In such cases one of the innermost electrons of the atom may combine with a proton to make a neutron. Such an electron-capture process will always release energy provided that the reverse process—the transformation of a neutron into a proton and an electron—is connected with an energy deficit. Thus, excluding the possibility of a really exact coincidence of two energies, one of the two transformations from neutron to proton or proton to neutron will always be possible. It is one of the most firmly established laws of nature that energy is always conserved. One would therefore expect that the energy of a beta ray would be exactly equal to the difference between the energy of the nucleus before the beta decay and the energy of the nucleus after the beta decay. As a matter of fact the energy of a beta ray is found never to be as great as this amount. Frequently it is much less. Some of the energy has apparently disappeared and the suspicion has been voiced that energy may not be conserved after all. It has turned out, however, that the missing energy is smuggled out of the nucleus, and the smuggler (who has only recently been caught) is called the neutrino. The neutrino is an electrically neutral particle, like the neutron, but its weight, like the weight of a ray of light, is equal to zero. Like such a ray, it moves with the velocity of light. The energy released by the nucleus in the beta-decay process is shared more or less equally between the neutrino and the beta ray. We shall see later that the electron gives rise to a number of effects. Some of these are harmful. The neutrino, however, is not in the least dangerous. Like an ideal smuggler it passes unnoticed and practically without a trace. It interacts so slightly with matter that several billion of them may go right through the whole sphere of our earth before a single collision occurs. Very recently this strange little particle has upset one of our most unquestioned concepts about symmetry. We have always believed that nature made no distinction between her right hand and her left hand; that for every natural process that exists, there exists also the mirror image of this process. The neutrino, however, is an exception. It has a definite symmetry, like a screw.[5] This fact may turn out to be most important in the development of science. It has no bearing, however, on the questions to be discussed in this book. Neutrinos reach us from some distant and hidden places like the interior of our sun and of exploding stars. It may become possible to use neutrinos as messengers to reveal the kind of nuclear reactions from which the energy of the stars is derived. Neutrinos are also emitted every time we release some nuclear energy. Among all the remarkable practical consequences of nuclear energy, the neutrinos have a unique distinction: they are never useful, and they are never harmful. They have not even been suspected of any mischief.