I. THE PRINCIPLE OF RELATIVITY
An objection to the idea that all motion is relative is sometimes stated in the form of a common sense example: "It is simply not believable to think of the tracks under a moving train as being in motion while the train is standing still." I am tempted to grant this objection but still affirm that motion is relative. That may sound paradoxical and therefore requires an explanation and analysis.
We can begin by dissecting the concept of motion. There are two or three simple, basic types of motion depending on how we classify these. On the one hand there is rotation, such as the rotation of a wheel or a revolving door; and similarly, there is oscillatory motion, such as the wings of a butterfly, or a bird, or even a pendulum. Neither of these two types requires more than a standard clock in terms of which we can state the number of revolutions or oscillations per second or per hour.
In contrast to these we have linear motion, a speeding train along a track, a speeding bullet, or a pulse of light. In order to quantify linear motion we need more than a standard clock; we need a standard ruler, and even more, we need a body or system in which the distance traveled in a given time can be measured. Then we can say, for example, "the bullet fired by a gun situated at the end of the train, in the direction of travel, traveled sixty feet in one millisecond". Of course we are measuring the distance on the train and forgetting about the fact that the train is itself in motion. (For the present we can forget about the movement of this bullet relative to the ground.)
Whether, in fact, we get the same result if we repeat the experiment by firing the gun from a fixed point along the track and measuring on the ground the distance traveled in a millisecond – that is a separate question. It is the latter question that exemplifies Einstein’s first principle, which asserts that if two systems can be considered as inertial systems, and they are in relative linear motion, the results, i.e. the laws of physics, will be the same.
So we see that linear motion is always relative to the system in which the experiment is being conducted, and the motion is measured. Asserting that linear motion must always be measured relative to some body, or inertial system (and that in fact this differentiates linear motion from rotary or oscillatory motion), is not the same as asserting that the train can be stationary while the tracks move. There will be cases when it does not matter which of two bodies is considered at rest, as when two space ships approach or recede from each other. These are exceptions, but do not imply that it makes sense, for example, to imagine the world rising up to meet a body that is suspended in space.
In the absence of impediments, or forces, such as gravity, all motion, circular, oscillatory, or linear, is perpetual. This is evident from the fact that after billions of years of travel a photon keeps moving along – oscillating as it goes.
Rotation of one body about another is a sign that the system consisting of these two bodies is not inertial – that gravity is in operation. So the principle of relativity cannot be applied in such a case.
Einstein’s principle could be thought of as a pessimistic one. It holds that we can never assert that one of two systems in relative linear motion is to be preferred, since the results of relevant experiments will be the same. In the nineteenth century scientists were looking for THE preferred system in which electromagnetic waves are carried. The principle of relativity tells us we won’t find it.
At the same time this is also a liberating principle. We don’t have to worry about the speed of the earth around the sun, or the speed of the sun with respect to the Milky Way. When measuring velocities here on earth - at least for relatively short experiments, we can assume that these motions are approximately uniform and linear. Consequently these terrestrial results will have general or universal validity.
Whether the pessimism of Einstein’s first principle is justified, is a different issue from whether linear motion is relative. I have seen no evidence to suggest that either of these assertions is unjustified. In any case, rejecting the principle of relativity does not mean that we have to reject the idea that linear motion is relative – in the sense explained above.
When we think of a light source in motion with respect to an observer, we should imagine him, or her, to be standing on a track, and the light source stationary on a moving train. That makes clear that movement of the source implies, and generates an additional inertial system. It is in that system that the velocity of light is necessarily constant.
In the principle of relativity, Einstein echoed what was becoming clear early in the twentieth century. There are no absolutes. We must adjust our vision of the universe to find truths in the absence of guideposts based on tenaciously held dogmatic beliefs. If only we could transpose the principle of relativity from physics to religious fundamentalism it would lead to a peaceful, enlightened and civilized world.
These remarks should not detract from the conclusion that SRT is an unconditional failure.
HOME PAGE
INTRODUCTION
THE PRINCIPLE OF RELATIVITY
SPECIAL RELATIVITY - 1905
SPECIAL RELATIVITY - 1917
THE MICHELSON-MORLEY EXPERIMENT
THE LORENTZ TRANSFORMATION
MICHELSON-MORLEY PLUS LORENTZ, CONDENSED
THE DOPPLER EFFECT
A COUNTER EXAMPLE TO SRT
A COUNTER EXAMPLE TO GRT
THE AGE OF THE UNIVERSE
THE SPEED OF LIGHT
CONCLUSIONS
APPENDIX I: TYPE 1A SUPERNOVAE
APPENDIX II: A EUCLIDEAN MODEL OF THE UNIVERSE
APPENDIX III: MASS AND ENERGY
All contents copyright 1997, 2006