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Gravity and its effect on particles/molecules and bonds.

  1. Jul 21, 2008 #1
    To start off with I am only a layman in the subject, in actual fact I've just finished my GCSE year. Please forgive me if I have misinterpreted any area. To this end, my question is whether particles/molecules that are not moving are affected by gravity? Using Einstein's theory of gravity as opposed to the Newtonian theory, a disruption in the space-time has to occur for gravity to exist. Ergo, I am lead to believe that if a sole particle/molecule is not moving, it would not be affected by gravity. However, is it the energy in bonds that form between the particles/molecules that disrupt the space-time and hence will induce gravity? Furthermore, could the moving electrons, whether they are in cloud or particle form, induce gravity by their movement and unequal spread of negativity? As I said I am only a layman so please if any off this question doesn't make sense or is wrong, please correct me. Thank you in advance.
  2. jcsd
  3. Jul 21, 2008 #2
    Not sure how to interpret your question. On the one hand, nothing is ever "not" moving strictly speaking. Leaving that nit-pick aside, neither GR or Newtonian gravity care whether the particle is moving. Not sure what your'e getting at.

    It's definitely not just the binding energy that causes gravity. It's the total mass/energy - which includes the binding energy of course. But solo protons and electrons and neutrons still fall to the earth.
  4. Jul 21, 2008 #3
    For a precise description of the 'motion' of particles/molecules, one needs to invoke the quantum theory. Now, if something is not moving, its momentum is zero (so that it is known precisely) and so by the uncertainty principle, its position has infinite uncertainty meaning that it can be anywhere. So except for a free particle (one that is not subject to any potential, or force, if you will) everything else will have non-infinite uncertainty in position.

    Now, whether or not something moves, gravity will certainly be acting on it but the small scales at which quantum behavior is predominant will usually not allow for a great variation in or strength of the gravitational force. Thus, while theoretically these particles are affected by gravity, the force is usually neglected (this is not to be confused with the quantum theory of gravity, which is under development at the moment).

    The mere presence of matter in spacetime without motion will be sufficient for it to experience a force of gravity.

    Let me see if I can come up with a better explanation...
  5. Jul 22, 2008 #4
    Does a free electron not fall to the ground at 9.8 m/s^2?
  6. Jul 22, 2008 #5
    Peter, does it have to "fall"? I'm sorry but I don't understand your question. Perhaps you're trying to point out a mistake in my post..please elaborate.

    And btw, my post was about the classical picture. I am trying not to get into quantum mechanics here. But I believe Sakurai has a brief theoretical description of a situation in which gravity does play a role in quantum mechanics...I'll find the exact reference and post it back here.

    EDIT: This is page 138 of JJ Sakurai's book "Modern Quantum Mechanics", Revised Edition..under the section "Gravity in Quantum Mechanics."

    To g__ritchie: I think I made too classical a statement there. The point I was trying to make was that the force of gravity is always there, even in the quantum 'realm'. So just ignore what I said in the paragraph quoted by peter0302. As for your last question, the essence of GR is captured in "Wheeler's Ditty":

    Matter tells spacetime how to curve.
    Spacetime tells matter how to move.
    Last edited: Jul 22, 2008
  7. Jul 22, 2008 #6

    Of course gravity affects quantum particles....The so called COW experiment ,1975,(Coella, Overhauser, and Werner) (in cold neutron interferometry) has shown that the phase of a neutron wavefunction changes as a result of its different position in a gravitational potential.

    More recent experimental studies have indeed shown that quantum particles (neutrons) in a confining potential can only fall in quantum 'jumps' in earth's gravitational field...see http://www.nature.com/nature/journal/v415/n6869/full/415297a.html

    More here:

    BTW, Good discussion...

    Typically a neutral particle is used in these type of experiments since the electric field strength of a charged particle would overwhelm the gravitational force measurement.

    Last edited by a moderator: May 3, 2017
  8. Jul 23, 2008 #7
    Well I think the confusion is in the OP, but the reason I ask that is because I thought he was asking whether two objects with no relative motion between them will still feel a gravitational force. When you said gravity was negligible in subatomic particles, that's certainly true when you're talking about the gravitational force between them, but the gravitational force - in classical gravity or GR - between the earth and an electron should be just as observeable as the gravitational force between the earth and a baseball. So it shouldn't matter whether the electron is moving or is bound to an atom. A free electron feels the earth's gravity like anything else and should fall at 9.8 m/s^2 in a vacuum.

    Maybe that wasn't the question but if not I'm totally confused.
  9. Jul 24, 2008 #8
    Yes, in a purely classical space it should fall, i.e. if you totally disregard quantum behavior.

    To the OP: But if you bring in quantum mechanics, the equation of "motion" of the electron would be Schrodinger's equation with the potential equal to the gravitational potential. This is in contrast to classical mechanics, where the equation of motion is Newton's second law, which would give you the acceleration of the electron as that due to gravity (with mass of the electron playing no role).
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