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PHY452H1S Basic Statistical Mechanics. Problem Set 2: Generating functions and diffusion

Posted by peeterjoot on January 26, 2013

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This is an ungraded set of answers to the problems posed.

Question: Diffusion

The usual diffusion equation for the probability density in one dimension is given by

\begin{aligned}\frac{\partial {P}}{\partial {t}}(x, t) = D \frac{\partial^2 {{P}}}{\partial {{x}}^2}(x, t)\end{aligned} \hspace{\stretch{1}}(1.0.1)

where D is the diffusion constant. Define the Fourier components of the probability distribution via

\begin{aligned}P(x, t) = \int_{-\infty}^\infty \frac{dk}{2 \pi} \tilde{P}(k, t) \exp\left( i k x \right)\end{aligned} \hspace{\stretch{1}}(1.0.2a)

\begin{aligned}\tilde{P}(k, t) = \int_{-\infty}^\infty dx P(x, t) \exp\left( -i k x \right)\end{aligned} \hspace{\stretch{1}}(1.0.2b)

This is useful since the diffusion equation is linear in the probability and each Fourier component will evolve independently. Using this, solve the diffusion equation to obtain P(k,t) in Fourier space given the initial \tilde{P}(k,0).

Assuming an initial Gaussian profile

\begin{aligned}P(x, 0) = \frac{1}{{\sqrt{2 \pi \sigma^2}}} \exp\left(-\frac{x^2}{2 \sigma^2}\right),\end{aligned} \hspace{\stretch{1}}(1.3)

obtain the probability density P(x,t) at a later time t. (NB: Fourier transform, get the solution, transform back.) Schematically plot the profile at the initial time and a later time.

A small modulation on top of a uniform value

Let the probability density be proportional to

\begin{aligned}\frac{1}{{L}} + A \sin(k_0 x)\end{aligned} \hspace{\stretch{1}}(1.4)

at an initial time t = 0. Assume this is in a box of large size L, but ignore boundary effects except to note that it will help to normalize the constant piece, assuming the oscillating piece integrates to zero. Also note that we have
to assume A < 1/L to ensure that the probability density is positive. Obtain P(x,t) at a later time t. Roughly how long does the modulation take to decay away? Schematically plot the profile at the initial time and a later time.


Inserting the transform definitions we have

\begin{aligned}0 &= \left( \frac{\partial {}}{\partial {t}} - D \frac{\partial^2 {{}}}{\partial {{x}}^2} \right) P \\ &= \left( \frac{\partial {}}{\partial {t}} - D \frac{\partial^2 {{}}}{\partial {{x}}^2} \right) \int_{-\infty}^\infty \frac{dk}{2 \pi} \tilde{P}(k, t) \exp\left( i k x \right) \\ &=\int_{-\infty}^\infty \frac{dk}{2 \pi} \left(\frac{\partial {}}{\partial {t}} \tilde{P}(k, t) + k^2 D\tilde{P}(k, t) \right)\exp\left( i k x \right),\end{aligned} \hspace{\stretch{1}}(1.0.5)

We conclude that

\begin{aligned}0 = \tilde{P}(k, t) + k^2 D\tilde{P}(k, t),\end{aligned} \hspace{\stretch{1}}(1.0.6)


\begin{aligned}\tilde{P}(k, t) = A(k) e^{-k^2 D t}.\end{aligned} \hspace{\stretch{1}}(1.0.7)

If the Fourier transform of the distribution is constant until time t, so that \tilde{P}(k, t < 0) = \tilde{P}(k, 0), we can write

\begin{aligned}\boxed{\tilde{P}(k, t) = \tilde{P}(k, 0) e^{-k^2 D t}.}\end{aligned} \hspace{\stretch{1}}(1.0.8)

The time evolution of the distributions transform just requires multiplication by the decreasing exponential factor e^{-k^2 D t}.

Propagator for the diffusion equation

We can also use this to express the explicit time evolution of the distribution

\begin{aligned}P(x, t) &= \int_{-\infty}^\infty \frac{dk}{2 \pi} \tilde{P}(k, 0) e^{-k^2 D t}\exp\left( i k x \right) \\ &= \int_{-\infty}^\infty \frac{dk}{2 \pi} \int_{-\infty}^\infty dx' P(x', 0) \exp\left( -i k x' \right)e^{-k^2 D t}\exp\left( i k x \right) \\ &= \int_{-\infty}^\infty dx' P(x', 0) \int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left( -k^2 D t + i k (x - x') \right)\end{aligned} \hspace{\stretch{1}}(1.0.9)

Our distribution time evolution is given by convolve with a propagator function

\begin{aligned}P(x, t) = \int dx' P(x', 0) G(x', x)\end{aligned} \hspace{\stretch{1}}(1.0.10a)

\begin{aligned}G(x', x) =\int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left( -k^2 D t + i k (x - x') \right)\end{aligned} \hspace{\stretch{1}}(1.0.10b)

For t \ge 0 we can complete the square, finding that this propagator is

\begin{aligned}G(x', x) &= \int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left( -k^2 D t + i k (x - x') \right) \\ &= \exp\left( \left(\frac{i (x - x')}{2 \sqrt{D t}} \right)^2 \right)\int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left( - \left(k \sqrt{D t} + \frac{i (x - x')}{2 \sqrt{D t}} \right)^2 \right)\end{aligned} \hspace{\stretch{1}}(1.0.11)


\begin{aligned}\boxed{G(x', x) =\frac{1}{\sqrt{4 \pi D t}} \exp\left(-\frac{(x - x')^2}{4 D t}\right).}\end{aligned} \hspace{\stretch{1}}(1.0.12)

A schematic plot of this function as a function of t for fixed x - x' is plotted in (Fig1).

Fig1: Form of the propagator function for the diffusion equation


For the Gaussian of 1.3 we compute the initial time Fourier transform

\begin{aligned}\tilde{P}(k) &= \int_{-\infty}^\infty dx \frac{1}{{\sqrt{2 \pi \sigma^2}}} \exp\left(-\frac{x^2}{2 \sigma^2}-i k x \right) \\ &= \frac{1}{{\sqrt{2 \pi \sigma^2}}} \exp\left(-\left( \frac{\sqrt{ 2 \sigma^2}}{2} k i\right)^2\right)\int_{-\infty}^\infty dx \exp\left(-\left( \frac{x}{\sqrt{2 \sigma^2} } + \frac{\sqrt{ 2 \sigma^2}}{2} k i\right)^2\right) \\ &= \exp\left(-\frac{\sigma^2 k^2}{2}\right).\end{aligned} \hspace{\stretch{1}}(1.0.13)

The time evolution of the generating function is

\begin{aligned}\tilde{P}(k, t) = \exp\left(-\frac{\sigma^2 k^2}{2} - D k^2 t\right),\end{aligned} \hspace{\stretch{1}}(1.0.14)

and we can find our time evolved probability density by inverse transforming

\begin{aligned}P(x, t) &= \int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left(-\frac{\sigma^2 k^2}{2} - D k^2 t + i k x\right) \\ &= \exp\left(i \frac{x}{ 2 \sqrt{\frac{\sigma^2}{2} + D t} }\right)^2\int_{-\infty}^\infty \frac{dk}{2 \pi} \exp\left(-\left(k \sqrt{\frac{\sigma^2}{2} + D t} + i \frac{x}{ 2 \sqrt{\frac{\sigma^2}{2} + D t} }\right)^2\right)\end{aligned} \hspace{\stretch{1}}(1.0.15)

For t \ge 0 this is

\begin{aligned}\boxed{P(x, t) =\frac{1}{{\sqrt{2 \pi \left( \sigma^2 + 2 D t \right) } }}\exp\left(-\frac{x^2}{2 \left( \sigma^2 + 2 D t \right) }\right).}\end{aligned} \hspace{\stretch{1}}(1.0.16)

As a check, we see that this reproduces the t = 0 value as expected. A further check using Mathematica applying the propagator 1.0.12, also finds the same result as this manual calculation.

This is plotted for D = \sigma = 1 in (Fig2) for a couple different times t \ge 0.

Fig2: Gaussian probability density time evolution with diffusion

Boxed constant with small oscillation

The normalization of the distribution depends on the interval boundaries. With the box range given by x \in [a, a + L] we have

\begin{aligned}\int_a^{a + L} dx \left( \frac{1}{{L}} + A \sin( k_0 x) \right) dx=1 - \frac{A}{k_0} \left( \cos( k_0(a + L) ) - \cos( k_0 a ) \right)\end{aligned} \hspace{\stretch{1}}(1.0.17)

With an even range for box x \in [-L/2, L/2] this is unity.

To find the distribution at a later point in time we can utilize the propagator

\begin{aligned}P(x, t) = \int_{-L/2}^{L/2} dx' \frac{1}{{2 \sqrt{\pi D t} }} \left( \frac{1}{{L}} + A \sin( k_0 x' ) \right) \exp\left( - \frac{(x' - x)^2}{2 \sqrt{D t} }\right)\end{aligned} \hspace{\stretch{1}}(1.0.18)

Let’s write this as

\begin{aligned}P(x, t) = P_{\mathrm{rect}}(x, t) + P_{\mathrm{sin}}(x, t)\end{aligned} \hspace{\stretch{1}}(1.0.19a)

\begin{aligned}P_{\mathrm{rect}}(x, t) =\frac{1}{{2 L \sqrt{\pi D t} }} \int_{-L/2}^{L/2} dx' \exp\left( - \frac{(x' - x)^2}{2 \sqrt{D t} }\right)\end{aligned} \hspace{\stretch{1}}(1.0.19b)

\begin{aligned}P_{\mathrm{sin}}(x, t)=\frac{A}{2 \sqrt{\pi D t} } \int_{-L/2}^{L/2} dx' \sin( k_0 x' ) \exp\left( - \frac{(x' - x)^2}{2 \sqrt{D t} }\right)\end{aligned} \hspace{\stretch{1}}(1.0.19c)

Applying a u = (x' - x)/\sqrt{4 D t} change of variables for the first term, we can reduce it to a difference of error functions

\begin{aligned}P_{\mathrm{rect}}(x, t) &= \frac{1}{{L}} \int_{-L/2}^{L/2} dx' \frac{1}{{2 \sqrt{\pi D t} }} \exp\left( - \frac{(x' - x)^2}{2 \sqrt{D t} }\right) \\ &= \frac{1}{{L \sqrt{\pi}}}\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du e^{-u^2} \\ &= \frac{1}{{2 L}} \left( \text{erf}\left( \frac{L/2 -x}{2 \sqrt{D t}} \right)-\text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} \right)\right)\end{aligned} \hspace{\stretch{1}}(1.0.20)

Following Mathematica, lets introduce a two argument error function for the difference between two points

\begin{aligned}\text{erf}(z_0, z_1) \equiv \text{erf}(z_1) - \text{erf}(z_0).\end{aligned} \hspace{\stretch{1}}(1.0.21)

Using that our rectangular function’s time evolution can be written

\begin{aligned}P_{\mathrm{rect}}(x, t)=\frac{1}{{2 L}} \text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} ,\frac{L/2 -x}{2 \sqrt{D t}} \right)\end{aligned} \hspace{\stretch{1}}(1.0.22)

For L = D = 1, and t = 10^{-8}, this is plotted in (Fig3). Somewhat surprisingly, this difference of error functions does appear to result in a rectangular function for small t.

Fig3: Rectangular part of the probability distribution for very small t

The time evolution of this non-oscillation part of the probability distribution is plotted as a function of both t and x in (Fig4).

Fig4: Time evolution of the rectangular part of the probability distribution

For the sine piece we can also find a solution in terms of (complex) error functions

\begin{aligned}P_{\mathrm{sin}}(x, t) &= A \int_{-L/2}^{L/2} dx' \frac{1}{{2 \sqrt{\pi D t} }} \sin( k_0 x' ) \exp\left( - \frac{(x' - x)^2}{2 \sqrt{D t} }\right) \\ &= \frac{A}{\sqrt{\pi}}\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du \sin( k_0 ( x + 2 u \sqrt{D t} ) ) e^{-u^2} \\ &= \frac{A}{2 i \sqrt{\pi}}\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du \left( e^{ i k_0 ( x + 2 u \sqrt{D t} ) } -e^{ -i k_0 ( x + 2 u \sqrt{D t} ) }\right)e^{-u^2} \\ &= \frac{A}{2 i \sqrt{\pi}}\left( e^{ i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du e^{ -u^2 + 2 i k_0 u \sqrt{D t} } -e^{ -i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du e^{ -u^2 -2 i k_0 u \sqrt{D t} }\right) \\ &= \frac{A}{2 i \sqrt{\pi}}e^{ -k_0^2 D t } \left( e^{ i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du e^{ -(u - i k_0 \sqrt{D t})^2 } -e^{ -i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}}}^{\frac{L/2 - x}{2 \sqrt{Dt}}} du e^{ -(u + i k_0 \sqrt{D t})^2 }\right) \\ &= \frac{A}{2 i \sqrt{\pi}}e^{ -k_0^2 D t } \left( e^{ i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}} - i k_0 \sqrt{D t}}^{\frac{L/2 - x}{2 \sqrt{Dt}} - i k_0 \sqrt{D t}} dv e^{ -v^2 }-e^{ -i k_0 x }\int_{-\frac{L/2 +x}{2 \sqrt{D t}} + i k_0 \sqrt{D t}}^{\frac{L/2 - x}{2 \sqrt{Dt}} + i k_0 \sqrt{D t}} dv e^{ -v^2 }\right) \\ &= \frac{A}{4 i }e^{ -k_0^2 D t } \left( e^{ i k_0 x }\text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} - i k_0 \sqrt{D t}, \frac{L/2 - x}{2 \sqrt{Dt}} - i k_0 \sqrt{D t} \right)-e^{ -i k_0 x }\text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} + i k_0 \sqrt{D t}, \frac{L/2 - x}{2 \sqrt{Dt}} + i k_0 \sqrt{D t}\right)\right)\end{aligned} \hspace{\stretch{1}}(1.0.23)

This is plotted for A = D = L = 1, k_0 = 8 \pi, and t = 10^{-8} in (Fig5).

Fig5: Verification at t -> 0 that the diffusion result is sine like

The diffusion of this, again for A = D = L = 1, k_0 = 8 \pi, and t \in [10^{-5}, 0.01] is plotted in (Fig6). Again we see that we have the expected sine for small t.

Fig6: Diffusion of the oscillatory term

Putting both the rectangular and the windowed sine portions of the probability distribution together, we have the diffusion result for the entire distribution

\begin{aligned}\boxed{\begin{aligned}P(x, t)&=\frac{1}{{2 L}} \text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} ,\frac{L/2 -x}{2 \sqrt{D t}} \right) \\ &+\frac{A}{4 i }e^{ -k_0^2 D t + i k_0 x }\text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} - i k_0 \sqrt{D t}, \frac{L/2 - x}{2 \sqrt{Dt}} - i k_0 \sqrt{D t} \right) \\ &-\frac{A}{4 i }e^{ -k_0^2 D t -i k_0 x }\text{erf}\left( -\frac{L/2 +x}{2 \sqrt{D t}} + i k_0 \sqrt{D t}, \frac{L/2 - x}{2 \sqrt{Dt}} + i k_0 \sqrt{D t}\right)\end{aligned}}\end{aligned} \hspace{\stretch{1}}(1.0.24)

It is certainly ugly looking! We see that the oscillation die off is dependent on the \exp( -k_0^2 D t) term. In time

\begin{aligned}t = \frac{1}{{k_0^2 D}},\end{aligned} \hspace{\stretch{1}}(1.0.25)

that oscillation dies away to 1/e of its initial amplitude. This dispersion is plotted at times t = 10^{-5} and t = 1/(k_0^2 D) for L = D = 1, k_0 = 8 \pi and A = 1/2 in (Fig7).

Fig7: Initial time distribution and dispersion of the oscillatory portion to 1/e of initial amplitude

Similar to the individual plots of P_{\mathrm{rect}}(x, t) and P_{\mathrm{sin}}(x, t) above, we plot the time evolution of the total probability dispersion P(x, t) in (Fig8). We see in the plots above that the rectangular portion of this distribution will also continue to flatten over time after most of the oscillation has also died off.

Fig8: Diffusion of uniform but oscillating probability distribution

An easier solution for the sinusoidal part

After working this problem, talking with classmates about how they solved it (because I was sure I’d done this windowed oscillating distribution the hard way), I now understand what was meant by “ignore boundary effects”. That is, ignore the boundary effects in the sinusoid portion of the distribution. I didn’t see how we could ignore the boundary effects because doing so would make the sine Fourier transform non-convergent. Ignoring pesky ideas like convergence we can “approximate” the Fourier transform of the windowed sine as

\begin{aligned}\tilde{P}_{\mathrm{sin}}(k) &\approx A \int_{-\infty}^\infty \sin (k_0 x) e^{-i k x} dx \\ &= \frac{A }{2 i} \int_{-\infty}^\infty \left(e^{i (k_0 - k) x} -e^{-i (k_0 + k) x} \right)dx \\ &= \frac{A \pi}{i} \left( \delta(k - k_0) - \delta(k + k_0)\right)\end{aligned} \hspace{\stretch{1}}(1.0.26)

Now we can inverse Fourier transform the diffusion result with ease since we’ve got delta functions. That is

\begin{aligned}P_{\mathrm{sin}}(x, t) &\approx \frac{1}{{2 \pi}} \frac{A \pi}{i} \int\left( \delta(k - k_0) - \delta(k + k_0)\right)e^{-D k^2 t} e^{i k x} dk \\ &= e^{-D k_0^2 t} \frac{ e^{i k_0 x} - e^{-i k_0 x}}{2 i} \\ &= e^{-D k_0^2 t} \sin( k_0 x )\end{aligned} \hspace{\stretch{1}}(1.0.27)

Question: Generating function

The Fourier transform of the probability distribution defined above \tilde{P}(k) is called the “generating function” of the distribution. Show that the n-th derivative of this generating function \partial^n P(k)/\partial k^n at the origin k = 0 is related to the n-th moment of the distribution function defined via \left\langle{{x^n}}\right\rangle = \int dx P(x) x^n. We will later see that the “partition function” in statistical mechanics is closely related to this concept of a generating function, and derivatives of this partition function can be related to thermodynamic averages of various observables.


\begin{aligned}{\left.{{ \frac{\partial^n}{\partial k^n} \tilde{P}(k) }}\right\vert}_{{k = 0}} &= {\left.{{ \frac{\partial^n}{\partial k^n} \left( \int_{-\infty}^\infty dx P(x) \exp\left( -i k x \right)\right)}}\right\vert}_{{k = 0}} \\ &= {\left.{{ \left(\int_{-\infty}^\infty dx P(x) (-i x)^n\exp\left( -i k x \right)\right)}}\right\vert}_{{k = 0}} \\ &= (-i)^n \int_{-\infty}^\infty dx P(x) x^n \\ &= (-i)^n \left\langle{{x^n}}\right\rangle\end{aligned} \hspace{\stretch{1}}(1.0.26)


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