In a previous blog post I suggested that some ideas in physics that seem strange when formulated in a continuous framework based on infinite sets make more sense in a discrete framework based on finite sets. In this blog post I will attempt to flesh this out in more detail. Consider a particle moving in discrete time and let x(t) denote the position of the particle at (discrete) time t. Let dt denote the increment between successive time steps. There are two ways to define velocity at time t: a ‘forward’ velocity u(t) = [x(t+dt)-x(t)]/dt, and a ‘backward’ velocity u(t) = [x(t)-x(t-dt)]/dt. We can already see therefore that it doesn’t make any sense to talk of ‘the’ velocity at time t. In a discrete context there is no notion of instantaneous velocity.
Measure the position of the particle at time t, and you get x(t). Then measure the forward velocity of the particle, and you get [x(t+dt)-x(t)]/dt. Now measure position again, and you get x(t+dt) because the time has shifted to the next increment in order to allow the velocity measurement. In order to measure velocity, the position is necessarily shifted to its value at the next time step. In this sense, position and velocity measurements cannot commute in a discrete framework. Which is to say that the order you take these measurements in affects what measurements you get. This is the key idea that motivates our constructions. In a discrete time framework, position and velocity are not defined concurrently, as position is defined at a single point in time, whereas velocity is defined using two points in time.
Suppose that we want to multiply together the position and forward velocity of the particle. If we measure position first we will get x(t)u(t), whereas if we measure velocity first we will get u(t)x(t+dt). Let us define the difference or commutator between the two as [x,u](t) = x(t)u(t)-u(t)x(t+dt). Then we have [x,u](t) = [x(t+dt)-x(t)]x(t)/dt-x(t+dt)[x(t+dt)-x(t)]/dt = [-x(t+dt)2+2x(t)x(t+dt)-x(t)2]/dt, and therefore [x,u](t) = -[x(t+dt)-x(t)]2/dt = -u2dt. Now suppose that our particle is undergoing a random walk, so that at each time step it moves a short distance dx to the left or right, each with probability p ≤ 1/2. Then u2 = dx2/dt2 with probability 2p, and u2 = 0 with probability 1-2p. Then using revised definition of the commutator [x,u](t) = E[x(t)u(t) – u(t)x(t+dt)] where E is the expectation operator, gives [x,u] = -2pdx2/dt.
Let f(x,t) denote the probability that the particle will be in position x at time t. Then, using the fact that the particle stays in the same place with probability 1-2p, we can see immediately that the function f() satisifies the relation f(x,t+dt)=pf(x+dx,t)+(1-2p)f(x,t)+pf(x-dx,t). Analogously to the continuous case, we can define discrete partial derivatives as fx(x,t) = [f(x+dx,t)-f(x,t)]/dx and ft(x,t) = [f(x,t+dt)-f(x,t)]/dt. Then from our expression for f() above, we have ft(x,t) = p[f(x+dx,t)-2f(x,t)+f(x-dx,t)]/dt. Let us also define the second partial derivative of f() with respect to x as fxx(x,t) = [fx(x,t)-fx(x-dx,t)]/dx. Expanding this expression gives fxx(x,t) = [f(x+dx,t)-2f(x,t)+f(x-dx,t)]/dx2, and therefore ft = pfxxdx2/dt. In other words, the function f() satisfies a discrete version of the diffusion equation, with diffusion constant pdx2/dt.
Now for the interesting bit. Let m denote the mass of the particle and set dx = ħ/mc, set dt = iħ/mc2, and set p = 1/2, where ħ is the reduced Planck constant, c is the speed of light, and i is the imaginary constant, so that i2 = -1. Then pdx2/dt = (ħ2/2m2c2)/(iħ/mc2) = ħ/2im = -iħ/2m, so [x,u] = iħ/m, or equivalently, [x,mu](t) = iħ, which is a discrete version of the canonical commutation relation between position and momentum. Furthermore, the function f() satisfies a discrete version of the Schrödinger equation: ft = -iħfxx/2m. We have just derived discrete versions of two of the most important relations in quantum mechanics using nothing but high-school algebra. Moreover, in deriving these results we may have uncovered some insights into why they hold, as we will now see.
If m is the mass of the particle then ħ/mc is referred to as the particle’s Compton wavelength, defined as the wavelength of a photon whose energy is the same as the rest energy of that particle. Similarly, ħ/mc2 is referred to as the particle’s Compton time, which is simply the time of a step length of the Compton wavelength taken at the velocity of light. These are considered to be fundamental quantum mechanical properties of a particle. Now suppose that m is the Planck mass, the largest possible mass of a particle, so that m = √(ħc/G), where G is is Newton’s gravitational constant. Then ħ/mc is the Planck length, the smallest unit of length, and ħ/mc2 is the Planck time, the smallest unit of time. These amounts of mass, length, and time are constructed from the values of fundamental physical constants.
Thus, our choice of units in the derivation above was far from arbitrary. What about the fact that we multiplied the Compton time by the imaginary constant i? This was not arbitrary either. The concept of ‘imaginary time’ is well established in physics, and was popularized by Stephen Hawking in his book The Universe in a Nutshell. It should be noted that the imaginary constant i is no more ‘imaginary’ than any other number; this unfortunate misnomer is only retained as it his become so well established in the mathematics world. I would argue that the imaginary constant i is more real than most ‘real’ numbers, as it can be constructed from the axioms of mathematics in a finite number of steps, whereas most ‘real’ numbers cannot.
We have seen therefore that in a discrete framework, we can derive fundamental relations of quantum mechanics in a natural way without having to resort to the usual heavy-duty Hilbert space machinery. Furthermore, these derivations used parameters based on fundamental physical constants with a clear physical interpretation. Admittedly, we have only derived these relations in a special case, and more work would need to be done to see if these results could be generalized. There is no reason to think they couldn’t be though. In my view, a discrete framework is much more natural for quantum mechanics than a continuous framework. After all, quantum mechanics is fundamentally concerned with quanta – the smallest possible, and therefore indivisible, units of nature.