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Re: Plasma



I am glad to be back on phys-l. I should note here that I read the
digest version, so my replies may seem a bit slow, and I may duplicate
something that has already been said by someone else. I don't really
want to subject myself to the flood of emails nor enslave myself to my
laptop that answering in a more timely manner would require. I also
excise only the relevant parts of postings to which I reply (and I wish
others would do so). Thank you for your patience with me.

Ludwik Kowalski writes:

On Monday, Nov 24, 2003, at 13:38 US/Pacific, Leigh Palmer wrote:

> . . . Is it not true that the great majority of nuclear
> reactions taking place in the universe are driven
> by gravity? . . .

Yes, but you were referring to the "intergalactic
space that is nearly completely ionized," not to
stellar interiors.

Yes, that is true, but we are not talking about nuclear reactions in
intergalactic space. I merely wanted to establish that much higher
energy processes are also driven by gravity alone. Many texts and
teachers seem to overlook this fact, leaving students with the
impression that gravity is somehow intrinsically "weak". What I am
about to show is that gravity in clusters of galaxies is easily strong
enough to produce the very hot plasmas that are observed in
intergalactic space.

> . . . It is thought that the ionization is due to
> gravitational interaction. . . .

That is very interesting and puzzling. I would think
that partial ionization of intergalactic atoms is due
to cosmic rays, UV, etc. But conditions near the
"edges of the infinite universe" are likely to
generate a lot of surprises. How reliable and
trustworthy is "experimental evidence"?

In astronomy we refer to "observational evidence". I would like to
champion the use of a new word, "empiry", that refers to the union of
observation and experiment. The union of observational evidence and
experimental evidence would then be called "empirical evidence", which
is very close to the meaning it presently has. "Empiry" used to be in
the OED (see first edition) with a different meaning. I see that it has
almost vanished from the second edition, being listed now only as a
variant of "empyre".

We are not surprised to find very hot plasmas in rich clusters because
we observe velocity distributions with a dispersion of order 1,000 km/s
in these clusters. We infer that the high velocities are a
(gravitational) consequence of the large masses of these clusters.

A hydrogen atom having a velocity of 1,000 km/s has (if I did the
arithmetic right in SI) a kinetic energy of 5 keV. This sort of result
becomes less surprising with time, but I do find that many physicists
are unaware of it.

John Denker writes (replying to Ludwik):

It's puzzling because it's wrong.

I don't understand. Usually you support such statements with argument;
here you do not do so.

Why do you feel it's wrong?

> I would think
that partial ionization of intergalactic atoms is due
to cosmic rays, UV, etc.

Well, that correctly explains how they *get*
ionized. But then you need to explain why
they *stay* ionized. (In epidemiological
terms, this is the difference between incidence
and prevalence.)

That is not what I was taught, and it should not be taught. According
to the standard model of cosmology the atoms recombined from the
primordial plasma about 300,000 years after the Big Bang. They then
interacted gravitationally, condensing about mass density fluctuation
maxima, getting hotter by gravitational acceleration as they did so.
When such mass concentrations got sufficiently hot (i.e. the atomic
velocities got sufficiently large) the atoms were ionized by collision.
There is no reason to believe (and plenty of reason to disbelieve) that
UV or cosmic rays ionize the gas. It's hot; that's why it's ionized.

Why is it hot?

Gravity!

Before I got to it Ludwik gave this answer to John's question (above):

They would stay ionized if the responsible agent
was constantly present, for example, very strong
flux of cosmic rays.

The ions in the plasma continue to move in the gravitational well of
the cluster. They do radiate away their energy (which we see as x-rays)
but it is replenished by gravitational interaction with the galaxies in
the cluster. The responsible agent, always present, is

Gravity!

I know that these results are surprising to many, apparently to the
extent of disbelief in some, but the concepts are really elementary,
and recourse to the Saha equation, the virial theorem, and other more
advanced topics is unnecessary. I hope that I have put them into a form
that can be used even in high school teaching.

Leigh

(Gravity does lots of counterintuitive things. Realize, for example,
that as gravitational collapse proceeds we observe a rising temperature
as internal energy diminishes by radiation. In a sense that means the
system has a negative heat capacity.)