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In this scenario is faster then light speed possible?

  1. Jul 1, 2008 #1
    Take this scenario for instance. Now from the position of the Earth if you were to lookup into the night sky the distance between stars can seem like mere inches, but in actuality it could be 100’s of light years between the two. Now, for this scenario you are a person standing on the Earth looking up into the night sky at two stars. From your perspective the distance between these two starts is only a few inches. Yet, in actuality these stars are hundreds of light years away, and hundreds or light years apart from one another. Now, in your hand you hold an unbreakable and unbendable rod that stretches from Earth hundreds of light years all the way to one of these two stars. Now, you have this unbendable, unbreakable rod in your hand and you are pointing it at one of these two stars, and then you take the rod and quickly point it at the other star. Now, here on Earth your hand only moved a few inches in a few seconds from pointing at one star to pointing at the next. Yet, the end of the rod that was at the first star you were pointing at it would have to move faster then the speed of light to travel the hundreds of light years from the first star to the second star to keep up with the part of the rod in your hand. Now, remember this is an unbendable and unbreakable rod. So, what would happen?
  2. jcsd
  3. Jul 1, 2008 #2


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    The simple answer is that perfectly rigid objects of the type you describe are impossible in relativity. Any change in velocity of one end of the rod will not influence the motion of the other end until a wave traveling at the speed of sound in the material has had time to move from one end to the other (and the speed of sound in any physically possible material is always less than the speed of light in a vacuum). See here for a little more info.
  4. Jul 1, 2008 #3
    Ok. So, if we were to remove the unbendable part out of this scenario what would happen? Would the rod simply curve from my hand through space until it hit the speed of light and stopped bending, and the rest of the rod would simply travel at the speed of light? Also, since as you progress closer and closer to the speed of light you begin to infinitely stretch. So, would the rod at the far end become infinatly wider then the section of the rod in your hand? Would the rod not only curve but also progressively widen as it curved?
  5. Jul 1, 2008 #4
    Since both rods are rotating the changes in direction inside the rod will be propagated at the speed of sound.
  6. Jul 1, 2008 #5
    The rod would bend. What you are describing is impossible.
  7. Jul 1, 2008 #6


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    Two threads on the same topic merged.
  8. Jul 1, 2008 #7
    Here's a quick rule of thumb for all FTL questions.

    In any scenario involving a massive object with ordinary (real, positive) mass, relative velocity at or above the speed of light is impossible.

    The only theoretical means of travelling _apparently_ faster than light involve warping spacetime, but even then the local velocity is always sublight (and the energy requirements are far too massive to be practical, leaving aside causality paradoxes).
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