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The idea of the speed of light being as fast as something can go is pretty well accepted. I'm wondering if the top speed might be (very slightly) higher? Lemme explain.

I was reading about particle acceleration, and how accelerating a particle to the speed of light would require infinite energy.

According to a quick Google search, the LHC is getting particles up to 0.999999991 c. For someone such as myself, I might as well consider that the speed of light. I mean, it's so close, right?

So what if in a similar vein, the speed of light is very, very close to the absolute top speed? Is that possible?

If the speed of light in a vacuum was something like 0.9999999999999999999999999999999999 to the absolute top speed in the universe, would we ever know the difference? (this number is just an example to show how the speed of light could be imperceptibly close to the absolute top speed, and yet still not be the fastest speed possible).

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marked as duplicate by Brandon Enright, John Rennie, Kyle Kanos, David Z Jul 16 '14 at 14:44

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Seems like a duplicate of this, perhaps, and/or probably several other questions on this site. $\endgroup$ – David Z Jul 16 '14 at 3:32
  • $\begingroup$ MGaz: "what if the speed of light is something like 0.9999999999999999999999999999999999 to the absolute top speed in the universe." -- Speed of light (in vacuum), a.k.a. signal front speed is exactly 1.0 to the absolute top speed (of any thing/signal being exchanged; not to be confused with "phase speed" etc.) due to the definition of what we mean by (how to measure) "speed" (and "light") in the first place. Related: "Is the Speed of Light an universal spacetime constant ... ?" and many more $\endgroup$ – user12262 Jul 16 '14 at 3:34
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    $\begingroup$ Note that the LHC does not hold the record for human-created particle beam speed. That distinction belongs to LEP if you count beams in collider systems or some high energy neutrino beam or another in the unlimited open division. $\endgroup$ – dmckee Jul 16 '14 at 4:20
  • $\begingroup$ I think the problem with this question is - this really is two separate questions. 1. The question in the title, for which everyone can answer YES, as per the current understanding. 2. The question in the body "What if c is slightly lower than the absolute maximum speed", which will certainly involve opinion-based answers and hence, may be closed by the moderators. @MGaz - you may want to edit out your question and be more specific about what exactly you want to ask. Also, take precaution so that it doesn't get closed as a duplicate of the Q that David Z mentioned. $\endgroup$ – 299792458 Jul 16 '14 at 5:47
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    $\begingroup$ I don't think you should flag my question as a duplicate of one that was closed for being unclear/poorly asked. As I understand it, those questions are supposed to be re-asked. $\endgroup$ – Michael Gazonda Jul 16 '14 at 12:51
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We would know the difference. Here are two possible ways we could tell:

One is that Maxwell's equations tell us exactly how fast light has to go, always. It doesn't matter how you're moving, theory predicts that every beam of light will always be moving at the same speed relative to you. There is only one speed for which that property can hold true, and that's the ultimate speed limit. So if light moves at a speed that's always the same, it must be moving at the ultimate speed limit.

Another way we could tell is that any particles that don't have mass must always travel at the exact same speed as all other particles that don't have mass, and that speed has to be as fast as anything could possibly go- the ultimate speed limit. If light had mass, then lower energy photons (like radio waves) would move slower than higher energy photons (like x-rays), and we would be able to measure that. We can't measure any such difference, which means that light travels at the same speed no matter what energy it has, therefore it must be massless and traveling at the ultimate speed limit.

Ergo, yes, the speed of light and the top speed through space must be equivalent. Top speed cannot be even a teensy bit higher.

There is, however, a loophole- space itself can move faster than light. As a result of that fact and the expansion of the universe, the farthest away galaxies that we can see are actually moving away from us faster than the speed of light, because the space that they occupy is expanding away from the space that we occupy. So the speed of light is not quite "the top speed inside the universe", for appropriate definitions of "speed" and "inside the universe".

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  • $\begingroup$ "There is only one speed for which that property can hold true, and that's the ultimate speed limit. So if light moves at a speed that's always the same, it must be moving at the ultimate speed limit." - But how would we know the difference between the ultimate speed limit and a speed as close to it as I suggest in the OP? $\endgroup$ – Michael Gazonda Jul 16 '14 at 12:48
  • $\begingroup$ @MGaz Like I said in my answer: the math does not lie. Either the laws of electromagnetism are incorrect, or light has to move at exactly the ultimate speed limit. There are no extra terms, even infinitesimal ones, in the equations of electromagnetism that could allow it to be anything else. Additionally, if it was a teeny bit smaller, it couldn't be constant, and it would be possible to low it down more. We can't do that either, so we know that there can be no difference, not even an infinitesimal one. $\endgroup$ – Logan R. Kearsley Jul 16 '14 at 14:41

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