Undergrad Speed of gravity vs. speed of light

[exponent137]

GW170817 showed that gamma rays came 2 seconds after gravitational waves. (after 130M years) Are these two seconds uncertainty at determination of difference of these speeds? Or is it possible to explain, why 2 seconds of difference, or is it possible to explain, at least, a part of this difference?

 

[jedishrfu]

I think it has more to do with the collision process and the geometry of the collision than a speed difference but I'm sure folks are actively investigating this difference to explain it.

http://ligo.org/science/Publication-GW170817GRB/flyer.pdf

Of course this could be nature's way of letting us see both events ie grav wave signal gives us a clue of where to point our scopes and then the gamma rays arrive for us to record them.

It's also possible, though remote, that they signal different events entirely although the article says that's a 1 in 20 million chance in which case the 2 second delay is just coincidental.

 

[haushofer]

I'd say gravitational waves don't suffer from the same amount of scattering as light waves. Light can be scattered or bend from dust or other types of mass, while gravitational waves (to first order!) are not.

<< Note -- typo corrected >>

 

[Vitro]

GW170817 showed that gamma rays came 2 seconds after gravitational waves. (after 130M years) Are these two seconds uncertainty at determination of difference of these speeds? Or is it possible to explain, why 2 seconds of difference, or is it possible to explain, at least, a part of this difference?

Those gravitational waves arrived here a long long time ago. They were gradually increasing in amplitude and frequency as the binaries were getting closer together and were detected when they exceeded the sensitivity threshold of the instrument. There was no discrete arrival moment.

The gamma burst on the other hand is probably a discrete event happening at a specific moment during the merger.

 

[PeterDonis]

GW170817 showed that gamma rays came 2 seconds after gravitational waves. (after 130M years) Are these two seconds uncertainty at determination of difference of these speeds?

No, they're showing that whatever process produced the gamma ray burst did not happen at the exact same moment as the peak of the gravitational wave burst. Since the gamma ray burst was probably produced by something happening in a cloud of plasma surrounding the black hole merger, there's no requirement that it had to happen at exactly the same instant as the gravitational wave peak from the merger itself.

 

[exponent137]

No, they're showing that whatever process produced the gamma ray burst did not happen at the exact same moment as the peak of the gravitational wave burst. Since the gamma ray burst was probably produced by something happening in a cloud of plasma surrounding the black hole merger, there's no requirement that it had to happen at exactly the same instant as the gravitational wave peak from the merger itself.

According to Jedishrfu link : http://ligo.org/science/Publication-GW170817GRB/flyer.pdf figure 2 - the fourth panel - the signal was stopped 1.7 seconds before gamma rays, also because LIGO-VIRGO cannot measure very high frequencies.

Let us imagine that it possible to measure any frequency of gravitational waves. Would this reduce this difference 1.7 s? Is any estimation, how?

 

[Wes Tausend]

No, they're showing that whatever process produced the gamma ray burst did not happen at the exact same moment as the peak of the gravitational wave burst. Since the gamma ray burst was probably produced by something happening in a cloud of plasma surrounding the black hole merger, there's no requirement that it had to happen at exactly the same instant as the gravitational wave peak from the merger itself.

I think your explanation must be correct.

It intuitively seems that if we assume that a gravity wave and electromagnetic move at the same velocity, we should be able to locate a ring location around the star merger center in which the gamma ray burst would have to be subsequently triggered for it to take the later eta unless it somehow came from partially behind the merger center. Does that make sense?

I suppose the resolution may not be fine enough to differentiate any of this in actual observation.

Wes

 

[pervect]

Has anyone looked at how much the interstellar medium would slow the gamma rays? It'd be a tiny effect, but so is 2 seconds in 130 million years. I have no intuition on the numbers offhand.