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Bending of Light due to Gravity

  1. Sep 11, 2014 #1
    Why does light bend due to gravity? I thought hard, read a lot and I found 3 reasons I can give as the answer. But first let me tell you what Newton said.

    According to Newton light (Photons) is massless so light cant bend due to gravity because only things with mass can be affected by gravity. But as per General Relativity Einstein says light bends due to gravity. Below are the reasons as per my understanding as to why light bends.

    Reasoning 1

    In this scenario we assume Newton was right. So we say that massless particles aren't affected by gravity. But light has mass which is called "Effective Mass". In solid state physics, a particle's effective mass is the mass that it seems to have when responding to forces. This effective mass can be acted upon by gravity which only cares how much mass a particle has; alternately, gravity only cares about how much mass or EQUIVALENT ENERGY a particle has given by E = m c^2. Also, if you prefer the particle description of physics over the wave description, you can approximate all photons as 'bullets' each carrying a mass of m = hf/c^2 and traveling at the speed of light.

    EFFECTIVE MASS, is the same as the KINETIC ENERGY of photons divided by c2 (C is the speed of light).

    Thus, even though light has no REST MASS (because it can never be at rest!), it does have an effective mass which (it turns out) has all the properties one expects from MASS - in particular, it has weight in a gravitational field [photons can "fall", which happens in a black hole] and exerts a gravitational attraction of its own on other masses.

    Reasoning 2

    Here we say that even though photons (Light Particles) have no mass they still have momentum. Photons cannot have mass because they travel at the speed of light, but they must have momentum for events like photoionization to occur. Photoionization means the removal of one or more electrons from an atom or molecule by absorption of a photon of visible or ultraviolet light. Also known as atomic photoelectric effect. (Remember Einstein's photoelectric equation).

    It is generally agreed that mass is not a requirement for having momentum. Particles made up entirely of energy can also have momentum according to modern physics. So as you know gravity affects anything that has energy and Light has energy as well as momentum.

    So you will say that momentum= mass x velocity. But remember Momentum is also p=E/c where p is the momentum and E is the energy also C is the speed of light. Photons have energy, and c is a constant.

    Reasoning 3

    Light gets bent because it travels in space time that is warped around massive objects.

    Light sometimes passes through space (or space-time) that is warped or bent because of a nearby object having very strong gravity. The light passes through this space in what (from the light's point of view) is a straight line. To other observers the light may appear to have followed a bent path. So gravity warps space-time, and light appears to bend as it travels through this warped space-time. The light isn't doing anything except following what is a completely natural path through space.

    What general relativity says is that any massive object warps the spacetime around it. You can think of this with a simple analogy. Imagine a stretched rubber sheet that is completely flat. This represents the spacetime when there is no mass. Now, if you put a heavy ball in the rubber sheet, it will cause a distortion in the sheet. This is exactly what happens in space, except that it is in 3 dimensions instead of two.

    Further, a photon always travels by the shortest distance between two points. As spacetime is warped, the light appears to bend around a massive object. In reality, it is not that the object is attracting light, but it is just that the photons are traveling by the shortest distance in a curved spacetime.

    Photons of light are not technically affected by large gravitational fields; instead space and time become distorted around incredibly massive objects and the light simply follows this distorted curvature of space.

    Please tell me which of the above reasons is correct.
  2. jcsd
  3. Sep 11, 2014 #2


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    #3 is definitely correct but I'm not clear that the others are totally Incorrect
  4. Sep 11, 2014 #3


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    Reasoning 3 is the right one and it seems you have a clear understanding of it.
    In GR, any kind of energy(including mass, which after SR, is just another kind of energy) distorts Space-Time and so for distorting Space-Time you need to have some kind of energy but for being affected by that distortion, there is no requirement beyond being inside Space-Time!
    As phinds said, you can't say the other two are wrong. You may build theories on top of GR that tell you those, but they don't seem needed and so we don't consider them.
  5. Sep 11, 2014 #4


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    Where did Newton say this? I read the opposite:

    http://www.colorado.edu/philosophy/vstenger/Cosmo/NewPho.pdf [Broken]
    Note that Newtonian gravitational acceleration is independent of the accelerated test mass, so it doesn't matter that the test mass is zero.

    As others said #3 is how GR states it: by geometry.
    Last edited by a moderator: May 6, 2017
  6. Sep 11, 2014 #5


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    Well Newton happened to be right at first, but the correct answer is taken from GR...
    Using Newtonian calculations you get different bending's (I think with a factor of ~2), so they are not correct.. the correct result is taken from GR (geometry!)
  7. Sep 11, 2014 #6


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    I like number 1:
    But this is absolutely correct:

    If you try to view the photon as a bullet, it has to be a pretty fancy bullet (one with a number of internal variables or properties) that allow the photon to be out by a factor of 2 in its gravitational pull, plus it has some other properties like wave/particle duality, where energy is stored as if the photon was a harmonic oscillator.
  8. Sep 11, 2014 #7

    Jonathan Scott

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    It is important to be aware that photons passing a gravitational source are effectively being bent by two effects.

    Firstly, Newtonian gravity applies to everything including photons, giving an acceleration of ##g##. This can be considered to be due to the curvature of space with respect to time, in that if you plot the radial distance against time, a free fall line curves towards the source.

    Secondly, space itself is curved by gravity, which has the effect that something moving through it is accelerated proportionally to ##v^2/r## where ##r## is the radius of curvature. According to GR, the radius of curvature for a weak Newtonian gravitational field ##g## is ##c^2/g## so the resulting acceleration component due to velocity is ##v^2/((c^2)/g) = (v^2/c^2) g##. This means that a photon is effectively accelerated by twice as much as a slow-moving object.

    Photons are not special in this way. A particle moving at very nearly the speed of light will follow a similar path to a photon, and a photon trapped between the walls of a reflective box whose sides appear to be parallel using local rulers will fall with the same local acceleration as a brick (noting that the rulers measuring horizontal distance will appear curved to an external observer).
    Last edited: Sep 11, 2014
  9. Sep 11, 2014 #8
    Photons and gravity

    If photons have no mass and are not subject to the dilation of their mass, when they travel at the speed of light, then why are they affected by gravity. I thought for something to be affected by gravity it had to have mass.


  10. Sep 11, 2014 #9
    I think reason #3 is the most correct one, but I also agree with phinds in that I'm not sure that #1 and #2 are totally incorrect.
  11. Sep 11, 2014 #10


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    I think it is even more subtle than #3 implies. light itself bends gravity too. so, even if there was only light in the universe, light would bend gravity and influence the path of other light, due to the effect of light on the gravitational field.
  12. Sep 11, 2014 #11


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    This is a common misunderstanding. The acceleration due to gravity is independent of the mass in Newtonian gravity, so a massless particle is accelerated by gravity just as much as a massive one.
  13. Sep 12, 2014 #12
    Reasoning 4

    Reasoning 4

    Mass isn't the only thing affected by gravity, energy is too. Einstein's famous equation E=mc^2 tells us the equivalence of energy and mass.

    In actual fact, mass and energy are the same things, much like ice and water are the same things, just slightly different forms of the same thing. In fact, mass is just a very static version of energy. All energy travels at the speed of light, but mass travels at less than the speed of light. Energy would also create a gravitational field, except that it's moving so fast that it doesn't have a chance to sink into space and produce a static source of gravity, like mass does. Energy is moving through space like a lizard which can run across a surface of water, without sinking in, because it moves so fast.
  14. Sep 12, 2014 #13


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    Einstein introduced the Equivalence Principle in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g is equivalent to the acceleration of an inertially moving body - such as observed in a hypothetical elevator in free space being accelerated at a rate of 1g.

    He then combined the EP with special relativity to predict that light rays bend in a gravitational field, even before he developed the concept of curved spacetime.

    Imagine a hole on one side of the accelerating elevator just large enough to allow a beam of light to enter. As the beam crosses to the opposite side of the elevator, the acceleration will cause the elevator to 'travel' faster than it was when the light beam entered. The beam will end up striking the opposite wall of the elevator lower than the height it entered on the other side.

    I think that light will "fall" in a gravitational field just as quickly as a bowling ball. It's just that light moves so darn fast that it's difficult to see/measure. I'm looking towards our expert members to please check me on this! :uhh:
  15. Sep 12, 2014 #14


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    Yes, as Jonathan said, light is not special.
  16. Sep 12, 2014 #15
    #1 and #2 might be reasonable arguments from the Newtonian model, but the Newtonian model breaks down when it comes down to considering how light actually bends. The famous Eddington solar eclipse expedition had predictions for bending of light due to the Newtonian model, and bending of light due to the Einsteinian model. The Einsteinian model predicted about twice the bending of the Newtonian model, and that's the bending that was observed. So #3 is the actual model that fits reality best (so far...)
  17. Sep 12, 2014 #16
    In empty space Joe shoots with a laser gun and with a revolver on a target. Joe and target are inertial and co-moving.

    Bob, who is accelerating and observing the shooting, says that the bullet and the laser pulse and the target are all accelerating the same way.


    And then Bob must conclude that shooting happened in non-curved space?


    Let's put Joe in a container that is free falling in the gravity field of the earth. The same shooting experiment works the same way. So an observer standing on the surface of the earth must conclude that the space is not curved near the earth?
  18. Sep 12, 2014 #17
    Photons have no rest mass. But they have energy - which is why you shouldn't stand in front of an industrial laser. By mass-energy equivalence (e=mc2) photons have a mass equivalent to their energy.
  19. Sep 12, 2014 #18


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    Only if the box is small. The equivalence principle only holds locally.

    Locally the tidal effects of intrinsic curvature are negligible, so one treat small areas as flat.
  20. Sep 12, 2014 #19


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    It seems to me that Newtonian gravity as described by Newton could not predict an effect on something mass-less.

    Newtonian gravity is a force that exists between masses (##F=G m_1 m_2/r^2##). In his equations there would be zero gravitational force acting on zero mass. Since ##F=ma##, ##F/m=a##, but in this case that is ##0/0=a## which is undefined.
  21. Sep 12, 2014 #20


    Staff: Mentor

    $$\frac{G M m}{r^2} = m a$$
    $$g m = m a$$
    $$g = a$$

    The acceleration is independent of mass, and is well defined.
    Last edited: Sep 12, 2014
  22. Sep 12, 2014 #21
    Acceleration due to gravity.

    You are right mentor. I think I can explain that so that others can understand.

    On further reading I realize that I was wrong about newtons theory where I mentioned massless particles aren't effected by gravity. But actually bending of light could be calculated by newton's theory but bending of light under general relativity would be twice that of newtons theory.

    In Newtonian mechanics, the acceleration of object A toward object B is not dependent on the mass of object A but on the mass of object B and the distance between objects A and B. Because the mass of object A does not affect its acceleration due to the gravitational influence of object B, does Newtonian mechanics predict that a massless particle (e.g. a photon) would be gravitationally affected by an object with mass?

    Newton obviously knew that the mass of an object falling under the influence of Earth's gravity has no effect on its acceleration, i.e., all objects accelerate toward Earth at 32 ft/sec/sec regardless of their mass ("weight"). Therefore it follows that an object with no mass, such as a photon, would follow the same rule: it accelerates to earth at 32 ft/sec/sec. (The reason we don't notice this is that photons spend so little time between the object we see and our eyes due to their extreme speed.)
    Newton obviously knew this, and logically concluded that photons from distant stars grazing the Sun's limb (edge) would "fall" just a bit towards the Sun as they passed by, resulting in a slightly curved trajectory.

    Now I explain why the acceleration is independent of mass of the object falling down.

    By Newton's Universal Law of Gravitation, the force between two object due to gravity


    where M and m are the masses of the two bodies attracting each other, r the radius of the Earth (or distance between the center of the Earth and you, standing on its surface), and G is the gravitational constant.. Let's say M is the mass of the earth and m of the object we're dropping.

    Using F=ma=GMm/r2

    We can rearrange to find a=GM/r2 (m gets cancelled out), independent of the mass of the object.

    Thus all object accelerate at the same rate under gravity alone.
  23. Sep 12, 2014 #22


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    After posting my message it occurred to me that this is true as ##m>0## (as ##m## approaches 0, not sure of correct notation). My argument leads to an undefined result whereas your algebra (which I admit is correct) does not.
  24. Sep 12, 2014 #23


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    Can you make that step, if m = 0?
    $$2 * 0 = 0 * 1$$
    $$2 = 1$$
  25. Sep 12, 2014 #24

    Jonathan Scott

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    Even if the maths is a little dodgy, the physics holds. In general relativity, the rate of change of momentum is proportional to the total energy, not the rest mass, but the momentum also involves the total energy, not the rest mass. So in that case both sides are still non-zero and equal.
  26. Sep 12, 2014 #25


    Staff: Mentor

    You can make that step if m is arbitrary to show that a does not depend on m. If it does not depend on an arbitrary m then it does not depend on a zero m.

    Your above equation should be:

    ##(2-1)\;m = 0##

    So your equation above is not a proof that 2=1, but that m=0. In other words, that equation is not independent of m.
    Last edited: Sep 12, 2014
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