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I have seen many folks doing this Moon shadow experiment concluding that shadow from point A on the Moon can travel to point B on the Moon faster than light.

What I fail to understand here (and I am sure I am wrong), is nothing can travel faster than light. I also read about Relativity where Einstein stated that every event happening for you depends on how fast the light gets from where the event is happening to you.

This is my particular problem with this Moon shadow experiment. You cannot make a shadow travel faster than light, because in order for the "disappearance" of light to get to the Moon, it has to travel at light speed.

Here is one way to understand my point. Consider we shine a flashlight on Moon, like the guy in the linked video did, we move the finger across the face of the flashlight, before we moved the finger it was casting the shadow at Point A on Moon, after we finish moving our finger, the shadow is at Point B on the Moon.

When the finger reaches the end of it's moving length, the light waves that have left the flashlight before the finger reached there will have to hit Point B and it will take them a second and a half to get there, and another second and half for us to see that shadow on point B, which makes it 3 seconds + finger moving time for us to actually see the shadow at point B. That means shadow traveled (say) across the diameter of the Moon in 3 seconds.

So. about 3000 Km in ~3 seconds make it 1000 Km per seconds which is very tiny compared to the speed of light. How can these experiments conclude that Shadow could be made to travel faster than light? What is the obvious clue that I am missing here?

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Imaginary things can "travel" faster than light

A shadow or a light spot can seem to travel faster than light, because it's not a particular physical thing, but a series of separate things, separate physical particles emitted at different time and at different locations.

Imagine that you have launched a lot of tiny bots into space with a very accurate clock and a single LED, spaced out in a straight line with a 1 km distance between each of them.

If you program them to blink their LED at particular times – say, the first one blinks at midnight, the second one at midnight + 1 second, the third one at midnight + 2 seconds, then you'd see a spot of light moving at 1 km/s across this line. If you program them to always be on except for a particular moment arranged in the same manner, then you'd see a "shadow" moving at 1 km/s.

If you'd do the same, but set the intervals when your bots light up to 1 millisecond instead, you'd see that the signal is "moving" at 1 000 km/s. If you would have them light up at 1 microsecond difference between the neighboring bots, almost at the same time, then you'd see that the signal is "moving" at 1 000 000 km/s, much larger than the speed of light – but note that there is nothing that's actually moving there, the bots are stationary.

The same applies for true shadows – they're reflected off of something that's not moving (as much), the reflected photons for each moment are different reflected photons, and the fact that a moment ago there was a reflection much further – the "reflection has moved" at above speed of light, describes only the distance and time between two separate events, not an entity that has moved anywhere.

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    $\begingroup$ +1 for the tiny bots analogy. should make it very clear for people :) $\endgroup$
    – Piotr Pytlik
    Commented Jun 9, 2017 at 14:10
  • $\begingroup$ I have heard that microwaves traveling down a rectangular wave guide move at an angle, bouncing off of one wall and then the other. The two parts of the wave moving in different directions produce an interference pattern which moves down the tube at a speed which can exceed the speed of light. $\endgroup$
    – R.W. Bird
    Commented Mar 7, 2020 at 3:40
  • $\begingroup$ Re, "you'd see that the signal is 'moving' at 1 000 000 km/s." The fact that the thing seems to move faster than the speed of light is proof that the thing is not a signal. Signals carry information. Your apparently moving dots of light and shadows on the moon do not carry information (at least anyway, they don't carry information in the direction that they seem to be moving.) $\endgroup$
    – Solomon Slow
    Commented Dec 9, 2020 at 14:51
  • $\begingroup$ Okay, I hate to point this out but there is a means of communication that depends upon the ABSENCE of light and not its presence, it is called Morse Code and if anyone is suggesting that a shadow can travel faster than the light beam that is producing it, they are also suggesting that FTL communication is possible using Morse Code and I frankly don't think that is possible. $\endgroup$
    – Richard Stanzak
    Commented Oct 15, 2023 at 3:51
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I think you've said a lot of things that are correct, you've just come to the wrong conclusion.

You're right that that it will take us three seconds to SEE the shadow moving across the moon, because that's how long it takes the light to get there and back. But what we'll see is not the shadow slowly moving across the moon over a period of three seconds. What we see is: nothing happens for three seconds (the shadow stays at point A); then, the shadow races across the moon to point B, possibly exceeding the speed of light.

The reason this can happen is simply that shadows are not made out of particles, so there's no reason to think they can't go faster than lightspeed. And as @brucesmitherson pointed out, there's actually no information being transmitted faster than light, since the shadow still takes three seconds to start moving.

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It is known that moving a lantern will result in a spot or projected light that travels faster than light is the screen is far enough. However, this effect and similar ones do not refer to the motion of actual objects at faster than light speeds, nor this process allows transfer of information faster than light.

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  • $\begingroup$ So basically we are saying, if blocking a light source would cast a shadow on the projected area in 0 seconds, that would allow us to move the shadow FTL on the Moon. But to actually "see" the shadow go across the Moon FTL is impossible? $\endgroup$
    – fahadash
    Commented Jun 4, 2016 at 16:49
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    $\begingroup$ sorry, I have not seen the video and I used the example of the projected light from a lantern instead of a shadow. You can see the shadow moving at faster than light speed, but the source will not cast a shadow in 0 sec, it will take time for the information reach the shadow (to propagate from the light source to the shadow), that is why you cannot transfer information even if the shadows moves at faster than light speed $\endgroup$
    – user83548
    Commented Jun 4, 2016 at 16:53

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