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I Is light speed limit an artifact of the math of measurement

  1. Mar 4, 2017 #1
    A delightful video here
    A lecturer derives special relativity in a world of bats and echo location using a bat (sound) clock.
    All communication is limited by the speed of sound. (so a parallel of our world and light speed communication)
    He ends up observing that in no cases for any observer can you ever measure a bat travelling at more than the speed of sound ...but obviously in reality a bat could at least theoretically be super sonic.
    Question I have then is if the parallel derivation or relativity is fair where does that leave our understanding of the universal limit of light speed ...seems at first glance to suggest that light speed limit is not a real physical limit at all.
    In this view the reason we can't make protons go faster than light in synchrotrons for instance is not because they can't but because we can't calculate how to do so ..or even if we guessed the magnets timing correctly to get the photon at 1.5c we could neither detect nor confirm it did so. of course that's different isn't it from what I understand Einstein's physics where C is a real - actual limit. e.g The infinite energy requirement to get to C then is our maths going haywire. Or is it? Thanks for any responses.
     
  2. jcsd
  3. Mar 4, 2017 #2

    phinds

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    No, it is utter nonsense
     
  4. Mar 4, 2017 #3
    Sorry that doesn't help much - what is 'utter nonsense'? the use of an imagined world limited by the speed of sound, his derivation of special relativity or the inference I seem to be able to make about it?
     
  5. Mar 4, 2017 #4
    Light doesn't behave like sound: sound is carried by the air, and the speed of sound you measure will depend on whether you are moving towards or away from the source of the sound--or, more generally, if you are moving though the air that is carrying the sound. (And of course it is possible to travel faster than sound.) But every non-accelerating observer will always measure the same value for the speed of light, no matter where it come from or how they are moving. In this respect light is completely different from anything in our daily experience, but it's a measured and confirmed fact.

    This behaviour also leads to lots of measurable consequences, notably that it gets increasingly hard to accelerate an object the closer it gets to the speed of light. (The theory implies that it would take an infinite amount energy to actually accelerate anything to the speed of light. ) This behaviour and plenty of similar consequences have been precisely confirmed--for example in cyclotrons and other particle accelerators that have been used for decades: the predictions of relativity have to be built into their designs, or they wouldn't work. The actual energies of very high speed particles are among the predictions that are routinely confirmed.
     
  6. Mar 4, 2017 #5

    Dale

    Staff: Mentor

    That is a misunderstanding I also had when I first encountered relativity. It turns out that the key point of the derivation is not that c is the limiting speed of communication or information. The key point of the derivation is that c is invariant. The speed of sound is not, so careful measurements by bats without light would still lead to the same Lorentz transform with the same c.
     
  7. Mar 4, 2017 #6
    I have seen a similar derivation before - it is essentially a derivation of Lorentz Ether Theory (LET), and I think the maths is sound.
    I don't think your conclusion is correct. At the end of the video he is not suggesting that a bat might fly faster than sound - he is saying that if bat A hears bat B travelling at half the speed of sound and B hears C going at half the speed of sound the A will hear C at 80% of the speed of sound, which is exactly the same calculation as in SR.
     
  8. Mar 4, 2017 #7

    phinds

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    Sorry about that. I was in a hurry when I replied, getting ready to SKYPE with a relative. Glad to see others have jumped in to give you more information than my brusk reply.
     
  9. Mar 4, 2017 #8
    Thanks Dale - "The key point of the derivation is that c is invariant" ..gee I thought he had attended that but I could be wrong and too with respect to what Charles has said I'm having a re-watch. And thanks Charles.
     
  10. Mar 4, 2017 #9
    Charles he is saying a bat may travel faster than sound - but the ...constriction of the maths on the measurement of that speed will never produce a result greater than the speed of sound. So he is saying - bat A hears bat B travelling away at say 0.7 of the speed of sound - and B hears C going same direction away at 0.7 of the speed of sound nevertheless A will detect C travelling away at .93.
    Reality has it that we know speed of bat C wrt to A is travelling 1.4 times the speed of sound.
    Isn't that a mathematical and or measurement artefact rather than an actual limit on relative bat speed?
    And regards Dale's point I guess he is not claiming the speed of sound is invariant with respect to any bat but he is implying it - calculating as if it is ...isn't he? ...or if not how does a 1d special relativity calculation vary from that shown?
    So he derives length contraction - time dilation - and he didn't derive the increase to infinite mass as one approaches 'c' ..but isn't that a deduced calculation from time and observed speed - that is if ones calculation will never provide a figure of 1 or greater for speed - it's inevitable that imagining an infinite energy as required to get to 1.
    Look I'm sure half of you throw your hands up ..not again ...and as advised all of this is experimentally confirmed and has been for decades - but I still see relative bats travelling at greater than the speed of sound despite the maths never able to confirm it.
    Help me out if you think I'm redeemable ..or just able in any case thanks for your time. :)
     
  11. Mar 4, 2017 #10

    Dale

    Staff: Mentor

    I am sorry, I got just a few minutes in and then realized how long the video is. I hadn't the patience to watch. But you cannot derive relativity without the second postulate.
     
  12. Mar 5, 2017 #11
    A couple of thoughts after having watched the video:

    He's a good lecturer. He doesn't actually claim he's doing more than providing an instructive parallel to Einstein's theory. He demonstrates some of the "geometrical" features like time dilation and the combination of velocities.

    But he doesn't address the invariance of the speed of light or relativistic accounts of momentum and energy (there's a rather well-known equation missing from his talk).
     
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