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The speed of light in a vacuum?

  1. Sep 18, 2006 #1
    Premise A: The constant c is the speed of light in a perfect or absolute vacuum.

    Premise B: A perfect or absolute vacuum is a practical impossibility.

    Conclusion (C): The speed of light in a vacuum is a purely theoretical concept.

    Note that I am, or consider myself, a philosopher. If I am a physicist, I am at best a classical physicist. I am not asking for highly advanced equations as a response to my argument. I just want to know whether it is valid (i.e., whether the conclusion follows from the premises) and whether it is "true", i.e., whether it is probable that my premises are correct. Thank you.
     
  2. jcsd
  3. Sep 18, 2006 #2
    vacuums are not impossible. Yes, a reasonably sized vacuum is impossible, there will always be radiation inside of it. However if you go sub-microscopic there are vacuums all over.
     
  4. Sep 18, 2006 #3
    How do you know? Have you run any experiment that proves your statement?
     
  5. Sep 19, 2006 #4

    HallsofIvy

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    If I remember correctly there is about 1 hydrogen atom per cubic mile in deep space. Pretty good vacuum between hydrogen atoms isn't there?

    I presume that your "A perfect or absolute vacuum is a practical impossibility" was referring to "in the lab" which is irrelevant to the question of "speed of light in the vacuum".
     
  6. Sep 19, 2006 #5

    ZapperZ

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    First of all, you need to understand WHY light appears to have a different speed in a medium. Once you understand that, THEN you can figure out why, for example, it is so difficult to detect any variation of the speed of light in air, for example, when compared to a "vacuum". Thus, even in air, we already have, for all practical purposes, the vacuum speed of light.

    Furthermore, your "logic" failed to consider the fact that if such an assumption isn't valid, we would have detected a slew of problems already. Satellite transmissions, GPS, etc.. etc... ALL make the assumption on light speeds. Considering that you directly depend on such things, I would point to you and what you use as evidence for its validity.

    Zz.
     
  7. Sep 19, 2006 #6
    Why does light appear 2 hav a different speed in a medium? Tiny spacetime warps caused by the matter (just 2 throw a guess at it)?
     
  8. Sep 19, 2006 #7

    ZapperZ

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    Read our FAQ in the General Physics forum.

    Zz.
     
  9. Sep 19, 2006 #8
    I see now:

    "A vacuum is a volume of space that is substansively empty of matter"
    http://en.wikipedia.org/wiki/Vacuum

    "In physics, matter is commonly defined as the substance of which physical objects are composed, not counting the contribution of various energy or force-fields"
    http://en.wikipedia.org/wiki/Matter

    I do count that contribution.
     
  10. Sep 19, 2006 #9

    ZapperZ

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    Then, do you also count the gravitational field from Alpha Centauri in your everyday life and therefore, argue against all obervations that neglect it?

    Zz.
     
  11. Sep 19, 2006 #10
    like zapper said, its ridiculous to actually count this stuff. If you count force-fields then there is no such thing as a vacuum. However, Im not sure if curvatures in space-time slowdown light, which would mean that only energy could distinguish a vacuum from a non-vacuum. Like I said before, although there is a lot of radiation, if you shrink your scope enough you will be able to find many vacuums void of any energy.
     
  12. Sep 19, 2006 #11
    But I think those vacuums are functions of the energy surrounding them. If there was no energy, there would be no vacuums.
     
  13. Sep 19, 2006 #12
    Light is itself energy. The presence of light itself makes space not a vacuum, under your expanded definition.

    In the strictest of senses, I would say yes, it is a purely theoretical construct. Of course, there are some implications of this, combined with current knowledge of the universe, which make it an eminently practical one to cling on to.

    Physicists and engineers don't like to speak in the strictest of senses, because frankly, that way lies nihilism and stagnation.
     
  14. Sep 19, 2006 #13

    ZapperZ

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    And how are you able to come up with this, considering that you don't think that there is such a thing as a "vacuum"? What is the justification for that statement? [This is, after all, physics and not philosophy.]

    Zz.
     
  15. Sep 19, 2006 #14

    DaveC426913

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    I think the number is more in the m^3 to cm^3 range.

    Not that it matters. Even at one atom per cm^3 is a vast, vast playground for light to romp around in and not have to go play with the big atoms.


    Question: why, simply because we can't make a perfect vacuum, can we not determine the speed of light? We can determine c over very short distances, short enough that we don't have to worry about atoms.
     
  16. Sep 19, 2006 #15
    what??
    yes, light is energy, along with all other radiation. However, there isnt radiation in every single point in the universe. There are still many small areas that lack any radiation.
     
  17. Sep 20, 2006 #16
    They are relative, not absolute, vacuums: relative to the energy around them.
     
  18. Sep 20, 2006 #17
    But theoretically, light cannot pass through these, for when it does, there is radation there. I admit this is a sophistry, like the tortoise and the hare.

    Supposing there are areas which, in themselves, are absolutely empty, I think these are a function of what surrounds them, by which I mean that the concept of such an area is due to our idea of particles, of substance, which still reigns supreme: even a wave, or a quantum, is such a mathematical idea which, in reality, does not exist.

    Question: do you think there can be equal quanta?
     
  19. Sep 20, 2006 #18

    ZapperZ

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    Are you making this up as you go along?

    What is a "relative vacuum"? How is such a term defined in physics?

    Before you proceed any further, please keep two things in mind:

    1. EVERY single concept and terms in physics has a clear, unambiguous underlying mathematical description. This means that things in physics simply can't be mixed and match to our heart's content, because often, this will lead to absurdity. This is what you are trying to do. You are using terminologies that have clear definitions in physics, yet in ways in which they simply makes no sense.

    2. Re-read our PF Guidelines regarding speculative, personal theories. While you might think such speculative, unsupported statements are fine in philosophy, it is not tolerated here in the physics forums. Thus, unless you are able to cite for me established physics or peer-reviewed papers to support whatever it is you're trying to push, then you are involation of the Guidelines that you have explicitly agreed to. You have to submit your "ideas" to the IR forum to continue this line of discussioin.

    Zz.
     
  20. Oct 27, 2006 #19
    Maybe Sauwelios intended to ask if it's possible that light, that is an EM field, could change the EM properties of vacuum itself?
     
  21. Oct 27, 2006 #20

    pervect

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    Note that we have a philosophy forum. Generally speaking, philosophical questions about 'existence' will find a much better home there, than in the physics forum.

    As far as whether or not your statements are correct, first tell us if you exist, and if you think you do exist, why you think you exist :-). But please, do it in the philosophy forum, not in the physics forum.
     
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