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Why can't we see the Gravity?

  1. Feb 5, 2007 #1
    We can see the Effect of the force of gravity. But how come we can't see the force itself?
  2. jcsd
  3. Feb 5, 2007 #2
    I think we are "looking" real hard, at least for a particle or wave.

    But on another level, why suppose that our eyeballs are built to see much at all? Extraordinary instruments with a huge dynamic range, ultimate sensivitity of close to a single photon, that they are, but in the end very narrowband EM detectors. Gravity is not EM.
  4. Feb 5, 2007 #3


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    Your question is rather puzzling.

    You can see the effect of wind, but you can't see wind itself.

    You can see the effect of electrostatic force, but you can't see the force itself

    You can see the effect of x-ray, but you can't see x-ray itself

    You can see the effect of heat, but you can't see heat itself


    So, why did you pick on "gravity" only?

    Gravity (and forces in general) isn't an object. It has no physical substance for visible light to be reflected on and then hit your eyes, where by a series of nerves and electrical signal transfer that to your brain and you "see" it. However, this kind of "seeing" is very limited in range. Your eyes can only see a very limited range of EM spectrum. It is not a very good indicator of what are around us in our world.

  5. Feb 5, 2007 #4
    You make a good point ZapperZ.
  6. Feb 5, 2007 #5

    D H

    Staff: Mentor

    First off, what do you mean by "see"?

    No device can be built that directly detects the force due to gravity because any gravitational field is locally equivalent (i.e., indistinguishable by measurement) to a uniform gravity field. Consider, for example, an accelerometer.

    An accelerometer is a physical device comprising a case that loosely holds some object (e.g., a mass attached to a spring). The loosely held object will accelerate with respect to the case if the case is subjected to some external force that does not affect the subject object. Equipment inside the accelerometer measures the motion of the sensed object with respect to the case.

    A uniform gravity field affects the case and the sensed object equally. There is no way to measure the acceleration due to gravity in a uniform gravity field. The equivalence principle says that on a small enough scale, any real-world gravity field is indistinguishable from a uniform gravity field.

    An accelerometer can measure the departure of the gravity field from uniformity. However, such departure is immeasurably small for typical accelerometers. A properly constructed accelerometer (one in the loosely held object is shielded from all external forces but gravity) can measure any force other than gravity. But it cannot measure gravity.

    A gravitometer, for example, is a kind of accelerometer. Despite its name, a gravitometer does not measure gravity. Objects fixed on the surface of the Earth experience an upward force exerted through contact with the Earth's surface that counteracts the gravitational force. Gravitometers measure this contact force, not the gravitational force.
  7. Feb 5, 2007 #6


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    Is it wrong to still think that gravity is bending of spacetime, because of mass? Or is the most common way (and most expected way) to think of gravity as gravitons, or a certain particle/wave that makes gravity?

    I find the bending of spacetime so nice to think of that I want it to be that way.
  8. Feb 5, 2007 #7

    D H

    Staff: Mentor

    "Bent" with respect to what? And what causes this "bending"? Gravity is tautological in general relativity. Physicists like to look for root causes. Tautologies are not root causes.
  9. Feb 5, 2007 #8


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    Now you should know better than to start something like that in this thread. The OP appears to ask a rather elementary question about "force" in general. We should not complicate things further by bringing in General Relativity.

    Furthermore, there have been tons of thread and posts in the GR forum as it is already that have already answered this question. So let's not hijack this thread into something more convoluted.

  10. Feb 5, 2007 #9


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    Doesn't an Etovos device "see" gravity?
    Much like a charged particle sees another charged particle.

    It doesn't seem to qualify as an accelerometer.
    No springs.
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