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Speed of light , true vacuum

  1. Jul 31, 2013 #1
    Hi , well we all know that the speed of light in vacuum is measured at c and it slows down when it enters other mediums.
    Now we always say speed of light is c in vacuum but hence I believe the speed is c there because there is nothing in the way that could slow light down like there is in other mediums like water , air , different types of gasses etc.
    But we also know that there are different types of vacuums out there , some mediums are closer to a true vacuum and some are not , like there is a partial little vacuum in the intake manifold of a internal combustion engine but we wouldn't say that light would travel at c in there would we?

    So the question then is , how "empty" the vacuum needs to be for light to travel at c in it ?

    For example would the vacuum which is commonly used in a CRT tube is enough ?
     
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  3. Jul 31, 2013 #2

    Drakkith

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    I would guess that you need a perfect vacuum to get light to travel exactly at c over a significant distance.

    Given the density of gas in a very very good vacuum is extremely low, my question is what effect would a single particle have on the expanding wavefront? Does it slow it down momentarily? How much of the wavefront does it affect?
     
  4. Jul 31, 2013 #3

    nsaspook

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    Air is still very empty at typical RF (< 1 ghz) frequencies as the size of the molecules in air are tiny when compared to the space between them and the wavelenght of the EM waves. The EM field gradient across any one molecule would be a very small fraction of the total field strength at any point in space so it's interaction with that energy would be very small if that molecule was isolated from others.

    The best vacuum levels we see on normal measurement devices in the semiconductor industry might be in the e-12 Toor range (using Ion pumps) for a very good CD SEM at the field emission tip.

    http://www.virginia.edu/ep/SurfaceScience/class2.html

     
  5. Jul 31, 2013 #4
    Hmm so , let me guess when we measured light here on earth with our apparatus we didn't have a perfect vacuum not even close? Now I believe they came to the final speed taking inaccuracies into context or they just did a better job by calculating the speeds from cosmic intergalactic distances yet as the speed of c is also used to say the distances between cosmic objects they first had to know the speed precisely in order to tell how far something is.
     
  6. Jul 31, 2013 #5
    why not turn it on its head?
    If you measure the speed of light in a region of space and get 2.99792458 x 10^8 m/s then that is a perfect vacuum.
     
  7. Jul 31, 2013 #6

    BruceW

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    or, from an experimental viewpoint: if the pressure of your 'almost vacuum' is some value epsilon, then by decreasing epsilon, you should be able to get a value of the speed of light to be closer to c. (Until epsilon gets small enough that quantum effects come into consideration).
     
  8. Jul 31, 2013 #7
    Well as technician said I believe the best measurement they have done is up in space.
     
  9. Jul 31, 2013 #8
    In the end they just decided that measuring the speed of light was not worth it, and just DEFINED it to be a particular number of meters per second. And then they defined one meter to be the length light propagates to with a particular period of time.

    So now they don't measure the speed of light anymore; they measure the meter.
     
  10. Jul 31, 2013 #9

    jbriggs444

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    My take is that they decided that measuring the speed of light was already being done so precisely, so accurately and was so easily reproducible and that measuring the length of the second was being done so accurately, so precisely and was so easily reproducible that one can do a better job establishing a standard of length based on a standard second and the speed of light than by depending on some carefully inscribed scratches on a standard hunk of metal stored in a laboratory.

    That said, I expect that the techniques used to realize the current length standard probably have little to do with what one would ordinarily think of as measuring the speed of light.
     
  11. Jul 31, 2013 #10

    Drakkith

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    Here you go: http://en.wikipedia.org/wiki/Speed_of_light#Measurement
     
  12. Jul 31, 2013 #11
    Not really. They understood that the speed of light was a fundamental constant. So instead of having two units defined independently and with some independent uncertainty, the left just one - the time unit - defined with an uncertainty, and the other one related to it exactly via the constant.
     
  13. Jul 31, 2013 #12

    Nugatory

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    Be careful here. Light slowing down in a medium isn't like a car being slowed by air resistance or you finding that wading upstream against a current is more work than wading downstream with the current. Instead, the light is traveling at c through the vacuum between the particles of the medium; but when the light interacts with the particles it can be absorbed and reemitted after a time delay, reflected, bounced off in a different direction. It's not so much that the light is slowed because something is "in the way"; it's more that if you look closely enough it travels in fits and starts.

    How good are your measuring instruments? For any practical problem that I can think of offhand, a CRT tube is a good enough vacuum that you can treat it as perfect. For that matter, as long as we're working with light you don't need a vacuum at all; the speed of light in ordinary air is only about 100 km/sec less than it is in the hardest of hard vacuums. That's not a big number compared with 3x[108 km/sec.
     
  14. Aug 1, 2013 #13

    BruceW

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    (I think) You can either talk about a photon quasiparticle, or talk about actual photons, which are absorbed and emitted. I agree though, in either 'picture', we don't have actual photons travelling at some speed other than c.
     
  15. Aug 1, 2013 #14

    D H

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    That is not how things proceeded. It took 96 years of ever improving measurements of the invariance of the speed of light, and 78 years of experimental verifications of special relativity before the meter was redefined as the "length of the path travelled by light in a vacuum in 1⁄299,792,458 of a second".

    Not worth it? Those ever improving measurements of the speed of light, along with those ever improving demonstrations that the speed of light is invariant were absolutely critical in making this redefinition acceptable to the international metrology community.
     
  16. Aug 1, 2013 #15
    @ D H

    My remark was supposed to be somewhat humorous.

    Anyway, I do think that "not worth it" captures the idea that with the invariance of the speed of light widely recognized as a fundamental principle of nature, measuring it really means measuring the units of time and/or distance, hence the re-definition.
     
  17. Aug 1, 2013 #16

    russ_watters

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    I'd just say it a different way: the meter and C were redefined because it was realized that the speed of light is more accurately known and fundamental than the the meter, so the meter should be defined based on the speed of light and not the other way around.

    Edit: voko beat me: that's close to my characterization.
     
  18. Aug 1, 2013 #17

    Dale

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  19. Aug 1, 2013 #18
    Ok so basically we could say that light travels at c at any given moment and medium but hence we usually measure the distance that light has traveled in the macro level we say that light travels at c only at vacuum because at other mediums the absorption and re-emission of individual photons takes time so hence the light hits the measuring target later than if it would have been "free" on it's way all the time.
     
  20. Aug 1, 2013 #19
    So...am I correct in saying that now the speed of light is a defined quantity and that the metre is a derived quantity?
    Or....is 'derived' the wrong word to use in relation to the metre.
    Can you recommend any references in this area?


    I have just checked: 1 metre is defined as :
    "length of the path travelled by light in a vacuum in 1⁄299,792,458 of a second".
    So it is the metre that is defined
     
    Last edited: Aug 1, 2013
  21. Aug 1, 2013 #20
    I think this argument is more about the sake of it not about some reality or some real quantity we use everyday.
    For example , measuring one metre can vary depending on the man doing the measurement on the measuring tape on the object that is being measured etc.
    Now on the other hand we have found out that light is fixed by nature at a given number on a given medium.Now when we have found out the speed of light we say that one metre is a given distance that light travels in one sec.
    So with this we have two out of three , we have second as a unit of time , we have the total speed of light which light can travel in that one second and then we have a very small distance which light has covered in that one second while traveling with it's fixed speed so we now call that one metre.

    Even though I believe that this accuracy for metre is more like a physics theoretical problem than a real one.It was just an academic hole that needed to be solved rather than a real problem.
    It's like fixing all the currencies to gold so that we have one pretty much universal point of reference because without a accurate point of reference everybody can be right and wrong in the same time...
     
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