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Comment by ForOldHack | Hacker News Reader

The speed of light, because it is so fast, is the most precise physical constant known : 299 792 458 m / s. Less than 7/1000ths away from 300,000,000 m / s. I am not going to sweat this in the least.

So light travels only 0.3 m / nano second, or 11.802 inches.

https://www.youtube.com/watch?v=9eyFDBPk4Yw

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Therefore in one cycle of a 3GHz processor, light travels about 4 inches. Wild.

My favorite riff on this: at a previous job we worked on a 12GHz SERDES, which meant each inch of the cable had a different data bit on it

Which is maybe part of the reason RAM needs to be so close to the CPU? Granted though most RAM access takes several CPU cycles.

This is exactly it, the longer your traces (or any sort of write I guess) between processor and memory, the longer delay you have to have for things to "settle". Additionally increasing capacitance means that it takes more effort (read, even longer) for the line to settle.

Does make me realise I don't know so much about the super low level parts of it beyond electrons and holes. One end of the wire needs to be at a lower potential to encourage electrons to flow from that side to the other. Hmmm.

Armed with this fact thinking about electronic devices. How the front of the signal travels and how suddenly the distance matters. Was blown away when I first thought about this.

Speed of light has recently been redefined, and is now _exactly_ 299792458 m/s. It’s no longer a measurement, it’s a definition.

That's a little misleading: either the meter or the second has been redefined to make c = 299792458 m/s.

Since 1983, the meter is defined in terms of the distance traveled by light in a vacuum during 1 second, see https://en.wikipedia.org/wiki/Metre#Speed_of_light_definitio...

The second is (and has been) defined independent of length for a while. It's the time it takes for a certain number oscillations of a caesium atom.

The meter was redefined as the distance light travels in a specific time. So you could say that either the meter or the speed of light was redefined to make the speed of light a round constant, but not the second.

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