🚀 go-pugleaf

>>>>> "Dave" == Dave Barlow <db@decuk.uvo.dec.com> writes:
In article <3s916t$b2i@mrnews.mro.dec.com> db@decuk.uvo.dec.com (Dave Barlow) writes:

    Dave> Ever heard of the problem of Helium-4 rotation in the
    Dave> superfluid state.  If you have a rotating container with He4
    Dave> in it, cool it below the super-fluid critical temperature
    Dave> then all the fluid stops rotating or all of it rotates.
    Dave> This is independant of frame of reference, the container
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Whether or not something is rotating is definitely frame dependent.  I
assure you that physicists and engineers often use rotating coordinate
systems.       

    Dave> still rotates.  If you stop rotating th econtainer there is,
    Dave> scant, evidence that the He4 is rotating. The question is
    Dave> with respect to what is at rest, rotating to ?  It is
    Dave> thought that this is a case of an Einstein-Bose Quantum
    Dave> Fluid.

    Dave> It indicates that the He4 is stationary, rotating wrt the
    Dave> "Fixed Stars".  -- Dave Barlow - Lurker at the Threshold
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         By this I assume that it *is* rotating with respect to the
laboratory attatched to the rotating earth?  Sorry, it has been a
while since I have thought about this problem, and never have I
thought about it in depth.

You raise an interesting point that I should have addressed in my
previous post.  When speaking of GR and the fixed stars, we must of
course specify the boundary conditions at infinity.    When I choose a
system of coordinates "at rest wrt fixed stars" the choice of boundary
conditions is almost trivial (asymptotically flat; no rotation).  When
I choose a rotating coordinate system, the boundary conditions on my
fields are somewhat more perverse in order to predict the correct
motions of distant objects: another good reason *not* to use such a
coordinate system when interested in describing distant objects.
(In some sense Mach's conjecture is that such boundary conditions
should be "forced" on us, rather chosen so that they work.)

I must stress however, that using the ideas of Mach is far deeper than
one needs to dig here.  Whatever equations for physics you believe in
for a nonrotating coordinate system, I can do a straightforward (but
extremely messy) coordinate transformation and tell you what those
equations  would look like in a rotating system.  Simple undergraduate
mechanics exercise.

- Tom




	
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