Nobody knows. Even Aether Wave theory alone doesn't supplies direct answer, as it relies on the inertia concept by itself and it just enhances the inertia effect by introducing of energy into waves in subsequent generations - but isn't able to create it from scratch. We can create a standing wave of arbitrary mass just by introducing a sufficient energy into vibrating inertial environment, but such environment should always have some at least infinitesimal inertia at the very beginning. It's obvious, the recursive wave equation concept alone isn't able to generate inertia from emptiness or even just from random fluctuations of vacuum.

Such fluctuations should always be inertial by it's very nature, it means, it should be organized into harmonic waves, thus creating gradient for being perceivable at all. The surprising is. without some oriented gradient the Universe would appear a quite empty even it would be full of random fluctuations. Lets try to imagine common TV set under lost of signal: you'll see just a random flickering on screen, but no motion, no time, no energy spreading, no space, no matter, no reality. The true is, the energy is able to spread just through elastic inertial environment, i.e. the environment, which contains some. at least short distance gradients. So we have to create some gradients at first from random fluctuations.

My speculation is, the source of inertia and the time delays in energy spreading is the manifestation of diffusion of such fluctuations, which is solely entropic process. The interesting thing is, the diffusion of randomly spreading particles isn't dependent on the inertia of such particles, it simply doesn't requires the inertia at all, being just geometrical concept of space. But it's able to create a gradual concentration ramp, i.e. the required gradients and it needs some time. It means, such diffusion of random fluctuations can be the fundamental process, which is able to create starting inertia. The subsequent recursive phase transform of such gradient is able to create inertial waves based on common energy/inertia concepts, and such of this it's able to enhance such gradients until the topology of space convolution allows.

By such concept, the Aether is the result of two parallel processes in mutual equilibrium: A) the diffusion, which destroys the gradient under formation of inertia/time delay and Β) inertial wave spreading, which uses such inertia for renewing of inertial gradients. The true nature of fluctuations remains unclear at this point and it's probable, it directly leads into requirement of infinite universe recursion. We are able to observe just only the gradients in terms of reality and the concept of gradient requires some continuous environment, which can be created just the big number of other gradients, recursively.

The true is, the infinite recursion of wave equation doesn't necessarily means, the concepts described above will be repeated into infinity without modification. From Godel's incompleteness theorem follows, each the recursion described some limited number of laws has it's own limits, after certain number of recursions it'll be alternated by some other recursion based on less or more different principle(s). At least we can estimate, the recursive wave equation solution will be replaced by quite complex mutation speed dependencies after some number of Universe evolution cycles, because of cumulation of Universe matter.

The Universe foam can form a real complex fractal patterns and the most interesting thing is, such complexity can be the result of just random fluctuations diffusion followed by wave equation transforms! It's obvious, in sufficiently prolonged perspective the influence of all the possible effects will lead into new randomless, from which the whole mechanism can be repeated under some modification.