Does quantum entanglement allow information to travel faster than light?

[DevilsAvocado]

Okidoki

 

[SpectraCat]

You seem to keep missing the point. After you made the comment about the wave function spreading out over space in an earlier post, I thought you would understand that the wave function provides the state information for the particles at the instant of collapse, which answers the problem of Faster-Than-Light communication from particle to particle. Just because QM does not provide a detailed description of the collapse mechanism does not take away from the physics of the global wavefunction.

Our problem is still fundamentally the mystery of the two slit experiment. The entanglement issues all spring from that fundamental phenomena. After all of these decades (going all the way back to Planck and Einstein's papers around the turn of the 20th century) physics still does not answer the fundamental questions about the dual particle and wave properties of photons, electrons, muons, taus, Up quarks, Down quarks, Strange quarks, Charmed quarks, Top quarks and Bottom quarks--much less the entangled coherent combinations of these elementary particles forming complex systems.

I am certainly not the one who will unravel this fundamental mystery and was not trying to in my post, and I don't expect to see it unraveled on this forum--but I'll be cheering on any honest efforts.

But I still maintain that you are missing the point of the global character of the wave function that produces a system of coherent particles at the instant of collapse (in other words, don't look for Faster-Than-Light communication between the particles--the wave function already had that taken care of that--even if we don't understand the mechanism).

I am not missing the point at all ... in fact you are repeating some of the same points I have made in my posts, but seem to be attaching different significance to them. QM tells us what the entangled wavefunction looks like before measurement, and it tells us about the results after the measurement is complete. This seems clear for both of us. However, QM does not tell us anything about the nature of the transition between the two states (i.e. what is conventionally called "collapse"), or the timescale on which the transition between the two states occurs ... THAT is the point.

The phrase you used in your post, "the instant of collapse", is in fact not well-defined, but it does serve to emphasize the point I have been making, namely that there is a conflict between this colloquial description and relativity, which says that events separated by a space-like interval will not be found to be simultaneous in all reference frames. Thus from the point of view of relativity, it is problem that QM tells us that a local interaction of the entangled wavefunction with a measuring device ends up determining the state of an unentangled particle (here I refer to the post-measurement state) at an arbitrary separation from the site of the measurement, in a way such that the initiation of the measurement and the "appearance" of the unentangled particle appear simultaneous to all observers in all reference frames.

The existence of the conflict is not just hypothetical, because the collapse DOES seem to be instantaneous, or at least superluminal, based on experimental evidence. The experiments that have been done to test this put a lower limit of ~10,000 times the speed of light on the "speed of collapse". Thus, as I said, it is a completely legitimate scientific question to ask, "how can that be consistent with relativity"? This is what bothered Einstein so many years ago, and as far as I can tell, that question have never been answered.

Let me be clear that a NO point have I stated, nor intended to imply, that whatever is happening when the entangled wavefunction collapses allows information to be transmitted between human observers.

One thing we definitely do agree on is that the fundamental mystery at issue is the apparent non-locality of quantum mechanical wavefunctions, which lies at the root of both entanglement and the double-slit experiment.

 

[bobc2]

...It's better scientifically to just say that the nature of the collapse is not understood and leave it at that ...

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles. Thus, no Faster-Than-Light communication between particles is implied.

 

[SpectraCat]

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles.

That is a very vaguely phrased statement .. what precisely are you trying to say? What does "the information spanning the space of the wave function" mean? How is it "adequate to assure an allowed state for the particles"? Take the case of counter-propagating, polarization entangled photons as an example, since that is probably the case where the most experimental work has been done.

Thus, no Faster-Than-Light communication between particles is implied.

Right, and it's not ruled out either. QM simply doesn't have anything to say about the question, which is the point of this whole discussion.

One thing that may be worth pointing out is that the degrees of freedom involved in these collapse events are purely quantum mechanical in nature, since they are associated with the spins of the particles. Thus, since the particle spins are not associated with any spatial degrees of freedom (at least in non-relativistic QM), it may be that the collapse of the spin-entangled wavefunction doesn't need to propagate through space-time in the way that is assumed when we impose the restrictions of special relativity on the propagation of information. (That is just a personal speculation).

Since spin is a property that arises from the proper consideration of symmetry in the full relativistic description of QM, it may be that deeper answers lie within the framework of that theory. Sadly, I have not had the time to study relativistic QM in any depth yet, although it is on my to-do list.

 

[bobc2]

I've sketched a space time diagram for two entangled photons in an attempt to illustrate one of the issues arising with entanglement and special relativity.

A red guy moving along his X4'' world line at some relativistic speed (with respect to stationary black reference coordinate) while a blue guy moves in the opposite direction at the same relativistic speed as red. Red measures an UP state of the left moving photon at event A. And a short instant later red measures a DOWN state of the right moving photon at event B.

But the event at B occurs in the blue's instantaneous 3D cross-section of the 4-D universe at tB', whereas the original event A does not occur for Blue until tA', much later than event B. So, for the blue guy, event A happened after event B, whereas for the red guy event A was first followed an instant later by event B.

 

[xts]

bobc2: such experiment had been made over 10 years ago by N.Gisin and his group.

See: http://arxiv.org/abs/quant-ph/0002031
Journal of Physics A: 2001, Volume 34 Number 35

Experimental test of relativistic quantum state collapse with moving reference frames
H Zbinden, J Brendel, W Tittel and N GisinAn experimental test of relativistic wave-packet collapse is presented. The tested model assumes that the collapse takes place in the reference frame determined by the massive measuring detectors. Entangled photons are measured at 10 km distance within a time interval of less than 5 ps. The two apparatuses are in relative motion so that both detectors, each in its own inertial reference frame, are first to perform the measurement. The data always reproduces the quantum correlations and thus rule out a class of collapse models. The results also set a lower bound on the `speed of quantum information' to 2/3 ×10^7 and 3/2 ×10^4 times the speed of light in the Geneva and the background radiation reference frames, respectively. The very difficult and deep question of where the collapse takes place—if it takes place at all—is considered in a concrete experimental context.

 

[edpell]

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum. It is like they are two disjoint effects.

I think it would be worthwhile pushing the experimental lower limit on the speed of the QM collapse.

 

[xts]

I think it would be worthwhile pushing the experimental lower limit on the speed of the QM collapse.

It had been done already! See the article I quoted few posts above.

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum.

What even more important: no information is exchanged! The information is created (not exchanged) at the time of collapse and made available for both parties (but is not passed from one to another). It may seems to be weird (Einstein never could accept it), but that is what Bell->Aspect->Gishin had proved.

 

[edpell]

It had been done already!

Yes I see the lower limit of .6E7 times c for a separation of 10km. I am saying it would be good to do it for a separation of 100km and try to push the lower bound to .6E8 times c.

 

[bobc2]

bobc2: such experiment had been made over 10 years ago by N.Gisin and his group.

See: http://arxiv.org/abs/quant-ph/0002031
Journal of Physics A: 2001, Volume 34 Number 35

Experimental test of relativistic quantum state collapse with moving reference frames
H Zbinden, J Brendel, W Tittel and N Gisin

Great reference. Thanks.

An experimental test of relativistic wave-packet collapse is presented. The tested model assumes that the collapse takes place in the reference frame determined by the massive measuring detectors. Entangled photons are measured at 10 km distance within a time interval of less than 5 ps.

In which reference frame is the 5 ps measured?

The two apparatuses are in relative motion so that both detectors, each in its own inertial reference frame, are first to perform the measurement. The data always reproduces the quantum correlations and thus rule out a class of collapse models. The results also set a lower bound on the `speed of quantum information' to 2/3 ×10^7 and 3/2 ×10^4 times the speed of light in the Geneva and the background radiation reference frames, respectively. The very difficult and deep question of where the collapse takes place—if it takes place at all—is considered in a concrete experimental context.

Notice that in the space diagram representation in my previous post that you always get the quantum correlation, regarless of which instantaneous 3-D slices of the 4-D universe that you choose. So, there is no conflict with the experimental results in your reference.

But the locations of the particle pair is ambiguous when you consider the red observer's 3-D space compared to the blue observer's 3-D space.

 

[xts]

In which reference frame is the 5 ps measured?

In Swiss Alps frame, but it doesn't really matter, as the relative speed of the detectors were pretty nonrelativistic: 100 m/s - "Ferrari may do it!" (Gisin's comment)

One more great reading (quite easy, even for non-physicists!) on this thread's topic:
David Mermin, "Is the moon there when nobody looks? Reality and the quantum theory", PHYSICS TODAY / APRIL 1985 PAG. 38-47,
http://www-f1.ijs.si/~ramsak/km1/mermin.moon.pdf“Anybody who’s not bothered by Bell’s theorem has to have rocks in his head.”
To this moderate point of view I would only add the observation that contemporary physicists come in two varieties.
Type 1 physicists are bothered by EPR and Bell’s theorem.
Type 2 (the majority) are not, but one has to distinguish two subvarieties.
Type 2a physicists explain why they are not bothered. Their explanations tend either to miss the point entirely (like Born’s to Einstein) or to contain physical assertions that can be shown to be false.
Type 2b are not bothered and refuse to explain why. Their position is unassailable. (There is a variant of type 2b who say that Bohr straightened out the whole business, but refuse to explain how.)If you don't want to be of type 2b - you must read it!

 

[DevilsAvocado]

But the global wave function is understood to a large extent. It is defined and propagates in a precisely defined and predictable way. And it carries the information needed to produce an allowed particle system state upon collapse.

Again, physics has no detailed information about the process of the collapse itself, but the information spanning the space of the wave function just before the collapse is adequate to assure an allowed state for the particles. Thus, no Faster-Than-Light communication between particles is implied.

I’m walking on thin line here... since I’ve been 'nagging' about "NO FTL!"... which is true.

But I have to ask you; you’re not saying that "the information the wave function carries" is all we need, right?

Because that is not true, the polarizer’s ('rotating' randomly at very high speed) are outside each other’s light-cone, and the total time for the final setting of the polarizer + the measurement registration is only a few nanoseconds, and at a distance of 10 or 18 km there is no way for this information (the final polarizer setting) to reach the other photon/apparatus.

We need TWO 'things': The final setting of the polarizer’s + the shared (global) wavefunction.

Only the information the wave function carries is not enough for EPR-Bell experiments.

 

[DevilsAvocado]

I do not see any contradiction between the quantum wave collapsing faster than c and SR. In the QM case no photons are exchanged, no mass, no momentum. It is like they are two disjoint effects.

The contradiction becomes clear for Relativity of Simultaneity. In one frame of reference Alice will perform the measurement first. In another frame of reference Bob will perform the measurement first.

Problem: There is only ONE shared wavefunction, which can only collapse ONCE, hence you get in conflict with good old Albert...

 

[xts]

Problem: There is only ONE shared wavefunction, which can only collapse ONCE, hence you get in conflict with good old Albert...

You may try to see that in the following way: shared wavefunction collapses not when Alice or Bob measure their parts, but at the moment when they meet to check correlations between their observations.

Such approach is valid from any of: Alice's, Bob's and their dad's points ov view.

 

[bobc2]

In Swiss Alps frame, but it doesn't really matter, as the relative speed of the detectors were pretty nonrelativistic: 100 m/s - "Ferrari may do it!" (Gisin's comment)

One more great reading (quite easy, even for non-physicists!) on this thread's topic:
David Mermin, "Is the moon there when nobody looks? Reality and the quantum theory", PHYSICS TODAY / APRIL 1985 PAG. 38-47,
http://www-f1.ijs.si/~ramsak/km1/mermin.moon.pdf


“Anybody who’s not bothered by Bell’s theorem has to have rocks in his head.”
To this moderate point of view I would only add the observation that contemporary physicists come in two varieties.
Type 1 physicists are bothered by EPR and Bell’s theorem.
Type 2 (the majority) are not, but one has to distinguish two subvarieties.
Type 2a physicists explain why they are not bothered. Their explanations tend either to miss the point entirely (like Born’s to Einstein) or to contain physical assertions that can be shown to be false.
Type 2b are not bothered and refuse to explain why. Their position is unassailable. (There is a variant of type 2b who say that Bohr straightened out the whole business, but refuse to explain how.)


If you don't want to be of type 2b - you must read it!

Nice post! Thanks.

 

[DevilsAvocado]

You may try to see that in the following way: shared wavefunction collapses not when Alice or Bob measure their parts, but at the moment when they meet to check correlations between their observations.

Such approach is valid from any of: Alice's, Bob's and their dad's points ov view.

That’s a philosophical interpretation! We are talking real hardware here!

But okay, let’s do it this way. Two Howitzers is connected to the electronics, and fire in left or right really thick metal plate (kinda "EPR punched card"), for up or down. What kind of wavefunction will put the Howitzers and the "EPR punched cards" in superposition until Alice and Bob meets??


PS: Seriously, this not one of my own "inventions", this issue was put forward by John Bell himself (a few months before he died).

PS2: The Howitzers is of course on my tab!

 

[bobc2]

I’m walking on thin line here... since I’ve been 'nagging' about "NO FTL!"... which is true.

But I have to ask you; you’re not saying that "the information the wave function carries" is all we need, right?

Because that is not true, the polarizer’s ('rotating' randomly at very high speed) are outside each other’s light-cone, and the total time for the final setting of the polarizer + the measurement registration is only a few nanoseconds, and at a distance of 10 or 18 km there is no way for this information (the final polarizer setting) to reach the other photon/apparatus.

We need TWO 'things': The final setting of the polarizer’s + the shared (global) wavefunction.

Only the information the wave function carries is not enough for EPR-Bell experiments.

I agree with you for the most part. You certainly need more than the wave function to get the final state of the particles. The wave function cannot predict that. There is no way to start with the wave function and predict from it the actual result of the collapse. However, I was trying to emphasize the point that the wave function does specify which final SYSTEM STATES are possible (with the ability to predict the probabilities for those allowable states). It is in this sense that I feel that the fundamental state information involves all of space at once--so there is no need for the particles to communicate, because when you make the measurement you collapse the SYSTEM wave into a set of particles that appear as components of the selected particle SYSTEM STATE, whose polarizations, etc., are possibilities and are embodied in the wave function.

Thus, the wave function collapses as one state is selected, but is is a system state, specifying an UP for one particle and a DOWN for the other, for example. It's not like the wave function collapses, putting one particle into an UP, which then communicates its existence to another particle to assure it will appear as DOWN.

I recognize that there are different interpretations of QM, and I am certainly no expert to be looked to for the authorative pronouncement. For example the Many Worlds interpretation would probably avoid the collapse all together.

 

[DevilsAvocado]

Agreed!

 

[xts]

That’s a philosophical interpretation! We are talking real hardware here!

As long as different approaches lead to the same predictions, consistent with experimental results, all of them are equally 'real', and chosing one rather than others is purely 'philosophical'...

 

[DevilsAvocado]

As long as different approaches lead to the same predictions, consistent with experimental results, all of them are equally 'real', and chosing one rather than others is purely 'philosophical'...

I don’t agree. With your interpretation Alice & Bob could continue to do entangled measurements after the first one (i.e. run thru another apparatus, and another apparatus, etc), but they can’t. The entanglement gets 'lost' after first measurement; the shared wavefunction is 'broken' once and for all. Not much you can do about that.

But if you still insist, try this 'variant': Alice & Bob do not travel to for a 'personal meeting'. Instead they send their measurement data to each other (by 'normal' messaging ≤ c), at a predetermined time. They analyze their data by computers (working at the same speed).

EDIT: Better, they send the data (encrypted) back to the 'source team', and then the source team sends the data simultaneously to Alice & Bob. This way we can guarantee that they get the data at exactly the same "relative time".

In one frame of reference Alice will analyze the data first. In another frame of reference Bob will analyze the data first.

Back to square one!

 

[xts]

I don’t agree. With your interpretation Alice & Bob could continue to do entangled measurements after the first one

OK. I agree, I was wrong. So let me correct my philosophical interpretation: from someone's point of view collapse occurs, when this person receives results of measurment of any branch of entaglement.

Anyway: my philosophical point is that the collapse is not a real, objective behaviour of the world, but it is rather related to our knowledge about the world. Forget for a moment about entaglement, and take old simple QM example. For poor cat the collapse occurs when machine kills him or let him alive. For Schrödinger the collapse occurs when he opens the cage. For me the collapse occurs when I watch TV news about German scientists cruel to animals.

But for cat the wavefunction to collaps is a WF of the machine. Schrödinger must deal with much more complicated WF of the compound system of the cage, cat and machine. I must deal with the WF of even more complicated system, containing also TV reporters, studio, broadcasting, and the whole part of the world which may influence their broadcast.

But from everyone's perspective the results are the same: before the collapse we did not know if cat survive (we know only probability), after the collapse we know that (let's assume for a moment, that the poison does not act immediately, so for a while cat may know he is dying...)

EDITED:

Or maybe I was not wrong? Once again it is more philosophy and discussion about meanings of the words rather than issue having any impact on experimental results.

As Alice measure her branch she gets only partial knowledge about the system. So if Alice is interested only in results within her branch - the collapse occurs as she measures it. But if she is also interested in results in other branch (which are measured with different angle of polarizator) the collapse occurs when she receives that information from Bob.

 

[DevilsAvocado]

OK. I agree, I was wrong. So let me correct my philosophical interpretation: from someone's point of view collapse occurs, when this person receives results of measurment of any branch of entaglement.

Anyway: my philosophical point is that the collapse is not a real, objective behaviour of the world, but it is rather related to our knowledge about the world.

Okay, agreed. Maybe it’s too early to 'measure' how 'upset' good old Albert really should be... we have the (still unsolved) http://en.wikipedia.org/wiki/Measurement_problem" [/I]).

http://en.wikipedia.org/wiki/Measur...s_the_measurement_process_violate_locality.3F

Does the measurement process violate locality?

In physics, the Principle of locality is the concept that information cannot travel faster than the speed of light (also see special relativity). It is known experimentally (see Bell's theorem, which is related to the EPR paradox) that if quantum mechanics is deterministic (due to hidden variables, as described above), then it is nonlocal (i.e. violates the principle of locality). Nevertheless, there is not universal agreement among physicists on whether quantum mechanics is nondeterministic, nonlocal, or both.

EDITED:

Or maybe I was not wrong? Once again it is more philosophy and discussion about meanings of the words rather than issue having any impact on experimental results.

As Alice measure her branch she gets only partial knowledge about the system. So if Alice is interested only in results within her branch - the collapse occurs as she measures it. But if she is also interested in results in other branch (which are measured with different angle of polarizator) the collapse occurs when she receives that information from Bob.

Well... I’m not completely convinced about this specific 'angle'... if you look at my previous post, there should be no problem arranging a "separate analyze" (18+ km), photon by photon, and AFAIK you’re back to square on when it comes to make a relativistic decision on which of Alice or Bob whom makes the "collapse/decoherence analyze" first... and it can only be made once...


PS: I agree that MWI seems to solve this problem with RoS, but IMHO, multiple splits of the entire universe are a 'quite costly solution' to get past this 'minor problem'...

 

[xts]

``Nevertheless, there is not universal agreement among physicists on whether quantum mechanics is nondeterministic, nonlocal, or both.''

I think Wiki exaggerates a bit. It is backed by reference to the paper by Hrvoje Nikolić, quoted from ArXiv, never published by any peer-reviewed journal. But OK, you made me for that - I'll try to go through those 45 pages tomorrow...

In my view, we have (since Aspect's experiment had been performed) common agreement that it is local and nondeterministic:

We all agree (at least since Aspect tested experimentally Bell's inequality) that the world cannot be simultaneously local and deterministic. At least one of those must not apply to our world.

No one ever demonstrated any violation of locality (as Wiki defines it: Principle of locality is the concept that information cannot travel faster than the speed of light), moreover nonlocality would lead to unsolvable paradoxes. What Gisin's extensions to Aspect's experiment had shown is that nonlocality (spooky actions), if we assume them as a mechanism explaining Bell's violation, would not only work with speed much higher than c, but they would have to act backward in time in frames of both observers.

Although for many people (starting from Einstein) determinism is intuitively necessary, we may drop it without falling in paradoxes and without introducing so exotic mechanisms like information exchange going backward in time.

So, having a choice to either:
1. reject determinism, which is counterintuitive, but self-consistent and not contradicting other branches of Physics;
2. reject locality, which leads to paradoxes or to rejection of Special Relativity or to acceptance for backward-in-time causality;

I believe vast majority of physicists opt for 1)

EDIT >>
Above I used the term 'locality' after Wiki article, quoted by DevilsAvocado. Here 'nonlocality' == 'information exchange faster than c'
Personally I dislike this definition as misleading and prefer the terminology used by Gisin:
- 'nonlocal' :== 'the same (possibly random) information may manifest itself at two locations' (weaker definition than the one by Wiki)
- 'signalling' :== 'able to communicate faster than c' - what Wiki article and me above called 'nonlocal'
- 'deterministic' :== 'fully determined by some local hidden variables and other pre-existing common information'

Using Gisin's terminology, our world is nonlocal, nonsignaling, nondeterministic. And I believe it is a view of vast majority of physicists.
<<``we still don’t know if the true nature of the world is non-local or/and non-real''

If the non-locality would demonstrate directly having experimentally verificable behaviour (e.g. by delivering yesterday an e-mail I send to you now), I would have to accept it.
If non-locality is only intrinsic to QM model, explaining Bell's violations, but do not demonstrate directly, I don't like this concept, as violating Occam's principle (when compared to non-determinism).
What is "true nature of the world" and "real/non-real"? I hate such questions.
For my taste Duns Scotus, Roscellinus and Abelard discuted them too long already.

``there should be no problem arranging a "separate analyze" (18+ km), photon by photon''

It had been done in 2000. Read the full report of the experiment, not just abstract. 4 pages only.
On 10km distance, not 18. Makes it fundamental difference?
Here is the reference once more:
H Zbinden, J Brendel, W Tittel and N Gisin
Experimental test of relativistic quantum state collapse with moving reference frames
Journal of Physics A: 2001, Volume 34 Number 35
http://arxiv.org/abs/quant-ph/0002031

And much more accurate and described with details static long-distance experiment:
Tittel W., Brendel J., Gisin N. & H. Zbinden,
Longdistance Bell-type tests using energy-time entangled photons,
Phys. Rev. A, 59, 4150-4163 (1999).
http://arxiv.org/abs/quant-ph/9809025v1

And one more worthy reading, explanation of locality problem, as it is seen by Nicolas Gishin.
A bit long, but pretty easy to understand and very nice to read:

Can relativity be considered complete ? From Newtonian nonlocality to quantum nonlocality and beyond.
http://arxiv.org/abs/quant-ph/0512168v1