Why FTL travel of light spot can't carry any information?

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  • #2
JesseM
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http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3
But it didn't explain why.
I just don't get it, let's say the spot travels from A to B, why can't I say some information travels from A to B, for example the shape of the spot?
But the shape of the spot at B wasn't caused by anything at A, it was caused by the source of the spot (the laser). The spot isn't a single moving object but just a series of photons hitting different positions along the line from A to B, and each individual photon traveled from the source at c.
 
  • #3
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But the shape of the spot at B wasn't caused by anything at A, it was caused by the source of the spot (the laser). The spot isn't a single moving object but just a series of photons hitting different positions along the line from A to B, and each individual photon traveled from the source at c.
Actually there's a problem bothering me constantly, when we say speed, we usually refer to the instantaneous speed. But for the speed of transmission of information it seems to be the other way around.
Let's say the source of spot is located at C, and A want to give a message to B, but A tells the message to C first and then make C tell B by sweeping the light spot from A to B.
If we calculate the average speed of this piece of information, it's certainly smaller than c, but if we calculate the so called "instantaneous speed" like what we calculate for a physical object, obviously in some time interval the "instantaneous speed" of the information is FTL.
So can I conclude that "information can't travel faster than light" is just an another ambiguous expression, and we'd better just say event at A can't affect event at B faster than light?
 
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  • #4
JesseM
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Actually there's a problem bothering me constantly, when we say speed, we usually refer to the instantaneous speed. But for the speed of transmission of information it seems to be the other way around.
Let's say the source of spot is located at C, and A want to give a message to B, but A tells the message to C first and then make C tell B by sweeping the light spot from A to B.
If we calculate the average speed of this piece of information, it's certainly smaller than c, but if we calculate the so called "instantaneous speed" like what we calculate for a physical object, obviously in some time interval the "instantaneous speed" of the information is FTL.
But why would you say the spot itself is carrying the information? Suppose A and B have agreed that A will flip a coin, if it's heads he'll send a message to C who responds by sweeping a laser beam across the path from A to B, so if B sees the spot he'll know A's coin came up heads rather than tails. But in this case you could think of it in terms of C sending a bunch of separate messages to different locations with different photons, with the only photons relevant to B being the ones that were aimed at him. If instead of photons we imagine C sending postcards saying "HEADS" to a bunch of different addresses, isn't it only the postcard sent to B that's relevant to how fast the message to B traveled? We could imagine that the addresses were houses all in a row, and that each postcard arrived at each successive house slightly later than the previous one, but surely the speed of the imaginary "spot" defined by the position of the house that had most recently received a postcard would not be relevant to the speed the information in the message was traveling.
So can I conclude that "information can't travel faster than light" is just an another ambiguous expression, and we'd better just say event at A can't affect event at B faster than light?
That's one way of thinking about it, although in some interpretations of QM there may be FTL "effects" that cannot actually be used to transmit information...perhaps you could say that if some event occurs at A, there is no way for B to learn about the outcome of the event FTL (unless the outcome can be deduced from other events in the past light cone of A which are also in the past light cone of B).
 
  • #5
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But why would you say the spot itself is carrying the information? Suppose A and B have agreed that A will flip a coin, if it's heads he'll send a message to C who responds by sweeping a laser beam across the path from A to B, so if B sees the spot he'll know A's coin came up heads rather than tails. But in this case you could think of it in terms of C sending a bunch of separate messages to different locations with different photons, with the only photons relevant to B being the ones that were aimed at him. If instead of photons we imagine C sending postcards saying "HEADS" to a bunch of different addresses, isn't it only the postcard sent to B that's relevant to how fast the message to B traveled? We could imagine that the addresses were houses all in a row, and that each postcard arrived at each successive house slightly later than the previous one, but surely the speed of the imaginary "spot" defined by the position of the house that had most recently received a postcard would not be relevant to the speed the information in the message was traveling.

That's one way of thinking about it, although in some interpretations of QM there may be FTL "effects" that cannot actually be used to transmit information...perhaps you could say that if some event occurs at A, there is no way for B to learn about the outcome of the event FTL (unless the outcome can be deduced from other events in the past light cone of A which are also in the past light cone of B).
I guess you're right, but I just don't understand why traveling information must be carried by physical objects, what about quantum teleportation?
 
  • #6
russ_watters
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I guess you're right, but I just don't understand why traveling information must be carried by physical objects...
I don't know about quantum teleportation, but a physical object need not move from point A to point B to transfer information from point A to point B. If you blast sound from point A to point B, you've just moved particles back and forth and none of them move from point A to point B.

The problem with the scenario in the OP isn't really that the spot isn't a physical object. The problem is that the spot is not being created at point A and then sent to point B, it is being created at point C and then sent to point A and B.
 
  • #7
JesseM
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I guess you're right, but I just don't understand why traveling information must be carried by physical objects, what about quantum teleportation?
Quantum teleportation doesn't work unless information is sent by a "classical channel" from the location of one entangled system to the other (see the second and third of the "remarks" here). There are general proofs that quantum entanglement can't be used to transmit signals FTL.
 
  • #8
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Quantum teleportation doesn't work unless information is sent by a "classical channel" from the location of one entangled system to the other (see the second and third of the "remarks" here). There are general proofs that quantum entanglement can't be used to transmit signals FTL.
I know we can't use it to transmit signals FTL, but the quantum channel do transmit part of the information , doesn't it?
 
  • #9
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I don't know about quantum teleportation, but a physical object need not move from point A to point B to transfer information from point A to point B. If you blast sound from point A to point B, you've just moved particles back and forth and none of them move from point A to point B.

The problem with the scenario in the OP isn't really that the spot isn't a physical object. The problem is that the spot is not being created at point A and then sent to point B, it is being created at point C and then sent to point A and B.
I guess I'm just confused by the expression "information can't travel ftl", or the word "information" is just ambiguous per se. I think I'll just use the expression
"if some event occurs at A, there is no way for B to learn about the outcome of the event FTL" like Jesse said
 
  • #10
JesseM
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I know we can't use it to transmit signals FTL, but the quantum channel do transmit part of the information , doesn't it?
It depends if you subscribe to an interpretation of QM that explains entanglement effects in terms of the different parts of the entangled system "communicating" in some hidden way.
 
  • #11
russ_watters
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I guess I'm just confused by the expression "information can't travel ftl", or the word "information" is just ambiguous per se. I think I'll just use the expression
"if some event occurs at A, there is no way for B to learn about the outcome of the event FTL" like Jesse said
"Learn about the event" is too vague. Right now, I am aware of events happening in far distant places. I'm even aware of many events before they happen. This does not in any way imply FTL communication or even precognition. If point C fires two photons via a beam splitter and one goes to point A and the other to point B, someone at point B can be "aware" of the arrival of the photon at point A the instant it happens. But again, that information is not being transmitted from A to B, it is being transmitted from C to B....and being combined with foreknowledge of the experimental setup.

What can't happen is for point A to transmit the information to point B FTL.
 
  • #12
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"Learn about the event" is too vague. Right now, I am aware of events happening in far distant places. I'm even aware of many events before they happen. This does not in any way imply FTL communication or even precognition. If point C fires two photons via a beam splitter and one goes to point A and the other to point B, someone at point B can be "aware" of the arrival of the photon at point A the instant it happens. But again, that information is not being transmitted from A to B, it is being transmitted from C to B....and being combined with foreknowledge of the experimental setup.

What can't happen is for point A to transmit the information to point B FTL.
It's not just "Learn about the event", it's "Learn about the outcome of event", you can't be aware of the outcome, that would be pretty much just a guess.
 
  • #13
russ_watters
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If "the event" is just a timing signal, for example, and you know the setup of the apparatus, two points far away from each other can both know when (or even before) the timing signal arrives at the other point.
 
  • #14
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If "the event" is just a timing signal, for example, and you know the setup of the apparatus, two points far away from each other can both know when (or even before) the timing signal arrives at the other point.
Emm, I never think of it this way. Ok then, but what's your definition of "information"?
 
  • #15
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"Learn about the event" is too vague. Right now, I am aware of events happening in far distant places. I'm even aware of many events before they happen. This does not in any way imply FTL communication or even precognition. If point C fires two photons via a beam splitter and one goes to point A and the other to point B, someone at point B can be "aware" of the arrival of the photon at point A the instant it happens. But again, that information is not being transmitted from A to B, it is being transmitted from C to B....and being combined with foreknowledge of the experimental setup.

What can't happen is for point A to transmit the information to point B FTL.
Just curious. If somebody inserted a mirror between point C and Point A so that the photon intended for point A was deflected to say point D. Would the the assumption of the observer at point B, that a signal arriving at point B indicates the arrival of the other photon at point A would be wrong, or am I missing something?

[EDIT] Basically, I was making the same point that Jesse expressed much better in the following post. The prediction can be FTL, but the information confirming the event is not.
 
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  • #16
JesseM
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If "the event" is just a timing signal, for example, and you know the setup of the apparatus, two points far away from each other can both know when (or even before) the timing signal arrives at the other point.
you don't really know the timing signal reached the distant apparatus until you get a signal from that apparatus confirming it--until then you can predict it'll reach the apparatus, but it's always possible something blocked it.
 

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