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B Is there an Ether or not?

  1. Sep 7, 2016 #1
    From everything that I have read, it seems fairly non-controversial that there is no Ether and that the only thing that remains constant (or that can be consistently measured against) is the speed of light. But how do we measure the speed of light? If the speed of light is constant, isn't there some type of "Ether" by which the speed of light is governed? Or is the speed of light always c in relation to the speed at which an object is traveling? There is probably a very simple answer to this question that I am overlooking...

    Thanks in advance,
  2. jcsd
  3. Sep 7, 2016 #2
    I think to measure the speed of light, you can do a "little" experiment here.
    You have to have two parties which are comoving. Comoving is speeding at the same direction and at the same speed. And for all practical purpose, you can call the parties stay relatively to each other. Doppler effect can confirm that. And you measure the distance, then you beam a light from A to B and reflects the light back to A and divide the time taken by two.
    Yes, But how can you measure it anyway?
    All you can do is this simple thought experiment here.
    B and C are comoving and traveling from A at some speed.
    A beams light to B, and after the light reach B, then the light will reach C at some time later. Then C will record the time when C see the light and compare it to the time recorded by B. And they will confirm the speed of light is c, some 300000km/second.
    Of course this would be in B and C frame, from A frame, well... the distance between B and C is contracted. And again A will record that the time difference when (from A point of view) the time reaches B and when (from A point of view) the time reaches C is somewhat shorter that what was recorded by B and C.

    You're welcome.
  4. Sep 7, 2016 #3


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    No. I just said no. The answer is no.

    That is correct. It doesn't matter how fast something is traveling relative to you, it always sees light as traveling at c relative to it. Google "Special Relativity"

    EDIT: this is, by the way (and fairly obviously) totally counter intuitive. If a passing spaceship, traveling at .5c relative to you, shines a laser at you, the spaceship sees the light leave at c. You see the light arrive at c, even though the source is moving relative to you. You DO see it red-shifted, so the frequency is different but the speed is still c.
  5. Sep 7, 2016 #4


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    Historically, by bouncing a light signal over a known distance and finding the travel time. Today, the speed of light is defined so you do not measure it. Instead, the above procedure would calibrate the distance. This is because the most accurate way of defining a length unit is in terms of how far light travels in a given time.
  6. Sep 8, 2016 #5
    or to calibrate clock?
    I just realize now. How to measure distance? If you have a very accurate clock then we can calibrate distance, by bouncing light. But..., how to have a very accurate clock?
    I know this is just a technical problem. Is there a solution for this problem?
  7. Sep 8, 2016 #6


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    No. The definition of a second is not based on the speed of light.

    The definition of a second is defined using the frequency of the radiation from a particular caesium atom.
  8. Sep 8, 2016 #7
    Yes, thanks. And from then we get the definition of a meter. Which was stored in Paris? The standard meter.
  9. Sep 8, 2016 #8


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    The meter used to be defined using a standard rod. This is no longer the case. It is now defined in terms of how far light travels in a given time.

    The only SI unit currently defined using an artefact is the kilogram.
  10. Sep 8, 2016 #9


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    Even that will go away soon and be tied to Planck constant. Till then, don't breath around the kg standard.

  11. Sep 8, 2016 #10


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  12. Sep 8, 2016 #11
    It is uncontroversial that the mechanical, 19th century "ether" concept was wrong. In particular there is no frame in which the laws of nature are preferred. "Ether" may simply correspond to the concept that there is something in nature that determines such things as the speed of light, which is the limit speed for everything. This speed is independent of the speed of the source, and independent of the speed of a target at which it ends. Other words that are used try to capture that idea are for example vacuum, space and spacetime. In science we can only describe what we measure (observe); and we cannot measure anything else but particles and fields.
    Oops, correction: funny enough I forgot to mention radiation. But then, many people include that in "particles"; and if I'm not mistaken, QFT includes everything in "fields". Only the phenomena themselves are uncontroversial.
    Last edited: Sep 8, 2016
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