Bertrand Russell - The ABC of Relativity

By

Bertrand Russell

1925

The Great Library Collection

By

R.P. Pryne

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Everybody knows that Eins tein did something astonishing, but very few people know exact ly what it was. It is generally recognised that he revol utionised our conception of the physical world, but the ne w conceptions are wrapped up in mathematical technicali ties. It is true that there are innumerable popular accounts o f the theory of relativity, but they generally cease to be intel ligible just at the point where they begin to say somet hing important. The authors are hardly to blame for thi s. Many of the new ideas can be expressed in non-mathemati cal language, but they are none the less difficult on that account. What is demanded is a change in our imaginative picture of the world - a picture which has been handed down from remote, perhaps prehuman , ancestors, and has been learned by each one of us in early childhood. A chan ge in our imagination is always difficult, especially w hen we are no longer young. The same sort of change was demanded by Copernicus, who taught that the earth is no t stationary and the heavens do not revolve about it on ce a day. To us now there is no difficulty in this idea, because we learned it before our mental habits had become fixed. Einstein's ideas, simil arly, will seem easier to generatio ns which grow up with them; but for us a certain effort o f imaginative reconstruction is unavoidable.

In exploring the surface o f the earth, we make use of all our senses, more particularly of the sen ses of touch and sight. In measuring lengths, parts of the human body are employed in pre-scientific ages: a 'foot', a 'cubit', a 'span' are defined in this way. For longer distances , we think of the time it takes to walk from one place to another. We gradually learn to judge distance roughly by the eye, but we rely upon touch for accuracy. Moreover it i s touch that gives us our sense of 'reality'. Some thin gs cannot be touched: rainbows, reflections in looking-glasses, and so on. These things puzzle children, whose metaphysical speculations are arrested by the information that what is in t he looking-glass is not 'real'. Macbeth's dagger was unreal because it was not 'sensible to feeling as to sight'. Not only our ge ometry and physics, but our whole conception of what e xists outside us, is based upon the sense of touch. We carr y this even into our metaphors: a good speech is 'solid', a bad speech is 'gas', because we feel that a gas is not quite 'real'.

In studying the heavens, we are debarred from all senses except sight. We cannot touc h the sun, or apply a foot-rule to the Pleiades. Nevertheless, astronomers have unhesitatingly applied the geomet ry and physics which they found serviceable on the surface of the earth, and which they had based upon touch and travel. In doing so, they brought down trouble on their heads, which was not cleared up until relativity was discovered. It turned out that much of what had been learned from the sense of touch was unscientific prejudice, which must be re jected if we are to have a true picture of the world.

An illustration may he lp us to understand how much is impossible to the astronom er as compared with someone who is interested in things on t he surface of the earth. Let us suppose that a drug is admi nistered to you which makes you temporarily unconscious, and that when you wake you have lost your memory but no t your reasoning powers. Let us suppose further that whil e you were unconscious you were carried into a balloon, whic h, when you come to, is sailing with the wind on a dark ni ght - the night of the fifth of November if you are in En gland, or of the fourth of July if you are in America. You ca n see fireworks which are being sent off from the ground, f rom trains, and from aeroplanes travelling in all directions , but you cannot see the ground or the trains or the aeroplanes because of the darkness. What sort of picture of the world will you form ? You will think that nothing is permanent: there are only brief flashes of light, which, during their short existence, travel through the void in the most various and bizarre curves. You cannot touch these flashes of light, you can only see them. Obviously your geometry and your physics and your metaphysics will be quite different from those of ordinary mortals. If an ordinary mortal were with you in the balloon, you would find his speech unintelligible. But if Einstein were with you, you would understand him more easily than the ordinary mortal would, because you would be free from a host of preconceptions which prevent most people from understanding him.

The theory of relativity depends, to a considerable extent, upon getting rid of notions which are useful in ordinary life but not to our drugged balloonist. Circumstances on the surface of the earth, for various more or less accidental reasons, suggest conceptions which turn out to be inaccurate, although they have come to seem like necessities of thought. The most important of these circumstances is that most objects on the earth's surface are fairly persistent and nearly stationary from a terrestrial point of view. If this were not the case, the idea of going on a journey would not seem so definite as it does. If you want to travel from King's Cross to Edinburgh, you know that you will find King's Cross where it has always been, that the railway line will take the course that it did when you last made the journey, and that Waverley Station in Edinburgh will not have walked up to the Castle. You therefore say and think that you have travelled to Edinburgh, not that Edinburgh has travelled to you, though the latter statement would be just as accurate. The success of this common-sense point of view depends upon a number of things which are really of the nature of luck. Suppose all the houses in London were perpetually moving about, like a swarm of bees; suppose railways moved and changed their shapes like avalanches; and finally suppose that material objects were perpetually being formed and dissolved like clouds. There is nothing impossible in these suppositions. But obviously what we call a journey to Edinburgh would have no meaning in such a world. You would begin, no doubt, by asking the taxi-driver: 'Where is King's Cross this morning?' At the station you would have to ask a similar question about Edinburgh, but the booking-office clerk would reply: 'What part of Edinburgh do you mean? Prince's Street has gone to Glasgow, the Castle has moved up into the Highlands, and Waverley Station is under water in the middle of the Firth of Forth.' And on the journey the stations would not be staying quiet, but some would be travelling north, some south, some east or west, perhaps much faster than the train. Under these conditions you could not say where you were at any moment. Indeed the whole notion that one is always in some definite 'place' is due to the fortunate immobility of most of the large objects on the earth's surface. The idea of'place ' is only a rough practical approximation: there is nothing logically necessary about it, and it cannot be made precise.

If we were not much larger than an electron, we should not have this impression of stability, which is only due to the grossness of our senses. King's Cross, which to us looks solid, would be too vast to be conceived except by a few eccentric mathematicians. The bits of it that we could see would consist of little tiny points of matter, never coming into contact with each other, but perpetually whizzing round each other in an inconceivably rapid ballet-dance. The world of our experience would be quite as mad as the one in which the different parts of Edinburgh go for walks in different directions. If - to take the opposite extreme - you were as large as the sun and lived as long, with a corresponding slowness of perception, you would again find a higgledy-piggledy universe without permanence - stars and planets would come and go like morning mists, and nothing would remain in a fixed position relatively to anything else. The notion of comparative stability which forms part of our ordinary outlook is thus due to the fact that we are about the size we are, and live on a planet of which the surface is not very hot. If this were not the case, we should not find pre-relativity physics intellectually satisfying. Indeed we should never have invented such theories. We should have had to arrive at relativity physics at one bound, or remain ignorant of scientific laws. It is fortunate for us that we were not faced with this alternative, since it is almost inconceivable that one person could have done the work of Euclid, Galileo, Newton and Einstein. Yet without such an incredible genius physics could hardly have been discovered in a world where the universal flux was obvious to non-scientific observation.