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Can there be a place in space where no gravity exists?

  1. Jun 10, 2012 #1
    Today I was thinking about the possibility that there could exist a place (or point) in spacetime where [STRIKE]no effects of gravity are felt[/STRIKE] gravitational fields cannot be detected (edited for clarification). Most of the thoughts I've had say this is impossible.

    Here are my ideas:

    No, it's impossible:
    1. Since gravity is an effect on spacetime and its effects are extremely far-reaching, perhaps the spacetime is itself simply a byproduct of gravity, in which case without any gravity there is no spacetime.
    2. Perhaps dark matter is imparting the gravitational force that makes up the underlying blanket of spacetime, and this could have existed before the big bang (in the case that it's real matter in another universe), meaning there could be gravity far past the expansion point of spacetime. Similarly, if the universe in our dimension is infinite, it probably loops back on itself, in which case there is no "end", or place in spacetime where it's unlikely that any particle has reached or will reach with any frequency.
    3. If no gravity exists in the place, than how can you even quantify space, or time, so perhaps this question is irrational.

    Yes, it's possible:
    1. If gravity has a force-carrier particle (graviton), then perhaps this question is the same as "is there a place in spacetime where no light (well, photons/EM radiation really) is/are detectable from mass-bearing objects?". With this reasoning, I would think that if you travel far enough, you could get to a place where it's improbable that any graviton would hit you in a reasonable amount of time (say, one day).

    Btw, the thing that got me thinking about this is the discovery that the our galaxy is likely to collide with Andromeda, meaning the gravity is strong enough in "empty space" to pull two galaxies together across over 2 million light years in 4 billion years, which I'd say is pretty incredible!
     
    Last edited: Jun 10, 2012
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  3. Jun 10, 2012 #2

    marcus

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    According to the prevailing ideas about it, gravity=geometry.

    Your question amounts to asking about a "place in space where no geometry exists?"

    A freely falling observer in a uniform grav. field does not FEEL any "effect" of gravity, if I understand what you asked about "feeling effects".
    Locally it is just the same as being in a region where the geometry is flat---has zero curvature.

    There is always geometry, and at places it can certainly have zero curvature. And since geometry is everywhere, gravity (which is nothing but geometry) is everywhere. The gravitational field IS "place". It is the geometry upon which the other fields are defined.

    My two cents anyway.
     
  4. Jun 10, 2012 #3
    Thanks for the reply! The essence of my question was whether you could escape all traces of gravity, such that every mass-bearing particle was so far away, that no traces of gravity exist (on average).

    Very interesting reply - this is sort of the same train of thought I had; so under the assumption that gravity=geometry (of spacetime), considering a place where there is no gravity is irrational, since "place" by definition is a location in space and time (sort of my point in cons #1 and #3).
     
  5. Jun 10, 2012 #4

    marcus

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    At the quantum level, think about the analogy with photons of the EM field. One cannot say of a place that "no photon is there".

    One can only say that no photon of some range of wavelength is DETECTED.

    Whether or not a detection event occurs is probabilistic and depends on things like the size of the antenna or (more generally) the sensitivity of the detector at various wavelengths. And the energy a photon carries is inversely proportional to wavelength so longer-wave quanta are feebler and harder to detect.

    Presumably some gravitons have wavelengths the size of the solar system and some have wavelengths the size of the Milkyway galaxy. Ripples caused by orbiting bodies. To have any chance to detect them one would need detectors on that order of size. And incredible sensitivity.

    But whether or not one detected them one could not say that they were not there, since a detection event is indeterminate. There can be a ripple in the field but your detector may or may not feel it.

    Probably simpler to stay at classical level on this one. Going to quantum level and talking about gravitons (the quanta of the geometry field) raises a bunch of new issues.
     
    Last edited: Jun 10, 2012
  6. Jun 10, 2012 #5
    Indeed, that makes sense. So no matter how far you go out, away from matter, the chances that you will be "hit" by a photon or graviton are probabilistic, not deterministic (granted, at the quantum level, they are non-deterministic anyway), so you can never say that a graviton will no be detected there, you can only say that the probability is extremely low; additionally, since gravity (presumably via gravitons) travels at the speed of light without losing energy, even if only one graviton was transmitted, it could still hit that spot, and you can't escape it without exceeding (or circumventing) the speed of light.
     
  7. Jun 10, 2012 #6

    marcus

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    I think I agree with the spirit of what you are saying. To say that "geometry is everywhere" is essentially a tautology (maybe it is whereness :biggrin:).

    You might enjoy reading some of the QG research literature. The abbreviation can stand for either or both quantum gravity and quantum geometry. And a lot of the research activity is in applying QG to cosmology, which means getting rid of the singularity and modeling various possibilities of what may have actually happened around the start of expansion.

    The graviton, as a mathematical concept, does not come into QG at a fundamental (non-perturbative) level. It only comes into the picture when you fix a background geometry and study perturbations of it (eg ripples imposed on a flat background). That is a useful approximate analysis. But the most basic QG theory is non-perturbative and does not represent geometry by a bunch of gravitons. Rather, by a network of geometrical measurements, bits of volume, area, angle, etc. These are what is uncertain and it is their evolution that is governed by the quantum model. It is an interesting research area, but off-hand I can't think of a good paper to recommend as introduction.
     
    Last edited: Jun 10, 2012
  8. Jun 10, 2012 #7
    lol - indeed, this solves my problem! Whereness :biggrin:!

    Thanks for the suggested reading. I'm an entrepreneur/developer by day, but I may find some time for this fun stuff at night :)
     
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