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Let me get one thing straight first. I am not saying that fundamental constants like the speed of light don't have the value that we know they have today. What I am asking is whether the value of a constant could change with time. For example, we measure today that the speed of light is almost $300000000\,m/s$, but this measurement was made over the last century. But what if a long time ago (billions of years ago) that constant had a different value?

So, it's always the limit of how fast something can move, but that limit slowly changes with time.
To conclude,i am asking if there is any kind of evidence that any of the fundamental constants like the speed of light remain constant in time or if we just took that for granted.

TheQuantumMan
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    Just to be clear, are you asking us what evidence we have that the speed of light hasn't changed? Or are you asking us what it would mean for physics if it had changed and we just haven't noticed? – Jim Jul 14 '15 at 13:05
  • I know that there is experimental evidence for the speed of light and it being a fundamental constant for velocity.I have read some interesting points from some physicists that explain some of the early processes or the universe(or some of the paradoxes in some of our measurements from the universe) and they say that the speed of light might not have always have the value that we know that it has today.So,i am asking if there is evidence that the speed of light is constant through time – TheQuantumMan Jul 14 '15 at 13:09
  • i edited the question – TheQuantumMan Jul 14 '15 at 13:10
  • You could also edit the title. Or better did a search before asking. I see 4 topics on the right pane that ask the same questions. – Val Jul 14 '15 at 13:12
  • No,now that i edited the question,i believe the title is appropriate – TheQuantumMan Jul 14 '15 at 13:14
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    Possible duplicates: http://physics.stackexchange.com/q/21721/2451 and links therein. – Qmechanic Jul 14 '15 at 13:15
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    The question and title are clear to me. We have evidence that the distance between objects increases because the nature of space is changing - space is expanding. This is too small to measure in a room, but shows up for distant galaxies. You are asking if similar effects are present for fundamental constants like the speed of light. What evidence is there? – mmesser314 Jul 14 '15 at 13:16
  • @mmesser314 yes that is what i am asking(the speed of light is just an example) – TheQuantumMan Jul 14 '15 at 13:18
  • @mmesser314 : the speed of light varies in the room you're in. If it didn't, NIST optical clocks at different elevations would stay synchronised, and your pencil wouldn't fall down. See this Baez article: "A curvature of rays of light can only take place when the velocity [speed] of propagation of light varies with position". This difference in speeds is precisely that referred to above by ceiling and floor observers. Also see the Shapiro delay. – John Duffield Jul 14 '15 at 18:55
  • @JohnDuffield The expansion of space is too small to measure in a room. – mmesser314 Jul 15 '15 at 04:27
  • @mmesser314 : agreed. But the variation in the speed of light isn't. Here's the old Baez article by the way. See the General Relativity section and note how it contradicts itself. Most unfortunate! Fortunately editor Don Koks sorted that out. – John Duffield Jul 15 '15 at 07:26
  • see also https://physics.stackexchange.com/questions/212287/why-should-the-planck-constant-be-a-constant-throughout-all-space – ProfRob Jul 29 '21 at 12:52

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We don't know that fundamental constants don't slowly change over time. Au contraire, you can find articles like Changes spotted in fundamental constant: "The researchers found that the fine-structure constant, known as α, has changed in both space and time since the Big Bang".

The thing to note is that some "fundamental constants" aren't constant at all. See the NIST article on the fine-structure constant: "Thus α depends upon the energy at which it is measured, increasing with increasing energy, and is considered an effective or running coupling constant". It's a running constant, which means it isn't constant.

Thee are other "constants" like this. See http://arxiv.org/abs/0705.4507 and the second paragraph here. Jo Magueijo proposed that the speed of light was faster in the early universe. You can see a mention of that in this Wikipedia article. I'm pretty certain he's wrong myself - it wasn't faster, it was slower.

PS: the Wikipedia article is wrong about Einstein. He didn't abandon the varying speed of light in 1911. You can find plenty of examples whicih dmeonstrate that in the Einstein digital papers. However in some the English translation used the word velocity rather than speed, causing much confusion.

John Duffield
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  • Thank you for a very nice answer.Also,via what experiment or theoretical procedure did you prove that it was slower?Also,what does this change of the speed of light mean for the world?Does it change anything fundamental? – TheQuantumMan Jul 14 '15 at 19:09
  • My pleasure Landos. There's plenty of experiments that demonstrate that light goes slower when it's lower. For example see this interview with David Wineland of NIST: "If one clock in one lab is 30cm higher than the clock in the other lab, we can see the difference in the rates they run at" The important thing to appreciate is that a clock doesn't actually measure the flow of time like some kind of cosmic gas meter. There is no time flowing in that NIST optical clock. It goes slower when the light goes slower. – John Duffield Jul 14 '15 at 20:09
  • As for what it means, I rather think that recognition of what Einstein said about the speed of light varying with position will eventually remove an impasse that has been impeding scientific progress. It's like for want of a nail, but the other way around. – John Duffield Jul 14 '15 at 20:12