re:Flow of energy

[brian whatcott <inet@intellisys.net>]

At 06:56 9/8/97 EDT, in answer to my ill-posed question you wrote
this rather fine response:

> [Brian]
> > When a particle transfers momentum to another particle so that 
> > 'heating is done' the momentum is said to transfer by 
> > electromagnetic force, at least at one level of explanation.
> > Feynman was fond of charting transactions of several kinds.
> >    How would he have shown the transfer?
> >
> > Dumb question; Is it by means of a photon?
> > If not, then what?

> At the level of individual fundamental particle interactions it is not true
> that 'heating is done'.  Heating and heat are *macroscopic* concepts which
> appear as a statistically emergent phenomenon in branches of physics which
> deal with *macroscopic* systems such as in thermodynamics and statistical
> mechanics. ...

It would have been better if I had more carefully specified a scenario of
this kind; in an ordinary gas of which each molecule has two atoms at
standard temperature and pressure, I expect ( and Einstein dealt with) a
distribution of molecular velocities existing in three axes.

 Considering the situation where a molecule near the 'fast' end of the
speed distribution collides with a molecule near the 'slow' end some
momentum is redistributed.
 This is the conceptual model that comes to mind when I think of a hot gas
mixing with a cool gas.

I understand your point that heat is a statistical or emergent property,
but I hope you will not demur at the use of average particle or molecular
momentum as the underlying basis of 'temperature' in this simple case,
where I do not need to press you to define 'microstate'.

 The reason I am unreasonably insisting on inspecting individual particles
where statistical quantities are really called for, is because a few years
ago, I attended an interesting talk given by a research engineer or
physicist from one of the big commercial labs - perhaps 'son of Bell labs'.

 He set out to describe his laser-cooling method of reducing macroscopic
particles to millikelvin temperatures, perhaps much lower, I cannot recall.

The details grow dim, but I recall his lasers in several orthogonal axes
and in each direction were able to couple energy into particles moving at
particular speeds, by means of tuning the laser frequency to interact with
the doppler-shifted frequency corresponding to particular speeds.

 He was able to segregate particles all of a closely controlled speed,
 and he claimed that by definition, where internal random motion was
suppressed, this corresponded to reducing the internal temperature.

 This naturally raised the possibility that photons can provide a momentum
carrying mechanism in my mind.

Having said thanks for your careful and generous exposition, I would like now
to offer a word of complaint about the passage quoted below:

> OTOH, for a macroscopic process which is characterized by adiabatic work the 
> system's energy level spectrum is disturbed because the Hamiltonian has been
> changed by the working process which changed one or more of the macroscopic
> defining parameters (such as the volume in the case of a fluid) on which the
> Hamiltonian depends....

> David Bowman
> dbowman@gtc.georgetown.ky.us

I take exception to the idea that a working process can change the
Hamiltonian, thereby disturbing the system's energy levels.
 I expect this is a mathematical shorthand ( but it seems quite on a level
with 'energy flowing' to me) because a Hamiltonian is simply a math construct.
 A math construct only ever describes some change - it never causes the
change, to my view...

So finally Thanks again. And is it possible to say a photon is involved
with transfering momentum between gas molecules?

Regards

brian whatcott <inet@intellisys.net>
Altus OK