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Can you actually turn if you are travelling at c?

  1. Aug 5, 2011 #1
    It just occured to me, can you turn if you travelling at the speed of light? I thought it was always a constant and constant speed = no acceleration??

    If it can, does that mean it will take infinite energy to turn?
  2. jcsd
  3. Aug 5, 2011 #2


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    Not to sound rude in any way but it is useless to ask what will happen at the speed of light to a massive particle because in the context of SR and GR massive particles cannot reach that speed. However, photons do get bent in curved space - times so they do in effect "turn" to put it loosely. There is no speed change though because the photon, traveling on some geodesic in asymptotically flat space - time simply starts following the geodesics of the curved space - time and so remains in free fall.
    Last edited: Aug 5, 2011
  4. Aug 5, 2011 #3
    No! An accelerating body can have a constant speed (think of the Earth accelerating around the sun). Constant velocity = no acceleration.
  5. Aug 5, 2011 #4
    Well...This kinda bugs me. The earth does not have a constant speed. If it did it would not be accelerating by definition. Dimensions are orthogonal and should be treated as such. If you are measuring the earth in any given dimension you will see that the speed is constantly changing.
  6. Aug 5, 2011 #5


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    You seem to be confusing "speed" and "velocity". Velocity is a vector- it has both magnitude and direction. Speed is only a number- the magnitude of the velocity vector.

    No, the earth does not have constant speed or velocity. But an object moving in a circle, at constant speed, has acceleration- its direction is changing. In fact, one can show that, in such a case, the acceleration vector points toward the center of the circle.

    Light, while moving at a constant speed does "accelerate"- its direction of motion changes. The fact that the path of light curves due to the gravitational pull of bodies it passes was an early bit of evidence for general relativity.
  7. Aug 5, 2011 #6
    I don't think of it that way. If I turn in my car, I am slowing down in the direction that i was travelling, so I feel a force in that direction. I understand your reasoning though it seems more complex than it needs to be. To each his own, both views are right but mine explains WHY changing direction is acceleration, rather than trying to say that there are two forms of acceleration.
  8. Aug 5, 2011 #7
    yeah I don't know why I use "speed", but I didn't care about it when since we were talking about c.

    What if you collide with something moving at c? Forcing it to "turn". can that happen?
  9. Aug 5, 2011 #8
    Good question! For many years, physicists thought that light could only be curved by gravity. However, in 1989 theorists at Stanford proposed that it may be possible to build a device that could change the direction of photons under laboratory conditions. After years of work, last month a group at CERN demonstrated a working prototype. More information can be found http://en.wikipedia.org/wiki/Mirror" [Broken]

    A spokesman for the mysteriously well-groomed team of physicists and engineers has stated that while their device is a great step forward for pure science, technological applications are probably decades away.
    Last edited by a moderator: May 5, 2017
  10. Aug 5, 2011 #9
    I hate you lol
    Last edited by a moderator: May 5, 2017
  11. Aug 5, 2011 #10
    Probably you know that matter cannot travel at the speed of light. And constant speed is not no acceleration. As others already explained, an object can move in a circle at constant speed.

    It can also be confusing that also "velocity" is still used for "speed", as many old texts do.

    However, there is an interesting point that you raised: relativity theory models light propagation as the propagation of a wave. As a consequence of the Huygens principle,
    http://en.wikipedia.org/wiki/Huygens-Fresnel_principle ,
    according to special relativity a light ray in vacuum cannot bend.
    But according to general relativity, a light ray can bend due to a speed gradient near matter (which made Einstein famous).
    - http://www.alberteinstein.info/gallery/gtext3.html [Broken]

    Last edited by a moderator: May 5, 2017
  12. Aug 5, 2011 #11


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    LOL. Dude I broke both my ribs laughing at that. Classic material right there brah.
    Last edited by a moderator: May 5, 2017
  13. Aug 5, 2011 #12
    Last edited by a moderator: May 5, 2017
  14. Aug 5, 2011 #13
    It is a scam. They are producing a new light ray via conductors that are excited by the incoming one.
    Last edited by a moderator: May 5, 2017
  15. Aug 5, 2011 #14
    Dang. Knew it was too good to be true. Throw this in the pile with perpetual motion and cold fusion, I guess.
  16. Aug 5, 2011 #15
  17. Aug 5, 2011 #16
    More specifically, the bending of light in GR is exactly twice what is predicted in Newtonian gravity. That's the confirmation, not the mere fact of bending.
  18. Aug 5, 2011 #17
    You're basically defining "speed" to mean what the rest of the world knows as "velocity." So, what word have you invented to mean "the magnitude of speed (your definition of speed)," meaning without direction?
  19. Aug 5, 2011 #18
    I am saying that the act of turning is to slow down in one direction and speed up in another direction. But your still slowing down, changing your distance over time, this is why you feel a force not because there is some magical effect to turning.
  20. Aug 6, 2011 #19
    Then, if you drive your car around a bend on the highway without slowing down (you keep it at a constant speed of 100 km/h), you confront yourself with a paradox. :tongue2:
  21. Aug 6, 2011 #20


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    It's not really a question of "how you choose to look at things," but rather a question of definitions. Speed is defined as the magnitude of your velocity vector. And the *magnitude* of your velocity vector never changes during uniform circular motion. Only its direction does. You feel a force because although your inertia wants to keep you moving in a straight line tangential to the circle at that point (i.e. towards the outside of the turn) the car door or the seat or something stops you and applies the centripetal force needed to keep you moving on the same curved path as the vehicle.
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