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Question:
I have often read that gravity propagates at the speed of light or less. What experiments have been performed that verify this?



Replies:
Gravitation and general relativity are not my forte' but I hope I can give you a good (and correct) answer that is up to date. That said... We currently have not detected, explicitly, gravitational waves. And, as such, the velocity of those waves has never been directly observed.

Now, there are several interesting points here. First, people have been attempting to detect gravitational waves directly for a few decades now and the research continues today. Initial Weber (from U of Maryland) began attempting to observe these waves. While his group reported that they had found such signals, all subsequent experiments by other scientists have failed to make any conclusive observations. Further investigations have led many to the conclusion that Weber's observations were not correct.

Despite the controversy over his initial results (and eventual dismissal), Weber went on to suggest the detection of the waves by a laser interferometry experiment. Currently such experiments are in progress around the world. Thus far they have yet to reliably, conclusively detect a gravitational wave. However, each year they improve the experiment and push their accuracy even higher. If they fail to observe anything, it will not be for lack of effort, skill or cunning. Look up the LIGO experiment, http://www.ligo.caltech.edu/

Likewise, general relativity has come under scrutiny from another direction. We can observe the effects of gravity on the large scale. Galaxies form, planets orbit stars, and we are attracted to the earth. However, people have been attempting to find violations of the inverse-square law at very short distances. Some theories of high-energy physics predict violations at small length scales. Finding and observing any such deviation would be a huge discovery. The University of Maryland and University of Washington physics departments(among others) have ongoing experiments to observe gravity at the small scale.

So currently there is (to my knowledge) a good deal of experimental evidence that General Relativity is correct over many decades of length and mass scales. However, there still remain many scales on which is has not been measured (that is a bad pun) and the gravitational waves predicted by the theory have never been directly observed.

One further note: We always talk in terms of "the speed of light." Therefore many people find it at least curious, if not confusing, that other things should travel at the same velocity as light. In fact, it is not a fundamental property of electromagnetism per se. Rather, "the speed of light" is the speed with which all massless particles and waves travel through the universe. Light moves at this speed, but so does everything else that is massless.

Michael S. Pierce
Materials Science Division
Argonne National Laboratory



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