I was musing last night about computers and processors and this video of a 6-bit digital adding machine, and I know that 4 GHz seems to be the practical limit of mass-produced CPUs of the current era. Why is that the case? I was trying to imagine something like that 6-bit adding machine * 11 (ok, 10.6666...), lined up side by side, and operating 4 billion times in a second. It's mind-bending to me. XKCD has another take on it.
So, for all of you with much more background in physics than I, I guess there are a few questions rolled into one: Why does 4GHz seem to be the practical limit for processors? Is it that silicon can't switch faster than that (and would another element help)? How far do electrons go in 1/4,000,000,000 of a second, and how long are the electrical paths on, say, a Core 2 Duo processor, and a motherboard? Is 4 GHz the limit because electrons can't get from the processor, say, in and out of memory fast enough while traveling at the speed of light?