

Very long string vs speed of light - pchristensen
http://michaelgr.com/2011/06/01/very-long-string-vs-speed-of-light/

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noonespecial
I always heard the paradox as a light-year long tube filled with pingpong
balls. Stick a ball in at one end, how long does it take the one at the other
end to fall out. From the other perspective (space) its often called the
"ladder through the garage" paradox.

The answer, of course, is that simultaneity is observer relative as well.

 _What if you and I are at each end of this string, and I pull on it (or I
have an incredibly powerful machine do it for me). How fast would you feel the
pull?_

I would feel the pull at the instant _I_ saw the string get pulled by your
machine with my giant telescope.

This ought to help: <http://en.wikipedia.org/wiki/Relativity_of_simultaneity>

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dspillett
I've heard a variant where the tube is curved such that the other end is also
at your location, so no giant telescope is needed to see the end that inserts
the new ball (or pulls the string) from the end where you are waiting for the
action to have the consequence you are looking for. This adds the complication
of you being both observers despite the effect needing to travel a huge
distance, so one observer has two relativistic frames of reference.

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archgoon
All physical objects are made up of atoms. To a first approximation, this
means everything is connected by springs, which means the assumption of zero
elasticity is impossible. 'Inelastic' objects just have very large spring
constants, they don't deform much, but they still deform. By the way, long
before we talk about speed of light considerations we run into the speed of
sound. This is material dependent, true, but that'll again be dictated by the
physics of atoms.

Or alternatively (if you want to start imagining crazy ass materials made of
dark matter), zero elasticity is defined by invariants of length. Length is
not something which is preserved by the symmetries of special relativity
(Lorentz Transformations), so a rigid object is not a compatible notion with
the idea that the laws of physics are invariant under Lorentz Transformations.

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noonespecial
I think maybe you can have a perfectly inelastic object in a "spherical
chicken of uniform density" type thought experiment. The catch is that space
is deformable(1) so it makes the experiment not especially useful. And that
might be the point.

(1) Unless you assumed that your material did not deform with the space that
contained it, which would make the experiment nonsense.

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archgoon
No you can't have an inelastic object and at the same time say that "Special
Relativity is Correct". They are incompatible definitions.

It's like talking about all even primes greater than 2. Or green objects that
aren't green.

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runaway
There is a physical limit to rigidity. You basically cannot have a giant
string that doesn't deform. Still limited by the speed of light.

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tomelders
Assuming the most obvious obstacles were solved (rigidity, how you tug on a
frictionless string [great point mike] etc), and you had the perfect string
for this experiment: Nothing would be moving faster than the speed of light.
Every single inch of the string would be moving at the same speed, which would
be a lot slower than the speed of light. But every inch would move in unison.
I don't see how the "faster than the speed of light" thing has any bearing.

That said, it's still a physically impossible experiment.

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mikegreenberg
I'll repeat the question I asked on the site... how do you pull on a
frictionless string? (Not that any other part of the question was very
plausible.)

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shubber
Tell you what: assume it's a loop, and I'll hook it to something that does
have friction.

Or, we could just nail it to something.

I mean, if we're talking about inelastic megastructures, the logistics of
manipulating a frictionless string seems a little silly to worry about.

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ggchappell
What this really is, is a nice proof by contradiction, that objects with
_absolutely_ no elasticity, cannot exist.

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magicbv
What if the sun was to magically disappear. how much time would it take for us
here on earth to feel the effects of it. approximately 8 (light) minutes.
which is the distance between sun to earth. my point is, nothing can travel
the speed of light. not even information.

