicists of the 1920s could force atoms to smash together hard enough to reach and shake the nucleus. ”

“ With the discovery of the pion, Yukawa’s theory was nailed down and any lingering doubt as to the validity of the proton-neutron theory vanished.
C. F. Powell
(Actually, it turns out that there are two forces. The one with the pion as exchange particle is the “strong nuclear interaction”. Another, involved in beta particle emission, for instance, is a “weak interaction”, much weaker than the electromagnetic but stronger than the gravitational.)
The working out of the details of the strong nuclear interaction explains further the vast energies to be found resulting from nuclear reactions. ”

“ In short if you have an even number of particles in a nucleus, each with a spin of +1⁄2 or -1⁄2, then the total spin is either zero or a whole number, no matter what combination of positive and negative spins you choose. ”

“ Unfortunately, the bombarding particles at the disposal of physicists in the 1930s possessed far too little energy to knock mesons out of nuclei, assuming they were there in the first place.
There was one way out. In 1911 the Austrian physicist Victor Francis Hess (1883-1964) had discovered that earth was bombarded from every side by “cosmic rays”. These consisted of speeding atomic nuclei (“cosmic particles”) of enormous energies—in some cases, billions of times as intense as any energies available through particles produced by mankind. ”

“ Since a nucleus is made up of a whole number of protons, its mass ought to be a whole number if the mass of a single proton is considered 1. (The presence of electrons would add some mass but in order to simplify matters, let us ignore that.) ”

“ The gallon of gasoline has a mass of 2800 grams and combines with about 10,000 grams of oxygen to form carbon dioxide and water, yielding 1.35 million billion ergs. That’s a lot of energy and it will drive an automobile for some 25 to 30 kilometers. But by Einstein’s equation all that energy is equivalent to only a little over a millionth of a gram. You start with 12,800 grams of reacting materials and you end with 12,800 grams minus a millionth of a gram or so that was given off as energy. ”

“ Uranium-238 nuclei, for instance, have a mass of 238.0506, so that each of the 238 protons they contain has a mass of 1.0002.
By 1927 Aston had made it clear that it is the middle elements in the neighborhood of iron that are most closely and economically packed. If a very massive nucleus is broken up into somewhat lighter nuclei, the proton packing would be tighter and some mass would be converted into energy. Similarly, if very light nuclei were joined together into somewhat more massive nuclei, some mass would be converted into energy. ”

“ Chemical energy was easy to initiate and control, since that involved the shifts of electrons on the outskirts of the atoms. Raising the temperature of a system, for instance, caused atoms to move more quickly and smash against each other harder, and that in itself was sufficient to force electrons to shift and to initiate a chemical reaction that would not take place at lower temperatures. ”

“ Ordinary objects never moved so fast as to make their lengths and masses show any measurable change. What about subatomic particles, however, which moved at tens of thousands of kilometers per second? The German physicist Alfred Heinrich Bucherer (1863-1927) reported in 1908 that speeding electrons did gain in mass just the amount predicted by Einstein’s theory. The increased mass with energy has been confirmed with great precision in recent years. ”

“ At the speed of light, which Einstein showed was a limiting velocity that an observer would never reach, the discrepancy became so great that the speed of the light source, however great, added or subtracted zero to or from the speed of light. ”

“ He suggested that attractions and repulsions were the result of particles being constantly and rapidly exchanged by the bodies experiencing the attractions and repulsions. Under some conditions, these “exchange particles” moving back and forth very rapidly between 2 bodies might force those bodies apart; under other conditions they might pull those bodies together.
In the case of the electromagnetic interaction, the exchange particles seemed to be “photons”, wave packets that made up gamma rays, X rays, or even ordinary light (all of which are examples of “electromagnetic radiation”) . ”

“ Radioactive changes gave off much more energy per atom than chemical changes did, and the percentage loss in mass was correspondingly greater. The loss of mass in radioactive changes was found to match the production of energy in just the way Einstein predicted. ”

“ The starting 82 mass is 0.0461 greater than the final mass and it is this missing mass that has been converted into energy and is responsible for the gamma rays and for the velocity with which alpha particles and beta particles are discharged.
Nuclear Bombardment
Once scientists realized that there was energy which became available when one kind of nucleus was changed into another, an important question arose as to whether such a change could be brought about and regulated by man and whether this might not be made the source of useful power of a kind and amount undreamed of earlier. ”

“ Suppose a ball were thrown forward at a velocity of 20 kilometers per hour by a man on top of a flatcar that is moving forward at 20 kilometers an hour. To someone watching from the roadside the ball would appear to be travelling at 40 kilometers an hour. The velocity of the thrower is added to the velocity of the ball.
If the ball were thrown forward at 20 kilometers an hour by a man on top of a flatcar that is moving backward at 20 kilometers an hour, then the ball (to someone watching from the roadside) would seem to be not moving at all after it left the hand of the thrower. ”

“ Second, the alpha particles that are sprayed toward the target cannot be aimed directly at the nuclei. An alpha particle strikes a nucleus only if, by chance, they come together. The nuclei that serve as their targets are so unimaginably small that most of the bombarding particles are sure to miss. ”

“ This demonstration that energy was released in any shift away from either extreme of the list of atoms according to atomic number fits the case of radioactivity, where very massive nuclei break down to somewhat less massive ones. ”

“ He is a prolific author who has written over 150 books in the past 20 years, including about 20 science fiction works, and books for children. His many excellent science books for the public cover subjects in mathematics, physics, astronomy, chemistry, and biology, such as The Genetic Code, Inside the Atom, Building Blocks of the Universe, Understanding Physics, The New Intelligent Man’s Guide to Science, and Asimov’s Biographical Encyclopedia of Science and Technology. ”

“ A billiard ball, colliding with a cannon ball, will just bounce, moving just as rapidly afterward as before, though in a different direction. If a billiard ball strikes another billiard ball, it will set the target ball moving and bounce off itself with less speed. ”