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Photons and Bose Einstein statistics 
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#1
Nov3009, 06:18 PM

P: 630

Can a photon gas undergo a Bose Einstein condensation ?



#2
Nov3009, 08:30 PM

P: 4,513

First: does the question make sense in some way? What's a BoseEinstein condensate?
Second: if it does, it's a question of human or natural capability to make it happen. For the first, isn't a resonate cavity of monochromatic radiation already a condensate? 


#3
Nov3009, 09:14 PM

P: 483

BoseEinstein condensate is a welldefined term, and it's clear what the OP is asking, so your "First" argument isn't necessary at all.
I am not sure what you mean by resonate cavity of monochromatic radiation though. You mean "light" in a cavity resonator? 


#4
Nov3009, 09:15 PM

P: 483

Photons and Bose Einstein statistics
To the OP:
Lasers could be thought of BoseEinstein condensates of photons. 


#5
Nov3009, 09:17 PM

P: 4,513

Look, I could go to wiki and read in paragraph one about how condensate bosons are weakly interacting. But photons are not weakly interacting. I can then go to paragraph two and read about photons as the inspiration for condensate. It's good to define, in common, what we are talking about. 


#6
Nov3009, 10:07 PM

P: 483




#8
Nov3009, 10:15 PM

P: 483

Here is one from Eric Cornell>
http://www.fortunecity.com/emachines/e11/86/bose.html "This BoseEinstein condensate (BEC), the first observed in a gas, can be thought of as the matter counterpart of the laserexcept that in the condensate it is atoms, rather than photons, that dance in perfect unison." 


#9
Dec109, 03:06 AM

Sci Advisor
P: 1,626

There are well defined differences between lasers and BECs. For example the BEC is supposed to be a macroscopic population of the ground state in thermal equilibrium, while lasers need population inversion and are therefore completely out of equilibrium.
In my opinion the closest thing to a BEC of photons is the topic of nonequilibrium polariton condensation, which is an "intopic" since 2006 (see for example Nature 443, 409414 (2006) by Kasprzak et al. http://www.nature.com/nature/journal...ture05131.html). Here you have a microcavity resonant with some quantum well exciton transition. If you are in the strong coupling regime you get a dressed quasiparticle with mixed excitonic and photonic content. The photonic and excitonic content can be tuned by changing the detuning between cavity and bare exciton. Therefore you can get quasiparticles with a photonic content (and extremely light mass) of 50% to condense. However, this also means extremely short lifetimes of this quasiparticle so that you do not get equilibrium with the lattice, but only an equilibrium of the quasiparticles. As it is basically a 2Dsystem this is not a "complete" BEC, but rather a Kosterlitz Thouless phase transition. Nevertheless it shows some of the essential features of the BEC like macroscopic population of the ground state, spontaneous buildup of coherence and linear polarization, a linear Bogoljubov (Goldstone) mode, quantized vortices and halfvortices and a second order intensity correlation function differing from the value expected for a laser. However, whether this should be called BEC is still a debate in the scientific community. However, at the moment those in favour of BEC write the "heavier" papers. 


#10
Dec109, 05:07 AM

P: 630

Dono. Do you make sense in some way ? Its a physics forum, for god's sake. If you dont understand the terms, don't bother to answer. 


#11
Dec109, 06:16 AM

P: 630




#12
Dec109, 09:20 AM

P: 2,158

You can also think of the classical Maxwell equations to be analogous to the GrossPitaevski equation that describes a BEC.
If you have a gas of a large number of N particles, then the gas can be described by a wavefunction of N position variables (if the gas is completely isolated, otherwise one needs to describe it using a density matrix). If all the N particles are in the same state, then the gas can be described by a wavefunction that depends on a single position variable. The Schrödinger equation for the Nparticle wavefunction is a linear differential equation, but if you want to rewrite this in terms of the single particle wavefunction, you get a nonlinear differential equation, because the original linear equation contains interaction terms between particles that have to be accounted for by an effective self interaction term. This then becomes the GrossPitaevski equation, a.k.a. "nonlinear Schrödinger equation". You can interpret this as a classical field equation, similar to the Maxwell equation. 


#13
Dec109, 11:26 AM

P: 483




#14
Dec109, 07:41 PM

P: 4,513




#15
Dec109, 08:07 PM

P: 630




#17
Dec109, 11:28 PM

P: 4,513

I would like to start over. I'm sorry to have insulted your intelligence. Really, I mean it. I was attempting to ask "does this question make sense?" Scrolling down through unanswered threads, I thoughtand still think, that your question deserved a bump. It's a good one. In fact, it was Einstein who asked himself, can massive bosons behave like coherent light (also bosons) in indistinguishable energy states? 


#18
Dec209, 05:49 AM

P: 630




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