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Re: [Phys-L] collisions



On 01/24/2014 09:50 AM, Carl Mungan wrote:

2. Let's expand a bit on the oven ideas. I assume one starts with a
little oven (or fridge) full of a large number of hydrogen atoms. So
the temperature of the gas is well defined. Then I assume the
launchers consist of drilling small holes in the oven (opening into
a vacuum chamber).

OK. That sort of oven-with-hole has been used as a source in
atomic-beam experiments for a verrry long time.

Just to clarify my understanding of the question: Suppose
we replace the atoms with photons. Then there are two
answers:

Two flashlights is equivalent to one flashlight + beam splitter

Two lasers is NOT equivalent to one laser + beam splitter

because the laser might have a significant amount of coherence,
whereas the flashlight does not.

Returning to the original question: an oven is like a flashlight,
not like a laser. If you wanted to, you could design a launch
system to produce correlated atoms, but it wouldn't look like an
oven. It would be vastly more complicated than that.

==========================

I can compute lambda1 = h/p where p is a momentum of a single atom. I
can also compute lambda2 = h/sqrt(2*pi*m*k*T) while an atom is in the
oven, where T is well defined.

Lambda1 is the plain old wavelength.

The thermal de Broglie length (lambda2) does not behave like
a wavelength; it has more to do with the envelope-size of
the wavepacket. This is spelled out, with diagrams, at:
http://www.av8n.com/physics/thermal-wave-packet.htm

See also
http://www.av8n.com/physics/exchange.htm

There is an unfortunate tradition of calling lambda2 the thermal
de Broglie «wavelength» but AFAICT that is a complete misnomer,
the kind of misnomer that leads to deep-seated misconceptions.

Suggestion: Train yourself -- and your students -- to call it
the thermal de Broglie /length/. This won't solve all the world's
problems, but it will at least not create new ones.