Quizzes & Puzzles13 mins ago
Quantum Entanglement - Information Faster Than Light?
16 Answers
I'd be interested to hear your thoughts on what this could mean when applied to any number of processes... I'm told we are able to contain subatomic particles and allow them to become 'entangled,' which simply means they share opposite spin (i.e. one clockwise and the other anti-clockwise.)
Then, and here's the tricky part, we can separate them by astronomical distances if we wanted, and by measuring the spin of one particle to force it so spin in one direction, instantaneously, the other particle begins to spin the opposite way, therefore information can be transferred at once, irrespective of time and distance.
Great discovery, but not sure how it works. What is the science behind this? Which particles exert this property? Can we apply this to communications? Space exploration? Computing?
IHI
Then, and here's the tricky part, we can separate them by astronomical distances if we wanted, and by measuring the spin of one particle to force it so spin in one direction, instantaneously, the other particle begins to spin the opposite way, therefore information can be transferred at once, irrespective of time and distance.
Great discovery, but not sure how it works. What is the science behind this? Which particles exert this property? Can we apply this to communications? Space exploration? Computing?
IHI
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There are some good answers in the "related" thread linked below. For convenience I'll copy the link here:
http://
My own answer is as follows. No. No information is travelling faster than light, in the sense that anybody sitting next to the second particle, whose spin is not measured, won't know the outcome of the first measurement any faster than the signal can get to him -- ie the first observer says something to him, sending a message at the speed of light. The particle itself finds out, and collapses in to the opposite spin state. But the second observer would have to perform his own measurement to work out the spin state of the particle.
I'll try to provide a better explanation of the full effect later this evening, but for the time being I'll stick to my assertion that while the quantum system is entangled so that the two particles appear to communicate faster than light, the information itself does not move faster than light, so I'm almost certain that you cannot use this to communicate faster than light.
http://
My own answer is as follows. No. No information is travelling faster than light, in the sense that anybody sitting next to the second particle, whose spin is not measured, won't know the outcome of the first measurement any faster than the signal can get to him -- ie the first observer says something to him, sending a message at the speed of light. The particle itself finds out, and collapses in to the opposite spin state. But the second observer would have to perform his own measurement to work out the spin state of the particle.
I'll try to provide a better explanation of the full effect later this evening, but for the time being I'll stick to my assertion that while the quantum system is entangled so that the two particles appear to communicate faster than light, the information itself does not move faster than light, so I'm almost certain that you cannot use this to communicate faster than light.
You can not pass information that way, merely be aware of what is spinning far away having looked at something near, so as a communications device, it's ability is limited. I suspect anything I add to this thread will verge on wishful thinking and fantasy :-) I suspect everything is connected somehow I can't even begin to picture. That there is some kind of reality in the idea that everything does everything along the line of Feynmann's particle that takes all the paths at once and is seen to travel the most likely. Perhaps it lets the far entangled particle how it should be spinning en route :-D Ah it's fun to let the imagination run wild.
Orson Scott Card is a fairly well known Sci-Fi author, and he wrote a series of books based around a novella (later expanded to a novel, and now shortly to be a film, I believe) called Ender's Game.
In it, he made liberal use of the Ansible, a communication device which was sort of based upon quantum entanglement - Does not move the conversation on a lot, but I thought it was interesting, and besides, QE, like Quantitative Easing, makes my brane hurt......
In it, he made liberal use of the Ansible, a communication device which was sort of based upon quantum entanglement - Does not move the conversation on a lot, but I thought it was interesting, and besides, QE, like Quantitative Easing, makes my brane hurt......
This has a history
It first comes from the EPR paradox ( the E is for Einstein who spent much of the latter years of his life desperately trying to disprove QM - in vain)
This along with Podolsky and Rosen was his best shot - pitting QM against relativity.
It was 1980 before the experiment could be done - when Alain Aspect managed it in Paris - Relativity lost!
However - and this is crucial - no information can be transmitted faster than light in this way.
Your system sends two particles with opposing spin out to two different planets and are captured by two people in some clever contraption.
They have not measured the spin so both are in a comopsite state - both up and down.
The first person observes his electron and finds in in state UP
The second instantaneously becomes down.
* he cannot select this state* it is what he gets probabistically - he therefore cannot use it to transmit information
Information does not travel faster than light - causality is preserved - and we all get to go to bed tonight after we got up and not before! :c)
It first comes from the EPR paradox ( the E is for Einstein who spent much of the latter years of his life desperately trying to disprove QM - in vain)
This along with Podolsky and Rosen was his best shot - pitting QM against relativity.
It was 1980 before the experiment could be done - when Alain Aspect managed it in Paris - Relativity lost!
However - and this is crucial - no information can be transmitted faster than light in this way.
Your system sends two particles with opposing spin out to two different planets and are captured by two people in some clever contraption.
They have not measured the spin so both are in a comopsite state - both up and down.
The first person observes his electron and finds in in state UP
The second instantaneously becomes down.
* he cannot select this state* it is what he gets probabistically - he therefore cannot use it to transmit information
Information does not travel faster than light - causality is preserved - and we all get to go to bed tonight after we got up and not before! :c)
As quantum entanglement involves 'particle's moving at the speed of light I fail to see where this is in disagreement with relativity but rather supports it in that within relativity time and distance are not relevant to objects moving at c, that is, within their rest frame they would be present at both the beginning and end of their journey at the same instant.
Ah Mib
In EPR you have two particles in an entangled state travelling in opposide directions apart at close to c
Think 2 electrons 1 up 1 down
until they are measured they are in a superpositional state up and down.
Then observer a beasures one and the wave function instantaneously collapses at at both
one electron seems to have communicated to the other at faster than the speed of light.
This isn't a bad page
http://
The experiment was reformulated in something called the Bell inequality in the 60's and then Aspect's experiment in '82 I think it was nailed it.
One of the best experiments of the late 20th century IMHO
In EPR you have two particles in an entangled state travelling in opposide directions apart at close to c
Think 2 electrons 1 up 1 down
until they are measured they are in a superpositional state up and down.
Then observer a beasures one and the wave function instantaneously collapses at at both
one electron seems to have communicated to the other at faster than the speed of light.
This isn't a bad page
http://
The experiment was reformulated in something called the Bell inequality in the 60's and then Aspect's experiment in '82 I think it was nailed it.
One of the best experiments of the late 20th century IMHO
Couldn't one use it like a binary message system (on/off, light/dark, 0/1) and have it register up or down states? Perhaps another set up would be to have a device to 'be aware' that its state is both or becomes fixed once observed from the sender of the message.
Could bot particles be observed at the same time but alternating at fixed intervals, as each detector measures the particle, it captures it's state that was held by the other detector, overlapping and maintaining the spins. Then by failing to detect the particle one end, the other becomes aware of this break when it next measuresit own particle, allowing a message to be interpreted.... Any of these ideas possible?
IHI
Could bot particles be observed at the same time but alternating at fixed intervals, as each detector measures the particle, it captures it's state that was held by the other detector, overlapping and maintaining the spins. Then by failing to detect the particle one end, the other becomes aware of this break when it next measuresit own particle, allowing a message to be interpreted.... Any of these ideas possible?
IHI
How can you use it as a binary message system ? You can't transfer what you do at one end, merely view it and know what the other end is doing. But because you can't set it you can't control what you see at your end nor what someone else can see at the other end.
The other end will not fail to detect the particle. They either observe the one they have or they do not. They have no notion as to whether the other end observed their particle before them.
The other end will not fail to detect the particle. They either observe the one they have or they do not. They have no notion as to whether the other end observed their particle before them.
Then would a single bit of information be transferable by having one detector constantly measuring it and the other periodically checking if it has changed? This could allow a pre arranged command to commence if the receiver detected an alternate spin, although it may not be instantly picked up, the ability to make an order on Earth, for it then to be carried out the other side of the galaxy would then mean information from point A is understood in point B faster than light can travel... - Sorry if I've missed the mechanics of it, but that scenario seems plausible!?
IHI
IHI
The problem with your idea is twofold, though both of these are subtleties that may not be covered properly in popular Science books. Firstly, there is no such thing as "the same time" for two different places in Relativity, so that if observer A measured one particle at an agreed time and Observer B looked at his particle a vanishingly small amount of time later... well, A and B couldn't agree on that time so that their watches would be out of sync somehow, or at least the two planets could argue later who measured their particle first. So Special Relativity tells you enough to know that it's impossible to coordinate matters, so that this system or a more complicated one could not be used to transmit information faster than light.
The second problems are quantum mechanical and come in a pair. Firstly neither observer has control over what they will measure originally -- that's the point of the entire set-up, after all -- which means that other than finding that "Oh, A measured the particle in spin-up", B learns nothing from looking at his end. Perhaps in a seriously sophisticated entangled system more information might be conveyed, but it would only be about the state of that system and not anything else attached to it, and again I stress that neither observer has control over what they see. Therefore also they have no control over the "information" that can be transmitted. Even associating say "spin-up" with "Attack at dawn" and down with "run for your life", what B does is random and not determined by A!
The other quantum-mechanical problem is that systems like this are interesting because the state of each particle changes over time, meaning that the two entangled particles are constantly changing spin state. This means in practice that, on the instant that observer A measures particle 1 in spin-up, B would see particle 2 as in spin down, but the next instant after both particles revert to changing spin states randomly. For a short time after the first measurement, particle 2 might be more likely to still be seen as spin down, but (depending on the exact nature of the system) that short time could be as short as a tiny fraction of a second. The "collapse of the wave-function" is then both an instantaneous effect, and also an effect that quickly disappears as the quantum state starts messing up again. What this means for the particle that observer B is watching is that if he measures it in spin state down he can infer that particle 1 was in spin-up at that point, but he cannot infer that A had recently measured it in spin-up.
I think that's right, anyway.
The second problems are quantum mechanical and come in a pair. Firstly neither observer has control over what they will measure originally -- that's the point of the entire set-up, after all -- which means that other than finding that "Oh, A measured the particle in spin-up", B learns nothing from looking at his end. Perhaps in a seriously sophisticated entangled system more information might be conveyed, but it would only be about the state of that system and not anything else attached to it, and again I stress that neither observer has control over what they see. Therefore also they have no control over the "information" that can be transmitted. Even associating say "spin-up" with "Attack at dawn" and down with "run for your life", what B does is random and not determined by A!
The other quantum-mechanical problem is that systems like this are interesting because the state of each particle changes over time, meaning that the two entangled particles are constantly changing spin state. This means in practice that, on the instant that observer A measures particle 1 in spin-up, B would see particle 2 as in spin down, but the next instant after both particles revert to changing spin states randomly. For a short time after the first measurement, particle 2 might be more likely to still be seen as spin down, but (depending on the exact nature of the system) that short time could be as short as a tiny fraction of a second. The "collapse of the wave-function" is then both an instantaneous effect, and also an effect that quickly disappears as the quantum state starts messing up again. What this means for the particle that observer B is watching is that if he measures it in spin state down he can infer that particle 1 was in spin-up at that point, but he cannot infer that A had recently measured it in spin-up.
I think that's right, anyway.
There are probably a couple of small mistakes in my previous post above, though I stand by the gist of it.
Mainly in the third paragraph. Depending on how you set up the system it is possible for the system to be stable rather than for the state of particle 2 to suddenly evolve away from spin-down after particle A was measured in spin-up. So the time-delay changing the result doesn't matter if you set things up properly. But the second paragraph is still correct, i.e. A has no control over his own measurement so that he could not use this to transmit a pre-determined instruction.
Mainly in the third paragraph. Depending on how you set up the system it is possible for the system to be stable rather than for the state of particle 2 to suddenly evolve away from spin-down after particle A was measured in spin-up. So the time-delay changing the result doesn't matter if you set things up properly. But the second paragraph is still correct, i.e. A has no control over his own measurement so that he could not use this to transmit a pre-determined instruction.
jake-the-peg
Ah Mib
In EPR you have two particles in an entangled state travelling in opposide directions apart at close to c
Think 2 electrons 1 up 1 down
until they are measured they are in a superpositional state up and down.
Then observer a beasures one and the wave function instantaneously collapses at at both
one electron seems to have communicated to the other at faster than the speed of light.
This isn't a bad page
http://
The experiment was reformulated in something called the Bell inequality in the 60's and then Aspect's experiment in '82 I think it was nailed it.
One of the best experiments of the late 20th century IMHO
18:03 Fri 01st Mar 2013
Thanks Jake,
From the author of the above reference (perhaps his explanation makes more sense to you) - "Here's my take on it: Imagine two entangled photons are emitted from a source. As time progresses, they move apart in space, so we view them as separate. However, if we view the entire spacetime block (as opposed to a single moment in time) then we find the two photons form a single object made up of the worldlines of the two photons. Hence, the impression that they are actually separate point particles in space is really a false impression. A particle is really a line in spacetime. And two particles are really two lines in spacetime, which can be joined. Hence, separation is an illusion."
Ah Mib
In EPR you have two particles in an entangled state travelling in opposide directions apart at close to c
Think 2 electrons 1 up 1 down
until they are measured they are in a superpositional state up and down.
Then observer a beasures one and the wave function instantaneously collapses at at both
one electron seems to have communicated to the other at faster than the speed of light.
This isn't a bad page
http://
The experiment was reformulated in something called the Bell inequality in the 60's and then Aspect's experiment in '82 I think it was nailed it.
One of the best experiments of the late 20th century IMHO
18:03 Fri 01st Mar 2013
Thanks Jake,
From the author of the above reference (perhaps his explanation makes more sense to you) - "Here's my take on it: Imagine two entangled photons are emitted from a source. As time progresses, they move apart in space, so we view them as separate. However, if we view the entire spacetime block (as opposed to a single moment in time) then we find the two photons form a single object made up of the worldlines of the two photons. Hence, the impression that they are actually separate point particles in space is really a false impression. A particle is really a line in spacetime. And two particles are really two lines in spacetime, which can be joined. Hence, separation is an illusion."
So because it's a 50/50 chance event, one could never deduce a deliberate action forcing the spin? Couldn't the law of probability help here though? (I'm aware I'm taking about this again, but it's for a completely different thing! Honestly!)
Sender (s) has particle (a), receiver (r) has particle (b) and, possibly, a sophisticated computer system compensates for any time lag due to the movement involved in separating the (r) and (s). Systems are capable of detecting spin at 1,000,000,000 dps (detections per second) and record up (u) or down (d) result. The laws of chance predict the spin ratio will be even 500,000,000 up 500,000,000 down.
(r) now continually observes (b) at a trillion dps. (s) can observe (a) in a fixed state meaning (b) spin is fixed too. If a predetermined amount of detections was agreed to be satisfied the fixed state isn't by chance, say a google of detections, you could interpret that a message has been sent. If you add further periods of fixation in a sequence it can build up to an enormous number that would be so completely unlikely to occur, it could be relied upon.
(s) + (a) = (u)
[(u) x google] / trillion dps = 90.7182818285 seconds (or 1:30 mins)
(s) releases (a) and begins to observe again until (d) is found and can be fixed for 90 seconds. This step can repeated any number of times to decrease probability of pattern emerging by chance. In 10 minutes one can be fairly sure the message is real, and a predetermined action should happen. If (s) + (a) were on Earth and (r) + (b) were on Pluto and you waited 20 minutes to observe patterns of fixed length which you then interpret as the signal, you'd be passing information faster, right?
It wouldn't matter that it is likely by chance therefore it could be a false signal, because in the real instance that the message is sent, that truly is info faster than light?
What can prevent that scenario? (Apart from the probability of a natural pattern occurring) Is the physics sound?
IHI
Sender (s) has particle (a), receiver (r) has particle (b) and, possibly, a sophisticated computer system compensates for any time lag due to the movement involved in separating the (r) and (s). Systems are capable of detecting spin at 1,000,000,000 dps (detections per second) and record up (u) or down (d) result. The laws of chance predict the spin ratio will be even 500,000,000 up 500,000,000 down.
(r) now continually observes (b) at a trillion dps. (s) can observe (a) in a fixed state meaning (b) spin is fixed too. If a predetermined amount of detections was agreed to be satisfied the fixed state isn't by chance, say a google of detections, you could interpret that a message has been sent. If you add further periods of fixation in a sequence it can build up to an enormous number that would be so completely unlikely to occur, it could be relied upon.
(s) + (a) = (u)
[(u) x google] / trillion dps = 90.7182818285 seconds (or 1:30 mins)
(s) releases (a) and begins to observe again until (d) is found and can be fixed for 90 seconds. This step can repeated any number of times to decrease probability of pattern emerging by chance. In 10 minutes one can be fairly sure the message is real, and a predetermined action should happen. If (s) + (a) were on Earth and (r) + (b) were on Pluto and you waited 20 minutes to observe patterns of fixed length which you then interpret as the signal, you'd be passing information faster, right?
It wouldn't matter that it is likely by chance therefore it could be a false signal, because in the real instance that the message is sent, that truly is info faster than light?
What can prevent that scenario? (Apart from the probability of a natural pattern occurring) Is the physics sound?
IHI
The thing you have to remember is that anything A can do, B can do too. The general point of entanglement is that what A does affects the measurement result at B. So in the same way anything B does affects the measurements at A. Regardless of how the experiment is set up, so regardless of the probabilities of each particular measurement, if both A and B are continuously measuring their particles then the situation is radically different from the original set-up, which is B waits until "after" A has measured.
Without knowing the precise details of how the set-up works, what is being measured, how the system was prepared and so on, I can't perform a full analysis. But in general there are three points:
i) Whenever you perform a quantum measurement you have no control over the outcome.
ii) Whenever someone at either end measures an entangled system that system's wavefunction collapses acorrding to the outcome, so that both measurements affect each other -> so that the measurements are not independent.
iii) The long-term results of the experiment depend on whether or not your states are time-dependent. Any time-dependence changes the analysis, but in a time-independent system the rule would be that A might measure either a million ups or a million downs a second but not a mixture of the two, i.e the first measurement completely determines the rest.
The final point is that you need to think a bit about what the second measurer sees. His experiment will tend to give spin down or spin-up, and while we from the outside know that A measured first, the results performed at B in fact cannot distinguish between whether A measured first or B did. So if B gets spin down, well he could have got that 50% of the time anyway, whether or not A measured first. The effect is that while, after measuring his spin, A knows what B will get, B still in is the dark about his result. And even performing the experiment does not give B any information about what A did, or did not, do.
Without knowing the precise details of how the set-up works, what is being measured, how the system was prepared and so on, I can't perform a full analysis. But in general there are three points:
i) Whenever you perform a quantum measurement you have no control over the outcome.
ii) Whenever someone at either end measures an entangled system that system's wavefunction collapses acorrding to the outcome, so that both measurements affect each other -> so that the measurements are not independent.
iii) The long-term results of the experiment depend on whether or not your states are time-dependent. Any time-dependence changes the analysis, but in a time-independent system the rule would be that A might measure either a million ups or a million downs a second but not a mixture of the two, i.e the first measurement completely determines the rest.
The final point is that you need to think a bit about what the second measurer sees. His experiment will tend to give spin down or spin-up, and while we from the outside know that A measured first, the results performed at B in fact cannot distinguish between whether A measured first or B did. So if B gets spin down, well he could have got that 50% of the time anyway, whether or not A measured first. The effect is that while, after measuring his spin, A knows what B will get, B still in is the dark about his result. And even performing the experiment does not give B any information about what A did, or did not, do.
-- answer removed --
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