# Reply to post: Speed of Gravity

### Boffins' gravitational wave detection hat trick blows open astronomy

#### Speed of Gravity

The LIGO result does put an upper bound on the speed of gravitational waves - the two interferometers (one in Washington State, the other in Louisiana) are far enough apart that light would take about 10 milliseconds to travel between them (The great circle distance is roughly 3000 km, and light travels at very close to 300,000 km per second - I'm not bothering to work out the straight line distance rather than great circle [if you want to work it our yourself, the Earth's circumference is 40,000 km, near enough])

The signal, seen by both detectors, was offset in time by about 7 milliseconds. If the event occured exactly on a line extended between the two interferometers and out into space, this means the maximum possible 'speed of gravity' is roughly 1.4c as the distance between the detectors is the maximum possible difference in path length for the signal (to a first order approximation). If the event occurs at any point on a plane bisecting the distance between the two interferometers, the difference in path length would have been zero, and we would have no information on the maximum possible speed of gravity.

There are very good theoretical reasons for expecting the 'speed of gravity' to be the same as the speed of light, but I believe this is the first direct measurement/test. (There are indirect methods, see https://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements).

When more detectors come on line, we will be able to get better experimental bounds (using this direct measurement method) on the value. Very few people would expect it to to differ from c, but it is always worth measuring to check.

Note, the detector being built in India is being built there because the Indians agreed to pay.

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