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Light and the ether

  1. Apr 6, 2003 #1
    Hello everyone,
    I was thinking about the Doppler shift of light and I had a disturbing thought. It seems that the only way that light could maintain a constant speed and have shorter or longer wavelengths is if it is being carried by some kind of ether, which has long since been rejected. For example, if you're standing beside a train track on a windless day and a train is approaching, the sound from the train will have a higher pitch because the train is moving relative to you and to the stationary air(ether). The sound will have a higher frequency and shorter wavelength, but the two effects will cancel each other to produce a constant speed. It seems that if there is no ether, then the light will always have the same wavelength, but then the frequency would need to remain constant in order for the light to have a constant speed.
    Have I misunderstood something?
    Any thoughts on this anyone?
     
  2. jcsd
  3. Apr 6, 2003 #2
    To me, ether has both substance and preferential motion. Spacetime, which readily explains the redshift, has neither. Rather, it is a geometry which can stretch and deform according to mass and energy density, and is universal overall.

    O Great One
    Why would light always have to have to same wavelength without ether? (The analogy between light and sound goes only so far.) Spacetime, an intimate background, is defined in terms of light in vacuuo, of which constant light speed is standard. Stretch spacetime, wavelength increases, and vice versa. Gravitational accelerative redshift (like near a star) cannot be explained in terms of ether, eventually because of singularities justifiable by spacetime alone.
     
  4. Apr 6, 2003 #3
    The idea of the ether was never rejected, but it was Enstein that said what is the point of worring about it if you can never measure it.
     
  5. Apr 6, 2003 #4

    russ_watters

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    Yeah. Air is *NOT* ever stationary. The earth is rotating and revolving around the sun. The sun is orbiting the center of the galaxy, and the galaxy is itself moving. In truth, we have no idea exactly how fast that air is moving. And yet, our doppler shift equations still work. WHY? Frame of reference doesn't matter. Thats Special Relativity. The laws of physics are the same regardless of your frame of reference.

    The difference between your sound wave and the speed of light, is that the speed of light is a universal constant. So applying Special Relativity, that means that regardless of your inertial frame of reference, the speed of light is the same.

    And for both the light and the sound wave, since frame of reference is irrelevant, there is no need for aether. It just so happens that for sound, the medium is physically observable, so we use it. But for teh sake of doppler shift calculations in "still" air, it isn't needed.
     
  6. Apr 7, 2003 #5
    Yes, but can anyone explain why the speed of light is constant to observers travel at any speed, because I cannot imagine how that is possible.... wouldn't the idea of an "ether" be necessary?

    If someone shines light parallel to my motion and I travel at 50% of the speed of light, then I would still observe light at 300,000,000 m/s, doesn't that mean the speed of light is 300,000,000 m/s * 150%?

    Thank you very much for your help....:smile:
     
  7. Apr 7, 2003 #6

    russ_watters

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    Its the fact that the speed of light is constant to all observers that makes the ether unnecessary, not the other way around. The reason why is that time is variable. Every observer agrees on how fast light is traveling but not necessarily how long or how far. Essentally, the faster YOU are traveling, the slower time goes by for you.
     
  8. Apr 8, 2003 #7
    There have been 100s of books on the subject of relativity, I don't think you can explain it here.
     
  9. Apr 9, 2003 #8

    russ_watters

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    Staff: Mentor

    I can try ;) The basics of it really aren't that complicated.
     
  10. Apr 9, 2003 #9
    I meant that the subject is too indept for a blog
     
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