# Border Guards

## Quantum Soccer

### Mathematical Details

The energy eigenfunctions of Schrödinger’s equation for a two-dimensional square-well potential with infinitely high walls are:

 φn,p(x,y,t) = (2/√(LM)) sin(nπx/L) sin(pπy/M) exp(–2πiEn,pt/h) (1)

where L and M are the dimensions of the well in the x– and y-directions (x and y being zero at one of the corners of the well), n and p are integers greater than or equal to 1, specifying the number of half-wavelengths that fit across the well in each direction, h is Planck’s constant, and En,p is the energy of the eigenfunction, given by:

 En,p = (h2/8m)((n/L)2 + (p/M)2) (2)

where m is the mass of the particle. The applet computes the superposition of these energy eigenfunctions for the current set of amplitudes, an,p:

 φtotal(x,y,t) = Σn,pan,pφn,p(x,y,t) (3)

on a grid of x, y values, and interpolates to find the contours of |φtotal|2. (The particle mass and the value for Planck’s constant are chosen to produce interesting effects on the length and time scales involved.)

To ensure that the probability of the ball lying in each goal is independent of the grid size, this is computed from first principles (rather than just adding up the probabilities for the grid points within the goals). The integral of |φtotal|2 over an arbitrary rectangular region isn’t too difficult to work out, and the symmetry of the goals then leads to some simplification, but the result is too messy to quote in full here. Basically, for any given set of amplitudes the probability consists of a constant plus a number of cosine waves at beat frequencies between the eigenfunctions — the constant term coming from the “beat” between eigenfunctions with themselves (n=k and p=l in Equation (4) below), and with other eigenfunctions with identical energies (other cases where En,p=Ek,l). Because of the symmetrical placement of the goals, the coefficients In,kJp,l for the beats between some pairs of eigenfunctions is precisely zero, hence the unchanging probability for the goals in the game’s starting configuration.

 ∫∫goal|φtotal|2dx dy = Σn,pΣk,l|an,pak,l|In,kJp,lcos(2π(En,p–Ek,l)t/h+arg(an,p*ak,l)) (4)

The coefficients In,kJp,l in this expression are the components (in eigenfunction coordinates) of a Hermitian matrix that represents an observable equal to 1 when the ball is in the goal and 0 when it’s not. The best possible goal-scoring spectrum (which the applet displays when you shift-click on the blue spectrum panel) is found by computing the dominant eigenvector of this matrix (the eigenvector with the greatest eigenvalue). In the infinite-dimensional case, there’d be an infinite subspace with eigenvalue 1, but the truncated matrix has a single optimum solution; this maximises the probability of the ball being in the left-hand goal when t is zero (or a multiple of 100, restoring all the phases) but also yields the same probability in the right-hand goal when certain phases are reversed (which happens when t is an odd multiple of 50).

The players are modelled as rectangular blips in the potential energy experienced by the ball, too small to have any significant effect on the shape of the wave function when they’re stationary. The matrix for this potential in eigenfunction coordinates is:

 H'kl,np(t) = Vδ2/LM [cos((k–n)πx(t)/L) – cos((k+n)πx(t)/L)][cos((l–p)πy(t)/M) – cos((l+p)πy(t)/M)] (5)

where V is the height of the potential, δ is its width, and x(t) and y(t) are the player’s coordinates. When a player moves across the field at a constant rate, x(t)=vxt, y(t)=vyt, these matrix elements oscillate; in general, there are eight harmonic components, all of equal amplitude (give or take a sign), with frequencies:

 fkl,np = (k±n)vx/2L ± (l±p)vy/2M (6)

The time evolution of the eigenfunction amplitudes obeys the set of equations (equivalent to Schiff’s Equation 35.5):

 ak,l(t) = (–2πi/h) ∫ Σn,pH'kl,np(t) an,p(t) exp(2πi(Ek,l–En,p)t/h) dt (7)

The integral in Equation (7) works as a Fourier transform, extracting any harmonic components of H' matching the transition frequency (Ek,lEn,p)/h, or its opposite. So the only significant coupling between eigenfunctions comes when the value of one of the frequencies in Equation (6) matches ±(Ek,lEn,p)/h.

The applet integrates these equations numerically, including only those cases where the frequencies match. For the particular values and units used (h=1, L=100, M=50, m=0.00125, En,p=(n2 + 4p2)/100), a transition occurs from φn,p to φk,l when:

 (k2–n2) + 4(l2–p2) = ±[(k±n)vx/2 ± (l±p)vy] (8)

Reference: Quantum Mechanics by Leonard I. Schiff, McGraw-Hill, 1968, sections 9 and 35.  Border Guards / Quantum Soccer (Technical Notes) / created Monday, 26 April 1999 / revised Thursday, 13 May 1999