# Fundamental postulates of SR

could someone be kind enough to explain to me why the fundamental postulates of special relativity introduced

thanks

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#### Fredrik

Staff Emeritus
Gold Member
I'm not sure if you're asking what the postulates of special relativity are, or why they are what they are.

In one formulation there's only one postulate:

Space and time are represented by Minkowski space.

That's really all you need, but you would have to know differential geometry to understand why. (Minkowski space has a non-trivial group of isometries...yada yada yada ).

However, special relativity is usually taught to people who don't know differential geometry, so classes and books about the subject tend to focus on the physics instead of the mathematics. In this formulation, two postulates are used. You can find them here.

There are two reasons why Einstein could be certain that the postulate about the speed of light was correct:

1. It was confirmed by an experiment in 1905.
2. It was a mathematical consequence of Maxwell's equations.

#### robphy

Homework Helper
Gold Member
One could view SR in this way:
All of the laws of physics are the same for every inertial observer.
(The Principle of Relativity applied to all---not just the mechanical--laws of physics.)

One of the laws is that the speed of light [in vacuum] is always the same, i.e., the value predicted by, for example, Maxwell's Equations.

Fredrik said:
I'm not sure if you're asking what the postulates of special relativity are, or why they are what they are.
firstly thanks for the replies, but to clarify: i already know what they are, just curious as to why they are what they are

#### Chronos

Gold Member
firstly thanks for the replies, but to clarify: i already know what they are, just curious as to why they are what they are
Good question. No one knows why the fundamental constants are why they are what they appear to be. At some time, even the axioms break down.

G

#### Gonzolo

Postulates are statements allowing a mathematical argument or physical theory to be developped. Rules of a boardgame or sport can also be seen as postulates. All physical theories have postulates, whether they be from experimental observation (Maxwell's equation) or non-obvious "guesses" (Shrodinger's equation).

The equations of SR would be useless without anyone agreeing to the postulates. Adding these two statements to much of the physics that is classically known (consevation laws etc.) allows SR to be developped. None of SR would be any good if the laws of physics were different depending on where you stand or if c wasn't a constant. The first is agreed upon by most people who are sane, and the second can be seen as a guess that works (because the resulting equations are verified experimentally).

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#### geometer

firstly thanks for the replies, but to clarify: i already know what they are, just curious as to why they are what they are
Here's another way to look at it. Einstein developed the Special Theory of Relativity to deal with certain contradictions in standard Newtonian physics that become apparent at high velocities, and these postulates were what were needed to eliminate those contradictions.

#### yogi

The first postulate that says the laws of physics are the same in all non-accelerating frames would seem to be logical in view if some underlying principle (that exists but is not yet revealed) The second postulate re the constancy of the measured velocity of light in uniformly moving reference systems is counterintutive and this leads to many investigations and alternative theories. In order to explain the MMx null results, Einstien could have simply asserted the round trip velocity of light is always measured to have the same value - but he went further - and postulated that the one-way velocity of light is also isotropic. Almost all validations of SR are based upon over and back measurements - so until there is a free space experiment establishing the veracity of the one way velocity, there will continue to be questioners. as there should be.

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