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Does speed of light change in strong gravitational fields?

  1. Aug 21, 2014 #1

    I am in a debate with some creationists. They are claiming that the speed of light changes in strong gravitational fields and can be slower or faster.

    My view is there can be time dilation effects. For example light towards the centre ( a black hole ) will be red shifted, and away from the centre will be blue shifted. But the speed of light is unchanged.

    The time dilation would make things appear slower, or faster. A satelite falling into a black hole would seem to take forever.

    I have tried googling but I cannot find any good links to that. The closest was

    But I feel that is somewhat ambiguous, as in one place it is saying it constant, whilst saying that there is a theoretical case where it may change

    What are people's thoughts?

    Thanks in advance

  2. jcsd
  3. Aug 21, 2014 #2
    What does them being creationist has to do with this? :tongue:

    It depends on how you define speed. The speed of light does not change locally.

    See this http://en.wikipedia.org/wiki/Shapiro_delay

    Edit: Not faster than c (just noticed the "faster" in your post)
  4. Aug 21, 2014 #3


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    'c' is invariant. Strong gravitational fields redshift light, but, do not affect on its speed.
  5. Aug 21, 2014 #4
    Any observer should measure the speed of light the same wherever they are.

    As an example, think of light escaping from a black hole that is being pulled in by the gravity so is taking a lot of time to escape.

    An observer near that black hole will be also be stretched and time dilated precisely by the amount that would make it appear to them that the light escaping from a black hole has the usual speed c.

    This only works locally (over very small distances), on larger distances it gets more complicated. (e.g. for an observer far away from the black hole it would appear that everything near a black hole is slowing down - even light, but this is considered an optical illusion)
  6. Aug 21, 2014 #5
    Thanks everyone.

    @ Mr-R. The creationist are saying that the universe is 6000 years old, and using some ideas about "thick and thin" gravity to say that the speed of light does slow down, and somehow, speed up light. Time dilation can give the illusion of speeding up, but nothing as changed locally

    I know that they are talking nonsense. I am well aware of the invariance of C in an inertial frame of reference. But non-inertial frames, and gravitational fields, do complicate things.

    Many thanks again
  7. Aug 21, 2014 #6


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    Even without gravitational sources, light-speed is only constant in inertial coordinates. Around a massive body it is impossible to have global inertial coordinates, so light speed only locally equals c. The average speed for a round trip can differ from c, when measured with a single clock at begin/end. See the link on Shapiro delay.
  8. Aug 21, 2014 #7


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    The speed of something depends on what coordinate system you use. Nothing, not even light, has the same speed in every coordinate system. Now, what is special about light is that there is a particular kind of coordinate system, a local inertial coordinate system, that will always give the same answer, c, for the speed of light. That's true regardless of whether there is gravity or not. A local inertial coordinate system is the sort of Cartesian coordinate system that you would produce using standard clocks and metersticks.

    However, if there is gravity, then a local inertial coordinate system can only be used locally, for a small region. So they are really not convenient for describing large-scale phenomena. For that, people often use noninertial coordinates. For the gravitational field of a spherically symmetric mass, such as a planet or a star, a convenient coordinate system is Schwarzschild coordinates. Using those coordinates, the speed of light is not constant. For light that is traveling radially away from the gravitational source, the speed is given by:

    [itex]\dfrac{dr}{dt} = 1 - \dfrac{2GM}{c^2 r}[/itex]

    where [itex]M[/itex] is the mass of the source and [itex]r[/itex] is the radial coordinate, and [itex]G[/itex] is Newton's gravitational constant. From this formula, the speed of light is slower the closer to the source, and actually goes to zero at [itex]r = \dfrac{2GM}{c^2}[/itex] (the so-called "event horizon") But as I said, that shouldn't be taken too literally, because you can use different coordinates that give a nonzero value for the speed of light at the event horizon.
  9. Aug 21, 2014 #8


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    So where are the strong gravitational fields that are doing this? Note that it would have to be happening to *all* the light we see, from all directions, everywhere; so we, here on Earth, would have to be deep inside a very strong gravitational field. But we're not.
  10. Aug 22, 2014 #9
    Light travels at the barrier c [speed limit]. Which is 186,000 miles per second in vacuum. The speed limit c is constant but only constant to the area through which it travels as it travels through it. Maybe read up on Relativity. Once you understand it then you will understand that from our perspective [which is not that of exactly where a light wave is] that the speed of light does change from an outsider's experience. Light moving through a prism, near a planet or through a galaxy is moving slower in relation to other perspectives. It is really all a matter of space-time. The higher the energy (such as gravity) of a region then the more the space is contracted and the time (space-time) is slowed. (check out Lorentz contraction) So yes. Light going through some regions are [as seen by outsiders](those not in the space-time the light travels through) seen as moving slower. It is Relativity. Just like light through air, water, crystals (prisms). That light does not appear to be traveling at 186,000 miles per second. It is slowed. Which means that as light travels through those substances and nearer atoms the space is contracted and the time is slowed. Look up the more recently discovered Bose-Einstein video experiment documentaries. You can see light moving at a crawl as it is stuck in a contracted space and slowed time.
    Last edited: Aug 22, 2014
  11. Aug 22, 2014 #10


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    My main thought is that there isn't any simple "hack" to GR and/or modern cosmology to make it consistent with the creationists desire to make the universe 6000 years old. I doubt that they will listen to this, though.

    As far as "speed of light" goes, more detail is needed. GR predicts that if you use local clocks and rulers (said rulers being based on a prototype meter standard) to measure the speed of a local lightbeam, you'll always get "c", as others have remarked.

    But it's not clear if that's the sort of "speed" that's being asked about. Unfortunately the popular concept of "speed" is flexible enough that we'd need more detail of what experiment was being used to measure "the speed of light" to answer the question.

    There have been suggestions that the fine structure constant could vary, the relationship between the fine structure constant and the speed of light would be another longish post. The highlights would be that nowadays physicists talk about the fine structure constant varying rather than the speed of light varying, for reasons that make sense to physicists but may not make sense to laymen. Therefore, if one is interested in modern papers on the variation of the speed of light one should also take a look at papers on variations of the fine structure constant, something that a layperson interested in the idea might miss, not realizing the relationship.

    I'll also add that since the fine structure constant can be defined as ##e^2 / 4 \pi \epsilon_0 \hbar c##, if you believe that the elementary charge e, the force between two elementary charges one meter apart (which defines ##\epsilon_0##), and planks reduced constant ##\hbar## are all constant, you must believe that if c varies, the fine structure constant varies.
  12. Aug 22, 2014 #11
    It's not right to say that it shouldn't be taken literally because different coordinates yield different results. That's true of a great number of things in physics and has always be true of all velocities in pre-relativistic mechanics.

    The coordinate speed of light most certainly does vary with speed. Not only did Shapiro prove that but he phrased it as such in the papers where he published the results of his work.
  13. Aug 26, 2014 #12
    c is considered an invariant per frame. That is a fairly well known "given" but why is c a constant per frame?

    I really would like an answer to that question: Why is c as fast as light can be observed to move by frame of reference?
    If you were on the edge of a Bose-Einstein condensate where [to the outside observer] the light can be seen to move at 17 meter per second but at the point of the light would be velocity c.
    Regardless of where: The question remains; Why is c the barrier?

    Someone, somewhere must have a theory of some type to describe why c is the velocity barrier to any lights of the entire EMF spectrum.

    Please don't throw "Fine structure constant" at me. That's just another way of saying "[to the observer] the speed of light changes depending on the energy level of a frame" which I know already and is sort of a given of Relativity since the increase in velocity increases energy.

    I'm not asking for speculation. Who has and where are the theories for why c is the barrier for all EMF light?

    Where can I go to find the paper that says light does not move faster than 186,000 miles per second in vacuum here because of reason X. And by X I mean a valid reason that is not a reworded repeat of the Relativity.

    Something like; X because for light to hold more energy it would become too slow in time and a particle so it rubber bands or is repelled back to wave and a certain velocity at which it retains a lower localized energy level.

    Some semblance of an excuse of a theory to show that there is someone, anyone, out there somewhere with a higher degree in physics that has read, understood and expounded on their understanding of General Relativity and space-time as set forth more than 100 years ago and who can convey that understanding in English terms.
  14. Aug 26, 2014 #13


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    Maxwell's equations, discovered in 1861 long before relativity, describe the classical behavior of electrical and magnetic fields. Their solution is electromagnetic waves traveling at the speed ##c##. So ##c##, the speed of light, is determined from classical E&M and that's where you go for an explanation of the speed of light.

    The speed of the observer does not appear in Maxwell's equations, so all observers regardless of their speed should get the same value for ##c##. Otherwise, we'd be in the weird position of needing different laws of electricity and magnetism according to our speed - and we see that that doesn't happen.

    The great unsolved problem of physics during the half-century after Maxwell's laws were discovered was figuring out how to reconcile Maxwell's E&M with classical mechanics (which of course predicts that the speed of light, just like the speed of everything else, should be different for different observers moving at different speeds). Special relativity in 1905 was the answer. The title of Einstein's paper is "On the electrodynamics of moving bodies" because he was proposing a solution to that problem.
  15. Aug 26, 2014 #14


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    There is no theory that explains this. It's simply an observed fact that light travels at c in all inertial reference frames. The consequences of this fact are what relativity explains.
  16. Aug 26, 2014 #15


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    That is not the correct explanation of why light is slowed by a medium. See the following link.

    https://www.physicsforums.com/showthread.php?t=511177 [Broken]
    Last edited by a moderator: May 6, 2017
  17. Aug 26, 2014 #16


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    See, there's your problem right there. It is not possible to "debate" such folks because they do not care about facts, only about belief. They HAVE to bend the facts to suit their beliefs and no amount of logical or factual argument is going to change that.

    If you do manage to get to the point where they cannot refute your factual arguments, they will simply fall back on something like "well, I can't argue the physics with you but you have to have it wrong somewhere because the Earth IS 4000 (or whatever) years old because the bible PROVES it!".
  18. Aug 26, 2014 #17


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    I don't believe this is necessarily true - in general the speed of light in a media will not be the constant "c", and your BEC is a media (to get such a low speed of light, a rather special one).

    It might be helpful to think of the speed of light as being an infinite rapidity, rather than a finite velocity. Velocities do not add in special relativity, but rapidities do add. What I mean by "add" might need a but of explanation. You "add" velocities by creating a chain of observers, for instance consider three obsevers, such that the velocity between the first and second is a", the second and third being "b". The "sum" of the velocities a and b is defined by the relative velocity between the first and third observer.

    If you have an infinite chain of observers, each moving with a velocity of 1 meter/second relative o each other, you will find that according to the postulates of relativity, the velocity between the first and last observer in the infinite chan will appraoch "c".

    There isn't any "barrier" here. You can add together as many velocities as you like. It is the way velocities add together that results in an upper bound on relative velocity.

    Note that this chain of reasoning leads you to the conclusion that c is the maximum speed limit for anything physical involving cause and effect, it's not just a feature of light alone.

    You can find more info on rapidiity (and the velocity addition law) in the Wikipedia.

    However, science doesn't necessairly have, or need, an answer to "why" questions. A theory is allowed to make whatever assumptions it needs to make. What's important is if the predictions made from those assumptions match experiments. Relativity has been well tested in this manner, the experimental results match the predictions in a wide variety of experiments.
  19. Aug 26, 2014 #18


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    The claims in the paper are rather incorrect.

    For a vertical version of the Michelson measurement the result is the same.
    But in the ideal conditions only, means in the vacuum.

    Michelson measured in the air, thus in this case the air condition: pressure, density, temperature, change with the elevation; so, it's practically impossible to measure correctly in the vertical configuration, thus they measured horizontally.
  20. Aug 26, 2014 #19
    "c" in a media is constant. To the observers who are [not in the media itself] the light does not appear to be moving at 186,000 miles/second "c". This is a very interesting and quite obvious example of relativity. Relativity shows that higher velocity means clock rates that are slower to the outside observer [i.e. BEC experimental results]. Very well known with proven time dilation effects. It is also known that higher velocities translates to higher kinetic energy. Meaning there is more energy in higher velocity space. So by way of logic it should be evident that the amount of energy in a space is what determines the clock rate of that space-time relative to the outside observers space-time. Granted ,that to be easily observed, the energy or velocity differences must be extreme.

    I disagree with "why" not being of concern to science. Maybe a "why" is not needed for those who implement the findings of scientists. But how can a person be a true scientist if that person is not inquisitive? Was Isaac Newton not a scientist?

    Minkowski taught Einstein some and Einstein examined Poincare's work. Years later Einstein at first believed Minkowski's four dimensional space-time manifold math to be a trick. No surprise since Einstein wasn't a giant of a mathematician like Poincare. How Einstein came up with "c" squared from Poincare's "stumped line of thinking" of E=mc ...I do not know exactly when but Einstein used Minkowski's math later in General Relativity. What you can find written in the back of Einstein's book on General relativity is a note that describes Minkowski's space-time field as being similar to but different from a strict gravity field. There would be no potential for better understanding, no GR and no handy GPS navigations if people hadn't been asking "why". I do not think Einstein got it all right but I am sure he got some things right.

    If anyone wants to understand dark matter or dark energy private message me. I know the "why".
  21. Aug 26, 2014 #20


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    "c" is a constant that has nothing to do with a medium. Light in a medium does not move at a velocity equal to the value "c".

    What is meant is that there are always certain things are simply observed facts with no underlying theory to explain them. Once you develop a theory to explain these facts, you again run into the problem that the new theory has unexplained facts.

    I doubt that. If you do, please publish your theory in a reputable journal for peer review so we can discuss it here on PF. Until then, please do not claim that you do and ask people to PM you. It's against PF rules.
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