Everybody sees the same elephant (says Carlo Rovelli)

[marcus]

thanks for your comments vanesch!
I think we are beginning to arrive at a fair, undismissive understanding of this kind of new Copenhagen or "Marseille Interpretation"

what I like to see emerging, in your (and like) comment is an idea of similarities AND differences and what this may have to offer a.f.a. resolving contradictions.

============
BTW I am finding the Stanford Encyclopedia article on RQM helpful now
(I see it now as very much like usual Copenhagen but with some unnecessary premise discarded. However it fits much better at first sight with Heisenberg picture----schrödinger has to be reconstructed by giving each observer his own realistic, and therefore quantum and fallible, clock so that he can observe correlations with this real physical clock)

I am increasingly enjoying getting to understand the "marseille interpretation" and am glad to have company of the other PFers also engaged in doing this!

 

[Careful]

**thanks for your comments vanesch!
I think we are beginning to arrive at a fair, undismissive understanding of this kind of new Copenhagen or "Marseille Interpretation"

what I like to see emerging, in your (and like) comment is an idea of similarities AND differences and what this may have to offer a.f.a. resolving contradictions.

**

Vanesch is just explaining in full detail here, what I said from the beginning (thanks for taking this job upon you - I can imagine it took quite a while to type that out for the 50'th time on this forum). In case you might have missed it, me nor Josh1 were ever dismissive about RQM, we just do not understand the FUZZ you are making about this (and neither does Vanesch I think - and highly likely the authors themselves find this a miracle too ).

Cheers

Careful

 

[hossi]

still confused

As far as Hossi goes there is only one Hilbertspace, hers, and just this one state of the precious pair of particles which as a token of friendship she has divided with the Dwarves. This state represents her Knowledge...

Hi marcus, thanks for the dwarves, very nice. Let me play this game too, since I still don't get it. I don't have any problem with my Hilbertspace or with the fact that every observers observation is relative until compared, i.e. I know that my knowledge is limited...

I prepare some entangled state of two particles with total spin=0, send one of them in a box to the dwarves at pluto. Then I open my box and find, it's spin 1. From this I conclude nothing since I have read this thread.

Should the dwarves just not have opened their box, there is no problem. I send them a letter, saying: I have spin 1, and when they open their box, they better find -1. No surprise in that, no problem, not even with usual QM.

So, the interesting case is when the dwarves have opened their box before we could have been in causal contact after I opened mine. Have they found spin=-1, fine. Should that always be the case, there is the miracle, alias, non-local collapse of the wave-function. Instead, I thought, it's now possible they also find 1. That however, does not bother me, since I don't know that. Now I send them a letter saying, I have spin=1. What happens when they receive it?

Or are you trying to say, that the very notion of spin = +/- 1 is relative to the measuring apparatus? But then, what happens to conservation laws or, again, how does/can this relativeness change over spacelike seperations.



B (who just found out that her new neighbor has a wireless...)

 

[vanesch]

what I like to see emerging, in your (and like) comment is an idea of similarities AND differences and what this may have to offer a.f.a. resolving contradictions.

Well, my feeling is that philosophical interpretations of quantum theory have the same "function" as theology has wrt to religion: to make you accept the latter within a picture which doesn't run in direct contradictions with what you think are absolute truths. In other words, they help you to picture the thing without running into obvious contradictions.
On one hand, we have a formalism that spits out mountains of useful results which compare with experiment, and which even help in desiging apparatus. On the other hand, it doesn't make much sense. So the need for the "pastor" to come along and help you fit this at first sight totally nonsensical way of reasoning into an acceptable "world picture".
No matter how you turn quantum theory, there's always something crazy about it. For some, we suddenly shouldn't talk about what "is" but just about "observations" or "information". For others, there's a distinction between "normal" objects and "quantum objects". For still others, the mistake was relativity, and if we allow again for a Newtonian picture, we can do so (the Bohmians). Others (like me) prefer to take the formalism (which works well) as litterally as possible, and dig into the philosophy bag to see if nothing can save us there (and we find stuff about minds and so). And finally, there are those that hope secretly (or openly) that this was just the 20th century bad movie, and that we'll soon see the flaws of it.
But all these views have something profoundly disturbing to them ; so you're just supposed to pick that one which fits the best with your temperament and move on.
After all, each of these views have something crazy and something plausible to them. Real breakthroughs do not come from this ; they come from formal breakthroughs, or experiment.

 

[Careful]

**
After all, each of these views have something crazy and something plausible to them. Real breakthroughs do not come from this ; they come from formal breakthroughs, or experiment. **

As always, you are to the point in a diplomatic way. To make it entirely clear: I do not think the Schrodinger/Dirac equations and so on BY THEMSELVES were a bad move - of course not, that would be stupid! As you know, it is **very** difficult to do better, what I find a bad movie however is the pertinent reluctance of trying to do better and taking such philosophical issues SERIOUSLY especially by those who are doing fundamental physics (I am not aiming to anyone in particular here). There is nothing wrong with the shut up and calculate mentality, as long as you are working in a part of QM which is undesputably tested and when your work is phemenologically inspired.

Cheers,

Careful

 

[marcus]

Well, my feeling is that philosophical interpretations of quantum theory have the same "function" as theology has wrt to religion: to make you accept the latter within a picture which doesn't run in direct contradictions with what you think are absolute truths. In other words, they help you to picture the thing without running into obvious contradictions.

I agree with the analogy but my attitude is different in the two cases. I do not WANT to be made to believe in religion and so I do not bother to read theology intended to make it palatable to reason.

On the other hand I WANT to understand the messages nature is sending us through these seeming paradoxes. If we can take seriously all the strange-seeming lessons and find what, of our prejudices, we must give up in order to harmonize, then maybe hopefully we can put it together in a new way that makes sense. I want this, and so I am prepared to listen to an occasional voice like Rovelli who offers to try to do it.

Normally I do not bother with philosophical questions of interpretation but in this case yes*.

...
No matter how you turn quantum theory, there's always something crazy about it. For some, we suddenly shouldn't talk about what "is" but just about "observations" or "information".

I hear you! But for me the idea that QM is about information transactions---one system getting information from another---is more and more seeming NOT CRAZY. Rovelli (and also the late Asher Perez a founder of quantum information theory and someone whose wisdom I respect) have just now TURNED IT A NEW WAY for me so that I see it slightly different and not so puzzling.

But I respect your puzzlement, if it looks crazy to you no matter how you have turned it at least you are trying for it to make sense and you are open to new perspectives, which not all people are.

...
But all these views have something profoundly disturbing to them ; so you're just supposed to pick that one which fits the best with your temperament and move on.
After all, each of these views have something crazy and something plausible to them. Real breakthroughs do not come from this ; they come from formal breakthroughs, or experiment

I disagree here. I think major breaks can come from taking seriously something that our human/animal intuition cannot yet assimilate. The example is Einstein 1905. the FORMALISM of lorentz transf already existed, what he had to do was take it seriously and get over the weirdness. Breaks are sometimes achieved at a PHILOSOPH level and they are often profoundly CONSERVATIVE. Like in 1905 he wanted to conserve and take seriously BOTH maxwell and gallileo----and only he saw the universally accepted obvious truth that you had to give up: simultaneity. So a REVOLUTION IN PHILOSOPHY IS NOT JUST A MATTER OF TASTE as you suggest there, sometimes there is a right way to go

Maybe this is not too much of an oversimplification
1905: one world but no absolute clock---each good-faith observer has his own clock
2005: one world but no single official list of facts---each good-faith observer has his own list of facts labeled his.

but that's just a rough paraphrase, everybody should read the RQM articles--------------------------
* I will always remember how the reactions of people at PF helped me to estimate the value and importance of this recent Rovelli EPR paper. One just has to know how to interpret the signals.

 

[Kea]

Thank you, Vanesh, for writing on your MWI interpretation for the umpteenth time! I agree that what Rovelli says does not seem very different from many MWI viewpoints. However, he has a very nice way of phrasing it, which Marcus for one seems to appreciate, and that makes the paper useful. I have to say that, on your point of dispute re minds, I tend to side with Rovelli. For instance, for electrons going through slits - I think you are making a terribly large assumption that the human observer's slit is important in the state space of the electron. Rovelli does not assume this.

 

[Careful]

**
I disagree here. I think major breaks can come from taking seriously something that our human/animal intuition cannot yet assimilate. The example is Einstein 1905. the FORMALISM of lorentz transf already existed, what he had to do was take it seriously and get over the weirdness. Breaks are sometimes achieved at a PHILOSOPH level and they are often profoundly CONSERVATIVE. Like in 1905 he wanted to conserve and take seriously BOTH maxwell and gallileo----and only he saw the universally accepted obvious truth that you had to give up: simultaneity. So a REVOLUTION IN PHILOSOPHY IS NOT JUST A MATTER OF TASTE as you suggest there, sometimes there is a right way to go **


How, how, Einstein gave up Galileo and later also the inertial observer (special relativity); and up to date there is NO UNIFICATION between GR and Maxwell theory, that is to find a *geometrical* unification between both jeopardizes causality. Also quantum gravity theories are effectively expected to break Lorentz invariance at very high energies, so perhaps promoting Lorentz invariance as a fundamental principle of nature was not such a great idea. But Marcus, you seem to forget the lesson that nobody cared about good old Albert at that time, actually he only became truly recognized after Eddington shouted out that relativity might be a good theory after all (14 years later). Moreover, you seem to overlook the obvious fact that Lorentz still believed in an ether theory and as such must have thought that LI is not a fundamental symmetry for ALL laws of nature. Hence, Einstein REFUTED the ether idea, which was a new PHYSICAL statement about the vacuum. Rovelli's QM does no such thing, it gives indeed some more confort to those trying to interpret quantum gravity amplitudes in a MWI or relative state picture. To be honest, I find it extremely grotesque that you speak about the ``Marseille interpretation´´ and ``a new historical landmark´´.


**
--------------------------
* I will always remember how the reactions of people at PF helped me to estimate the value and importance of this recent Rovelli EPR paper. One just has to know how to interpret the signals. **


I actually read Rovelli's RQM one year ago - so against your opinion I am open to new suggestions, since I know we are still looking for answers to some questions (that is why I truly read some papers still). Rovelli's particular suggestion creates comfort for locality (obviously since it sacrifies realism - which actually has nothing to do with a superobserver) something MWI did 50 years before, but it does not adress the question for a unified dynamics (even) at the quantum level (and as such works with distinguished elements - a very anti Einsteinian thought actually).

Cheers,

Careful

 

[marcus]

... However, he has a very nice way of phrasing it, which Marcus for one seems to appreciate, and that makes the paper useful...

A propos style, Kea, I was just reading Asher Peres on this with considerable pleasure. In the exerpt here (which is germane to our topic) the italics are his.

---quote Peres---
In the EPR article, the authors complain that “it is possible to assign two different wave functions to . . . the second system,” and then, in the penultimate paragraph, they use the word simultaneous no less than four times, a surprising expression for people who knew very well that this term was undefined in the theory of relativity. Let us examine this issue with Bohm’s singlet model. One observer, conventionally called Alice, measures the z-component of the spin of her particle and finds +hbar/2. Then she immediately knows that if another distant observer, Bob, measures (or has measured, or will measure) the z-component of the spin of his particle, the result is certainly -hbar/2. One can then ask: when does Bob’s particle acquire the state with s_z = -hbar/2?

This question has two answers. The first answer is that the question is meaningless — this is undoubtedly true. The second answer is that, although the question is meaningless, it has a definite answer: Bob’s particle acquires this state instantaneously.
---endquote---

BTW have you by ANY improbable chance read Asher Peres remembrance of childhood/adolescence in 1930-1950 Europe called I am the cat who walks by himself
title=quote from Kipling, if you'd had a look I don't think you would've forgotten.

Nathan Rosen was Asher Peres PhD thesis advisor----and in an affectionately remembered incident gave Peres (after some hesitation) his last-but-one reprint of the EPR article.

 

[Kea]

BTW have you by ANY improbable chance read Asher Peres remembrance of childhood/adolescence in 1930-1950 Europe called I am the cat who walks by himself
title=quote from Kipling, if you'd had a look I don't think you would've forgotten.

Marcus

No, I can't say I have. Sounds interesting.

 

[marcus]

Marcus

No, I can't say I have. Sounds interesting.

http://arxiv.org/abs/physics/0404085
I am the cat who walks by himself
Asher Peres
Comments: To be published in a special volume of "Foundations of Physics" honoring the 70th birthday of the author

"The city of lions. Beaulieu-sur-Dordogne. The war starts. Drole de guerre. Going to work. Going to school. Fleeing from village to village. Playing cat and mouse. The second landing. Return to Beaulieu. Return to Paris. Joining the boyscouts. Learning languages. Israel becomes independent. Arrival in Haifa. Kalay high school. Military training. The Hebrew Technion in Haifa. Relativity. Asher Peres. Metallurgy. Return to France. Escape from jail. Aviva."

----------------------------------
The one that is (explicitly) GERMANE, since we should always stay focused and on topic is

http://arxiv.org/abs/quant-ph/0310010
Einstein, Podolsky, Rosen, and Shannon
Asher Peres

"The EPR paradox (1935) is reexamined in the light of Shannon's information theory (1948). The EPR argument did not take into account that the observers' information was localized, like any other physical object."

 

[wolram]

I have one thing to say, and that is prismatics and distorting mirrors, if everyone sees the same thing , it is only after these these (distortions)
are negated, muted.

 

[marcus]

I have one thing to say, and that is prismatics and distorting mirrors, if everyone sees the same thing , it is only after these these (distortions)
are negated, muted.

very true. also one assumes the observers must be competent and of good faith.
we don't allow incompetents who wouldn't know @rse from pickax, and we exclude liars---the kind who insist the elephant is a zebra just to make trouble

but if the observers are all right then EVEN THOUGH THEIR ACCOUNTS may differ in detail, we trust it can all be sorted out
thanks, good point

 

[marcus]

Kea, I mentioned this of Peres

http://arxiv.org/abs/quant-ph/0310010
Einstein, Podolsky, Rosen, and Shannon
Asher Peres

"The EPR paradox (1935) is reexamined in the light of Shannon's information theory (1948). The EPR argument did not take into account that the observers' information was localized, like any other physical object."

You may know it, if not I hope you read it---real nice and only two pages! He likes to make his points by stories.

towards the end of the article, on page 2, Peres says:

"...For Bob, the state of his particle suddenly changes, not because anything happens to that particle, but because Bob receives information about a distant event. Quantum states are not physical objects: they exist only in our imagination. [/color][/size]..."

Einstein Podolsky Rosen asked "Can quantum mechanical description be considered a complete description of physical reality?" As I understand Peres conclusion, he says YES IT CAN and different observers will give different descriptions.

My comment: If Peres is right, then like it or not (I like it) that is just how the world is.
People who like it that way should be happen then.
No one official list of facts that we can say about the world. No one official "wave function".
And people who don't like it should just get over it and get on with their lives.

Great guy.

 

[Careful]

Hi Thomas,

Sorry for the delay, but I liked to take a bit of time to make the phrasing accurately.


**Thank you. Coming from you I take that as a serious compliment.**

Come, come, I am not that severe .

**
I am not sure what these problems are.
**

I was alluding to the usual problems interactions bring along, Haag type misery although you do not work in the interaction picture of course.

**
On page 16 I specialize to Minkowski spacetime. By replacing the geodesic equation by d^2 q/dt^2 = 0 I made t physical, defined in terms of the gravitational field. This could have been empasized more, though.**

I did not miss that but it troubles me somehow. To be precise: if *t* is to be interpreted as an *inertial time function* on spacetime then it would set up an identification between a spacetime vectorfield and \partial_t, which would jeopardize the \partial_t \psi(x,t) = 0 constraint. The same can be said by regarding *t* as eigentime on the selected worldline of the observer where the latter constraint would force zero relative energy. Hence, perhaps it is better not to make such identification at all and quantize the ``full´´ geodesic equation written out wrt to a general non-affine parameter. Then, you could truly speak about the eigentime as a quantum operator with respect to a classical ``life-time parameter´´ t (that is: where is the observer ``alive´´ on the worldline - just a small philosophical remark). I guess this is not such a straightforward thing to do, since in the corresponding equation the eigentime operator will be in the denominator and the spectrum contains zero's as well as purely imaginary numbers which make your life miserable even if you start out with an observer state which is sharply peaked (at t = 0) around some localized (in spacetime) timelike vectorfield. At least, I would naively expect this to be the case .


**
This is of course the problem with doing something that nobody cares about; there is no careful proof-reader.
**

I guess you could submit your papers and at least force the referees to read it through carefully no?


**You mean non-causal separation, no? **

sure- sorry for the typo.

**
Since all points on the observer's trajectory are supposedly causally related, and the Taylor coefficients live on this trajectory, the notion of spacelike separated events disappear from the horizon. This is confusing, and I am not sure that I have digested it yet. However, it is the same miracle that underlies the notion of analyticity.**

Well, this won't be true for the quantum worldline I guess anymore (see my comment above) - though this is negligible of course. But I would like to see that miracle to happen *explicitely*. Generally, I think it would be good if you would somehow clean up the paper and focus on providing rigorous results concerning the issues I adressed during our conversations here (and undoubtedly some others I missed in my quick first reading). In case you manage to do that, I am certainly interested in learning more about it.

Cheers,

Careful

 

[vanesch]

I disagree here. I think major breaks can come from taking seriously something that our human/animal intuition cannot yet assimilate.

Yes, that was in fact the point: you first need that "something" before you can take it seriously. You first needed the Lorentz transformations (which were "too crazy to be really true") before you could take them seriously. My point was - probably I was not very clear - it is not by doing the philosophy that you find the "new something", the philosophy is there to help you take it seriously.

This is BTW, why I prefer an MWI style scheme over a Copenhagen like scheme, or a "modification to introduce collapse": the MWI style scheme is the only one who takes the axioms of quantum theory seriously, all the way. Copenhagen-like versions seem to claim that, no matter the superposition principle, you DO have a classical world, with classical "measurement information" and all that, and there is "of course" no superposition principle valid for macroscopic objects. This sounds a bit like the ether interpretation of Lorentz transformations, which was nothing but a "trick" to calculate outcomes of measurements.
I think that _or_ quantum theory is *totally* misguided, in which case fiddling with it doesn't really help, or it is *fundamentally* correct, in which case we have to take it seriously all the way.

So, let us say that, what leads the game is the formalism, and what "delivers the mind" is the philosophy, which avoids you to cling onto the old paradigms in which you try to force the new scheme.
But without formalism in the first place, there's not much hope to get the view right.

Like in 1905 he wanted to conserve and take seriously BOTH maxwell and gallileo----and only he saw the universally accepted obvious truth that you had to give up: simultaneity. So a REVOLUTION IN PHILOSOPHY IS NOT JUST A MATTER OF TASTE as you suggest there, sometimes there is a right way to go

Let's say that the "right" philosophy should follow the formalism, and not have its own prerogatives.
So what I consider an error is to say that quantum theory is "only concerned with certain quantum systems" or "a trick to calculate measurement outcomes". This is forcing it into the classical paradigm somehow.
If, when applying quantum theory strictly, I arrive at describing my body state as a superposition of two totally different states, and as I clearly don't see this, then of two things one:
- or this is in a certain way correct, and I then have to explain WHY I don't see this (MWI style, relative-state approaches in all their variants)
- or this is fundamentally wrong, and what I see is correct. In that case, quantum theory is fundamentally wrong, and all ways to "weasel out" by saying that it is "about information I have, not what's going on" or "there's no such thing as a quantum description of my body" or the like are, exactly, philosophical attempts to remain in the old classical paradigm.

 

[Careful]

**
I think that _or_ quantum theory is *totally* misguided, in which case fiddling with it doesn't really help, or it is *fundamentally* correct, in which case we have to take it seriously all the way. **

This I find a bit simplistic, what is wrong e.g. with the point of view of 't Hooft who takes QM as an operational scheme very seriously? I mean Lord Kelvin did not have to understand the kinetic theory of gasses and solid state physics either in order to find out about the laws of thermodynamics. And as you know, in this context, it is still troublesome to describe something like friction starting from a time reversible microscopic dynamics.

**
So what I consider an error is to say that quantum theory is "only concerned with certain quantum systems" or "a trick to calculate measurement outcomes". This is forcing it into the classical paradigm somehow. **

Well, in any case, it tells us a lot about potential classical alternatives too.

**
If, when applying quantum theory strictly, I arrive at describing my body state as a superposition of two totally different states, and as I clearly don't see this, then of two things one:
- or this is in a certain way correct, and I then have to explain WHY I don't see this (MWI style, relative-state approaches in all their variants)
- or this is fundamentally wrong, and what I see is correct. In that case, quantum theory is fundamentally wrong, and all ways to "weasel out" by saying that it is "about information I have, not what's going on" or "there's no such thing as a quantum description of my body" or the like are, exactly, philosophical attempts to remain in the old classical paradigm.**

Well, the first option leads to a dual world view (whatever you try out) and the second one is not for tomorrow (at least ).

Cheers,

Careful

 

[Thomas Larsson]

**
This is of course the problem with doing something that nobody cares about; there is no careful proof-reader.
**

I guess you could submit your papers and at least force the referees to read it through carefully no? Careful

This paper and its two follow-ups were written as a preparation for an article appearing in a book which will be available very soon: https://www.novapublishers.com/catalog/product_info.php?products_id=3848 . I am sure that submitting it to an independent referee would be useful (although perhaps would lead to copyright violations), but since I am lazy and have no use for an academic CV, it did not happen. However, the two key papers on the representation theory, http://www.arxiv.org/abs/physics/9705040 and http://www.arxiv.org/abs/math-ph/9810003 , did appear in CMP.

At any rate, what you found are nitpicks compared to the flaws I mentioned myself in my previous post (overcounting for the harmonic oscillator and the absense of an inner product). These are the areas that need to be, and are being, addressed first. But I agree that my discussion on reparametrization invariance is confused. I thought it was clear when I wrote it, though.

I am not particularly happy about having to invent a new formulation of QM. However, I see no alternative. The key lesson from the multi-dimensional Virasoro algebra is that all fields have to be expanded in a Taylor series around the observer's trajectory. To profit on this insight, I need a formulation of QM written solely in terms of Taylor expandable objects. Non-local integrals, like the Hamiltonian and the action functional, do not satify this criterion, but the Euler-Lagrange equations do. Therefore, I need a formulation of QM where these encode the dynamics.

Finally, let me comment again on the relation between observer independence and QG infinities. The philosophical kinship between MCCQ and Rovelli's RQM is observer dependence, and that the observer possesses physical properties. AFAIU, the only property that Rovelli attaches to the observer is position, but this is naturally generalized to other physical properties, in particular energy (or mass) and a clock.

In conventional QM, time just marches on independent of what happens. Time must operationally be defined by ticks on the observer's clock, and thus the observer does not accelerate. However, observation means that the observer interacts with the system, experiencing a force F = ma. If F != 0 and a = 0, the observer's mass m is necessarily infinite. This is no problem we observe an electric phenomenon, say. Then F = ma = qE, where q is the observer's charge and E the electric field generated by the system. That q and E are non-zero and a = 0 is OK, since m = infinity is a good approximation to reality.

But when we introduce gravity, the force on the observer is rather F = ma = mg, where g is the gravitational field generated by the system. This leads to the equation a = g, which is clearly incompatible with no observer acceleration (a = 0) and non-zero gravitational field (g != 0). That the inertial and gravitational masses are the same thus immediately implies that ignoring observer acceleration leads to inconsistencies in quantum gravity.

This is a happy thought indeed!

 

[Careful]

**but since I am lazy and have no use for an academic CV, it did not happen.**

I do not see how this fits with your wish to have a careful proofreader.

**
At any rate, what you found are nitpicks compared to the flaws I mentioned myself in my previous post (overcounting for the harmonic oscillator and the absense of an inner product). These are the areas that need to be, and are being, addressed first. But I agree that my discussion on reparametrization invariance is confused. I thought it was clear when I wrote it, though. **

That went without saying no, I felt no need to comment about the latter issues since you were so forthcoming about this yourself.


**
In conventional QM, time just marches on independent of what happens. Time must operationally be defined by ticks on the observer's clock, and thus the observer does not accelerate.**

? There is no problem whatsoever in defining QFT with respect to (non-uniformly) accelerating observers (and bending the foliation according to local eigentime - as long as one does not encounter focal points).


** However, observation means that the observer interacts with the system, experiencing a force F = ma. If F != 0 and a = 0, the observer's mass m is necessarily infinite. **

Obviously a is not zero.

** This is no problem we observe an electric phenomenon, say. Then F = ma = qE, where q is the observer's charge and E the electric field generated by the system. That q and E are non-zero and a = 0 is OK, since m = infinity is a good approximation to reality. **

? The m in the Newton formula is the physical mass and not the bare mass, for an electron that is still the very tiny number of 10^{-30} kilo at least when it moves smaller than c wrt an inertial observer. You can find such information in Eric Poisson, ``An introduction to the Lorentz Dirac equation´´ gr-qc/9912045 where such understanding is offered at a classical level.

The rest of your comments get equally many question marks.

 

[hossi]

still confused

Hey brainy guys,

me and my dwarves, we would really like to play your adult's games. But I still did not get the point.

I have observers A and B, measuring alpha and beta when not in causal contact. That does not neccessarily have to be the same time (which is not well defined anyway), but let's say in whatever slicing its the same time t_0. We know the total spin is zero. Let us say they measure

S_A,alpha = 1
S_B,beta = 1

which is not a problem, because they have not compared their stuff. Now go to time t_1 when they are in causal contact and measure the other part of the previously entangled state. They find

S_A,beta = -1
S_B,alpha = -1

Now, I would have thought S_B,beta is what B has measured for beta at t_0. According to Rovelli, the important thing is now to let A ask at t_1 what B has measured. This is

S_AB = S_A,beta = -1

which is not what B has measured at t_0. Has B changed his mind concerning the measurement of beta from 1 to -1? Or has he not changed his mind but A always hears the answer he wants to hear? If so, does that make sense macroscopically?



B.

 

[marcus]

Hey brainy guys,

me and my dwarves, we would really like to play your adult's games. But I still did not get the point.

.

Hi Biene, I am glad that you did not get too offended by my beginning that story about Hossi and the Seven Dwarves. It seemed charming at that moment and then later I was afraid it was too unserious.

Anyway now it IS Easter, happy easter everybody, and you remember that today is the day that Hossi looks at the spin of her particle!

She measures it in EastWest direction and Ach! Behold! it is East, as is just right for Easter.

BUT the confusion-loving dwarf, Careful John, has meanwhile distracted the other six Dwarves so that they have FORGOTTEN what direction they want to measure spin. they have totally forgotten everything, those poor Dwarves!

So they just measure their particle spin in any old random direction!

What do you think? They do this year after year and the spin of the dwarfs particle is always acting just as if random. Because you see they DO NOT KNOW that their friend Hossi has observed hers. And they do not Know what is their friend's result.

TO BE CONTINUED

(in case anyone did not see the beginning, this is a continuation of post #44 story
https://www.physicsforums.com/showthread.php?p=963986#post963986 )

 

[marcus]

Recap of what the thread is about

To recapitulate, this thread is about Rovelli RQM (relational quantum mechanics).

RQM was defined by Rovelli in 1996 in the first paper he wrote about it, a paper called, appropriately enough Relational Quantum Mechanics
quant-ph/9609002
So to have a clear authoritative idea of what the term means (and not just what this or that person might SAY it means) I can think of nothing better than simply to read the first paragraph of Rovelli 1996

---quote Rovelli 1996 RQM paper---
I. A REFORMULATION OF THE PROBLEM OF THE INTERPRETATION OF QUANTUM MECHANICS

In this paper, I discuss a novel view of quantum mechanics. This point of view is not antagonistic to current ones, as the Copenhagen [Heisenberg 1927, Bohr 1935], consistent histories [Griffiths 1984, Griffiths 1996, Omnes1988, Gell-Mann and Hartle1990], many-worlds [Everett 1957, Wheeler 1957, DeWitt 1970], quantum event [Huges1989], many minds [Albert and Lower 1988, 1989, Lockwood 1986, Donald1990] or modal [Shimony 1969, van Fraassen 1991, Fleming 1992] interpretations, but rather combines and complements aspects of them.

This paper is based on a critique of a notion generally assumed uncritically. As such,it bears a vague resemblance with Einstein’s discussion of special relativity, which is based on the critique of the notion of absolute simultaneity.

The notion rejected here is the notion of absolute, or observer-independent, state of a system; equivalently, the notion of observer-independent values of physical quantities.

The thesis of the present work is that by abandoning such a notion (in favor of the weaker notion of state-–and values of physical quantities–-relative to something), quantum mechanics makes much more sense.

This conclusion derives from the observation that the experimental evidence at the basis of quantum mechanics forces us to accept that distinct observers give different descriptions of the same events.

From this, I shall argue that the notion of observer-independent state of a system is inadequate to describe the physical world beyond the hbar->0 limit, in the same way in which the notion of observer-independent time is inadequate to describe the physical world beyond the c->oo limit.

I then consider the possibility of replacing the notion of absolute state with a notion that refers to the relation between physical systems.[/color][/size]
-------endquote-------

So you get the idea of what RQM is about. The main idea is to get rid of the notion of an absolute state---an official right set of facts or measurements.

The RQM point is that it is not realistic to expect nature to provide one official right list of facts (any more than it is realistic to expect all observers to agree on the sequence in which some events occurred and which ones were simultaneous). So since it isn't realistic to expect that, we need to get over it---and adjust our expectations to be in line with what QM has been trying to tell us Lo these many years.

That is just my paraphrase of what he's saying. If you want to be more sure what Rovelli is saying please read the paper---it is just 20 pages or so, and not difficult.

So that is what RQM is. RQM is not NOTATION. It has the same equations as usual old Copenhagen. It calculates the same numbers as any normal version of QM. And it is not an imagining of some MECHANISM by which probabilities come about, as by branching which explore alternative lives in alternative universes. It does not add any novel mechanisms to believe in. It doesn't add anything that I can see: it just THROWS OUT SOMETHING. It discards the notion of an absolute or observerindependent state----it chucks out what i suspect is an UNREASONABLE EXPECTATION.

So here is a good test to see what SOME OTHER PROPOSAL MIGHT HAVE IN COMMON with RQM. If some other other person's proposal for adapting how we think about, i.e. interpret, QM is primarily focused on DUMPING THE ABSOLUTE STATE then that other interpretation is fundamentally similar. If it is not aimed at discarding the absolute state then it may be superficially similar, but does not capture the essential.

It is the essential of RQM that I want to focus on in this thread, although some people may wish also to talk about superficial resemblances with this or that. ROVELLI HIMSELF talks in the paper about some overlaps with other interpretations---which is fine, it just doesn't interest me especially

 

[koantum]

In the Quantum Physics Forum there is a thread called https://www.physicsforums.com/showthread.php?p=965424&nojs=1#goto_threadsearch".

 

[Chronos]

Rovelli's relational quantum states proposition is very attractive. I think it also yields testable predictions. The concept of observer independent quantums states is intuitively wrong, IMO.

 

[vanesch]

I have observers A and B, measuring alpha and beta when not in causal contact. That does not neccessarily have to be the same time (which is not well defined anyway), but let's say in whatever slicing its the same time t_0. We know the total spin is zero. Let us say they measure

S_A,alpha = 1
S_B,beta = 1

I think that the point of Rovelli is, that you have to look at things from the point of view of one observer at a time. As there is no SINGLE observer who can see both S_A and S_B at this moment (in whatever frame), it doesn't make sense for him to talk about "S_A has measured 1 and S_B has measured 1". You have to put yourself into the skin of A OR of B because there's no single observer seeing both.

So, S_A ONLY knows about "A saw 1", not about B and vice versa.

which is not a problem, because they have not compared their stuff.

Exactly.

Now go to time t_1 when they are in causal contact and measure the other part of the previously entangled state. They find

S_A,beta = 1 [note: I corrected this, think it was a typo to say -1]
S_B,alpha = -1

Now, I would have thought S_B,beta is what B has measured for beta at t_0. According to Rovelli, the important thing is now to let A ask at t_1 what B has measured. This is

S_AB = S_A,beta = -1

which is not what B has measured at t_0. Has B changed his mind concerning the measurement of beta from 1 to -1? Or has he not changed his mind but A always hears the answer he wants to hear? If so, does that make sense macroscopically?

Again, I think you have to see things from the PoV from ONE single observer, even when they meet.
Let's pick A. Back when A did his measurement, A knew of HIS result, but didn't know anything about B, so from A's PoV, B was in a superposition of states. And when A met B, this resulted in a kind of collapse, which put B into one definite state. So first A had his result (+1), and next, he meets B which has a result (-1). No problem. The -1 of B comes from the collapse of the superposition of B (before measurement by A) and its measurement by A.

So from A's PoV everything is ok.
You could do a similar reasoning from B's point of view.

And this is what I've been claiming all along: if you step back, and you consider ALL of these PoV's together, you have MWI.

Because from B's PoV, we first had +1. Ok. And from B's PoV, A was in a superposition until they met. So B will "measure" A in the -1 state when he meets. But that means that the "B" of the second story is NOT THE SAME B as the B in the first story (written from A's PoV).
In other words, they are in different branches. But as long as you look upon things from a single observer viewpoint, you don't have to think about this and this is what Rovelli does. He works from the PoV of ONE observer, for which the second observer is still in a superposition until he's observed by the first observer. And then he says that this is not something that happens to that second observer, but a description from the PoV of the first observer. This argument comes close to "Wigner's friend" (Eugene Wigner, 60ies).

So in a certain sense, Rovelli's viewpoint, that quantum states (using projection and all that) have only a meaning relative to an observer, is very similar to saying that coordinates in relativity have only a meaning relative to an observer. This is very true of course. But in relativity, one doesn't reject, because of that, the *underlying* geometrical object of spacetime, which is simply differently explored by different observers. So it is not because Rovelli rightly gives us a view on how "standard quantum theory with projection" is something which is relative to an observer, that this implies that the underlying objective structure does not exist.

And in an MWI view, that's exactly what happens. So it seems that Rovelli says something which is the equivalent of "(x,y,z,t) coordinates are to be seen relative to the observer" and MWI says the equivalent of "there is an underlying geometry from which we can derive that for each observer, things appear in an (x,y,z,t) coordinate frame".

 

[marcus]

Hi hossi and vanesch, BTW an incidental observation about coordinates.
In Gen Rel coordinates (t,x,y,z) do not have physical reality.
Events do though.

Rovelli has an interesting discussion of this in his book, including some quotes from Einstein. One could find it by looking up "Hole Argument", I expect, in either the index or the TOC.

I seem to recall some AE quote like "the princ. of gen. covariance removes the last vestige of physical reality from points of spacetime"
something like that. Anyway coordinates are just something you compute with, and have no real meaning: only events matter. (and the gravitational field, which is defined as an equivalence class making it coordinate-free)

I imagine that side observation is thoroughly familiar to you both, but let's not worry about coordinates. I think they are probably even off topic

 

[hossi]

But that means that the "B" of the second story is NOT THE SAME B as the B in the first story (written from A's PoV).
In other words, they are in different branches. But as long as you look upon things from a single observer viewpoint, you don't have to think about this and this is what Rovelli does.

Hi vanesh, thanks for the explanation. I think I finally get it. The observers can indeed have different measurements, but they agree whenever they exchange information, because that implies a quantum measurement. Can't say I like it.

B.

PS: I don't get your typo correction, the total spin is zero, and A measures =1 for particle alpha, then he should measure -1 for particle beta, when they come in causal contact, no?

 

[marcus]

dear Bee, my condolences to the federal republic for not catching a fine fish with the bright bait of an Emmy Noether. but it was just one month and some stiff-necked bureaucrat that screwed them out of it. you would have looked very nice in hamburg---in my way of thinking better there even than on the beach at SanBa
it is one of the world's good cities and also I like the freihaffen where I worked for a little while unloading ships
anyway it is all extremely sad and I am sorrier for them than for you.

 

[hossi]

dear Bee, my condolences to the federal republic for not catching a fine fish with the bright bait of an Emmy Noether.

Dear marcus,

thanks for the nice words. It's good to hear someone not telling me that I am stupid. I still hope that they are able to fill my position with someone else. It would be too bad, when the opportunity to build a group on phenomenological quantum gravity would be lost in Germany! There are just too little people working on physics beyond the standard model and quantum gravity.

I am kind of afraid, the priorities set for physics research in German are completely missing the importance of the field.

Anyway, I certainly don't regret going to PI

Take care,

B.

 

[marcus]

Dear marcus,


Anyway, I certainly don't regret going to PI

Take care,

B.

Hey! CONGRATULATIONS HOSSI![/color][/size]

 

[marcus]

I watched the video of your talk at PI, and I was wondering...when a person goes somewhere and gives a seminar talk then often later there could be discussions and a possible offer...but I don't read everything and I hadnt heard. I was just wondering.

this is GREAT NEWS!

and at least now we have gotten you off the beach at Santa Barbara

I was afraid you would be turning into a real California-German. Northerners should experience freezing weather on a regular basis and Waterloo Ontario will do fine.

==========================

about them not succeeding in putting together a Hamburg QG phenomenology team, that sux.

If we believed in duty, to the Fatherland and to the Scientific Establishment and for the honor of the Human Mind and the Future of Colonies in Space, and things like that. If we believed in dark Gothic letters written PFLICHT. Then we might say that you have a HIGHER DUTY to, at all costs, make sure that the QG phenomenology team is formed.

But it is a no-brainer to build such a team and I feel somewhat confident that after a little fumbling they will go ahead and do it without you. Or even offer you part-time. It is such an obvious thing to do. An "idea whose time has come" than which nothing is more powerful as they say.

Good luck at perimeter! this is such excellent news! when do you go?

 

[hossi]

Then we might say that you have a HIGHER DUTY to, at all costs, make sure that the QG phenomenology team is formed.

I will find some way. With or without the Germans.

But it is a no-brainer to build such a team and I feel somewhat confident that after a little fumbling they will go ahead and do it without you. Or even offer you part-time. It is such an obvious thing to do. An "idea whose time has come" than which nothing is more powerful as they say.

Nice that you are so optimistic. I will keep you updated but I wouldn't put my hopes too high. The Germans are pretty conservative, and quantum gravity is *uhm* no butter and bread physics (not yet, not in europe).

Good luck at perimeter! this is such excellent news! when do you go?

September. B.

 

[hawk00321]

CONGRATULATIONS TO HOSSI
I am very curious about QG.if I want to study it,how can I do ?Can u give me some advice.thank u very much in advanced.:-)

 

[hossi]

Hi hawk,

I have no idea - I don't even know what quantum gravity is.

Try to start with some of this and follow references therein, according to your interests:

"[URL Gravity
Enrique Alvarez[/URL]

"[URL Quantum Gravity
Dagny Kimberly, Joao Magueijo[/URL]

or, if you are really tough

"[URL Quantum Gravity
Carlo Rovelli [/URL]

Don't forget to have fun

B.

 

[hawk00321]

Dear hossi ,thanks

Hi hawk,

I have no idea - I don't even know what quantum gravity is.

I only know some QFT and QCD.

Try to start with some of this and follow references therein, according to your interests:

"[URL Gravity
Enrique Alvarez[/URL]

"[URL Quantum Gravity
Dagny Kimberly, Joao Magueijo[/URL]

or, if you are really tough

"[URL Quantum Gravity
Carlo Rovelli [/URL]

I think that I am tough enough but as like you said:quantum gravity is *uhm* no butter and bread physics of course ,it will be lost in China too.I am very sorry.

Don't forget to have fun

haha ,That's my dream in all life.U MUST BE A PRETTY GIRL

BEST WISHES FOR U

 

[marcus]

hello hawk, Sabine suggested several things to read. If you still want some ideas of beginning reading in QG, just say. someone will come up with more suggestions. I would try to think of some, if you want. but you may be quite content with what Sabine already mentioned.

I assume that you live in China, from what you said in your post, and have studied physics at college or university level (you may have introduced yourself to others but I didnt see---I don't read everything at the forum)

there are quite a few people at Beijing who do Loop Quantum Gravity and also a lot who do superstring theory (the majority field). In the summer of 2006, in fact quite soon, there will be a String conference at Beijing.

If you live in China perhaps you know what the two large university in Beijing are called. As a foreigner I call the one which has QG physics group by the name "Beijing Normal University". Probably you have a different name.

 

[Kalimaa23]

Nice that you are so optimistic. I will keep you updated but I wouldn't put my hopes too high. The Germans are pretty conservative, and quantum gravity is *uhm* no butter and bread physics (not yet, not in europe).

Then why are half the theoretical physics groups in Belgium working on String Theory?

 

[marcus]

Then why are half the theoretical physics groups in Belgium working on String Theory?

Could it be that they feel obliged to imitate the Americans?

 

[hossi]

Then why are half the theoretical physics groups in Belgium working on String Theory?

oh well, sorry, I consider string theory to be conservative

B.

 

[Kalimaa23]

@Marcus

@Hossi : I can see what you mean... but being newer doesn't mean it's "better". Don't get me wrong, I'm no Motl-type LQG hater, but I must admit that the more I read about it the less appealing it seems. Not that I think ST has the answers though... As a MS student in the field I'm taking the current pragmatic attitude that String Theory is a useful tool to learn about QFT, new mathematics, and who knows, will maybe show the way to true quantum gravity.

 

[hossi]

Hi Dimitri,

well, the LQG people like to claim their approach is much older, not newer. I can toally relate to your not-liking of LQG, it's definately not appealing. Anyway, what I wanted to say...

...what did I want to say...

...sorry, long day...

...essentially: anything is better than nuclear physics and solid state



B. (gotta go home get some sleep :zzz:)

 

[Kalimaa23]

...essentially: anything is better than nuclear physics and solid state

Amen!

 

[Kea]

To come back to the Elephant, although an extravagance perhaps, consider this quote from 1943:

The only possible alternative is simply to keep to the immediate experience that consciousness is a singular of which the plural is unknown; that there is only one thing and that, what seems to be a plurality, is merely a series of different aspects of this one thing, produced by a deception (the Indian Maya); the same illusion is produced in a gallery of mirrors, and in the same way Gaurisankar and Mt Everest turned out to be the same peak seen from different valleys.

There are, of course, elaborate ghost-stories fixed in our minds to hamper our acceptance of such simple recognition.

Erwin Schroedinger, What is life? 1943

This is taken from a set of lectures given by Schroedinger in Dublin, which became very well known and were subsequently read by Watson and Crick and others hot on the trail of the mystery of genes.

many worlds but one elephant

 

[marcus]

nice quote from Schrödinger

this Kea koan not bad either

many worlds but one elephant

 

[marcus]

Ten Commandments Of The Church Of The Smaller Hilbert Space

found this on the web, Matt Liefer, a guy at Perimeter, had it on his blog
http://mattleifer.wordpress.com/2006/04/13/the-church-of-the-smaller-hilbert-space/

http://www.perimeterinstitute.ca/personal/mleifer/humor/commandments.pdf

in quoting I dropped some technical notation---the link has Liefer's original
=========quote========

TEN COMMANDMENTS OF THE CHURCH OF THE SMALLER HILBERT SPACE

With apologies to Charlie Bennett and, of course, God.

1. I am ρ , your state, who brought you out of wavefunction realism, the place of orthodox dogmatism.

2. Do not have any other states except Me. Do not represent states by false purifications, conceived as ontological states of the Platonic forms above, of reality below, or of the space-time foam underlying reality. Do not bow down to such states or worship them. I am ρ your state, a state that demands exclusive belief.

3. Do not announce your state ρ in vain. ρ will not allow the one who announces it in vain to go unpunished by a Dutch bookie.

4. Remember the CP-map E to keep dynamics meaningful. You can work things out using six different Kraus decompositions or Steinspring dilations and do all your tasks. But the CP-map E is an equivalence class to ρ your state. Do not do anything that attaches meaning to the arbitrary tools you choose to work with. This includes your Hilbert Space basis..., your purifications...

5. Honor your forefathers by using the Hilbert space algorithm they handed down to you to calculate your expected utilities. You will then live long on the land that ρ your state describes your beliefs about.

6. Do not commit murder, since there is no other “branch of the wavefunction” in which your victim will survive.

7. Do not adulterate the Schrödinger equation by adding nonlinear terms designed to cause collapse.

8. Do not steal from classical physics by insisting that particle position or field configuration variables must evolve deterministically.

9. Do not testify as a false witness to the existence of histories of events that do not appear in the empirical records.

10. Do not be envious of your neighbor’s state σ. Do not be envious of your neighbor’s dynamical CP-map ..., his POVM elements..., his update CP-maps..., his Kraus operators..., his donkey, or anything else that is your neighbor’s, for they only describe his beliefs (except for the donkey), which naturally differ from yours.

===endquote===

 

[marcus]

Bee has introduced another animal into the picture, this time a dog.

A number of people were discussing this at her blog and I posted twice already and still have a bit more to say, so I will say it here.

In Bee's story there are two experimenters A and B and they agree to take their entangled electrons to separate places and on a certain day each perform an measurement. Let's make them women for a change.

They agree to point the machines to the East and read the spin.

Each person has a hilbertspace and each person has a wavefuntion or state in that space which describes what they have learned about life so far, and about the universe, like what to expect if you go out with Italian men---what to do if you see an elephant, and so on.

A and B are well-educated so they expect that one of them will get +1 and one get -1. Everything is crystal clear to them. Off they go to their respective stations, which are quite far apart.

the day arrives and A does her measurement and she gets -1, so she applies the appropriate projection operator and collapses part of her wavefunction to show the new information she has about HER electron.

She also has in her wavefunction or state vector some experience of how RELIABLE the other experimenter, B, is. And how often B's location is hit by hurricanes. In the hypothetical situation that B is TOTALLY reliable, always remembers to do what she is supposed to, TOTALLY competent, always gets her lab machines to work perfectly, and NEVER hit by hurricanes, then of course B would be expected to be reading +1 right now, because the spins on a given axis add up to one.

But that would not be realistic. So A does not commit herself right away, she doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't have that information yet.

Before she does that she will at least telephone, or maybe even go and check out the other station, where B is. Before recording any information about B's electron, she has to get in CAUSAL CONTACT.

SO THE COLLAPSE OF A's wavefunction is LOCAL. Somebody had to get in somebody else's lightcone, or even go over and stand next to them at the same spot, for it to happen.

 

[marcus]

I guess the point is that even if A knows B to be admirable in every respect---very reliable and competent in the lab etc.---maybe B is just then having an argument with her boyfriend, or maybe she has a hangover, or there has been a supernova explosion, or it just simply isn't her day. This happens sometimes. So you can't be sure.

the advice to A is, DONT COLLAPSE YOUR WAVEFUNCTION UNTIL YOU SEE THE WHITES OF THEIR EYES and you have definite information, don't go collapsing it based on some suppostion about somewhere you are not in causal contact with, some spacelike separated place where you can't have any idea what is happening there.

so now what about the DOG?

Well Sabine ups the narrative stakes by having B take along a dog to her station, and if her reading comes out +1 then B should SHOOT THE DOG. And conversely if the measurment comes out -1 then she should not shoot the dog.

There is a picture of the dog, which is an unpleasant overweight bulldog which it would be tempting to shoot regardless how the experiment turned out. I will get a link so you can go read further discussions of this

http://backreaction.blogspot.com/

http://backreaction.blogspot.com/2006/05/nonlocality.html

 

[vanesch]

A and B are well-educated so they expect that one of them will get +1 and one get -1. Everything is crystal clear to them. Off they go to their respective stations, which are quite far apart.

the day arrives and A does her measurement and she gets -1, so she applies the appropriate projection operator and collapses part of her wavefunction to show the new information she has about HER electron.

She also has in her wavefunction or state vector some experience of how RELIABLE the other experimenter, B, is. And how often B's location is hit by hurricanes. In the hypothetical situation that B is TOTALLY reliable, always remembers to do what she is supposed to, TOTALLY competent, always gets her lab machines to work perfectly, and NEVER hit by hurricanes, then of course B would be expected to be reading +1 right now, because the spins on a given axis add up to one.

But that would not be realistic. So A does not commit herself right away, she doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't have that information yet.

Before she does that she will at least telephone, or maybe even go and check out the other station, where B is. Before recording any information about B's electron, she has to get in CAUSAL CONTACT.

SO THE COLLAPSE OF A's wavefunction is LOCAL. Somebody had to get in somebody else's lightcone, or even go over and stand next to them at the same spot, for it to happen.

Yes, this is a perfectly all right view from a "solipsist" viewpoint: there is only ONE observer in this world, in casu "A". We never talk about the experience "B" lives, we only talk about what A OBSERVES from "B"s state.

As such, one can indeed see quantum theory as the theory that explains A's experience: first A sees her local result (local collapse of A's state), then A encounters B (local collapse of A's experience of B's state)...
So collapse occurs when A becomes, say, consciously aware of something.
This is all fine and well.

The problem arrives when we want the theory to describe at the same time also what B experiences, from its viewpoint.
Now, you can of course say that we should now apply the formalism on B's side, but there's a problem.
When A became aware of her result, and locally collapsed HER wavefunction, what can we say on B's side, from B's point of view ?
If you have A's wavefunction collapse from B's side too, then we are in contradiction with what we tried to establish, namely only "local collapse upon local becoming aware of the result". But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world.

From A's PoV:

State before A measures:
|a+>(u |b-> + v |b+>) + |a-> (x |b+> + y |b->)

A measures and "experiences" the result, it is +:

--> collapse (for A) into |A+>(u |b-> + v |b+>)

B might measure far away, but A doesn't know yet...

A meets B, and "measures" B's state, result of B is +:

--> collapse (for A) into |A+> | B+>

fine.

From B's PoV:

State before A measures:

|a+>(u |b-> + v |b+>) + |a-> (x |b+> + y |b->)


A measures, but this doesn't change anything to B yet.

B measures now, finds -

--> Collapse on B's side: (u |a+> + y |a->) |B->

B meets A, finds -:

--> Collapse on B's side: |A->|B->

All this is nice and well, except that from A's PoV, A and B got + and +, while from B's PoV, they both got - and -


This can be solved in a MWI scenario, by replacing collapse by "branch is consciously observed by"

Take # the "is conscious state by A" tag, and * the "is conscious state by B" tag, then we just have the state:

Before measurement, all states are still part of the "relative state" of A and of B:
|a+#*>(u |b-#*> + v |b+#*>) + |a-#*> (x |b+#*> + y |b-#*>)

A does a measurement, and found +, only changes the accessible states of A ; we remove what is NOT anymore part of "A's conscious world" (is projected out by the "collapse according to A":

|a+#*>(u |b-#*> + v |b+#*>) + |a-*> (x |b+*> + y |b-*>)

B does a measurement and found -:

|a+#*>(u |b-#*> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

Now, A and B meet.
First, A "measures" B (that is, A learns about the "B" state) A learns that B saw +, so this gives:

|a+#*>(u |b-*> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

About at the same time, B measures A, that is, B learns that "A saw -":

|a+#>(u |b-> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

What now emerges is that, in "A's mental world", symbolized by #, the state of the first particle is seen to be a+ and the state of the b particle is seen to be b+. In "B's mental world", symbolized by *, the state of particle a is -, and the one of b is - too.

So both "relative views" intermixed is simply a many worlds view where the "objective" wavefunction didn't collapse, but where "the awareness of a state" narrowed down its scope as a function of what it was made aware off, to the piece of the overall wavefunction that corresponds to its measurement results.

As such, the Rovelli flavor of the relative state view is the "one-observer" version of the many worlds view (where many observers are considered in parallel).

 

[vanesch]

A and B are well-educated so they expect that one of them will get +1 and one get -1. Everything is crystal clear to them. Off they go to their respective stations, which are quite far apart.

the day arrives and A does her measurement and she gets -1, so she applies the appropriate projection operator and collapses part of her wavefunction to show the new information she has about HER electron.

She also has in her wavefunction or state vector some experience of how RELIABLE the other experimenter, B, is. And how often B's location is hit by hurricanes. In the hypothetical situation that B is TOTALLY reliable, always remembers to do what she is supposed to, TOTALLY competent, always gets her lab machines to work perfectly, and NEVER hit by hurricanes, then of course B would be expected to be reading +1 right now, because the spins on a given axis add up to one.

But that would not be realistic. So A does not commit herself right away, she doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't have that information yet.

Before she does that she will at least telephone, or maybe even go and check out the other station, where B is. Before recording any information about B's electron, she has to get in CAUSAL CONTACT.

SO THE COLLAPSE OF A's wavefunction is LOCAL. Somebody had to get in somebody else's lightcone, or even go over and stand next to them at the same spot, for it to happen.

Yes, this is a perfectly all right view from a "solipsist" viewpoint: there is only ONE observer in this world, in casu "A". We never talk about the experience "B" lives, we only talk about what A OBSERVES from "B"s state.

As such, one can indeed see quantum theory as the theory that explains A's experience: first A sees her local result (local collapse of A's state), then A encounters B (local collapse of A's experience of B's state)...
So collapse occurs when A becomes, say, consciously aware of something.
This is all fine and well.

The problem arrives when we want the theory to describe at the same time also what B experiences, from its viewpoint.
Now, you can of course say that we should now apply the formalism on B's side, but there's a problem.
When A became aware of her result, and locally collapsed HER wavefunction, what can we say on B's side, from B's point of view ?
If you have A's wavefunction collapse from B's side too, then we are in contradiction with what we tried to establish, namely only "local collapse upon local becoming aware of the result". But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world.

From A's PoV:

State before A measures:
|a+>(u |b-> + v |b+>) + |a-> (x |b+> + y |b->)

A measures and "experiences" the result, it is +:

--> collapse (for A) into |A+>(u |b-> + v |b+>)

B might measure far away, but A doesn't know yet...

A meets B, and "measures" B's state, result of B is +:

--> collapse (for A) into |A+> | B+>

fine.

From B's PoV:

State before A measures:

|a+>(u |b-> + v |b+>) + |a-> (x |b+> + y |b->)


A measures, but this doesn't change anything to B yet.

B measures now, finds -

--> Collapse on B's side: (u |a+> + y |a->) |B->

B meets A, finds -:

--> Collapse on B's side: |A->|B->

All this is nice and well, except that from A's PoV, A and B got + and +, while from B's PoV, they both got - and -


This can be solved in a MWI scenario, by replacing collapse by "branch is consciously observed by"

Take # the "is conscious state by A" tag, and * the "is conscious state by B" tag, then we just have the state:

Before measurement, all states are still part of the "relative state" of A and of B:
|a+#*>(u |b-#*> + v |b+#*>) + |a-#*> (x |b+#*> + y |b-#*>)

A does a measurement, and found +, only changes the accessible states of A ; we remove what is NOT anymore part of "A's conscious world" (is projected out by the "collapse according to A":

|a+#*>(u |b-#*> + v |b+#*>) + |a-*> (x |b+*> + y |b-*>)

B does a measurement and found -:

|a+#*>(u |b-#*> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

Now, A and B meet.
First, A "measures" B (that is, A learns about the "B" state) A learns that B saw +, so this gives:

|a+#*>(u |b-*> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

About at the same time, B measures A, that is, B learns that "A saw -":

|a+#>(u |b-> + v |b+#>) + |a-*> (x |b+> + y |b-*>)

What now emerges is that, in "A's mental world", symbolized by #, the state of the first particle is seen to be a+ and the state of the b particle is seen to be b+. In "B's mental world", symbolized by *, the state of particle a is -, and the one of b is - too.

So both "relative views" intermixed is simply a many worlds view where the "objective" wavefunction didn't collapse, but where "the awareness of a state" narrowed down its scope as a function of what it was made aware off, to the piece of the overall wavefunction that corresponds to its measurement results.

As such, the Rovelli flavor of the relative state view is the "one-observer" version of the many worlds view (where many observers are considered in parallel).

 

[marcus]

**But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world.
**
I think we didnt lose the possible linkage because
a two-way link occurs when the two are in the same place----linkage (one way or two way)follows causal contact

there is more on this at Bee's blog. To my considerable pleasure and surprise she indicates agreement to some extent. I don't know how far her agreement goes, so you'd best consult the blog or her directly.

Im happy for you to totally disagree! I just can't take the time to discuss this too much. I imagine Rovelli would consider his picture "multiple-observer" and "single world". Rovelli picture is multiple-observer in the sense that it ALLOWS for having one or more observers. You can have one, or you can have 100. You have as many hilbertspaces as you do observers. Objective reality arises from the AGREEMENT of these guys. I personally see no connection with "many worlds".

But if you see a connection with "many worlds" that is fine!