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Thread: information transfer

  1. #1 information transfer 
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    Suppose I have an infinitely taut wire in my hand, and my friend is holding the other end of the wire at the opposite side of the room. I pull on the wire. How fast does the "information" that I pulled on the wire travel from my hand to my friend's hand?

    You could imagine doing the same experiment across a distance of lightyears. How fast would the information travel over a distance like that? My intuition says that as soon as I pull on one end of the wire, the wire is "instantly" felt moving at the other end. Obviously this can't be true because that would mean information is traveling faster than the speed of light.

    I'm no expert in physics (I'm a code-monkey by trade): what am I missing here? My best guess is that when I pull on the wire, the other end of the wire only moves because my pull gets propagated through every individual chemical bond along the wire, and over a distance of a light-year it would have to take at least a lightyear for my pull to propagate.

    If the above is the reason, then would I be able to get "instantaneous" information transfer if my wire was somehow able to be composed of only one particle? Of course I know there's no such thing as a particle that could stretch over a distance of lightyears, but if that's theoretically possible then information transfer faster than the speed of light is also possible over distances as large as the widest existing particle. Shoot me down, please!


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  3. #2 Re: information transfer 
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    Quote Originally Posted by hivebright
    Suppose I have an infinitely taut wire in my hand, and my friend is holding the other end of the wire at the opposite side of the room. I pull on the wire. How fast does the "information" that I pulled on the wire travel from my hand to my friend's hand?

    You could imagine doing the same experiment across a distance of lightyears. How fast would the information travel over a distance like that? My intuition says that as soon as I pull on one end of the wire, the wire is "instantly" felt moving at the other end. Obviously this can't be true because that would mean information is traveling faster than the speed of light.

    I'm no expert in physics (I'm a code-monkey by trade): what am I missing here? My best guess is that when I pull on the wire, the other end of the wire only moves because my pull gets propagated through every individual chemical bond along the wire, and over a distance of a light-year it would have to take at least a lightyear for my pull to propagate.

    If the above is the reason, then would I be able to get "instantaneous" information transfer if my wire was somehow able to be composed of only one particle? Of course I know there's no such thing as a particle that could stretch over a distance of lightyears, but if that's theoretically possible then information transfer faster than the speed of light is also possible over distances as large as the widest existing particle. Shoot me down, please!
    What you have found is the reason that in special relativity there can be no such thing as a rigid body or an infinitely stiff wire. That would violate the proscription on trasmission of information faster than the speed of light which would in turn violate causality.

    What happens in reality is that there is no such thing as an infinitely stiff material. When you pull on a wire you establish a stress wave that propagates at approximately the speed of sound in the material. It is that stress wave that controls the speed of the signal to the other end.

    Elementary particles are points, and quantum objects at that. They don't have definite positions. Your model breaks down at that level. At a higher level you are back to elastic materials.


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  4. #3 well 
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    causality. and the speed off light can brake .
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