Principle of Relativity (before Einstein)

Many think relativity is a concept introduced by the genius Einstein. It is wrong. Relativity actually dates back to the times of Galileo. But it was Einstein who extended and improvised the idea and showed that it was not a paradox but a reality of nature by his special and general theory of relativity.

So, let’s first go to the origins of relativity. The principle of relativity is considered to be first proposed by Galileo in 1632 in his 'Dialogue Concerning the Two Chief World Systems'. Galileo gave the first law of relativity. And then, when Newton proposed his three laws of motion in his infamous 'Principia Mathematica', the second law of relativity was derived and the first law was confirmed.

The principle of relativity before Einstein originally had the following postulates-

1st- Laws of nature appear to be same for all observers moving with constant velocity.

2nd- Absolute velocity is has no physical meaning, only relative velocities are meaningful.

Consider a completely sealed chamber whose interior is completely free from any outside interference. This chamber is occupied by an intelligent scientist who has access to a lot of scientific instruments and is free to conduct any kind of experiment, but the condition is that he can conduct his experiments within the premises of the chamber not outside it (which means he even cannot send or receive any sort of signals from the outside). Now, without knowledge of the scientist, the chamber is strapped on to a rocket flying at a high but constant speed heading straight towards an active volcano! Now, he is given a challenge, that if he could anyhow find out the speed of the rocket (while within the chamber), then only he will be released.

So, can he do it?

The answer is NO! Because the first postulate of relativity forbids him to do so. According to the first postulate, laws of nature appear to be same for all observers moving with constant velocity. So, any experiment would appear to be the same for objects stationary on the ground and objects moving with a uniform velocity. So, any experiment conducted by the scientist within the chamber on the rocket will give him normal results (just as he was stationary on the ground). If he hadn't even been told that he was on a rocket he wouldn't even have known it and would have certainly met his death.

Therefore, there is no way he could have found out the speed of the rocket, no matter how intelligent he is.

Let’s come to the 2nd rule of relativity. The 2nd rule states that no one can calculate ones absolute velocity. You can only calculate your velocity relative to something else. So, if the scientist could have carved out a window out of the wall of the chamber, he could have got an idea of his speed relative to the wind blowing backwards. (But that would be cheating of course).

Now, consider a train moving at 100 mph. Now, this speed is not its absolute speed because, as earth spins around its axis we are being carried at 1000 mph, as it orbits the sun we are being carried at 70,000 mph,  as the sun orbits the milky way galaxy, we are moving at 500,00 mph. Also our galaxy is moving at some 800,000 mph through the cosmic microwave background. So, what is the actual or absolute speed of the train? Theory of relativity states that one’s absolute velocity is unobtainable. We can only state that the train is moving at 100 mph relative to the earth.

So, due the rules of relativity, we could never know what our actual position is or what our actual state of motion is. We can only state that we are here relative to something else or we are moving relative to something else. So, the original theory of relativity completely eradicated the concept of an absolute space, which was believed in until then.

Relativity proved that space is not absolute. The position of events occurring in space depends on the one who observes it. Different observers might disagree on the actual position of an event. But, the all are correct. Because it is what they observe according to their frame of reference, (provided that the frame is inertial, as relativity does not apply to non inertial frames just as Newton’s laws of motion does not apply in non inertial frames.) For example, a person on the train might argue that the train is stationary and the earth is moving relative to it and there is no reason to disagree with him.

So, basically relativity eradicated the notion of an absolute reality which was unacceptable by many. Although Newton’s laws of motion indicated absence of an absolute space, he denied to accept it because it did not agree with the notion of an absolute god.