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Has the speed of light always been the same figure?

  1. Dec 15, 2015 #1


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    I have always felt that the inflationary period immediately following
    the 'big bang' is a contrived theory, and this leads me to ask if the
    speed of light has always been the same figure.

    The reason I ask this is that to explain the initial rapid expansion,
    I wondered if the speed of light could have been virtually infinite at
    the moment of creation and subsequently slowed down on an
    exponential curve? I feel that this might be a more natural way to
    account for inflation.

    This would mean that speed of light continues to slow, although
    at the present time the change would be very hard to detect.

    If the speed of light must always be the same figure, why that
    particular number?
  2. jcsd
  3. Dec 15, 2015 #2


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    The speed of light is more than just the speed of light. It is a fundamental constant that contributes to determining the strength of the electromagnetic force. If we were to say the speed of light were faster in the past, I expect this would have observable consequences in distant galaxies that would be different from the redshift we see already. In particular, we might see a frequency dilation (a spreading of the spectrum) rather than just a frequency shift from the doppler effect.
  4. Dec 15, 2015 #3
    It's possible that the physics that governed light was slightly different during inflation, but that's not the same thing. The speed of light is not special, the speed of causality is. Light happens to travel at the speed of causality because it has no mass, everything without mass travels the same speed. It's that number because of the fabric of spacetime and how space and time they are connected mathematically.
  5. Dec 15, 2015 #4


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    The number c has the numerical value it has simply because of our choice of units. More detail here:


    Because c has units, it doesn't make sense to ask whether it has changed over time. More detail here:


    No, you can't conclude this. What's observable is if a unitless constant such as the fine structure constant has changed.

    No, it's not meaningful to talk about whether the speed of light was different at another time.

    All of the things people have said about c being more than just the speed of light are true, but not relevant.
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