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Comment by everybodyknows | Hacker News Reader

Anyone care to give us a Too Long; Didn't View?

Here's a summary, but really you should watch the video, it is great.

Take 1km of wire, a resistor lightbulb, and a power source. Connect oscilliscope around power source. Flip switch.

For 1 light-speed delay, you actually see current going across the lightbulb! After the speed of light delay, current starts flowing normally as expected. (Why? How? Is this spooky action at a distance?)

You're actually seeing cross-talk between the wires where the build-up of electrons on the high side actually moves electrons on the (spatially) adjacent side of the loop!

Then to really complete the puzzle, sever the end of the wire. You still see the exact same behavior up to the speed-of-light time! (Which, how could you not! You need to wait a speed-of-light delay to _discover_ that the wire was severed, so you MUST see the exact same behavior.)

When you close a switch to initiate an electrical current down some long wires it takes a measurable time before you know what's at the far end of the wire.

Therefore if the wire is either intact or broken at the far end, over a 500m distance the first 1.6us of measurement looks exactly the same.

Technically there is a very small current that flows through the circuit almost right away due to capacitance and inductance properties of the circuit, so Derek was right technically. That current won't be enough to turn on the light bulb. The rest of the current that actually turns on the light bulb takes some time to reach the bulb because the wire is so long and electrons travel with the speed of light along the wire [0].

[0] actually electrons don't move that much, instead electrons pushing against each other on the wire cause a ripple, it is this wave that moves along the wire and turns on the bulb.

200uA of current flows through the lightbulb instantly, but it doesn't reach full current until speed of light delay.

A follow-up question: would there be no tiny change in the graph if the wires exiting the battery were further away from the wires immediately entering the light bulb?

Yeah -- nothing mysterious is happening, the electrons coming out of the battery are just causing an electric field, which causes current in the wire near the 'lightbulb.' This electric field obeys the c speed limit, so if the battery terminals were far away from the 'lightbulb,' there'd be a fundamental physics limitation, the current couldn't start until at least d/c (where d is the distance).

More likely the electric field would be too weak to measure, but that's not fundamental -- you would just have trouble finding a sensitive enough 'lightbulb.'

The capacitive effects will decrease and noise created by inductive effects will increase due to a larger area.

I don't think this is possible. If it were true, then you could add a transistor and an LED on the other side of the 1000M wire, and light up the LED instantly as soon as you press the switch. (Transistors and LEDs only take a few nanoseconds to turn on.) It's not possible to send any kind of signal or information faster than the speed of light.

Watch the video, read the other comments. The "instant" current flow comes from crosstalk. While the resistor and the battery are separate by 500m worth of ire on each side, they are actually located next to each other (so both places can be measured simultaneously).

In the video he explains that the "faster than light" effect happens due to the magnetic field traveling through air (because the light bulb is so near by). At least that's how I understood it; and hence my question about changing the position.

Parent's use of "instantly" is a bit imprecise. Here "instantly" means after 1 ns, whereas "speed of light delay" means the delay over the full 500m distance out and back.

The point here is that the LED is 1m away from the source. The signal still reaches it after a light-speed delay, but it doesn't travel along the wires, it travels through the air.

Watch the animation starting at 17mins. The electrons at the light bulb end start moving immediately because of the magnetic field and the electrons at the far ends don’t move. Would have been cool to have oscilloscopes at the far ends and somehow sync them all up to show nothing happens at the far ends.

roughly : electric power (and resulting work) is transmitted at the speed of light in the form of magnetic waves, not by the movement of the electrons in the wire the current goes through

Some small part of it is. The vast majority is transmitted along the wires.

I think the most interesting part is from 16:45 to 18:00

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