Re: Is evolution an example of decreasing entropy?

rja.carnegie@xxxxxxxxxx wrote:
> David Ewan Kahana wrote:
> > Navillus wrote:
> >
> > > I'm going to contend that gravity is a key force and probably
> > > the FUNDAMENTAL force in the universe that drives increases in
> > > orderliness, eventually leading to complex life.
> >
> > Gravity is a key force in driving the collapse of large amounts
> > of matter into small spaces, given that there was some
> > initial inhomogeneity in the matter distribution.
> >
> > However, other forces, especially the electromagnetic
> > force are equally critical to producing complex life.
> >
> > Gravitational collapse does not violate the second law
> > of thermodynamics.
> >
> > Your `orderliness' seems to be an extremely vague
> > concept.
>
> That's a fair statement, I think. The 2LoT is good solid science.
> Worrying about gravity isn't.
>

Worrying about gravity is OK and may be perfectly good
science; but imagining that gravitational collapse violates
the 2LoT is simply wrong.

> At the same time, the 2LoT is rather statistical in character, although
> that doesn't seem to help people who want to undo it, much.
>

That's correct ... it is a statistical law, and fluctuations
may cause it to be violated.

> I understand that gravitational collapse got the universe moving in
> interesting ways from a state closer to thermal equilibrium - and then
> fusion really heated things up.

It took a combination of generalized expansion of the
universe, everywhere in a state close to equilibrium, and
localized gravitational collapse to get things really
moving.

Probably the most standard story these days is that the
universe was initially in a supercooled state: this led to
rapid expansion of the universe. As the energy associated
with supercooling was used up, the inflationary expansion
terminated, and the universe was heated up, producing the
initially large entropy and high temperature state which is
an input to the hot big bang model.

Initially temperatures would have been much higher than is
necessary for fusion, so high in fact, that nucleons could
not even have existed as separate particles.

As the universe expanded further it cooled enough to allow
the condensation of nucleons from quarks, and finally when
the temperature had cooled enough to allow deuterons to
exist as stable nuclei, an epoch of nuclear fusion occurred
in which the main nuclei constituting the current universe
were made up. These most abundant nuclei are hydrogen and
helium, of course.

Later on, gravitational collapse led eventually to the
formation of stars, which heat up in their cores. This then
allows fusion to occur.

Almost all of the elements heavier than helium were produced
in stars (there's no other plausible mechanism), and then
spread into the interstellar medium by supernovae. Our solar
system has about 2% of heavier nuclei ... and these are
absolutely essential for life.


> But fusion needs high temperature,
> doesn't it?

Generally yes, it does.

Fusion in most stars actually occurs very far out on the
tails of the thermal distributions. If you have a huge
number of protons around and gravity holds them in place so
that they must continue to collide over and over with
each other, then you don't actually require that high a
temperature for fusion to go on at a significant rate.
In fact, the very long lifetime of the sun is in part
due to the fact that the temperature is not too high,
but mostly due to the fact that the major reaction
in which two protons combine to form a deuteron is
a weak process with a very small reaction rate in
the core of the sun. Heavier stars live for a much
shorter time, and achieve higher core temperatures.

Temperatures in the core of the sun are rather modest
compared to nuclear Coulomb barriers: T=10,000,000 K
corresponds to a thermal energy of about 1 KeV, while
typical barriers to fusion are on the order of 1 MeV.

David

.

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