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Gravity speed vs light speed experiment

  1. Are gravimeters sensitive enough?

    1 vote(s)
  2. Can a gravity peak be determined?

    0 vote(s)
  3. Can extrapolation methods be accurate?

    0 vote(s)
  4. Has this experiment been done before?

    0 vote(s)
  1. May 8, 2010 #1
    I've read the Fomalont and Kopeikin's experiment, and though very interesting and supposedly proves the speed of gravity, I feel there may be a very "simple" experiment that will make a comparison of the speeds, with an event that occurs every day, and may be tried by many groups with the measurement capabilities that are currently available on the ground.

    This is the experiment I'm proposing:

    Use gravity sensors of the highest sensitivity to determine the "peak" of solar gravitational pull in the vertical direction, and compare this peak time with the peak of solar radiance that occurs at noon.

    If the peaks are coincidental, then the speeds are the same. If there is a lag of eight minutes of the peak of solar radiance, then gravity is not light speed limited.

    We know the sun's actual position is eight minutes shifted from the observed position, but I have not seen any experiments that try to demonstrate this.
  2. jcsd
  3. May 8, 2010 #2


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    What do you think is the reason we see the sun at its brightest at midday? Do you think the sun itself gets brighter at 1152 and then we see the increase at 1200? :smile:

    Once you've answered that, you should see there's a flaw in your proposal.
  4. May 8, 2010 #3


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    FAQ: How fast do changes in the gravitational field propagate?

    General relativity predicts that disturbances in the gravitational field propagate as gravitational waves, and that low-amplitude gravitational waves travel at the speed of light. Gravitational waves have never been detected directly, but the loss of energy from the Hulse-Taylor binary pulsar has been checked to high precision against GR's predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity's description of gravitational waves.

    It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., Brans-Dicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances propagate at any other speed than c. Without a test theory that predicts a different speed, it becomes essentially impossible to interpret observations so as to extract the speed. In 2003, Fomalont published the results of an exquisitely sensitive test of general relativity using radar astronomy, and these results were consistent with general relativity. Fomalont's co-author, the theorist Kopeikin, interpreted the results as verifying general relativity's prediction of propagation of gravitational disturbances at c. Samuel and Will published refutations showing that Kopeikin's interpretation was mistaken, and that what the experiment really verified was the speed of light, not the speed of gravity.

    A kook paper by Van Flandern claiming propagation of gravitational effects at >c has been debunked by Carlip. Van Flandern's analysis also applies to propagation of electromagnetic disturbances, leading to the result that light propagates at >c --- a conclusion that Van Flandern apparently sincerely believes.

    Fomalont and Kopeikin - http://arxiv.org/abs/astro-ph/0302294

    Samuel - http://arxiv.org/abs/astro-ph/0304006

    Will - http://arxiv.org/abs/astro-ph/0301145

    Van Flandern - http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

    Carlip - http://xxx.lanl.gov/abs/gr-qc/9909087v2
    Last edited by a moderator: May 4, 2017
  5. May 8, 2010 #4
    DrGreg, I'm not referring to how bright the sun is, but the exact time when it is directly overhead. If it is sensed with a slit that passes a line across a sensor, then the area at the sensor gets brighter and brighter until the suns direct rays strike the sensor. If it is a spot then the same thing will happen.
    An single aperature would act as a lens, which is not what is best, but a series of aperatures with black side cavities would allow most of the light to be trapped except what is direct to the sensor.
    And the sensor can be biased to ignore light levels below the threshold of trigger, so that only the direct solar rays would be sufficient to trigger the sensor.
  6. May 8, 2010 #5
    Hi bcrowell,

    I'm not interested in looking for any waves. That's a different game. But if you want to think of it this way, a big gravity wave goes overhead every day. It has a peak to peak period of 24 hours. Ocean tides are the evidence of this intense gravity wave.

    What I'm trying to get is the steady state pull "peak" (time of solar gravity pull that is directly overhead) that can be compared time wise to the time when the observed sun is directly overhead. We know the suns actual position is eight minutes ahead of the suns observed position.

    If this steady pull peak can be determined, then it can be compared to the exact time of the observable light peak. If these peaks are coincidental, then gravity is at light speed, but if the light peak lags, then the gravity effect is faster than light speed.

    Let's get some observations going, and then when the results are in, the ones who like theories can make up whatever they like to explain the data. Just please don't massage (bias) the data to make it fit the pet theories.
  7. May 8, 2010 #6


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    DrGred had something else in mind. Is there day and night because the sun is revolving around the earth, or is earth rotating? What does this mean for your experiment?

    Further, it's a bit more difficult than you think.
    For example, if the sun were electrically charged, and we could measure the field of this charge here on earth, we would measure its direction to be not where we see the sun, but (almost exactly) where it ought to be now. For gravity, this "coincidence" is even more exact.
    It's hard to explain in a few words, but those "forces" point to the momentary position rather than the retarded position of the source. Has to do with the absence of the respective modes in the radiation (no monopole radiation in EM, no monopole or quadrupole radiation in Gravity).
    Read http://arxiv.org/abs/gr-qc/9909087" [Broken].
    Last edited by a moderator: May 4, 2017
  8. May 8, 2010 #7
    bcrowell, and DrGreg,

    Solar gravity is the most powerful and immediately available force that can be studied by anyone with reasonably sensitive instruments. Even amateur researchers like myself, can make experiments that may be repeated and verified by more advanced and learned researchers.
    With a forum such as this one, the value of an experiment may be determined by brilliant minds from all over this planet, and with enough amateur and professional experimental input from all over the world, the results may be reinforced and verified by experts with the result that possibly new knowledge may be obtained on the physics of the world that surrounds us. New knowledge is what science is all about.
  9. May 8, 2010 #8
    Hi Ich,

    By this experiment, I'm not trying to show what gravity is or isn't, but just trying to record the solar gravity effect upon the earth in a time frame that can be compared to the light arriving from the sun.
  10. May 8, 2010 #9


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    The "speed of gravity" can only apply to changes in gravity. The Sun's gravity is pretty constant, on a day-to-day scale, and the only significant changes we detect on Earth are due to the rotation of the Earth, not due to changes propagating from the Sun. Similarly the apparent brightness of the Sun is due to the Earth's rotation, not due to some effect propagating from the Sun.

    So the two variations you propose to measure are both originating locally on the Earth, not eight light-minutes away. To put in another way, if you were in a spaceship hovering above the Earth, the 24-hour variation you refer to wouldn't occur at all.

    An analogy: Suppose you are outside, and it so happens that there is wind blowing towards you from the same direction that you see the Sun. You spin round on a vertical axis and notice that the amount of sunshine in your face reaches a peak at the same time that the wind blows strongest into your face. Does that mean the speed of the wind equals the speed of light?
  11. May 8, 2010 #10
    Hi DrGreg,
    Yes it is true, the earth rotates instead of the sun going around the earth, but for experimental purposes the sun still goes to its zenith at a certain TIME of the day. Its brightness is of no concern other than to trigger the sensor at the precise TIME it is at its zenith.

    The solar gravity pull also rotates around the earth, with the peak of the distortion always being on the sun side. The ocean tides are a visual of this pull.

    And no your analogy doesn't hold. These effect are not local, they both are influences on the earth from the very distant sun, so that even a spacecraft in orbit will sense the distortion caused by the solar gravity pull when it is on the sun side. Close to the earth it is in milligravs, but it is still measurable.

    Since gravity cannot be stopped, chopped, throttled, or in any way modulated by man to determine its speed of propagation, with the possible exception of some recent magnetic antigravity effects, we have no way that I know of to measure its effects other than by its steady pull in relation to its instantaneous position in space from a known point on the earth at a specific time.

    The earths distance from the sun can be a useful dimension to determine the time frames of light speed, in order to make these comparisons.

    Regards, Bob.
  12. May 8, 2010 #11


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  13. May 9, 2010 #12


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    First of all, even the most sensitive detectors are still well below the sensitivity required to detect gravity waves, due largely to ever-present mechanical vibrations.

    Second, even if they were perfectly sensitive the experiment would not accomplish your goal. As atyy and Dr. Greg (and perhaps others) already mentioned the gravitational field of the sun is not changing. So your experiment would only measure our motion through a static gravitational field, not the propagation speed of gravity waves. It turns out that gravitational waves require a quadrupole source, so we need to look at least at binary pairs of very massive objects in very tight orbits.
  14. May 9, 2010 #13

    Once again I am NOT trying to detect or explain gravity "WAVES". And yes, todays absolute gravimeters have the sensitivities necessary for this experiment to proceed.

    By the way, these gravity waves are still only a theory, yet so many people have described their properties, how they behave, and arguing whether they're being monopolar or quadrupolar, and on and on.

    Hey, how about lets discover their existence first and then carefully observe and document their characteristics.

    The same thing for the BIG Bang theory.
    Nothing to date has proved the theory is viable that hasn't been shown by other theories that have verifiable evidence based on positional observation data.

    Imagine if you will, being on a merry-go-round, that is located on the periphery of a much larger merry-go-round, which is located on a still much larger merry-go-round. Now try from your point of observation, to determine the speeds of motion, direction, and positional relationships of an object that is very far removed from all of these merry-go-rounds.
    It is possible, but the possibility of error from so many unknowns, prohibits a conclusive determination of characteristics of the observed element.

    Has it been determined yet where the center of rotation of our solar system is, or if it is revolving around some other much larger system?

    If the current gravity data is examined carefully it can readily be seen that there is not a steady gravitational field, but a continuous moment by moment changing field. These changes haven't been adequately explained yet, but they are there to be examined.

    Some amateur researchers have proposed that this changing field is the instantaneous response of overhead gravity influences from distant celestial bodies and systems. Maybe, but nevertheless they are readable, and open for interpretation.
    Last edited: May 9, 2010
  15. May 9, 2010 #14


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    Then the title of your thread and your entire OP make no sense whatsoever. It is only gavity "WAVES" that can be associated with a speed. When you are dealing with a static spacetime there is no sense in which gravity has a speed.

    This is similar to EM. EM "WAVES" propagate at c, electrostatic fields do not have a speed.
  16. May 9, 2010 #15
    On a day-to -day scale the sun's gravity is pretty constant.What about from year-to-year or rather six months-to-six months.
    Eliptical orbit of Earth, closest to the sun in winter, furthest away in summer, gravity stronger in winter than summer, do experiment.
  17. May 9, 2010 #16
    I think you're misunderstanding their responses.

    For simplicity, imagine if the earth were not orbiting the sun and rotating on its axis. Instead, let the earth be fixed and let the sun fly past the earth (just once, with constant velocity. Let's say the closest point of its trajectory is 8 light-minutes away, and it is at 0.5% of lightspeed so that at peak it still advances through our sky a quarter degree per minute).

    They're saying, you will indeed measure the 8 minute lag! But it's even worse than you expected: say you also try to measure the slight electric field of the sun. Then you'll measure the same lag again: the direction of the mean electric field will be 2 arc-minutes of a degree ahead of the direction of mean solar EM wave irradiation. The mathematics that supports this is worth reading.

    Do we know it? Can you cite some reference to show that fact isn't just false?

    The above example needed 0.5% of light-speed to imitate (by linear motion) its apparent motion (as we pretty much "rotate on the spot") (!). Actually, the relative motion due to Earth's orbit is only 0.01% (of light-speed) plus 0.0001% for the surface's rotational speed. That should correspond to only a couple arc-seconds (at 8 light-minutes), a lag of a tenth of a second from the observed position..

    But as before, it doesn't prove anything new, even if you can detect the visual position lagging the gravitational tidal force by a tenth of a second. You'd need to be able to detect whether or not gravitational fluctuations that are caused by non-terrestrial disturbances like solar flares arrive eight minutes early. To build such a sensitive detector is basically what the laser interferometer people are trying.
    Last edited: May 9, 2010
  18. May 11, 2010 #17

    For an answer to your question;
    Originally Posted by 1bobwhite
    "We know the sun's actual position is eight minutes shifted from the observed position"
    "Do we know it? Can you cite some reference to show that fact isn't just false?"

    I an using information from www.metaresearch.org[/URL] :

    [How then does the direction of Earth’s acceleration compare with the direction of the visible Sun? By direct calculation from geometric ephemerides fitted to such observations, such as those published by the U.S. Naval Observatory or the Development Ephemerides of the Jet Propulsion Laboratory, the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes. In other words, the acceleration now is toward the true, instantaneous direction of the Sun now, and is not parallel to the direction of the arriving solar photons now. This is additional evidence that forces from electromagnetic radiation pressure and from gravity do not have the same propagation speed.]

    This explanation also helps to stimulate the search for that gravity peak for the time check, to verify the propagation speeds experimentally.
    Last edited by a moderator: Apr 25, 2017
  19. May 12, 2010 #18
    Last edited by a moderator: Apr 25, 2017
  20. May 12, 2010 #19


    Staff: Mentor

    You are very confused. First, you speak of the speed of gravity, then you say that you are not speaking of waves, now you say that you are interested in propagation speed. A static spacetime does not propagate, only waves propagate.
  21. May 12, 2010 #20
    Spacetimes in GR, including non-stationary spacetimes, which by definition include gravitational waves, never change as they include time.
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