4 ms·

Good point about isolation and noise. I got the propagation numbers from Wikipedia: > the refractive index of glass is typically around 1.5, meaning that ligh

Good point about isolation and noise. I got the propagation numbers from Wikipedia: > the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200000 km/s [0] > the refractive index of air for visible light is about 1.0003 [0] > Wires have an approximate propagation delay of 1 ns for every 6 inches (15 cm) of length. [1] The speed of light in air is c/1.0003, or 299,700 km/s. In glass, it is c/1.5, or 200,000 km/s. If Wikipedia is correct, wire propagation delay is about 15 cm/ns, or 150,000 km/s, which is still slower than in glass. If that number is off, would you please update the Wikipedia article with the correct number? It would be good to include a better reference than the university textbook it currently cites. Also, I don't think we should assume that a future processor optical mesh will carry signals in glass. I expect it will use micro-randomized mirrors and protect the optics with inert gas. The randomized mirrors will form random optical lines between cores, on the same chip or between chips. The lines will spontaneously reconfigure due to mechanical shock or changes in temperature. The OS will detect such changes and automatically reconfigure the software to optimally utilize the current connections. This should allow hot-swapping CPU cores, and upgrading or downgrading processors on live computers. There is a scene in the film 2001 Space Odyssey where (MOVIE SPOILER) the protagonist removes pieces of a computer and the AI running on it becomes slower and slower. Such a thing may actually happen. [0] https://en.wikipedia.org/wiki/Speed_of_light https://en.wikipedia.org/wiki/Speed_of_light [1] https://en.wikipedia.org/wiki/Signal_propagation_delay#Electronics https://en.wikipedia.org/wiki/Signal_propagation_delay#Elect...

ncmncm 5y ago
Propagation delay of electrical signals depends on properties of the insulation. In coaxial cable, the insulation is foam, mostly air, so faster than light in glass. On a chip, the wire often has glass and silicon on one side, organic passivation goo on the other. So, propagation there will be slower than in coaxial cable, and could even be a little slower than in fiber. Optical interconnect via straight-line paths through inert gas, steered by mirrors, could indeed be strictly faster than through fiber. At the same time, propagation across a wafer cannot take less than a nanosecond, at best, so 1.0ns, 1.5ns. The total delay will be a product of message marshalling and routing, typically dozens or more times that. Real-time adaptation may be necessary for managing flaky quantum bits, but the more of it that happens, the less total speed you get.

ncmncm 5y ago

Propagation delay of electrical signals depends on properties of the insulation. In coaxial cable, the insulation is foam, mostly air, so faster than light in glass. On a chip, the wire often has glass and silicon on one side, organic passivation goo on the other. So, propagation there will be slower than in coaxial cable, and could even be a little slower than in fiber. Optical interconnect via straight-line paths through inert gas, steered by mirrors, could indeed be strictly faster than through fiber. At the same time, propagation across a wafer cannot take less than a nanosecond, at best, so 1.0ns, 1.5ns. The total delay will be a product of message marshalling and routing, typically dozens or more times that. Real-time adaptation may be necessary for managing flaky quantum bits, but the more of it that happens, the less total speed you get.