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Comment by userbinator | Hacker News Reader

Over a decade later, even overclockers haven't managed to reach 10GHz --- but some have come close:

http://www.tomshardware.com/news/amd-fx-8150-overclock-9ghz-...

At 10GHz, light travels approximately 3cm between each period of the clock.

Note that transistors which operate above 10GHz are not rare and used in microwave applications; as I understand it, the difficulty is in creating logic circuits with them and at a scale suitable for a CPU.

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The difficulty is thermal. Microwave stuff operates in linear region, extremely fast and often extremely inefficient. Logic stuff almost always operates in saturation mode where its very slow but efficient. Old timers will remember emitter coupled logic ECL which went to great efforts to operate in linear mode at amazing power use, like 100 MHz IBM mainframes needed water cooling back when 2 MHz Z80s were considered fast. You could make something like ECL circuits using modern ICs but the thermal load would be like the surface of the sun and its cheaper to break the problem down into 10000 parallel processes that to make some kind of ECL.

Funny you say that given I discovered ECL looking up ways to do fast CPU's on human-verifiable nodes. Might try to make some of them in the future.

> 10GHz, light travels approximately 3cm between each period of the clock

Light doesn't change speed, so this statement confuses me.

10GHz is the clock frequency, not the light frequency ;)

The frequency is what's changing. How far does light travel in 1/10,000,000,000 of a second?

> Light doesn't change speed, so this statement confuses me.

Practically speaking, it does change speed. The speed of light in a vacuum is fixed, but it varies based on what it's traveling through.

The propagation velocity of an EM signal in RG coax cable is 80% of c*, PCB traces can be as low as 50% and somewhat surprisingly both fiber optic cable and cat6 cable are about the same, ~60-70%. I don't know if there is any good public information about the velocity factor with modern CPU transmission lines.

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