Answers to the various questions about gravitation that have hitherto proved puzzling are largely (and I hope wholly) implicit in the above list of statements. Those that have been asked in Chapter 22 are answered below in the order in which they occur.

Question: Can gravitation be brought into association with the main body of science?

Answer: The reason why this could not be done in the past was that the traditional theory is based on an ad hoc hypothesis. This was that a specific law, applicable only to objects with inert and gravitational mass, requires them also to have something quite different, namely attracting mass.

It has been explained in Chapter I that ad hoc hypotheses and unified theories are incompatible and a brief recapitulation will be helpful here. Before Newton there was the ad hoc hypothesis that a specific law, applicable only to planets, required these to move in elliptical orbits. Had Newton been content with this he would not have achieved the unification of all mechanics that has done so much for science. He would have left many questions about planets, and other things, unanswered that he clarified when he showed that many apparently isolated phenomena are really special manifestations of general principles. Once the elliptical orbit of planets was no longer explained by an ad hoc hypothesis it found its place in the main body of science.

It is similarly pointed out in the third of the above statements that, according to the new theory, gravitation is one of many manifestations of a general principle, namely Symmetrical Impermanence. This alone would suffice to bring gravitation within the main body of science. But the unification goes even further.

The Principle of the Symmetrical Impermanence of Matter is, in turn, not based on an ad hoc hypothesis, but is an inference from an even more general principle. This is the Principle of Minimum Assumption. Sym- metrical Impermanence appears as but one of very many manifestations of this. A more general principle could hardly be thought of. It both precludes all ad hoc hypotheses and provides a roof over a complete and self-consist- ent structure of scientific thought in physics in which gravitation finds a place and bears a logical relation to all other parts.

Question: Why do inert and attracting mass always occur in association?

Answer: They do not. Matter has no gravitational effect during its continued existence. Each particle exercises this effect only at the moment of its extinction.

From statistical considerations one should, of course, expect to observe strict proportionality between the quantity of inert mass present in a body and the gravitational field that surrounds it. For the extinctions that produce the field must occur at a rate proportional to the amount of mass. But we have been misled in the past by this proportionality into the belief that every atom is contributing to the field all the time.

It is interesting to note that a similar hypothesis never seems to have been seriously entertained about radio-activity. The notion that every atom in a lump of radium contributes to the radiation all the while has had little, if any, support. We should today smile at anyone who insisted that it must be so. We know that the activity occurs only at the moments when an atom is disintegrating. We do not regard radio-activity as the signature tune of each atom of radium but as its swan song. Yet the metaphor of signature tune is still commonly regarded as the only appropriate one for gravitation.

Question: Why does gravitation seem to contradict the Principle of Stabilization, according to which every process in a self-contained system tends to reduce its cause?

Answer: It only seems to do so and this because we have hitherto wrongly assumed the system in which a gravitational field occurs to be self-contained. Such a system is one in which matter neither originates nor becomes extinct. That in a gravitational system processes tend to increase their causes does not prove, therefore, that the principle does not hold universally. What it provides evidence for is that a system in which there is gravitation is not a self-contained one.

Question: Why do gravitational fields always have the same sign?

Answer: Because they have only been observed where the mass density enormously exceeds the equilibrium value. The rate of extinctions per unit volume therefore enormously exceeds the rate of origins. As the sign of the field depends on which rate exceeds the other, one must expect the sign in the vicinity of massive bodies always to be the same. But the observed constancy of the sign is the result of statistical laws and not of a specific law.

A region where the rate of origins exceeds the rate of extinctions per unit volume must be surrounded by a field of the opposite sign. In such a region the mass density is below the equilibrium value.

Question: Why is there a limited number of constant ratios between unit electric charge and the inert mass of an elementary particle?

Answer: This question is not about gravitation at all. It is about inertia and electric charge. One must not expect the new theory to provide an answer and it cannot do so. But, nevertheless, the question has some relevance here. For the conclusion that gravitation is quantized may give a new urgency to a search for a principle by which all the indivisible physical quantities are co-related.

Question: Why is the gravitational field attributed to an elementary particle vastly weaker than the electrostatic one?

Answer: It is not necessarily so. Let t be the time during which an extinction is being effective and let 1 / n be the fraction of the particles present at the source of the field that are becoming extinct during this time. Of course, n is a very large number. The contribution to the field made by a particle is then n times the contribution attributed to it by the traditional theory and may be of similar magnitude to the electrostatic field around an electron.

There must be a quantitative relation between the duration of a pulse of gravitation, t, the number n, and the half-life of matter. If two of these quantities were known, the third could be calculated. But at present none of them are known. It will be worthwhile to look for pointers to the value of these three quantities.

Question: Why is gravitation uncontrollable?

Answer: Because the events that cause it, namely extinctions, are uncontrollable. Their uncontrollability is, indeed, intrinsic and it would be idle to attempt to overcome it. According to Symmetrical Impermanence, origins and extinctions are not associated with anything in the existing state of affairs. If they were, they would not be true origins and extinctions, but only conversions. Hence anything that one could do to influence the existing state of affairs could not have any effect on origins and extinctions. It follows that a given inert mass must always be surrounded by the same constant gravitational field whatever be done to it or to anything else.

Question: Why is gravitation the only phenomenon that does not seem to be the consequence of any change?

Answer: It is not. Like every other known phenomenon gravitation is the consequence of a change and the change is the most radical that can be conceived. It is the change between existence and non-existence of an elementary component.

Question: How can one reconcile the conceptual difficulty of action at a distance with what is known about gravitation?

Answer: The distance between the place where the extinction occurs and the place where a particle is accelerated is spanned by the movement of a wave of curvature. One can think of this as a pulse of gravitation. Like a letter, a projectile or a photon of radiation, it is a carrier of something and transfers a change in the geometry of space from one place to another. There is, therefore, no action at a distance. The action of gravitation, like that of light, is effected by the transmission of a signal.

I should hesitate, however, to suggest that the pulse of gravitation can helpfully be regarded as a carrier of energy. I am strongly inclined to deny it. It is true that each particle over which the pulse passes is accelerated to a velocity, v. We can speak of the particle as having acquired the kinetic energy, 1/2 mv². But we may not speak of this as having been acquired at the expense of the energy in the pulse; for the energy in the pulse does not seem to be changed by the encounter. If it were changed, there would be such a thing as screening against gravitation and we know that there is not.

It must not be forgotten that gravitational fields can only occur because the system is not self-contained. Hence the Principle of Conservation of Energy is not applicable and one must be very cautious about drawing inferences from this principle.

Question: There is, further, a question that occurred in Chapter 7 Why do we not observe an infinite intensity of gravitation here and now?

Answer: It will be remembered that we should expect to observe an infinite intensity of radiation if it were not for the fact that the universe is expanding. A consequence of this is that there is an optical horizon. No radiation can reach us from places beyond this.

According to the new theory there must also be a gravitational horizon. There must be a critical distance beyond which any two objects are being separated at a rate greater than the velocity of propagation of the gravitational waves.

As the velocity of the waves is not known, this distance is not known either. Perhaps a method for discovering its value will be found some day.