The Basic Conflict Between the Concepts of General Relativity and of Quantum Mechanics
General relativity’s basic observable quantities are space-time distances between events, the events being, at least in principle, crossing points of the world-lines of two objects, one of which is, usually, a light signal. It is assumed, therefore, that such crossing points, that is the positions and times of collisions, are accurately localized in space—time. This is denied by quantum mechanical theory in which the location and time of the collision plays no role. Its collision matrix describes only the end result of the collision, the momenta and average positions of the colliding particles after the collision has taken place. It is true, on the other hand, that the description of the observation of these disregards the gravitational effects of the instruments used for the observation of the colliding particles. This is, of course, in practice negligible for the collisions of interest in quantum mechanics, yet present in principle. The basic events of the two theories, of general relativity and quantum mechanis, are truly different — in fact quantum mechanics denies the observability of general relativity theory’s basic event. It is not unreasonable to say that the general relativist’s attention is focused on macroscopic objects the quantum uncertainties of the position of which are negligible, whereas the quantum theorist’s attention is concentrated on microscopic objects, atoms and particles, the gravitational interaction of which he can neglect when compared with their other interactions. Both theories aim at the description of the behavior of isolated systems, not subject to outside influences. Yet, as has been pointed out by Zeh, macroscopic objects can hardly ever be considered to be “isolated” — their microscopic structure is affected even by particles a 100 or so yards off. This, together with the conflict between the present concepts of relativity and quantum theories, indicates that our physics will undergo fundamental modifications even if applied to inanimate objects — a recognition which surely was not foreign to Einstein.