How can a shadow move faster than light?



I just read about this but… can quite wrap my head around it. Is it even possible?

In: Physics

[Discovery posted a thought experiment trying to explain it.](

In essence it can “move” faster than light Only because darkness itself is a *lack* of something rather than a something like photons or atoms or electrons. It’s something of a gimmick really, in the truest technical sense when your finger hits that lens it will still take X amount of time for the shadow to appear on the planet surface to begin with, just like it takes 7 minutes or whatever for Sunlight to reach us and if the Sun suddenly went out we wouldn’t see it for 7 minutes either.

Uhh not a physicist but basically a shadow isn’t a thing. Specifically it is the lack of light in an area.

Say you have the world’s strongest spotlight pointed at the moon. Now, if a ball were to go past the light, it would cast a shadow on the moon. If the ball were moving fast enough, the shadow could “move” faster than the speed of light.

But the shadow isn’t actually moving. The spotlight is emitting photons towards the moon, and the shadow is what happens when you interrupt the stream of photons. The only thing moving is the photons from the spotlight to the moon.

Think of it like shooting water out of a hose. The water moves at a constant speed, but you can move the stream to cover a large area pretty quickly.

Because the speed of light limit applies to physical things. A shadow isn’t really a thing. Its a just a region of low light due to something blocking the source.

You can make light “break the speed of light” too. Lets say you had a really powerful laser that you could shine at the moon. You shine it at the moon and approx. 2.6 seconds later you see the dot on the surface (about 1.3 light second journey for the light each way). Now you rapidly move your laser to sweep the dot across the surface. It is easily possible to sweep the laser over fast enough that the dot “moves” across the face of the moon faster than the speed of light. This is possible because you are just moving the point of impact of the light rapidly, but the light itself is still only traveling at 1 c.

This question reminded me about Ask A Ninja – [](

Anyways, darkness (shadows) is the absence of light. It’s not something that is cast from a flashlight but rather the areas a light is not shining on. When you look at your shadow outside cast by the sun your shadow’s shape is actually tied to the speed of light — the light going around your body is lighting up the ground around you creating the shape of your shadow. The darkness inside of your shadow however, the darker it is, is the absence of light, which was always there until the sun or other lights started to brighten it up.

The shape of your shadow is the result of a light reaching the area around you, lighting everything BUT your shadow. What’s inside of your shadow, the area your body prevented the light’s source from reaching, was always there (darkness).

Imagine swinging a ball on a string around a lamp. Consider the speed of the shadow on the wall compared to the ball. Every time the ball goes around the light, the shadow crosses the entire length of the walls. The speed of the shadow is related to the speed of the ball and the distance that thw shadow is projected. If the ball is going 10 feet per second, and the wall is 5 times as far from the lamp as the length of the string, the shadow would move 50 fps. If you extend the walls out infinitely far, the shadow “moves” infinitely fast.

Imagine we shine a powerful laser pointer at the surface of the Moon, so that it produces a visible red dot. Now suppose we point the laser pointer at one edge of the Moon, and then quickly move it to the opposite edge, crossing through the center. If we do this in one second, the laser dot will move across the entire visible surface of the Moon in a second. This is a distance of about 5000 km. So the laser dot has moved at a speed of 5000 km/s.

It wouldn’t be hard to flick the laser pointer faster. For instance, say we used a machine to spin the laser very fast, so that the laser dot sweeps across the surface of the Moon in 10 ms. This wouldn’t be hard – the machine wouldn’t have to spin that fast (on a scale of what is possible). But it would mean that the dot “traveled” across the surface of the moon at 500,000 km/s. For comparison, the speed of light (in a vacuum) is “only” 300,000 km/s.

The important thing to note is that no light actually traveled at 500,000 km/s. Each photon in the laser beam left Earth and traveled to the moon at about 300,000 km/s (or a bit slower as it passed through Earth’s atmosphere). It’s just that one photon arrived at point A on the moon, and another photon arrived at point B, and their arrivals were spaced very close together in time. However, *no photons actually traveled from A to B*. So nothing actually moved at 500,000 km/s.

Importantly, this also means that no *information* actually traveled faster than light. Suppose we used our laser to send information to the Moon. For instance, let’s say we can change the color to mean different things: red means “Earth is under attack” and green means “Earth is fine”. As we spin the laser, this means that the information about Earth’s safety reaches point A 10 ms earlier than point B. If point A had sent this message *to point B* in 10 ms, that would have broken the speed of light, since that would have required the information to travel at 500,000 km/s. But that’s not what happened, and indeed we cannot use our spinning laser pointer to send information from A to B – only to send some information to A and some information to B.