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Gravitation among individual particles

  1. Nov 5, 2009 #1

    I've been pondering something and hope someone here can help me understand this. Simply put, have any experiments confirmed that individual atoms and/or free particles like neutrons, protons, electrons, anti-particles, etc are affected by gravity when they are not constituents of a larger body?


  2. jcsd
  3. Nov 5, 2009 #2


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    The force of gravity on particles is so much less than electrical forces.

    Take proton of mass 1.67x10-27kg and multiply by g=9.81 m/s2 to get an idea of the magnitude of force.

    The gravitational force between particles is also very small compared to electrical and nuclear forces.
  4. Nov 5, 2009 #3
    Right; that's what makes direct measurement difficult. I am wondering if anyone has ever devised a clever way to measure gravitational forces for particle.
  5. Nov 5, 2009 #4
    I think we have seen cold neutrons fall, so that's a direct measurement of one microscopic particle's gravitational interaction with a rather large body (the Earth). I may be able to dig that up.
  6. Nov 5, 2009 #5
    Thanks; that's exactly the type of experiment I was wondering about.

    Are the results of the cold neutron falling experiments consistent with both the standard model and with GR? One would assume any conflict with either theory would have been pretty big news.
  7. Nov 5, 2009 #6
  8. Nov 5, 2009 #7

    George Jones

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    The Physics World general article:

  9. Nov 6, 2009 #8


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    There are setups called 'Atom fountains' that rely on the gravitational attraction of Earth upon individual atoms.

    In a vacuum chamber a very small amount of matter is cooled to a temperature that is below a few millionths Kelvin.
    The atoms are suspended in the center of the vacuum chamber, they are more or less confined there. Some atoms escape the confinement in upwards direction. The upward moving atoms are slowed down by gravity, they reach a highest point, and then drop back down.

    The aim of the scientists is to do measurements at the point where the atoms are their highest point, because that is the closest they'll ever get to being motionless.

  10. Nov 6, 2009 #9
    Thanks again; I will check the reference.

    Regarding your comment above I am trying to understand it but am having trouble with the wording. Can you please help me understand what you mean there?

    Again, thank you!
  11. Nov 6, 2009 #10
    Again, exactly the type of result I was wondering about. Thank you.

    So would it be safe to say that, experimentally, it has been confirmed that neutrons and atoms experience gravity as predicted by Newtonian Physics? If so, how about protons and electrons? I would expect the experiments to be much more difficult with regard to charged particles since the EM forces would need to be avoided or cancelled.
  12. Nov 6, 2009 #11


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    I believe so. For atoms there is confirmation that microscopic matter (matter at atomic level) is subject to gravity in the same way that macroscopic matter is.
    Note that any other outcome would have caused great surprise: the supposition has always been that the macroscopic behavior arises as the sum of the gravitational properties of individual atoms.

    As an aside: terrestrial gravitational experiments are not accurate enough to distinguish between Newtonian and Relativistic gravity. The confirmation of Relativistic gravity comes from astronomical observations, such as the precession of Mercury's perihelion. Hence my uncommitted phrasing: atoms are subject to gravity in the same way that macroscopic matter is.

    I expect that in the case of charged particles EM forces will always swamp any gravitational effects.

  13. Nov 6, 2009 #12
    Understood; thanks.

    I guess the next step for me is to understand whether the accepted thinking in physics is as follows:

    - a neutron, proton, electron, or atom will distort space-time as predicted by GR, and it is that distortion that produces the apparent gravitational forces responsible for the cold neutron falling experiments and the atomic fountain experiments.
    - at the scales involved in terrestrial testing of such phenomena, GR can be accurately approximated by Newtonian gravity, but GR still defines the actual reality.

    Is that the accepted thinking?

    Again, I truly appreciate you folks taking the time to help me. What a wonderful time we live in that sites such as Physics Forums provide laypersons with such access to experts!
  14. Nov 6, 2009 #13
    all particles that carry mass have a gravitational force, its zero mass particles that are questionable
  15. Nov 6, 2009 #14


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    Initially I concentrated on atoms and subatomic particles being subject to gravitational influence.

    You have explicitly informed as to source of gravitational influence, so I'm moving to a deeper level now.

    The more general point of view (necessitated by relativistic physics), is that any amount of energy confined to a finite volume of space will act as a source of gravitational influence. (And yes, in terms of relativistic physics distortion of spacetime acts as mediator of gravitational influence.)

    The inertial mass of any particle is a source of gravitational influence, but nuclear binding energy is also a source of gravitational influence.
    Another way of saying this is that nuclear binding energy contributes to the gravitational mass of a nucleus.

    (Gravitational mass and inertial mass are equivalent. If they wouldn't be we'd have noticed that by now, so it has been elevated to the status of physics principle: gravitational and inertial mass are equivalent.)

    Illustrating that nuclear binding energy has inertial mass: the nucleus of a particular Uranium isotope has a particular inertial mass. If that Uranium isotope fissions, then the inertial mass of the breakup products combined is less than the inertial mass of the original nucleus. (The rest mass of a particle can be counted in terms of energy by figuring how much energy would be released in a mutual annihilation with an anti-particle.)

    Here's another interesting example, a thought experiment.
    Construct a chamber with perfectly reflective walls. Pump light into that chamber. As long as there is influx of light the luminosity in the chamber will keep rising, as the walls reflecting all the light. So that chamber confines an amount of electromagnetic energy to a finite volume of space. That energy will act as a source of gravitational influence.

    The confinement is key: if an object moves in a straight line it has a lot of kinetic energy, but that's not confined, the line along which that object moves can be extended infinitely.

  16. Nov 6, 2009 #15

    What I am trying to understand is at what scale the transition from GR to the SM is generally accepted to be required to get results that conform to experiment.

    Is GR accepted as describing gravity down to the electron/proton/neutron level? If so, does that mean the SM picks up only at the levels of quarks? If not, at about what scale does the transition from GR to SM generally take place? Atoms/Molecules/Marbles/Bowling-Balls/planets/stars/etc?

    Or is it more accurate to say that GR works well for astrophysics, Newtonian Gravity is fine for terrestrial work down to the neutron/proton/electron, and no one is sure how gravity behaves at a scale below that?

    Thanks again!
  17. Nov 6, 2009 #16


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    Well, the GR description of gravititation should be good down to the smallest distances.
    However, at those smallest distances other interactions are far, far stronger. At the atomic scale taking gravity in consideration is useless, so physicists don't bother.

    It's not a matter of scale. (I can imagine though, how superficial introductions may have wrongfooted you into thinking that.)

    Take the example of a neutron star. It has gravitationally collapsed. A neutron star is so dense that the usual atomic configuration cannot exist. If memory serves me: roughly a neutron star can be thought of as a single lump of matter with the density of an atomic nucleus.
    The fact that a neutronstar does not collapse all the way to a black hole is attributed to a quantum mechanical principle that is called the http://en.wikipedia.org/wiki/Pauli_exclusion_principle" [Broken]. Certain types of particles are barred from occupying the same quantum state, so when they are together they have to take up some volume.

    So it's not scale that is at play in whether General Relativity or Quantum mechanics applies.

    Rather, it is very, very rare to have circumstances where gravitational effects and quantum effects are in the same league. The example of a neutron stars came to my mind as an example where gravitational contraction and quantumphysical opposition to compression are a match for each other.

    - Quantum effects can be relevant on a large scale, but it's very rare.
    - For gravity: on small scales (say, beginning at micrometer scale) gravity is always outgunned by other effects.

    Last edited by a moderator: May 4, 2017
  18. Nov 7, 2009 #17
    I understand in general the types of situations where GR, Newtonian Gravity, and the Standard Model are usually used.

    What puzzles me is this: since the LHC is being used to look for the Higgs Boson, that suggests that the SM can explain gravity if the Higgs Boson is found.

    If that is indeed the case, will the SM and Higgs Boson explain all that GR does and predict all that GR predicts, thus replacing GR, or will GR and the SM still be separate entities that are not connected in any obvious way?
  19. Nov 7, 2009 #18


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    Well, the Standard Model is a quantum theory that is compliant with the postulates of special relativity.
    The Higgs mechanism, designed to be part of the Standard Model, complies with the postulates of Special Relativity.

    Gravitation, however, is outside the scope of the Standard Model, just as gravitation is outside the scope of special relativity.

    (By the way, a quantum physics can also be formulated for newtonian space and time. To be self-consistent it's not necessary for quantum physics to comply with special relativity. Early on in the development of quantum physics observation confirmed the expectation; only the SR compliant version agrees with observation.)

    As I understand it: in itself confirmation of the Higgs mechanism will not establish connection between the Standard Model and General Relativity. Still, it is hoped that the LHC results will provide leads to a successor to the Standard model.

  20. Nov 7, 2009 #19

    D H

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    The Higgs mechanism explains why leptons and quarks have the mass they are observed to have. The Higgs mechanism does not explain gravity.
  21. Nov 7, 2009 #20
    Great information from all parties; thanks!

    So if I understand correctly, GR explains gravitational force as arising from a distortion of space-time by mass. The standard model will have an explanation for why mass exists if the Higgs Boson is discovered. But even now, physicists are comfortable with gravity affecting quarks and leptons due to mass-induced space-time distortions exactly as predicted by GR. Even at that scale (leptons and quarks), physicists believe GR behaves the same as it does for large masses like planets, stars, and galaxies.

    Have I got it?

    thanks again,

  22. Nov 8, 2009 #21


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    It's difficult to say whether you 'got it'. For some reason you keep suggesting there are scale issues, in places where there aren't any.

    Let me begin with recapitulating how Electromagnetic interaction is conceived. It's thought of as being mediated by a field, and that EM-field is an occupant of spacetime. The EM field resides in spacetime, just as particles reside in spacetime.

    In the case of gravitational interaction the mediator is not thought of as a field that resides in space, the very spacetime itself is thought of as being distorted.
    That idea should make anyone uncomfortable. Can we still think of particles as being subject to gravitation? Arguably everything in spacetime (curved or uncurved spacetime) is just floating along, in inertial motion. It's very counter-intuitive.
    Anyway, there is no room for any scale distinction: celestial bodies or subatomic particles, everything is in spacetime, floating along in spacetime. In terms of GR space is assumed to be smooth, GR gives no hint to suppose otherwise, so in terms of GR floating in (curved) spacetime is the same at any scale.

  23. Nov 8, 2009 #22
    Thanks again Cleonis.

    If there is no scale distinction then are we left with a situation where the gravitational force is understood to result, fundamentally, from mass-induced space-time distortion, whereas other forces like the electromagnetic force result from a completely different phenomenon (force-carrier particles)?

    For example, do physicists believe that coulomb forces are the result of a space-time curvature caused by charge, or is it only gravity that is caused by space-time curvature? If that's the case, it's interesting that such basic forces are seen as arising from such dissimilar causes.
  24. Nov 8, 2009 #23


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    Yes, Electromagnetic interaction and Gravitational interaction are described with very different kinds of theories. In their current forms quantum physics of the Standard Model and General Relativity cannot be unified into a coherent structure.

    We have to assume that Nature itself is a coherent structure.
    (If Nature itself would be unstructured, how come theories can be formulated at all?)

    The general expectation is that somewhere down the line a successor to the current theories will be developed/found, achieving unification, but so far none of the avenues that have been explored have been fruitful.

  25. Nov 8, 2009 #24
    Thank you VERY much, Cleonis. I truly appreciate the time you and others have spent helping me to understand this better. It is very generous of you and the other posters.
    Last edited: Nov 8, 2009
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