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Is light speed really constant?

  1. Feb 23, 2016 #1
    Big masses like planets and stars have gravitational fields.
    Gravitational fields curve space around them.
    So in theory a particle having mass moving at fixed speed, from its own perspective, will accelerate when moving closer to such a planet. It'll be moving at fixed speed in a curved space. This is similar to having a car move "straight" on a curved road (although I'm talking about moving toward and not beside).
    Light is also affected by such a mass. We know this because light curves away (changes direction) when passing close to such a mass.
    If light is affected, then it should also be affected when moving directly towards the mass.
    So it should be that although from the single photon perspective it's moving in constant speed (light speed), from an observer's perspective it's actually moving faster as it nears the planet, hence light changes its speed.
    Is this true?
    If not, what's the explanation?
  2. jcsd
  3. Feb 23, 2016 #2
    Its speed is fixed by the laws of spacetime, so when acceleration is applied, there is a Doppler shift to add the energy to its frequency.
  4. Feb 23, 2016 #3
    Sorry. I don't understand.
    Can you elaborate with an example, or say it in layman language?
  5. Feb 23, 2016 #4
    Photons (the individual "chunks" of light) have a very simple equation for their energy: Since we are talking about them moving at their maximum speed it makes it simple to think about their energy being constant as they fly through empty space (vacuum). If it heads towards the sun, it can't speed up, its already going as fast as it can! But it "feels" the pull of the Sun growing as it gets closer, so it must add energy. This force can't make whole chunk of waves move any faster through space, so it makes the frequency of the waves move faster.
  6. Feb 23, 2016 #5


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    Please read our FAQ.

    Single-photon perspective is a fallacy. There is no physics that works at the "single photon perspective", because that violates Special Relativity (are you proposing to violate such a thing?). Again, read our FAQ.

  7. Feb 23, 2016 #6
    So my description is wrong? Or is it just the wrong way to visualize it?
  8. Feb 23, 2016 #7
    This is understood.
    What you said does not contradict what I said but rather aligns with it.
    1. The particle needs not change speed from its own perspective. It can be thought of as a viewpoint thing.
    2. If it "adds energy" then it must add mass, because mass is condensed energy. That's problematic because that would slow it down considerably and it would no longer travel at the speed of light.
    3. If you suggest Doppler effect, then that means adjacent particles must travel at different speeds (faster and faster as one gets closer to the big mass).

    So the original question remains.
  9. Feb 23, 2016 #8
    No. It can't. It's a global relativistic constraint. The viewpoint only rationalizes why relative observations differ.
  10. Feb 23, 2016 #9
    Can you supply a link to a specific topic in the FAQ? it's quite elaborate.
  11. Feb 23, 2016 #10


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    Didn't I linked to TWO specific topics there?

    One explains what it means that light is "affected" by gravity. It isn't light, it is the spectime "geodesic"!

    The second explains why you can't "work" at the speed of light and still use the physics that we know of, because that will violate Special Relativity.

    The whole starting premise of this topic is faulty, i.e. you need to learn basic Special Relativity first, such as starting with its postulates. And please refrain from using the phrase that light "condenses" into matter. That is false!

  12. Feb 23, 2016 #11
    That's the religious reply.
    ZapperZ suggested that my whole viewpoint is wrong and wondered if I want to violate special relativity.
    I'm just wondering about this topic.
    If that turns out to be a breakthrough in physics - it's a bonus :))
  13. Feb 23, 2016 #12
    All you are achieving is broken physics.:frown:
  14. Feb 23, 2016 #13


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    But there's a strong likelihood that this will turn out to be crackpottery. I know where I'm betting my money on.

  15. Feb 23, 2016 #14
  16. Feb 23, 2016 #15
    None of this is correct.
  17. Feb 23, 2016 #16
    I've now read more about special relativity.
    The point is that fixed light speed in an axiom (meaning assumed and not proven or witnessed). Wikipedia also says there are experiments proving light speed in fixed.
    The question is do you know of an experiment (can you supply a link) that checks what I asked above:
    Light travelling toward a big mass?
  18. Feb 23, 2016 #17


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    The GPS system relies on radio signals between satellites and the ground. It includes relativistic maths. It works. Ergo the behaviour of light in a gravitational field is as per relativity.
  19. Feb 23, 2016 #18

    Jonathan Scott

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    The speed of light is always the same locally, as according to Special Relativity. On a larger scale, gravity changes the shape of space-time, effectively making it a fraction "denser" closer to massive objects. If one tries to map space-time using a flat coordinate system for a region affected by gravity, then relative to that coordinate system the speed of light appears to vary slightly. However, at any local point the speed of light has its usual value.

    A static gravitational field does not affect the frequency of a beam of light. However, observers at different gravitational potentials will see the frequency of the beam to be slightly different because their clocks will be running at slightly different rates.
  20. Feb 23, 2016 #19
  21. Feb 23, 2016 #20
    Thank you, wonderful tidbit I was neglecting...
  22. Feb 23, 2016 #21
    Here is an introductory explanation that someone posted in another discussion in these forums

    It explains what zz implied when he posted:

    "Single-photon perspective is a fallacy.....and.....


    " .....The numbers said that the wave moved through space a certain distance every second. (The distance is about seven times around Earth.) They didn’t say what was sitting still. They just said a certain distance every second.

    It took people a while to realize what a huge problem this was. The numbers said that everyone will see light going that same distance every second, but what happens if you go really fast in the same direction as the light? If someone drove next to a light wave in a really fast car, wouldn’t they see the light going past them slowly? The numbers said no—they would see the light going past them just as fast as if they were standing still..... And the numbers said that no matter how fast you move, light moves past you {Locally} at a certain distance every second {the same fixed velocity}.

    In other words, no matter how fast you go, light still passes you locally at the same old velocity, "c". You cannot catch up to light,never....you can never get to velocity 'c' locally.
  23. Feb 23, 2016 #22
    When you say that "you cannot catch up light, never....you can never get to velocity 'c' locally," what does that mean? Does "you" refer only to human observers? If two photons are traveling next to each other in the same direction, and they approach and pass a photon traveling in the opposite direction, I don't understand how each photon can be said to move "past" the other photons at the same fixed velocity.
  24. Feb 23, 2016 #23


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    Whatever you do to it, something with mass (that includes humans, even supermodels) will never be able to travel at the speed of light. Furthermore, it will always measure a pulse of light passing it at c, however much you accelerated it.

    To you, or any other observer, you just have two photons going in one direction at speed c and one in the opposite direction at speed c. That's all there is to say about your scenario from that perspective. I think you are asking about the photon's perspective. Don't. Any attempt to describe a "photon's perspective" is self contradictory, since it would require a description in which light is stationary. However, one of the postulates of relativity is that light is always travelling at c. Photons don't have a perspective. Trying to reason as if they did is where the OP in this thread is going wrong.
  25. Feb 23, 2016 #24


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    The problem here is that you think you CAN be in a photon's perspective.

    Think about it. Special Relativity, which is what all of this is based on, says that the speed of light is the SAME in ALL inertial reference frame. PERIOD!

    This means that if you can imagine yourself being with the photon, then SR is no longer valid, and you will have to invent a whole NEW PHYSICS to do your description. You can't use any of the existing ones. It also means that the concept of "observers" and "observing" will have to be redefined and reinvented for that reference frame. All of SR's equations and descriptions can't be used, because there were never meant to be in a frame where the speed of light is zero in that frame.

    Now, you may want to say that in that case, we need to invent new ones. But that's a different topic entirely, and one that will require someone to first publish those new set of rules of physics in a peer-reviewed journals BEFORE they can be applied here in this forum. Till that happens, all we have, and the BEST we have, are SR/GR. If you want to follows SR/GR's rules, then being in the frame of a photon doesn't work.

  26. Feb 24, 2016 #25
    When you say vary slightly, do you mean up to and never past the limit of c?
    Or do you mean it can appear to pass that limit? i.e. appear to travel faster then light speed?
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