# So what actual physical event would allow us to observe the speed of gravity?

I have read, if the sun disappeared it would take 8 minutes before this could be detected on Earth due to speed of gravity being $$c$$. Of course, the sun can't disappear. Perhaps realizing this impossibility, people write of the sun exploding instead. But if the sun literally exploded the mass would still be present for a while, just dispersed.

So what actual physical event would allow us to observe the speed of gravity? If rotating massive objects, how? And if the sun exploding would allow this observation, how so?

• So what actual physical event would allow us to observe the speed of gravity? As S. McGrew’s answer explains, we have already observed the speed of gravity (i.e., of gravitational waves). – G. Smith Sep 15 at 23:30
• @G.Smith There is a delay for the wave to reach the different GW detectors. That can be used, I guess? – Sidarth Sep 16 at 6:42
• @Sidarth That’s insufficient unless you know which direction the wave is coming from. I was referring to the fact that we observed the gravitational waves at nearly the same time as the electromagnetic waves. – G. Smith Sep 16 at 16:15

You need to have a changing quadrupole moment in order to see gravitational waves. A spherical explosion would not produce any gravitational waves at all, so exploding is not the answer.

What you would want is to collapse the sun into two half-suns and then have the two half-suns spin around each other very rapidly. Of course, that wouldn’t conserve angular momentum so you would probably have to make two pairs and have them counter-rotate

• correct, I have deleted my comment. – Árpád Szendrei Sep 16 at 15:13
• Really nice answer. – Árpád Szendrei Sep 16 at 16:24

Colliding stars and merging black holes produce gravitational waves that we have detected. Occasionally we have been able to locate the source and correlate it with electromagnetic emission. By comparing time of arrival of the two signals and knowing the distance to the source - and knowing the composition of the intervening space - we can compare the speed of the gravitational and electromagnetic signals.

The last section of this Wikipedia article describes such an event.

• This is a correct answer, though, the question I believe is about changes in a static gravitational field (like in the example, the Sun disappears), and how fast those changes propagate, the answer is of course the same, c, but this is a theoretical limit, and I do not know if there are experiments to test this (changes in a static gravitational field, and how fast those propagate) specifically. – Árpád Szendrei Sep 16 at 5:41
• I have been trying to formulate a question about your response, I hope this is reasonable: Do any movements of mass cause gravity waves but we need movement at high speeds of very large masses to create waves big enough to detect? If so, maybe blowing up the sun would cause waves although the idea was detecting absence of gravitational attraction which I think is different and would not work since as mentioned the mass would remain. – releseabe Sep 16 at 9:27
• Any movement of mass that comprises a change in quadrupole moment will cause gravity waves, but gravity is so weak that the waves are not detectable unless the mass and movement are enormous. Blowing up a spherical sun symmetrically would not change its quadrupole moment, so would not produce gravitational waves. However, an oblate spheroid does have a quadrupole moment and the sun is a slightly oblate spheroid, so blowing it up would likely change its quadrupole moment and produce very weak gravitational waves. – S. McGrew Sep 16 at 13:48