Re: GR and Thermodynamics


"Barry" <Sirdry@xxxxxxxxxxx> wrote in message
news:1162611276.719433.92040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Bill Hobba wrote:

Barry wrote:

Bill Hobba wrote:

Barry wrote:

Entropy is concerned with the flow of heat.

Not quite - it is concerned with how ordered a system is. That often
corresponds to heat but is not the same. See for example the Feynman
Lectures Chapter 44.

In Volume 1, Chapter 44, 44-6, he says that entropy is heat divided by
temperature.

In Chapter 46, 46-5, he relates entropy to disorder, mathematically.
But that does not change the fact that entropy is concerned with the
flow of heat.

Whoa. Wrong. As Feynman says 'We measure disorder by the number of ways
that the insides can be arranged so that from the outside it looks the same.
The logarithm of that number of ways is the entropy'. The immediate
consequence is it can be applied to areas beyond heat. BTW that is a rather
loose definition.


The Second Law of Thermodynamics says that the entropy of a closed
system cannot decrease and is related to using the flow of heat to do
Work.

Your logic is simply incorrect. The fact it is defined in terms of
order/disorder means it is applicable beyond heat.

At maximum entropy, heat is uniformly distributed and can do no
more work.

So? What has that got to with the fact the concept of entropy is applicable
beyond its use as a measure of heat?.


In any case, be careful whom you quote. In Volume 1, 15-1, he says
that " Newton's Second Law ... ...was stated with the tacit assumption
that m is a constant, but we now know that this is not true, and that
the mass of a body increases with velocity".

Well he actually states elsewhere it does. But in fact on that point he is
not in tune with modern usage - mass in modern times is generally considered
invariant.
http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html



This became interpreted as
a probabilistic matter, and is often viewed in terms of information
and disorder. The fact that it is fundamentally about heat flow gets
almost forgotten.

You are confused. Entropy was originally introduced for heat (by Carnot
if
memory serves me correctly - Feynman in the reference given previously
gives
the exact details) - sure.

I don't feel confused, entropy is *defined* as heat/temperature.

You even quoted the page where it is defined differently.

19th
century statistical approaches focussed on the relationship between
entropy and order. But that does not change what entropy *is*.

The modern definition does. The moment the modern definition was accepted
then its applicability beyond heat was assured.



In GR spacetime, world lines describe the flow of mass/energy.

They do not. They describe the path of free particles. They form
geodesics
due the principle of maximal ageing.

World lines describe the paths of all particles, free or not.

Well is a certain sense you are correct in that it is a plot of position vs
time. If that is the sense you mean it then the statement 'In GR space-time,
world lines describe the flow of mass/energy' is vacuous.

Matter tells spacetime how to curve Curved spacetime tells matter how to
move.

Correct.

Bill






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