Messengers of Force by D. H. Wilkinson , F.R.S. Professor of Nuclear Physics. at the Clarendon Laboratory, Oxford The problem of action at a distance-the way in which forces are propagated through even a vacuum-is an intriguing one. The idea that there is no agency is abhorrent to the physicist and in getting round the problem he postulates that there is some sort of particle involved. In the case of electrical and nuclear forces these particles have been identified, but what about the phenomena of magnetism and gravity?