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

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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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


    User Avatar
    Science Advisor
    Gold Member

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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


    Staff: Mentor

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
  22. Jan 30, 2009 #21
    To further explain this point, in general relativity mass and energy are the same thing essentially. E=mc^2, it therefore makes sense in objects with mass to talk about energy rather than mass which tends to confuse the issue, it also makes the idea that mass bends space, or rather gravity does more easy to comprehend in real time, at least mathematically if you ask me. Objects without mass have their own specific rules like e=hf.
  23. Jan 30, 2009 #22
    It occurred to me discussions on gravitational lensing might offer insights.
    Wikipedia says:

    As usual wiki offers no constraints nor assumptions so it's again difficult to interpret the scope of applicability of this quantification. Taken at face value looks like light color plays no part.

    If different light frequencies did curve differently would not gravitational deflection of light produce a rainbow spectrum like a prism?? Does it produce such a spectrum??
  24. Jan 30, 2009 #23

    I tried this formula in google and it did not work, E=(mass of Earth)c^2. What is the energy of Earth? And show me an equivalent equation to
    (G * mass of Earth) / (radius of Earth * (c^2)) = 6.95453588 × 10^-10
    that uses energy rather then mass.
    Thank you
  25. Jan 30, 2009 #24
    I don't understand what you are asking me, are you questioning the validity of e=mc^2 or just that the Earth doesn't have a uniform mass?

    Either way both special and general relativity owe their derivation to this equation, so that is all that matters. If you're suggesting that energy isn't equal to mass times the speed of light squared, that the two terms either side are not equivalent or that they cannot go from one form to the other and back in a relation suggested by that equation, and that the laws of special and general relativity are wrong, I think you're in the wrong part of the forum.
  26. Jan 30, 2009 #25


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The answer you got was really lame. He just told you what everyone knows. I would find that annoying too. But if you meant to suggest that only a person who can tell you the location, size and shape of a photon can claim to know what a photon is, I have to disagree. The answers to 1-3 are all "not well defined" (which was probably your point), and I'm not sure that question 4 even makes sense.

    In case you're wondering, I would say that what you need to know to understand what a photon "is", is a) the theory of unitary representations of the Poincaré group's universal covering group, and b) how to construct the appropriate representation using the appropriate quantum fields. So it definitely takes more than a single university course in quantum mechanics (or a quick look at a Wikipedia article :smile:) to really know what a photon is.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook