Special relativity postulates that the speed of light (while traveling through vaccum) is a constant ($c = 3 \times 10^8 m/s$). But, when light is refracted and change the medium in which it's traveling, its speed changes. The question is:

If the new medium has $n=2$, for example, then the speed of light in this medium is $v=1.5 \times 10^8 m/s$. Does this new speed become a limit to the speed of objects in this medium? And the Lorentz transformations still unchanged or they'll use this new speed too?

  • $\begingroup$ It should be noted that special relativity specifies $c$ as the limiting speed of information transfer in a vacuum. The speed of light in a given medium is not necessarily the new limiting speed of information transfer, especially since there could be other particles/waves that do not interact nearly as much with this medium that could travel through it faster than a beam of light (even other wavelength photons that can travel faster). As far as special relativity is concerned, it's usually the case that $c$ remains the speed limit $\endgroup$
    – Jim
    Dec 1, 2017 at 14:36
  • $\begingroup$ Adding to Jim's answer, you may find cherenkov radiation specifically interesting. Here, the speed of electrons is greater than the phase velocity of light in the medium. Although still not greater than group velocity, there are nuclear reactions in which the particles do achieve greater speeds. More on this in en.wikipedia.org/wiki/Cherenkov_radiation $\endgroup$ Dec 1, 2017 at 14:50

2 Answers 2


Does this new speed become a limit to the speed of objects in this medium?

Not in the same sense that the speed of light is a limit in vacuum. There is a well-known phenomenon which is caused by a charged object moving through a medium faster than the speed of light in that medium: Cherenkov radiation.

Now, this radiation emits energy, so a charged particle moving faster through a medium with a speed greater than $c/n$ will rapidly lose energy until its speed falls below this threshold. In this sense, $c/n$ is a kind of limit for charged particles moving through the medium; one wouldn't expect to find charged particles traveling long distances at a speed higher than this. But it's not a fundamental limit the way $c$ is a limit in vacuum; neutral particles could conceivably travel at speeds greater than $c/n$.

And [are] the Lorentz transformations still unchanged or they'll use this new speed too?

The index of refraction of a medium can be thought of as arising from the interaction between an external EM wave and the skillions of tiny dipoles inside the the medium. Importantly, the assumption that's usually made in this derivation is that the dipoles are at rest. This means that the index of refraction is implicitly defined in a particular reference frame. We therefore shouldn't expect Lorentz symmetry and Lorentz transformations to transfer over to a medium in the "naïve" way, where we just replace $c$ with $c/n$ everywhere and call it a day; in a medium, we wouldn't expect there to be symmetry between inertial reference frames.

(Actually working out how Lorentz symmetry extends to linear media is a complicated business. See this answer from a related question for more information on the subject.)

  • $\begingroup$ Thank you Michael! I'll will look after studying these observations you made! $\endgroup$
    – K. Lemos
    Dec 1, 2017 at 15:11
  • $\begingroup$ What do you mean by the Lorentz symmetry not holding in a medium? It sure holds for neutrinos passing through. If you mean that the Maxwell equations are not Lorentz invariant in a medium, it doesn't mean that the Lorentz symmetry is broken. $\endgroup$
    – safesphere
    Dec 1, 2017 at 15:24
  • $\begingroup$ @safesphere: Your point was what I was trying to address in my last parenthetic paragraph. Let me see if I can edit to clarify. $\endgroup$ Dec 1, 2017 at 15:31

Massless particles like photons move in vacuum with the maximum speed allowed in the hybrrboluc geometry of spacetime. Here geometry is primary while the fact that light moves with this speed is a consequence. We can clearly see this distinction in a refractive medium where light moves slower, but the spacetime geometry is essentially unchanged.

To answer your question, in $n=2$, light slows down twice, but the maximum speed remains unchanged and the Lorentz transformation remains unchanged as well. For example, if neutrinos travel at $0.99c$ in vacuum, they would pass through this medium still at $0.99c$.

The asymptotically local maximum speed in the local frame of the observer, in spacetime of any smooth curvature, is always 1 light-second per second. Based on this constant we define 1 meter as exactly 1/299,792,456 part of 1 lught-second.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.