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Is Faster than Light travel impossible?

  1. Nov 24, 2009 #1
    Or is it just that it can't be observed? This is a question that has been on my mind for some time, so i thought it worth asking here.

    Imagine the following scenario: Two space craft travel towards each other. Each travels at 0.6C relative to their point of origin (the points of origin being stationary relative to each other). Now as they both approach each other at 0.6, observers at the points of origin will be able to observe the distance between the two craft closing at a rate of 1.2C, breaking the rule of not travelling at a speed greater than 1C.

    What am I missing here?

  2. jcsd
  3. Nov 24, 2009 #2


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    Objects CAN recede and approach eachother at a speed greater than the speed of light from a different rest frame (namely,the origin you speak of). However, SR tells us that in each of the spaceships frame, the other spacecraft is not moving at a speed greater than the speed of light towards it.
  4. Nov 24, 2009 #3


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    The rule is that information (eg. matter, a signal) cannot travel faster than light.
    It's possible for mathematical points to travel faster than light.
    The distance between the spaceships doesn't transfer any data, there are a couple of other examples such as a projected laser spot moving across the moon (lighthouse paradox) or even in theory the closing point of a pair of scissors.
  5. Nov 24, 2009 #4
    SR's velocity addition equation solves this.
  6. Nov 24, 2009 #5


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    SR velocity addition deals with what one ship's velocity will be in the other ship's inertial rest frame; in this case the answer is always less than c. However, the question was about how fast a third inertial observer who sees both ships moving at 0.6c would measure the distance between them to be shrinking, something known as the "closing velocity". Here the distance measured by the third observer would indeed be shrinking at 1.2c.
  7. Nov 25, 2009 #6
    Thanks for the replies. I find it fascinating that even though an outside observer would witness the two ships approaching at 1.2C, any observers on-board those vessels would be observing the other object approach at something less than 1C. I'm thinking that this must be explained by time running differently on the moving craft relative to the "stationary" observer.

    Can anyone point me to a good book / site that describes the theory of SR in laymen friendly language (I'm thinking something like "The Selfish Gene", but for SR). I'm not really interested in the mathematics of it yet, so something lite on the formulas would be good.

    Jimmy McGrath
    http://www.zizee.com" [Broken]
    Last edited by a moderator: May 4, 2017
  8. Nov 25, 2009 #7


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  9. Nov 25, 2009 #8


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    I like Martin Gardner's Relativity Simply Explained. Just be aware that it's a little out of date in some spots.
  10. Nov 25, 2009 #9
    So I watch some Discovery sci-flick and where they yap about the usual stuff - aliens and wormholes and stuff. But then it occurred to me that for all the science and relativity theoretical mumbo-jumbo I know I can't answer a very simple question. Then I come here and - lo and behold, there's already such a topic among the first ones in the forum.

    So - having in mind this post:
    I'll paraphrase it a bit: Is it possible for one and the same piece of information that is in one place to come to be in another place faster than it would take the speed of light to cover the distance between these 2 places?
  11. Nov 25, 2009 #10


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    hmmm... well, to give a straight answer: as far as I know, it is not possible for any piece of information that is in one place to come to be in another place in less time than the shortest possible time in which light in the one place could come to be in the other place. (I say "shortest possible" because there are sometimes different paths that light can take to get from point A to point B, and they may take different amounts of time)
  12. Nov 25, 2009 #11
    Was just wondering (and I think this links on to what 'matrix' was saying). What happens if you were to hypothetically have a rod, which is so long that each end of it is separated by say, the distance it would take light to travel in an hour.
    Now if you had two people on either end of the rod you could set up a system whereby one guy could prod the other guy in order to send some sort of message, Say they had a code that told them two prods = send me a light pulse, one prod = don't.
    Using this system is there any physical reason why you could not send a message (via prods) to the other person. The rod would only be moved a few feet at a time so no part of it would be exceeding the speed of light, however the message would surely reach the receiver before a beam of light. I know this would defy causality, but is there any physical reason why this would be impossible.

    -That was a lot longer than I hoped it'd be. Anyway, if anyone can be bothered to read all that (and if it makes sense), could you explain why it wouldn't work.
  13. Nov 25, 2009 #12


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    The "prods" travel through the rod at the speed of sound in the rod, so the messages are transmitted far slower than the speed of light.
  14. Nov 25, 2009 #13


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    No - thats the whole point of SR
    (except - there is some experimental evidence that's it's possible to transfer information a very short distance in some special quantum situations)

    That comes up regularly.
    The 'push' travels through the rod at the speed of sound (in the rod), which unless you have an infinitely stiff rod is a lot less than the speed of light.
    It feels instant because the speed of sound in a metal is a few km/s
    Last edited: Nov 25, 2009
  15. Nov 25, 2009 #14
    Ah right. Cheers, that makes a lot of sense. The questions been annoying me all day. I knew something had to be wrong with it, i just couldn't figure out what.
  16. Nov 25, 2009 #15
    Sure. Simply put, when the rod is "prodded" at one end, the other end doesn't instantaneously move. There is no perfectly rigid rod. The propagation speed of the prod from one end of the rod to the other is limited to the speed of sound in the rod.

    A perfectly rigid rod would be one that the speed of sound in the rod equals the speed of light, and all rods are less than perfectly rigid.
  17. Nov 25, 2009 #16


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    Here's a nice formula-lite online book written in a Q&A format:

    Last edited by a moderator: Apr 24, 2017
  18. Nov 25, 2009 #17
    This is not true.
    The SR velocity add equations deals with a third observer observering 2 relative moving frames limitied by the speed of light.

    SR velocity addition deals with what one ship's velocity will be in the other ship's inertial rest frame;
    This is LT.
  19. Nov 25, 2009 #18
    I am not sure that I fully understand this correctly but the theory of Special Relativity is derived from the Mitchelson Morley and similar experiments. An inherent assumption in the experiment is that light travelling within the apparatus can travel faster then light. Check out Einstei8n's 1905 paper. The experiment assumes that light moving towards the observer is travelling at (c + v) where c = the speed of light and v = the speed of the apparatus, and therefore the observer on the apparatus sees light traveling faster than the speed of light.

    Later, of course, it is concluded, from special Relativity, that nothing travels faster than the speed of light.

    How does an experiment which is based on the theory (and math) that light travelling faster than the speed of light can end up concluding nothing travels faster than light ?

    It makes no senese to me.
  20. Nov 25, 2009 #19


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    Not quite, Special Relativity is based on simply assuming that the principle of relativity and Maxwell are both right. The michelson-morely experiment is used in some books because it's an easy way to derive the lorentz-fitzgerald contraction ( ie. the famous sqrt(1-v^2/c^2) )

    The experiment was trying to determine if you could measure absolute motion, which would disprove relativity - Special relativity and length contraction explains the experiment but came 20years later
  21. Nov 25, 2009 #20
    Thanks for the reply, however I am still stuck at the earlier point - is faster than light travel possible. Doesn't (c + v) represent something faster than c ?
    If Special Relativity explains the experiment then Special Relativity must justify a situation in which a speed of (c + v) can exist.

    Does it matter whether the experiment explains SR or whether SR explains the experiment ? If either is the case then they should agree v - but they don't. In the experiment there is a velocity of light relative to the apparatus of greater than c.
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