THERE are two or three particular and personalised images that stick in my mind from my father's efforts to explain to me, aged nine, Einstein's General Theory of Relativity. These images arose from the conjecture that light itself had weight, so that it could be bent or pulled in the proximity of matter by what used to be called 'gravity': that if there is enough matter in the universe (which Einstein thought there was) then space itself would be bent or curved - and it would be just such curvature that
could properly be called gravity. The particular images suggested by my father ... are, firstly, of a small group of people standing back to back on a vast and lonely plain; they are looking outwards; they are trying to see something other than just their surroundings and themselves. But they can never by the nature of things see anything outside the curve of their own universe, since gravity pulls their vision back (my father drew a diagram of this) so that it comes on top of them again like falling arrow s. The second image is that of a single person on this vast and lonely plain who has constructed an enormously powerful telescope; by this he hopes to be able to break at last out of the bonds of his own vision; ... he sees - what appears to be a new star! Then he realises that what he is looking at is the back of his own head - or the place where his head now is billions of years ago, or in the future, or whatever.... Then there was a third image ... which is of gravity being like the effect of two people sitting side by side on an old sofa so that the springs sag and they are drawn together in the middle: and there were my father and I sitting side by side on the sofa in his study.