Science is hydra-headed. The answer to one question raises others. If Symmetrical Impermanence solves some problems, it gives rise to many more. Two of them are: What is an elementary component of the material universe? How does its extinction affect the nucleus of an atom?

I can only suggest tentative answers to these questions and should like to draw attention to their difficulty and their significance.

I have taken care throughout this book to avoid the impression that an elementary component of the material universe is necessarily any of the elementary particles with which nuclear physics has made us familiar. It is partly for this reason that I chose this non-committal, if cumbersome, name. The constituents of matter that, according to Symmetrical Impermanence, originate and become extinct may, for all that can so far be proved to the contrary, be found among the protons, electrons, neutrons, and other charged or uncharged particles that have so far been studied. They may, of course, be smaller. Nothing has so far been said to refute the hypothesis, if anyone cares to form it, that they are sub-particles and that the familiar particles are structures produced by a process of synthesis. But it is implicit in Symmetrical Impermanence that the elementary components are basic.

A few other considerations deserve brief mention.

Origins and extinctions should be expected to leave the average ratio of mass to charge for the whole universe unaltered, for if they did not this ratio would change with time. This suggests that whatever originates and becomes extinct as a unit is neither mass nor exclusively charge, but either a combination of both or something common to both.

Some objections to identifying those constituents of matter that I have called elementary components with protons, neutrons or electrons only have been mentioned in Chapter 3, Section 4. Mr Wilson, of the Research Laboratories of the Central Electricity Generating Board, and Dr Sciama, of Cambridge, have drawn my attention to another objection to doing this. It leads to the second question, how does the extinction of an elementary component affect the nucleus of an atom?

If an elementary component were a neutron or any charged particle and became extinct in the complex nucleus of a radio-active substance, the balance of forces in the nucleus would be upset. The consequence would be the emission of other particles and of radiation, while what was left of the nucleus would become that of a different chemical element. There would, in short, be characteristic radio-activity. There would also be a residue of the chemical substance into which the disintegrating atoms were converted.

If the extinction of elementary components were the only cause of such radio-activity, the half-life of the substance would equal the half-life of matter. If there were additional causes, the half-life of the substance would be shorter. But it could never be longer than the half-life of matter.

Now good reasons have been found in preceding appendices for believing that the half-life of matter is likely to be not more than 4 x 10⁸ years. If what became extinct were a proton or a neutron, no substance could have a radio-active half-life greater than this value. But many substances do have substantially greater radio-active half-lives. The radioactive half-life of most substances is indeed to all intents and purposes infinite.

From this it is clear that not every extinction of an elementary component causes radio-activity. It may be that none does so. But if some do cause radio-activity, it can be only a small proportion. It would not be so if the elementary component were a neutron or a proton forming part of a nucleus.

As mentioned above, the elementary component must have something that is common to mass and charge and so its extinction must result in the immediate, or delayed, disappearance of both from the nucleus. Therewith the loss of stability would not be as great as it would if mass or charge only were to disappear. It is worth investigating whether every nucleus that suffered such combined loss would necessarily show radio-activity. If the balance of forces was not greatly disturbed and the probability that further particles would be emitted were small, there would be no reason why the radio-active half-life should not greatly exceed the half-life of matter.

But the further difficulty mentioned above would remain. What was left of the nucleus in which an extinction had occurred would be the nucleus of a different chemical element. Extinctions must occur in nonradio-active elements and, according to this theory, a consequence of the continuous extinction of matter is a slow change of their chemical constitution. With a half-life of 4 x 10⁸ years such a change would presumably be easily detected. If it does not occur, a different answer to the question has to be found.

One answer would be that a complete nucleus becomes extinct. If so, there is no chemical residue, no radiation, no emission of particles. An observable effect would be the pulse of gravitation that was emitted by the extinction. But the objection to this answer is that we regard the nucleus as a very composite affair, as anything but an elementary component. But I should not reject this answer out of hand. Conceptual distinctions do not necessarily coincide with real ones and it is not impossible that what is composite from the point of view of nuclear physics may be simple from the point of view of Symmetrical Impermanence. It has to be remembered that the individual constituents of the nucleus are not observed while they are forming a part of it, but only while they are outside. In Appendix H it will be shown that the nucleus of any element may well be a more basic unit than is usually supposed.

If this is accepted many puzzles concerning the nucleus disappear. At the same time the question is answered satisfactorily why extinctions are not accompanied by radio-activity and why they do not leave a chemical residue. The answer is that the unit to become extinct is always an entire nucleus.