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- Newsgroups: sci.physics
- Path: sparky!uunet!zaphod.mps.ohio-state.edu!cs.utexas.edu!tamsun.tamu.edu!zeus.tamu.edu!dwr2560
- From: dwr2560@zeus.tamu.edu (RING, DAVID WAYNE)
- Subject: Bose condensation for photons?
- Message-ID: <29JUL199217001157@zeus.tamu.edu>
- News-Software: VAX/VMS VNEWS 1.41
- Sender: news@tamsun.tamu.edu (Read News)
- Organization: Texas A&M University, Academic Computing Services
- Date: Wed, 29 Jul 1992 22:00:00 GMT
- Lines: 46
-
- This is yet another posting from the fortune cookie factory. :)
- John Baez asked me to post for him and I have included a few comments.
-
- I write...
- >So now I think the thing which makes the photon classically 'different' from
- >other particles is that it's a neutral boson. Like helium. Hmmm...
-
- John:
- I guess another thing that makes photons weird compared to, say, atoms
- is that there is not even approximate conservation of photon number in
- most situations. That's why there's no exact analog of Bose
- condensation for photons. (Lasers are not examples of Bose condensation
- because laser light is not an equilibrium state the way, say, superfluid
- helium is. The equilibrium state of photons is blackbody radiation and
- this never exhibits Bose condensation.)
-
- Dave:
- My book says that the condensation temp _would_ be ~1,000,000 K , but we
- assumed constant N, which is wrong, so there is no condensation.
- I don't understand this, one does not need to demand N be conserved, one
- just needs to observe the value that N takes.
-
- John:
- Bose condensation assumes that N is conserved.... just as maximizing
- entropy subject to constant energy gives you temperature as a Lagrange
- multiplier, maximizing entropy suubject to constant N (or density)
- gives you chemical potential as a Lagrange multiplier, so working out
- the Gibbs state for a system of conserved particles is quite different
- than for nonconserved ones such as photons. If photon number was
- (almost) conserved with time there would be Bose condensation. Since
- it's not, there's not - one gets blackbody radiation.
-
- Dave: Hmmm... but physical properties are supposed to be independent of
- which ensemble you use. If, instead of holding temperature fixed, you
- measured or calculated the expectation of energy, and then demanded
- energy be conserved to this value, you would change to the microcanonical
- ensemble, but all physical properties would be unchanged.
-
- Dave Ring
- dwr2560@zeus.tamu.edu
-
- and
-
- John Baez jbaez@math.mit.edu
-
- sorry I can't get your sig right. :)
-