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Light as being constant

  1. Dec 25, 2014 #1

    I'm so new to this. So I would like to know what is meant when it is said that light is a constant. Is it the speed they are talking about.is it that light never increases speed or decreases speed? Does the speed never change? Thank you
  2. jcsd
  3. Dec 25, 2014 #2

    Doug Huffman

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    No. The speed changes with the density of the medium, gives us rainbows. The speed is constant to all observers. Run, shine a flashlight ahead and the speed is the same. Run, shine a flashlight behind and the speed is the same.
  4. Dec 25, 2014 #3


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    The speed of light is assumed to be in vacuum and assumed to be with regard to equipment that is inertial (not accelerating). If you wanted to measure the value of the speed of light, you would have to set up a round-trip experiment with a clock or timer at one end and a mirror some measured distance away. People who have done this in the past have always gotten the same value so now scientists have defined that value to be an exact value and they use it to calibrate their distance device.

    Einstein's second postulate is not about the constant value of the speed of light. Instead, it is with regard to the one-way speed of light which cannot be measured. It assumes that the light takes the same amount of time to reach the mirror as the reflection takes to get back to the source of light.
    Last edited: Dec 25, 2014
  5. Dec 25, 2014 #4
    As a non-expert, I'll give it a try in simple language to provide something to work with, and gladly entertain any and all corrections...

    All unaccelerated observers despite moving at various relative speeds among themselves always measure the speed of light* to be the same.. that is the starting observational thing to be accounted for. The first problem is that if you assume that time, distance, and simultaneity are absolute everywhere for all these observers then this does not make sense, seems paradoxical, and one must come up with some adjustments that would invent some very strange properties of light to account for it.

    Relativity takes the observations as fundamental and the approach that the speed of light is assumed to be a constant as has always been observed... and then accounts for this by taking away the assumption of the absoluteness of time, distance, and simultaneity. This is revolutionary, but it ensures that all observations of light speed are constant and shifts the needed adjustments away from light and into the time, distance, and simultaneity calculations, with equations and concepts of time dilation, length contraction, and disagreement on simultaneity when one observer is accounting for what is happening to another.

    Relativity adjusts these formerly absolute concepts in order for light to be assumed constant speed as observed. The nature of these adjustments conspire perfectly to make all light speed measures consistent for all observers, and amazingly the effects of these adjustments become transparent and unmeasurable to the one apparently subject to them when being measured and observed by another. This works so well that two observers will each observe and measure these adjustments in the other, but not in themselves.

    This whole approach was based on a very careful step by step analysis of exactly what it means to measure time, distance, and simultaneity using light and the assumption that light speed is always constant.

    * Light remains a peculiar thing in that one can't measure the one-way direction speed of light; the speed is always calculated from a two-way direction measurement (in which the speed in both directions is assumed to be the same).
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