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Photons and gravitons

  1. Apr 26, 2007 #1


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    I'm an engineer, not a physicist, so you'll forgive my pre-uni knowledge of all things physics. I was discussing electromagnetic fields with someone, we got into it, photons being messenger particles, then onto gravitation and gravitons and so on.

    I got slightly confused by a few things. Photons have no mass and aren't affected by gravity (they have no gravitons I guess), but they are affected by gravitational lensing, because space-time is curved. But then what's the point of the graviton particle? If there was only one mass object in the universe, and it was bending EM waves around it, but it wasn't doing this with gravitons, what do gravitons actually do (in theory)?

    Where am I going wrong? Thanks,

  2. jcsd
  3. Apr 26, 2007 #2


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    Some people would say "No point"

    The graviton as a mathematical convention is useful in some kinds of (approximate) analysis. It comes up especially in situations where spacetime is approximately flat----where the density of matter is low.

    Then one can approximate the "weak field" by imagining it as a perfectly flat geometry with some little ripples or "perturbations" superimposed on it.
    But this is not a fundamental description. A fundamental description has to be able to handle more extreme situations----high density, high curvature, dynamically changing geometry. In extreme situations, the "graviton" description breaks down.

    So you see a kind of division in people's attitudes between

    A. Particle theorists (and their relatives the string theorists)

    B. Other Quantum Gravitists (who do non-perturbative quantum geometry, not using a flat or other fixed background geometry).

    Particle theorists are accustomed to working with fields defined on the flat spacetime called Minkowski space (the standard geometry of special relativity) and analyzing these fields perturbatively, in terms of particles.

    Other QG people come at the subject from GENERAL relativity, where you don't use a fixed background and the number of particles may not even be well-defined. For them, Minkowski space is just the version of geometry you get as a solution to the Einstein equation when there is no matter in the universe: the empty version.

    For them, gravity is not particles, it is geometry.

    So you get a kind of semantic split in the discussion.

    You say you are confused by this. My response is that you are right to be confused.
    I haven't heard of any neat way to resolve these issues, at the present time. I can't advise you as to what you should think.

    Smolin's book "The Trouble with Physics...and What Comes Next" is pretty good. He does an honest job of laying out the issues, making ordinary language substitute for mathematics. So you get a good sense of what's involved in the current struggle to join General Relativity and Quantum Mechanics into a (for the first time) general relativistic quantum physics.
    Last edited: Apr 26, 2007
  4. Apr 26, 2007 #3


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    Classical GR does not use gravitons. You are correct in noting that photons do not have mass, however photons do have energy. Because they have energy photons contribute to gravity, though the effect is usually negligible (they don't have enough energy to matter). It's a Newtonian idea that "mass" causes gravity - GR basically says that "energy" causes gravity. More precisely, it's the "stress-energy tensor" that is the source of gravity in Einstein's GR field equations.

    Where gravitons may play a role is in quantum gravity - but they aren't needed and are not used in classical non-quantum gravity.
  5. Apr 26, 2007 #4


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    Thank you both for your replies, marcus I'll leave interpretation of all of the points you made to some future time, when I have more of it to read up these things.

    Pervect, thanks for the clarification, I guess it should be intuitive since mass-energy are interconnected, but I must admit I never looked at it this way, silly me.

    If anyone is interested in extending this discussion into EM fields, I will pose a few questions. EM radiation/fields do not alter space-time, or do they? Is it the same argument as pervect said, the energy they carry (although insignificant) does actually cause some kind of effect on its surroundings (like gravity). I realise EM is a separate force from gravity, so is the model which is used to describe it the following:

    Photons are like messenger particles, they reveal charges to each other, and the force of attraction/repulsion is the difference in charge, which attempts to "correct" itself.

    Is that a fair/accurate summary? Are there other models, as you have described for gravity?
    Last edited: Apr 26, 2007
  6. Apr 26, 2007 #5


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    whether photons have mass or not depends on what you mean by mass. i am not a physicist either, so i try to tread lightly in opposing the semantic of pervect, but there are physicists, some who write papers, that disagree with this semantic that photons are unconditionally "massless".

    no one disputes that photons have energy and that photons have momentum. if you take the energy (relative to some observer) that photons have and divide by c2, you will get a quantity of mass as a result. if you take the momentum that photons have and divide by their velocity c, you will get the same quantity of mass. this would correspond to the "relativistic mass" sometimes called "inertial mass" of particles in motion (relative to the same observer) that physicists like pervect do not like as expression, but do appear in some physics texts. so rather than just say "photons are massless" without qualification, i would suggest the more precise "the rest masses (also called 'invariant mass') of photons are zero". i don't think that pervect would disagree with that statement (i tried to make it safe). what pervect and other physicists on this forum don't like to hear (or read) is something more assertive like "photons are not massless, but they have no rest mass".

    just to be clear, i am not disputing any tangible physics with pervect and the other physicists here that do not like this semantic, i only take issue with the semantic because i think that it leads to errors in some non-physicists and your statement: "Photons ... aren't affected by gravity" is an example of such an error.

    the issue isn't just their effect in "generating" gravity (which they do to the same degree of any particle of mass when compared using the c2 conversion factor). it's that they, the photons, are affected by gravity. and that amount they are affected, their deflection in Euclidian space, by gravity is (virtually) the same as any test particle (of velocity approaching c) that you would agree has mass.

    pervect, i know you real physikers don't like it, but this is a good example, in my opinion, where the absolute and unqualified "photons are massless" causes more confusion than it attempts to prevent.
  7. Apr 26, 2007 #6


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    Leaving aside the issue of the contribution that light makes to gravity, remember that particles do not need mass to 'feel' gravity. Remember that even in Newtonian physics there is the equivalence between gravitational and inertial mass that causes all objects to be accelerated equally by a gravitational field.

    It's a common misconception to think that things need to have mass in order to 'feel' gravity. This is certainly not required in either GR or even Newtonian gravity.
  8. Apr 26, 2007 #7
    This question was like asking, since electric charges already attract/repel one another anyway, what do mediating photons do?

    You can describe gravity as a mostly Newtonian force acting over great distances (basically the PPN approach), or as a curvature of the space-time geometry, or as a quantum particle interaction mediated by gravitons. Pick one, don't try to use different descriptions at the same time.
  9. Apr 27, 2007 #8


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    btw I love this quote "Do not bodies act upon light at a distance, and by their action bend its rays; and is not this action the strongest at least distance?"

    Guess who said that?

    It was Newton in 1704 in his book 'Opticks'. The details of how Newton linked light and gravity turned out to be wrong, however the principle of light being influenced by gravity is a lot older than GR.
  10. Apr 27, 2007 #9


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    Are you being sarcastic, or just reiterating? That was, in some sense, the question, a clarification on what EM force is, the mechanisms the current model implies.

    That's becoming more clear with each reply, I see my mistake with respect to those points.
  11. Apr 29, 2007 #10
    No it wasn't sarcasm, just a simile intending to help (having assumed you had a deep understanding of EM, to hopefully draw on in understanding gravity).

    According to (the not yet properly existant theory of) quantum gravity, it is specifically by gravitons that mass will "bend space-time" (or affects other trajectories). Analogously, you can in principle solve EM problems either classically (by calculating the "electromagnetic" forces between separated charges) or through quantum field theory (where virtual particles are created, and basically replace the "electromagnetic force" entirely). I don't really know if this analogy holds all that well, the point was just that gravitons have no place in GR whatsoever, and that particle physics are looking at the exact same phenomena from just a completely different angle.
    Last edited: Apr 29, 2007
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