The second postulate of STR was made exclusively for light (electromagnetic waves)?

If gravitational waves also travel at velocity $c$, then are they obliged to fulfill this second postulate?

Any wave of field (anti matter, dark matter, dark energy), traveling at velocity $c$, must obey this same postulate?

Since gravitational waves are not contained in the second postulate, does the invariance of these waves need to be proved?

Is Ligo results a proof of the velocity $c$ of gravitational waves?

Can Ligo prove the invariance of gravitational waves?

  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/5456/2451 and links therein. $\endgroup$ – Qmechanic May 17 '19 at 4:19
  • $\begingroup$ All massless particles travel at the speed of light. No massive particles do. It is the invariant speed, related to the geometry of spacetime. There is nothing light-specific about it. $\endgroup$ – G. Smith May 17 '19 at 4:34

What you are describing, as well as why electromagnetic waves are constrained to $c$ can be seen as a consequence of STR. Much like time dilation and length contraction the $c$ limit is a result/implication of STR's model Lorentz transformation.

The same goes for gravitational waves, before General relativity it was a wide point of discussion that a faster than $c$ propagation speed would lead to issues with one of the implications of STR, which is local time. Einstein would go on to prove the propagation speed of $c$ in a very elegant way (proof starts in page six).

There exists several ways of actually proving this experimentally. One of the first was studying orbital decay rates of binary pulsars that orbit around each other quickly and as such propagate a lot of energy through gravitational waves.

Hope this helps somewhat.

  • $\begingroup$ I agree with the limit c, but my questions are focused on the invariance that the second postulate imposed only for the light. $\endgroup$ – João Bosco May 17 '19 at 3:37
  • $\begingroup$ I guess one way of thinking about this would be that $c$ arises in the second postulate because of the implications that come from SR. That is to say the postulates were a good way of coming up with SR because they were established constants. But I fear at that point I might be getting into speculation. On that you could read several discussion on the two postulates. One thing to note is that the two postulates sort of arose out of other observations such as Maxwell's equations. But there exists other postulates from which SR may be derived. $\endgroup$ – ElectrumHafnium May 17 '19 at 3:48

It is a misconception that SR second postulate was made for light. In reality, what it says is (local measurement):

  1. particles with rest mass that travel in vacuum with speed less then c will always travel with speeds less then c

  2. particles with no rest mass that travel in vacuum with speed c will always travel with speed c

  3. particles that travel in vacuum with speed faster then c will always travel faster then speed c (this part is theoretical)

Now EM waves are quantized, and the particles that make up light are photons, that are massless particles, and they always do travel at speed c in vacuum when measured locally.

Now GWs are theoretically said to be quantized too (there is no experiment to prove this), and the particles that make up GWs are gravitons (theoretical), that are said to be massless particles, that travel at speed c in vacuum, when measure locally.


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