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I am referring to this video, on YouTube, by minutephysics, which has quite a lot of views.

In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of light on the moon will travel 20 times the speed of light. Now don't get me wrong, I do like their videos, just this one seemed a bit fishy to me. At first I thought it all practically made sense, then I realised something...

In my mind, I would think that light particles (photons) travel from the laser to the moon and bounces off the moon and back to your eye (it doesn't just stay there, in place, so you can't move it around). Now, what he is stating is that if you flick your wrist these photons that have travelled to the moon will move along with your wrist. Wouldn't these photons be bouncing off of other objects or still travelling to the moon by the time you flick your wrist? i.e. dissipating, therefore new photons will be travelling to the moon (from the laser directly).

For example: let's say you point the laser at the moon, and once it reaches the moon, you wait a couple of seconds and then flick your wrist. The laser that you have flicked will emit photons in every direction that your wrist was in, correct? i.e. The photons would shoot out in a straight line (unless disrupted) continuously, with your wrist taking no affect on the speed of the photons.

So back to the question, is this video wrong?

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Related: physics.stackexchange.com/q/6912/2451 –  Qmechanic Jan 5 '13 at 1:05
Apologies for the thread archeology, but I'm still missing one thing - if I understand correctly, the photons forming the dot at it's 'destination' on the moon, are those that left the laser after your hand stopped (unless flicking your wrist affects in any way photons already 'in flight'). If so, those take far longer to reach that new spot (the Earth - moon distance being larger than the diameter of the latter). So not only won't the spot on the moon move faster than the speed of light, it will move significantly slower (around 1/100 c if I got my basic facts right ). What am I missing? –  decPL Mar 26 at 14:28

2 Answers 2

up vote 43 down vote accepted

The photons move at the speed of light in a straight line from the laser to the moon and back. The spot on the moon can move faster than light. There is no law against that. The spot is not a physical object, just an image. When you turn your wrist nothing happens to the photons which are already on the way to the moon - they continue on the same trajectory. But new photons are emitted in the new direction of your laser. It's like waving a garden hose back and forth.

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So basically, the spot isn't moving at all; it's rather being re-created in a different position continuously. –  miguel.martin Jan 4 '13 at 10:13
Yes, exactly right. –  Michael Brown Jan 4 '13 at 10:15
+1 Nice answer. I always try to filter these faster than light confusions by asking "is it just a locus?", because loci can do whatever they like! –  twistor59 Jan 4 '13 at 12:03
Which from memory is part of the point being made in the video. –  Daniel Blay Jan 10 '13 at 13:38
@miguel.martin, just to try to convince you that this isn't totally trivial, what this means in solving a physics problem, say in special relativity, is that you can define some coordinate x(t)=1.1*c*t (where c is the speed of light) and have this be sensible, representing something real (like a laser beam shining off the moon). (also, "it's not moving it's being recreated in a different position continuously" sounds like a phrase I'd use to annoy someone during any physics problem. Preceded by "Almost. Technically...") –  NeuroFuzzy May 31 '13 at 20:52

I too am a big fan of minute physics and YouTube, but I think it might be correct.

Here you are not talking about speed of light, you are talking about speed of flicking of wrist. Let's say that I flick my wrist by an angle $\alpha$. The distance traveled by the spot will be $\tan \alpha$ times the distance between Earth and Moon.

(First imagine the scenario, and then continue reading this.)

$\tan \alpha$ is not necessarily between $-1$ and $1$, but $\alpha$ is surely a small angle (if the angle is $45^{\circ}$, then the flicking will lead it to the distance away (the distance between Earth and Moon), which definitely is not on the Moon! So it should be much smaller). So $\tan \alpha$ is smaller than one. But also it took mere seconds for me to do so and the distance is also big. So the speed should be a big number.

Doing the math, if you flick your hand by $0.00000001^{\circ}$, the point would cover one meter on the moon. And the time it takes to move your hand is obviously in huge negative powers of ten (it might be even less than $3.33564095 \times 10^{-9}$, which light takes to cover one meter). And this number's reciprocal is the speed by which the point moved.

Looks like I proved my point...

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A laser beam is not a solid object. You completely ignored the fact that individual photons each travel at the speed of light in different directions as you flick your wrist. –  Brandon Enright May 31 '13 at 20:22
Brandon, the laser is emitting the photons exactly in the direction where it is pointed to. After I flick my wrist, the photons which are already moving towards the point, which will leave us a trail, and here we are already talking about the head of that trail, which is moving accordingly. Photons might deflect, I am not opposing to that, but in the end that too will just be the thing I am talking about right now. –  Rohinb97 May 31 '13 at 20:46
Speed of light is 3e8 m/s. Diameter of moon is a bit larger than 3e6 m, so it takes about .01 sec for light to traverse the moon. So you have two lasers A pointed at right side of moon, and B pointed at left. A fires a pulse, and .001 s later B fires a pulse. So B's pulse arrives at the moon .001 second after A's. Now if you thought it was one laser being moved, not 2, you would say "Gosh, the point moved at 10c, how can that be?" –  Mike Dunlavey May 31 '13 at 20:58
Mike, now you're talking relative (motion). Firstly, I am talking about the point, which is not a solid thing, that is the reason you did not break the speed of light. (I am clarifying the thing discussed above). And mike, you are relatively moving at 10c w.r.t. The second beam. Considering one point relative to the actual setup, I moved the pointer greater than the speed of light. ( 20c according to Henry(the minute physics guy, I think he did the remaining math)). –  Rohinb97 May 31 '13 at 21:51

protected by Qmechanic Dec 26 '13 at 8:06

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