I understand that the speed of light c is derived from the self-interaction between elections/photons, and is thus the maximum speed of anything composed of electrons/photons.

Suppose that there is a particle that is made up of stuff that is not electrons and photons. Is it possible in principle for this particle to travel faster than c? Or does c somehow apply to absolutely everything in the universe, even as yet undiscovered particles?

If it has already been proven that c applies to absolutely everything in the universe, even as yet undiscovered particles, then c is really a property of space itself, rather than a property of electromagnetism.


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  • $\begingroup$ Electromagnetism is a property of space itself. That is one speaks of EM waves propagating through vacuum, without medium. Look into special relativity. $\endgroup$ – Keefer Rowan Dec 18 '18 at 23:27
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    $\begingroup$ "the speed of light c is derived from the self-interaction between elections/photons and is thus the maximum speed of anything composed of electrons/photons" Where did you read these statements? $\endgroup$ – my2cts Dec 19 '18 at 0:43
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    $\begingroup$ @KeeferRowan "Electromagnetism is a property of space itself" - Can you provide a reference to this claim? Photons flying in space "without medium" doesn't prove it. $\endgroup$ – safesphere Dec 19 '18 at 0:50
  • $\begingroup$ @my2cts My understanding that the speed of light derives from the speed at electrons interact with themselves comes from a few YouTube videos that I've watched about it by Lawrence Krauss and similar top notch physicists. $\endgroup$ – Steve Dec 19 '18 at 2:52
  • $\begingroup$ @my2cts From what I've watched, the speed of light can be derived from Maxwell's equations, and the meaning of the manipulation of the equations leads physicists to conclude that c represents how electromagnetism itself propogates. $\endgroup$ – Steve Dec 19 '18 at 2:54

The fact that we can't exceed the speed of light is a fundamental property of spacetime. It stems from the fact that the speed of light is constant in any reference frame. If you turn on a laser pointer, you will observe the beam propagate away from you at the speed of light, regardless of whether you are standing still or zooming across the universe at 99% the speed of light. One of several interesting consequences of this is that no matter how fast you go, light will always go faster.

This might seem a little hand-wavy, but that's the gist of it. If you want to go deeper, there are some great videos on YouTube about special and general relativity. Wikipedia also has a lot of good information.

  • $\begingroup$ @Jack Nick Porter Your answer completely ignores the whole point of my question. My whole point is that I was asking if it is possible for particles that are not composed of electrons/photons to exceed c. Your answer immediately refers the speed of light being constant. $\endgroup$ – Steve Dec 19 '18 at 2:57
  • $\begingroup$ @Steve to put it another way, the speed of light is a fundamental property of spacetime and must be respected by all matter, including things that aren't made of electrons/protons. $\endgroup$ – Allure Dec 19 '18 at 5:03
  • $\begingroup$ @Steve Try to avoid confusing $c$, a physical constant with units of velocity that is a fundamental property of spacetime, with the speed at which light travels, a speed which happens to be equal to $c$. Other things also move with speed $c$, such as gravitational waves and gluons, and they do this because of the nature of spacetime, not because light moves at that speed. $\endgroup$ – eyeballfrog Dec 19 '18 at 5:57
  • $\begingroup$ @eyeballfrog Can you provide a reference to a measurement of the speed of gluons or an observation of the graviton? $\endgroup$ – safesphere Dec 19 '18 at 15:45
  • $\begingroup$ @safesphere Gravitational waves were observed two years ago and are known to travel at speed $c$ to within one trillionth of a percent. Surely you remember it. $\endgroup$ – eyeballfrog Dec 19 '18 at 16:09

The speed of light is not derived from the self interaction of photons or electrons. Photons and electrons are part of physical theories which are described in terms of space and time. The description of space and time themselves, without introducing any other physical theory, already includes a speed limit, which is referred to as $c$. That speed limit must be respected for any massive or massless particle, including, but not restricted to, photons and electrons.


In special and general relativity, "distance" between two "events" is the square root of the difference between square of the spatial separation and the square of c times the time separation. For things moving at the speed of light, that "distance" is precisely zero.

So you are right, the speed of light is an aspect of spacetime.

The existence of "tachyons" has been proposed, but is only rarely seriously considered. It was thought that particles with imaginary mass (mass x i) could travel faster than light. However, it has been determined that even particles with imaginary mass could not exceed light speed. See "Tachyon" in Wikipedia.

  • $\begingroup$ You say "for things moving at c, that distance is 0". Is this true for particles that are not composed of photons/electons? $\endgroup$ – Steve Dec 19 '18 at 3:06
  • $\begingroup$ Yes, because it's inherent in the geometry of spacetime. The type of particle is irrelevant. $\endgroup$ – S. McGrew Dec 19 '18 at 3:39

The answer to your question, as written in the details, is no for all known particles. Empirical evidence comes from the fact that in particle colliders like the Large Hadron Collider but also its many earlier antecedents, beams of protons and heavy nuclei - composed of protons and neutrons - can be accelerated arbitrarily close to, but no faster than, the speed of light. These particles are not photons, and do not contain electrons anywhere in their composition - they are instead blobs of quarks. Moreover, muons - a particle similar to the electron in some ways but equally an elementary particle that is not composed of anything, including electrons - provide less direct tests for this idea.

The reason that this is to be expected is that it is not so much that the "speed of light" arises from "interaction between electrons and photons" - which I believe you're using to intend to really mean the electromagnetic interaction - but rather it comes from the geometry of spacetime, and in fact it is this geometry of spacetime which shapes electromagnetic interactions, not the other way around. In particular, it both limits the speed of light and creates the possibility of magnetic fields. So yes, your last hunch at the end of your post is, to the best of our knowledge, correct.

The key fact about the geometry of spacetime is that it is "Lorentzian", which means that the mathematical relations describing how the point of view of an observer changes when sie changes speeds - which effectively amount to its "rotations" - are such that there is a particular speed which is not changed by them, and this speed is what we call the speed of light. This speed is not necessarily a limit, but the Lorentzian geometry also tells you that if you could somehow exceed it, you would be able to travel or at least send messages backwards in time. The facts that we have seen nothing to do so, including that we have received no messages from our future selves, that such backwards-in-time travel creates interesting logical problems like the so-called "grandfather paradox", and moreover the sheer mountain of experimental evidence which verifies to very high precision (including all tests of the universality of the light speed limit but also of related consequences of the geometry like that of time dilation, which the aforementioned muon studies deal with) the validity of the Lorentzian geometry as being the geometry on which our Universe is based, are all good reasons to consider the "speed of light" as the absolute maximum value allowed for the speed of any moving objects in the real Universe.


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