# Does quantum entanglement allow information to travel faster than light?

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!

Gold Member
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... :uhh: 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.

Gold Member
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.

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.

Gold Member
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!! :grumpy:

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!!
:rofl:

I’m walking on thin line here... :uhh: 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.

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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'...

Gold Member
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!

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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.

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Gold Member
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" [Broken][/I]).

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'...

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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

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