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Comment by cenamus | Hacker News Reader

Light speed is ~3ns per metre, so maybe the lowered speed through the fibre?

Speed of electricity in wire should be pretty close to c (at least the front)

Velocity factor in most cables is between 0.6 and 0.8 of what it is in a vacuum. Depends on the dielectric material and cable construction.

This is why point-to-point microwave links took over the HFT market -- they're covering miles with free space, not fiber.

I always thought it was about reduced path length. Interesting.

It's both. Those links try to minimise deviation from the straight link (and invest significant money to get antenna locations to do that), but they also use copper/coax cables for connecting radios as well as hollow core fibre for other connections to the modems.

I misremembered the speed of electrical signal propagation from high school physics. It's around 2/3rds the speed of light in a vacuum not 1/3rd. The speed of light in an optical fibre is also around 2/3rds the speed in a vacuum.

It seems there is quite a wide range for different types of cables so some will be faster and others slower than optical fibre. https://en.wikipedia.org/wiki/Velocity_factor

But the resignalling must surely be unrelated?

> Light speed is ~3ns per metre, so maybe the lowered speed through the fibre?

Obligatory Adm. Grace Hopper nanosecond reference:

* https://www.youtube.com/watch?v=si9iqF5uTFk&t=40m10s

It's c, but not the same c as in air or vacuum. The same applies in optic fibers. They're both around two thirds of the speed of light in vacuum.

c is the speed of light in vacuum.

EM signals move at about 0,66c in fiber, and about 0,98c in copper.

More like 0.6c to 0.75c in Cat6 Ethernet cable.

The insulation slows it down.

Don't know why you were downvoted, this is true. RF energy is carried primarily (solely?) by the dielectric, not the copper itself, simply by virtue of the fact that this is where the E and M fields (and therefore Poynting vector) are nonzero. It's therefore the velocity factor of the dielectric which is relevant.

What if I make the copper wire hollow inside, filled with vacuum? Will the signal travel at c?

Nope, I wouldn't say it's carried solely/primarily by the dielectric, since the material the conductor is made out of also matters when you are considering losses. (Someone correct me if I'm wrong.)

Also, you've got this weird thing called skin effect where the current mainly flows on the surface of your conductor. 8.5mm deep for 60Hz, but 2μm for 1Ghz. So what's in the center of your conductor doesn't really matter.

However, if you want your signal to travel at c, surround your conductor with vacuum instead of insulation. I think to actually reach exactly c your vacuum would have to cover an infinitely(?) large area around it.

If you want something more practical, air has a relative permittivity of 1.0006 (vacuum is 1.0), so if you surround your uninsulated conductor with air, you get a velocity of 0.9997c.

Yes, if the conductor is poor, it will have losses -- but still it is not the material by which power is conducted to the load. (Rather, because its resistivity creates an E field along its length and within it, you now have a nonzero Poynting vector within the conductor -- one which points outward into the environment!)

Poynting vectors are bizarre and magical.

They are definitely bizarre, and in my limited analysis of things electrical, they hinder rather than help. So much of electrical/electronic stuff can be simply modeled as flowing through the conductor, and remember a few caveats (velocity factor is caused/affected by insulation not conductor).

What if it's something in between, say cross-section is not O-shaped, like in a hollow wire, but C-shaped or almost closed circle? Where one side is vacuum and another a dielectric.

Thinking about it I believe it would be one of three possibilities: 1 slow the signal to the slowest side, 2 increase circuit resistance, or 3 "smear" the wave, so a short sharp signal would arrive long and dull (increased reactance?).

(Having pondered this a day) this is a good question and I do not know! I think this would be straightforward to perform a waveguide simulation of though.

There are people trying to sell fiber with air instead of glass (hollow-core fiber). As far as I understand, it's still too expensive (and possibly too fragile?) to be a serious contender over long distances, but if you could get it down to, say, twice the price per meter of normal glass fiber, it would be huge.

Not what's within each wire, but what is between the pair of wires is what matters. (Assuming of course the wires conduct well, as BenjiWiebe points out.)

And yes, using air instead of dielectric results in signal velocity near c. (A good example of this is ladder-line.)

c is constant, the speed of light is not.

c is the speed of light in a vacuum, but it is not really about light, it is a property of spacetime itself, and light just happens to be carried by a massless particle, which, according to Einstein's equations, make it go at c (when undisturbed by the medium). Gravity also goes at c.

I've always considered C the speed of light and gravity goes at the speed of light, not that light and gravity both go C, which is a property of spacetime. This is a much simpler mental model, thanks for the simple explanation!

I've always thought of c as the speed limit of causality

You can think of c as the conversion rate between space and time; then, light (and anything else without mass, such as gravity or gluons) travels at a speed of 1. Everything else travels at a speed of less than 1.

(Physicists will in fact use the c=1 convention when keeping track of the distinction between distance units and time units is not important. A related convention is hbar=1.)

You can tell that c is fundamental, rather than just a property of light, from how it appears in the equations for Lorentz boosts (length contraction and time dilation).

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