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Light and gravity

  1. Jun 2, 2008 #1
    if light can be affected by gravity then why is it said to have no mass? and sience light is affected by gravity.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole?? this light would never be seen because it will never escape or plummet but would probably be intence enough to vaporize anything that passes through it...
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  3. Jun 2, 2008 #2
    if light can be affected by gravity then why is it said to have no mass?

    1) gravitational time dilation should slow light (think refractive index).
    2) it isnt said to be massless
  4. Jun 2, 2008 #3


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    Photons are massless. Massless particles always move with speed c on the straightest possible path through space-time, but the geometry of space-time around near a star is such that the corresponding path through space is actually a curved path in space.

    I think there are circular paths around a black hole that light can actually follow (in principle), but those orbits are unstable. If the photon isn't going in exactly the right direction, it would either fall in or move away from the black hole.
    Last edited: Jun 2, 2008
  5. Jun 2, 2008 #4
    if light can be affected by gravity then why is it said to have no mass?

    Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. The rest mass of such an object is zero, and the only mass which the object has is relativistic mass-- a quantity which depends on the observer."

    Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

    .. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole??

    I am not sure what you mean by 'perfectly fixed gravity', but light can get trapped in a circular orbit by the gravity of a black hole based on the trajectory of the light entering it. See this: en.wikipedia.org/wiki/Blackhole#Photon_sphere
    Last edited: Jun 2, 2008
  6. Jun 2, 2008 #5


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    The red stuff should be included too. This is the reason I've never liked this explanation.
  7. Jun 3, 2008 #6
    actually, you're also wrong about the energy not escaping. energy is slowly leaked out of black holes, and the black holes themselves destingerate after immensly long period of time.
    An, the light affected by gravity is explained in the local frame/closed rocket acceleration thought expiriment done by einstein. it also explains y planets close to the sum have time differences in their orbits which can't be explained unless it's the theory of genral relativity.
    Last edited: Jun 3, 2008
  8. Jun 3, 2008 #7


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    Remember the old experiment attributed to Galileo where he dropped heavy and light objects off the Tower of Pisa and they all fell at the same rate? The effect of gravity is independent of the mass of the object (for tiny objects near huge planets) so even something with zero mass should (and does) accelerate the same way.

    It is possible for light to orbit a black hole, but how would it get there? Any light approaching the hole would be on the wrong trajectory to orbit -- it would either fall in or else pass by and escape. Something (e.g. a collision) would have to move it onto a different trajectory. Only a small number of photons would get captured this way -- and after capture could later escape by the same mechanism -- so there wouldn't be an intense "belt" round the hole.
  9. Jun 3, 2008 #8
    Think of it this way. If an observer went to the point outside a black hole where the gravity was at the right amount that photons could orbit. Can you see light. Yes. and if you move along this sphere so as to position a specific star at they proper angle so that its photons are being caught you would see them enter their orbit. the preceding was only for thought. Light comes from every star at every conceivable angle, so of course you should have a steady flow filling this sphere of light. Read about the photon sphere on this page http://en.wikipedia.org/wiki/Black_hole
  10. Jun 5, 2008 #9


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    Light hitting the photon sphere from outside wouldn't normally get captured, it would pass straight through it, spiralling into the black hole. Only those photons that are already in orbit within the photon sphere are at the right angle to remain in orbit. For a photon to be captured it would have to change direction, perhaps by collision, or by the influence of another massive object passing by. More likely, a photon would be produced as a result of particles colliding just as they pass through the photon sphere. This would be only a slow trickle of photons being added.

    More importantly the orbits are "unstable" which means only those photons travelling in exactly the right direction can orbit indefinitely (and even then, only if they don't collide or interact with any other particle in the Universe). The tiniest error results in a slow spiral, inwards or outwards, so almost all the photons which appear to have been added will in fact gradually escape orbit some time later.
  11. Jun 11, 2008 #10
    This is incorrect. Light is affected by gravity, not because it has relativistic mass.
    The reason is space-time is curved and light has to follow the curved path, called geodesic
  12. Jun 11, 2008 #11
    Last edited by a moderator: Apr 23, 2017
  13. Jun 11, 2008 #12
    If you agree with her/him, you will get only half of the bending, which Einstein did for his first try.
    You will get the same answer using Newton's theory of gravity.
    After Einstein developed GR, he use GR to calculate the deflection of light, he got the right answer which is twice as large. This is due to curved space
    Last edited by a moderator: Apr 23, 2017
  14. Jun 12, 2008 #13
    everybody has mass
    we can calculate his total energy from
    the equation:
    if light has mass
    we can calculate his Energy from: E=mc^2
    and his speed is v=c
    when a mass is moving a speed of light it will be infinited
    that means E=infinite energy (by Joule) and that impossible of course
    so....we calculate Energy of light from E=hv
    h=planck constant
    v=frequency of light electromagnetic waves
    so the light has body with no mass! (Specifically the photon)
    Last edited: Jun 12, 2008
  15. Jun 12, 2008 #14
    Why do you think that is a contradiction?

    Mass affects the curvature of spacetime. The worldline of unaccelerated obects follow the geodesics of spacetime. It is not the case that if they have no mass they can ignore those geodesics.
    Last edited: Jun 12, 2008
  16. Jun 12, 2008 #15
    the light (photons) has body with no mass
    photons belong to the bosons
    the bosons have spin=0,1,2...........
    like gluons (weak nuclear force)
    i dont know how photons affect by gravitational fields...
    i didnt read the General Relativity to answer this...
  17. Jun 12, 2008 #16
    Actually it is quite correct. Both Einstein and Feynman stated this explicitly themselves.
    That is not a reason. That is a description. And even that description is in accurate since its possible to have a gravitational field in the absense of spacetime curvature (e.g. a uniform gravitational field has zero spacetime curvature). It is important to understand that light has gravitational mass because without that knowledge one has no reason to believe that light is deflected in a gravitational field. The fact that light has gravitational mass is the reason Einstein was led to postulate the equivalence between a uniform gravitational field and a uniformly accelerating frame of reference.

    I recommend that you look this up in Eintein's book "The Evolution of Physics" by Einstein and Infeld. I'm out of town for the next week or two so I can't tell you where in that book Einstein says that. When I get back home I can give you the precise reference and quote.

    Best wishes

  18. Jun 12, 2008 #17


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    Pete, thinking about it that way only made sense before GR was discovered. The reasoning would have to go something like this:

    1. We can prove that a massive particle at rest has an energy mc2.
    2. We can define a velocity-dependent "relativistic mass" such that the energy of a massive particle is always mc2.
    3. This suggests that maybe it's E/c2 that should go into Newton's law of gravity, not the rest mass.
    4. A photon must have some energy, so if we were right in step 3, then light should be affected by Newtonian gravity too.

    This doesn't really have anything to do with GR. Saying that the world line of a ray of light is a null geodesic is definitely a much better "explanation" of why the path of light is bent by a heavy object than the idea that it's the energy/c2 that should go into Newton's law of gravity instead of the mass.

    I'm also surprised that you're characterizing the GR explanation as a "description" and the SR+Newton explanation as the "reason" why gravity bends light.

    If the relativistic mass of a photon is the reason why it's affected by gravity (as defined by GR), then its world line would be a time-like geodesic, not a null geodesic.
  19. Jun 12, 2008 #18
    I disagree of course. The assertion "only made sense" implies that the arguement for it can be proven to be wrong and I certainly don't believe that is true. In fact I know of no valid arguement which could possibly prove it wrong. Especially since it makes perfect sense. And just because that reasoning is what led to GR it does't mean that it can or should be dismissed after its discovery. Each of us is different and as such we all think differently. We all arrive at ideas along different paths. To say that one way of thinking is right and another, equally valid line of thinking, is wrong is a very wrong statement. In fact it is for that reason that both Feynman and Einstein presented those arguements.

    Let me ask you this: If it indeed makes no sense then why do you think Einstein and Feynman explained the defection of light using the mass-energy equivalence arguement? Let me quote them

    The Evolution of Physics - from Early Concepts to Relativity and Quanta, Albert Einstein and Leopold Infeld, Simone & Schuster (1938), page 221 - Einstein commented on an observation made by an observer inside an accelerating elevator that light is ‘weightless’ Einstein wrote
    This same sentiment was expressed by Feynman in Feynman Lectures Vol - I, page 7-11. Section entitled Gravitation and Relativity
    Of course it does. What possible reason could you have for saying otherwise?

    By the way, Einstein never proved that a body at rest has energy. When Einstein first derived the expression E = mc2 he started the derivation with the assumption that a body which was at rest had energy. As such the body was able to emit radiation and this requires that the body had energy, E, to begin with. What Einstein showed was, not that a body at rest has energy, but that the energy of a body at rest is related to its proper mass, m, by E = mc2.
    Why do you consider that a "better" explanation??

    In any case it can't be taken as an explaination since it is a description of the phenomena and not an explaination. Its quite wrong to consider things like this as "explanations" since laws of physics are formulated, not to explain the phenomena in nature that we observe, but to describe the phenomena. This is a fundamental fact of all the sciences. Unfortunately many people miss this very important fact. If you ever have the chance I highly recommend reading the first chapter of Fritz Rohrlich's book "Classical Charged Particles" The author is very good and very well known physicist. He's top notch as a matter of fact, a first rate physicist. He certainly knows what he's talking about and that chapter does an excellant job at explaining the philosophy of physics. You'd do yourself a great service by studying that chapter and knowing the material by heart. I know it did me a world of good .. although I knew the material anyway. The author just does a wonderful job at it. :smile:
    Me? I wish I could take the credit but this is due to Einstein and not myself. Eddington himself explained that GR is not an explanation of gravity but a description of it. This must be kept in mind so that one has a good understanding of the relationship between mass and gravity in both its active and passive aspects.
    Because timelike geodesics are for particles which have a non-zero proper mass (aka rest mass).

    I hope you're not confusing the notion of rest mass with the gravitational mass of light?

    By the way. The definition of relativist mass is as follows: If "m" is the inertial mass (aka relativistic mass) of a body, v its velocity and p its momentum then p = mv. In otherwords inertial mass m is defined as the m such that the quantity mv is conserved in elastic collisions. This quantity is then given the name (i.e. defined) as the relativistic momentum of the object.

    Best wishes


    ps - By the way. One can go on and on about this and never get past semantics and opinions. If this thread gets to that point then I'll bow out.
    Last edited: Jun 12, 2008
  20. Jun 13, 2008 #19


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    I think some of the contributions to this thread are missing the point. If we are talking about a tiny mass near a huge mass (say a 1 kg mass near a planet) then the mass of the small object is irrelevant. A 1 kg mass, a 1 g mass, or a 1 nanogram mass all behave identically -- they all follow the same geodesics generated by the large object and we can ignore any minuscule spacetime curvature introduced by the small object. By that logic, a zero-proper-mass object should also behave identically (the only difference being a difference in velocity). It doesn't matter whether that object has some other sort of "mass" or not. For example, a high-energy high-frequency photon follows the same path as a low-energy low-frequency photon (in vacuum), energy being a frame-dependent concept.

    The mass of the "small" object only becomes relevant when it is much larger, e.g. another planet.
    Last edited: Jun 13, 2008
  21. Jun 13, 2008 #20
  22. Jun 13, 2008 #21
    If light has mass, its speed would slow down when it climb out gravitation field
    This is the essential difference between Newton theory and GR
  23. Jun 13, 2008 #22
    To a local observer the speed of light is constant everywhere in the gravitational filed and to a coordinate observer who has the "big picture" the speed of light speeds up as it climbs out of a gravitational field. I think Newton thought of a photon as a "corpuscle" (with mass?) so presumably Newton would have predicted the opposite (the light would slow down as it climbs out of a gravitational field).
  24. Jun 13, 2008 #23


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    The assertion is that gravity bends light because of its relativistic mass. I have thought about this some more and I think I have to agree. I haven't seen a way to prove that E/2 should go into Newton's theory of gravity instead of mass, but it doesn't matter. Even if that was just a guess, the result is a valid theory (which I will call "SR+Newton" for the rest of this post) that says that gravity bends light. So saying that the relativistic mass is the cause is no different in principle than saying that the gravity of the moon causes tides on Earth.

    I have a hard time believing that it did.
    The reason is of course that we're obviously talking about Newtonian gravity in the framework of special relativity. This doesn't even suggest that space-time might be curved. There certainly is no obvious connection to GR. The insight that's needed to take the step from SR to GR is that objects in free fall follow geodesics, and that there might be a way for this to remain true even when gravity is included. I really don't see how the idea that maybe E/c2 should go into Newton's law of gravity instead of mass takes us even a little bit closer to that insight.

    Do you realize that you're asking why I consider GR a better theory than SR+Newton? (And that the double question marks suggests that you think it's absurd to do so). GR is a better theory than SR+Newton for the same reasons that "the Earth is round" is a better theory than "the Earth is flat". (It agrees with experiments to a higher degree).

    That's not the only reason why I consider GR better, but it's the only one that I'm sure is objectively true. I think these may be valid reasons too, but there is some subjectivity in all of them: GR is simple and beautiful. It's just one theory, and not two theories glued together like SR+Newton. It doesn't require action at a distance.

    This is junior high school semantics, and I'm very familiar with it. What you're saying is certainly true in the sense that any explanation leads to more questions, so nothing can really be explained. "Description" may be a more appropriate word than explanation, but if we're going to be that pedantic we might as well eliminate the word "explanation" altogether, since there are no better explanations around for anything than scientific theories supported by vast amounts of evidence.

    Anyway, you seem to have misunderstood which one of your statements in #16 I objected to. I don't mind so much that you described GR as a "description" rather than an "explanation". That's just a matter of semantics. The thing is, you did this immediately after saying that light is affected by gravity because it has relativistic mass! So you dismissed GR as a "description", but you didn't do the same to SR+Newton. It's that last part I have a problem with. SR+Newton is clearly inferior to GR in every way that matters, but you said that SR+Newton is the real reason why gravity bends light while GR is not a reason at all! So you're implying that SR+Newton is superior to GR, and even has a property that you claim that no theory can have (the property of being an explanation).

    I'm not saying that you don't understand these things well enough. I'm just saying that you chose your words very poorly.
  25. Jun 14, 2008 #24
    I don't see how that would be neccesary. Seems to me that you're using Newtonian intuition. In any case the speed of light does change as it moves through a gravitational field.
    For now I'd like to retract that since I only saw Einstein state that explicitly in his book which was written much later. He did however base the equivalence principle on the fact that all matteer falls at the same rate regardless of its mass. If Einstein believed that light didn't have mass then I see no reason for him to state the equivalence principle. In Einstein's 1907 paper in which he announced the equivalence principle he argued for the deflection of light based on the variation of speed with gravitational potential. I believe that Einstein derived the fact that matter moves on geodesics by using the equivalence principle.
    There is nothing in any comment that I've read or posted that would indicate that we're talking about Netonian gravity. And the presence of tidal forces is only required when there are tidal gradients. In, e.g., a uniform gravitational field there are no tidal forces and thus no spacetime curvature. Yet there is a gravitational field in which light and all other matter is affected by gravity.
    Then I guess there is no reason for me to continue.

    Last edited: Jun 14, 2008
  26. Jun 15, 2008 #25

    You mentioned spacetime isn't curved in a uniform gravitational field. What's that? Seems to me like anything producing a gravitational field inherently causes a 'gravitational gradient' with distance. The only circumstance under which I can imagine a uniform field is at the center of a symmetrically oriented and moving group of bodies, like for instance a tetrahedron shape of 4 equal-mass bodies all slowly traveling, spinning and co-orbiting together, where the uniform field would be at the very center, equidistant from each, although for 4 bodies to be oriented that way, much less to have equal mass, would be a long damn shot. Also the 'center' would be a 'snapshot' and not really legitimate. Is there any other way to get a uniform field and thus 'flat' spacetime?

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