Am 25.02.2012 12:46, schrieb Warp:
>    How about this theory:
>
>    Make one portal on the ground and the other on the ceiling right above it.
> Then take a metallic rod that's exactly the same length as the height of
> the room, pass it vertically through the portal, and then weld its ends
> (iow. the rod gets welded to itself).
>
>    Now the rod can be moved horizontally and vertically, but it cannot be
> turned around a horizontal axis (because it stops itself from being turned).
> It's effectively an infinite rod. Now let the rod go.
>
>    The rod will start falling.

Why should it? Downward motion won't put the endless rod into a state of 
lower potential energy, so it will not happen.

To the contrary, every coaxial acceleration of the rod would impose an 
ever so slight length contraction due to relativistic effects, leading 
to buildup of internal stress (and hence internal energy - not sure how 
the expert would call this type), while deceleration would reduce the 
internal stress, so deceleration is likely to happen spontaneously while 
acceleration would require external energy input (and I mean energy 
input, not just some force). So even if the rod was falling in the first 
place, given sufficient time its motion will actually /stop/.

(You /could/ force it into motion by heating it up though: As the 
material would try to expand in all directions, again internal stress 
would be induced, and as any increase in speed - whether up- or 
downwards - would reduce this stress due to length contraction, a tiny 
push /would/ eventually get it up to relativistic speed - provided the 
rod doesn't melt long before due to air friction. Or at least that's how 
the thought experiment goes - maybe there's a flaw in it as well.)

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