Survey: When does the wavefunction collapse?

[RJBeery]

Represent two quantum particles, A and B, in a unitary state having complementary spins (as a result, let's say, of a typical EPR-type experiment) in a Minkowski space of a single spatial dimension.



'|........\----A------------------B----/
'|..........\---A----------------B---/
'|............\---A------------B---/
'|..............\--A1--------B2--/
'|................\--A------B--/
'|..................\-A---B1-/
'|....................\-AB-/
T|______________________E________________________
''X


In this picture the diagonals are the light cone of the particles' creation event E. A and B are emitted at E and move away from each other at a velocity < c. At A1 the particle A's spin is measured, and particle B is never measured but instead runs off into the infinite blackness of space.

There are 2 observers local to A1. Due to Relativistic effects, Observer 1 is moving such that A1 and B1 are considered to be simultaneous, however he does not choose to observe A's spin at the time of measurement - he is merely "present". Observer B is stationary relative to particle A, and calculates that A1 and B2 are considered to be simultaneous.

So the question is this: at what point does particle B's wavefunction collapse?

1: E because B's spin was determined from the time it was emitted
2: B1 because of Observer 1's presence at A1
3: B2 because Observer 2 actually observed A's spin
4: None of the above because B is never actually measured so B's spin is never determined

Thanks for your perspective!

[RJBeery]

Arrgghhh! I can't figure out how to edit the survey options. The options should be E, B1, B2, and None as described in the post rather than E, A1, A2, and None as listed in the survey. My apologies.

[funkstar]

If I've understood correctly, B doesn't have a separate wavefunction if A and B are entangled...

[RJBeery]

funkstar, that is correct. Their wavefunctions are entangled, and A's wavefunction supposedly collapses at point A1. My question is basically "when" does B's wavefunction collapse if Relativity blurs the meaning of "instantaneous"? I'm surprised there aren't more people willing to share their opinion on this.

[RJBeery]

OK I posted my vote (B's spin existed at the point of emission) which reflects my block time stance. Also I would like to thank whichever moderator corrected the poll choices (BenTheMan? - if it was you I wish you would've taken the time to share your opinion!)

[BenTheMan]

I thought I already explained this in another thread?

There is no idea of simultanaeity in special relativity.

Either way, let me edit your post, to what I think you mean. Then you can see how to make the ASCII figures work, and re-edit the figure to your liking...

[BenTheMan]

In a frame where A1 and B1 are simultaneous, the wavefunction collapses there. In that frame, the particle was initially never at rest. In a frame where A1 and B2 are simultaneous (i.e. the particles rest frame), that's where the wavefunction collapses.

Note that the two frames are related by a Lorentz transformation, as I said in the other thread.

[Pete]

Do physicists in this area think that the wave-function collapse is something real?
Or is it simply a paradigm that gives good results?

(I think I know... but I want to see what RJBeery and Ben think about what physicists think.)

[BenTheMan]

To be honest, most physicists that I have talked to (including one author of a Quantum Field Theory text book who claims to be an "unqualified believer") are proponents of the many worlds interpretation, although that interpretation suffers from a poor name. I've heard someone quote Stephen Hawking, that it is "trivially true". The essential idea is that the universe exists in some giant superposition of states.

I don't know what I think about it. The problem with wavefunction collapse is that it's a non-unitary process, and quantum mechanics is an inherently unitary theory. The problem with the many worlds interpretation of quantum mechanics is that I don't observe the state that is a linear superposition of a dead cat and an alive cat, even though it exists in the hilbert space.

The problem may be deeply tied to (...wait for it gluon...) consciousness, but I don't know. One doesn't need to know how to resolve this problem to calculate physical observables, and one cannot distinguish between interpretations of quantum mechanics---otherwise there wouldn't be more than one. Either way, it isn't science, and it shouldn't be confused as such.

Contemplating interpretations of quantum mechanics is what physicists do to waste time, in between publishing papers. Typically we discuss our pet theories over beers or something---it's a sort of common ground, because it's something we all think about. It's a much better conversation starter than the weather :)

[iceaura]

So the question is this: at what point does particle B's wavefunction collapse? The question seems to assume yet a third, special, frame - ours - in which we can compare the other two frames against a fixed reality.

No frame with that property exists.

[RJBeery]

Ben, please edit the picture to make it pretty if you want, thanks. The question in the other thread that I never felt you answered is this one (http://www.sciforums.com/showpost.php?p=2153296&postcount=48) where I ask: Or are you saying that the superposition state of B may reduce to a definite spin for one reference frame BUT NOT NECESSARILY FOR THE OTHERS?

I have a real problem with this because it seems like, strictly from the B particle's perspective, it has different realities at simultaneous periods in its own timeline. In other words, if I understand you correctly, this would be claiming that B was in a state of superposition until B1, then it would exist both in a state of superposition AND in a collapsed state from B1 to B2, depending upon who is doing the calculating. This is a form of solipsism and to me is equivalent to saying that the moon exists for those people looking at it while at the same time does not exist for those that don't. I'm not REJECTING this stance (in fact I find it interesting) but I am surprised if this is really what you meant to say. :eek:

Pete said:
Do physicists in this area think that the wave-function collapse is something real?
Or is it simply a paradigm that gives good results?

You are getting at the very heart of my point here. In my experience the sole justification for using statistical interpretations and "wavefunction collapsing" is that it does in fact produce good results. But IMHO when you actually analyze the situation and ask questions the wavefunction paradigm falls apart. I could just as easily assign a wavefunction to a die that is hiding under my hand and "prove" the wavefunction's validity by statistical analysis of the die throw's results. Point being that one cannot conclude (solely from the accuracy of the wavefunction predictions) that the die was in a state of superposition until I looked at it.

Iceaura, if you wish, place a third observer at rest with particle B and have all observers meet at some point in the future to discuss what time they calculate (on the third observer's clock) when B's state of superposition has "collapsed".

[Pete]

You are getting at the very heart of my point here. In my experience the sole justification for using statistical interpretations and "wavefunction collapsing" is that it does in fact produce good results. But IMHO when you actually analyze the situation and ask questions the wavefunction paradigm falls apart. I could just as easily assign a wavefunction to a die that is hiding under my hand and "prove" the wavefunction's validity by statistical analysis of the die throw's results. Point being that one cannot conclude (solely from the accuracy of the wavefunction predictions) that the die was in a state of superposition until I looked at it.

My (very unreliable) understanding is that there is a very important difference between the analysis of dice and quantum mechanics:

The results of throwing dice can be explained by proposing a local state attached to each die that is independent of the state of any other die.
But for quantum systems, that explanation isn't sufficient - the state of one particle in a wavefunction is dependent on the state of other particles in the wavefunction.


I guess you could describe a single dice using a wavefunction if you wanted to? What about a pair of dice? What would be the result of entangled dice throws?

[RJBeery]

Pete, you are right, the state of the die (which is the word for a single "dice") can be proven to be independent of other dice which is not the case with entangled particles. But that wasn't my point. My point was that just because a wavefunction could effectively describe what is happening with the resulting throws of the die does not lend any merit to the idea that the wavefunction is a LITERAL PHYSICALITY of the die.

With dice, we know that their behavior arises from the manner in which they are thrown (the hand muscles, the table top, wind, etc). With quantum particles it is generally accepted that there is no such analogous "causal" explanation for their behavior but that doesn't rule out the possibility that there is another (retro-causal!) explanation for their behavior. Does that make sense?

[Pete]

Pete, you are right, the state of the die (which is the word for a single "dice") can be proven to be independent of other dice which is not the case with entangled particles. But that wasn't my point. My point was that just because a wavefunction could effectively describe what is happening with the resulting throws of the die does not lend any merit to the idea that the wavefunction is a LITERAL PHYSICALITY of the die.

With dice, we know that their behavior arises from the manner in which they are thrown (the hand muscles, the table top, wind, etc). With quantum particles it is generally accepted that there is no such analogous "causal" explanation for their behavior but that doesn't rule out the possibility that there is another (retro-causal!) explanation for their behavior. Does that make sense?

Yes it does, but the problem with your analogy is that it seems to imply that QM is misguided by something like the following argument:

- Quantum systems can be described by wavefunctions
- Dice throws can be described by wavefunctions
- Dice throws have a simpler and better explanation
- So, there is obviously a simpler and better explanation for quantum systems as well
- Therefore, wavefunctions are a misguided idea

Yes, there might be a better explanation... but that is not controversial. I think that most physicists would agree that there is a very strong chance that there is a better explanation of QM systems than collapsing wavefunctions. I notice you didn't actually answer that question before... what do you think that physicists think about wavefunction collapse? That it's something real, or something useful?


I think the question we should be discussing is whether your retro-wave idea is worth pursuing. However, I don't think that decrying current QM is the right approach - we all agree that QM might not be the Truth, and we all have occasional ideas about alternative approaches, but it's the best thing out there so far, and it has proven itself to be surprisingly unshakable.


My (amateur) experience is that the best way to approach this kind of discussion is with a very healthy dose of humility. This is the kind of discussion that Ben mentioned before:
Typically we discuss our pet theories over beers or something

Here's something I wrote a while ago, talking about the coffee table/bar round stage of scientific development:
All new ideas are thoroughly hammered at this stage, and most are subsequently discarded. That's the only way to filter out the crap and find the quality gems.
Good scientists are smart enough to brush away the rubbish, alert enough to spot the promising rocks, patient enough to polish them up and search for flaws, pragmatic enough to throw away the fractured stones and fools gold, smart enough to enlist and accept critical assistance, hardworking enough to collect a reasonable cache of small jewels over their career, hopeful enough to think that maybe one day they'll be at the coffee table when a flawless diamond shows up, and realistic enough to know that it's probably never going to happen to them.

So, my advice is this:
Don't worry about convincing people that wavefunctions might not be the real explanation. They already agree.
Do be humble. Accept that the chance of any given crazy idea (including you own) being worth publishing is low, even for the professionals.
Expect your idea to be hammered during discussion. Accept this as a mark of respect, not as an attack on your intelligence.
Don't ever stop thinking up crazy ideas. Just be sure to examine them carefully and critically, and discard the flawed ones without mercy.

[Pete]

Here's an interesting blast from the past which might be worth reading. I ended up moving it to Pseudoscience (I was moderator in Physics and Maths back then), which might not have been the best thing to do:
7 Reasons to Abandon Quantum Mechanics-And embrace this New Theory

There was a follow up thread (actually a sidetrack which Stryder separated from the other thread) in which I explained why it was moved:
7 Reasons to Abandon Sense



(I'll stop the self-indulgent BS now before I disappear completely up my own ass!)

[Pete]

Ha! Synchronicity! I just found this comic strip today (Thanks to Pandaemoni):
http://abstrusegoose.com/strips/moment_of_clarity.JPG

Abstruse Goose (http://abstrusegoose.com/93)

[RJBeery]

Pete, you make more sense to me than anyone I've encountered on this thread. You exhibit intellectual security which I am as yet unable to emulate. I think the reason for this is that I have had nothing but (vehement!) opposition to any unorthodox ideas in QM interpretations which has made me very cynical of the foundations of current QM culture.

[BenTheMan]

I think you may be less confused by my answer if you rephrase the question:

At what point should the passing observer preform his experiment, so as to be simultaneous with the observer at point A?

(At least, this is how I rephrased the question for myself.)

Then, you can use your intuition about special relativity and simultaneous events---the event in this case is a spin measurement. The frame of the passing observer B and the particles rest frame (which you called E) are now related by a Lorentz transformation.

[RJBeery]

Ben I'm sorry but I just can't understand your rephrasing of my question. There is no passing observer B. E does not represent the particles' rest frame but rather their creation (emission) event. Maybe the way you phrased it is equivalent to what I asked but I just don't get it.

[BenTheMan]

E does represent the rest frame of the particle, because the initial particle was at rest, and decayed into two electrons traveling in opposite directions at the speed of light.

The "passing observer" is the one who sees B1 and A1 as simultaneous events. An observer in the rest frame (i.e. lab frame) of the particle sees A1 and B2 as simultaneous events.

The two frames are related by a Lorentz transformation.

[RJBeery]

Got it, thanks. An ancillary question is: Does the wavefunction of B collapse at B1 whether the passing observer looks at the results of the A particle's spin measurement or not? I was prepared with all kinds of "what if's" (such as "what if the passing observer was only 80% sure that he 'read' the results of particle A's spin correctly?") but it appears that the Physicists in the audience do acknowledge, under specific questioning, that contradictions do in fact exist. Pete acts as if this is common knowledge but he is the only one that I have found that admits this.

[BenTheMan]

An ancillary question is: Does the wavefunction of B collapse at B1 whether the passing observer looks at the results of the A particle's spin measurement or not?

You should be able to answer that---of course it collapses.

but it appears that the Physicists in the audience do acknowledge, under specific questioning, that contradictions do in fact exist.

Umm...what contradictions?

The fact that there is no idea of simultaneity is well-understood from special relativity, right?

[RJBeery]

OK you guys are running me in circles...
Pete says
Yes, there might be a better explanation... but that is not controversial.

and Ben says
Umm...what contradictions?

Pete makes me think that I'm analyzing a problem that is old-hat in the Physics community while at the same time Ben is acting like he has an explanation of the OP that eliminates all controversy.

Let me ask this: Pete, do you understand BenTheMan's explanation and can you put it in layman's terms?

[Pete]

Well, I don't know. But maybe...

In a frame where A1 and B1 are simultaneous, the wavefunction collapses there. In that frame, the particle was initially never at rest. In a frame where A1 and B2 are simultaneous (i.e. the particles rest frame), that's where the wavefunction collapses.

Note that the two frames are related by a Lorentz transformation, as I said in the other thread.

So the change of B's condition from a superposition of states to a well-defined state isn't supposed to be a well-defined event?

[BenTheMan]

Special relativity deals with events. The collapse of the wave function is not an event, however, a device which measures the collapse is an event. This means that if observer B preforms a measurement simultaneously with observer A, then he will measure the same thing as observer A. If observer B is not at rest with respect to observer A, it is conceivable that the two events at A1 and B1 are simultaneous in B's frame. In this case, the results must be consistent. Note that in this frame, which is boosted wrt the lab frame, the particle is not initially at rest in B's frame (as it is in As frame).

Again, "simultaneous" is defined in the special relativity sense of the word.

If B is at rest with respect to A and the initial particle (which we assume is at rest before it decays), then the points A1 and B2 will be simultaneous in the reference frame of the particle (at rest, i.e. the lab frame) and A (which is also at rest wrt the particle).

Now remove the measuring device.

I agree that it is confusing, but this is the conclusion that I come to. If someone thinks I am wrong, you have to explain to me why a measurement isn't an "event" in the sense of special relativity, or why the presence or non-presence of a measuring device in B's experiment collapses the wave-function.

Now, it could be that the inherent non-locality of wave-function collapse kills whatever intuition I have about special relativity, but I don't think so.

[RJBeery]

Ben and Pete, I admit that I become confused in your web of Lorentz transforms and SR frames (not to say that I don't understand the theory, just this application), but let's make this simple: FROM B PARTICLE'S PERSPECTIVE, at what point along B particle's timeline does B particle's spin become a physical reality? At the heart of the EPR 'paradox' is the following paraphrased statement:

"If a piece of knowledge is calculated about a system with certainty then such knowledge is intrinsically represented by the system in the form of a physical reality without the need for a verifying measurement."

IMO to conclude something other than the fact that B possessed a certain spin at the time of its creation requires one of the following:

1) A rejection of the EPR presumptive statement as a whole (which violates common sense and maybe logic but that's not the first time QM has done this)
2) A rejection that such knowledge may be known with certainty until a measurement is made (which, in my example, violates conservation of angular momentum)
3) A Many Worlds Interpretation where B does not have a distinct spin, ever, when taken across the aggregate "many" worlds (which has many problems that I would be happy to discuss)

I don't feel an affinity to any of these options. I'm not trying to say anyone is wrong here, I'm trying to show the logical path that I followed to conclude block time. If B always had a definite spin, but that spin is determined by the axis of measurement upon A, then either block time exists or there is some form of retro-causal signal traveling back to E from A1 (and I'm not convinced that there is a difference - I think if you accept retro-causality you might be stuck with block time). Pete says my objections are well known but if that were the case why is he the only poster on this site to even acknowledge the plausibility of block time? Because I'm arrogant? Don't confuse arrogance with passion, I'm well aware of my insignificance. Also, if you look at my thread history you will see that most of the time I'm asking questions of those smarter than me rather than claiming SCIENCE IS WRONG WOO WOO!! :)

[BenTheMan]

Your "logical path" is flawed because you don't understand the definition of "event", and you don't understand how to do a Lorentz transformation between the two frames. This is one of the reasons why it is important to understand the calculations, before you understand the physics. Once you understand the results you SHOULD get, via a proper application of mathematical axioms to a physical situation, only then can you speculate at all.

Once observer B preforms his measurement (or doesn't), the electron's wavefunction is collapsed. Simultanaeity between A's measurement and B's measurement is defined differently whether B is at rest wrt to A or B is moving wrt A.

B can only preform one measurement. B can only measure the spin of the particle at B1 OR B2. B cannot be BOTH boosted AND static. Or, said another way, you can only collapse the wavefunction once.

[RJBeery]

Ben, so your answer is that I cannot understand your answer because I don't know how to do Lorentz transformations [I do] and I don't understand the definition of "event" [I believe I do]. Tomorrow I will repost the question with specific relative velocities listed. Please let me know what other objections you will raise before you admit that you don't have an answer that doesn't involve retro-causality or block time. Yes, Pete, I am showing my intellectual immaturity again and have blown my chances of Ben publicly agreeing with me but I had some vino... :D

[iceaura]

Iceaura, if you wish, place a third observer at rest with particle B and have all observers meet at some point in the future to discuss what time they calculate (on the third observer's clock) when B's state of superposition has "collapsed". Why would their calculations, suitably transformed to the third observer's clock, disagree?

[BenTheMan]

Ben, so your answer is that I cannot understand your answer because I don't know how to do Lorentz transformations [I do] and I don't understand the definition of "event" [I believe I do].

If you understood these things, I wouldn't have to explain it to you for the Nth time.

Tomorrow I will repost the question with specific relative velocities listed.

The thread will be locked, as we already have one thread on the topic.


I'll explain this for you one more time.

A (spin 0) particle at rest in frame E decays into two electrons, whose spins are correlated (i.e. must add up to 0). The electrons move in opposite directions, at the speed of light.

Consider two separate experiments.

1.) Frame A and frame B are in the particle's rest frame, equidistant from the original decay. The electron passes A's and B's detectors at the same time according to an observer situated in frame E. An observer in frame A measures the electron's spin. The observer in B either measures the spin or he doesn't at point B2. The act of A measuring the spin of his electron collapses the wavefunction so that the result of B's measurement is already determined.

2.) In the second experiment, B is a boosted observer, with a non-zero velocity relative to the initial rest frame, E. In observer B's frame, the particle which decays is initially NOT at rest. Observer A measures the spin of the first electron. Because observer B is now boosted wrt the original particle, observer B sees events at A1 and B2 as simultaneous. The act of A measuring the spin of the electron at A1 collapses the wavefunction, so that when B measures the spin of the particle (at B2, which is now simultaneous with A1, according to B), the result is already determined.

[Pete]

Ben and Pete, I admit that I become confused in your web of Lorentz transforms and SR frames (not to say that I don't understand the theory, just this application), but let's make this simple: FROM B PARTICLE'S PERSPECTIVE, at what point along B particle's timeline does B particle's spin become a physical reality?
I have no idea. I don't know enough about QM to know if the question makes sense in that model.

Pete says my objections are well known
Umm, not those particular objections. I don't actually understand enough about QM to follow your objection.

...but if that were the case why is he the only poster on this site to even acknowledge the plausibility of block time?
Well, you're not actually asking people whether "block time" (a spacetime manifold) is plausible in itself... you're asking about your model of quantum reality. Or, perhaps you mean something more by "block time" than "a defined manifold of space and time"?

[Bishadi]

Represent two quantum particles, A and B, in a unitary state having complementary spins (as a result, let's say, of a typical EPR-type experiment) in a Minkowski space of a single spatial dimension.



'|........\----A------------------B----/
'|..........\---A----------------B---/
'|............\---A------------B---/
'|..............\--A1--------B2--/
'|................\--A------B--/
'|..................\-A---B1-/
'|....................\-AB-/
T|______________________E________________________
''X


In this picture the diagonals are the light cone of the particles' creation event E. A and B are emitted at E and move away from each other at a velocity < c. At A1 the particle A's spin is measured, and particle B is never measured but instead runs off into the infinite blackness of space.

There are 2 observers local to A1. Due to Relativistic effects, Observer 1 is moving such that A1 and B1 are considered to be simultaneous, however he does not choose to observe A's spin at the time of measurement - he is merely "present". Observer B is stationary relative to particle A, and calculates that A1 and B2 are considered to be simultaneous.

So the question is this: at what point does particle B's wavefunction collapse?

1: E because B's spin was determined from the time it was emitted
2: B1 because of Observer 1's presence at A1
3: B2 because Observer 2 actually observed A's spin
4: None of the above because B is never actually measured so B's spin is never determined

Thanks for your perspective!

a great 'thinking' thread....

keep this thread for framing as it allows many to observe what entanglement is.

Keep in mind that A and B began by event E with (x); both share a potential.

And to measure either side, the initial 'event' becomes a whole unit of (xE) in time. (cat is both dead/alive at once)

The missing aspect is the 'potential' shared (some call it spin) (x).

See it as 'cupids' arrow, hitting a par of lovers, who don't even know each other; but the potential is still there because the arrow was shot. (entangled by the event in time; think of gravity)

post #9 is heart felt

i will leave this with you guys

thanks again :)

[BenTheMan]

Discussion continued at : http://www.sciforums.com/showthread.php?p=2180449