[ExI] Interstellar FedEx

spike spike66 at att.net
Tue Sep 29 01:40:49 UTC 2009


> ...On Behalf Of Anders Sandberg

> ...Hmm, so what is the most entropic kind of stable matter?
> Anders Sandberg,

Anders, I need to review my thermo books to be sure, but I think the answer
to that question is molecular hydrogen in the form of a cloud in
interstellar space.  It might depend on how you define stable matter.: the
answer might be atomic hydrogen.

I have been thinking about the original question of how to derive the
equation for inherent entropy production in moving mass.  I have a thought
experiment for you.

Imagine a rigid structure deep in the hard vacuum of interstellar space.  It
might help to visualize it if I give it some arbitrary dimensions: this
structure is a tube a light second in length with identical coil springs at
either end.  The springs have a ratchet mechanism which will hold the spring
compressed, and a trigger which will release the ratchet, allowing the
spring to decompress.  Now imagine the spring at the east end of the
structure compressed and locked, with a mass on the end.  The spring at the
west end of the tube is uncompressed.

Imagine pulling the release trigger, which accelerates the mass to a
micro-c, or about 300 meters per second.  I chose that speed because anyone
who has ever flown commercially can easily visualize that, since you have
been close to that speed.  OK so the spring decompresses and launches the
mass along the tube, so that a million seconds later, or about 12 days, when
the mass arrives at the west end of the tube, it collides with the west
spring and compresses that spring, which then locks at maximum compression.

Both before and after the 12 day flight, the tube is at rest with respect to
an external observer, and a spring is compressed before and after, and a
mass has been moved a light second, so one might argue there has been no
entropy production, but this is not exactly right, as will be shown.  

Consider the east spring.  At the point in time where the east spring
releases the mass, the mass is travelling at 300 meters per second, but
notice that the end of the east spring is also moving at 300 meters per
second, so two things can be said.  The east spring is now oscillating, a
form of energy storage.  This is completely unavoidable, no matter what you
do.  The third law is out to get us.  So the identical west spring cannot
compress as far as the east spring was to start with.  The energy lost will
be back at the oscillating first spring.  

Before the event you have one compressed spring and everything is still and
quiet.  After the event, you have moved the mass a light second and have one
oscillating spring.  You can launch the mass back to where it started, but
after that initial trigger release, you always have at least one oscillating
spring.  In general, the energy of oscillation in that spring cannot be

In that particular thought experiment, you demonstrate that moving a mass
cannot be made perfectly reversible.  The third law is called a law for a
reason.  It isn't just a suggestion.  It means business.



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