Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EM-radiation (light) realllllly have gravity?

  1. May 20, 2012 #1
    Look, I am familiar with einstein's work, and the mass-energy equivalence principle, and the idea that even LIGHT exhibits gravitational properties.

    I've looked at other threads on the forum, but there is one crucial thing that really needs to be answered:

    has light been EXPERIMENTALLY VERIFIED to exhibit gravitational properties??? -or any instance of 'mass' that is not hard matter (ie made up of leptons,) that has its own gravitational field?

    because, look, I am looking at a theoretical system that suggests that force-carriers CAN act as conducive to a material bodies' gravitational field, but IN THEMSELVES and INDEPENDENT of a material body, force-carriers DO NOT have their own gravitational field.

    So, please, if anyone can explain to me or prove to me that light, (or anything that isn't fundamentally made up of leptons,) DOES INDEED exhibit the property of gravity, PLEASE PLEASE PLEASE tell me

    because otherwise, I am fighting with einstein on this one. without the experimental (or observational) verification, it is JUST theory and not proven FACT

    thank you, in advance-- it means alot to me
  2. jcsd
  3. May 20, 2012 #2
    err, or matter made up of baryon's, either..

    I mean anything that has an atomic (or even positronic) structure
  4. May 20, 2012 #3
    I'm not an expert, but isn't it fairly well established that light bends due to the influence of heavy astrophysical bodies such as stars etc. Isn't this the whole principle behind the (observed) phenomenon of gravitational lensing?

    In Einstein's theory gravity couples to energy and momentum via the energy-momentum tensor (and thus to mass).
  5. May 20, 2012 #4
    Why does light bend?

    Well, acc. to Einstenian theory and my interpretation of it, light travels in a straight line, always.

    But, it travels in a straight line w.r.t. space-time fabric. If we bend the fabric, we have bent the light! Since gravity bends fabric, and bent fabric bends light, we have light bending under gravity.
  6. May 20, 2012 #5
    While I don't think light itself has been experimentally shown to gravitate, virtual photons (the electromagnetic force binding electrons to nuclei) have been shown to affect the masses of atoms (the binding energy reduces the mass of the atom below the sum of the masses of the nucleus and electrons) (reference link). Since light and the electromagnetic force are intrinsically linked whether considered classically or quantum mechanically, I think this is strong evidence supporting light gravitating.
  7. May 20, 2012 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    I don't think there's any truly direct experimental evidence, but GR would certainly be inconsistent if light didn't gravitate. You'd need to have gravity "dissappear" when particles and antiparticles annihilated, for instance.

    There's probably some indirect evidence, but you might need certain modest assumptions to use it.
  8. May 20, 2012 #7
    Gravity couples to the stress-energy tensor not mass. As far as I know no experiment has, or can be expected to measure the gravitational interaction between photons. However the theory predicts that photons travelling parallel to each other will not attract but those that are anti-parallel will. This is because some of the stress-energy components cancel when the photons are parallel.
  9. May 20, 2012 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The best indirect evidence for photons as a source of gravity is found in measurements of the cosmic microwave background (CMB). From a few minutes to around 400,000 years after the big bang, the energy density of the universe was dominated by photons. At the end of this photon epoch, electrons became bound to nuclei (recombination) and the photons effectively decoupled from matter, leaving the CMB pattern measured by experiments like WMAP.

    The primary anisotropies in the CMB arose from physics that took place before decoupling, so they are sensitive to the evolution of the universe during the radiation dominated phase. Analysis of the WMAP data is made under the assumption that photons do produce gravity, specifically through their contribution to the Friedmann equations. It is hard to guess how any sensible results could have come out if this assumption were wrong. If you have some other model of how photons interact with gravity, then the WMAP data would be an important testing ground.
  10. May 20, 2012 #9
    Yes, gravitational-lensing of light occurs because light travels in accordance to the warps in Space-Time produced by other material bodies. It is not necessary that light exhibit gravitational fields of their own.

    Please remember that the focal point of this argument lies in comparing the standard einstein theory supporting (independent) photonic gravity,

    to a model that suggests that photons (or force-carriers, generally,) can only act as *conducive* to the gravitational force exerted by a material body-- ie, in themselves and independent of the material body, force-carriers such as photons do not exert gravitational fields.

    fzero, yes, I believe you are right on track-- we would need to follow the implications into the early universe.

    in the radiation-dominated era, there would be very little gravity. In the phase-transitions prior baryogenesis, (before atomic structure,) there would be no gravity at all.

    I must say, I am not advanced enough to begin to prove, or re-model, the the early universe according to these implications. I am merely a hobbyist.

    I am going to assume that might theory of gravity (as stated herein,) is as valid a theory as einstein's, until experimental verification can confirm the results either way.

    does anybody have any suggestions as to what exactly the implications of a delayed- phase-transition for gravity might have for the early universe?

    *also, the fact that gravity travels at the speed of light might somehow have relevant consequences. If one can fathom how. ...and that in regards the expansion of the universe, the speed of the expansion of SPACE, not necessarily being considered energetic phenomena, might not be restricted by the speed of light. are these puzzle-pieces relevant to the theory presented..? hmmm....
  11. May 21, 2012 #10
    you need A LOT of photons to generate matter/mass, and in turn you need A LOT of mass to produce measurable gravity.
    On the other hand I doubt if "something" needs to have the ability to generate gravity before it can be affected by gravity.
  12. May 21, 2012 #11
    I'm a physics hobbyist as well.

    From what I read in blogs and popular books, there is no direct evience as to whether EM energy attracts other objects gravitationally. There is a considerable amount of indirect evidence like the two kinds mentioned earlier in this thread.

    This is the same situation with neutrinos and with anti-matter: it's hard to capture a big lump of them together and measure gravitational effects. It's not even known yet if antimatter is attracted or repulsed by gravity.

    But if you want to keep the principle of conservation of energy your theory must include gravitational attraction caused by any kind of energy. Also, as you said, otherwise gravity would effectively start and stop in different stages in the evolution of the universe.
  13. May 21, 2012 #12


    User Avatar

    Staff: Mentor

    Please click the "Rules" link at the top of any page here and read the section Scientific Discussion Guidelines. It is not part of our mission to help people develop unpublished theories.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook