# The Naked Scientists Forum

### Author Topic: Can a light beam move faster than light?  (Read 8004 times)

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #25 on: 08/10/2012 21:23:21 »
Oh, then you've basically compressed the beam in time.  But the total time from sending to receiving each bit of information is still longer than if you sent it directly to him.  Draw the line of each ray that hits the sphere and bounces to his eyes--it's longer than if you sent it directly to him, and that's basically the proof.  You can roughly treat each ray as carrying a piece of information--to get any piece of information to transmit FTL, it would have to take a shorter path than a straight line to him, which is impossible.

In other words, he gets the information very quickly, but it takes him much longer to get the first bit of it than if you sent it directly to him, so overall the total time to get the signal is slower than if you sent it directly to him.  So no piece of information went from the sender to receiver FTL.
« Last Edit: 08/10/2012 21:29:22 by JP »

#### Guthers

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##### Re: Can a light beam move faster than light?
« Reply #26 on: 09/10/2012 15:08:40 »
I've thought a bit more about what an observer standing on the shell would actually see.

Let's say you have a flashlight which can emit a beam of light while rotating at a constant rate. The emitted beam falls on a distant spherical shell upon which the observer is situated.

From the flashlight's initial orientation (A) the beam hits a point on the shell (P) (arbitrarily) 10 light minutes away from the observer, and after 10 seconds the flashlight has rotated to its final orientation (B) so it is pointing directly at the observer (O). After hitting the shell some of the light is scattered so that the observer can see it. It can be seen that the point of illumination on the shell might be considered to be travelling at considerably more than the speed of light, 10 light minutes in 10 seconds (although changing position would be more accurate).

When the flashlight reaches B, the light emitted from A is already 10 seconds away on its journey, and this difference is maintained until the light reaches the shell a time t seconds later, so it is obvious that while the light from B reaches O after t seconds, light from A, travelling the path APO by scattering, takes t + 600 seconds. In fact it can be seen that all light which reaches O after being scattered off the shell will not arrive there until after light seen directly from B.

What O will see then, is a flash directly from the flashlight, then a bright spot on the surface of the shell receding from O, until it appears to disappear at P, 590 seconds later, representing the time at which the flashlight was switched on.

Incidentally, if O has a powerful enough telescope she could observe the flashlight start to rotate 10 seconds before seeing the flash of light from B.
« Last Edit: 09/10/2012 15:12:20 by Guthers »

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #27 on: 09/10/2012 21:23:13 »
JP, what I'm using is the idea of light being instantaneous for that observer, giving him a arc of light to observe, if we assume the lateral motion to be 'continuous' at that distance. Then all colors will reflect from that sphere simultaneously from that observers point of view, as a assumption. The other way to see it is to assume that as each 'lightpath' must be transmitted at 'c', and each one belonging to the motion of that wrist, furthermore sent in 'steps' as in a casualty chain. Then that beam, although more 'stretched out' laterally the more you remove the 'sink' from the 'source, still need to obey 'c'. But, assuming this I find it real hard to accept the idea of a continuous arc of light from the far observers view, and then the question of what 'continuous' energy those detectors would measure at each point of the sphere? It's just though experiments from my side but bringing with it very strange conclusions, if they are possible to follow that is :)

PS: just been undergoing surgery so if my thoughts seem jumbled up, I will blame it on that :)

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #28 on: 09/10/2012 21:31:05 »
Guther, I will need to reread that :)

What you might be referring to is that geometrically one could assume each light path, now thinking 'photons', should become more removed from its closest neighbors in time as it propagate in SpaceTime, which may be one way to see it? But then we have the wave/particle duality and a wave should indeed be continuous, without breaks in its light paths (in time and seen as a causality chain)?

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #29 on: 10/10/2012 14:22:43 »
PS: just been undergoing surgery so if my thoughts seem jumbled up, I will blame it on that :)

Hope you have a speedy recovery!  I didn't quite follow your post, but we can continue when you're feeling better.

#### bizerl

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##### Re: Can a light beam move faster than light?
« Reply #30 on: 11/10/2012 06:48:15 »
Another summary for this armchair scientist.

I'm not entirely sure why people think that this is a way for information to be sent FTL. If I have a beam and I sweep it between two observers between two points on this enormous sphere, the signal will pass between them FTL but any information and any changes I make, still take the 1/2 a year to get to the edge of the sphere, and there is no way for one observer to control the information being swept across without relaying information back to me at light speed, then me changing the signal and sending it back, which would take double the time.

If I have two beams pointed at two different observers on the sphere, I can transfer the information simultaneously to both observers, but no information has been exchanged between the two observers.

I'm sure the light from distant stars are reaching other observers at the same time they are reaching us, despite being light years away, but there is still no "FTL travel" in all of this, either with information or particles.

Again, have I missed something?

Oh, and yes, get well soon yor_on!

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #31 on: 11/10/2012 16:27:42 »
Thnx. The surgery went fine, now it's just convalescence and painkillers :)

Anyway, if you think of it as particles, 'photons' assuming paths to them it must follow that as the distance grows each unique path must find itself for ever more separated to the next 'photon path' as the flashlight moves its arc. If you instead consider it waves then? Well, can we spit a wave into 'quantum bits' at a smallest scale? If they are a continuous phenomena then we have this unbroken beam laterally too, although we then have to redefine photons as 'particles' as I see it. You could assume a photon a excitation in a field, but what would that make a wave?

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #32 on: 11/10/2012 17:13:34 »
Good to hear you're doing well.  :)

Bizerl's point is pretty much the point I'm making, too.  Information might hit other observers at the same time as you, but until they pass their information to you, you can't make use of what they got.  That transfer from them to you limits the whole process to the speed of light.    As I mentioned before, since light moves at a constant speed, the signalling time is proportional to the length it has to travel.  The shortest possible path from source to observer is a straight line, indicating direct communication.  Any path involving hitting a sphere and reflecting off it is automatically longer than that straight line, so it's slower than that limit.  The only way you'd have FTL communication is if you somehow had a line that was shorter than a straight line.

As for breaking up a signal into pieces, the way you do it isn't really important.  The easiest way is to think of the laser as being flicked off or on to indicate a stream of bits.  There are more sophisticated ways to encode information on a classical wave, but you can rigorously show that the limit is the same.  The problem is that even if this on/off beam is spread across the entire sphere at the same time, the observer has to somehow obtain the whole signal to get all the information contained in it.

You can generalize all this to photons, but its not really worth going down that path until you understand the classical case fully, since it builds upon the classical case, since photons aren't "little bullets" and are difficult to deal with in a rigorous way.

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #33 on: 13/10/2012 14:51:41 »
Yeah :)

B u t ::))

To make the statement work, that you have a continuous beam moving laterally at our 'sphere', you need to some pretty drastic brain gymnastics as it seems to me? Because the further away you place that sphere the more pronounced your motion of the wrist must become as that beam finally hits. And that's why I put it in terms of the energy receives at each 'spot'' of the sphere, all of this assuming our standard interpretations of radiation being timeless etc. And that one is not 'observer dependent' as you can assume that no matter what your frame of reference is, radiation will move at 'c' relative it.

Or alternatively build a case in where the only thing differing that sphere from the lights origin is the 'distance' that light has propagated. Gravity, etc, being the same. To me it becomes geometry, and assuming 'photons' existing, which they do, their 'paths' (relative each other that is:) if seen classically will widen. As for a wave you then have the question of the energy received at the 'sphere'? by that waves motion laterally over each spot, as defined by the spots light detectors changing output? If you would find the energy to change with distance it would become a problem, ignoring expansion here. But if it doesn't change with the distance? Where would the extra energy needed come from? Always assuming a smooth motion of that beam laterally?
==

What is the time for correcting spelling, words, etc for TNS those days?
To short for me anyway :)
=

It's geometry, and then a question of the duality relative a smooth lateral beam motion moving over the sphere. And thinking of it able to do it FTL just make it become more interesting to me :)
« Last Edit: 13/10/2012 15:04:43 by yor_on »

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #34 on: 13/10/2012 15:38:58 »
First, let me nitpick a bit, since this is one of my pet peeves about SR:
. . .assuming our standard interpretations of radiation being timeless etc. . .
Light (I'm assuming that's what you mean by radiation) is not timeless.  Special relativity says it doesn't make sense to consider the reference frame of light, which is very different than light being timeless.

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you can assume that no matter what your frame of reference is, radiation will move at 'c' relative it.
That is 100% correct, and doesn't require light to be "timeless."  :)

Quote
Or alternatively build a case in where the only thing differing that sphere from the lights origin is the 'distance' that light has propagated. Gravity, etc, being the same. To me it becomes geometry, and assuming 'photons' existing, which they do, their 'paths' (relative each other that is:) if seen classically will widen.
This is another problem.  Photons are not little bullets following paths between the source and detector.  They're smeared out over all space somehow.  You can detect them at a point, but their quantum wave is very non-bullet-like.  The "little bullet" paths you can draw out are actually rays, which are distinctly not photons and describe light's travel in the classical wave theory.

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As for a wave you then have the question of the energy received at the 'sphere'? by that waves motion laterally over each spot, as defined by the spots light detectors changing output? If you would find the energy to change with distance it would become a problem, ignoring expansion here. But if it doesn't change with the distance? Where would the extra energy needed come from? Always assuming a smooth motion of that beam laterally?

Ok, what happens is that the classical model breaks down when the energy per unit area gets small enough.  The continuous classical wave model works because you have so much energy and so many photons at each point that you can average over them and treat everything as continuous.  If you make the sphere really big, the classical energy at each point will be small enough that you need to use a quantum/photon model.  This means you'll actually pick up discrete photons at the detector rather than a continuous wave.  There's no problem with energy conservation, since the total sum of photon energy received has to add up to what you sent out initially.

There is still no FTL signal, though proving so gets much harder if you rigorously deal with photons and not "little bullets."

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #35 on: 13/10/2012 18:31:45 »
Sweet stuff JP. Although, heh, astronomically the definition of light or any radiation is that it has to be timeless, as I understands it? If you assume otherwise you get so called 'tired light'. But I'm guessing that you might say that as it 'propagate' (speed), according to what we observe and define, it also takes a 'time' according to us?

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #36 on: 13/10/2012 19:03:38 »
Sweet stuff JP. Although, heh, astronomically the definition of light or any radiation is that it has to be timeless, as I understands it? If you assume otherwise you get so called 'tired light'. But I'm guessing that you might say that as it 'propagate' (speed), according to what we observe and define, it also takes a 'time' according to us?

Again, it's important to be precise.  Timeless would mean that a photon has a reference frame in which it's clock measures no time passing.  That is in no way supported by relativity or any other physical theory that I know of.

Some tired light models propose that photons lose energy over time without interacting with matter.  As far as we can tell, photons are very stable (not losing energy or decaying unless they interact with other matter).  But that doesn't mean timelessness.

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #37 on: 13/10/2012 20:44:00 »
A photon seems very much a invariant relation to any thought up 'frame of reference' to me in its modern interpretation. Tired light assumes it not being of a constant 'energy' as I understands it. And it does take time to 'propagate' from any frame of reference observing it. I think I'm rather precise myself :) well, at times at least.

In the absence of proofs for a 'internal clock' I will consider it 'timeless' over those astronomic distances we've measured, ahem :)

#### JP

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##### Re: Can a light beam move faster than light?
« Reply #38 on: 13/10/2012 22:26:48 »
In the absence of proofs for a 'internal clock' I will consider it 'timeless' over those astronomic distances we've measured, ahem :)

You are precise when you talk about it's invariant speed in any inertial reference frame and the lack of decay/energy loss in free propagation, but that is not equivalent to timelessness in terms of physics terminology.  But physics does not equate the word "timeless" with these properties, so it's a bit imprecise to use that term if you're trying to discuss physics.

#### yor_on

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##### Re: Can a light beam move faster than light?
« Reply #39 on: 14/10/2012 14:51:35 »
I stand corrected JP :)

And maybe I might agree, it opens for interesting possibilities, assuming a propagation and that a geodesic becomes a very strange thing in a vacuum, whose metric is defined as 'gravity'? Weird stuff.

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##### Re: Can a light beam move faster than light?
« Reply #39 on: 14/10/2012 14:51:35 »