Faster than light thought experiment

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  • #1
leebobandido
Hi :)

I just had a (most likely totaly unoriginal) thought and thought I should try to ask someone who's thinkings on physics are somewhat more refined than mine. So after thinking twice about private messaging Mr Tyson and Mr Cox :) I decided to google 'physics forum'.

So here I am with my stupid question for you to groan at.

As follows:

Lets say we had the technology to create stable structure of extraordinary length I.E a solid graphene arm of some descrption and then attached that to a small pivot, if the arm was long enough then quickly turning that pivot, lets say east to west would result in the far tip of the arm moving from point A east to point B west faster than that of light travelling between those points.

Thanks for reading.. tune in tomorrow for my clockwork electron hypothesis :P
 
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  • #2
Ibix
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A shockwave propagates along the rod at the speed of sound in the rod (much less than the speed of light, and cannot exceed the speed of light even in principle). Only when that shockwave reaches the other end of the rod does it start to move. And it necessarily moves slower than light. If naive calculation shows that the end of the rod would move faster than light then the rod would disintegrate due to centrifugal forces.

The atoms of the rod are held together by electromagnetic forces. Changes in such forces propagate at the speed of light - which is the underlying reason that the rod cannot move faster than light, no matter how rigid you make it.
tune in tomorrow for my clockwork electron hypothesis
Not sure if you're joking. We're happy to answer questions, but discussion of personal theories is against the rules here.
 
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  • #3
PeroK
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Hi :)

I just had a (most likely totaly unoriginal) thought and thought I should try to ask someone who's thinkings on physics are somewhat more refined than mine. So after thinking twice about private messaging Mr Tyson and Mr Cox :) I decided to google 'physics forum'.

So here I am with my stupid question for you to groan at.

As follows:

Lets say we had the technology to create stable structure of extraordinary length I.E a solid graphene arm of some descrption and then attached that to a small pivot, if the arm was long enough then quickly turning that pivot, lets say east to west would result in the far tip of the arm moving from point A east to point B west faster than that of light travelling between those points.

Thanks for reading.. tune in tomorrow for my clockwork electron hypothesis :P
This question comes up quite often. So, as you guessed, it's not original.

The first problem is that there is no such thing as a truly rigid structure. The end of the arm does not move as soon as the pivot acts at the centre. In fact, the motion propagates along the arm at the speed of sound in that material.

The other problem is that as the arm accelerates, the amount of energy needed increases without limit as any part of the arm approaches the speed of light. In many ways this is no different from linear motion: even with unlimited energy the speed of light can never be attained for a massive particle. Hence cannot be attained for any part of the rod.
 
  • #4
leebobandido
Thankyou both.

I very much appreciate the response and i'm happy to be educated :)
 
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