Quantum Mechanics Series Lecture 2
Lecture 1 gave us one of the cleanest facts in quantum mechanics
If ψ(x,t) is the state, then ρ(x,t) = |ψ(x,t)|² behaves like a probability density, and Schrödinger evolution makes that density satisfy a continuity equation.
So, if ρ flows like a fluid, then what determines the flow?
To answer that, write the wavefunction as
ψ(x,t) = r(x,t) exp(iθ(x,t)).
Now the picture sharpens. The magnitude r tells you how much probability is present at a point. The phase θ tells you how that probability is directed. But how exactly does phase enter the motion?
When you expand the current,
j = (1/m) Im(ψ* ∇ψ),
it reduces to
j = (ρ/m) ∇θ.
That is the key step. The current is driven by the phase gradient. So the flow lines are not decorative. They are the geometry of phase made visible.
Then another question appears. Can the phase wind around space in any arbitrary way?
The answer is no, because ψ has to come back to the same value after going around a closed loop, the total phase change cannot be arbitrary. It must be an integer multiple of 2π:
∮ ∇θ · dl = 2πn.
That is where quantized vortices come from. They are not features we bolt onto the theory. They are topological defects the mathematics allows only in discrete units.
In the 2D render reveals the underlying mechanism of probability current bending around isolated vortex charges.
The math breakdown:
We describe the state by a complex field ψ(x,t) on the plane, with x in R². The Born rule defines the probability density
ρ(x,t) = |ψ(x,t)|².
Schrödinger evolution, in units with ħ = 1, is
i ∂ψ/∂t = [ −(1/2m) ∇² V(x,t) ] ψ.
Why does this preserve probability?
Start from
ρ = ψ*ψ.
Differentiate:
∂ρ/∂t = ψ* (∂ψ/∂t) ψ (∂ψ*/∂t).
Now insert Schrödinger’s equation and its complex conjugate:
∂ψ/∂t = (1/i) [ −(1/2m) ∇²ψ Vψ ]
∂ψ*/∂t = (−1/i) [ −(1/2m) ∇²ψ* Vψ* ].
What survives after substitution?
The potential terms cancel. The rest rearranges into the continuity equation
∂ρ/∂t ∇·j = 0
with probability current
j = (1/2mi) ( ψ* ∇ψ − ψ ∇ψ* )
= (1/m) Im(ψ* ∇ψ).
So ρ really does behave like a conserved fluid density, and j is its flux.
But what actually steers that flux?
Write ψ in polar form:
ψ(x,t) = r(x,t) exp(iθ(x,t)).
Then
∇ψ = exp(iθ) (∇r i r ∇θ).
So
ψ* ∇ψ = r (∇r i r ∇θ).
Taking the imaginary part gives
Im(ψ* ∇ψ) = r² ∇θ = ρ ∇θ.
Hence
j = (ρ/m) ∇θ.
That is the central statement of this lecture:
The phase gradient sets the direction and speed of the flow, scaled by the density and the mass.
Now one last question. Why are the vortices quantized?
Because ψ must be single-valued. If you go once around a closed loop and return to the same point, the complex field must return to the same value. That forces the total phase winding to satisfy
∮ ∇θ · dl = 2πn, with n in Z.
The integer n is the vortex charge. At the vortex core, ρ is nearly zero, the phase becomes undefined, and the current circulates around that defect.
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