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Other Special Speeds? (than light)

  1. May 27, 2010 #1
    It seems to me that the speed of light and its constancy have very important implications in physics as we know it. Isn't light a disturbance in the electromagnetic field? If so, shouldn't there be special speeds related to the other fields, (namely the Gravitational field?)
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  3. May 27, 2010 #2


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    Disturbances in the gravitational field also propagate at the speed of light.
  4. May 27, 2010 #3


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    There can only be one invariant speed. The invariance of c results in it being the "speed limit" for the Universe. Speeds greater than c don't exist, so you can't have any invariant speed greater than it, and if you had an invariant speed less than c, then it would be the speed limit of the universe.
  5. May 27, 2010 #4
    "Speeds greater than c don't exist"

    Uh, how does that sit with the data transfer rate between entangled photons ? IIRC, ~10^6 c over ~kilometre separation, though an upper limit was not established due to limits on the timing.
  6. May 27, 2010 #5
  7. May 27, 2010 #6
    It seems to me that JonnyB42 deserves better answers than the kneejerking so far, for he has asked a profound question.

    Yes special speeds abound in Physics.
    A simple example would be critical velocity in fluid mechanics, but I'm sure many could present better examples.

    Of course the exact nature and implications of each special speed will depend upon its field of application so will affect more phenomena than others.
  8. May 27, 2010 #7
    If you want to get more profound, try http://arxiv.org/abs/physics/0302045" [Broken]. You will see that symmetry and reciprocity leads to a single "invariant" speed, which happens to be a maximum speed.

    The real question is why does anything travel slower?
    Last edited by a moderator: May 4, 2017
  9. May 27, 2010 #8

    Andy Resnick

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    Sure- the speed of sound is another example: the speed at which elastic disturbances propagate. Shock waves are what happens when that limit is exceeded.
  10. May 29, 2010 #9
    Hey, thanks. This is just what I've been looking for. I posted a similiar question last year. Good things come to those who wait!

    Last edited by a moderator: May 4, 2017
  11. May 29, 2010 #10
    It's likely instantaneous. It has little to do with speed and travel, and more to do with the fact that the assumption that for two things to influence each other, they must be adjacent.

    Essentially it can be looked at as evidence for the nonfundamentality of the concepts of space, rather than speed.
  12. May 29, 2010 #11


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    I heard that the only thing faster than light was human thought, but surely this can't be true? Or maybe it "takes time" to think of something. I don't know how quick that is...
  13. May 29, 2010 #12


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    I heard that the only thing faster than light is bad news. Someone actually built a spaceship that was powered by bad news, but they weren't welcome anywhere.

    Seriously though, the actual thought doesn't have a position, so you can't associate a speed with it. The place where the thought occurs is just your brain, and it's not moving very fast, certainly not faster than c.

    Edit: Some of you might be interested in my thread here.
    Last edited: May 29, 2010
  14. May 29, 2010 #13
    It seems we can respond to a question on several different levels.

    Of global significance, I've included zero speed in the set; {0,c}. I'm hindsight, I'm not sure there's any justification for it, Is there?

    Any candidate speed, other than c itself, is not invariant under a Lorentz transform.
  15. May 29, 2010 #14
    Why so?

    Surely any limit that depends upon the ratios of the velocities of two particles in the same system will be invariant?
  16. May 29, 2010 #15
    I'm not sure what you mean. Say we had some force that was mediated by a particle having a velocity u. Under a Lorentz boost v,
    u' = (u-v)/(1-uv/c2).​
    I'm sure there are characteristic velocities in particular media, if that's what you mean.

    Under the more general question "Are there any special speeds other than 0 and c?", I can't think of any. If we posit additional, rolled-up dimensions, you could imagine a difference characteristic speed, b not equal to c, for these dimensions, but b/c may just be an immeasurable scale factor.

    We might consider the magnitude of the 4-velocity to widen the search.
  17. May 29, 2010 #16


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    Off-topic, but the speed of thought is in fact quite measurable.

    Broadly speaking, the brain takes about a fifth of a second to act in habitual fashion and around half a second to react in attentive fashion. The distances that neural signals have to travel explain why one is quicker than the other.

    And indeed the very question that got psychology started (Helmholtz, Wundt, Donders, etc) was about the speed of various mental processes, following measurements of nerve conduction speeds in frog's legs.
  18. May 30, 2010 #17
    Imagine a substantial disk of elastic material.

    Place stations, A and B, on opposite edges of the disk and let the stations interchange elastic waves through the material of the disk.

    Now imagine an observer, O, to be moving at a velocity v, a substantial portion of c, along the axis of the disk.
    What velocity would this observer make for the speed of the elastic waves between A and B?

    What would this observer now make of the critical velocity of a third station, D, moving from A to B in relation to the elastic waves?
  19. May 30, 2010 #18
    Zero is not special. Only c is special, being invariant for all observers. Zero, like 10mph, is relative to your reference frame.

  20. May 30, 2010 #19
    There are quantites associated bodies such as mass, energy, length, and time interval that are minimal or maximal when the relative velocity is zero.
  21. May 30, 2010 #20
    Saying there are no speeds greater than c may not be accurate.

    Tachyons are a hypothetical subatomic particle that travels faster than the speed of light.
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