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RELATIVITY

I have been asked to give a brief account of Einstein's theory of Relativity. I cannot do so adequately in 1.200 words, nor without mathematical symbols. Nevertheless I shall try my best.

Newton's physics already go half-way to Einstein, and if you really understand Newtonian relativity, you will not find it so hard to grasp the more modern theory. Imagine a com­partment^lin a train going very smoothly at forty miles an hour. A man sitting in it drops a stone. To him it seems to fall vertically. That is to say it only moves vertically and not horizontally in relation to the other things in the compartment.

But if a man on a station platform watches the falling stone, he says it is moving horizontally at 40 miles an hour, besides its vertical movement. To him the stone appears to fall in a parabola, and to have moved forward for 29 feet whi­le it fell the first four feet. Both these men are right in the sense that their thinking is this-sided, and will enable them to calculate correctly at what distance the stone will be at any time from objects on the train or the platform respecti­vely.

The mechanics of a system are independent of the speed at which it is travelling, provided this speed remains unchanged in magnitude or direction.

Your feelings, or a plumbline² hung in the compartment,are affected if the train slows down, accelerates, or goes rounda curve. But no mechanical observations inside the train willtell you which way the train is going. Nor will they tellyou anything about the speed with which the station platformis moving round the earth's axis, or the much greater speedwith which the earth is moving round the sun. You can mea­sure the changes in these speeds by a gyrostatic compass orin other ways. But you cannot measure the speeds themsel­ves.

Clearly unless there is some way of detecting absolute rest and measuring absolute speed, space and time are mixed up in a curious way. For one man says the stone is moving in a straight line, and the other in a curve, and one seems to be as right as the other.

It was long thought that there was such a way, namely by means of light. One of two things might have been true. Light might have moved at a constant speed relative to ab­solute space. If this were so the speed of light moving east­ward relative to a measuring apparatus would be different at noon and midnight on account of the earth's spin on its axis. And the difference between the speeds in January and July would have been still greater.

Or else the speed of light from an object moving toward us, for example, Venus as an evening star, would be greater, re­lative to objects on the earth, than when it is moving away. Neither of these is true. All kinds of light move at the same speed relative to objects on the earth, regardless of the time which the measurement is made, or the object which sends out the light.

Since radio waves behave like light, no method has been discovered to find out how fast an object is moving, and ac­ cording to Einstein's theory there is no way of finding it out. In fact, the question is a meaningless one. We can only find the speed of one thing relative to another thing. This is quite a simple idea, but it leads to very odd consequences. For one thing measurements of moving objects are slightly affected. The moving train is slightly shorter when measured by a man on the platform than when measured by a man on
the train with the same foot-rule.

Measurements of time are also affected. A watch in the train will record slightly less time in a given interval than a similar watch on the platform. The differences are much too small to measure at present when the relative motion is as small as that of a train relative to a platform. But unless they are there, there is a way of determining absolute rest and motion.

Again, different observers would disagree as to what events happened at the same time. The disagreement would only be measurable if the observers were moving at enormous speeds relative to one another, speeds which were an appre­ciable fraction of that of light, but there is no way of getting round it.

One cannot in practice get two observers moving fast enough relative to one another to make such measurements. But one can get small particles moving quickly enough to show that their mass and weight increase with their speed, as they should on the theory of relativity.

In fact, the calculations as to the energy liberated by ato­mic fission are based on the theory of relativity. For some of the weight of uranium or plytonium is due to the high speed of the particles inside their nuclei, which get out when there is an atomic explosion.

So far, everything is comparatively simple. I have des­cribed what is called the Special theory. But this theory does not deal with the mechanics of a system whose speed is chan­ging. This is the province of the General theory of Relativity. Almost all physicists agree with some of this theory, but not many agree with all of it, and indeed, it is far from complete.

If we accept the theory of relativity we have to admit that space and time are nothing absolute, but merely ways in which we describe the relations between different events.

This has often been taken to be idealism, but it is not. The falling stone in the railway carriage moves in a straight line relative to the carriage and a curve relative to the plat­form. This does not mean that the stone, carriage, and plat­form are not all real.

It does mean that our ordinary notions of space and time are not sufficient to describe the relations of three objects in motion relative to one another. Why should they be so?

Geometry is derived from two Greeks words, geo meaning land, and metron meaning a measure. It was originally a set of rules for measuring land, or surveying, and for this purpose it works extremely well. It now turns out that it is not suf­ficient to describe what happens if the surveyor is moving over the land, and a train is moving over it also.

One cannot deal with such a system without using a sys­tem of measurement which includes time as well as space, and is naturally more complicated than geometry.

That does not mean that the surveyor, the land, and the train are not real.

It does mean that philosophers like Zeno, Gegel, Marx and Engels were quite right in saying that our ideas about motion were self-contradictory. However, they had not the special knowledge needed to resolve the contradictions.

Einstein has only done so partially. Very small distances and times are even more troublesome than very high speeds, and the combination of them is still worse. However the difficulties are gradually being overcome. And each step in overcoming them leads to a greater control over matter.

1948

Література

 John B.C. Haldane. Reader of Popular Scientific Essays. - Изд-во«Наука», М., 1993. - 235 с.