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Why doesn't a photon orbit?

  1. Jul 1, 2004 #1
    Just a question that has puzzled me for a while.

    We know that light is bent as it passes a large gravitational mass. ( yes?)

    The question as implied in the thread heading is why doesn't light orbit that mass?

    Possibly the velocity is such that it can escape that pull of that mass but in extreme cases like black holes etc would not we see photons orbiting that mass at 'c'?

    Sorry if this question appears silly but some understanding would be appreciated.
  2. jcsd
  3. Jul 1, 2004 #2


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    when the light orbits inside of a black hole.. surely you wont see it because no photons would escape, and even if one lucky mofo does.. surely he wont reach you the guy on planet Earth and jump into your fancy telescope and register on the computer screen :D

    so basically... i think photons dont assume the orbit because of their high velocity.. again im not familiar with the calculations - im simply speculating.. idonno how much gravitational pull you need to exert on a photon to get it to orbit.. probably a black hole will do
  4. Jul 1, 2004 #3
    We'd never to be able to see photons orbiting something unless we stood in their orbit (or unless photons escaped).
  5. Jul 1, 2004 #4
    If we assume that photons are orbiting then any reflective object entering the path would reflect those photons. So we would see that the photons were in fact orbiting by the angle of the reflection ( I would think)
  6. Jul 1, 2004 #5
    I somehow don't think that would be the case. Given that the only area where a photon could conceiveably orbit a mass is in the photosphere of a blackhole (and it is unstable I might add) it is a prett much lost cause for any object to reflec the light back simply because it wouldn't have enough energy. Add to the fact that unless the object were orbiting at c as well, it would just fall into the black hole anyways.
  7. Jul 2, 2004 #6
    Hi Scott,

    Photons are massless.....Cheers.
  8. Jul 2, 2004 #7
    But that doesn't mean that it can't be travelling through bent space.
  9. Jul 2, 2004 #8
    I am mainly thinking about the peripheral horizon were the pull is just enough to force an orbit but not enough to restrain reflected light from escaping.

    The object(s) would be seen to fall through this periphery and show us something don't you think?
  10. Jul 2, 2004 #9
    this may be the case but I sort of figure is that if their path can be bent then the path could be an orbit as well, assuming that a photon, is as often posed, an independant, massless particle.

    sort of like you can't have it both ways. if the path can be bent then the logic would follow that the photon can orbit. What do you think?
    Last edited: Jul 2, 2004
  11. Jul 2, 2004 #10


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    An orbiting photon implies a closed geodesic, this is a very real possibility inside the event horizon of a black hole. So the answer to your question is... It can happen, in very special regions of space/time.
  12. Jul 2, 2004 #11
    yes a proton can "orbit"

    gravity doesn't only affect mass, it literally curves space.

    it's been proven that light (massless particles) are bent when they travel near a massive object (like the sun).

    extrapolating that, a thought experiement can help you think about the edge of a black hole and what it really means. the edge, where no light comes out, means that all light inside is "sucked in". but, just as a mass may be sucked into our solarsystem it doens't just dissappear... it goes into orbit in our solar system.

    but, the inside of a blackhole is a complete mystery.

    at the center there may be some weird "singularity" that the photons can orbit... but the same thought experiement makes some very difficult questions..

    if there singularity were infinitely dense (not infinitely massive) it would have a volume approaching zero... so, from all the eliptical orbits particles, dogs, cats, or whatever else that falls into a blackhole could choose to take, none would ever "hit" the singularity and become a part of it...

    until the space between the singularity and the edge of the hole were too filled with particles so particles were bouncing into eachother and then they would all eventually crunch in.. so long as more mass kept falling into the black hole.

    of course i just pulled this all out of my ass, but it sounds good.
  13. Jul 2, 2004 #12
    So we are in some form of agreement that a photon can orbit under special conditions.

    The interesting thing is, if this is the case then incoming photons would continue to add to the ones already orbiting, I would think. Thus we would see and intense amount of photonic activity within this special orbit or space surrounding the black hole.

    Because the orbit is closed wouldn't an amassing of photons in orbit eventually cause something to be observed. I really have no idea what we should see but.

    Anything reflective passing through this orbit on it's way to the centre would reflect some of the photons outward and we would I guess actually see the black holes location during this passage of the object towards the centre.
    And because the photins are coming in from every direction the orbits would form a sphere of orbiting photons.

    Of course this is pure speculation. But I wonder...........
  14. Jul 2, 2004 #13


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    Unfortunately, if you read Scott's response based on your reply here, there is a tendency to "over interpret". That will usually happen if we don't make it clear that a lot of these things are still speculation, and not based on established and accepted physics. For example, your caveat about the fact that "... inside of a blackhole is a complete mystery" seems to have missed him. If there is a "closed geodesic" for light, inside a blackhole (or the event horizon) would be where this would occur, and we know next to NOTHING of the physics within this region.

    Furthermore, it needs to be clearly emhasized that it is the space-TIME "geodesic" that is affected by gravity, not just "space", which is where we have conical path in a typical central force problem.

    My advise, for whatever it is worth, is that unless we know that the person who asked the question has the capabilities to understand the principle involved in our explanation, maybe we should just stick to what is known and has been verified. If not, then our answers will be wildly extrapolated beyond its boundaries of validity and can result in some really amusing but nonsensical conclusions.

  15. Jul 2, 2004 #14
    Zapper, you are of course quite right in what you say and as I stated at the end of my last post that this is all just pure speculation.
  16. Jul 2, 2004 #15


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    Disagree Zapper, it is almost universally agreed that the laws of physics don't breakdown until the singularity and that solution that extend spacetime into the region bounded by the event horizon are valid. So though we can be certain that light may not orbit the inside of a Schwarzchild black hole, though infact there is an unstable orbit of light around a Schwarzchild black hole outside of the event horizon.
  17. Jul 2, 2004 #16
    out of curiocity would you agree that theoretically as light is coming in from all directions the orbits of all would describe an unstable sphere?
    maybe I am assuming incorrectly that a black hole event horizon is spherical?
  18. Jul 2, 2004 #17


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    I'm not exactly sure what you mean by "universally agreed". Unless I missed something, (i) there hasn't been any experimental verification that what we know works within the event horizon, and (ii) for something to be "universally agreed" upon in physics, point (i) is a necessary criteria. I know what GR would predict if we want extend it into this region. I certainly am perfectly willing to accept the idea of a closed spacetime metric in here. But I certainly would not translate my agreeable position into something that is accepted upon as established physics.

    In any case, my intrusion into this string was not to debate the validity of the physics, as should be clear from my previous posting. The physics was merely an example on how things that are clearly defined in physics can be easily miscontrued (gravity doesn't warp space, it "warps" the Minkowski spacetime, to put it crudely). The "orbits" based on this isn't your classical orbits. That alone should put the "fear of god" into anyone wishing to extrapolate scantilly-understood principle. Unfortunately, it doesn't. Important subtleties are often missed in cases such as this.

    I think I should stop now before I start using more colorful metaphors and adjectives. :)

  19. Jul 2, 2004 #18
    Zapper I appreciate your concern. I for one do not expect any verification or resolution to this topic. As per current thinking this is not really possible...agreed.

    I also don't expect any "land Mark" discovery either. I am just wanting to discuss a question that has bugged me for a while and maybe provide an interest to other readers.

    I understand that the physics involved is beyond my level of learning as my posts suggest.

    The thing I would ask though is it a valid position to take that if the photons managed to orbit would we some how be able to see this?
    As a lay person I would intuitively expect to be able to see something from this orbit. But as usual these things can be very counter-intuitive hence the question.
  20. Jul 2, 2004 #19


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    But black holes until recently were entirley theoretical objects and they have hardly been studied up close and personal, infact the observational evidence for their existance almsot rests on accepting the theory behind them. That's why I'm quite happy to accept what theory says as it's not the kind of situation where I expect GR to breakdown.

    There's no reason to think that GR doesn't apply inside the event horizon as the singularity at the event horizon AFAIK is almost universally agreed to be removable and it's just a matter of using a different co-ordinate system.
  21. Jul 2, 2004 #20


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    I will illustrate here what I meant by an extrapolation that you are doing in which you may not be aware that it may be more than what you think.

    What EXACTLY do you mean by "orbit"? I tend to think you are picturing the very same closed orbits that we see celestical bodies make. Unfortunately, these orbits are not (and cannot) be the same thing as what photons can make. Planetary orbits can be accurately described as a "stationary" solution to the central force probelm IN SPACE. The "geodesic" of a photon ISN'T! It is in a more complicated SPACETIME geometry. (I apologize for the capitalization, but after 2 postings, somehow that complexity that is involved here is being missed). It isn't just space that is being distorted. A curved spacetime geometry is anything but simple central force orbits.

    Secondly, IF such a geodesic exist, it has been pointed out that this would occur within the event horizon. Thus, when you ask "would we some how be able to see this?", it makes me wonder if you either missed the fact that these complicated geometry are inside the event horizon, or if you are not aware what an "event horizon" mean.

    This is the very reason why I would not want to use the physics of blackholes to validate the question you asked. There are simply too many unverfied IF's for us to even be able to ADDRESS the question, much less answer it in any reasonably definitive manner. This was the main point of my first posting on this string. However, if people are comfortable with making such multilayered speculation, then who am I to argue?

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