# Speed of EM waves differ from GWs?

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:

http://iopscience.iop.org/article/10.3847/2041-8213/aa920c/pdf

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.

Question:

1. How can there be experimental evidence of non-zero difference between the speed of EM waves and GWs?
• Quick comment: "because the meter is defined as the distance that light travels in a second" - I understand what you mean by this but the statement is obviously not true - a meter is not 1 light-second. The meter is defined as the distance light travels in 1/299792458th of a second. Commented Oct 4, 2018 at 17:08
• @ÁrpádSzendrei It’s a good question so I upvoted. I would like to see how they are proven to be non-zero also. Ultimately I believe it will be found that they both come from the same thing or are the same thing. Commented Oct 4, 2018 at 17:19
• Having looked over the abstract, it seems this paper is talking about constraining an upper bound on the difference of speeds between EM waves and GWs (quite stringent at 1 part in $10^{15}$), and doesn't say that there is a difference in the speeds. Does the paper claim to have found a non-zero difference in the speeds later on? One would think that such a claim would be found in the abstract. Commented Oct 4, 2018 at 17:36