I understand that light travels at speed c in vacuum, when measured locally. This speed has an exact value, not an approximation, because it is defined as 299 792 458 m / s.
It is an exact value, because the meter is defined as the distance that light travels in a 1/299 792 458th of a second.
Now as per SR, all particles that have no rest mass must travel at speed c in vacuum, when measured locally.
This goes for photons (that build up the EM waves), and the (hypothetical) gravitons (that build up GWs). So both photons and gravitons have to travel at speed c. Both EM waves and GWs have to travel at this speed in vacuum when measure locally.
So theoretically there can be no difference between the speed of EM waves and GWs (assuming they propagate both in vacuum).
Then I found this:
And it talks about an experimental evidence about non-zero difference between the speed of EM waves and GWs. How is that possible? I thought the speed of light is an exact value, and it has to go for all massless particles.
- How can there be experimental evidence of non-zero difference between the speed of EM waves and GWs?