Decay of waves IIa: Minkowski background, homogeneous case

Now let us get into the mathematics. The wave equations that we will consider take the form

Equation 1
-\partial_t^2 \phi + \triangle \phi = F

where \phi:\mathbb{R}^{1+n}\to\mathbb{R} is a real valued function defined on (1+n)-dimensional Minkowski space that describes our solution, and F represents a “source” term. When F vanishes identically, we say that we are looking at the linear, homogeneous wave equation. When F is itself a function of \phi and its first derivatives, we say that the equation is a semilinear wave equation.

We first start with the homogeneous, linear case.

Homogeneous wave equation in one spatial dimension

One interesting aspect of the wave equation is that it only possesses the second, multidimensional, dispersive mechanism as described in my previous post. In physical parlance, the “phase velocity” and the “group velocity” of the wave equation are the same. And therefore, a solution of the wave equation, quite unlike a solution of the Schroedinger equation, will not exhibit decay when there is only one spatial dimension (mathematically this is one significant difference between relativistic and quantum mechanics). In this section we make a computation to demonstrate this, a fact that would also be useful later on when we look at higher (in particular, three) dimensions.

Use x\in\mathbb{R} for the variable representing spatial position. The wave equation can be written as

-\partial_t^2 \phi + \partial_x^2\phi = 0

Now we perform a change of variables: let u = \frac{1}{2}(t-x) and v = \frac{1}{2}(t+x) be the canonical null variables. The change of variable formula replaces

Equation 2
\displaystyle \partial_t \to \frac{\partial u}{\partial t} \partial_u + \frac{\partial v}{\partial t} \partial v = \frac{1}{2}\partial_u + \frac{1}{2}\partial_v
\displaystyle \partial_x \to \frac{\partial u}{\partial x} \partial_u + \frac{\partial v}{\partial x} \partial v = -\frac{1}{2}\partial_u + \frac{1}{2}\partial_v

and we get that in the (u,v) coordinate system,

Equation 3
-\partial_u \partial_v \phi = 0

Read the rest of this entry »