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Plutonium-186, then, would have a charge of 94, which makes it plutonium, and its atomic weight would be 186. The number of protons in the nucleus would be 94, to balance the negative charge of the electrons. If one subtracts 94 from 186, one arrives at the number of neutrons in the nucleus: 92 not enough neutrons to keep the protons from repelling each other. Thus plutonium-186 is impossible. The plutonium nucleus that we can analyze in our world has an atomic weight of 242, which means that the 94 protons have 148 neutrons to hold them together.
Asimov was faced with the logical problem of rationalizing the existence of plutonium-186. It could exist, he realized, only in an alternate universe in which the strong nuclear interaction was even stronger than in our universe perhaps one hundred times stronger in order to keep the protons together. Asimov could have written a story about such an alternate universe a place in which plutonium-186 could exist and eventually he did. But that alone would not have met the challenge. A universe such as that, with no connection to our world, would have been remote from the concerns of the reader. Asimov wanted to bring the plutonium-186 into our universe, and he did by exchanging it for an isotope of tungsten with an atomic weight of 186. Tungsten-186, which has a charge of 74, has 74 protons in the nucleus but 112 neutrons to hold them together. How could it become plutonium-186? By changing twenty neutrons in its nucleus into twenty protons.
The scientific background of the story must have taken shape in Asimov's mind in much this way: in an alternate universe that has a much stronger nuclear interaction, plutonium-186 could exist but tungsten-186 could not. It would be unstable because it has too many neutrons (or too few protons). In our universe, tungsten-186 is stable, but plutonium-186 is unstable. If quantities of the two elements were exchanged between the two universes, power would be released in each of them: in our universe plutonium-186 would emit positrons, as protons within the nucleus were converted into neutrons, and in the alternate universe tungsten-186 would emit electrons, as neutrons were changed into protons. In each universe positrons would annihilate electrons and produce energy. In the process, our universe would lose twenty electrons and the alternate universe would gain twenty. The exchange could mean a clean, inexhaustible power source for both universes.
One question Asimov did not raise (or answer), possibly because it might sabotage the scientific basis for his novel, was the amount of energy required to transfer the materials between the universes. If the

 
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