Comment by jfoutz | Hacker News Reader

Comment by jfoutz | Hacker News Reader

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I'm not even remotely qualified to answer. But anyway, a charge moving down a wire makes a magnetic field. Sending little packets of 0 or 1 means there's a lot of little fields forming and collapsing around the wire. Motherboards are very small, so that field may be big enough to push up against the field of the next wire. That's the bottleneck. electrons in a conductor are crazy fast (i think a big fraction of c). The problem is sending enough of them that you get a meaningful signal, but not so many they interfere with their neighbors.

i'd guess the speed improvements come from much more precise timing and voltages, so you can get better guarantees about interference. if voltage is +/-10%, that field will be bigger, and interfere more. If the timings are +/-10% the field will be there when you don't want it to for the next signal.

Anyway, i'm sure there are much more knowledgeable people who can give you much better insight, but i think that's the physics 101 kinda answer.

Electrons in the conductor are actually kinda slow, it's on the order of millimeters or centimeters per second. It's the electromagnetic field propagation that is fast. That is, if you have a conductor cable, and apply voltage to it, the individual electrons inside it will proceed very slowly, but they all will start moving almost at the same time along the length of the wire.

PCIe uses differential signaling which should approximate AC so the electrons do not have a net movement down the wire at all, they more or less just wiggle back and forth. In reality there is likely some DC bias resulting net migration but a signal trace should be very low current; you're probably looking at electron drift velocities in the range of a 1-2 millimeters per hour if were to hand-wave a guess.

The way I like to visualize it is a line of billiard balls. You smack one end and all balls move a bit maybe one place over but the force (charge) moves through them moving the last ball. (Yes charge isn't force but can be made into a electromotive force EMF via inductors, coils etc.).

This is also a flawed analogy as the whole line of balls will still move slow. It's more like tiny balls on a loudspeaker. When you apply energy to the loudspeaker a pattern depending on energy on frequency will develop. It's more the speed at which this pattern can change rather the speed of movement of the individual balls

I'm thinking more along the lines of the spot where the balls were is a hole which the balls now occupy. The balls moving isn't the point I was trying to make it was the force from one end to the other. Not the greatest analogy I'll admit.

I'm not sure I understand what you're saying it almost sounds like you're referring to impedance.

no way electrons speed is at centimeters or even millimeters per second, electrons travel at or close to speed of light.

Electrons move fast, though not nearly as fast of the speed of light. However, they don't go in a straight line. So if you mark a single electron and look at it from afar, you will see it move very slowly. But if you look closer, you will see it jiggling around with a slight bias toward a direction.

What goes at nearly the speed of light is the "message" that electrons should move a certain way. If you want an analogy, if you blow into a flute, even though the air is moving slowly, the sound travels fast.

You're both right. Individual electrons can move at high speeds (not c), but the overall flow of electrons in a wire is very slow. This happens because individuals move in random directions but the group has a slow push with the current.

Imagine people going to the shopping mall. Once in a while someone goes in and an hour or two later they come out the other door 10m away. But they travelled a lot more than 10m. You just didn't notice from outside.

> Individual electrons can move at high speeds (not c)

My memory may be off but as I recall the fermi speed in copper is only around 0.5% of c, rather far off. What does propagate at speeds on the order of c is the EM field, which is what most people are actually talking about when they think of electricity moving down a wire. But, it's still something around ~60% of c in a copper transmission line.

This is factually false: https://en.wikipedia.org/wiki/Drift_velocity

this is actually correct, electric signals are actually about 6inches or 15cm per nano second (not seconds)

https://en.wikipedia.org/wiki/Signal_velocity

You're describing the EM field, not the electrons themselves. Actually the two are moving in opposite directions as well.

one possible way to think about it is to realize that all wires can be modeled as a transmission line (most accurately) with finite (but non-zero) R(esistance), L(inductance) and C(capacitance) components.

signals propagate over the transmission line as a wave by alternatively transferring energy from electric to the magnetic fields i.e. between L & C.

which is where the delay comes from...

it is fairly trivial to derive the wave propagation equation for the above model (assuming ofcourse that leakage conductance is zero). when considering lossy transmission lines though, things get quite complicated, but you can always (almost) get away with numerical techniques...

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