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Experimenting with the speed of gravity

  1. Aug 5, 2006 #1
    So I was sitting around, thinking of nothing in general, when the idea struck me that to me gravity seems instantaneous. Realizing this is rather unlikely (many people at one time would have regarded light as instantaneous as well), I googled speed of gravity. Well, it seems the only experiments conducted as yet have been flawed, and this puzzles me.

    You see, it occurs to me that if the speed of gravity is different from light, then we have in our hands an entirely new way of communicating, among other things. If this is significantly faster than light, then the possibilities in my mind are endless (and I shall not list them all here.. lol).

    So I propose two experiments, one to determine the speed of gravity here on earth, and one to determine its speed in a vacuum, if different (but somehow, I doubt it is... common sense just tells me otherwise):

    1. We place an electromagnet on one side of the earth. On the opposite side we place a detector. We pulse the magnet. Given the diameter of earth (about 7900 miles) and the speed of light (about 186,000 miles/second in a vacuum), there should be a .0425 second delay if the speed of gravity is the same as that of light (in a vacuum). This is a very detectable delay by modern electronics. In fact, a shorter distance should easily suffice; say, Caltech to MIT or something. This would decrease the necessary strength of the magnet considerably.

    2. Same thing, but from a satellite to something placed on the moon, or another satellite, etc, provided they are in a high-enough orbit to be effectively in a vacuum, and far enough apart for the delay to be detectable.

    Now. Surely I am not the only one with this thought, so why hasn't this happened. Can we not detect changes that minute in the gravitational field? Can we not create a magnet powerful enough? It wouldn't need to be pulsed long enough to start drawing in cars, buildings, lab techs with too high a blood-iron count, etc., so there shouldn't be a "public danger" argument... Why can't this be done??
    Last edited: Aug 5, 2006
  2. jcsd
  3. Aug 5, 2006 #2
    I know there are 4-tesla magnets used for MRI at several academic centers. Would those be strong enough?

    But here is a different way: observe the changes in the paths of orbiting moons of Mars as one gets near the other.

    But then again, is gravity is only a deformation of space-time, it is instantaneous, isn't it? Or are you assuming it's really a force. What do people say it is these days?
  4. Aug 5, 2006 #3
    Well, that's the question, isn't it? If it's instantaneous, as in Newtonian mechanics, then there wouldn't be a measurable delay. But in relativity, it is assumed to be equal to the speed of light.

    So while my experiment wouldn't determine it to be instantaneous, it could determine it to be equal to or a good deal greater than the speed of light.

    The sat-moon or sat-sat idea would only be interesting if we already determined a speed for gravity from the first experiment. If we did, it would be useful to see if there was a significant difference in its speed through a vacuum.

    It would be nice for the speed of gravity to be instantaneous. Most things that once seemed immeasurable now have quantities, however. But that doesn't rule out the possibility that the speed of gravity is, for 3D purposes, instantaneous.

    Your MRI idea is the same as mine; surely those are sensitive enough? So now we're back to my original question... why can't this be done? Or am I truly the first person to think of such a thing? (VERY unlikely.)
  5. Aug 5, 2006 #4
    I fail to see how this experiment would determine the speed of gravity, and even if it would I don't understand why you would use a magnet and what using the magnet would tell you about the speed of gravity as opposed to just about the speed of light.
  6. Aug 5, 2006 #5
    I think the best way to test it might be astronomical observations. If a planet or star's motion was shown to be affected by a change in another heavenly body's motion faster than could be explained by the "force" of gravity travelling between the two at the speed of light, the result would be positive for a superluminal speed of gravity.There must be data interactions out there that have already been observed.

    I also failed to see the connection between magnetism and gravity but I assume you know of one?
  7. Aug 5, 2006 #6
    I don't see how an experiment with magnets has anything to do with gravity, there's no experimental evidence supporting such a thing.

    This is the 21st century, we can easily measure nanosecond time delays, in which light travels about a foot. There's no need to have your detector thousands of miles away.

    Btw, you say "gravity seems instantaneous" - where did you get this notion?
  8. Aug 5, 2006 #7
  9. Aug 5, 2006 #8
    I was under the impression that gravity affects magnetic fields.
  10. Aug 5, 2006 #9
    You're right, to disprove the speeds being equal, it wouldn't need miles and miles. But that gives us a large margin by which we could measure something: let's say we measured the delay between our 7900 miles to be .00002 seconds. While gravity is now established as far faster than light, it does have a speed as well. And if we detect nothing, getting an experimental delay of zero is far more significant at 7900 miles than at one foot.

    And when I say gravity seems instantaneous, I mean to me. It seems it would need to be. And while I can't prove or disprove that, the ability to prove or disprove its speed being equal to light would be useful, since that is the current debate.
  11. Aug 5, 2006 #10
    I don't think I've phrased this well at all. My point is this: We should feel the affects of an object's position relative to us in location A slightly before (that is, if location A is closer to the Object) than in location B. That is to say, the "gravity waves" or whatever you want to call them should have an easily detectable delay (.0425 sec at 7900 miles), if they travel at the speed of light.

    And since gravity affects the earth's magnetic field, this should be possible.
  12. Aug 5, 2006 #11

    Doc Al

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    Staff: Mentor

    Sorry folks, but this is not the place for idle speculation. (And why was this posted in the quantum physics forum? Before I close it down, I'll move it to the relativity forum--general relativity still represents our best understanding of gravity.)

    Read this: http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html" [Broken]
    Last edited by a moderator: May 2, 2017
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