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Science

Experiment This Weekend To Measure Speed Of Gravity 91

Posted by timothy from the ch-ch-ch-changes dept.
An anonymous reader writes: "Is gravity an instantaneous phenomenon, as we were taught in high school, or is its speed, like all other Einsteinian phenomena, bounded by the speed of light? A radical new experiment, proposed by Sergei Kopeikin, and involving the Very Long Baseline Array, is set to occur this weekend, and results should be known within about two weeks."
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Experiment This Weekend To Measure Speed Of Gravity More

Experiment This Weekend To Measure Speed Of Gravity

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  • I thought that this was a well understood topic. I'm suprised that this hasn't been tested before.

    Experiemnt (or observation) is good for science. But I'll still bet anyone that the current theories will be supported by the new evidence.

    • Re:I thought (Score:2, Insightful)

      by unDiWahn ( 599102 )
      I'll bet that the current theories won't, and they'll have to invent something new to account for the difference, ala Tachyons or Dark Matter

    • Of course it is understood (Score:1, Informative)

      by Anonymous Coward
      According to relativity, gravity propagates at the speed of light. Since the Earth is attracted to where the Sun *was* 499 sec ago, you'd expect wierd orbits that don't follow the experimental data. It so happens that the curvature of space-time caused by the Sun pushes the orbit in the other direction and compensates exactly up to the 4th order.

      In other words, speed-of-light gravity + curved space-time (Einstein) = instantaneous gravity + Euclidean space (Newton) + 4th order error.

      That 4th order term fixes the discrepency in Mercury's orbit, so Einstein's theory wins over Newton's because it explains Mercury's orbit. Speed-of-light gravity it is.

      • Re:Of course it is understood (Score:1, Informative)

        by Anonymous Coward
        Except that because of minor effects due to factors like the Sun being not perfectly spherical, electromagnetic interactions with the Sun's field, and such like, that 4th order error term should not match what Einstein predicts.

        Somebody cooked the books.

  • If it's instantaneous... (Score:5, Interesting)

    by crow ( 16139 ) on Thursday September 05, 2002 @11:48AM (#4200372) Homepage Journal
    If it's instantaneous, then that would provide a means of faster-than-light communication. Of course, it's probably impractical, as the amount of mass we would have to move to be detectable at a significant distance would be prohibitive. ...unless you had some way to implement gravity shielding, and turn it on and off like smoke signals.

    Fun to think about. Probably more practical for a science fiction story than reality.
    • the amount of mass we would have to move to be detectable at a significant distance would be prohibitive

      Well, perhaps a very finely-tuned vibration of a specific type could show up in a Fourier spread, even if the power were incredibly low. But even so.....

      Probably more practical for a science fiction story than reality.

      Undoubtedly! But fun to speculate.
      • the amount of mass we would have to move to be detectable at a significant distance would be prohibitive
        Well, perhaps a very finely-tuned vibration of a specific type could show up in a Fourier spread, even if the power were incredibly low. But even so.....

        It goes both ways though. If you can move something large back and forth quickly to transmit via gravity, then you also endure resistance (energy loss) remotely.

        This would mean faster-than-light energy transmission. Again, though... at very low magnitude.

    • Well on that theory we just need to instantly convert that mass to energy and back again.

      I'll go file a patent just in case someone figures out an implementation of e=mc^2.

      Don't you love solving one neigh impossible problem with another?

      • Re:If it's instantaneous... (Score:4, Interesting)

        by crow ( 16139 ) on Thursday September 05, 2002 @12:48PM (#4200731) Homepage Journal
        My understanding was that you don't convert between mass and energy. Mass and energy are one. It all depends on how you observe it. If my understanding here is correct, then energy produces gravity, just like any other mass. However, because of the huge c^2 factor, it would take a huge amount of energy to test this; sounds like another good experiment.

        Or is my understanding whacked?
        • No, your understanding is spot on.

          I'd phrase it as: it all depends on what units you prefer. The difference between joules and kilograms as units of mass-energy is in many ways similar to the difference between metres and parsecs as units of distance. They are just multiples of each other, and one unit is a very large multiple of the other.

          Paul
    • Fun to think about. Probably more practical for a science fiction story than reality.

      Definitely. The nearest place I can think of where faster-than-light communication might be useful would be Mars, and then, only for ultra-time-critical purposes. Like playing the stock market, perhaps. A market 'rise' could be a lessening of gravitational pull, a market 'fall' could be an increase.

      And a market 'crash'.... Oops. Scratch that, bad idea.
    • My first thought was:

      Imagine RFC10023: TCP/IP and IPv6 Over Gravity Waves. :o)
    • ``unless you had some way to implement gravity shielding, and turn it on and off like smoke signals.''
      Which would then have to be faster than light, too, or it wouldn't help much.
      • Not true.. Instantaneous communication would still be possible, the speed of the gravity shielding would only limit the rate of communication ( bit-rate? )
        • I was thinking the same thing, until I realized that of course the object's gravity that would be moved, could only move at the speed of light, but once it had moved, then the gravity change detected at the other end would be received right away.
          This would get around light speed restricted communication, so I doubt it will ever be possible. I hope I am wrong however.
          • Good point.. but then again there is always that quantam entanglement/spooky action at a distance thing with entangled particles what we could use to get around light speed restricted communication.

            I remember hearing that they pulled that off between Britain and main-land Europe some time last year..
    • If it's instantaneous, then that would provide a means of faster-than-light communication.

      Isn't this the basic idea behind Star Trek's "subspace" communications?

  • Does it have a speed? (Score:4, Interesting)

    by brejc8 ( 223089 ) on Thursday September 05, 2002 @11:58AM (#4200429) Homepage Journal
    I was on the impression that gravity was instant.
    Imagine two balls spinning around eachother in space. each ball sees the other one a bit in the past. They will get faster and faster as they see eachother infront of themselves. Sortof surfing eachothers gravitational field.

    Or am I just wrong?
    • The idea, I think, is that the time it takes before the two objects move together is equal to the amount of time that a beam of light would between them.

      The "message" that tells an mass that it is being acted on by another mass is relayed, theoretically, by the elusive "graviton", a particle which so far has been unobservable, except during episodes of Star Trek Voyager. The speed of the graviton travelling from one mass to another would always be no greater than the speed of light.
    • Imagine two balls spinning around eachother in space. each ball sees the other one a bit in the past. They will get faster and faster as they see eachother infront of themselves. Sortof surfing eachothers gravitational field.
      I think that effect is so slight that it is nomally unmeasurable. But if two objects are very massive, and orbitting very close to one another, they will slow down measurably due of this effect, and radiate energy in the form of gravity waves.

      This is analagous to electrons in an atom dropping into a lower energy state and radiating photons.

      I've never studied the physics of GR, but I assume it is somewhat analagous to the retarted potentials you study in undergraduate E&M.

    • IANAP, but at the same time A sees B "a bit in front of itself", B, on the other side of the circle is seeing A "a bit in front of itself", so whether or not there is a time delay, it seems gravity is still conserved. This is not to say that we won't see these type of effects for instance when objects pass near each other (but are not in a steady orbit). In fact, that's exactly what this experiment is testing, if I read it correctly. The quasar's halo should be "delayed" as the gravity "travels" toward it (however you want to conceptualize this). Basically like red-shifting, but for gravity.
    • This effect is very small, as you would expect, since light is very fast. In fact it is so small that it gets mixed up with all the other small ways in which general relativity predict deviations from Newtonian gravity, like gravity waves. The net effect is that two spinning boies do VERY SLOWLY radiate their rotational energy away as gravity waves, and will eventually collide. For most reaosonable objects, however this will take longer than the lifetime of the universe. Only very dense objects rotating very fact (like binary neutron stars) actually show a measurable effect.

  • Speed of anti-gravity (Score:5, Funny)

    by GuyMannDude ( 574364 ) on Thursday September 05, 2002 @12:09PM (#4200475) Journal

    I'm not sure about the speed of gravity (the force reponsible for attraction between two bodies) but I've noticed that the speed of anti-gravity (the force responsible for repulsion between two bodies) is roughly the speed of light. Everytime I see some hot babe in a bar and start walking towards her, she turns and runs away in the opposite direction when she sees me coming.

    If independent verifcation of this experimental result is needed, I can get my buddies (who always guffaw when this happens) to pledge that these results are repeatble!

    GMD

    • I think you are observing Arrid's first law, which dictates the speed of body odor in a confined space. It appears that your body odor has a particularly strong force, which causes the repulsion you mention.

  • gravity doesn't exist, per se (Score:4, Interesting)

    by medcalf ( 68293 ) on Thursday September 05, 2002 @12:15PM (#4200511) Homepage
    OK, that's a bit strong of a statement. My physics prof uncle taught me at a very young age that all of science is a best approximation of how things work, and that we sometimes realize that we were way off in cause, even though our theories do a good job of describing effect. A case in point is Newtonian physics vs. Relativity. Newtonian physics works until you get to very large or very small measurements, and then it breaks down completely.

    As I understand it, Quantum theory describes the very small very well, and Relativity describes the very large very well, and each describes the middle (our normal perception) fairly well (particularly relativity). There has been a search to unify these into a single theory, but it keeps breaking down, and my understanding is that it's gravity that generally gets in the way.

    As a result, I've come to the conclusion that we are very wrong about gravity at a fundamental level, though our understanding is certainly good enough to get from place to place in space. The problem is, I don't know what to replace that underlying understanding with. My cosmology isn't complete there.

    It seems that either way that this experiment turns out, it is going to be one of those events which is looked back on as pivotal in our understanding of the world.
    • Clarification:

      Quantum Theory describes phenomina on very small distance scales. And does it very well - in fact, it's often heralded as the most successful theory of all science.

      General Relativity describes interactions between very heavy objects over very large distances.

      Where these two break down is when very heavy objects interact over very small distances - such as inside a black hole, and this is where new theories like superstring theory or M-Theory are attempting to fill the gap. Problem is, we don't understand enough mathematics to even write the complete equations down!

    • Re:gravity doesn't exist, per se (Score:5, Informative)

      by Theaetetus ( 590071 ) <theaetetus,slashdot&gmail,com> on Thursday September 05, 2002 @01:32PM (#4200989) Homepage Journal
      Superstring or Membrane-theory attempt to join Newtonian and Quantum physics together, and so far seems to be doing so, at least to our admittedly weak understanding (we are just now creating the math to solve the equations).

      In Superstring theory, gravity is not a force, but more is due to a fundamental warping of the multi-dimensional space our universe exists in due to concentrations of matter/energy (yes, according to string theory, and as an outgrowth of relativity, concentrations of energy also create gravity). Thus, if the warping is instantaneous, so would the apparent effect of gravity... but if the warp takes time to propagate, then 'gravity' would travel at a slower speed.

      Interesting result of this, though, is that if gravity is instantaneous, we get an easy FTL communication method... But if gravity isn't instantaneous, then there are all sorts of conservation of energy questions (for example: take the sun away... if gravity is not instantaneous, then the Earth continues to orbit empty space for another 8 minutes... where does that energy come from?)

      Check out The Universe In A Nutshell by Hawking. There's also a _really_ in depth book called Supersymmetry, but I forget the author.

      -T

      • It's true that general relativity raises some thorny issues to do with conservation of energy, but your example isn't actually one of them, since according to GTR, it is impossible for the sun to "suddenly disappear". This was one of the most beautiful things about GTR which distinguishes it from other field theories: the equations of motion follow from the equations of the gravitational field, rather than being tacked on later. In electromagnetism, on the other hand, there are equations describing the field, and then the equation qE+qv\cross B = m dv/dt that follows from Newton's laws and describes the motion of the charges. Thus it's possible to have a consistent EM field with stationary charges, raising questions of what holds the charges stationary, whereas it is not possible to have a consistent gravitational field with stationary masses.
      • It comes from the work it took to make the sun disappear.

      • Re:gravity doesn't exist, per se (Score:1, Insightful)

        by Anonymous Coward
        Where from? A flow of gravitons? Who knows.

        I wonder if the speed of magnetic effects can be measure more easily that gravity.
      • Interesting result of this, though, is that if gravity is instantaneous, we get an easy FTL communication method.

        For arbitrarily large values of "easy".

        -
      • Actually it was Einstein's theories that postulated gravity as a distortion of spacetime.

        Speaking of that...if gravity is a warping of spacetime, how is it that a body is ripped apart as it enters the event horizon of a black hole? Any of my physics professors that described black holes always said that a body would be ripped apart by the difference in gravitational forces as approaching a black hole. If gravity is a warping of spacetime, wouldn't mass in the warped space be warped as well, and therefore maintain its structural integrity?

        I must be missing part of the picture here...I do actually have a bachelor's in physics, but this is something that I've wondered about for a long time.
        • Speaking of that...if gravity is a warping of spacetime, how is it that a body is ripped apart as it enters the event horizon of a black hole? Any of my physics professors that described black holes always said that a body would be ripped apart by the difference in gravitational forces as approaching a black hole. If gravity is a warping of spacetime, wouldn't mass in the warped space be warped as well, and therefore maintain its structural integrity?

          It's not so much dependent on the event horizon, but rather the gradient of gravity change (tidal effects)...
          For instance - here on Earth, there's a very slightly greater force pulling your feet towards the ground than is pulling your head, because your head is an additional ~1.5 meters away (miniscule difference compared to the radius of the earth, which is why it's barely measurable).

          However, if you have something as dense as a black hole, with such a tiny radius, then the 1.5 meter difference (or 100 meter for your spaceship) starts to really matter... Though the whole ship is being pulled in, the nose is being pulled in a lot harder than the tail. In fact, eventually, the atoms are getting pulled in with enough of a gradient that they get ripped apart from each other.

          As another example, think of a drop of water, rolling over something spherical, like a ball... It starts to elongate as the tip of it goes onto a more vertical section than the back of it. If it were an even more pronounced curve, then the tip of the drop would break off, leaving the back to slowly roll after it.

          -T

      • And what exactly is that FTL communication method? We haven't quite mastered control of gravity yet. If concentration of matter or energy is required for these instantaneous gravitational changes, then we're required to break some thermodynamic laws to communicate FTL.
        • And what exactly is that FTL communication method? We haven't quite mastered control of gravity yet. If concentration of matter or energy is required for these instantaneous gravitational changes, then we're required to break some thermodynamic laws to communicate FTL.

          Well, the easiest theorized one (blame Robert L. Forward... see the book "Indistinguishable from Magic") is to take a really dense and small mass (such as a quantum black hole... real dense, real small, no Swartzchild radius), spin it up and charge it by shooting electrons at it, and then use big, powerful magnets to move it back and forth very quickly. The oscillations, provided gravity is instantaneous, would be measureable at a long distance (the bigger the mass, and the more you move it, the farther away you could measure it).

          Then, just move it in a sine wave as a carrier, and do some FM modulation on top of that carrier. Easy FTL device, provided gravity is FTL.

          -T

      • Where does the energy come from? That's like asking where the light comes from. It's already on it's way. Its in space en reute to its destination, earth.

  • Press Release (Score:2, Informative)

    by greenhide ( 597777 )
    The University of Missouri [missouri.edu] has also released a press release [missouri.edu] concerning the experiment.

  • A copy of the actual paper (Score:5, Informative)

    by MarkusH ( 198450 ) on Thursday September 05, 2002 @12:22PM (#4200544)
    This [arxiv.org] is Mr. Kopeikin's actual paper in PDF format. You can go to this site [arxiv.org] for other formats.

    Warning: The paper contains some very seriously heavy math. It uses things like the Euler Gamma function, Lorentz factors and stuff like that. You have been warned.
  • It may be instantaneous, or it may not be. It definitely doesn't take 2 weeks though.
  • From the article, the results should be available in two months, not two weeks as supposed by /.

    I have a really great .sig, but I'm not going to give it to you.

    • My bad. (Score:1, Interesting)

      by Anonymous Coward

      Sorry.

      On a related note, I was just reading a page at the VLBA [nrao.edu], and their data collection methods sound rather archaic:

      Astronomical data from the observations are recorded on digital tape at each antenna site. The tapes are then shipped to the Socorro Operations Center where they are correlated and the results sent to the scientists.


  • Gravity is not a "speed" but an acceleration. The difference being m/s versus m/s/s as this article and most other people have failed to realize.
    • The article refers to the SPEED OF PROPAGATION of
      the acceleration.
    • I think you have misinterpreted what they are doing here. They are measuring the speed of propogation of gravity. Sure, things in a gravity well will accelerate toward the center of the well. What is being measured here is how fast the effects of gravity are felt at a distance.

      In other words, if a black hole with 10 million times the gravitational force of the sun instantly appeared in place of the sun would it:

      a) take the 8 minutes (that's how long light takes to get to earth) for us to all get sucked off the face of the earth, or will it

      b) instantly suck us off the face of the planet?

      I kinda like the instant sucking off myself!

      Vincit qui se vincit.
  • All Einstein showed was that nothing could accelerate beyond the speed of light. Nothing prevents something from moving beyond the speed of light provided it always moves faster than light.
    • Einstein didn't "show" that. Einstein developed a mathematical theory that makes certain experimental predictions. Whether those predictions are correct or not needs to be verified experimentally. Just because a few predictions have been verified doesn't mean that the whole theory is true. In fact, we already know that the whole theory cannot be true--at best, it can be an approximation.
      • You're correct, which doesn't invalidate the point I made, does it? That makes the value of your contribution... well, let's just say I wish the moderation system included "whiny anal nitpicky bastard" as an option.

        • Given the complexity of the topic of discussion, I think nit-picking is appropriate.

        • Yes, it does invalidate the point you made. Your point was that "Einstein showed something". But he didn't. One can show (=prove) that pi must be irrational, but one can't show that objects can't accelerate faster than light (even if it seems plausible and likely). The distinction is particularly important for physicists to keep in mind; too bad that it is lost on many of them.
          • Yes, it does invalidate the point you made. Your point was that "Einstein showed something". But he didn't. One can show (=prove) that pi must be irrational, but one can't show that objects can't accelerate faster than light (even if it seems plausible and likely). The distinction is particularly important for physicists to keep in mind; too bad that it is lost on many of them.

            I see I have to get nit-picky too.

            My point definitely was not that "Einstein showed something".

            The point I made was based on accepted theory which SUGGESTS (happy?) something may travel at speeds in excess of the speed of light, and that only acceleration to and/or beyond the speed of light is impossible because the energy required would be infinite.

            The original article contains an assumption on the author's part that the theory requires that the speed of light is an inviolable upper boundary. My point was simply that this is an inaccurate interpretation.

            Therefore, my point is not invalid. The worst you can say is that my choice of a single word was unfortunate. As another responder noted, perhaps this subject requires nit-picking to a degree which makes such distinctions important, but somehow I suspect my /. response isn't going to influence the researchers involved.

            So, as I noted previously, I concede that Einstein didn't SHOW anything in the sense that SHOW can be interpreted as providing incontrovertible evidence (and I suppose I could argue that point, too, if I didn't have a life to get back to). Nonetheless, accepted theory does allow for travel beyond the speed of light, and therefore the original article text's assumption that the speed of light was an absolute upper limit is based on an incorrect interpretation of the applicable theory.

            Hopefully you are happier with this over-anal-yzed wording.

    • I'm no physicist, but how do you move from the speed of light without accelleration?
      • how do you move from the speed of light without accelleration?

        Magic. Or rather, "it's just created that way". Consider an electron changing energy levels and emitting a photon. That photon does not accelerate from whatever speed the electron has up to the speed of light, it's created going at the speed of light. A hypothetical tachyon is created going faster than light. Such a tachyon also has an imaginary rest mass, but it's never at rest.
    • I've actually wondered about this. The simple version is if you look at e=mc^2, then all you need for something to move faster than the speed of light is a particle with less mass than a photon. This would then become the "nothing can travel faster than the speed of light" phrase with the assumption that nothing is lighter than a photon.
    • Einstein's theory says that nothing with mass can be accelerated beyond the speed of light, true, and this doesn't stop the idea of items that always travel faster than the speed of light.

      But AFAIK it also says (or this may be deduced from above) that "information cannot be transmitted faster than the speed of light", and if gravity acts simultaneously then I could transmit information by moving an object and letting you detect the change in the gravitational field. OK, me moving a rock might be hard to distinguish from a couple of light years away (anyone care to work out what the hamming distance would be, how many correction bits you'd need) but it is something I've always wondered about.

  • USENET sci.physic FAQ (Score:5, Informative)

    by g4dget ( 579145 ) on Thursday September 05, 2002 @02:34PM (#4201507)
    The USENET sci.physics FAQ has a pretty readable explanation [ucr.edu] of some of the speculation surrounding the speed of gravity.
  • If the experiment is successful it will provide a new independent test of general relativity in the solar system.

    If the experiment showed infinite propagation velocity, it would invalidate GR. But it is a common fallacy among physicists to claim that conducting an experiment that can invalidate a theory "tests" that theory. The problem with that view is that there are many other possible theories of gravity that differ substantially from GR but still have finite speeds of gravitational interactions. In fact, merely imposing finite speed on Newtonian gravity (and doing some fixing up to make the result consistent) gives you an interesting theory that is quite similar to the experimental predictions of GR in many ways.

    • You can indeed say that conducting an experiment that can invalidate a theory tests that theory. If the theory cannot explain the experimental results, then if the experiment was done carefully the theory failed the test and needs to either be modified or thrown out. That is a pretty good test. If the theory is in agreement with the experiment, then it passed the test, but that is about all you can say about it. That is pretty much the basis of science (physical, at least), not just physics.

      Maybe it is a semantic problem, but I guess I don't understand your point. If the experiment says that GR needs to be changed, what does that have to do with the other theories of gravity you mentioned? If the experiment showed infinite propagation velocity (though I don't know how you show that given experimental uncertainties), then I think the finite-velocity theories you allude to also take a hit, and I would say those theories didn't "pass the test." If the experiment shows a finite velocity, then those other theories pass the test and they get to stay on the island for another week.

    • But it is a common fallacy among physicists to claim that conducting an experiment that can invalidate a theory "tests" that theory.

      No, tests can have false positives or false negatives and still be "tests". Do you think the Miller-Rabin primality test is not a test because it is possible for composites to pass it? Where did you get this use of the word "test"?

      In fact, merely imposing finite speed on Newtonian gravity (and doing some fixing up to make the result consistent) gives you an interesting theory that is quite similar to the experimental predictions of GR in many ways.

      And if this theory makes predictions that differ from GR, one could use them to test GR. What's your point? No one's claiming that a theory cannot be wrong as long as it passes at least one test.

  • Results timeframe off (Score:3, Informative)

    by VisualStim ( 130062 ) on Thursday September 05, 2002 @03:14PM (#4201819) Homepage
    "... and results should be known within about two weeks."

    The articles says two months.
    • "... and results should be known within about two weeks."

      The articles says two months.

      It's a relativistic effect. Data moves really fast over the internet, so there is a time dilation.

      Obviously.

  • from what i've learned in highschool, college, and the tube... gravity is a constant... when you let go of an object it instantly falls because gravity was already acting on it before you dropped it... same goes for any other locale in the universe...
  • Perhaps I've missed something, but didn't Bell's theorem, with the help of Clauser and Freedman's experimental work, demonstrate that the entire concept of "locality" fails?

    In which case, the idea of a cosmic speed limit fails as well, since we measure velocities in terms of displacement per unit of time. Without the idea of locality, the first of those units ceases to exist, and the second comes under some serious suspicion...
  • Strange, I don't recall thier even being any sort of debate on the subject. Gravity travels at the speed of light.

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