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Light gravity

  1. Jul 31, 2003 #1
    [SOLVED] Light gravity


    Just an interrogation this morning.

    If, locally, I parallelize two laser beams separated by some distance "d" but pointing in the same direction. Would these beams converge and mix (after some distance), would they swirl forever, or would they ...?

    Last edited by a moderator: Jul 31, 2003
  2. jcsd
  3. Jul 31, 2003 #2


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    Re: (classical) Light gravity

    may we make some ideal assumptions? the beam from a real laser would not be perfectly collimated and would spread out---more like a cone than a column----but perhaps we can assume that these two beams have classical perfection?

    And we may assume you shine them into space so empty that they can travel undisturbed without limit? then one could imagine them having enough time to interact gravitationally and----if they were close enough together initially---weaving a kind of braid: crossing paths again and again.

    by "classical" is often meant un-"quantical"---not quantum-mechanical. Quantum mechanics imposes limits on geometrical perfection and would probably forbid this waltzing interaction of lightbeams. Some other PF poster may wish to discuss your morning interrogation with different assumptions.

    BTW if you carelessly make the distance d too large then even though the two beams start out parallel the expansion of space will (overcome their gravitational attraction for each other) and eventually pull them away from each other! The initial distance must be small enough so that they remain gravitationally bound.
    Last edited: Jul 31, 2003
  4. Jul 31, 2003 #3
    re Marcus-
    "overcome their gravitational attraction for each other"

    I know that gravity causes light to bend but where is the proof that this is due to 'attraction' rather than a variation in field density?
  5. Jul 31, 2003 #4

    Note he's talking in the classical sense.
  6. Jul 31, 2003 #5


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    Re: Light opera

    MessieursDames (Imagine, elas, neutroncount) my impression was that Imagine posed the question "lightly" and confess that my answer was in a similar vein

    in reality I cannot suppose one could ever detect the gravitational interaction of beams from two real lasers,
    so that how one answers this question will depend very much on how one idealizes
    the situation. I will be happy to withdraw or correct my account
    if anyone wishes to reply with more "gravity"
    and provide a serious quantum mechanical and general relativistic analysis.
  7. Jul 31, 2003 #6
    if they are parallel how would they converge they will never meet.
    am i missing something here?
  8. Jul 31, 2003 #7


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    Re: Re: Light gravity

    I believe that you are. If I understand Imagine's question correctly, he is asking if the two beams will exert gravitational attraction on one another.

    BTW, the more I hear about it, the more convinced I become that light has no gravitational influence.
  9. Jul 31, 2003 #8


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    Re: Re: Re: Light gravity

    I've nothing but admiration and respect for skepticism and, in particular, the steadfast refusal to believe experts
    but some of the pleasure of such a firm denial is missed if
    one does not appreciate the expert view, and so here is
    the conventional wisdom:

    cosmologists with online cosmo tutorials (George Smoot, Charles Lineweaver, Ned Wright, Eric Linder) will tell you that if the universe were suddenly made empty except for light, and there was enough light, it would begin to collapse to a Big Crunch

    and they can tell you how much joules of light per cubic kilometer would be needed.

    I dont recall how much exactly, maybe I can edit this later and put in a figure, but certainly ONE joule per cubic kilometer should be enough to make otherwise empty space collapse.

    (this does not involve any "dark energy" or "dark matter" or anything exotic, just empty space with plain old light in it, and even if there were howevermuch dark energy there as well, you could put enough light in to overwhelm the effect of dark energy and cause gravitational collapse anyway)

    this is what they say. the nice thing is that no one (unless you are taking a midterm exam) is obliged to accept it. So you can
    declare light to be a complete no-show when it comes to gravity and who knows maybe you are right and not they!
  10. Jul 31, 2003 #9
    Interrogation's Precisions:

    Yes, beams would not spread out, would be in empty space to travel enough to interact, and would be "near" enough to "keep" interacting. (Imagine: Similar to my sparse neurons)

    Source of my interrogation:

    If we launch, at the same time and speed, two electrons to follow initial parallel path (similar to laser beam setup), what would be their behavior? Note: Since they repulse eachother, after a certain time, they will have different speed components and therefore magnetic force... Right? In what the magnetic force would change their paths? Swirling?

    IMHO, light beams would attract instead of repulse but could they follow the similar behavior?

    P.S.: Nice morning
  11. Jul 31, 2003 #10
    Re: Re: Re: Light gravity

    Bonjour LURCH,

    Due to conservation laws and based on my modest opinion and knowledge, I invite you to think about momentum, electric charge, spin and energy instead of light, mass, and/or whatsoever. You could find some interesting similarities.:wink:
  12. Jul 31, 2003 #11
    Re: Re: Light gravity

    Bonjour lqg,

    Photons don't have sexe but, IMHO, they are attracted eachother. My "good morning" interrogation was concerning the existence of some derivative force.

    IMHO, upon Maxell's equation, magnetic field is derivative of electric field. Does exist some similitude when we are talking about gravitational field?
  13. Jul 31, 2003 #12


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    Re: Re: Re: Re: Light gravity

    Considering the universe collapsing on itself and two parallel laser beams are two very different cases.

    The answer is no they will not converge and will remain parallel, it is only systems of non-parallel photons that exert a gravitational force.
  14. Aug 1, 2003 #13
    Re: Re: Re: Re: Re: Light gravity

    Do you have an explanation?
  15. Aug 1, 2003 #14
    According to Enc. Brit. there is at present no generally accepted explanation for the transmission of light between galaxies. As for gravity surely everyone should by now be aware that relativity does not explain recent observations by the Hubble telescope.
    With the issues in so great a turmoil any opinion made as if it were a statement of fact cannot be justified. I believe the answer lies in a vacuum theory, but others prefer anti-gravity, strings and so on. Perhaps we should be debating the merits or otherwise of each proposal, using two light beams as the event to be explained.
  16. Aug 1, 2003 #15


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    Re: Re: Re: Re: Re: Re: Light gravity

    Imagine, I too am eagerly awaiting the explanation!
    When I picture two long thin beams of light starting out parallel,
    it is hard to see how the photons in one beam would not
    be influenced by the gravity of the photons in the other beam
    Last edited: Aug 2, 2003
  17. Aug 2, 2003 #16

    Ivan Seeking

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    Re: Re: Re: Re: Re: Re: Re: Light gravity

    Thank you Marcus! I have been trying to get an answer to all of this for some time.

    Marcus, as I understand you here, we can argue for gravity solely on the basis of the energy of the photons?
  18. Aug 2, 2003 #17


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    All I can say for the moment is that at ther moment I can't reemebr exactly how, but I'll get you an answer soon (IIRC it is a consequence of considering photons in GR).
  19. Aug 2, 2003 #18


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    Bravo jcsd, I can conceive of it's depending on how you picture the lightbeams.

    Two individual photons traveling side by side in parallel might have a different outcome from two thin extended beams (each containing many photons) starting out parallel.

    I hope you do find out a GR result of this type. It would come as an agreeable surprise.

    Also jcsd, I may say thank you for apparently corroborating what I said about a universe filled with nothing but light collapsing by its own gravity. It is a standard case modeled in cosmology. Light does attract light---generically speaking. What you are saying, if I understand you, is that it is a special case when two photons (or possibly beams) are traveling in parallel formation.


    Ivan S, thanks for the kind comment on my minor contribution. The main idea was contained in Imagine's "morning interrogation". It will be interesting to see how the discussion turns out. All I can say is that the otherwise-empty light-filled universe is a textbook example and that the energy density (joules per cubic kilometer) is, as always, the parameter that decides future evolution. If the energy density (merely that of light in this case) is greater than the "critical" density, which is calculated from the current expansion rate H0[/sup], then the U will eventually collapse. This is only an idealized model but it shows that cosmologists think light exerts a gravitational effect as do other types of energy.

    But this example has the light so-to-speak "in general position"---randomly directed and distributed. So we just have to wait and see if jcsd digs up anything about light that is parallel.
  20. Aug 2, 2003 #19


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    Re: Re: Re: Re: Re: Re: Re: Re: Light gravity

    one little footnote
    the prevailing model of gravity is of course the 1916 einstein equation and the righthand side of that equation is an expression for the energy-density and Pressure (!) of some
    region or locale

    usually the effect of pressure is neglible (in the examples we consider, we are just partly-evolved fish and do not think of everything all the time) so we almost always neglect to mention the pressure

    but you said "gravity solely on the basis of the energy of the photons" and that is not quite right, it is gravity solely on the basis of the energy density and PRESSURE of the photons.

    But for crying out loud light has almost no pressure at all, so let's neglect it!

    But in another thread of discussion one should talk about the fact that the SQUEEZING inside the core of a star actually contibutes to the gravlish attractiveness of the star. this is so strange as to be beautiful... the righthand side is called the "stress-energy" tensor and it is not merely an energy-density it also contains pressure terms

    so if you put some energy in a bag it will exert gravitational attraction, and how much attraction depends on how much energy you put----but it ALSO depends (which I find delightfully unintuitive!) on the pressure inside the bag.

    you should look at the Friedmann equation which is a simplified
    form of the 1916 GR equation and has a righthand side which is essentially (density + 3 times pressure)

    (rho + 3p)
  21. Aug 2, 2003 #20


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    Yep, Marcus the phtons of the universe defintely contribute to it's mass and the gravitational attraction, and yes it is related to the pressure exerted by light (the classic analogy is to consider a sealed box, containing photons, on a set of weighing scales).This wouldn't hold true for two paralell beams of phtons as they are acting in the same direction tho'(I'll try and get the correct equatins soon)>
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