# Confusion with Qm question

1. Feb 7, 2004

### Xgkkp

Hi,

I'm going through my problem sheet for my quantum mechanics course, and I've come across something I don't know what it means;

It says that a Bose-Einstein condensate can be described by the wave function:

http://xgkkp.no-ip.com/images/math/PsiCondensate.jpg [Broken]
Psi(x,t) = sqrt( n(x,t) ) * exp ( i*alpha(x,t) )

It then asks me to find the probability density, which I have (assuming it is equal to the (Sigma = |Psi|^2 = Psi.Psi*), and it came out as:

http://xgkkp.no-ip.com/images/math/sigma.jpg [Broken]
Sigma = n(x,t)

(I can only assume I am correct with this)

The next part has me stumped - it asks me to "Calculate the probability density as a function of n(x,t) and alpha(x,t), and their derivatives".

What does it mean? Does it mean to re-arrange the wave equation so that the functions n(x,t) and a(x,t) are the subject then calculate the probbability density for them? (and the derivative) or does it mean something else entirely?

Thanks,
Nick

ÏˆÎ±Ïƒ (testing - I don't know if greek letters work with the browser fonts)

Last edited by a moderator: May 1, 2017
2. Feb 12, 2004

### slyboy

Are you sure that both n and alpha are real-valued functions? If alpha is complex then the density would be more complicated.

3. Feb 12, 2004

### Tom Mattson

Staff Emeritus
For some reason, I can't view the images you attached, but I have one suggestion.

Take a closer look at the problem and see if it doesn't say to calculate the probability current density, which is:

j=(some factors)[&Psi;*(&part;/&part;x)&Psi;-&Psi;(&part;/&part;x)&Psi;*]

That's the only way I can see derivatives coming into it.

edit: typo

Last edited by a moderator: May 1, 2017