Minutephysics has a popular YouTube video called "How to break the speed of light". 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?

up vote 72 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.

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 $45°$, 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 $0.00000001°$, 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.

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    if there was no gravity, you can do the same thing with a garden hose. The water would reach the second point after it hits the first point faster than the sped of light. – Peter R Jun 15 '16 at 14:32
  • @PeterR However, the moon would travel a lot faster than the water from the garden hose. That is assuming that it didn't become an expanding cloud of rock. – wizzwizz4 May 26 '17 at 17:28

protected by Qmechanic Dec 26 '13 at 8:06

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