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Gravitational Lensing & Light?

  1. Aug 11, 2006 #1
    Why does light bend when it encounters a black hole or a star?

    Does a blackhole have a refractive index? It must depend on its mass if it does.

    A star can produce an effect knows as gravitational lensing which bends light in much the same way as an optical lens bends light, how does this happen since light is massless?
  2. jcsd
  3. Aug 11, 2006 #2


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    Welcome to Physics Forums, Cyclotron. It's not a case of refraction (which involves light going through some medium), but rather space itself being warped. See Einstein's Theory of General Relativity. Space is a bendable/flexible thing. The mass of a star (or anything really), curves space around it (this is the essence of gravity). The greater the mass, the greater the curvature. Now picture a beam of light travelling through space. The path of that light will follow the curvature of space. With no mass around, and no curvature of space, the path is straight. With a nearby mass, the path is altered. The amount of deviation in the light beam's path is most noticeable when the degree of curvature is high (like next to a massive star).

    Note: I should be saying "spacetime" instead of just "space"...but that would require some more explanation.
  4. Aug 11, 2006 #3
    Oh yes im well aware of Einsteins G.Relativity warping and bending spacetime. I just wasnt sure that if light followed the curvature of spacetime. But you made it clear now. Thanks!
  5. Aug 13, 2006 #4
    I'm sorry, but there isn't any proof that space is a bendable/flexible thing. You should rather say that light curves.

    Light is not massless. Its rest mass is equal to zero. A photon has a "motion" mass which is equal to h*f/c^2. Because of this mass, the photon follows the Gravity Law.
  6. Aug 13, 2006 #5


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    Better check that thought again..:confused:

    Something missing there.
  7. Aug 14, 2006 #6
    The energy of a relativistic particle is given by the relation:
    E^2=(p*c)^2 + (m*c^2)^2 , where E=energy, p=momentum, c=speed of light, m=rest mass.

    But, photon's rest mass is equal to zero and its velocity is the speed of light. So:


    E=p*c, we also know that E=h*f and p=m*u (p=m*c for photons)



    The mass above is not the rest mass of photon. It's the "motion" mass. Accordingly, light has "motion" mass. This mass is the reason of light curvature when it passes near masser objects.

    I don't see where is the argument.
  8. Aug 14, 2006 #7


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    Take a look at:

    http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html where it points out that:
    and other interesting points about photon "mass".

    So, a photon's path near a massive object isn't a Newtonian "G" relation; the path curves because the photon is following the straight line/path of curved spacetime.

    http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html makes this point with more emphasis (last paragraph).
    Last edited: Aug 14, 2006
  9. Aug 14, 2006 #8
    Do you like terminology? If so, could you please give me the definition of "bend"? Imagine an object. Any object... Let's say a remote control. The remote control takes up some spots of the space and others not. It's bend is obvious. That's the definition of "bend". There isn't any geometry which supports limits between space and non space. So, where does the space bend???

    I didn't say anything different. The photon's mass is h*f/c^2 , where f is frequency. Apparently, UV photons are more massive than visible photons. I don't think that spacetime bend is less obscure.

    I'm sorry, but I'm not ready to erase the whole Physics in order to accept a theory (General Relativity). I read your links and the main argument was that there isn't any experiment where the light is in rest. To be honest, I didn't expect that kind of argument because I can easily reverse it to you. Which experiment proves that the spacetime curves??? If there isn't any experiment, then you shouldn't claim that light curves because of the curved spacetime.

    Between the two answers I think that light curve because of gravity makes less "damage".
  10. Aug 14, 2006 #9


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    No problem, I believe that Albert was reasonably correct. You can stick with something else if you like.
  11. Aug 14, 2006 #10
  12. Aug 15, 2006 #11


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    Relativity does erase classical phyics, that's why is was so revolutionary. It has also been proven correct by every experiment conducted to date. Physics is hard.
  13. Aug 16, 2006 #12
    General relativity is just a theory. It may be spectacular but it's still a theory. Instead of trying to correct the theory in order to keep step with our natural experience, we are trying to justify its existance a priori. That's wrong. Even language isn't enough to explain the predictions of general relativity. I will ask again:
    Does anyone perfectly understand what is a spacetime bend??? Because I don't. There isn't any geometry which supports limits between space and non space. It seems outrageous.

    Over and above, there isn't any experiment which proves that spacetime is bendable. Experiment is the main component of all sciences. Why are we trying to remove experiment from physics???

    Erasing classical physics is wrong. Thanks to Classical physics we are so far in Physics science. You can't reach the second floor if you don't reach the first. In which experiments are you refering to? I hope you don't mean predictions or results.

    This is not an experiment. It's one of the results. Besides, I admit that some of the general relativity's results may be accurate. BUT, that's not enough in order to accept this theory. The "price" of this theory is huge.

    Einstein's theory was about to negate self-evident clauses. That's a major problem for me. Everyone is free to choose which theory is more efficient :biggrin:
  14. Aug 16, 2006 #13
    So you would like to stick with classical newtonian explanations for gravity despite the links I provided? Did you even look at the link? I fail to see how you can not think this is experimental.
    The newtonian regime is still used in lots of areas, but in some it breaks down and GR is a better descriptive theory for what we observe.
  15. Aug 16, 2006 #14


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    Efficiency is irrelevant. What is important is what works. Newton's theory gives wrong answers to some questions. It doesn't work in all the situations Einstein's does. In addition:
    That is an utterly useless statement to make in a science forum: everything we talk about in a science forum is theory and to use that fact in a misguided attempt to attack one theory is a crackpottery red-flag.
    I'm not sure what you mean here, but I suspect that is because you don't know what you are talking about either - it appears you don't even understand the concept of science itself. SR and GR came into being because of known flaws in the previous existing theory (Newton's gravity theory, Newton's laws of motion). It is popular today for one reason and one reason only: it makes accurate predictions about how the universe works and Newton's theories don't.

    That said, all theories by definition are incomplete. So your attempts to denigrate GR by saying it is 'only a theory' and that it is incomplete reduces to 'GR sucks because it is good science.'
    Physics is mathematical. Being expressable verbaly is a distant second (some would even say completely unimportant) to the need for a theory to be expressable mathematically.

    And I must remind you now of the forum guidlines regarding personal theories and overly speculative posts.
    Last edited: Aug 16, 2006
  16. Aug 16, 2006 #15
    First of all, I never said that I am a fan of Classical Physics! I said that I am totally opposite in erasing Classical Physics! I accept that Newton's theory gives wrong answers to some questions, BUT the same does General Relativity, Special Relativity, Quantum Mechanics... etc to some other questions.Neither General Relativity works in all the situations.

    No comment...:mad:

    Excuse me, but you are not there to JUDGE me! Could you please be more polite?

    I said that Physics is always based on experiments!!! There isn't any experiment which proves that spacetime is bendable (Do you consider the light curve as an experiment? In this case, I hardly can understand it.) Until then, we can't be totally sure if the predictions of General Relativity are correct.

    I totally agree in that. I only said that there are some parts in General Relativity which are very difficult to be understood (like the spacetime bend) and maybe somehow could be avoided.

    I'm not trying to denigrate General Relativity. I insist that it's only a theory because there aren't any experiments which prove that this is reality. In fact, I don't understand what's the concept of the spacetime bend.

    Yes, physics is mathematical. But in physics, we usually modulate maths in order to take solutions that exist in reality. For example, in Mechanics we don't accept solutions with infinite amplitude when we describe the movement of an oscillator.

    So, as I said before I was wondering if we could avoid these complicated and very revolutionary predictions by modulating maths.

    Did you find any personal theory or overly speculative post?:confused: If you are still talking about the light curvature, I inform you that the thought I mentioned is written in "Modern Physics" (by Raymond A. Serway, Clement J. Moses, Curt A. Moyer). Apparently, it's not a personal theory.
  17. Aug 16, 2006 #16


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    No, the curvature has been measured and agrees well with theory.

  18. Aug 16, 2006 #17


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    That is so wrong, I'm not sure where to start - how about gravity probe B?
  19. Aug 17, 2006 #18


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    I'm not sure why you're choosing to take this up in the astronomy forums...

    The "bending" of space comes from the geometrical interpretation of GR, which will almost certainly break down at the quantum level. I find it strange, however, that you consider this to be a problem with the theory. I think most physicists consider the geometrical aspects of GR to be far more elegant than their quantum counterparts. What is it about the "bending" that you don't understand?

    What predictions? What aspects of GR do you think are problematic? There are aspects of the theory that are generally accepted as problematic, but the "bending of space" is certainly not one of them.
  20. Aug 17, 2006 #19
    I believe that General Relativity works perfectly in universe so far. Its predictions match perfectly with the authentic orbits of the planets... etc. The problem is somewhere else:

    Exactly! For me, the misunderstanding is based on the geometrical interpretation of GR. Which geometry supports limits between space and non space? If there isn't such geometry, in where does the spacetime bend? That's why I was wondering if we can modulate mathematics in order to avoid these difficulties.

    Maybe I'm wrong, but I admit that I can't catch the idea of space curvature. It's hard to me to consider spacetime as an object.

    That's true. Gravity Probe B is the best experiment that ever took place about General Relativity. Βut, as you can see, there are still some doubts
    about the warped space due to earth. We are looking forward to the results. *

    I didn't know that. *

    *Thanks for the information.
  21. Aug 17, 2006 #20


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    This is indeed a somewhat difficult concept to internalize. The curvature is intrinsic to the spacetime manifold. That means it doesn't require anything outside of itself for the definition of the curvature. Basically the differential directions at each point define a set of tangents, which form a tangent space. As you move from point to point the tangent spaces change, and in order to be able to compare vectors at different points you need a map between nearby tangent spaces. This mapping (called a connection) defines the curvature. You can think of the curvature as a change a vector has to go through when you move it from place to place. If you move it around a simple closed curve, it will wind up pointing a different direction than when it started - really! This kind of thinking has to replace old intuitions about how curved surfaces "look" in three dimensional space.

    Because Einstein posited spacetime to be a very nice manifold (pseudo-Riemannian), the connection, and hence the curvature near each point, can be calculated in this case directly from the metric tensor defined there.
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