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The most popular theories say that yes, they do travel at the speed of light. Only a coincident detection of a gravitational wave and an electromagnetic counter
by sleavey 9y ago
The most popular theories say that yes, they do travel at the speed of light. Only a coincident detection of a gravitational wave and an electromagnetic counterpart would confirm this (such as from a binary neutron star coalescence; the binary black holes seen so far by LIGO didn't emit any visible EM as far as we know).
- tzs 9y agoWould even a binary neutron star coalescence give us the information we'd need to answer this? Interstellar space is not actually empty. It contains a low density plasma, and so the speed of light in it is slightly slower than the speed of light in a vacuum. I have no idea if that plasma would also slow down gravity waves, but even if it does it wouldn't necessarily be the same amount [1]. Maybe from the differences in arrival time of light at different frequencies we could figure out enough about the intervening plasma to figure out when the light would have arrived if the space had been empty, and check if that is when the gravity waves arrived? [1] Edit: I have done some Googling. Gravity waves would not be slowed down by the interstellar plasma. The slowdown of light through a medium depends on the existence of positive and negative charges in the medium that can form dipoles in response to the passing electromagnetic wave. For gravity all the "charges" are positive so there is no dipole formation.
- yorwba 9y ago> For gravity all the "charges" are positive so there is no dipole formation. I'm no physicist, but couldn't lack of matter in some region otherwise filled with particles play the part of "virtual" negative matter?
- imaginenore 9y agoWe could build a second detector, sync their clocks, separate them, and measure the speed directly.
- greglindahl 9y agoLIGO is 2 pairs of detectors.