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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?

David Z
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miguel.martin
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    Related: http://physics.stackexchange.com/q/6912/2451 – Qmechanic Jan 05 '13 at 01:05
  • This is kind of the same thing that is happening here: https://en.wikipedia.org/wiki/Superluminal_motion It's not the same photon you're seeing. – Jack R. Woods Jun 25 '17 at 03:32
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    Think of the beam like a jet of water coming from a hose rather than a beam of light from a laser. If you flick a hose around rapidly, what happens to the stream of water? – forest Jul 15 '18 at 01:08
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    This is asked here often, it takes approximately 3 seconds, c, for the photons you send to reach the moon and be reflected back to you, where ever on the moon you shine them. – Adrian Howard Jan 31 '20 at 06:24

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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.

Sklivvz
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Michael
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  • Continuing with the garden hose analogy, if the stationary spot consisted of $x$ amount of water/photon flux per second, then the same area (area of the stationary spot) in moving case would receive less amount of water/photon flux, am I right? The continuous flow of water from a pipe also breaks into blobs, so the laser spot motion won't look smoothly continuous as well, I think. – Apoorv Potnis Sep 16 '20 at 15:56
  • But is a physical object? The behavior of the dot of the laser in the moon can be accurately modeled by relativity, and those predictions can be confirmed by observation. It is by all means an object, and physical. It's true that it does not carry information nor violate causality. Maybe that's your definition of a physical object then, something that can be used to relay information. – Juan Perez May 31 '21 at 17:09
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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.

Rohinb97
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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
  • I don't get why this answer was just downvoted. The most voted answer well established why the devised scenario does not break any law. The question specifically asked by the user was "Is the video wrong?". I just did the math and gave an answer – Rohinb97 Feb 28 '20 at 05:03
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It is impossible to break the speed of light for a simple reason. The speed of light depends on the properties of empty space- permittivity and permeability values alone. Empty space has to change character before the speed of light is changed. This by the way is general for any wave motion and the present confusion could be comming from imagining light as particles.. The photon is not a particle, it is the probability of finding energy in a certain point- Otherwise you run into all sorts of difficulties like the size of a photon of extreemely long wavelenghts for example.

So even in sound waves in a homogeneous medium it is impossible for sound to exceed the speed of sound determined by the medium properties. Motion of objects producing and receiving sound can only change the frequency of it. The speed of sound can be truly changed only if the medium as a whole moves. Which in the case of light requires vacuum itself to move, which is unheard off if not impossible.

Then you need a method to determine the speed of a light pulse for example. You need to see the pulse as it starts and ends at a different position and time it to find out the speed. If you say you can see the two spots as they reflect back from the moon, this would then be like seeing two galaxies at the same time and reflecting this back on the speed of light. The two photons are coming from two different sources like the two galaxies.To do a proper test, you need to send a light pulse to a 45 degree mirror on the moon, travel to another mirror on its surface, then receive it back on earth and time the flight, which will clearly produce the usual speed of light.

Riad
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  • The photon is an elementary particle of zero mass in the standard model of particle physics https://en.wikipedia.org/wiki/Standard_Model . Look at single photon footprints in this experiment https://www.sps.ch/en/articles/progresses/wave-particle-duality-of-light-for-the-classroom-13/ . There is no problem with very large wavelength light composed out of a superposition of photons, It can be described mathematically in quantum electrodynamics, see this https://motls.blogspot.com/2011/11/how-classical-fields-particles-emerge.html – anna v Jan 22 '19 at 06:33
  • Thank you anna for very good references. But reading through you do see mention of probability of finding a photon''when each photon is forced to follow a specific path (right), its probability to be detected is higher than when it is left ..'' – Riad Jan 22 '19 at 07:14
  • The single spot seen on a detector that supposedly belong to a photon is only the spot where light energy and detector state are fit to produce an interaction- not a sign of a photon in my opinion. It is like putting a shallow tray with water on very low heat and observing small bubbles appearing singly and randomly here and there on the tray, then claim that phonons are particles hitting it. – Riad Jan 22 '19 at 07:20
  • except that phonons do not have a partilcle model behind them like the standard model of particle physics which models photons as particles on par with electrons and neutrinos. The SM is continually validated, and is not dependent on the medium as phonons are. – anna v Jan 22 '19 at 08:39
  • anna; I don't question the validity and importance of the photon concept as part of the SM or part of everyday understanding and applications. Thinking in terms of probability however helps clear mysteries around it. In fact this probability explanation was given to me by an eminent physicist contributer some time back, and found it convincing. It was in answer to a query I raised of how does a photon face dispersion as light from a single point expands to cover an infinite spherical surface at large distances. regards. – Riad Jan 22 '19 at 09:56