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Matter breaking the speed of light? [Nope]

  1. Sep 23, 2014 #1
    • moved from general physics
    after watching minutephysics video on breaking the speed of light with a laser pointer (kinda a cheat though, as you are not actually making anything move faster than light.)


    I came to wonder, what if instead of using beam a light and the image it creates to appear faster. you instead used an incredibly long, 99.999% rigid stick and swept it across the vacuum of outer space, would the end of stick travel fast enough to break the speed of light just as the photons from the laser sweep across the moon faster than light?
     
  2. jcsd
  3. Sep 23, 2014 #2
    See the following entry in the Relativity FAQ:
    https://www.physicsforums.com/threads/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod.536289/ [Broken]
     
    Last edited by a moderator: May 7, 2017
  4. Sep 23, 2014 #3
    yes i have read similar article to that one, but i am not looking for the speed at which a signal is sent (i understand the whole compression wave, speed of sound in an object deal), but more like how the surface speed on a rotating object is faster than the cores rotation speed (ex. a tire or a lathe, or more closely related, a baseball bat). i understand that the signal for the end of the object to begin to move would only arrive there after the compresion wave reaches the end, but when it does get to the end of the stick, wouldnt it snap through the sky at an extremely high speed?
     
    Last edited by a moderator: May 7, 2017
  5. Sep 23, 2014 #4
    No.
    An object with mass traveling at the speed of light would have infinite kinetic energy. It doesn't matter how long the rod or how stiff, if a part of that rod is to travel at the speed of light you will have to expend an infinite amount of energy to accelerate it to that speed. In your scenario that would mean applying an infinite torque to the end of the rod you are holding.
     
  6. Sep 23, 2014 #5
    eh, im pretty strong, i could do it. lol ok thanks, this helped answer a question that had me puzzled for a while now. good work.
     
  7. Sep 26, 2014 #6
    The video is wrong why would the laser pointed at the moon move across it faster than light.Each new position would require a photon travelling at light speed from the laser, to project itself onto the moon, therefore a time lag which would be the speed of light exists between each new position.
    Those pixells going on and off on a screen have nothing to do with pointing a laser at the moon and moveing it across it's surface from a distance.
    All It shows is that light's can illuminate in different parts of space with little time between the event's which is a different phenomina.
     
  8. Sep 26, 2014 #7

    A.T.

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    What do you mean here? Time and speed are different quantities.

    And that there is no lower bound on that time between those events. The time lag between those events has nothing to do with the speed of light, just with how fast you rotate the laser.
     
  9. Sep 27, 2014 #8
    The time would be equal to the speed it takes for each individual photon to arrive at the moon.They don't all arrive at once there is a time interval between each photon which is light speed.So the "lag" is the time between the first photon and the second photon illuminating the moon once you change it's position.

    Exactly the time lag between those events has nothing to do with the speed of light.
    Rotateing the laser won't produce that effect though.The beam won't appear to travell faster than light across the moon.
     
  10. Sep 27, 2014 #9

    Doc Al

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    Sure it takes time for a photon to travel from the laser to the moon. But that has nothing to do with the time between the first and second photons hitting the moon. Try this analogy: Imagine a rifle marksman shooting two bullets at a distant target. Imagine it takes 60 seconds for the bullets to reach the target. He fires two shots, one second apart. When the bullets reach the target, will they strike one second apart or 60 seconds apart?

    That much is true. The time between successive photons has nothing to do with the speed of light.

    Sure it will.
     
  11. Sep 27, 2014 #10

    PeroK

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    You can simplify this by imagining a series of lights being illuminated in sequence. This creates the illusion of motion. The less time there is between adjacent lights being lit, the faster the motion appears to be. Clearly you can make this illusory motion arbitrarily fast.

    In fact, if you then illuminate all the lights simultaneously, you may appear to see a signal travel instantaneously.
     
  12. Sep 27, 2014 #11

    A.T.

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    You seem to have the bizarre idea that the laser can only fire the 2nd photon, once the first photon has arrived at the moon. This is not the case.

    That is just gibberish. Light speed is not a time interval. They have different dimensionalities.

    Yes it will.
     
  13. Sep 27, 2014 #12

    Borek

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    Not "the beam" - whatever it means - but "the point in which beam hits the surface".
     
  14. Sep 27, 2014 #13

    Dale

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    No. The stick would just break.

    However, you could in principle build a machine gun firing bullets, each of which travels at .01 c, and arrange the line of impacts to move faster than c.
     
  15. Sep 27, 2014 #14

    A.T.

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    This and similar frequently asked question are answered here:

     
  16. Sep 27, 2014 #15
    The bullets will strike one second apart.

    I don't quite see how it does not the photons dont all arrive at the target at the same time.
    The second one won't arrive before the first it has a maximum speed which cannot be broken so the distance between the two photons is that speed.

    Not convinced it will.
     
  17. Sep 27, 2014 #16

    Doc Al

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    Good! Now apply that same reasoning to the photons striking the moon.

    Sorry, I cannot understand that statement. Distance is not speed. All photons have the same speed.
     
  18. Sep 27, 2014 #17
    I don't have the bizarre idea that the laser can only fire the second photon , once the first photon has arrived at the moon all I am saying is that there is a difference between each photon .You however seem to wan't to convince me that there is no time interval between photons and they all arrive together at the same instant.
     
  19. Sep 27, 2014 #18
    I agree all photons have the same speed but they are not all manufactured by the laser at the same time, so there must be an interval between each one produced, which is light speed.
     
  20. Sep 27, 2014 #19

    Doc Al

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    Right, just like the bullets in my rifle marksman analogy.

    Of course.

    Two points: One, "light speed" is not an interval. Two, the interval has nothing to do with the speed of light. Just like with the bullets--the interval between bullets had nothing to do with the speed of the bullets.
     
  21. Sep 27, 2014 #20

    phinds

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    Why do you keep equating an interval, a measure of time, with light-speed, a velocity? They just aren't the same thing, as has been pointed out to you a couple of time (and now once more).
     
  22. Sep 28, 2014 #21

    A.T.

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    The point is, that there is no lower bound on this time difference, so the the spot on the moon can change it's position arbitrarily fast. The change of the spots position is not subject to any speed limit.
     
  23. Sep 28, 2014 #22
    The interval does have something to do with the speed of light.
    Before a new laser position can be seen on the moon "the dot moves"your hand would have to change it's position.
    To do this you first move your hand then a new photon from the laser in the new position would have to travell the 240,000 miles at light speed from the earth to the moon before it hits the surface, where it could be seen in a different location.
    Just like the bullet it will not hit the target unless it's pointed in that direction each new target requires a new position and a new bullet from the fireing point or it won't have the right direction.
    The time it takes for the photon from the laser in your hand to the moon to register each new position is slower than the time it would take for light to travell from A to B on the surface of the moon.
     
  24. Sep 28, 2014 #23

    A.T.

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    The time interval the spot needs to cross the moon has nothing to do with the time interval the light needs to get to the moon, and hence nothing to do with the speed of light.

    It can be less time, or more time, depending on how far apart A and B are. In other words, those two intervals have noting to do with each other.
     
  25. Sep 28, 2014 #24
    I assumed that A and B would be the distance from one side of the moon say the left extreme to the right..
    Something like the co-ordinates that you would use in shineing your laser across the moons face.
     
  26. Sep 28, 2014 #25

    A.T.

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    Fine. The time in which the light spot moves from A to B still has nothing to do with the time in which the light moves from Earth to Moon. It could be more time or less time, depending on how quickly you rotate the laser.
     
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