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Why stop at speed of light?

  1. Jul 10, 2005 #1
    Why can nothing(in theory) go faster that light.

    what is stoping it and what governs the speed of light itself?

    ????
     
  2. jcsd
  3. Jul 10, 2005 #2

    James R

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    It is not true that nothing can go faster than light. For example, if you point a flashlight at the moon, then you can sweep the beam across the moon's surface in a fraction of a second. Work out the speed of the spot from the flashlight as it crosses the moon's surface, and you'll get an answer greater than the speed of light.

    What is true is that that no material object (with mass) can ever be accelerated to the speed of light. (The spot on the moon is not a material object.)

    The reason you can't accelerate an object to the speed of light is that you'd need to provide an infinite amount of energy to do that, and an infinite energy source isn't available.
     
  4. Jul 11, 2005 #3

    pervect

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    The speed of light appears to be a universal speed limit. Hypothetical particles called "tachyons" have been proposed which always go faster than light, and could never go slower than light, but have never been observed.

    As far as "what is stopping it" goes, if you add together a velocity of .99*c to itself 100 times, you still get a velocity that's below the speed of light. This happens because of the relativistic velocity addition formula.

    v1+v2 = v1+v2/(1+v1*v2/c^2)

    you can add together ANY number of velocities lower than 'c', and the result will still be lower than 'c'.
     
  5. Jul 11, 2005 #4

    Pengwuino

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    That is incorrect. Your transmitting a beam of photons at the moon. When you turn the flashlight, you are not moving anything at the speed of light, your simply re-directing the photons. A photon that just hit hte moon does not leave the moon simply because you change the direction of the flashlight after that.
     
  6. Jul 11, 2005 #5

    James R

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    Pengwuino,

    You misunderstand.

    If you were standing on the moon's surface, and I was standing on the opposite side of the moon, then you would see a spot of light appear to move from you to me across the moon's surface in a certain amount of time. Calculate the distance divided by the time it takes the spot to move, and you'll get an answer greater than the speed of light.

    I know that the same photon doesn't travel from you to me, but that's not what I'm talking about. I have demonstrated that something can move faster than the speed of light.
     
  7. Jul 11, 2005 #6
    Logically , for cause and effect to be seen in perfect order , the speed of light needs to be less than c.

    I would like to ask you 'What would be the the effect if the speed limit was 2c?"

    BJ
     
  8. Jul 11, 2005 #7

    Tom Mattson

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    That I can't agree with. It isn't simply a matter of logic, because the Galileo vs Einstein question cannot be decided by logic alone. The matter must be (and has been) decided by experiment.

    In that case the laws of electrodynamics could not be the same for all inertial observers.
     
  9. Jul 11, 2005 #8

    Pengwuino

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    This argument has been brought up before and no THING has moved faster then the speed of light. A perception has moved... but no THING has moved. No THING has been displaced at >c.
     
  10. Jul 11, 2005 #9

    Pengwuino

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    Actually it is logical... but its the NEW logic... or "improved" logic. Bring in the idea that time is a dimension, and all of a sudden its logical that nothing can go beyond c. Train a child that there are 4 usable dimensions and logic now dictates that nothing can go beyond C. Now... this is probably symantecs but i sure am in a typing mood today.

    Well since c is the speed limit of light, its impossible to say "what if its 2c" since in this new world, our 2c woudl be the new c at 600,000km/sec
     
  11. Jul 11, 2005 #10

    Integral

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    The universe has a nasty habit of doing things, then failing to tell us why. We can only observe and make predictions based on those observations.
     
  12. Jul 11, 2005 #11

    Pengwuino

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    We should blow up one planet for every problem we cant understand. The universe deserves it for not being nice to us.
     
  13. Jul 11, 2005 #12

    Gokul43201

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    Well, no massive "thing" at least. And James never claimed that a "massive thing" was made to move faster than c; in fact he specifically disclaimed this.

    Another simple example is the intersection of a pair of lines. This too, is a thing, but not a massive thing (even though the "lines" may be).

    To the OP : While SR explains (as James did) why it is not possible to accelerate something up to c, it does not insist that there not be things traveling at speeds greater than c. That, however, violates causality.
     
  14. Jul 11, 2005 #13
    If things could travel at a speed greater than light , we could 'see' the effect before the 'cause' . This is one of the basic things that I would like to introduce to 'Gaijin' . 'Perceiving' the images of an event is a surety that it has happened , we get these images in an ordered manner , that is from cause to effect . But objects exceeding the speed of light would reverse the order of images certainly.In that case we would have the subjects 'Future' instead of 'History' and History would have been unsure of. :uhh:

    Any spacetime point can be taken to be surrounded by spacetime region divided into three regions , two timelike and one space-like, this spacetime point can be affected in several ways . The same point in past-time coordinates would effect the point in present , and this point (at origin ) can affect the future, example it can shoot a bullet , which will hit the wall in the future . Future is always affected by past and present , Have you ever wondered why future cannot affect your present?? Gaijin?? :rolleyes: .... This is where the limits end.This is where the limit 'c' comes into play.
     
  15. Jul 11, 2005 #14
    I think there you're turning things upside down. As far as I know, Einstein started from the idea, that the speed of light is the same everywhere (in every inertial system) and thereby got to the conclusions that are now so popular: e=mc^2, length contraction, time dilatation...

    Your "explanation", that there would be needed an infinite amount of energy to accelerate an object to the speed of light, is no reason, it simply follows from the theory Einstein developed (and that was found to be correct by experiments).

    So, I would say that the best argument for the speed limitation by c is what Dr.Brain mentioned writing about cause and effect. This isn't so obvious, considering quantum theory, for example, where deterministic reasoning simply doesn't work anymore.

    @Pengwuino: Could you explain, what exactly you mean, by saying "logical"? If you feed a child the necessary information (that is: you keep on telling that c is constant), it will surely find it the most natural thing, just like religious people, that are grown up with their religion, find it to be obvious. But there's no logic in it, as far as I get your idea.

    greetings...Cliowa
     
  16. Jul 11, 2005 #15

    Pengwuino

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    No no i mean like... ok, bring yourself into before you were taught SR. You thought what was logical in your mind is that you can go as fast as you want and that time isnt a "dimension". That was your logic because all you were taught before learning SR is that you just add speeds and such and that time cant really be screwed around with.

    If you were being taught abotu velocities for the first time and were also taught about special relativity at that time, your "logic" would dictate that time is something that can be messed with like it is in SR and that you cant just constantly add up velocities.

    Its symantics I think because to me logic = obeys laws of physics. Since all the universe is is a bunch of laws of physics, nothing can be illogical to me. Well... ok, humans can be illogical for whatever reason they are but the rest of the world isnt :) hehehe. But then again on the more detailed level, whatever makes people do "illogical" things has a logical sequence of neurons firing and brain things doing this and that and im just still typing for no reason and i dont even know what im talking about anymore.... i think im gonna eat this taco now.
     
  17. Jul 11, 2005 #16

    Pengwuino

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    No, he said a light beam can move faster then c. But what is a light beam? A lot of photons. The photons dont move though lateraly (towards the other side of the moon) but the perceived "beam" does. The "beam" isnt a solid object however. Its like firing a stream of water at a house and then moving to fire it at a house 400 light years away. No molecule of water actually moved faster then allowed but the perceived "stream" moved.
     
  18. Jul 11, 2005 #17
    Well, the thing is, that the "old view" perfectly fits our experience! It is absolutely counter-intuitive that the Doppler effect at some point just won't "happen" anymore! Probably you're (at least partly) correct in calling the humans illogical and the universe logical, but i wouldn't call things this way. Because the physics theories are man-made, describing what is happening, but not why it is happening that way, I would take as the base the human being. Logic means getting from an idea to the conclusion in reasonable, rational steps. So it is pretty much our definition (what is rational). I would therefore take the human arguing and reasoning as being logic and the rest as not. But, as you see, I'm not really getting anywhere: it's a pure matter of definition (this is not to say it's trivial).

    What a pity I don't have a taco around.
     
  19. Jul 11, 2005 #18

    krab

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    ...but apparently, not in a spelling mood. "symantec" is a computer software company.
     
  20. Jul 11, 2005 #19

    Pengwuino

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    Well my other favorite forum is a computer hardware forum so I'm just thankful there were no famous physicists named Microcroft.
     
  21. Jul 11, 2005 #20

    Gokul43201

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    Saying the speed of light is frame-independent is not equivalent to saying that c is an upper limit (without the framework of SR). In other words, the "upperlimitness of c" is not a postulate of SR, it is a result.

    All explanations of physical phenomena must follow from some theory. Please explain anything to me without using the results of some theory.

    Causality alone (without SR) is not sufficient to make speeds greater than c impossible. It is the framework of SR that tells you that v > c violates causality. In Newtonian mechanics, for instance (with Galilean transforms), traveling faster than c would not violate causality.

    But one must note the important distinction between traveling faster than c and accelerating past c. The latter is impossible, according to SR, because it violates energy conservation (as James explained). The former is impossible because, according to SR, it would violate causality.
     
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