Solid state is limited to the speed of the movement of electrons through a circuit. An electron is a charged particle that has size and other physical factors. We are limited to how close we can place components together before we can no longer insulate the circuit and seperate and control the flow of electrons in the circuit.

Then there's photonics - I.E. light circuits. While light has line of sight issues, it can be reflected and refracted by a number of different means and doesn't have the size limitations that electrons do in a circuit.

Over a decade ago - a photonic processor was created and calculated extremely basic math at an impossibly fast rate.... but it was the size of a large coffee table.... and the trend was that everything was getting smaller and portable..... so photonics didn't go very far because there was still a lot of progress that could be made making electronics smaller and lighter.

So - now we are close to the stage where solid state electron technology has reached it's miniaturization limitations. We'll see more research in photonic research and miniturizing it into stuff that is light enough and mobile enough to be a consumer product.

IBM is working on another concept - quantum technology - I.E., getting away from the electron and going to even smaller nuclear particles to do the computing. As they have to move faster than electrons, we aren't totally certain what the limits of speed are on that or even how to fully utilize it.

I'd say that we will see solid state circuits go away and be replaced by fiber channels even within a PC. You'll see quantum processors and almost everything will have it's own processor - video, audio, com ports, - and in that way free up the bank of main CPU's for other calculations. The days of one CPU on a board are already going away. Now if only code writers would follow the trend and create more multi-processor code out there.