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Thread: Speed of Light - Can it Change?

  1. #1 Speed of Light - Can it Change? 
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    I was having an interesting discussion with a friend and this came up. I know it's probably completely bogus, but I was hoping to understand more about it.

    Would it be possible that the speed of light varied throughout different regions of the universe? To us, understandably, the speed of light would look the same always as our vision/detection of the light would always feed in at the speed limit of our "region".
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  2. #2  
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    Quote Originally Posted by Realsan
    Would it be possible that the speed of light varied throughout different regions of the universe? To us, understandably, the speed of light would look the same always as our vision/detection of the light would always feed in at the speed limit of our "region".
    Welcome to the forum,

    Yes. This kind of thinking came about when trying to understand the presence of life in the universe. The physical constants of nature seem to precisely "tuned" (bad word since it gives the wrong idea) that it was hard to understand the presence of life. So one way out of this was the suggestion that the physical constants of nature varies from place to place. The permittivity of free space is one of those parameters and is what the speed of light is a function of. For details please see
    anthropic principle

    The variety that I'm referring to is the one that says the following
    There are different regions in the Universe where the physical constants take on different values, we live in one of those regions.
    You should also know that the (coordinate) speed of light slows down in a gravitational field. This was predicted by Einstein in his general theory of relativity and measured by Irwin Shapiro.
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  3. #3  
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    This is brilliant! Exactly what I was looking for.

    Thank you!
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  4. #4  
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    Quote Originally Posted by Realsan View Post
    This is brilliant! Exactly what I was looking for.

    Thank you!
    You're very welcome, Realsan. That's why we're here.
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    It sounds good on the surface, but it's not exactly right. Not diametrically opposed to the right answer either.

    Light has no set speed relative to the "Aether", or background of space. All observers perceive it to move at a little bit under 300 meters per second, but it doesn't actually have a "true" speed. It only has an observed speed.

    Relativity is interesting because it explains how that is possible.
    A mathematician and an engineer were at a party. An older colleague of theirs was there with his daughter. The two each asked if they could speak to her. He said it was ok, but they had to approach her by going half way across the room, then stop, then half way again and stop and proceed in that manner. The mathematician realized that he would never reach her and gave up. The engineer determined that he could get close enough to talk. --Approximate retelling of a joke by my math teacher.
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  6. #6  
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    Quote Originally Posted by Kojax View Post
    It sounds good on the surface, but it's not exactly right. Not diametrically opposed to the right answer either.

    Light has no set speed relative to the "Aether", or background of space.
    Since there has never been a detection of the "Aether" (sic), one would not be able to measure a speed (let alone a "set speed") relative to it.

    All observers perceive it to move at a little bit under 300 meters per second,
    You missed 6 orders of magnitude there.

    but it doesn't actually have a "true" speed. It only has an observed speed.
    What's your definition of a "true" speed?

    Relativity is interesting because it explains how that is possible.
    I have no idea what you're trying to say; it's unnecessarily vague. What relativity does is allow observations in one frame to be translated into those in another. There is no one "true" frame.
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  7. #7  
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    Quote Originally Posted by tk421 View Post
    Since there has never been a detection of the "Aether" (sic), one would not be able to measure a speed (let alone a "set speed") relative to it.
    Right. But I'm looking for a clearer way to say that there is nothing even similar to an Aether. Nothing at all. "Space" is not a reference point, and accordingly C cannot be measured relative to "Space"

    Nothing moves at an "absolute speed", only relative speed.


    You missed 6 orders of magnitude there.
    Yeah. Sorry. It was late.

    What's your definition of a "true" speed?
    "True speed" appears to be what the OP is asking about. If relativity were false (and it isn't), and Galilean physics were genuinely descriptive of the way all things work in the universe (and unfortunately it is not), then you could choose any object or location, and measure all speeds relative to it if you wanted to.

    Let me requote the OP here:

    Quote Originally Posted by Realsan View Post
    I was having an interesting discussion with a friend and this came up. I know it's probably completely bogus, but I was hoping to understand more about it.

    Would it be possible that the speed of light varied throughout different regions of the universe? To us, understandably, the speed of light would look the same always as our vision/detection of the light would always feed in at the speed limit of our "region".

    So in Relativity, this last part about our perception changing to see the speed of light as always being the same - well that is pretty close to what relativity predicts.

    As for the rest, about the speed of light varying in some kind of "true sense", what I'm trying to say is that light has no "true speed". If you want to call that a "variable speed" you may feel free to, but there is nothing to vary.
    A mathematician and an engineer were at a party. An older colleague of theirs was there with his daughter. The two each asked if they could speak to her. He said it was ok, but they had to approach her by going half way across the room, then stop, then half way again and stop and proceed in that manner. The mathematician realized that he would never reach her and gave up. The engineer determined that he could get close enough to talk. --Approximate retelling of a joke by my math teacher.
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