Can a light beam move faster than light?

  • 39 Replies
  • 8576 Views

0 Members and 1 Guest are viewing this topic.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Can a light beam move faster than light?
« on: 05/10/2012 04:44:16 »
How many agree to this statement?


"If you wave a flashlight across the night sky, then, in principle, its image can travel faster than light speed (since the beam of light is going from one part of the Universe to another part on the opposite side, which is, in principle, many light years away). The problem here is that no material object is actually moving faster than light. (Imagine that you are surrounded by a giant sphere one light year across. The image from the light beam will eventually hit the sphere one year later. This image that hits the sphere then races across the entire sphere within a matter of seconds, although the sphere is one light year across.) Just the image of the beam as it races across the night sky is moving faster than light, but there is no message, no net information, no material object  that actually moves along this image."

From http://bigthink.com/dr-kakus-universe/what-travels-faster-than-the-speed-of-light

:) It's certainly thinking big, but?
« Last Edit: 08/10/2012 20:08:35 by chris »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #1 on: 05/10/2012 04:46:47 »
Let us dissect the statement slightly, either it presumes a difference between the light waves (photons) hitting the spheres surface or it has a magical component that reminds me of alchemy. If now this is possible, that light change its properties due to geometry then warp drives sounds very feasible. I bet Dr Kaku can build one in his backyard although it might go faster if his tools are one light year long.

The point is that we're not talking shadows and a geometry, but light which is mediated by radiation (and that's 'c').
=

It becomes a logical fallacy to me in that if light is governed by 'c' it doesn't matter if you assume 'information' being sent or not. What we would call information is our treatment of that light, but the intrinsic properties of light won't change due to that. One mans information is another mans BS if I may paraphrase slightly. What he most probably are associating to is entanglements, and their concept of 'information', but this is just radiation and radiation will obey 'c' from all frames of reference. Another way to express it is that he's mixing scales here, assuming 'as on a quantum plane, so macroscopically' which phrase then would be me lending from 'as above so below' or something to that similarity, which I believe being one of those ideas governing alchemy? Anyway, that statement in itself isn't necessary bs, but moving it to lights propagation most certainly becomes one, as I see it?

So, do you agree?
=

To express it clearer, what he assumes is that if we had one observer on earth and one 'observer' consisting of that 'intelligent sphere' and we had arrived to a understanding of communicating relative the flashlight sent out, he then must assume that this flashlight hitting the sphere suddenly would 'slow down'? Now that is a interesting point and feasible if we had some way to put some device on the moon for example testing for this, but, I soo very much doubt a such thing to happen.

You can also assume that he turns of and on the flashlight, sending a Morse code for example while moving it.


« Last Edit: 05/10/2012 05:44:59 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline butchmurray

  • Sr. Member
  • ****
  • 194
  • If I had my druthers, I would have druthers
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #2 on: 05/10/2012 05:31:51 »
The information reaches opposite edges of the sphere from the center at the speed of light. No information is sent from one edge of the sphere to the opposite edge of the sphere. Both edges receive the same information from the center of the sphere. The opposite edges are not in contact with each other or can know, as a fact, that the opposite edge received the information.
I was not smart enough to know it was impossible to do what I did.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #3 on: 05/10/2012 05:45:52 »
Yes, if you reread what I'm writing about you will see that I'm not questioning the idea of moving a flashlight in itself. You can as easily assume that if I cast a shadow above me head that shadows geometry must grow the further out in the universe I assume it to 'propagate' :)  What I'm questioning is how he relates it to 'information'. Lights propagation in a vacuum and 'information' as in a quantum entanglement are two different things as i see it. And he better come up with a experiment if he want me to believe that statement.
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #4 on: 05/10/2012 06:01:54 »
It is quite interesting though, assuming that we really find it having a continuous beam moving FTL at the spheres surface. What this then might be considered to proof is that light is a smooth experience, not 'photons' per se, as soon as that beam is proven to move across the surface continuously, faster than light, that as all photons are point particles :) And it would give us a interesting universe (as long as we're assuming wave/particle equivalence:) as directionality of the origin then would be a definition of 'c', whereas the beam moving across the sphere would surpass that. But to make that become 'information' :) Nah. Don't think so. And if it is, then my thought experiment above must 'slow it down' to 'c', so we can test for it. Actually I would expect this feasible on Earth, although I would prefer larger distances. Then there was something more irritating me, but I forgot :) later maybe?
« Last Edit: 05/10/2012 06:15:14 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline bizerl

  • Sr. Member
  • ****
  • 279
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #5 on: 05/10/2012 07:23:25 »
I'm trying to clarify what this experiment actually is.
As far as I can tell, the flashlight is at the centre of a sphere 1 light year in diameter.
The flashlight gets switched on and the light radiates out at the speed of light, hitting the sphere in half a year.

Now are you saying that because the light hits different parts of the sphere at the same time, that it is somehow travelling across the sphere? Because I'd have to disagree. The light that spreads out over the sphere is still dependent on the source in the centre. Any changes made in the centre would still take half a year to reach the sphere.

Have I missed some important point here?

*

Offline bizerl

  • Sr. Member
  • ****
  • 279
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #6 on: 05/10/2012 07:31:59 »
I've just re-read and I think I understand the thought experiment further, but wouldn't it be like saying that if you had a garden hose and sprayed it around fast enough and far enough, some drops could fall far enough apart in distance and so closely together in time, that light would not have been able to travel fast enough between the two.

Therefore you could say that water travels faster than light.

Again, am I missing something?

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #7 on: 05/10/2012 08:47:42 »
The idea is that the transverse movement of your hand moving that flashlight relative the sphere, or imagine a crosswise placed infinite 'plate/ground' one light year away, will translate that small motion you do into a (laser) beam, that one ly away will have to move faster than light to fit your motion done at earth. Turning your flashlight in a arc you should be able to cover almost half of the universe in that movement imaginatively :) And as light only annihilate but never dies you can then assume that you actually are able to get the beam propagating one light year.

There are several objection I can make to this idea in itself, but I won't at the moment. Instead I'm discussing the implication of the statement that this (FTL) should be see as allowable by the idea of 'information', as DR Kaku connect it to the idea of information in a entanglement. Light is either always a information, or never a information simply seen, as the idea of information in a quantum manner builds on the idea of us observers, at a astronomical distance from each other, first 'talking ourselves together' agreeing on what different types of light might mean, in entanglements mostly defined as its polarization (spin), or, in this case,  I see nothing hindering you from using Morse code:) to 'transmit that information. There is a third possibility too possibly? Having to do with the observers becoming in some way 'unique', in that they are conscious beings, so making this a unique system, not to be compared to some volcano emitting flashes of light propagating in the universe to some planet reflecting it.

But the point is that light is 'c', everywhere. And that its properties won't care for your definition of a 'communication' by it. They must stay the same, so assuming that the beam can do ftl due to it not being 'information' is meaningless to me. And if this was right you would also have to assume the same as with a entanglement, that it only can work when first being communicated as 'information' inside light speed. In this case the equivalence should be that the light sweeping FTL over the light sphere now would be forced to obey 'c'. If you by that FTL assume a entanglement effect.
=

Probably :) it's better to put it this way, it won't matter to light if you're communicating by it, or not. It will still have the same speed, so the 'no information theory' becomes meaningless.
==

I'm not sure if my first reaction is appropriate longer, need to get some sleep first, but my second statement is to the point, it makes no difference for light what you do with it, the speed is the same. Heh, I think I mixed it (my first argument) with a wave function breaking down in the measurement :) ala Copenhagen.

Ah well, Dr kaku is not alone in presumptions, and instinctive connections, I do them too :)
But 'c' is 'c'.
« Last Edit: 05/10/2012 09:18:30 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #8 on: 05/10/2012 09:25:11 »
As for your analogue with light traveling in densities larger than a vacuum Biz, it will still have a proper speed of 'c' as far I consider it. It's the interaction that present us with a different speed when measuring, similar to the way gravity 'bend a light path' as defined by the observer.  To assume otherwise makes 'c' a variable, and do not fit the experiments done.
=

The point with relativity is that it actually can explains lights behavior. It gives it one speed 'c' in all circumstances, except when interacting as that is when it annihilate. So interactions in densities is 'photons' annihilating creating a energy excess that results in another photon leaving whatever that first photon interacted with. And as we're now talking 'real particles' (matter) of rest mass those interactions must take a measurable 'time' for us. Relativity collect gravity space and matter as well as 'motion/accelerations' under one roof, defined by a constant 'c' and 'energy', and it has worked so far, and that's a hundred years :)

And people are still finding new way to interpret the equations. Doesn't mean that it has to be the final answer, but if it isn't, it still will be a very good approximative one.
« Last Edit: 05/10/2012 09:46:13 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #9 on: 05/10/2012 14:12:21 »
There is no problem in the thought experiment itself.  If you wave a flashlight about, the tip of the beam can trace out a surface faster than light.  If you had two flashlights aimed in opposite directions, the distance between the beam tips would increase faster than light as well. 

His claim about information is also true, but a bit confusing.  Let's say you modulated this beam so that you encoded Beethoven's 9th on it.  If someone on the other end set up a giant satellite dish on this sphere, they wouldn't be able to play back the signal faster than light, even though it might all hit the dish at the same time.  This is because the sphere is huge and they'd have to collect the signal from each point, back to their playing device, which would be constrained by the speed of light.

*

Offline syhprum

  • Neilep Level Member
  • ******
  • 3893
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #10 on: 05/10/2012 14:22:16 »
The spot on the screen of my expensive oscilloscope will move faster than c for what its worth.
syhprum

*

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Re: light moving faster than light in a vacuum.
« Reply #11 on: 05/10/2012 19:18:53 »
As Syhprum implies You do not have to go as far away as one light year to encounter the speed of light circle if you swept the beam through about 90 degrees  in one second it would happen closer than the moon a practical distance to doing the experiment.  The main point is that the light IS NOT TRAVELLING SIDEWAYS! the light is travelling in a straight line from the torch towards an observer looking back to the source and all the observe sees is a flash of light just like the beam from the lighthouse if there is empty space the observer looking sideways will not see anything as the light beam comes towards him.

However if there is a slight thin layer of mist showing the position of the beam somewhere a short distance closer to (or further from) the source some very interesting things happen.

The light beam as it passes through will create a moving spot in the mist.  This can be seen and measured as TRAVELLING FASTER THAN LIGHT!  However, again absolutely nothing is travelling faster than light and even if the beam was modulated with information it can easily be proved that no information has actually been transmitted faster than light because the delay caused by the time the light takes to travel from the bright spot to the observer prevents this.

"faster than light" apparent movements can sometimes be observed in astronomy when the light "flash" from a supernova peak brightness passes through a layer of dust some distance away from the source of the flash.
Learn, create, test and tell
evolution rules in all things
God says so!

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #12 on: 07/10/2012 14:57:26 »
Hmm, I can see what you mean JP, you might say that it then would prove the concept of anything over 'c' becoming a barrier for information. And so it made sense :) But it should still be testable to see if it will work out the way it is assumed, namely 'ftl'. The energy of the light will be there anyway, even if moving ftl and somehow I doubt this idea, I would really like to see it tested.
=

The point to me is that we have two movements. The photons path at each frozen instant, relative the arc done by the hand moving the flashlight, that then, one ly away will translate to a light spot moving (sideways) ftl (in a vacuum) as it meet the sphere. And here it seems to be assumed to be a continuous spot. I doubt it. You can if you like stretch that distance until you get a instant continuously unbroken 'band of light' for the observer, instantly there, and simultaneously covering ?? Whatever distance you like. Ah well, at least I see how he thought :)
« Last Edit: 07/10/2012 15:05:42 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #13 on: 07/10/2012 15:27:28 »
Thinking of this JP "His claim about information is also true, but a bit confusing.  Let's say you modulated this beam so that you encoded Beethoven's 9th on it.  If someone on the other end set up a giant satellite dish on this sphere, they wouldn't be able to play back the signal faster than light, even though it might all hit the dish at the same time.  This is because the sphere is huge and they'd have to collect the signal from each point, back to their playing device, which would be constrained by the speed of light."

That would demand that anything moving over 'c' won't be collectible faster than it would take for the same information to be sent in a 'straight path' to a receiver right? Such a statement should be able to be proved mathematically and geometrically, right?
==

Assume this unbroken band of light hitting a 'sphere' much further away than one ly. Then place a observer able to intercept each 'bit/modulated waves/' of information as it hit that sphere, maybe some ly away. Can it really be proven that he can't get the information faster than the time it would take for him to receive is 'spot like' without any sweeping 'ftl' motion?
« Last Edit: 07/10/2012 15:32:08 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #14 on: 07/10/2012 16:05:16 »
Yep.  It's pretty easy to prove.  Just draw a picture and calculate how long it would take the observer to get two bits of information sent at different times if (a) they were sent directly to him and (b), they were sent to different points on the sphere and had to be transferred to him (or alternatively, he moved to intercept them). 

(The answer is that it always takes the same time for a bit to go from the sender to the sphere, and it will only add time if that signal then has to be sent over the sphere to the observer, so that case is always slower than sending the signal directly to him.  Drawing it out makes it even more obvious.)

*

Offline simplified

  • Sr. Member
  • ****
  • 428
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #15 on: 07/10/2012 17:31:38 »
"faster than light" apparent movements can sometimes be observed in astronomy when the light "flash" from a supernova peak brightness passes through a layer of dust some distance away from the source of the flash.
What is the reason?

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #16 on: 08/10/2012 17:26:02 »
To be true this statement have to work over all distances right :) No matter if it is one ly or a billion ly. Assuming that you have a unbroken 'beam' of information sent one billion ly away. A minuscule movement of his wrist will cover several ly, how many is unimportant. but somewhere, at some distance, that movement must look as a 'unbroken' instant arc of light for the observer on the sphere as the sideway sweep is assumed to be 'continuous' for the light waves, and also have no 'speed limit'. Assume your disc receiving there instead of a sphere, are you sure that the information that observer receive must as slow as a direct one way transmission?
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #17 on: 08/10/2012 17:33:46 »
Maybe you can as the information must take longer time to transmit than his 'sweep' by the wrist, when measured over astronomic distances? Which then leaves to explain how this information will be presented at the end-station, as we're now talking about a instantaneous 'arc of light', as i think of it? If we have a 'bit' universe, then the sweep must 'jump' between bits, and then the beam can't be continuous, although you might argue that we can't measure the 'cracks between the bits', possibly? It is very weird stuff.
==

Another thing, assume instead of a disc that you have photo electric cells soaking up the energy from that arc? will that energy from the beam be diluted the further away you are, or not? If not, then there must be some limit somewhere, as it seems to me.
« Last Edit: 08/10/2012 17:43:12 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #18 on: 08/10/2012 18:02:35 »
Let's try this: say I'm an alien 1 ly away from earth and I want to send you a code that's either 00, 01, 10 or 11.  I do this by quickly sweeping a laser beam that sends either a 1 or 0  to the Mars rover and either a 1 or 0 to your house on earth.   The time between the signal hitting the Mars rover and the earth is 1 second, while the time light takes to travel from the Earth to Mars is 5 minutes (at that point in its orbit).  How long will it take you to recover all the information in that signal? 

*

Offline Guthers

  • Jr. Member
  • **
  • 36
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #19 on: 08/10/2012 18:08:14 »


"If you wave a flashlight across the night sky, then, in principle, its image can travel faster than light speed (since the beam of light is going from one part of the Universe to another part on the opposite side, which is, in principle, many light years away). The problem here is that no material object is actually moving faster than light. (Imagine that you are surrounded by a giant sphere one light year across. The image from the light beam will eventually hit the sphere one year later. This image that hits the sphere then races across the entire sphere within a matter of seconds, although the sphere is one light year across.) Just the image of the beam as it races across the night sky is moving faster than light, but there is no message, no net information, no material object  that actually moves along this image."

I think you've said it yourself - "no material thing is moving faster than light". Your inference depends on the 'image' of the light on the shell being a physical object when it is nothing of the sort. The only reality of that image is that it is a product of your brain's interpretation of photons hitting your retina having bounced back from the shell.

The only things that are moving, apart from you and your flashlight, are those photons, which are moving at the speed of light in the local medium.

Imagine an analogy of a rotating cannon firing a steady stream of small bearings, representing the photons emerging from the flashlight. You can now watch successive bearings striking the shell, at a certain distance apart depending on the distance of the shell and speed of the bearings. Now if the experiment were set up just right, the interval between successive hits could be made shorter than the travel time of an individual bearing from one hit to the next would be, analogous to the 'image' in your scenario moving faster than light. This would obviously be impossible for a single bearing. However you can see that this is explained by there being more than one bearing accounting for this phenomenon.

The image is then generated by the bearings rebounding into your retina and triggering the generation of reality in your concious mind.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: light moving faster than light in a vacuum.
« Reply #20 on: 08/10/2012 19:45:00 »
Let's set it up this way. Assume that I color my beam, green, red and blue in predefined variances, meaning different things. Then let the arc beam come to be at that far distance. Assume that the guy observing those colors are some lightyear/minutes away btw :) I have now sent information faster than light, although we still need to agree on what that information means beforehand, and that will need 'light speed' to get agreed on. you might want to think that there is some universal way of decoding information, as starting with primes etc and then building from that, but ignoring that? And it should be ignored as it only can be hypotheses. So what we call 'information' won't necessarily become some others as i think. Decoding a unknown piece takes time.

So what is information then? Well, if you believe what I say then information as a description is meaningless without a common ground to decode it from. And then we have the idea of a beam constantly 'hanging together' no matter how far it propagate (to then later) or what lateral speed it moves subsequently. That I addressed with those photo electric cells as an example asking about the 'energy' received? And that one gets weirder the further that beam travels as it seems to me?
==

You can't define 'photons' as losing energy, though it should be possible to relate it to relativity and frames of reference, finding them red shifted at both sides, becoming more so as the angle widen from your position? So maybe that could work out from each point on the disc as each point represent one 'frame of reference'. But each point for itself must receive the full impact of energy from that 'photon' finding its path ending there. if you see how I mean?

You seem to be referring to 'photons' Guthers? How would you define them as they sweep the disc FTL, elongated sideways? they are 'point particles' to me, or 'energy quanta'.
=

To clarify my thoughts. To make that work, assuming a equivalence between 'waves' and 'photons' (duality). you will have to give up the idea of x amount photons representing one type of wave/energy. Or do you know a way to fix it that I miss?
« Last Edit: 08/10/2012 20:21:34 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: Can a light beam move faster than light?
« Reply #21 on: 08/10/2012 20:28:13 »
Again, there's no way for the guy to observe at more than one point, so he can't get any information FTL.

There's no question that things can "happen" FTL, such as the beam sweeping out space in this case.  What's forbidden is to send information from one observer to another FTL.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Can a light beam move faster than light?
« Reply #22 on: 08/10/2012 20:39:20 »
Then one can refute this example JP? Because, as I think, that arc of light on the 'sphere' will be instantaneous at some point, assuming a infinite universe, and placing the guy some light year(s) away should give him a good view of it. Then he just need to know what the color combination stands for, and I think this to work even if placing him further away. And that would be FTL as I think. But it also assume there to be some common ground for understanding what he sees predefined. Although, trusting in the idea of mathematics as a 'universal language', you might take one step more then and assume that you don't need to know combinations beforehand, and even after decoding it still get the info faster than it being sent piecemeal.
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: Can a light beam move faster than light?
« Reply #23 on: 08/10/2012 20:58:48 »
This is a case where you have to be very precise in your thought experiments to see what's going on.  What's forbidden here is for me to send any part of a signal faster than light to another observer.  So you're dealing with sender-to-observer transfer here.  That's important.

You can swing your laser so the signal strikes many points on the sphere nearly instantaneously, but the sphere isn't an observer.  The observer lives at one point on the sphere and only knows about the light that has arrived at his eyes.  The instant the signal arrives, the observer doesn't know that it's splashed across the whole sphere.  He only has the one piece of it that hit him directly.  The next instant, he sees light that was reflected from nearby him that has moved (at the speed of light) into his eyes.  As time goes on, he sees parts of the beam that are reflected from points further and further away.  So even though the signal almost instantly hit a huge region of the sphere, the observer doesn't know this and doesn't know what information the signal carried until it reflects into his eyes, which takes time.

Once you understand that, it's easy to see why the signal doesn't reach the observer FTL.  The reflected light always takes a longer path from sender to receiver than if you sent it directly to him.  It has to hit the sphere and reflect to his eyes rather than going directly to his eyes, which will always take longer.

And that's the information limit.  There is no way to encode data onto that beam and have it reach the observer in less total time than if you sent it directly to him.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Can a light beam move faster than light?
« Reply #24 on: 08/10/2012 21:13:10 »
Nope JP, that's not my example. I'm placing him away from the sphere, at a comfortable distance receiving the reflected 'arc beam' I refer too, assuming the light to fill in all 'points' as it sweeps FTL. Then I assume some way of manipulating that original flashlight sending the beam to code information. I used colors but? I suspect you can get in more information other ways. What I'm using is the idea of the light moving transversely faster, the further away it gets, assuming it to become a instant phenomena for that far observer at some point, as we define it FTL and then we have no limits. And the other case is about the energy perceived at that far disc if we replace the disc with light sensitive detectors at each point. Will they all receive the same 'energy', will it have changed under its propagation, and what happens when we move the detectors one trillion ly further away and that beam gets even wider. It becomes illogic to me.
=

My spell correcter seems bugged today :)
« Last Edit: 08/10/2012 21:16:14 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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 »

*

Offline Guthers

  • Jr. Member
  • **
  • 36
    • View Profile
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 »

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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 :) 
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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)?
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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.

*

Offline bizerl

  • Sr. Member
  • ****
  • 279
    • View Profile
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!

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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?
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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 »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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.

Quote
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.

Quote
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." 

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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?
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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 :)
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
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.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
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.
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."