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Photon energy and gravitation

  1. Jan 27, 2009 #1
    Do higher energy photons have the same trajectory around planet/gravity as low energy photons? I.e: If you were shooting photons from the same position wrt a planet would their path be the same?
  2. jcsd
  3. Jan 27, 2009 #2


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    As long as the spacetime curvature caused by the photon is negligible compared to the curvature caused by the planet, then yes.
  4. Jan 28, 2009 #3
    Doeis it depend on photon's energy or on energy density? How big is a photon? And if you are talking of a laser beam, instead of single photons, does it really depend on wavelenght or on intensity? The latter, I presume; a photon's energy doesn't seem relevant to me.
  5. Jan 28, 2009 #4
    "The energy and momentum of a photon depend only on its frequency ν or equivalently, its wavelength." The entire spectrum travels at the speed of light and should follow the curvature of space/time. One possible exception being examined is a "gamma ray anomaly" but there is nothing conclusive. I am a layman so any other input or corrections are welcome.
  6. Jan 28, 2009 #5
    Can a photon cause a spacetime curvature? Please explain and include a link.
  7. Jan 28, 2009 #6
    Negligible but yes. Look at the Einstein Field Equation, right hand side is the energy-momentum tensor. So energy, as well as mass, cause spacetime curvature. There's a discussion at https://www.physicsforums.com/showthread.php?t=232899
  8. Jan 28, 2009 #7
    Interesting but "negligible" may be an understatement. It may be worthy of it's own thread for the rubber room folks but if it can't be measured, it probably should not be introduced as fact in this type of thread. That's my take on what I read. But again, I am very much a layman. :smile:
  9. Jan 29, 2009 #8
    Assuming (but not still proven, as far as I know) that a photon creates a spacetime curvature, according to the fact that it has energy, what counts is not *energy* but *energy density*.
    Let's talk about a simpler case: a continuous laser beam between a source and an absorber, which has specific cross section and lenght. If it's red but with high intensity, it will bend spacetime more than if it's blue but with low intensity.
  10. Jan 29, 2009 #9
    You say "but not still poven" and then say "If it's red but with high intensity, it will bend spacetime more than if it's blue but with low intensity". Is that a yes and a no? Can we say some think the math shows that Photons can bend spacetime but it has never been observed? There are many theories that deserve attention but until observations support the math, shouldn't we be careful not to present it as fact? I am new here and what to learn and would just like to have the facts straight.
  11. Jan 29, 2009 #10


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    It is undoubtedly true that general relativity theory says that photons cause spacetime curvature. I'm no expert but I doubt anyone has been able to experimentally confirm this, because the amount of curvature would be unfeasibly small to measure. You'd need an amount of energy that was a significant fraction of a planet's mass multiplied by c2 to get a big enough effect.

    However, just because we haven't measured it doesn't mean there's any doubt about it. If there were no curvature, it would mean the whole of general relativity were wrong, but we have no evidence to doubt its accuracy yet.

    Curvature-by-photon isn't some take-it-or-leave-it optional add-on to relativity, it's an intrinsic part of it.

    Similarly, no one has ever measured the gravitational attraction between two grains of salt, but there is no doubt that such attraction exists (either from the Newtonian or relativistic viewpoint). But when considering grains of salt orbiting a planet, we can dismiss such attraction as "negligible". (And the energy in a grain of salt is huge compared with one photon!)
  12. Jan 29, 2009 #11
    I thought GR had to do with mass and a photon has no mass. There is no doubt there is energy in a photon, is that what is being considered in GR? If so, it would seem the photon energy of the sun would be huge but is not even considered when calculating time on the GPS satellite clocks. When you say unfeasibly small, would it be like considering Earth's gravity 12 billion light years away? Thanks for your reply, I am trying to learn.
  13. Jan 29, 2009 #12
    "If it's red but with high intensity, it will bend spacetime more than if it's blue but with low intensity" is "the light beam", not "the photon" as I wrote. They are not the same thing. You don't need to talk about photons, to find (from GR) what I said.
    About if photons bend spacetime or not, I don't know because I don't even know what is a photon...
    If others are so sure that photons do that, I would like to know how much a photon bends spacetime in a reference frame where the photon has a much lower energy. The spacetime curvature *as far as I know* should *not* depend on the frame of reference.
  14. Jan 29, 2009 #13

    In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation.
  15. Jan 29, 2009 #14


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    Only a quantum theory of gravity can tell us how photons affect the geometry of spacetime, but it's clear that they must have some effect on the geometry since they carry energy and momentum.
  16. Jan 29, 2009 #15


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    Mass is not the source of gravity in GR, the stress-energy tensor is. There have been half a dozen posts on this in the last week.
  17. Jan 29, 2009 #16
    You see how I write, that is also how I read. :smile:

    After going through a few threads, it is like a F-16 over my head. Is there a layman definition in English anywhere? Just so I know what is being considered.
  18. Jan 29, 2009 #17


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    Try this one. Note that the stress-energy tensor has 16 components, but only 10 of them are independent. One of them is the energy density.
  19. Jan 29, 2009 #18
    First glance says I can get through it. Thanks.
  20. Jan 30, 2009 #19
    Ok, then, please, telle me:
    1. where is the photon between source and detector
    2. how big is it, width, lenght, thickness
    3. which shape it has
    4. what is made of.
  21. Jan 30, 2009 #20
    I understand that a photon could be the length of the known universe with a frequency from 1 through gamma rays. It has 0 mass and exhibits both wave and particle properties. In flat space (vacuum) it travels at the speed of light and that speed is relative to it's current location (frame). The link I provided has a lot of information.
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