Cern and String Theory

[Mark]

Well, if it wasn't so hard to separate out comments directed to the resident psychedelic from those directed at everyone else, I wouldn't mind. Presently, there is about a page of "answers"s, even at the "you have ... ignore" level, after most any substantive article. The last time I looked, 2/3 of them were answers to answers, as it were, which seems quite untoward, as well as a bit of a cause for concern of some sort.

Perhaps (I'll ask Jeff about this) it might be possible to flag replies to an ignored individual as "... ignored because it is a reply to ... who you have on ignore...", which would remedy the situation.

I don't care to have discussions with people who haven't any evidence for anything, but who use words like "educated-idiot" and the like. It's like trying to have a civil discussion with a drunk Interesting Ian who's even less interesting. The dualists on this board aren't impressing me with their rhetoric, I must say.

I was referring to Answer's vitriolic responses. That other people responded emotionally to his silly name calling was unsurprising.

 

[69dodge]

But when M2 is done on B while M1 is done on A they also give the same answer - so b2=0 implies

a2=0

Shouldn't this be the other way around? That is,<blockquote>But when M1 is done on B while M2 is done on A, they also give the same answer, so b1 = 0 implies

a2 = 0</blockquote>

 

[Tez]

Shouldn't this be the other way around? That is,<blockquote>But when M1 is done on B while M2 is done on A, they also give the same answer, so b1 = 0 implies

a2 = 0</blockquote>

Yep - my bad. Thanx...

This is when that 60 minute timeout for editing is annoying....

Someone mentioned Griffiths. If you deny free will to the experimentalists in the scenario I described, then one can construct a sort-of-satisfactory explanation for how the observables in the experiment evolve, and this is what Griffiths does. However, for me personally, its not a satisfactory approach.

 

[wipeout]

Someone mentioned Griffiths. If you deny free will to the experimentalists in the scenario I described, then one can construct a sort-of-satisfactory explanation for how the observables in the experiment evolve, and this is what Griffiths does. However, for me personally, its not a satisfactory approach.

I mentioned Griffiths. Are you sure that's what's he saying?

From the book Consistent Quantum Theory by Robert Griffiths:

Some quantum paradoxes are stated in a way that involves a free choice on the part of a human observer: e.g., whether to measure the x or the z component of spin angular momentum of some particle. Since the principles of quantum theory as treated in this book apply to a closed system, with all parts of it subject to quantum laws, a complete discussion of such paradoxes would require including the human observer as part of the quantum system, and using a quantum model of conscious human choice. This would be rather diffcult to do given the current primitive state of scientific understanding of human consciousness. Fortunately, for most quantum paradoxes it seems possible to evade the issue of human consciousness by letting the outcome of a quantum coin toss "decide" what will be measured. As discussed in Sec. 19.2, the quantum coin is a purely physical device connected to a suitable servomechanism. By this means the stochastic nature of quantum mechanics can be used as a tool to model something which is indeterminate, which cannnot be known in advance.

Seems to me that Griffiths replaces the free will of a human observer with the free will of the quantum equivalent of a coin toss.

 

[wipeout]

Whatever the cause if there is one,the nonlocality is 'an embarassing fact' (as Penrose put it in one of his books) for it seems to imply also that all particles are somehow connected.

Just thought I'd mention that according to Chris Isham in a lecture at Jim Hartle's 60th Birthday event, Roger Penrose is now a supporter of consistent/decoherent histories approach, which is supposed to remove this embarassing fact.

 

[wipeout]

This would seem to be consistant with my own understanding, and would seem to indicate that the non-locality issues arise from a situation where two things were in fact local when the 'decision' was made, and the only issue is that the decision is MEASURED after the two entities are separated.

Yes? No? What don't I understand here?

I'm learning the subject myself, so I'll just mainly stay to quoting the experts on the relevant topics.

More from Robert Griffiths, form the conclusion at the end of his book Consistent Quantum Theory.

Quantum mechanics, like classical mechanics, is a local threoy in the sense that the world can be understood without supposing that there are mysterious influences which propagate over long distances more rapidly than the speed of light. The idea that the quantum world is permeated by superluminal influences has come about because of an inadequate understanding of quantum measurements - in particular, the assumption that wave-function collapse is a physical process - or through assuming the existence of hidden variables instead of (or in addition to) the quantum Hilbert space, or by employing counterfactual arguments which do not satisfy the single-framework rule. By contrast, a consistent application of quantum theory provides a positive demonstration of the absence of nonlocal influences.

Like Robert Griffiths, Roland Omnes is an expert in the field of the interpretation of quantum mechanics and one of the four physicists responsible for the consistent/decoherent histories version of quantum mechanics, which are in full: Robert Griffiths, Roland Omnes, Murray Gell-Mann and Jim Hartle.

Roland Omnes says in the notes at the end of his book, Understanding Quantum Mechanics:

Some authors keep seeing a mysterious instantaneous exchange of information at long distance when the two measurements I mention in the text are made along the same direction (n=n'). They seem not to appreciate the fact that two previous elements of information must be available for the realization of these conditions: (i) an initial state involving spin correlations; (ii) a prior decision or information ensuring that the two spin measurements will be performed along the same direction. When this is taken into account, there is never a transmission of information nor a real prediction but only a correlation in a pre-set experimental arrangement.

So that's what a couple of high-level experts in the field who have been studying the problem for decades say. Make of it what you will.

 

[jj]

No - that would simply be classical correlation and wouldnt keep anyone awake at night!

Loosley speaking what happens is that once separated a choice of 2 measurements - call them M1, M2 - is made on each particle. Crucially: each experimentalist can choose (free will) which of the two measurements they want to make. Thus, over many runs, about 1/4 of the time they both perform M1, 1/4 of the time one performs M1 the other performs M2 and so on.

Say each measurement results in a 2-valued outcome - arbitrarily labelled 0 or 1.

Now If the experimentalists find that every time they both choose to do M2 the particles give opposite outcomes (0,1 or 1,0), but in all other cases (M1,M1 or M1,M2 or M2,M1) the particles give the same outcome they know something fishy is going on.

Why? Say particle A answers 0 for the M1 measurement being done on him. In obvious notation we can say

a1=0.

Well, particle B must also answer 0 if either the M1 measurement or the M2 measurement is done on him (since when M1 is done on A and either M1 or M2 is done on B they give the same answer). So

b1=b2=0.

But when M2 is done on B while M1 is done on A they also give the same answer - so b2=0 implies

a2=0

Now we have a contradiction with the fact that when M2 is performed on both particles they give opposite answers....



Detection efficiency is a red herring. E.g. in ion traps we have 99.9% detection efficiency for these sort of experiements...

Tez, once again I fail to see any problem. In the case where you get "same answer" you're performing another measurement in the middle, ok, so you changed the state of things. In the case they are different, you didn't.

I am still not seeing what is at all startling about this. You measure something, it can change state as a result of the measurement. Yes, we know that.

All it SEEMS that we know is that first the separation occurs, and then we do some measurements. Once we've done ONE Measurement, rules change. Obviously, yes?

????

 

[jj]

I was referring to Answer's vitriolic responses. That other people responded emotionally to his silly name calling was unsurprising.

Um, well, yeah, that's what I meant, too. But I don't need to see the replies, either. Answer is, as far as I can tell, purely an entropy source in the physical sense.

 

[Mark]

Um, well, yeah, that's what I meant, too. But I don't need to see the replies, either. Answer is, as far as I can tell, purely an entropy source in the physical sense.

"Entropy source in the physical sense."

I love that phrase! Accurate, too.

 

[TillEulenspiegel]

This thread started out really great untill it took a sharp left turn into hell.

Whipeout:" More from Robert Griffiths: " ...By contrast, a consistent application of quantum theory provides a positive demonstration of the absence of nonlocal influences...." "

I haven't read the book but what about the GHZ proof of non-locality and Stapp and Bradfords work regarding GHZ ? Anyone?

Roland Omnes: " They seem not to appreciate the fact that two previous elements of information must be available for the realization of these conditions: (i) an initial state involving spin correlations; (ii) a prior decision or information ensuring that the two spin measurements will be performed along the same direction. When this is taken into account, there is never a transmission of information "

Seemingly at least one layer of detachment between the observer and the experiment is present in the three particle group in GHZ. Other protochols have been proposed to elimnate determinist effects of the observer, but even those have a level of dependance on human input I.E. a computer generated (RND) number still relies on a seed number that is derived from human intervention. How many recursive processes would have to be employed to guarantee true randomness? Rhetorical question. ( Weren't Geiger counters used at one time =) )

 

[wipeout]

Wipeout:" More from Robert Griffiths: " ...By contrast, a consistent application of quantum theory provides a positive demonstration of the absence of nonlocal influences...." "

I haven't read the book but what about the GHZ proof of non-locality and Stapp and Bradfords work regarding GHZ ? Anyone?

Griffiths deals with Hardy's paradox instead in the book:

A paradox of a somewhat similar nature involving three spin-half particles was discovered (or invented) by Greenberger, Horne, and Zeilinger a few years earlier. The basic priciples behind this GHZ paradox are very similar to those involved in Hardy's paradox. We shall limit our analysis to Hardy's paradox, as it is a bit simpler, but the same techniques can be used to analyze the GHZ paradox.

At the end of the chapter, Griffiths says about Hardy's paradox.

...the basic difficulty with the argument in H1-H8 lies in an implicit assumption which is easy to make because it is always valid in classical mechanics. Incompatibility rather than some mysterious nonlocality is the crucial feature which distinguishes quantum from classical physics, and ignoring it is what has led to the paradox.

I'm not sure if it's the same for the GHZ paradox.

I don't know anything about Stapp and Bradford, but I did notice in a paper of Griffith's you can get online from October 2002, Consistent Quantum Counterfactuals, in the references, Griffiths thanks Stapp for "lengthy correspondence" on an issue with Hardy's paradox, so they're certainly talking about all this.

I haven't checked other papers yet for references either way.

 

[wipeout]

Actually, I just did a search and Stapp wrote a paper called Nonlocality, Counterfactual, and Consistent Histories.

http://www.arxiv.org/abs/quant-ph/9905055

 

[69dodge]

Originally posted by jj
Tez, once again I fail to see any problem.

The problem is, how can the particles guarantee that the measurement results will be as described, if they don't know, at the time they're separated, which measurement will be done on each, and if also they're too far apart, when the measurements are done, for them to communicate with each other?

If I do measurement M1 on particle A and the result is 0, I can't now do measurement M2 on A. (Well, I can, but you know what I mean.) However, I can try to figure out what the result of M2 on A would have been had I chosen to do M2 instead of M1. It turns out, as Tez explained, that had I chosen, on this particular run of the experiment, to do M2 on A and M2 on B, the results would have been 0 for both particles. Yet, in all previous runs where I did M2 on both A and B, the results for the two particles differed.

Isn't it lucky I didn't choose to do M2 on A this time?

 

[jj]

The problem is, how can the particles guarantee that the measurement results will be as described, if they don't know, at the time they're separated, which measurement will be done on each, and if also they're too far apart, when the measurements are done, for them to communicate with each other?

If I do measurement M1 on particle A and the result is 0, I can't now do measurement M2 on A. (Well, I can, but you know what I mean.) However, I can try to figure out what the result of M2 on A would have been had I chosen to do M2 instead of M1. It turns out, as Tez explained, that had I chosen, on this particular run of the experiment, to do M2 on A and M2 on B, the results would have been 0 for both particles. Yet, in all previous runs where I did M2 on both A and B, the results for the two particles differed.

Isn't it lucky I didn't choose to do M2 on A this time?

Could somebody write this as a state chart (or set thereof) or something? The language is NOT communicating whatever it intends to communicate. So far, I still don't see a problem, but that could always be the description.

 

[sorgoth]

Well, I was able to follow the discussion after I blocked Answer for a while, but you guys have lost me now.

Ok, so WHAT are you talking about?

There are two particles, and an experiment is done on one and the other seems to know?

Has this been tested? Wouldn't it imply information travelling faster than light?

Or is it just an 'if' scenario?


Oh, and kind of off topic, but what the h3ll is a wavefunction collapse (Or something like that)?

 

[LucyR]

jj, sorgoth,

I've also just come back to this thread so I hope this is on topic:

Try looking up the the Einstein, Podolsky, and Rosen Paradox.

For the record information is not transferred faster than light.

 

[espritch]

If you deny free will to the experimentalists in the scenario I described, then one can construct a sort-of-satisfactory explanation for how the observables in the experiment evolve, and this is what Griffiths does. However, for me personally, its not a satisfactory approach.

What does "free will" mean in this context? Are the experimenters choosing what to measure each time or are they basing the choice on some independent random criteria, like Griffith's "quantum" coin toss? And should it matter to the results how the choice is made (aside from possible bias introduced by the experimenter in favor of one test over the other)?

 

[Tez]

Tez, once again I fail to see any problem. In the case where you get "same answer" you're performing another measurement in the middle, ok, so you changed the state of things. In the case they are different, you didn't.

I am still not seeing what is at all startling about this. You measure something, it can change state as a result of the measurement. Yes, we know that.

All it SEEMS that we know is that first the separation occurs, and then we do some measurements. Once we've done ONE Measurement, rules change. Obviously, yes?

????

Forget about changing of states - its irrelevant to the problem.

You have some experimental data (a list of ones and zeros) that you collected in one location. A colleague has another list. Next to each piece of data you have noted whether this data is of the M1 type or the M2 type - a random choice you made while far separated. You note that the data has a correlation (described above) that is either incompatible with each of you having the independent free will to make a random choice about something, or incompatible with locality. No need to mention quantum mechanics or particles or state changes anywhere.

Every time we have this discussion it seems to me you know what you think/believe the problem is about and so you interpret what I'm saying in that context. I understand very well what your intuition is - its the same one we all carry into this problem, and as a student I spent weeks of my life trying to apply it to this problem without success. I remember the phase transition in my understanding when I finally grasped that it was impossible to carry such "realistic" thinking into the problem. It distresses me that I cannot seem to explain it better...

 

[Tez]

What does "free will" mean in this context? Are the experimenters choosing what to measure each time or are they basing the choice on some independent random criteria, like Griffith's "quantum" coin toss? And should it matter to the results how the choice is made (aside from possible bias introduced by the experimenter in favor of one test over the other)?

The point is that whatever coin an experimenter may use to decide which measurement to perform was at one point in its history in causal contact with the fields whcih are creating the entangled particles. At that point they could have conspired to fool us. Unlikely, I know, but possible. Thus for the sake of a thought experiment its better to insert random freewill choices of the people performing the measurements. Of course, the stuff in our heads was also once in causal contact with the stuff making the particles, and so it could be that each person cannot make an independent choice of settings on their side.

 

[Tez]

Could somebody write this as a state chart (or set thereof) or something? The language is NOT communicating whatever it intends to communicate. So far, I still don't see a problem, but that could always be the description.

Its best to simply imagine a game wherein you and a friend are going to be locked in separate rooms.

A game show host is going to randomly either say "What is M1?" to you or "What is M2?". A different host is going to do the same to your friend.

Each of you can answer either 0 or 1.

Now - how can you and your friend win the game, if every time that you are both asked "What is M2?" you must give opposite answers, but for all 3 other possible pairs of questions you must give the same answer.

No quantum mechanics necessary to see that its "impossible" for you to satisfy this....