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Is there a smallest unit of gravity?

  1. Oct 6, 2011 #1
    Is there a singular, smallest unit of gravity between 2 objects? For example, is it possible that a speck of dust on Earth, and a speck of dust on the opposite side of the universe, in the farthest away galaxy will have no gravitational attraction to each other? Or does the gravity will always exist between 2 objects? I know there is Plank scale for size, temperature and I also think time (?).
    I apologize if this question may seem silly to you, but I am only 14 years old and know almost nothing about physics. :(
  2. jcsd
  3. Oct 6, 2011 #2
    You know enough to ask a great question; one that should be testable, someday, somehow...

    I think that the geometric answer from Relativity at this point would be "yes", the attraction is there in all cases because the curvature never gets perfectly flat for any finite separation distance.

    The QM answer might be, "Wondering about that same thing..."
  4. Oct 11, 2011 #3
    When you look into Brownian Motion you might think of intermitent gravitational attractions as being part of the cause of this motion. I tend to think it could be possible that for very small particles gravity becomes patchy and hence objects experience large unexpected motions.
    Under the conditions of an Expanding Universe it seems less likely that Gravitational Fields extend to infinity.
  5. Oct 11, 2011 #4
    The Wikipedia article "Mass–energy equivalence" is very useful http://en.wikipedia.org/wiki/Mass%E2...gy_equivalence [Broken] and I can understand this very well. It only seems to breakdown when describing Gravitational Potential Energy. They have not worked out that it contributes to the Rest Mass of an object.
    Quoting from the article:
    "Note further that in accordance with Einstein's Strong Equivalence Principle (SEP), all forms of mass and energy produce a gravitational field in the same way.[15] So all radiated and transmitted energy retains its mass. Not only does the matter comprising Earth create gravity, but the gravitational field itself has mass, and that mass contributes to the field too. This effect is accounted for in ultra-precise laser ranging to the Moon as the Earth orbits the Sun when testing Einstein's general theory of relativity.[15]
    According to E=mc2, no closed system (any system treated and observed as a whole) ever loses mass, even when rest mass is converted to energy. All types of energy contribute to mass, including potential energies. In relativity, interaction potentials are always due to local fields, not to direct non-local interactions, because signals cannot travel faster than light. The field energy is stored in field gradients or, in some cases (for massive fields), where the field has a nonzero value. The mass associated with the potential energy is the mass–energy of the field energy. The mass associated with field energy can be detected, in principle, by gravitational experiments, by checking how the field attracts other objects gravitationally.[16]
    The energy in the gravitational field itself has some differences from other energies. There are several consistent ways to define the location of the energy in a gravitational field, all of which agree on the total energy when space is mostly flat and empty. But because the gravitational field can be made to vanish locally at any point by choosing a free-falling frame, the precise location of the energy becomes dependent on the observer's frame of reference, and thus has no exact location, even though it exists somewhere for any given observer. In the limit for low field strengths, this gravitational field energy is the familiar Newtonian gravitational potential energy."

    When they start talking like this "the precise location of the energy becomes dependent on the observer's frame of reference, and thus has no exact location, even though it exists somewhere for any given observer" I would suggest you can tell they have got it somewhat wrong!

    I will take a bet it will be shown one day that GPE is stored in the mass of the objects, even though I do understand there is a Gravitational Field around these masses which means that some of the Mass-energy is tied up in the production of this field. The fact that it is energy related would tend ultimately to allow for proposition of a quantum of Gravitational Field Energy, which then suggests it would only have a finite effect and not extend out infinitely into space.
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  6. Oct 11, 2011 #5
    i don't think you'll find any magnetic properties in the center; it's the quarks,photons,pions that support the mass surrounding.moment of inersia would not support your theory
  7. Oct 12, 2011 #6


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    Why ask this just about gravity? Both current quantum theories of electroweak interaction, and classical EM theory, say there is no distance limit on interaction between charges; nor is there a 'lowest energy photon'. So, quantization does not imply a smallest unit of energy or interaction. So why should gravity be any different, quantized or not? There is no reason to expect a lowest energy graviton.

    If you want to ask about all field theories: is there scale at which you can't take them to be exact? Most likely, we'll never know for sure. However good the technology, there is some sufficiently small energy scale that is undetectable. Similarly, there will always be unreachable high energy scales.

    A different question is whether there is any value to supposing such a low energy cut off in validity? Since it can't be verified, it would only be useful if it led to some simplification in a theory or its application.
  8. Oct 12, 2011 #7
    It was a while ago that I did my calculations, but the theory behind the calculation was based on the question, if the gravity field is sourced from either a proton or a neutron how far could it extend its own gravitational field into space before its own gravitational attraction brought the field back to itself. OK I know we are not meant to discuss unproven ideas on here, but I do believe it needs someone to double check it.
    At the time I calculated that a "graviton" could extend a field out to 96 light years into space, but not to infinity. Since then scientists have discovered that Galaxies are speeding away from the gravitational pull of the rest of the Universe. I can see a situation where matter (in a frame, so it still has gravity within itself only) if more than 96 light years from some other substantial gravitational mass, it becomes free to expand.
  9. Oct 12, 2011 #8


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    Sergent, Welcome to Physics Forums! Do not be confused by all the technical stuff above! I suggest you memorize this, and put your faith and trust in it. You will NOT ever be wrong. There is lots more to learn about Gravity, but start here.

    Newton's law of universal gravitation:
    "Every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them."

    As for a dust particle's mass here attracting a dust particle many lightyears away, the above law applies (it is Universal) but when you do the math it is so small as to be considered "negligible". Now, even though the force of attraction is "negligible" it always exists. I do not know about any lower limit.
    Last edited: Oct 12, 2011
  10. Oct 12, 2011 #9
    But don't you think that was a bit far fetched when his experiments were done on objects that were rather close together. Would if he was alive today be game enough to say the same thing with the same conviction. I can't see how anything is able to have that sort of effect on every point mass in the entire Universe. It is unimaginable, and borders on being a far fetched unproven theory.
    Last edited: Oct 12, 2011
  11. Oct 12, 2011 #10
    Do you really think a single electron has sufficient mass-energy to fill the whole 3 dimensional Universe with an EM field, even an ever so weak field?
    Any calculation like that would take all the mass of the electron and more to make a field like this, to the point that the electron would not even exist as its mass-energy would be entirely dissipated in the field! And if one electron can't do it on its own why should two of them work cooperatively?
  12. Oct 12, 2011 #11


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    What I think is irrelevant (my thoughts are not that powerful). Current, best supported, physical laws say so. As I said ealrier, we can never verify whether or not there is an infinitesimal energy cutoff. So I claim the only useful question to ask is whether physical law is made simpler by assuming such a cut off. If it is only made more complex, why bother assuming something unverifiable for the sole purpose of making laws more complex?
  13. Oct 12, 2011 #12
    I see what you mean. So you would be willing to drop the idea of this infinitesimally weak field extending throughout the Universe, as long as there was a benefit.

    Well I do see an immediate benefit in that each galaxy becomes an independent system no longer in any gravitational jeopardy from the billions of other ones in the Universe.
  14. Oct 12, 2011 #13


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    Ah, but that is demonstrably false; unlike the undecidable question of whether a free electron in the milkyway can have an infinitesimal interaction with the field emanating from a free proton in andromeda.

    Further, GR with a low energy cutoff has never even been formulated as a coherent theory. So a theory with no established errors should be abandoned for a theory that doesn't exist?
  15. Oct 12, 2011 #14
    Could you detail your "demonstrably false" comment please? I know there are colliding galaxies, but you can't show they were gravitationally drawn together and not just caught up with each other through kinetic motion. Ok once within gravitational range the story is chaos!
    GR has been said to not fit that comfortably with quantum physics. (I don't know enough to form an argument on it, but that was what I had read.)
  16. Oct 12, 2011 #15


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    1) Ever heard of galaxy clusters and superclusters? They are gravitationally bound.

    2) The total mass of matter determines the overall geometry of spacetime and its long term fate. Thus all galaxies in aggregate interact.
    Completely irrelevant, as quantum theories have no low energy cutoff principle. As I mentioned previously, there is no such thing as a lowest energy photon, nor do quantum gravity theories posit a lowest energy graviton. They may have high energy cutoffs (e.g. Planck scale), but no low energy cutoffs.

    More fundamentally, read what I wrote: there is no established observation inconsistent with GR at present. This is a simple fact. That theorists (and myself as a theory follower) believe that GR will 'soon' be replaced does not change this. If a simplification results from assumption of low energy cutoff, great; if theory becomes more complex, nope.
  17. Oct 12, 2011 #16
    I looked up those terms on Wikipedia: "Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation.[1] They form the densest part of the large scale structure of the universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters."

    So even they say superclusters are non-gravitationally bound OK. And even in the clusters the individual galaxies were moving away from each other rather than together. "Clusters are larger than groups, although there is no sharp dividing line between the two. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally bound by their mutual attractions,...".

    It appears that there is such a lot of gas between these galaxies they struggle to get free from one another. This might still allow for a noninfinite gravitational field as there is so much matter inbetween they never get to be more than the limit (96 parsecs, which remember is only a tentative figure, (one that represents something far less than infinity or the measurements of the Universe)).

    Regards the rest of your argument I'd need time to build a case.
    Last edited: Oct 12, 2011
  18. Oct 12, 2011 #17
    "there is no such thing as a lowest energy photon" ? is this so? What is the lowest frequency of photon ever recorded? In my way of thinking a photon with a wavelength of extreme length can this be called a photon at all?
  19. Oct 12, 2011 #18
    A single electron having a "field" extending throughout the universe is certainly hard to imagine. Thankfully, that's not how electric fields actually work; electric fields are composed of "virtual photons" (VP's). As you get further away from an electron, the less PROBABILITY you have of interacting with one of it's VP's. Read "The Strange Theory of Light and Matter" (Feynman) for a better idea of the inner workings of electric fields. As for gravity, we're still not quite sure whether it's a quantum function or continuous. The consensus seems to favor the quantum view, ie, gravitons (the VP's of gravity).
  20. Oct 12, 2011 #19


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    Recorded and predicted by theory are two different things. This is getting completely repetitive. I have said:

    1) There are practical limits on low energy detection.

    2) There is no reason to introduce a low energy cutoff into a theory that doesn't naturally have one. If the theory with and without the cutoff are indistinguishable in detectable predictions, why complicate with an additional feature only affecting the unobservable domain.

    As for the theory's natural predictions, given any photon, there exists a frame of reference in which it has any energy value greater than zero, including 10^-1000 ergs. A cutoff would violate Lorentz invariance.
  21. Oct 12, 2011 #20


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    Probability infinitesimal is not probability zero. There is no specific point that can be called the 'end of electric field of a proton'. Again, declaring a fixed end only complicates the theory without changing its predictions.
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