I came across an interesting question which was shown to me by my professor, it is as follows:
Investigation of an alternative Universe:
This Universe contains three spatial and one time dimensions, and space can be described as a large cuboid. The lengths of the sides of the cuboid are denoted by $L_{x},L_{y}$ and $L_{z}$, its volume is $V$ . The universe is prescribed cyclic boundary conditions. The alternative Universe is filled with a Bose-Einstein condensate which consists of ultra-cold atoms. The condensate is characterized by a complex wavefunction $\psi(r, t)$. The dynamics of the wavefunction is described by the Gross-Pitaevskii equation
$$\boxed{\mathrm i\hbar\frac{\partial{\psi(r,t)}}{\partial{t}}=\left(-~\frac{\hbar^2}{2m}\Delta-\mu\right)\psi(r,t)+g\psi^{*}(r,t)\psi(r,t)\psi(r,t)}$$
Where $m$ is the mass of the ultracold atoms, $\mu$ is the chemical potential and g is a constant which characterizes the atomic collisions. The number of condensed atoms is denoted by $N_{c}$. The local density of the condensate is given by the absolute value of the wavefunction squared. Let $c =\sqrt{\frac{gN_{c}}{mV}}$ denote a parameter which has the dimension of velocity and let $\xi =\frac{\hbar}{mc}$ denote another parameter, which is small, and which has the dimension of length.
The alternative Universe is habited by intelligent creatures and the creatures live in two colonies, whose distance is large. The habitants of the two colonies communicate by creating small wavelike perturbations in the otherwise homogeneous condensate. Due to the limitations of the moderately advanced technology of the creatures, they can only create waves in the condensate, whose wavelength is much larger than the length $\xi$.
In one of the colonies, the leading physicists of the creatures are arguing about the fundamental nature of the physical laws of their world. While Alice says that the aether should exist, Bob argues that the aether hypothesis is not needed, because their world is fundamentally Lorentz invariant.
Who is right, Alice or Bob?
To decide who is right in the controversy, write up the equation of the small perturbations of the complex wavefunction, and then make a real wave equation from this complex equation, which will contain derivatives higher order than the previous complex one. In this equation, the parameters $c$ and $\xi,$ which were introduced above, will appear. Analyze this equation from the perspective of the debate.
How do I go about solving this question?
My attempt:
I have found the solution to the Gross-Pitaevskii equation and have hence found the local density of the condensate. Bose-Einstein statistics tells us that the occupation of the ground state is unbounded as the energy of the system decreases. For dilute systems this can be achieved by reducing the temperature of a system below some critical $ T$, given by $Tc$, such that the particles want to be in the lowest state. For a free gas of non-interacting particles in 3D, with particle mass m, this critical temperature is given by : $$\boxed{T_{c}=\frac{2(\pi)\hbar^{2}}{mk_{B}}\left(\frac{n(r,t)}{\zeta\left(\frac{3}{2}\right)}\right)^{\frac{2}{3}}}$$ Where $\zeta$ is the Riemann Zeta function and $\zeta\approx2.612$ Below this temperature we would expect the onset of Bose-Einstein condensation. A quantum particle is smeared out over a region of space as a wavepacket. When the temperature of a system is decreased, these wavepackets (whose typical size is given by the de Broglie wavelength) increase in length and begin to overlap, behaving as one giant matter wave. The thermal de Broglie wavelength can be derived by equating the quantum mechanical kinetic energy of free particles, $E_{K} = \pi k_{B}T$, with kinetic energy in terms of momentum, $E_\textrm{kinetic} = \frac{p^2}{2m}$ and using the standard de Broglie wavelength $\lambda_{dB} = \frac{2\pi}{p}$ to obtain $$\boxed{λ_{dB} =\sqrt{\frac{2(\pi)\hbar^2}{mk_{B}T}}}$$
where we note $λ_{dB} ∝\frac{1}{T}$ so it increases in length as $T$ decreases. Note that using the former relation in terms of $λ$ we can write $nλ^3 ∝ nT^{\frac{-3}{2}} ≈ 2.612$.
At temperatures T much smaller than the critical temperature Tc, the BEC is well described by the macroscopic wave function $\psi=\psi(r,t)$ whose evolution is governed by a self-consistent, mean field nonlinear Schrödinger equation (NLSE) known as the Gross-Pitaevskii equation. If a harmonic trap potential is considered, the single particle equation becomes:
$$\boxed{i\hbar\frac{\partial{\psi(r,t)}}{\partial{t}}=-\frac{\hbar^2}{2m}\Delta(\psi)+V(r)\psi+NU_{0}\mid{\psi^2}\mid\psi}\tag{1}$$
Where $r=(x,y,z)^{T}$ is the spatial-coordinate vector, $U_{0}=\frac{4(\pi)(\hbar^2)a}{m}$ with a, the s-wave scattering; and $$V(r)=\frac{m}{2}(\omega_{x}^{2}x^2+\omega_{y}y^2+\omega_{z}z^2)$$is the harmonic trap potential.
Multiplying $(1)$ by $(\frac{1}{m\omega_{x}^2 a_{0}^{\frac{1}{2}}})$ and normalising the function I got:
$$\boxed{i\frac{\partial{\psi(r,t)}}{\partial{t}}=-\frac{1}{2}\Delta(\psi(r,t))+V(r)\psi+\beta\mid{\psi^2}\mid\psi(r,t)}\tag{2}$$
$\beta=\frac{4(\pi)aN}{a_{0}}$. Here positive/negative β corresponds to the defocusing/focusing NLSE, respectively.
Hence, $(2)$ is the local density, I.e., $\frac{N_{c}}{V}=\frac{N_{c}}{L_{x}L_{y}L_{z}}$
Now, I am aware of the dark solition (oscillating dark solutions-arxiv), am I to use that along with introducing perturbations? How do I write down an equation for the small perturbations in terms of a complex wavefunction and then take a real value of it? Any help will be deeply valued and appreciated.