Here’s why. Vilenkin notes that the Wheeler-DeWitt equation, like all differential equations, allows for an infinite number of solutions. To determine a unique solution—a unique universal wave function—theoretical physicists must carefully choose boundary conditions and impose them on the equation at the outset. Yet unlike the boundary conditions imposed on a vibrating string (recall the discussion in Chapter 13), no physical system yet exists that can determine the appropriate constraints on the Wheeler-DeWitt equation.
