# Perceived contradiction in non-locality principle

.Scott
Homework Helper
Let me go further. Because there is no result without a measurement, it doesn't make sense to talk about A causing B or B causing A - because you would be assuming that had only one measurement been made, it would have been the same as the measurement that was made.

Here's the extreme example that I gave in another thread - but now I'll work a little more with it. Entangled particles split. One is measure immediately, the other is stored in a box for a year. In all reference frames, measurement A is made before measurement B. While waiting for the year to go by, we can publish the result of measurement A - and it would be very tempting to say that measurement A is independent of whatever we will eventually do with B. But the Bell inequality shows us that would be an impossible interpretation to consistently maintain. If we don't take the A+B measurement as a single measurement in this case, we would be forced to make arbitrary choices in how we interpret situations where the time period (1 year) was so short that no before/after could be unambiguously determined.

Even more importantly, we could capture thousands of those particles, a thousand each at angle -45,0,+45, storing all of them for a year in boxes. And then at the end of the year, Bill could make measurement B of each one - randomly choosing angles among the -45,0,+45 degrees. Presuming that the Bell inequality was demonstrated, to explain the result in terms of A influenced B, we would have to presume that somehow the "A" measurement got into the "B" box, or influenced Bill's best effort to randomize his measurements.

entropy1
The fact that someone could fly overhead in a rocket at .3c and reverse the detection events is just academic.
Not entirely. Coincidence times in Bell tests have have the order of nano-second differences. Depending on how spacelike separated you can make your experiment, you don't need to be moving that fast to really shift the timing of events to make one detection before another in a different frame. At the low orbital velocity of the ISS (~7 km/s), a ground experiment with spacelike seperation of 20km in total would give a time shift of about 1ns if I got it right (using SR only). Anyway, you'd probably be better off putting the experiment in orbit for maximum spacelike separation if you really wanted to do this, but the point is that there probably have been actual human satellites which would have disagreed on which photon in earth based experiments was detected "first". This is an almost completely academic point yes, but not entirely academic.

Collapse is unambiguously defined if you introduce preferred frame simultaneity convention.
I would be interested in reading references which formalise this method if anyone has them. In particular if they deal with the problem of entanglement across multiple frames.

zonde
Gold Member
we would be forced to make arbitrary choices
What so bad about arbitrary choices? You mean that we could make the wrong choice and that is unacceptable?
But in science we only care about "does not work" and "works for now". And from that perspective we can make arbitrary choice (even the 'wrong' one) as long as it works.

zonde
Gold Member
Wavefunction-collapse seems to require a relativistic approach, and the approach won't hold, it seems to me. So you can throw out relativity, but why not throw out collapse?
Don't ask me too much about wavefunction-collapse. I just saw that you tried to discard it based on flawed argument.
Why you can't follow Nugatory's suggestion and ignore collapse idea? Is it because you want to sort everything into 'correct' or 'flawed'?

Gold Member
Why you can't follow Nugatory's suggestion and ignore collapse idea?
I may not ignore it, but I am challenging it, for I gather that some people treat it as real, whereas I don't.

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zonde
Gold Member
I may not ignore it, but I am challenging it, for I gather that some people treat it as real, whereas I don't.
You can challenge the idea on the grounds that:
- it's predictions do not agree with experiments;
- it gives ambiguous predictions (it is not self consistent).
As I understand you are skeptical that collapse approach gives unambiguous predictions, right?

Gold Member
As I understand you are skeptical that collapse approach gives unambiguous predictions, right?
No no. The expression "the wavefunction collapses" suggests that there is something in the real world 'collapsing'. But perhaps I am not familiar enough with the math. I still have to start with my first book.

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Demystifier
Gold Member
On the other hand: non-locality seems to imply FTL influence. This is not, on first sight, compatible with relativity.
But on second sight, it might be:
http://lanl.arxiv.org/abs/1002.3226 [Int. J. Quantum Inf. 9 (2011) 367-377]

.Scott
Homework Helper
What so bad about arbitrary choices? You mean that we could make the wrong choice and that is unacceptable?
But in science we only care about "does not work" and "works for now". And from that perspective we can make arbitrary choice (even the 'wrong' one) as long as it works.
No. I mean that we try to say measurement A causes measurement B or vice versa, we would have to describe very similar situations in opposite ways. And when the measurements we space-like separated, we would be making an arbitrary choice on how to interpret the measurements. That lack of time-based cause and effect is the core reason for taking the entire measurement process as one fully-integrated event. And, of course, the fact that the math that supports the modelling of the measurements also treats the measurements as a single operation.

entropy1
Demystifier
Gold Member
So how do I step out of the domain of relativity if I am an experimenter in a spaceship observing the experiment on earth?
Relativity, just like any other theory in physics, has its domain of established validity. Certainly there are domains which have not yet been investigated, which leaves a possibility that relativity might be violated in some of those domains. One frequently studied possibility is that relativity is violated at very small distances (say of the order of Planck length). At such small distances, the correct theory (still unknown to us) might contain an explicit non-local mechanism lying behind the known phenomenological laws of quantum mechanics (QM). The fact that we don't see any such mechanism in the standard laws of QM is then a consequence of the fact that those standard laws do not describe the "true reality" at such small distances.

So, if you are an experimenter in a spaceship observing the experiment on earth, either you are able to measure such small distances or you are not. If you are not (which corresponds to current technology), then you don't see those explicit non-local mechanisms and all what you see looks in accordance with relativity. If you are able to measure them, then you are able to see the violation of relativity at those small distances.

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zonde
Gold Member
That lack of time-based cause and effect is the core reason for taking the entire measurement process as one fully-integrated event.
Lets say you measure one particle from entangled pair and then depending from the measurement result you decide to perform or not perform entanglement swapping of second particle with a particle from another entangled pair.
Can you take it as one fully-integrated event?

.Scott
Homework Helper
Lets say you measure one particle from entangled pair and then depending from the measurement result you decide to perform or not perform entanglement swapping of second particle with a particle from another entangled pair.
Can you take it as one fully-integrated event?
Well, you can. But that makes things too easy. Normally you want to keep one measurement from having an obvious influence on the other so that there is no "hidden value" solution.
Perhaps more importantly, the measurement of just one particle is "random", he equivalent of a coin flip. It is only when you have a population of A measurements to compare to a corresponding population of B measurements that you see a pattern.