Why the speed of light is constant in every inertial frame of reference? Is it just a postulate of special relativity, a physical observation or is their a way to explain it theoretically?
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2Does this answer your question? Why does speed of light have to be constant? – Rishi Jul 25 '22 at 02:25
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Also relevant: What is so special about speed of light in vacuum? – John Rennie Jul 25 '22 at 03:46
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It is first of all an experimental fact or, as you put it, a physical observation. Second, it is a foundational principle of special relativity, which as a mathematical model of the way our universe works has been experimentally tested ever-more-precisely over the last 100 years and has passed all of those tests with flying colors. Finally, if the speed of light were not the same in all inertial frames of reference then for example it would be possible for us to see effects before their causes had happened, as if time were running backwards.
There are many aspects of our world which would appear completely different to us if special relativity were false. The fact that those effects have been diligently searched for and never seen is powerful evidence that special relativity is a correct description of our world.
niels nielsen
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One thing you hint at, which is not obvious: SR is far simpler than Galilean Relativity + Maxwell's Eq. That latter is a contradictory mathematical nightmare. – JEB Jul 25 '22 at 03:25
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The idea is simple... We once found that light was traveling at a constant and invariant speed in space experimentaly. Something that directly contradicted with Galilean relativity... Einstein wanted to explore how a Universe would "look like" if it wasn't governed by Galilean relativity, but it was governed by some new kind of relativity that allowed speeds in space to be like the speed of light (mainly constant and invariant)... So the quest of Einstein was to explore how such a Universe would behave... So he took the speed of light as an axiom and explored if his new theory would be usefull – Nuke Jul 27 '22 at 11:11
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And that's how physics works, we ultimately have to take some things for granted, and work our way up, trying to explore how this new theoretical Universe will ever behave, and whether or not it will ever be usefull for us to think of the Universe in that way! The only thing that we can ever prove in physics, is how much usefull a theory is, in any given framework. Other theories are more or less usefull than others and that's the only thing we can ever prove! Everything else is outside of the scope of physics, and enters the scope of philosophy! – Nuke Jul 27 '22 at 11:18