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Photon curvature

  1. Oct 1, 2006 #1
    I was wondering, how does light bend in very intense gravitational fields, if it has no weight? And does anyone have a good source for facts on spacetime curvature, gravity and such? Thanks
  2. jcsd
  3. Oct 1, 2006 #2
    Gravitational attraction attracts everything including things that don't have mass. It is a common misunderstanding that something needs to have mass to become attracted to a gravitational field.

    By the way in general relativity the photon itself simply follows the directions of space-time, so there is no acceleration or change in direction. But acceleration or change in direction can of course easily be observed from some particular frame of reference.

    By the way the gravitational field does not have to be very intense, all gravitational fields will bend light, but obviously the stronger the field the more bending.
    Last edited: Oct 1, 2006
  4. Oct 1, 2006 #3


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    "Gravitational attraction attracts everything including things that don't have mass."

    In fact, when you calculate the pull that a massive body has on a small body, the mass of the small body cancels out of the equation, meaning that it doesn't matter how small the body is in how it is being affected (which is how the space station and a stray bolt can follow the same orbit.)

    It also means the smaller body could have a mass of zero.
  5. Oct 1, 2006 #4

    Of course, light DOES have mass since the energy of a photon is Planck's constant times its frequency. Hence, the mass is given by this energy divided by c**2. Only the photon's rest-mass is zero but since the photon is not at rest, one should not concentrate on this.
  6. Oct 2, 2006 #5
    If I'm not mistaken, light has energy by virtue of its motion, E = pc = h nu. The equation E = m c^2 only holds for bodies at rest.
  7. Oct 2, 2006 #6


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    No. Light is energy.
  8. Oct 2, 2006 #7


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    Don't you mean that in calculating the acceleration due to gravity, the mass cancels out? It does not cancel out in calculating the gravitational force.
  9. Oct 2, 2006 #8
    E = pc but p is also defined as mv. So why don't photons in this
    Energy equation have a mass of m?
  10. Oct 2, 2006 #9


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    It depends on which equation you use.

    If you use the equation for invariant mass, m = sqrt(E^2 - pc^2) / c^2, so when E=pc, m=0.

    If you use the equation for relativistic mass, m = E/c^2, so m is not zero.

    Thus it is correct to say that a photon has relativistic mass, but it is generally regarded as incorrect (see the sci.physics.faq) to say that a photon has "mass". The best you can say for it is that it is ambiguous - many if not most people considers it to be wrong, not just ambiguous.

    http://www.math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html [Broken]
    Last edited by a moderator: May 2, 2017
  11. Oct 3, 2006 #10
    light falls into gravity wells because the space time is curved altering its path. All things do this regardless of mass (though some do it to a lesser extent due to high velocities, like light)

    The guys are right in that mass only really comes into it when you calculate force due to gravity. In the same way particles of energy like photons with no mass are affected by gravity wells, so can energetic things with no mass bend space time
  12. Oct 5, 2006 #11
    No, it holds always !
  13. Oct 5, 2006 #12


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    No it doesn't. Study up.
  14. Oct 5, 2006 #13
    I know my physics. No need to study further.

    E = m c**2 always. In fact, for a moving body m = m0/Sqrt (1 - v**2/c**2) in which m0 is the rest mass. For small values of velocity v , the Taylor expansion of the square root will lead to the expansion E = m0 c**2 + m0 v**2 /2, which shows that the energy in the formula E = m c**2 is the total energy and m is the total mass (which takes into account the mass increase due to movement). This is all very basic.
  15. Oct 5, 2006 #14
    Mass warps the fabric of space, which results in curved spacetime, otherwise known as gravity wells or dents in space and time. The light follows a curved path thru the warped spacetime, which is the energy free path. Matter follows curved paths as well, which is why we remain in orbit about the sun. Per Einstein, gravitation is not really a force in the traditional sense. Instead, the dents in a non euclidean spacetime direct a body's motion, whereby curved paths become the inertial paths.

    Light does not have rest mass, but it does have an effective mass and carries momentum.

    Here's a link about the mass of light, and which suggests that light produces gravitation ...

    http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html" [Broken]​

    There are many links which discuss the bending of light. I'm having a little trouble finding a good one though.
    Last edited by a moderator: May 2, 2017
  16. Oct 6, 2006 #15

    Hi, I took a look at the link you mentioned. I do not agree however with their opinion on "modern" and "outdated" definition of mass. For me, this is not just a matter of convention. For a moving particle, it is the relativistic mass which is determining the gravitational field its producing not the so-called rest-mass. For a photon, it is also its relativistic mass (nonzero) which will generate a gravitational field and not its rest-mass (which is zero). Or will those with the "modern" view of mass assert that light can not produce a gravitational field (however weak) ?
    Last edited by a moderator: May 2, 2017
  17. Oct 6, 2006 #16
    First off gravity bends light in all fields whether strong or weak. If the field is weak its just that much harder to detect. Second, Gravity is that field which attracts objects which have mass. Since Einstein showed that since light had energy it has mass. If you pick up a copy of the Feynman Lectures you'll read this - From the Feynman Lectures Vol -I page 7-11, Section entitled Gravitation and Relativity
    Best wishes

  18. Oct 6, 2006 #17
    I agree that the gravity well's configuration depends upon the total mass, ie. rest + relativistic, so the moving mass. Gravity is a function of the total mass, plain and simple.

    Regarding EM, some claim light produces no gravity, because it has no rest mass. Others, who assume that EM waves are ripples of the medium within the medium, suppose that the ripples produce a tension on the medium just as rest mass does. Or, higher graviton density if you prefer. But the gravitational effects of EM would seem negligable in comparison, if those effects exist.

    Einstein showed that a light source looses mass due to the EM radiated away. The light carries momentum, but not in the form of rest mass. The earth is struck by 4.5 lbs sunlight every second. So light is a conveyer of momentum from rest mass to rest mass. A body is struck by light, and momentum is conveyed to the recipient body. This changes its relativistic mass, because the rest mass changes in its state of motion due to the momentum transfer. That said, it seems reasonable that light may in fact possess the ability to pruduce gravitational effects, although minute, given it possesses momentum and alters the medium from flat uniform density.

    Last edited: Oct 6, 2006
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