Introductory remarks to the session on gravitational radiation were made by BONDI.
There are two quite different attitudes one can take toward the general theory of relativity. First, one can regard it principally as a theory of gravitation. Then one knows that it must be an open theory into which knowledge gained in other fields can be fitted. Secondly, one can take the attitude that relativity is more than other theories in that it says more than just something about gravitation; and then one supposes that there are somehow some wonderful clues hidden in it. The latter attitude, which is what the speakers of yesterday put forward, is one which Bondi cannot accept.
The theory of relativity is based on two principles: (1) the principle of equivalence, and (2) the general principle of relativity. The principle of equivalence, we all agree, is profound. The principle of general relativity, on the other hand, actually says nothing physical at all; it is purely a mathematical challenge, which has been used successfully in gravitational theory as a heuristic principle. On the basis of the first ideology, BONDI proceeded to discuss some of the principal questions in gravitational radiation.
The analogy between electromagnetic and gravitational waves has often been made, but doesn't go very far, holding only to the very questionable extent to which the equations are similar. The cardinal feature of electromagnetic radiation is that when radiation is produced the radiator loses an amount of energy which is independent of the location of the absorbers. With gravitational radiation, on the other hand, we still do not know whether a gravitational radiator transmits energy whether there is a near receiver or not.
Gravitational radiation, by definition, must transmit information; and this information must be something new. The picture of a gravitational transmitter Bondi has is a finite region of space, inside of which something is going on and outside of which space is empty. An example of a gravitational transmitter is a person sitting very quietly holding two dumbbells, who suddenly, unpredictably, starts taking exercise with them. What we want to know is what is the effect of his motion? Does it transmit information to other regions of space of what the person
taking exercise is doing, and does it transmit energy? In connection with these questions, BONDI reports on some work of L. MARDER carried out at King's College. The following is a summary of Marder's results: