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Re: Sink or swim?



Perhaps time dilation also plays a role in this relativistic buoyancy
(!) problem.

The buoyant force of the water on the sub depends partially on the
collision rate of the water molecules with the sub's hull, doesn't
it? At high relative speeds (and to say nothing of Bernoulli
effects), that collision rate would be reduced by time dilation.
I.e., the "moving clock runs slowly." So, in this case, the moving
clock involves not the period of a second hand's revolution but the
period of time between water molecules' collisions with the sub's
hull.

Therefore, wouldn't time dilation (alone) argue for a *decrease* in
buoyant force?

- Tucker

At 9:44 PM -0400 8/7/03, Ian Ellis wrote:
For your consideration, from Nature magazine
http://www.nature.com/nsu/030728/030728-3.html

-----

"Objects travelling at close to the speed of light appear to get shorter
when viewed by stationary observers. But from the viewpoint of those on
the moving object, the observers - who are receding at close to the
speed of light - appear shortened instead. Other dimensions remain the
same.

When these notions are applied to a submarine just below the water's
surface, an inconsistency seems to arise. Spectators on an anchored ship
would see the submarine shrink as it moves parallel to the surface at
near-light speed. The resulting density increase would sink the vessel.
The submarine crew would see the opposite: water rushing past them would
contract and get denser, making the submarine more buoyant and causing
it to rise.

Relativity insists that both viewpoints are equally valid - so does the
sub sink or swim?"