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I'm just a layman with no real science background

(a) I'm not really sure what a shadow is. Some people are saying that it's the absence of something (a photon?) and therefore it is not subject to laws of physics. But I know when I'm in a shadow and when in sunlight... that's "information", and I thought that information flow can't break the speed of light limit.

(b) Some others use the laser pointer/flicking wrist example to talk about laser dots moving faster than light speed. But that can't be right, can it? The photons from the laser point have to travel from A to B and (after wrist flick) from A to C ... but the photons are all moving at the speed of light - there's no law-breaking in moving the laser dot from point B to point C because the photons are moving either from A to B or A to C ... the distance from B to C is not material (is it?).

(c) Also, can a shadow be thought of as a gap in a photon flow? E.g. if I interrupt a beam of single photons then can I think of it as e.g. PPPSSSPPPSSSPPPSSSPPP, etc where P = photons and S(shadow) = no photon &, if so, the "SSS" is effectively travelling at the speed of light ... no faster.

marked as duplicate by Jon Custer, Yashas, John Rennie, Kyle Kanos, peterh May 27 '17 at 12:14

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    There seem to be a lot of questions here that aren't clearly broken out. AFAIK it's discussed here, by the sounds of it though you're right. You can't move things faster than the speed of light. You can change the location of a projected image faster, but everything involved in the transmittal of that image is still limited to light speed. – JMac May 26 '17 at 13:22
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    Possible duplicate of Speed of Dark experiment, does it *really* travel faster than light? – Firebug May 26 '17 at 19:44
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    A shadow is a region that is illuminated with lower intensity of light than the surroundings because something is blocking the light from some source of light to reach that region. – md2perpe May 26 '17 at 20:49
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    This actually has nothing to do with "Darkness vs. Light" perceived distinction. Light can "travel" faster than speed of light too. Suppose you are spinning a flashlight across a night sky, and suppose that your flashlight can be (eventually) seen in nearby star systems. Your spotlight can cover light-years in a matter of seconds. And yet no real faster-then-light travel occurs. – Alexander May 26 '17 at 22:42
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    VSauce - What is the speed of dark? - youtube.com/watch?v=JTvcpdfGUtQ – mmesser314 May 27 '17 at 13:00

Imagine lining up 10 alarm clocks. Now set the first to ring at 12 o'clock sharp, the second to ring 1 second past 12, the third to ring 2 seconds past 12, the fourth 3 seconds past etc.

It will look like a continuous flowing spread of ringing. As if the first alarm starts the second alarm, which starts the third, which starts the fourth etc. It looks like information is being spread. But we know that this is just an illusion - we know that they are all just preset to by coincidence ring in this order.


The alarm clock ringing depends on your presetting and not on the neighbour-clock. In the same way a photon appearing and disappearing depends on the light-source and not on the neighbour-point. That they seem ordered is just an illusion - a (deliberate) coincidence.

(You basicly explained it already yourself with the light-flickering example.)

  • So if it were not a coincidence, then it could be information being spread? – yters May 26 '17 at 16:34
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    @yters But it is, so it isn't. If it weren't a coincidence, then the spread would be limited by the speed of light. – Shufflepants May 26 '17 at 17:22
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    Note that you don't spread the information from the place where the shadow was a minute ago, but rather from the light source. So while the shadows moves faster than light, it cannot be used to carry information. – yo' May 26 '17 at 21:40
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    @yters I don't understand what you mean by "the initial conditions of the universe set up the sequence". I don't see how that's not still coincidental. – Shufflepants May 28 '17 at 19:00
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    @yters I still don't see why setting it up via initial conditions makes it not coincidental. Maybe you misunderstand what we mean by coincidental. Something is coincidental if there is no causal relationship in one direction or the other. If one alarm clock does not cause the next one to go off, then it's coincidental. If we could remove one alarm clock in the chain, and the others still go off, it's coincidental. – Shufflepants May 30 '17 at 13:56

The simple answer is yes; a shadow can move faster than the speed of light. But this doesn't break any physical law, because a shadow is not a real object. A shadow is the absence of light.

For example, if you cast a shadow of your hand on the moon, and then move your hand, your shadow would have travelled so many miles in just a second which would be more than the speed of light.

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    because a shadow is not a real object. A shadow is the absence of light Simple and best explanation to me! – Failed Scientist May 27 '17 at 0:30
  • @TalhaIrfan: You can do the same with the presence of light. Point a spotlight at the moon, then spin it, and the spot will move across the face of the moon much faster than c. – Beta May 27 '17 at 2:10
  • yes. Both are like two sides of same coin. – OFFplanet May 27 '17 at 6:53
  • @Beta Interesting! Can you kindly explain it a bit more? – Failed Scientist Jul 6 '17 at 12:41

A shadow isn't an object, you can make it travel as fast as you want. If you shed a light on a complicated surface, and turn the light off, depending on the topography of the surface, you'll find lines along which the obscured region spreads out at infinite speed.

Special relativity only prohibits1 superluminal communication. A phenomenon can move faster than light if there is no way to use that motion to send a message.

Suppose you are standing at the edge of a shadow cast by a moving object very far away. You can observe the shadow move, and its edge may indeed move faster than the speed of light.

Now, suppose that the object is moving back and forth because a friend of yours is moving it, and the two of you have agreed on a scheme for encoding a message in the way the shadow moves.

Can your friend send you a message faster than the speed of light? No. Why? Because each movement of the edge of the shadow propagates from them to you at exactly the speed of light.


1 Technically, it is not special relativity that makes the prohibition. What special relativity actually says is that whenever an information-carrying signal travels faster than light in some reference frame, there will be another reference frame in which it moves backward in time. Since both frames are equally valid, and physicists generally assume that backward time travel is impossible, they conclude that superluminal communication is also impossible.

protected by Qmechanic May 26 '17 at 14:29

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