Is QM Inherently Non-local in EPR and Bell Discussions?

[vanesch]

I do not know what MWI is

MWI = Many Worlds Interpretation, a fancy word for assuming that the observer is just as well suffering quantum evolution as everything else, so that an observation does not give rise to a projection, but that everything (including observation) is simply one big unitary evolution.
initial state:
A0, B0 and C0 are the states of the observers before they got involved in the measurement (before they underwent an evolution that entangled them with our system).
|A0)|B0)|C0) (u |a+) + v |a-) )
A "measures":
|B0)|C0) (u |A+)|a+) + v |A-) |a-) )
A+ is the body state of observer A where he saw a + result, and A- is the body state of the observer A where he saw a - result.
B "measures":
B+ is the state of the body of observer B when he's informed about the entire result of the B-measurement (so here we see - see further - than in order to be so informed, B actually has to have the entire B region in his past lightcone, but we're now pretending that this must not be the case).
|C0) u |A+) (|B+) (b+|a+) |b+) + |B-) (b-|a+) |b-))
+ |C0) v |A-)(|B+) (b+|a-) |b+) + |B-) (b-|a-) |b-))
C "measures":
u |A+) (|B+) (b+|a+) (|C+) (a+|b+)|a+) + |C-) (a-|b+)|a-))
+ |B-) (b-|a+) (|C+)(a+|b-)|a+) + |C-)(a-|b-)|a-)) )
+ v |A-)(|B+) (b+|a-) (|C+) (a+|b+)|a+) + |C-) (a-|b+)|a-))
+ |B-) (b-|a-) (|C+)(a+|b-)|a+) + |C-)(a-|b-)|a-) ) )
= |C+) {u |A+) (|B+) (b+|a+) (a+|b+)
+ |B-) (b-|a+) (a+|b-) )
+ v |A-) (|B+) (b+|a-) (a+|b+)
+ |B-) (b-|a-) (a+|b-) )}|a+)
+ |C-) {u |A+) (|B+) (b+|a+) (a-|b+)
+ |B-) (b-|a+) (a-|b-) )
+ v |A-)(|B+) (b+|a-) (a-|b+)
+ |B-) (b-|a-) (a-|b-) )} |a-)
The probability to get C+ is then the total length of the state vector which has the C+ body state as a factor:
|u|^2 (|U11|^4 + |U12|^4) + |v|^2 (U11.U21.U11*.U21*+U22.U12*.U22*.U12)
which is the same result as our first calculation.
Note that a priori we're in the same deep s**t, because if we don't let
A measure, then |A0) stays factored out, and the |A+) and |A-) terms are not
orthogonal anymore, but just add as amplitudes:
B "measures":
|C0) u |A0) (|B+) (b+|a+) |b+) + |B-) (b-|a+) |b-))
+ |C0) v |A0)(|B+) (b+|a-) |b+) + |B-) (b-|a-) |b-))
C "measures":
|C+) |A0) {u (|B+) (b+|a+) (a+|b+)
+ |B-) (b-|a+) (a+|b-) )
+ v (|B+) (b+|a-) (a+|b+)
+ |B-) (b-|a-) (a+|b-) )}|a+)
+ |C-) |A0) {u (|B+) (b+|a+) (a-|b+)
+ (b-|a+) (a-|b-) )
+ v |A-)(|B+) (b+|a-) (a-|b+)
+ |B-) (b-|a-) (a-|b-) )} |a-)
which will us probably give the same result as using projection.
But now we understand why ! The so-called B measurement cannot have taken place completely when C measures, so the B interaction (unitary) has to be split in 2 parts:
the one in the future lightcone of A (BL), and the one in the past lightcone of C (BR). Both interactions (unitary evolutions) BL and BR commute, and BL commutes with C, while BR commutes with A. BL does not commute with A and BR does not commute with C however.
Clearly, my 2-state example is not sufficient in this case to implement these operators, so I give up here for the moment, but I think that this will solve the issue.
In a way, you can say that (typical of the MWI approach) this splitting in BL and BR is part of what you require "detailling the detection procedure".
As far as I can tell, because the only evolution that could possibly influence C (as unitary evolution, using Green's functions all the way within the detector, brain, whatever), is BR, and whatever happens to BL and A should normally factor out, hence not influencing the entanglement of C with the state.
But I should work it out, and I think it's going to take more work and time than I have.
Nevertheless, interesting problem !

 

[Careful]

MWI = Many Worlds Interpretation, a fancy word for assuming that the observer is just as well suffering quantum evolution as everything else, so that an observation does not give rise to a projection, but that everything (including observation) is simply one big unitary evolution.
initial state:

I thought you quantum physicists called this environmental decoherence (I made the MWI guess myself but I was confused over what you meant by it) I thought MWI was just a particular way of envisaging the Schroedinger equation in the path integral framework, but ok now we speak the same language...

 

[Careful]

But now we understand why ! The so-called B measurement cannot have taken place completely when C measures, so the B interaction (unitary) has to be split in 2 parts:
the one in the future lightcone of A (BL), and the one in the past lightcone of C (BR). Both interactions (unitary evolutions) BL and BR commute, and BL commutes with C, while BR commutes with A. BL does not commute with A and BR does not commute with C however.
Clearly, my 2-state example is not sufficient in this case to implement these operators, so I give up here for the moment, but I think that this will solve the issue.

The B measurement can have taken place before C happens (see my comments about state reduction). Your entanglement to an observer is really not going to solve anything, it is just going to make the notation more heavy. B is a measurement which *cannot* be split into two parts by definition since is measures a non local property.

 

[vanesch]

(a) make your measurement procedure exact: you will have to apply a non local avaraging procedure as well in time as in space in order to interpret the result of this entanglement in a classical way.

But that's exactly what you DON'T want to do in MWI: you only consider a (pointlike) observer, which gets LOCALLY entangled (that means, whose state can only suffer a unitary evolution involving whatever is local at the spot of the observer).
If you want to do this "averaging" you should in fact construct several local observers at the different locations of the B region, which you make then travel (at less than lightspeed) towards the final B observer, and make them interact with this final B observer when they get there. It is only when that final B observer has encountered locally all of his "messengers" that he is finally entangled with the "B measurement" which is a very coarse-grained operation.
So I made one extra step, and considered "messengers from the BL and the BR" region, BL being in the future lightcone of A, and BR being in the past lightcone of C, both regions being disjunct.

(b) you say that it is only possible for a magical collapse to happen once an observer can have acces to the entire information of B. Now, this collapse is a non local procedure and happens on an entire spacelike hypersurface X containing this point like observer. It is no problem to put C to the future of this X unless X stays in the future lightcone of A which brings along other problems (so your claim is false there). Since this has to hold for any A your collapse has to happen on a null surface (and not even a differentiable one)!

Yes, that's why I consider collapse bull**** Except that it is damn practical to do calculations and that it comes out all the same as the MWI approach

(c) the only reasonable way to save your butt is by coupling realistic detector models to A,B and C and making a measurement theory for those. However, the dynamics to the quantum field under observation is not unitary anymore

Oh but of course it is ! That's the entire issue of MWI: stay unitary until you die (and beyond ) Once you accept that ALL is unitary evolution, maybe the respect of the "lightcone" will occur to you.
It is exactly what I try to argue with EPR situations: if you treat it the MWI way, you can stay local and nevertheless obtain the EPR correlations ; only, you can only observe them when BOTH Alice and Bob are in the past lightcone of this famous "correlation observer" (because Bob will have to travel to Alice, and Bob is in two states !) MWI is (according to me) the only way to reconcile relativity with QM.
In MWI, it is not you who collapses the state of the world, it is the world who entangles your body (and you only consciously experience one of those states, according to the Born rule) with the state of the world.
Now, I could argue of course endlessly over this, but I challenge you (for a change): describe me a way, in principle to DO this extended B measurement, so that we can turn it into a real FTL phone.
You can use screens, detectors, whatever. A plane wave (photon) is coming in, and A is going to decide to do, or not do, a measurement, while I'm C, doing a measurement, and you have to make me (at C) find out the result of your decision at A to do, or not, your measurement.

 

[vanesch]

B is a measurement which *cannot* be split into two parts by definition since is measures a non local property.

Ok, but then the exact unitary dynamics of that "measurement" will involve non-local hamiltonians and it will not happen using electroweak or strong interactions.

If you have such a physical process which can do something non-locally, you ALREADY screwed up relativity, and you've a preferred foliation of spacetime. The very definition of your measurement interaction did this. But, as I said, you're not going to be able to construct such a measurement apparatus whose function is based upon electroweak or strong interaction.

 

[vanesch]

I thought you quantum physicists called this environmental decoherence

There are subtle differences between the two concepts, but it is true that environmental decoherence does not make much sense if you do not adhere to an MWI-like view, because both are based upon the same idea: that what's called "measurement" is nothing special, and involves just unitary interaction (using hamiltonians in the usual way). This is in fact even present in the von Neumann view, and he calls this unitary interaction the "pre-measurement interaction". Only, von Neumann states that *at a certain point* (between the system and conscious observation) we have to make a break, and apply the projection postulate. Using the results of decoherence, one can then show that this comes down (FAPP = for all practical purposes) to just applying the projection postulate already on the system level - as it is taught in elementary textbooks.

MWI takes this one single step further, and allows everything (even your body, your brain and all that) to take part in the pre-measurement interaction, WITHOUT collapse. Problem is then of course that we've lost the Born rule. People have been struggling with that, I just assume (as others did) that we can just state that we consciously observe only one branch and that the probability of observing this is given by the Born rule.
There's not much difference between the von Neumann view and this view, in fact (FAPP, the calculations give the same results). It is only on the conceptual level that MWI is the only way to AVOID entirely this collapse, which is indeed highly non-local, badly defined (when exactly does it happen, and in what spacelike foliation?) and at least weird in that my brain can change the state of the universe somehow.Environmental decoherence comes of course to its "full glory" within such a view when there's no collapse... and also looses a part of its meaning: because environmental decoherence tries to explain the Born rule, by using the Born rule on a higher level of complexity. So contrary to what is sometimes claimed, environmental decoherence does not EXPLAIN the appearance of the Born rule in MWI. It just transports it from a high level of complexity down to the system level (as such, justifying the elementary textbook procedures). But the Born rule still has to come from some place. In von Neumann, that's clear. In MWI, you have to do it with what you call consciousness bull****

 

[Careful]

I can appreciate your comment about *collapse bull ***** since it leads to even further problems than I have mentioned. However, your interaction picture is unphysical (measurement does not happen at a spacetime point, it does need a spacetime region, an apparatus registers only when there is peak which goes over a certain threshold). If you want to interpret your measurement results, you have to trace out the degrees of freedom of the field under observation, the resulting dynamics of the traced out density matrix is not unitary although the total dynamics (field + observer) is. Similarly the dynamics of the field under observation is not unitary when you trace out the degrees of freedom of the detector. Therefore, neither of both fields have to statisfy causality constraints if you want to hint that unitarity implies causality. Moreover, there is no theorem which says that unitarity implies causality and vice versa (otherwise the wightman axioms would be abundant). The most you can argue is that unitarity is less troublesome than reduction postulates, but then again you have other problems. So unless you come up with a theorem, your argument is empty.

 

[ttn]

The probability to get C+ is then the total length of the state vector which has the C+ body state as a factor:

The *probability*??! To **get**?!

Maybe you should elaborate (for those who don't know it already) *your* "measurement" axioms which give these concepts meaning in (your version of) MWI.

 

[Careful]

Environmental decoherence comes of course to its "full glory" within such a view when there's no collapse... and also looses a part of its meaning: because environmental decoherence tries to explain the Born rule, by using the Born rule on a higher level of complexity. So contrary to what is sometimes claimed, environmental decoherence does not EXPLAIN the appearance of the Born rule in MWI. It just transports it from a high level of complexity down to the system level (as such, justifying the elementary textbook procedures). But the Born rule still has to come from some place. In von Neumann, that's clear. In MWI, you have to do it with what you call consciousness bull****

Ok, so that kills it off... You know what I hate the most about this kind of arguments, is that you always leave something unexplained (something weird, magical has to be there). The next step you have to take is to explain conciousness by a physical theory which uses consciousness as a fixed, postulated, concept. So, actually, you are not solving anything, you are just pushing a perverse scheme a step further. I would like to know from you where your consciousness was in the beginning of the universe, since clearly something must have reduced the state there (the universe is entirely classical...). Moreover, your consciousness does not solve many problems : I do not see for example how you would get out the second law of thermodynamics (this is much nastier at the quantum level than the classical one). If you like Penrose in that respect, then you must realize that the scheme he has for quantum gravity is not covariant ...

I also think that gravity is playing an important part in quantum mechanics, but then CLASSICAL gravity not some undefined dream as QUANTUM gravity.

 

[vanesch]

The *probability*??! To **get**?!

"The probability for the conscious observer to be associated with the body state who saw C+."

 

[vanesch]

You know what I hate the most about this kind of arguments, is that you always leave something unexplained (something weird, magical has to be there). The next step you have to take is to explain conciousness by a physical theory which uses consciousness as a fixed, postulated, concept.

I think that all this just indicates that we have not yet a full understanding of physics, which is - I would think - a totally trivial statement.
However, it is not by tossing out all we know that you get a better understanding of course.

So, actually, you are not solving anything, you are just pushing a perverse scheme a step further. I would like to know from you where your consciousness was in the beginning of the universe, since clearly something must have reduced the state there (the universe is entirely classical...).

Aren't you being a bit axiomatic here ? What says that the universe is entirely classical ? If that were true it wouldn't be necessary to use quantum theory of course. You can just as well state that the universe is entirely Newtonian or Aristotelian. It is not by stating this that things have to be this way. I'm just presenting a view of quantum theory, incomplete as it may be, that shows you that the problems that make you toss everything out of the window can be seen in a different light.
Of course Newtonian physics and Coulomb electrostatics are much nicer and better understood. They give less rise to interpretational problems... but then they don't correspond to observations in certain circumstances.

So we have a formalism that works (= makes correct predictions FAPP). You've presented us with a riddle as a gedanken experiment which uses non-existing interactions to provide for "extended measurements", and when you look at it through MWI glasses, you see simply more clearly that your "measurement interaction" cannot be compatible with known, local, unitary laws.

Moreover, your consciousness does not solve many problems : I do not see for example how you would get out the second law of thermodynamics (this is much nastier at the quantum level than the classical one).

Because I will always experience a branch with a (relatively) high Hilbert norm, and in those branches, that law is respected, no ?

If you like Penrose in that respect, then you must realize that the scheme he has for quantum gravity is not covariant ...
I also think that gravity is playing an important part in quantum mechanics, but then CLASSICAL gravity not some undefined dream as QUANTUM gravity.

The combination of gravity and quantum theory is still an open question, and it is silly to claim a priori what view will prevail. I can just as well claim that neither general covariance, neither the superposition principle will survive and that we will be in for something totally new. But all that is speculation, and one speculation is as good as the next. That is still no reason to toss out our actual knowledge and CERTAINLY no good reason to go back 90 years.

 

[vanesch]

(measurement does not happen at a spacetime point, it does need a spacetime region, an apparatus registers only when there is peak which goes over a certain threshold).

Measurement does not happen AT ALL.

If you want to interpret your measurement results, you have to trace out the degrees of freedom of the field under observation, the resulting dynamics of the traced out density matrix is not unitary although the total dynamics (field + observer) is. Similarly the dynamics of the field under observation is not unitary when you trace out the degrees of freedom of the detector.

You don't have to consider this approximated dynamics as anything real, do you. It is just a shortcut in a calculation.

Moreover, there is no theorem which says that unitarity implies causality and vice versa (otherwise the wightman axioms would be abundant).

Of course unitarity is not sufficient. You also need local dynamics ! Interactions that only act locally. That's what goes wrong with your extended measurement: you cannot build that using known interactions.

At least we know that electroweak and strong interactions satisfy these criteria...

 

[Careful]

That is still no reason to toss out our actual knowledge and CERTAINLY no good reason to go back 90 years.

I do not go back 90 years in time; there have been many sensible people in the last 90 years which have, although in unfortunate circomstances, conducted good research outside the mainstream. The shortage in your approach is not just a lack of respect for common sense but the absence of a good axiomatic system of physical principles (so all this is just patchwork). The sad thing about this whole story is that most people do not even research classical theories to the bone. There is an overwhelming number of staments done by quantum theorists concerning the presumed fact that only quantum mechanics explains some cherished experiments, most of these are plainly wrong! What I try to tell to people is that CLASSICAL gravity has surprising implications on the microscale most physicists are not even aware of, which come very close to quantum phenomena. It seems therefore logical that people explore this beautiful/rational theory to the end. As I said QM is for the moment a good effective scheme but certainly not a physical theory and filled with contradictions (I would like to see how your non-local consciousness state solves the measurement/superluminal signalling problem). Classical chaotic phenomena are not understood and progress in physics is not going to be made without a good axiomatic system.

 

[Careful]

Measurement does not happen AT ALL.
You don't have to consider this approximated dynamics as anything real, do you. It is just a shortcut in a calculation.
Of course unitarity is not sufficient. You also need local dynamics ! Interactions that only act locally. That's what goes wrong with your extended measurement: you cannot build that using known interactions.
At least we know that electroweak and strong interactions satisfy these criteria...

Look, you fall over words now. The question is how are you extracting a classical number which you note down on your sheet of paper from a quantal field. You need some non local averaging procedure for that; I call that measurement (how you implement it is your own business, but you should do it in a realistic way). This has nothing to do with strong or weak interactions, this has to do with when we put a cross and when not and that is clearly a real process. In my opinion it makes not even sense to take any quantum theory and speak about one measurement in a temporal sense (since quantum theory is about predicting results of a series of measurements). So far we have been speaking about reduction of density matrices in QFT, but this is not what we should actually do since these computations are really about non temporal experiments. But anyway, most people do not seem to bother about these *small details*.

I think this approximated dynamics is very very real since that is the only way you can put crosses. Moreover, it was not clear at all in the beginning you did not want to implement state reduction which we all know not to be local. The fact that I used a non local observable or not did not matter in that respect. So now, you still have to invent your scheme in which I can measure non local (and even local!) obervables without violating causality. Good luck!

Moreover, if you claim that general covariance does not survive, then (a) you have a hard job in explaining why GR is sooo good (as successful as you dear QM) and (b) why don't we go back to Newtonian days all together (go back 350 years back in time).

Moreover, most quantum gravitists expect QG only apply at the big bang and deep into black holes, the rest is entirely classical (apart from the cosmological constant perhaps). About the second law of thermo: my knowledge is that it is most of the time respected although not always (Poincare recurrency times seem to have more severe consequences in QM than in classical thermo).

 

[vanesch]

Look, you fall over words now. The question is how are you extracting a classical number which you note down on your sheet of paper from a quantal field. You need some non local averaging procedure for that; I call that measurement (how you implement it is your own business, but you should do it in a realistic way).

What I am trying to say is that this cross is put there locally (locally, on the time scale of writing down a cross, being several milliseconds, so what we call "local" here is a spacetime blob which extends over several milliseconds/lightmilliseconds in all directions) and that all "result of measurement" that resulted in me writing down that cross or not, if it came from a region a lightyear across (spacelike) needs to be totally in my past lightcone. So the averaging you are talking about (over the region a lightyear across) can be 'non-local' but will then take about half a year before it reaches me and I can decide to "put down a cross" or not.
This cross is there then "really" only for me ; I don't know how others experience this. So it isn't sure that "a real classical number has been extracted" in any way but my own conscious observation. In my conscious awareness of the world, this looks then now as a "real classical number", but it could just as well be that the paper is in a superposition, one state with, and another state without a cross on it, and I'm only consciously aware of the paper with a cross, while someone else may only be aware of the same paper without a cross.

This has nothing to do with strong or weak interactions, this has to do with when we put a cross and when not and that is clearly a real process.

As I try to point out, it isn't so clear that this is a "real process". It gives me maybe only the awareness of some reality, but that's just my experience, and maybe not someone else's. (I'm sure that this makes you jump up and down your chair ) But that's exactly MWI...

In my opinion it makes not even sense to take any quantum theory and speak about one measurement in a temporal sense (since quantum theory is about predicting results of a series of measurements).

I don't see why you say that. This is only the epistemological view of QM: a technique for calculating statistical predictions. But then it becomes very hard to implement physical principles into the theory. Even "locality" doesn't mean anything, because of course the numbers printed on a sheet are "local to the sheet".

Moreover, it was not clear at all in the beginning you did not want to implement state reduction which we all know not to be local. The fact that I used a non local observable or not did not matter in that respect. So now, you still have to invent your scheme in which I can measure non local (and even local!) obervables without violating causality. Good luck!

I think there is no difficulty there (but I told you that it will involve too much calculation on a too involved example to do it explicitly here). I take observers to be "pointlike" (at least on the scale of their conscious experience, which must be of the order of milliseconds or so), and all interaction, including "measurements" to be described by the standard unitary evolution dictated by the electroweak and strong interaction lagrangians or whatever, knowing that these have Green's functions which vanish outside of the lightcone. As such it should be clear that the state of entanglement of the local observer cannot be influenced by what happens outside of its past lightcone (as no unitary interaction will be able to propagate to it, using the Green's functions), and the state of entanglement of the local observer is exactly what describes the local observer's experience.

Moreover, if you claim that general covariance does not survive, then (a) you have a hard job in explaining why GR is sooo good (as successful as you dear QM) and (b) why don't we go back to Newtonian days all together (go back 350 years back in time).

I didn't say that general covariance will not survive ! I say it is an OPEN QUESTION.

Moreover, most quantum gravitists expect QG only apply at the big bang and deep into black holes, the rest is entirely classical (apart from the cosmological constant perhaps).

Black holes are BIG THINGS compared to people ! When you see that Hawking considers superpositions of spacetimes over billions of years (the time for gas to contract into a star, then a black hole, and then have the black hole evaporate in interference with the gas finally not contracting into a star) then having Alice in two states for a couple of years doesn't seem so extravagant !

About the second law of thermo: my knowledge is that it is most of the time respected although not always (Poincare recurrency times seem to have more severe consequences in QM than in classical thermo).

I am not so very fluent in these sophisticated applications of statistical mechanics.

 

[Careful]

QUOTE]
Just a few small notes for now :
(a) Black holes are not necessarily big things at all by any standard, they can be as small as the want to and there are VERY good reasons to think of elementary particles as black holes or similar gravitational configurations.
(b) My claim is that you HAVE to break covariance if you do stick to QM as it stands (if you are interested we can have a deeper chat about that)
(c) I am not going to answer on your consciousness crackpot stuff for now, just saw the lord of the rings and it indeed gives me the creeps but I am too tired to jump out of my chair :zzz:
(d) I am curious how you will put your cross at the orgin of the cosmic microwave background though.

I am off for the weekend so you can plunge yourself in your highly personal though universally connected consciousness state.

 

[Hurkyl]

So you seem to claim that performing the measurement at a, or not, when the B measurement is performed, changes the outcomes of C ?
Let us take an initial state |psi> which is u|a+> + v|a->, |a+> and |a-> being the two eigenstates of A (and also of C, since they commute).

As far as I know, there should be four eigenstates:

|a+, c+>
|a+, c->
|a-, c+>
|a-, c->

(Actually, there should be another parameter denoting the stuff that A and C don't care about)

 

[Hans de Vries]

(a) Black holes are not necessarily big things at all by any standard, they can be as small as the want to and there are VERY good reasons to think of elementary particles as black holes or similar gravitational configurations.

It's well known that the Schwartzschild radius only becomes equal to the
Compton Radius at Planck's scale. It's the very definition of Planck's scale!
This is 10¹⁹ times the energy scale of our common elementary particles.

That's how remote this proposal is from the generally accepted laws of physics...Regards, Hans

 

[vanesch]

As far as I know, there should be four eigenstates:
|a+, c+>
|a+, c->
|a-, c+>
|a-, c->
(Actually, there should be another parameter denoting the stuff that A and C don't care about)

Yes, you're right, I only treated a specific example where A and C were equal, and with a 2-state system (in a 2-dim space, I even think that you don't have a choice but to take their eigenspaces equal). I wasn't doing things in all generality. I probably should, and even do so with the BL and B

 

[vanesch]

Just a few small notes for now :
(a) Black holes are not necessarily big things at all by any standard, they can be as small as the want to and there are VERY good reasons to think of elementary particles as black holes or similar gravitational configurations.
(b) My claim is that you HAVE to break covariance if you do stick to QM as it stands (if you are interested we can have a deeper chat about that)
(c) I am not going to answer on your consciousness crackpot stuff for now, just saw the lord of the rings and it indeed gives me the creeps but I am too tired to jump out of my chair :zzz:
(d) I am curious how you will put your cross at the orgin of the cosmic microwave background though.
I am off for the weekend so you can plunge yourself in your highly personal though universally connected consciousness state.

Concerning your claim (b), I think you are right but I'm not expert enough in quantum gravity to understand this issue entirely. I think you're crashing into a wide open door if you claim that there is a conflict between QM and GR, and that it is an OPEN QUESTION on how to solve this. Your wishful dreams of particles being black holes are just as speculative and unfounded as any other random claim - who was the one requesting a theorem here ?
Concerning the "consciousness crackpot stuff", my point of view is this one: it is - in my opinion - the view that fits best with the formalism of QM as we have it today. I'm surely not entirely happy with it myself, but I stick to it as long as I have to stick to QM. Do I really think that this is how the world works ? My answer is simply that I don't know, and I think that anybody who claims he knows is deluding himself. As we don't have a final physical theory yet, we cannot say (and in fact, I don't know if we'll have such a theory one day - how can we know ?) My personal preference (but what does that mean?) would be that I somehow hope that this is NOT how the world really works. The only thing that this view allows me is to have a clearer understanding on how to apply the QM formalism - that's why I found your example very instructive.
On the other hand, even though I agree that it sounds crazy, it is not *that* crazy, you only have to get used to it. The idea that "observation" is something linked to a conscious experience is not so much far fetched after all ! In your beloved GR, one could call it just as well a crackpot idea that *time* is something observer-related. That you could go traveling and come back younger than your kids. Or that sitting on a high building just does the same (ok, the effect is tiny :-) That's also an idea to get used to.
At the end of the day, it doesn't matter what story we tell around a theory. What matters is the formalism and the general principles from which it is derived, and how well this formalism can explain experimental results. There, QM is for the moment unbeatable. And the day when there will be deviations it will be extremely interesting and instructive, but that isn't the case yet. So you're simply stuck with it for the moment.

 

[Careful]

It's well known that the Schwartzschild radius only becomes equal to the
Compton Radius at Planck's scale. It's the very definition of Planck's scale!
This is 10¹⁹ times the energy scale of our common elementary particles.

That's how remote this proposal is from the generally accepted laws of physics...


Regards, Hans

But that is not the issue: I did nowhere claim that the the Schwartzschild radius has to be of the order of the comption scale (of say an electron). What I alluded to however is, say, that indirectly (through the Einstein Maxwell equations) there are important gravitationally induced electromagnetic phenomena on the compton scale (of say the electron), see Rosquist 2004 (on gr-qc I believe and references therein). This does not involve a Kerr black hole solution (with an event horizon) but a spin dominated Kerr solution which gives a naked singularity (the electron is by far spin dominated if you believe in electron spin at least). You do NOT have to go to Planck scale energies to get interesting phenomena out (that is a common misunderstanding). This is a possible mechanism which might allow you to put forward a realistic continuum electron model and explain its structural stability, something particle physicists cannot even dream of.

cheers,

Careful

 

[vanesch]

But that is not the issue: I did nowhere claim that the the Schwartzschild radius has to be of the order of the comption scale (of say an electron). What I alluded to however is, say, that indirectly (through the Einstein Maxwell equations) there are important gravitationally induced electromagnetic phenomena on the compton scale (of say the electron), see Rosquist 2004 (on gr-qc I believe and references therein).

Hey, this post is an unexpected proof of MW ! You, Careful, went on a weekend, in your conscious experience:

I am off for the weekend so you can plunge yourself in your highly personal though universally connected consciousness state.

Nevertheless, in MY conscious experience, you came back and you posted again on PF

 

[vanesch]

see Rosquist 2004 (on gr-qc I believe and references therein).

I suppose you mean the paper:
gr-qc/0412064

It surely is thought-provoking !

 

[Careful]

What matters is the formalism and the general principles from which it is derived, and how well this formalism can explain experimental results. There, QM is for the moment unbeatable. And the day when there will be deviations it will be extremely interesting and instructive, but that isn't the case yet. So you're simply stuck with it for the moment.

On the other hand, even though I agree that it sounds crazy, it is not *that* crazy, you only have to get used to it. The idea that "observation" is something linked to a conscious experience is not so much far fetched after all ! In your beloved GR, one could call it just as well a crackpot idea that *time* is something observer-related. That you could go traveling and come back younger than your kids. .

I knew you wanted to go back 350 years back in time time is not observer related, time is the same for all inertial observers in minkowski (that is a common misunderstanding of the twin paradox). Time changes however when you accelerate and deviate from the geodesic path between spacetime points A and B, that is what you need to do in order get back home and such effects have been measured already.

Moreover, I wanted still to make a few comments on your ``measurement´´ procedure:
(a) I do not understand why you want to keep observables at all since they were introduced in the first place to make observation and you propose something which not related at all to this.
(b) in your ``reasoning´´ concerning the spacetime consciousness blob (did I understand that well ? ) you make a common mistake of introducing a global lorentz frame (since you speak about big spacelike distances), in which your consciousness must operate (so your observers are global at all and not local which is what I meant with your universal consciousness).
(c) It remains crystal clear that any copenhagen scheme is still in trouble when they use non local observables (in the standard way, involving QCD and all that)
(d) my claim for strong gravitationally induced gravitational effects at the compton scale is far from empty (see my reply to de Vries, also Carter and Wheeler have made similar observations at the end of the sixties even. ) and is very well supported indeed (for further references : see Cooperstock et al.)
I am one of the very few people around who don't go to the Planck scale at all in order to find interesting gravitational effects and to explain ``quantal´´ phenomena.

The rest is too crazy to answer, I am not stuck with QM at all, as I said many of its predictions have classical answers. There are a few challenges left true, but I one would succeed in solving these then one has ``quantum gravity´´ for free.

Cheers,

Careful

 

[Careful]

Hey, this post is an unexpected proof of MW ! You, Careful, went on a weekend, in your conscious experience:
Nevertheless, in MY conscious experience, you came back and you posted again on PF

I know, I cheated a bit
Now I have to go, otherwise my wife kills me... :!)

Cheers,

careful

 

[vanesch]

I knew you wanted to go back 350 years back in time time is not observer related, time is the same for all inertial observers in minkowski (that is a common misunderstanding of the twin paradox). Time changes however when you accelerate and deviate from the geodesic path between spacetime points A and B, that is what you need to do in order get back home and such effects have been measured already.

Just to avoid all misconceptions: I *know* that time is observer-dependent! But when you first hear it, being brought up in a Newtonian picture, you could have as a first reaction that this is a "crackpot idea". I wanted to draw the parallel that if you grew up with a classical relativistic picture, the idea that people could be in two places at once, though only observing one of them, can sound like a "crackpot idea" too. Nevertheless, that is the *fundamental idea* behind quantum theory: the superposition principle: if you can be here, and you can be there, then you can also be in both places at once.
The only small problem we have with this otherwise beautiful idea is that, well, we don't observe that (ahum... ). We do seem to observe the indirect consequences of it, however. So you need then to say that you will only be consciously aware of one of the states.

(a) I do not understand why you want to keep observables at all since they were introduced in the first place to make observation and you propose something which not related at all to this.

If by observables, you mean those famous hermitean operators with eigenstates in which you're supposed to flip ? They are only a useful mathematical summary of the very complicated unitary interaction - in fact environmental decoherence theory is the justification for that approach. A hermitean operator is nothing else but a "bag of orthogonal eigenspaces".
The "bag of orthogonal eigenspaces" is then nothing else but the family of disjoint, environmentally stable final states of the specific unitary evolution operator that describes the action of the measurement apparatus, calculated back to the states of the system to which it is to be applied (eg, the states of the system which will remain entangled with those environmentally stable final states of the overall system+measurement+environment).

(b) in your ``reasoning´´ concerning the spacetime consciousness blob (did I understand that well ? ) you make a common mistake of introducing a global lorentz frame (since you speak about big spacelike distances), in which your consciousness must operate (so your observers are global at all and not local which is what I meant with your universal consciousness).

? I do make the assumption that the piece of spacetime around the "conscious event" is about Minkowskian, but if it weren't, my body wouldn't be there ! I only wanted to say that at the end of the day, when you consciously look at your results, this takes a certain time, and occupies a certain space of course (the time of becoming consciously aware and the size of your brain, for instance) and that we should consider this blob as being "one event" and not going to subdivide this in smaller pieces of spacetime (the left side of my brain, or the right side, the beginning of my becoming aware, or the end...). This is just as in relativity books, where you consider the "explosion of a fire cracker" to be an event ; you shouldn't then nitpick over the length of the firecracker or the duration of the explosion.
I only wanted to point out that this "event" (this blob, if you want) must have the "experienced measurement interactions" entirely in its past lightcone. So the "measurement" is only complete at that point (and even a bit later). If you insist on using projection, you should only apply it when the measurement is complete, meaning, at that time (using of course a foliation of spacetime - the very reason I don't want to consider this projection because I don't want such a foliation).

(c) It remains crystal clear that any copenhagen scheme is still in trouble when they use non local observables (in the standard way, involving QCD and all that)

I think it is an abuse. You can DEFINE non-local hermitean operators, after all, why not. But it is an abuse to call it a measurement because you will not be able to construct a measurement apparatus which will involve a unitary evolution (due to the electrons and protons of its constituent parts) and couple in such a way to the environment that it will RESULT in an environmentally stable set of states and that can be traced back to the non-local hermitean operator, BEFORE a spacetime event is reached where the entire non-local region is in its past lightcone.

(d) my claim for strong gravitationally induced gravitational effects at the compton scale is far from empty (see my reply to de Vries, also Carter and Wheeler have made similar observations at the end of the sixties even. ) and is very well supported indeed (for further references : see Cooperstock et al.)
I am one of the very few people around who don't go to the Planck scale at all in order to find interesting gravitational effects and to explain ``quantal´´ phenomena.

If there's no error in the paper you cited (I'm not enough of a relativist to check, although I can follow the arguments), it is indeed surprising that GR phenomena appear already on the scale of 10^(-13) cm.
But there's a lot of work to do before you can claim "equivalence" with quantum phenomena.
What is a great success of QFT (dispite its lot of deseases) is, I'd think, the prediction of particles from fields: the very reason why the electon field comes in "lumps of equal electrons".

The rest is too crazy to answer, I am not stuck with QM at all, as I said many of its predictions have classical answers. There are a few challenges left true, but I one would succeed in solving these then one has ``quantum gravity´´ for free.

Of course. But as you say, there are still a few challenges left.

 

[Careful]

you shouldn't then nitpick over the length of the firecracker or the duration of the explosion.
I only wanted to point out that this "event" (this blob, if you want) must have the "experienced measurement interactions" entirely in its past lightcone. .
If there's no error in the paper you cited (I'm not enough of a relativist to check, although I can follow the arguments), it is indeed surprising that GR phenomena appear already on the scale of 10^(-13) cm.
But there's a lot of work to do before you can claim "equivalence" with quantum phenomena.
What is a great success of QFT (dispite its lot of deseases) is, I'd think, the prediction of particles from fields: the very reason why the electon field comes in "lumps of equal electrons".
Of course. But as you say, there are still a few challenges left.

Your blush makes you more attractive Ok, I am knitpicking over the length of the firecracker as you say it and I agree with you that the non local observables are an abuse (already for quite some time but I had too much fun with your consciousness). Now, is the firecracker important or not (is this tiny violation significant)? For all practical purposes (FAPP) of course not, however my evil mind could of course cook up an idealized thought experiment with many fire crackers placed in sequence as to violate causality with some big amount (although that is not possible in practice, but on a sufficiently long timescale it would be). The reason why I was knitpicking here is because it is IMPORTANT to know the details and we have now been talking for quite some time here (with many more details involved) about something which most people take to be as obvious.

My overall feeling in this, is that this FAPP attitude is not going to lead us anywhere (you accused me before of being too axiomatic when I said that your consciousness really does not solve anything and now you had to invoque another argument of knitpicking). Theories have to start from *exact* principles, and investigate the full consequences. The only theory really worhwhile doing this for is GR because of its immense axiomatic beauty and extreme accuracy. The paper of Rosquist is correct. Concerning your argument about what QFT is really good for, I cannot unfortunately disagree more. The particle concept of QFT is worthless. Its big succes however is the accuracy of S matrix predictions, a miracle indeed if you realize what mathematical ``magic´´ is needed for achieving this.

Have a nice weekend (true this time)

 

[vanesch]

My overall feeling in this, is that this FAPP attitude is not going to lead us anywhere (you accused me before of being too axiomatic when I said that your consciousness really does not solve anything and now you had to invoque another argument of knitpicking).

No, you miss the point - it is a pity you do not want, just for the sake of argument, place yourself in the MWI viewpoint. You seem to have the impression that it is all handwaving, but it is not that at all. There are of course very fuzzy concepts such as consciousness, but human perception IS a fuzzy thing in the end! In MWI, there is a very strict law of evolution, it is unitary evolution, period. As such, your body ends up in an entangled state with many other states, and the only extra thing that is claimed, is that you are consciously aware of ONE of these states, according to the Born rule.
The precise interaction doesn't matter - this is not a matter of FAPP ; it is a matter of what the experimenter, at the end of the day, when he studies the output of his computerized experiment, observes. If, at that point, you can say that the experimenter will experience his body state according to the Born rule, then, thanks to decoherence theory, this Born rule trickles down to the exact system his experimental setup has been studying. There's no "FAPP" here, this is entirely exact. The only vague point is what happens exactly when he reads his printout, but if it can be accepted that this results in applying the Born rule to this (exact interaction, using unitarity) superposition of body states in one way or another, we're home.
You can even trace this back 15 billion years if you want (except for the fact that we don't have quantum gravity), and have the experimenter choose from all possible states that occurred since the Big Bang - and nevertheless, he'll pick out a state (corresponding to the Born rule) which makes you apply the Born rule all the way back.
So the Earth did, and didn't form ; the sun did and didn't form... but we happen to have choosen a branch where it did form.

Theories have to start from *exact* principles, and investigate the full consequences. The only theory really worhwhile doing this for is GR because of its immense axiomatic beauty and extreme accuracy.

Newtonian mechanics is also in this case: a nice theoretical framework of immense axiomatic beauty and extreme accuracy. The dirty physics however, would also like to have predictive power that fits with experiment.

Concerning your argument about what QFT is really good for, I cannot unfortunately disagree more. The particle concept of QFT is worthless.

In free field theory, I'd say that the derivation of the particle concept is rather straight forward ! The problem comes in with the interactions.

Its big succes however is the accuracy of S matrix predictions, a miracle indeed if you realize what mathematical ``magic´´ is needed for achieving this.

I will not deny the mathematical difficulties of QFT. But there are A LOT of successes (like the recent hadron mass predictions using lattice QCD), which should make you wonder how it could be that a totally misguided idea leads to so much correct results.

 

[Careful]

The problem comes in with the interactions.
I will not deny the mathematical difficulties of QFT. But there are A LOT of successes (like the recent hadron mass predictions using lattice QCD), which should make you wonder how it could be that a totally misguided idea leads to so much correct results.

I will be short about this
(a) you have the comments of Penrose which are entirely justified (concerning the clever mixing)
(b) you have a preferred basis problem (which coarse graining do you apply?)
(c) you cannot explain the perception state (I already made that comment)
and so on and so on. It is true that in a *well defined QFT* involving *local* projection operators on a universal perception state, you can save locality but then try to give me an example of a local perception projection operator whose outcome corresponds to the measurement of a non local observable such as in our region B. The only way, in my opinion to solve schroedingers cat, is to make quantum theory non linear (another reason why I am keen on the self field approach) just as all realistic processes in nature are. About QFT ,who cares about the free field ?? (and even then you have to be careful). Look, as mentioned before, I think that QFT despite its contemporary uglyness, is a worthwhile theory, a bit in the sense that thermodynamics is. It does not give a realistic explanation of the processes in the microword, but it gives very good statistical predictions concerning outcomes of scattering experiments (as well as masses of gauge particles) just as thermodynamics can serve extremely well for engineering. I have nothing against QFT in that respect, I just think it cannot serve as a basis ingredient for a theory of quantum gravity.

There are other approaches in QM which do in my opinion a much better job in trying to solve the cat problem: this is penroses OR theory, and a theory which is called the Brussels Vienna interpretation of QM (very abstract for now still) but these two suffer still from incompatibility with special relativity as far as I know. A fully classical alternative (such as I try) is another logical way out.

By the way, do not dare to compare Newtonian mechanics to GR.

 

[vanesch]

(b) you have a preferred basis problem (which coarse graining do you apply?)

This could be solved "by postulate". Within the MWIers I'm kind of heretic in that I'm convinced that you cannot *derive* the Born rule logically from unitary quantum theory, so that you need to ADD extra postulates which describe conscious observation ; one of them being the Born rule, and why not, another postulating the basis in which it occurs. This comes in fact very close to von Neumann, except that the act of observation is not something that has physical consequences (as does the projection postulate), but only affects conscious observation. 'True' MWI-ers spend a lot of effort trying to derive that somehow from unitary QM, but I think that that is fundamentally impossible.

(c) you cannot explain the perception state (I already made that comment)
and so on and so on.

That is just added by postulate: you have the physical world, entirely gouverned by unitary QM, and you have the "mental world" which couples to the physical world through some extra postulates, which "sample" the wavefunction in a certain way so that this corresponds to our habitual perception.

It is true that in a *well defined QFT* involving *local* projection operators on a universal perception state, you can save locality but then try to give me an example of a local perception projection operator whose outcome corresponds to the measurement of a non local observable such as in our region B.

Well, build me an apparatus that does the measurement, and I'll give you the local perception operator! It will be nothing else but the unitary evolution operator associated with the physics of the apparatus.

The only way, in my opinion to solve schroedingers cat, is to make quantum theory non linear (another reason why I am keen on the self field approach) just as all realistic processes in nature are.

It is A way, but not the ONLY way. I would think that if somehow gravity could introduce, in a "natural" way, this non-linearity, that would be a good idea. Just adding non-linearity for the sake of obtaining a projection is a fudge factor. And we have to accept the possibility that the superposition principle IS fundamental and strict. If that's the case, I don't see any way out except the MWI style. And, you'll have to admit it, for the moment there's no experimental indication of a deviation from the superposition principle (except maybe for the trivial fact that we don't directly experience it )

Look, as mentioned before, I think that QFT despite its contemporary uglyness, is a worthwhile theory, a bit in the sense that thermodynamics is. It does not give a realistic explanation of the processes in the microword, but it gives very good statistical predictions concerning outcomes of scattering experiments (as well as masses of gauge particles) just as thermodynamics can serve extremely well for engineering. I have nothing against QFT in that respect, I just think it cannot serve as a basis ingredient for a theory of quantum gravity.
There are other approaches in QM which do in my opinion a much better job in trying to solve the cat problem: this is penroses OR theory, and a theory which is called the Brussels Vienna interpretation of QM (very abstract for now still) but these two suffer still from incompatibility with special relativity as far as I know. A fully classical alternative (such as I try) is another logical way out.

If by your comments, you want to state that there are still unsolved problems in physics, and especially with QFT, and that QM's interpretation is far from clear, and that it is very well possible that it will need modifications in the future, I couldn't agree more. However, as you also point out, many of the suggested ways out are in their infancy on the formal level, in that they introduce a fundamentally different formalism for which none has yet, in its whole, shown the same experimental accuracy and efficiency as quantum theory as it stands today. I think that the danger of following these paths is that one is blinded by the consistency of the formalism one is develloping, and forgets that it needs to work also in the lab - a bit the result of some nostalgia to times when physics was clearer. Of course, one cannot work on 10 approaches at the same time, and in the end it is just gut-feeling which guides one to choose an approach - as this is a very personal matter, it is hard to discuss.
My viewpoint is that any approach that will have a fundamental impossibility to explain entanglement in the way QM does, is a very dangerous bet, because too many close hits have confirmed QM when it uses entanglement, in very different circumstances.

By the way, do not dare to compare Newtonian mechanics to GR.

I don't see why. I would even say that Newtonian mechanics of fundamental elastic spheres, with Newtonian gravity and Coulomb electrostatics, is a very very clear, axiomatically well-defined and interpretationally totally clear theory. It doesn't fit certain experiments, but so what ? It is axiomatically even better defined than GR, so if you want to hide into a clear, axiomatic world view, why not go for that one ?
In the words of Weinberg: "I don't see what's wrong with a Newtonian universe with fundamental spheres. It's simply not ours."

 

[Careful]

This could be solved "by postulate". Within the MWIers I'm kind of heretic in that I'm convinced that you cannot *derive* the Born rule logically from unitary quantum theory, so that you need to ADD extra postulates which describe conscious observation ; one of them being the Born rule, and why not, another postulating the basis in which it occurs. This comes in fact very close to von Neumann, except that the act of observation is not something that has physical consequences (as does the projection postulate), but only affects conscious observation. 'True' MWI-ers spend a lot of effort trying to derive that somehow from unitary QM, but I think that that is fundamentally impossible.
."

I agree that it is impossible to do this (I don't see a real difference here with the environmental decoherence game). However, I by far do not agree with you that you could just take this as an extra postulate. Then, you are not really explaining anything - on the contrary - you are simply putting the possible outcomes of your pre-dediced experiments in by hand. Moreover, as I pointed out to you several times, such attitude is worthless if you speak about the entire universe. There, reduction happens without any experimentator being around for deciding which setup one shoud do, moreover in a sum over histories framework you would even have tremendous difficulties in defining the experiment itself when doing quantum gravity. As said before, you are really taking a perverse game one step further.

.
Well, build me an apparatus that does the measurement, and I'll give you the local perception operator! It will be nothing else but the unitary evolution operator associated with the physics of the apparatus.
It is A way, but not the ONLY way. I would think that if somehow gravity could introduce, in a "natural" way, this non-linearity, that would be a good idea. Just adding non-linearity for the sake of obtaining a projection is a fudge factor. And we have to accept the possibility that the superposition principle IS fundamental and strict. If that's the case, I don't see any way out except the MWI style. And, you'll have to admit it, for the moment there's no experimental indication of a deviation from the superposition principle (except maybe for the trivial fact that we don't directly experience it )

I do not have to build you this apparatus, you do! I pointed out to you that non local observables, which are widely used in QFT pose a potential measurement problem. You answered, that if I would take into account a perception field and state, and limit myself to local projection operators on these ``mental states´´ (you can even choose here to do the reduction or not) then I can still save locality. I agreed modulo the technical worries of a well defined QFT and physical worries as to the reality and dynamics behind the mental field and its coupling to physics fields. Now, I asked you to make this scheme concrete: try to link ``observables´´ of the real field to the singular projection operators on mental states, singular because they live on a set of measure zero. This is not my problem, and even if you could solve it, which is possible in principe, then still it is not a meaninful scheme in my opinion unless you go to a OR type of scheme. But then you would have to join you fellow MWI compadres.

Of course, this non linearity is to come from gravity. Moreover, there is no reason for keeping the superpostion principle at all (as you said, we do not see it and I thought quantum physicists are all strong positivists, so logically abandonning this should be of no worry). As said before, it is this very principle which is responsible for the measurement problem.

If by your comments, you want to state that there are still unsolved problems in physics, and especially with QFT, and that QM's interpretation is far from clear, and that it is very well possible that it will need modifications in the future, I couldn't agree more. However, as you also point out, many of the suggested ways out are in their infancy on the formal level, in that they introduce a fundamentally different formalism for which none has yet, in its whole, shown the same experimental accuracy and efficiency as quantum theory as it stands today. I think that the danger of following these paths is that one is blinded by the consistency of the formalism one is develloping, and forgets that it needs to work also in the lab - a bit the result of some nostalgia to times when physics was clearer. Of course, one cannot work on 10 approaches at the same time, and in the end it is just gut-feeling which guides one to choose an approach - as this is a very personal matter, it is hard to discuss.

Indeed it is hard to discuss, let's just respect each others approaches and argue on points of consistency.

My viewpoint is that any approach that will have a fundamental impossibility to explain entanglement in the way QM does, is a very dangerous bet, because too many close hits have confirmed QM when it uses entanglement, in very different circumstances.
I don't see why. I would even say that Newtonian mechanics of fundamental elastic spheres, with Newtonian gravity and Coulomb electrostatics, is a very very clear, axiomatically well-defined and interpretationally totally clear theory. It doesn't fit certain experiments, but so what ? It is axiomatically even better defined than GR, so if you want to hide into a clear, axiomatic world view, why not go for that one ?
In the words of Weinberg: "I don't see what's wrong with a Newtonian universe with fundamental spheres. It's simply not ours

[/QUOTE]

Concerning your entanglement impossiblity, I would say that this bet is as dangerous for you as it is for me. There is no distinction as yet and if the experiments would keep on turning out inconclusive (or in favor of realism) then from the esthetical point of view, the local realist attitude is certainly preferred. QM can only vindicate when the experiments are fully successful and even then there are dirty, realist ways out. Concerning your Newtonian comments, you really don't seem to appreciate the full beauty of GR: locality and space time coordinate invariance are the most powerful and simplifying principles in nature.

 

[vanesch]

Then, you are not really explaining anything - on the contrary - you are simply putting the possible outcomes of your pre-dediced experiments in by hand.

That's the unfortunate fate of anything that is introduced by postulate...

Moreover, as I pointed out to you several times, such attitude is worthless if you speak about the entire universe. There, reduction happens without any experimentator being around for deciding which setup one shoud do

Or it doesn't happen ! Why does reduction have to happen ? If there's no observer, there's no need to have the wavefunction reduce to one of the "observable" states, as there aren't any. This is one of the big advantages of considering the unitary evolution "all the way": you can have - in principle - a wavefunction of the universe - which poses indeed a problem if you need state reduction, because what observer is going to do so.
The nice thing about the MW view (also, probably more appropriately called relative state interpretation) is that you simply split the universe in "observer" + "rest of the universe" and that you consider that the observable states are those that are product states of the two subsystems.

, moreover in a sum over histories framework you would even have tremendous difficulties in defining the experiment itself when doing quantum gravity.

All this of course in a universe without gravity, because we can't yet do this...

Moreover, there is no reason for keeping the superpostion principle at all (as you said, we do not see it and I thought quantum physicists are all strong positivists, so logically abandonning this should be of no worry). As said before, it is this very principle which is responsible for the measurement problem.

Unfortunately it is also the principle at the basis of the successful quantum formalism !

Concerning your entanglement impossiblity, I would say that this bet is as dangerous for you as it is for me. There is no distinction as yet and if the experiments would keep on turning out inconclusive (or in favor of realism) then from the esthetical point of view, the local realist attitude is certainly preferred.

That's where we differ fundamentally in opinion: the tests are NOT inconclusive. The tests follow EXACTLY what is expected by quantum theory, including the description of the apparatus. There is not one of these situations where you simply present the description of the experimental situation to just any quantum physicist, and where he doesn't turn up, after some straightforward calculation, with exactly those numbers that are also found during the experiment. In doing so, he did use quantum entanglement.
Ignoring the indicative value of these experiments is what I call "a dangerous bet".
Let us take a (ridiculous) example: imagine that you have a world view where, for some or other fundamental philosophical reason, it is impossible to have gravitational attraction between a mass like the sun, and a planet like Jupiter, at the earth-sun distance. Imagine you're living in the 17th century and there's a crazy Brit, called Newton, which has a theory with gravity in 1/r^2, which goes against your world view.
Now, this theory works of course on the sun-mercury, venus ... distance, but of course it is "experimentally" impossible to put Jupiter on the Earth orbit. So you say, see, this Newton guy's theory doesn't PROVE that the Sun-Jupiter interaction, if it were at the sun-earth distance, would be there. All measurements done today confirm my statement and are inconclusive about a potential gravitational interaction of a Sun-Jupiter system at a Sun-earth distance. That's 50 years now that people have been trying to confirm, without success, that Jupiter, placed at the Earth orbit, would follow Newton's laws.
Wouldn't you find such a claim totally ridiculous, in that if the Newtonian scheme is *confirmed* by experiment for all the cases of the planets in the solar system, that there is very very little room for a view where it WOULDN'T work in exactly that situation which gives you a conceptual problem ?

Concerning your Newtonian comments, you really don't seem to appreciate the full beauty of GR: locality and space time coordinate invariance are the most powerful and simplifying principles in nature.

Oh, but I do ! I only wanted to indicate that it is not sufficient to have a beautiful, powerful, simplifying principle. It needs to fit experiment also. And I DO find Newtonian physics more "beautiful, powerful an simplifying" than GR or QM. It is much more intuitive and clear... only it doesn't work in all cases, so that's it.
I think that QFT and GR will, one day, go the same way.

 

[Careful]

Or it doesn't happen ! Why does reduction have to happen ? If there's no observer, there's no need to have the wavefunction reduce to one of the "observable" states, as there aren't any. This is one of the big advantages of considering the unitary evolution "all the way": you can have - in principle - a wavefunction of the universe - which poses indeed a problem if you need state reduction, because what observer is going to do so.
The nice thing about the MW view (also, probably more appropriately called relative state interpretation) is that you simply split the universe in "observer" + "rest of the universe" and that you consider that the observable states are those that are product states of the two subsystems.
All this of course in a universe without gravity, because we can't yet do this... .

Come on, you know as well as I do that this position leads to ridiculous situations where the moon is not there unless we consciously percieve a photon scattered by it (Penrose mocks with this for a good reason). Again, you do NOT explain what macroscopic is, this is my main comment and you put it away handwavingly.

Look, the nice thing about GR is that it tells us that everything is dynamical; so your split observer + rest is very very ugly from that point of view.

That's where we differ fundamentally in opinion: the tests are NOT inconclusive. The tests follow EXACTLY what is expected by quantum theory, including the description of the apparatus. There is not one of these situations where you simply present the description of the experimental situation to just any quantum physicist, and where he doesn't turn up, after some straightforward calculation, with exactly those numbers that are also found during the experiment. In doing so, he did use quantum entanglement.
Ignoring the indicative value of these experiments is what I call "a dangerous bet".
Let us take a (ridiculous) example: QUOTE]

Your example is truly ridiculous. Look, you see the issue too one sided. For me, it is ok that you take Schroedinger as a good description together with its linearity and I acknowledge that this theory has predictive successes. BUT at the same time YOU do not want to see that your cheap tricks are putting one and the same problem on more and more religious and fuzzy grounds. What I try to tell you all along is that this travesty is the result of a HISTORICAL process, not of any *absolute* scientific reason. It is in my opinion much more natural, if you start from classical Hamiltonian dynamics to derive the non linear self field equations than the N particle Schroedinger equation (and in this derivation you do not even need to speak about a product state, neither do you need second quantization to take into account a non stationary radiation field). What I want to say here is that the compelling need to fit with experiment would have gotten anothor face if people would have taken that road and there would not even be spoken about entanglement at all. There is in my view no need to go over to configuration space techniques and this comment has been around since the first days of QM (unfortunately, people wanted to push the program too fast). The ONLY thing you say is that QM can be fitted, by taking into account realistic measurement setups, to the experimental data, but in the same way can local realism do that. The only difference is that QM is more advanced but this in an issue of MONEY and policy and not of intrinsic scientific value. Both positions are logically consistent and that is why your example is ridiculous. I would even dare to say that QM at the macroscopic level is more underdevelopped as classical theories are on the microscopic level (but you have a fictitious mechanism which you do not wish to explain to dispose of that comment). Moreover quantum gravity is not a problem in my reasoning, it is however a terrible (and unsolvable one in my opinon) in your line of thought.

Cheers,

careful

 

[vanesch]

Come on, you know as well as I do that this position leads to ridiculous situations where the moon is not there unless we consciously percieve a photon scattered by it (Penrose mocks with this for a good reason).

What is fundamentally wrong with that idea ? Weird, true ; but fundamentally wrong ? Not really. I don't say that things *have* to be that way, but why *can't* they be that way ? In fact, if you take MWI a bit further, in most of the universal wavefunction, the moon and the sun aren't even there ! But we just happen to observe that part of it where they are. I don't see why this position is untenable or ridiculous. If a scientific theory can explain one's perception, isn't that all one can require of it ?

I agree that things would certainly be more intuitively confortable if we didn't have to go into these considerations, in the same way as things would be more comfortable if we could have a universal time. But if a successful formalism forces us in some way to take up these considerations, what's wrong with that ?

Look, the nice thing about GR is that it tells us that everything is dynamical; so your split observer + rest is very very ugly from that point of view.

You still seem to see an observer as something absolute. But it is not. A stone could be an observer - a conscious one. You'll never find out (that's a well-known philosophical problem). There is not more a fundamental problem in considering "observer + rest" this way, than to consider a world line of an observer in GR, and the way this observer sees the universe. In exactly the same way, a "quantum observer" will see what happens along its "quantum world line", this time including its entanglement with whatever it is interacting locally with and making his "Born rule choices" in tracing out its world line.
This is not so very much stranger than an observer falling into a black hole observing (just before getting crushed) the entire universe's future (and hence being fried by all the radiation he gets at once).

It is in my opinion much more natural, if you start from classical Hamiltonian dynamics to derive the non linear self field equations than the N particle Schroedinger equation (and in this derivation you do not even need to speak about a product state, neither do you need second quantization to take into account a non stationary radiation field).

It may sound natural, but this doesn't work ! Many *solved* problems in QM have not much hope of being correctly handled that way ; I gave you the examples of configuration interaction in quantum chemistry, but there are miriads of examples, in solid state physics and condensed matter physics.

The ONLY thing you say is that QM can be fitted, by taking into account realistic measurement setups, to the experimental data, but in the same way can local realism do that.

That's the point: it does NOT have to be "fitted". The photoelectric effect in the photomultipliers for instance can be quantum-mechanically described. The workfunction of the metal can be described quantum-mechanically. There are no specific "fudge factors" that apply in the case of these EPR experiments. The phenomena leading to the detection process are well-described. That doesn't mean of course that no empirical calibration is used, but NOT MORE than for any other experimental technique. No "new concepts" have to be plugged in the theory to have the natural descriptive machinery of the process, of the beam splitters, of the detectors etc...
All LR models have to propose new concepts made up for the purpose, and often involve "unknown" workings of the experimental material (such as the detectors). For instance, Santos' SED, as far as I understands it, posits that EM radiation with energy h omega/2 is "present" in every mode, but that photodetectors have calibrated that away in order to give out 0 in what we think is "dark" but is in fact the background radiation. But if you now apply *thermodynamics* to this, you'd find that a bottle of black ink would soon start to boil if it truly absorbed all this radiation ! The remark is then that SED is only meant to describe *optical* phenomena, and that it can mimick EPR results in low-efficiency detectors that way. Ok, but you can't have such an EM theory that works for optics, but not for thermodynamics !

The only difference is that QM is more advanced but this in an issue of MONEY and policy and not of intrinsic scientific value. Both positions are logically consistent and that is why your example is ridiculous. I would even dare to say that QM at the macroscopic level is more underdevelopped as classical theories are on the microscopic level (but you have a fictitious mechanism which you do not wish to explain to dispose of that comment).

Except that apart from *conceptual* problems (which I will not deny - although they may be less severe than you seem to imply), we have good formulas which work FAPP ! It serves no purpose to have a clear conceptual framework when you don't have working formulas !

Moreover quantum gravity is not a problem in my reasoning, it is however a terrible (and unsolvable one in my opinon) in your line of thought.

Ok, but quantum chemistry (amongst other things - like a lot of stuff in solid state physics) is a problem in YOUR reasoning. You have some vague hope that this can be solved, but it is on just as fuzzy grounds if not more, than the quantum gravity problems from the QM side. Only, on the quantum chemistry side, there's a lot of experimental data, while on the quantum gravity side, there's not much for the moment on the experimental side. So you should first get the data we already have, right, before tackling what we don't even have, don't you think ?
As I said before, the solution space in QM is much bigger than any classical field problem - so if you succeed in obtaining a *correct* way of doing, in a classical way, quantum chemistry, this will be computationally MUCH more efficient.

 

[DrChinese]

In case it wasn't clear, what I described is not an allegory - the game could be played by real prisoners and captors, and presuming the prisonors can carry concealed entangled particles and stern gerlach appartuses(!) their probability of being released goes up to 85%. And no, it doesn't allow for superluminal communication between the two prisoners, but it certainly would seem to require superluminal communication between the particles in order to achieve.

I know it's not an allegory, I'm familiar with the game and some of its variations. Depending on the exact version of the game (questions, rules, number of players, etc.), you play it different ways. But where is the magic? The quantum players get a "tool" the others players don't get. But you must always bring together the results before you can see anything special happening.

It is pretty cool that there seems to be FTL "something" even though there is no (apparent) way to exploit this "something".