# Does darkness have a speed?

It’s slightly fun to note that the equations that govern the conduction of heat through substances can also exactly model the conduction of “cold” through the same material. You can (at least in the simpler models) substitute “T0 - t” for “t.”

(You can put one end of a piece of conductive metal in a tank of ice water, and feel the “cold” creep up the rod, exactly the same way you’d feel the heat creep up it if you put one end in a flame.)

“Cold” really is “negative warmth.”

Electricity is the archetypal example, right?

We are so used to the principal charge carrier being labeled “negative” that we forget how odd that is. It’s because in the olden days of electricity research it was impossible to distinguish between charge carriers flowing one way, and “holes” flowing the other way. They had a 50/50 chance, and guessed wrong, but by the time we did figure out experiments to discern the charge carrier we were stuck with the convention .

In electronic terms the positive is the collector and the negative is the emitter. Electrons flow from the emitter to the collector. Conventional electricity is the opposite direction and positive is the “hot” side when in reality the negative is.

The fact that holes behave just like positively charged electrons is what allows us to make complementary semiconductors. So both NPN and PNP bipolar transistors. This is in contrast to thermionic devices (tubes, or valves, depending upon which side of the Atlantic you reside) where there is no mechanism to make a device where charge carriers flow from positive to negative.

So you can indeed make transistors where the electrons flow both from collector to emitter and emitter to collector. (In reality you don’t even need a complementary process, you can make any bipolar transistor work perfectly well upside down, it is just that you get little to no gain that way up. It is still actually a useful trick.)

The vagaries of the process means that complementary devices are never exactly mirror images of one another, but they are close enough.

This was also the joke/plot of a Garfield strip that I remember for some reason. (I could not even tell you for sure how long it has been since I have even seen a Garfield strip!)

2/12/1982

Darkness doesn’t have a speed, but it does have a hunger that’s insatiable.

(And lightness has a call that’s hard to hear.)

How far can you go with this analogy? Can you have a pair of entangled electron holes?

In the right kind of system the distinction between a hole and a particle is moot, and yes they can be entangled.

However in the context of the OP holes are kind of a diversion as you don’t get holes in systems of photons.

VSauce episode: What is the Speed of Dark?

https://youtu.be/JTvcpdfGUtQ

So a shadow can go faster than the speed of light, or not?

You…you just associated Tolkien and Lucas.

I’m going to assume you were drunk at the time.

Yes. It’s an illusion, of sorts: no actual “thing” is moving.

It’s like the cresting of a wave that is coming in to a beach at nearly ninety degrees, or the point of crossing of scissors blades. These points can move at speeds faster than light, but no actual object is moving.

Yes, in the same way that you can shine a laser at one star and then quickly move it so it’s pointing at another star. The “end” of the beam of light may have moved hundreds of lightyears in just a few seconds, but it is meaningless, the end of the light beam is not an object.

The end of the beam is not a photon, or a photon is not an object?

If you fire a bullet at one target, then point the gun at another target and fire again, has either bullet moved between the targets? No, just the barrel of the gun has moved.

Imagine the laser device as just a gun shooting out a constant stream of tiny bullets. Changing what you’re aiming at doesn’t affect the photons that have already left the device.

I have a problem with the idea that you can move the reflection (or shadow) faster than the speed of light in the first place. Doesn’t the light only reach our eyes after it has traveled all the way to the reflecting object and back?

I can probably more easily describe it using something moving much slower. Let’s say I’m throwing balls to someone across a distance, and they throw the balls back when they get them. Let’s we’re really good at throwing, and the balls take a good 5 seconds to reach the other person. But I throw them 1 per second.

The balls I throw are the light I’m sending, and the balls I get back are the light I see reflected. If I suddenly start throwing balls a different guy who throws the balls back, I don’t instantly stop getting the balls from the first person. There’s a delay that corresponds to the speed of the balls being thrown.

So, speed this up, and doesn’t that mean there’s a delay from when I move the light source, of the distance divided by the velocity, hence meaning I can’t actually move the reflection faster than the speed of light?

It’s irrelevant that all the photons are from the past…everything you see is from the past*. The reason that looking at two stars in quick succession doesn’t violate relativity is, as already pointed out, that a moving gaze doesn’t in itself transmit information.

• Although of course it depends what reference frame we’re talking about. It’s a convenience to say we’re looking 130 years back at something 130 light years away. There is no simultaneous “now”.

But yeah, your analogy works to illustrate how this is possible:

Let’s say the three of you are in a triangle and each side is 100m. So, the “speed of ball” is 20m/s (5 seconds to arrive at target).
If you ask the guys to shout when they receive a ball, you’ll hear a shout only 1 second apart (the speed of sound being so fast compared to speed of ball we’ll basically consider it instant).

So the balls “signal” just moved 100m between the two other guys in 1 second, faster than the speed of ball.

Or, if you prefer, everyone is mute and you have to wait for a return ball signal. If both the guys take the same amount of time to catch a ball and throw it back, you’ll receive the balls 1 second apart.

Pratchett, too. “Darkness is the only thing that moves faster than light, because wherever light goes, darkness was there first.”

This question came up once on (I’m pretty sure) Wait Wait Don’t Tell Me. They quoted some physicist(s) to the effect that the speed of darkness is 0, because darkness is always there waiting for the light to leave. Or something like that. Darkness doesn’t move, in other words.

If I recall correctly, some members of the audience actually booed this answer.