well. Atoms, it seemed, weren't playing by those rules. Energy and momentum were somehow getting lost along the way. So distressing was this finding that the distinguished physicist Niels Bohr openly discussed the possibility that atoms might at times violate some of the standard laws of physics.

Pauli wasn't as pessimistic as his Danish colleague. The Viennese physicist had an abiding faith that atoms were obeying the physical laws of the land. But to maintain that allegiance, Pauli took the rather radical step in 1930 of suggesting that an entirely new particle, invisible to ordinary instruments, had to exist to explain the discrepancies seen in the Cavendish experiments. Every time a nucleus undergoes beta decay, he proposed to some colleagues by letter, this neutral phantomlike particle is emitted and vanishes off into the night carrying off that extra bit of energy and momentum. Usually full of chutzpah, the young Pauli was actually intimidated by the outrageousness of this idea. "Dear radioactive ladies and gentlemen …," he teasingly wrote his friends, who were then attending a meeting in Tübingen, Germany. "For the time being I dare not publish anything about this idea and address myself confidentially first to you, dear radioactive ones, with the question of how it would be with the experimental proof of such a particle." It was only after the chargeless neutron was discovered in 1932 that Pauli finally got the courage to publish his unusual suggestion. The noted physicist Enrico Fermi soon dubbed Pauli's hypothetical mote the neutrino, Italian for "little neutral one." The name was apt. The neutrino seemingly had no mass, and it had no charge. Indeed, it was nothing more than a spot of energy that flew from a radioactive atom at the speed of light.

Fermi perceptively recognized that Pauli's idea would also require a whole new force, what came to be known as the weak nuclear force. It is this force that enables a neutron to convert into a proton, releasing the electron and the neutrino (actually an antineutrino) seen in beta decay. (Interestingly enough, the prestigious journal Nature rejected Fermi's idea on the weak force as too speculative and too remote to be of any interest to practicing scientists; a small Italian review, however, did publish the hypothesis. Thus, the weak force entered the world of physics with little fanfare.)

It took so long to prove that the neutrino was more than a figment of Pauli's imagination that some physicists began to call his neutrino "the little one who was not there." In fact, Pauli began to wonder whether he had committed the theorist's ultimate sin: postulating a particle that could not possibly be detected. He had good reason to be fearful.

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