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 in 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 the 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 for29 feet while it fell the first four feet. Both these man are right in the sense that their thinking is this-sided, and will enable them to calculate correctly at 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 traveling, provided this speed remains unchanged in magnitude or direction.
Your feelings or a plumbline hung in the compartment, are affected if the train slows down, accelerates, or goes round a curve. But no mechanical observations inside the train will tell you which way the train is going. Nor will they tell you anything about the speed with which the station platform is moving round the earth’s axis, or the much greater speed sure the changes, in these speeds by a gyrostatic compass or in other ways. But you can’t measure the speeds themselves.
Clearly unless there is some way of detecting absolute rest and measuring 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 the 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 methods has been discovered to find out how fast an object is moving, and according 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 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 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 plutonium 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. But the so-called Special 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 it, and indeed, it is far from complete