# The Principle of Invariant Light Speed

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1. Sep 14, 2014

### AbhiFromXtraZ

While reading Special Theory of Relativity from Feynman Lectures, I fell into the confusion about invariant speed of light.
No matter whether physical explanation or mathematical.
So my question is Why the speed of light is same for a person at rest and for a person moving with the speed of light?

2. Sep 14, 2014

### phinds

First off, there is no such thing as a "person moving with the speed of light". You can, theoretically, move very close to the speed of light but you can't get there.

The universal speed limit, which light obeys because it is mass-less, is a postulate of Special Relativity and and empirically demonstrated fact.

3. Sep 14, 2014

### AbhiFromXtraZ

Ok.. Forget about the value of speed of the person...
Can you now explain why the speed of light is INVARIANT of the motion of that person?

4. Sep 14, 2014

### Staff: Mentor

We don't know why the speed of light is invariant. It was postulated by Einstein when he developed Special and General Relativity and evidence supports the belief that it is invariant, but we don't know why this is so. Like many things in physics, it simply is.

5. Sep 14, 2014

### DiracPool

Nobody knows "why" the speed of light is a constant in all inertial reference frames, it just is. It was a stipulation laid down by Einstein when he was developing SR that was based primarily on the constant value of the speed of light implied in Maxwell's equations. The constancy of the speed of light was also demonstrated experimentally by Michelson and Morley (M&M) 20 years prior to Einstein's development of SR, although Einstein had stated that he was unaware of the study at the time he wrote the seminal 1905 paper. See: https://www.amazon.com/Einsteins-Mistakes-Human-Failings-Genius/dp/0393337685

We simply accept the constancy of the sped of light as fact today because the models developed around that stipulation predict accurately experimental measurements.

Edit: Drakkith beat me to it. I wasn't trying to be redundant

Last edited by a moderator: May 6, 2017
6. Sep 14, 2014

### vanhees71

This is one of the things you cannot explain. You can derive it, in a way, from simpler symmetry considerations about space and time: Assuming that space is Euclidean and time is homogeneous for any observer in an inertial frame, and that the principle of special relativity is valid, you'll find (up to trivial equivalence) two symmetry groups: the Galilei group and the Poincare group, implying the Galilei-Newton space time (a fiber bundle) and Einstein-Minkowski space time (a pseudo Euclidean affine manifold). Which structure describes nature best, is an empirical question, and as is well known nowadays, Einstein-Minkowski space time is the winner.

Even this is an approximation, however! When you need to take into account gravity, which is the case in astronomy and cosmology, then you must extend the description of space time to a pseudo-Riemannian manifold and use Einstein's General Relativity.

7. Sep 14, 2014

### AbhiFromXtraZ

Ok.. Now I get it... Thanks to all..

8. Sep 14, 2014

### Staff: Mentor

You might also look at this entry in the FAQ section at the top of this forum: