Superphysics
Section 18

# Special And General Relativity

##### 3 minutes  • 491 words

Special Relativity is the principle of the physical relativity of all uniform motion. Every motion must only be considered as a relative motion. In our earler train example:

1. The train is in motion relative to the embankment. Our viewpoint in this case is the embankment
2. The embankment is in motion relative to the carriage. Our viewpoint in this case is the train

For theoretical descriptions of motion, we do not care which viewpoint we take.

This is true for Special Relativity, but not for General Relativity.

But for real descriptions of motion, the laws of physics have the exactly the same form in both cases when=

1. The embankment is our viewpoint
2. The train is our viewpoint
general laws of nature use

This means that in the real world, neither of the viewpoints `K, K'` are unique as compared with the other. Unlike the first, this latter statement is only proven by experience.

Up to now, however, we did not say that all viewpoints or bodies of reference `K` are equal in terms of real motion.

First, we assumed that there exists a viewpoint `K` that is bound to the Galileian law: A particle left to itself moves uniformly in a straight line. All viewpoints or bodies of reference `K'` added to `K`:

• should be given preference
• should be exactly equivalent to `K` for the formulation of natural laws, provided that they are in a state of uniform rectilinear and non-rotary motion with respect to `K`.

All these bodies of reference are to be regarded as Galileian viewpoints. Special Relativity is valid only for these reference-bodies, but not for others which have a different kind of motion.

In contrast to this is General Relativity= All bodies of reference `K`, `K'`, etc., are equivalent for the formulation of the general laws of nature, whatever their state of motion.

General Relativity is more abstract than Special Relativity. People will be more curious about General Relativity.

A train travels at a uniform rate. Its occupant is not sensible of its motion. This is why he can say that the train is at rest, but the embankment is in motion. This is acceptable to Special Relativity.

If the train’s motion becomes erratic, such as if it hits the brakes, then its occupant jerks forward. This leads to mechanical behaviour of objects relative to him inside the train.

This mechanical behaviour is different from the behavior of those objects before the brakes were applied. This is why it is impossible that the same mechanical laws hold relatively to the erratically-moving train, as when it is at rest or in uniform motion.

The Galileian law does not hold with respect to the erratically-moving train. This is why we feel compelled to grant an absolute physical reality* to erratic motion in Newtonian Physics, which is different from General Relativity.

*Superphysics Note: This absolute reality is given by Newton as his concept of “Lord God” in Book 3 of Principia