Question on observer created reality

[gptejms]

No, the classical field satisfies microscopic causality (because of the fact that the differential equation (KG or Dirac) that it obeys is Lorentz-invariant).

By microscopic causality,I had loosely meant the commutation relation for the field at two spacetime points itself.I've just checked Bjorken and Drell for the definition of microscopic causality:-it says 'the condition of vanishing of the commutators for all space-like intervals,no matter how small,is referred to as the condition of microscopic causality".For a classical field the above should be satisfied for all intervals--I hope that is what you mean by microscopic causality for classical fields.

 

[gptejms]

It's a trick :-) In fact, it turns out that the wave function in the QFT Hilbert space (which can be identified with the Fock space), when reduced to the 1-particle subspace of Fock space, satisfies the same equation as the classical field. But you can already see the difference, for instance in the case of a real classical field: the field is supposed to be real, and the resulting NR Schroedinger equation gives you complex 1-particle wave functions! Another way to see it is: how does your field reduce to the wave function of TWO PARTICLES ? The field is function of (x,t) and the wave function is function of (x1,x2,t).

Can you elaborate on the above--it's not quite clear to me.

 

[vanesch]

By microscopic causality,I had loosely meant the commutation relation for the field at two spacetime points itself.

Yes, that is correct, but only makes sense in the Heisenberg picture of course, where the fields are Hilbert space operators, parametrized over the spacetime manifold.
The meaning behind it is of course that the value at one (x,t) point cannot be influenced by the value at another spacetime point (x2,t2) if the interval is spacelike. In the quantum version, this comes down to a condition on commutators, in the classical version, this comes down on a condition on the Green's function (propagator).

For a classical field the above should be satisfied for all intervals--I hope that is what you mean by microscopic causality for classical fields.

No, it wouldn't of course make sense to talk about commutators of the classical fields. What I meant was that the value of a classical field at an event (x1,t1) can only depend on initial values of the classical field which are within its past light cone ; which puts a condition on the Green's function. This is exactly what is re-worded in the quantum language.
I'm a bit out of my depth here, but I'd guess (someone correct me if I'm wrong) that the classical version of microscopic causality can be reformulated as Poisson brackets vanishing over spacelike intervals.

cheers,
Patrick.

 

[gptejms]

Let's stick to Bjorken and Drell's definition of microscopic causality.

The interesting part is the non-zero commutator for time-like separations.
Measuring a field at spactime point 1 and then at spacetime point 2 does not give you the same result as measuring it first at point 2 and then at 1.I interpreted this in one of my earlier posts as due to the fact that when you make a measurement at 1(or 2),you create a disturbance which travels to 2(or 1) at the speed of light and readjusts the field there.Do you agree with this interpretation?You don't as we'll see below.In my model(the above interpretation) there's nothing instantaneous.A measurement at 1 can't instantaneously establish a field value at 2.

In your model when you make a measurement at 1,a certain field configuration is established in the whole of space(not just at 2)at once--of course it has some prob. attached with it.So when you measure at 1,you get some value 'a' and the value at 2 say 'b' is automatically established.If you do the reverse,that is measure at 2 you get some value 'c' which establishes the value at 1 say 'd'.So the disturbance model is out.Well yours is the official picture,can't overrule it--even though I find the disturbance model less disturbing!

 

[vanesch]

Can you elaborate on the above--it's not quite clear to me.

I will try to find a reference...

cheers,
Patrick

 

[vanesch]

The interesting part is the non-zero commutator for time-like separations.
Measuring a field at spactime point 1 and then at spacetime point 2 does not give you the same result as measuring it first at point 2 and then at 1.

I'm sorry, but interchanging event (x1,t1) and (x2,t2) if t1 < t2 doesn't place suddenly the "second event before the first one". It is tricky to say what it means to "measure first at point 2" (which is an event, with a time coordinate!). So interpreting exactly what it means for NON-commuting observables at timelike separated events is tricky !
In fact, one usually talks about non-compatible observables AT EQUAL TIMES (in a certain reference frame). Time-like separated events are never at equal times in any frame, so it's going to be tricky.
It is in fact more instructive to say that space-like separated events have COMMUTING field operators - indeed, space-like separated events can be made equal-time events in a certain reference frame, and if you work in that frame, you can say that these field operators are dynamically independent.

I interpreted this in one of my earlier posts as due to the fact that when you make a measurement at 1(or 2),you create a disturbance which travels to 2(or 1) at the speed of light and readjusts the field there.Do you agree with this interpretation?You don't as we'll see below.In my model(the above interpretation) there's nothing instantaneous.A measurement at 1 can't instantaneously establish a field value at 2.

But entangled field states CAN be entangled at space-like separated events, and they do have space-like separated Bell-violating correlations. So this "field propagation" won't do. And it doesn't have to, in fact, because it is NOT the dynamics of individual field configurations that is at hand here (which have dynamics that is constrained by special relativity), but THE CHOICE BETWEEN DIFFERENT FIELD CONFIGURATIONS which is mastered by the quantum state (the wave function, if you want).

In your model when you make a measurement at 1,a certain field configuration is established in the whole of space(not just at 2)at once--of course it has some prob. attached with it.So when you measure at 1,you get some value 'a' and the value at 2 say 'b' is automatically established.If you do the reverse,that is measure at 2 you get some value 'c' which establishes the value at 1 say 'd'.So the disturbance model is out.Well yours is the official picture,can't overrule it--even though I find the disturbance model less disturbing!

Yes, except that the "disturbance model" doesn't explain EPR situations, which are supported by experiment as well as theory !

cheers,
Patrick.

 

[vanesch]

I will try to find a reference...

Indeed, not many QFT books mention this, so I went through Peskin, Zee, Stone, Ryder to no avail, but Brian Hatfield, "Quantum Field theory of point particles and Strings" has a very illuminating chapter on it in the beginning (chapter 2).

cheers,
Patrick

 

[gptejms]

I'm sorry, but interchanging event (x1,t1) and (x2,t2) if t1 < t2 doesn't place suddenly the "second event before the first one". It is tricky to say what it means to "measure first at point 2" (which is an event, with a time coordinate!). So interpreting exactly what it means for NON-commuting observables at timelike separated events is tricky !

No.The second part is hypothetical--a hypothetical observer at t2,disturbance going back in time followed by an observation at t1.Indeed,it's hypothetical,but it conveys the idea.

There's a footnote in Bjorken and Drell which says Bohr and Rosenfeld made a detailed analysis of the physical meaning of the commutation relations in terms of physical measurement in some obscure journal in 1933,and also in Phys. Rev,78,794(1950).Can you get hold of this---if yes,I'll be obliged if you can send me a copy.

Yes, except that the "disturbance model" doesn't explain EPR situations, which are supported by experiment as well as theory !

We have discussed EPR,polarizations can be entangled but not field intensities at space-like intervals.The former is not forbidden by the time-like commutator(I don't know what to call it,after reserving the term microscopic causality for space-like intervals,a la Bjorken & Drell!)

 

[gptejms]

Indeed, not many QFT books mention this, so I went through Peskin, Zee, Stone, Ryder to no avail, but Brian Hatfield, "Quantum Field theory of point particles and Strings" has a very illuminating chapter on it in the beginning (chapter 2).

Thanks,but I am right now in a place where we get no physics books or journals.Kindly illuminate me on the contents of the illuminating chapter!

 

[vanesch]

We have discussed EPR,polarizations can be entangled but not field intensities at space-like intervals.The former is not forbidden by the time-like commutator

I don't see why you make a difference between "intensities" and "polarizations" ? I mean, if we're talking about a vector field, then, can the x-component at A be entangled or not with the 45-degree component at B or not ? And can the x-component at A be entangled with the x-component at B ? And can the x-component at A be entangled with the y-component at B ?
Can the +45 component at A be entangled with the -45 component at A ? And at B ?

 

[gptejms]

Patrick,let's look at the time-like part of the commutator in a different way.The commutator implies that $$\Delta E_1(x_1,t_1) \Delta E_2(x_2,t_2) \geq \hbar(...)$$,i.e. field measurements E1 and E2 at two spacetime points follow the uncertainty principle.So if I make a measurement at x1,t1 and find a certain value,the value at x2,t2 is totally uncertain.But if I make a measurement at x2,t1 (or near about t1),the value of the field is certain.So you are also right and so is the disturbance model.

 

[Sherlock]

Would it be ok for you if I dropped the "as if" ?!Is that the only objection?Keeping it or dropping it is just a matter of taste--stick to whatever you like,it makes no difference.

The usage of terms like photon and electron isn't just
a matter of tasete. The terms, photon and electron, have
specific meanings. Photons and electrons aren't what
produce measurement values. They *are* the measurement
values. It isn't known what (in terms of modeled behavior in
some quantum or submicroscopic 'realm') produces certain
measurement values. It's only known that certain experimental
setups yield certain results at certain rates. There might
be *lots* of 'models' for a given setup, and which model
one chooses to use is what is due to taste or convenience.

If we say that it's "as if" the photon or electron is created
via measurement, then it's "as if" the terms photon or electron
refer to something other than formal constructs related to
detection attributes. But photons and electrons don't have
any objective (ie., verifiable) existence other than as formal
constructs related to detection attributes -- and this is why the
photon and the electron are *neither* particle nor wave in
any classically analogous sense (and why, by definition, they
*can't* exist, as physical phenomena differentiated from yet
associated with formal constructs, prior to detection).

There is a source of electrons and a screen a little away.Think of this as just a field,an 'electron field'.There is no pattern on the screen.You introduce the double slit in between the source and the the screen.You have imposed certain constraints,the field gets disturbed and readjusts to 'interference pattern' on the screen--this proceeds at the speed of light(consistent with microscopic causality).Till I do not make any measurement,I think of it as just a field--in fact I am not even allowed to think of it as particles,until I make a measurement and find a particle.In fact my assertion is that only interactions are quantum,otherwise it's just a field.

In 'realistic' terms, the medium that is transmitting disturbances from one
oscillator (an emitting medium) to another (a detecting medium) is "just a field" unless
set in motion by an interaction of some sort. If the oscillations of the emitter are quantized,
then so must the associated oscillations of the transmitting medium be quantized. And, all
of this is inferred from the quantized oscillations of the detecting medium.

This is a wave picture of course, and wrt to the two-slit setup where interference
patterns are built up quantum by quantum there is the problem of explaining how/why
two wave fronts emerging from two open slits, and interferring with each other, produce
only a single detected point of interaction.

Of course, we don't know that there's two wave fronts (associated with a single
detection attribute) emerging from two open slits. Maybe there's just one wave front,
or some sort of bubble or whatever, emerging from just one of the two open slits -- and
while this would seem to account for the single detection points ,,, how could it happen
that way, and how could that produce interference if there's nothing for the
singular disturbance to interfere with?

One way to talk about the measurement problem is that it isn't known
how a submicroscopic or quantum 'realm' is behaving -- or even if there exists,
independent of instruments, a submicroscopic or quantum 'realm' of behavior.

And, I don't think that your approach is providing any new insight wrt
the problem. But maybe I've missed something.

The kind of local realism I am talking of is dictated by the principle of microscopic causality(i.e.nothing but the commutation property of the field at two space-time points),which is the basic feature of QFT--how can anyone doubt it?And this kind of local realism dictated by microscopic causality does not contradict EPR situations as I have already said--in EPR you don't measure the field anywhere,you are just measuring polarization at two places (which due to entanglement leads to higher correlation than expected on classical grounds).

Entanglement *refers to* ( at least wrt certain setups) a "higher correlation than expected
on classical grounds", doesn't it? That is, this is one way to test for the presence of
entanglement. However, if, as you seem to indicate, entanglement is the *cause* of the
higher correlations, then what is entanglement?

Anyway, it isn't EPR setups that are at odds with local realism. It's the Bell-Bohm-etc.
setups that evolved from the EPR considerations that pose a problem. I want to think
of the precondition (at a submicroscopic or quantum level) necessary for the global
correlations (predictably produced by variations in the Theta of the analyzing crossed
linear polarizers), as a relationship between opposite-moving paired emissions that
is produced at emission. But, Bell's analysis doesn't seem to allow this way of
thinking about it.

It isn't clear to me how your approach deals with this problem.

 

[vanesch]

So if I make a measurement at x1,t1 and find a certain value,the value at x2,t2 is totally uncertain.But if I make a measurement at x2,t1 (or near about t1),the value of the field is certain.So you are also right and so is the disturbance model.

Not necessarily ! Commuting observables do not mean that their value IS, FOR ALL STATES, certain ! It means that THERE EXIST states for which both of their values can be certain (common eigenstates).
So it is not true in all generality that if I make a measurement of the field at x2, t1, that this means that now my value of the field at x1, t1 is certain. It might be, it might not be, and that depends upon the specific quantum state at hand I'm measuring. Most of the time, it will not be (Eigenstates are rare, your generic state isn't an eigenstate).

cheers,
Patrick.

 

[gptejms]

Not necessarily ! Commuting observables do not mean that their value IS, FOR ALL STATES, certain ! It means that THERE EXIST states for which both of their values can be certain (common eigenstates).
So it is not true in all generality that if I make a measurement of the field at x2, t1, that this means that now my value of the field at x1, t1 is certain. It might be, it might not be, and that depends upon the specific quantum state at hand I'm measuring. Most of the time, it will not be (Eigenstates are rare, your generic state isn't an eigenstate).

From the wavefunction approach,I know that if I make a measurement of the field at x1,t1,I instantaneously establish a certain field configuration in the whole of space.My basic idea was that this is allowed for by the time-like commutator(I wish I had a better word for it),and if the measurement is made at x2,t2(where x2,t2 is time-like separated from x1,t1) the value is completely uncertain--this substantiates the fact that a disturbance has traveled from x1 to x2 at vel. of light.So the disturbance model is also right.

 

[vanesch]

The usage of terms like photon and electron isn't just
a matter of tasete. The terms, photon and electron, have
specific meanings. Photons and electrons aren't what
produce measurement values. They *are* the measurement
values.

Yes, that's an epistemological view of nature. There's nothing really out there, we're just knowing about measurements, and our theories give us relationships between measurements. It is an acceptable view. However, I don't like it: I like to think that *something* is really out there (a certain ontology), and that our formalism maps onto that ontology in some way. The main reason for it is that it is an "inspiration killer" ! If you just think of physical theories as scratching symbols on a paper, and measurement apparatus as making pointers move on a dial, there's not much your imagination can cling on to try to see what grand principles underlie all of it. The theory "measurements happen" is also a good theory then.

If we say that it's "as if" the photon or electron is created
via measurement, then it's "as if" the terms photon or electron
refer to something other than formal constructs related to
detection attributes. But photons and electrons don't have
any objective (ie., verifiable) existence other than as formal
constructs related to detection attributes -- and this is why the
photon and the electron are *neither* particle nor wave in
any classically analogous sense (and why, by definition, they
*can't* exist, as physical phenomena differentiated from yet
associated with formal constructs, prior to detection).

Ok, that's the purely epistemological view concerning "particles".
But...

In 'realistic' terms, the medium that is transmitting disturbances from one
oscillator (an emitting medium) to another (a detecting medium) is "just a field" unless
set in motion by an interaction of some sort. If the oscillations of the emitter are quantized,
then so must the associated oscillations of the transmitting medium be quantized. And, all
of this is inferred from the quantized oscillations of the detecting medium.

This is a wave picture of course, and wrt to the two-slit setup where interference
patterns are built up quantum by quantum there is the problem of explaining how/why
two wave fronts emerging from two open slits, and interferring with each other, produce
only a single detected point of interaction.

We can now apply the same epistemology to "fields" and "waves" and say that they are just constructions according to our taste to explain measurements. They're not real either.

Of course, we don't know that there's two wave fronts (associated with a single
detection attribute) emerging from two open slits. Maybe there's just one wave front,
or some sort of bubble or whatever, emerging from just one of the two open slits -- and
while this would seem to account for the single detection points ,,, how could it happen
that way, and how could that produce interference if there's nothing for the
singular disturbance to interfere with?

In the same way as "particles" are attributes of a measurement, so are then "interference patterns", no ?

One way to talk about the measurement problem is that it isn't known
how a submicroscopic or quantum 'realm' is behaving -- or even if there exists,
independent of instruments, a submicroscopic or quantum 'realm' of behavior.

And, I don't think that your approach is providing any new insight wrt
the problem. But maybe I've missed something.



Entanglement *refers to* ( at least wrt certain setups) a "higher correlation than expected
on classical grounds", doesn't it? That is, this is one way to test for the presence of
entanglement. However, if, as you seem to indicate, entanglement is the *cause* of the
higher correlations, then what is entanglement?

In your view, "entanglement" is just another attribute of measurement results...

Anyway, it isn't EPR setups that are at odds with local realism. It's the Bell-Bohm-etc.

I'd say, if there is no reality, no point in discussing local realism !
"Measurements happen". See how it kills inspiration ?

cheers,
Patrick.

 

[vanesch]

From the wavefunction approach,I know that if I make a measurement of the field at x1,t1,I instantaneously establish a certain field configuration in the whole of space.

No, because the measurement in ONE spacetime point doesn't fix all of the field configuration, there is still a lot of degeneracy. You only project upon the eigenspace of field configurations that have this result at (x1,t1). It is like if you measured, say, the x-component of a particle in NR QM. This doesn't mean that the y and z component are now completely established !
If the x-coordinate is not entangled with the y coordinate (the wave function is a product of a function of x and of y), then the measurement of x will not influence the measurement at y (they will be statistically uncorrelated). If the wavefunction is not a product, then the "x-coordinate" is entangled with "the y-coordinate" and the measurement of x will be correlated with the measurement of y. It doesn't have to FIX it. It might. In that case, we have 100% entanglement, and we are using compatible observables (x and y).
All this being an example in NR QM of course.

My basic idea was that this is allowed for by the time-like commutator(I wish I had a better word for it),and if the measurement is made at x2,t2(where x2,t2 is time-like separated from x1,t1) the value is completely uncertain--this substantiates the fact that a disturbance has traveled from x1 to x2 at vel. of light.So the disturbance model is also right.

It can explain the LACK of correlations WITHIN the lightcone of INCOMPATIBLE observables. It cannot explain the Bell-type correlations OUTSIDE of the light cone of COMPATIBLE observables.

cheers,
Patrick.

 

[seratend]

Yes, that's an epistemological view of nature. There's nothing really out there, we're just knowing about measurements, and our theories give us relationships between measurements. It is an acceptable view. However, I don't like it: I like to think that *something* is really out there (a certain ontology), and that our formalism maps onto that ontology in some way. The main reason for it is that it is an "inspiration killer" ! If you just think of physical theories as scratching symbols on a paper, and measurement apparatus as making pointers move on a dial, there's not much your imagination can cling on to try to see what grand principles underlie all of it. The theory "measurements happen" is also a good theory then.

So you think you need constraints to have inspiration (a kind of causality or masochism?)?

Why do you seem to think an epistemic approach does not allow to build beautifull grand principles to describe reality?
Look at the powerfull of the approach: it contains surely the ontological view (while i am not sure for the converse) and there may be not a single principle, but a collection of powerfull principles or no grand principle at all. Your imagination is free to choose what it wants (the freedom of choice).

Seratend.

 

[Sherlock]

Yes, that's an epistemological view of nature. There's nothing really out there, we're just knowing about measurements, and our theories give us relationships between measurements. It is an acceptable view. However, I don't like it: I like to think that *something* is really out there (a certain ontology), and that our formalism maps onto that ontology in some way. The main reason for it is that it is an "inspiration killer" ! If you just think of physical theories as scratching symbols on a paper, and measurement apparatus as making pointers move on a dial, there's not much your imagination can cling on to try to see what grand principles underlie all of it. The theory "measurements happen" is also a good theory then.

I also think that *something* is really out there. My bit about
photons and electrons just had to do with how *those terms*
should be used -- to avoid confusion -- since we don't know the
physical characteristics of what corresponds *out there* to
photons and electrons.

We can now apply the same epistemology to "fields" and "waves" and say that they are just constructions according to our taste to explain measurements. They're not real either.

Of course they're real. Just like photons and electrons are
real. (But waves and particles are the only things mentioned
so far that have any meaning wrt our ordinary experience of
the world.) The only question is what are we referring to when we
use the terms. When we're talking within the context of an
instrumental theory like qm, then these terms have specific
meanings and don't refer to things *out there*. They refer
to specific instrumental configurations and formal constructs
associated with the instrumental configurations. What is
*out there* is a matter of speculation, and hopefully a
comprehensive picture of *out there* can eventually be
developed via the data and the many ways of interpreting it.

In the same way as "particles" are attributes of a measurement, so are then "interference patterns", no ?

Particles, waves and interference patterns refer to commonly
observable phenomena, and are often used interpretively, to help
develop a qualitative picture of *out there*.

In your view, "entanglement" is just another attribute of measurement results...

Quantum entanglement has a theory-dependent meaning.
It's not a cause, it's an effect.

I'd say, if there is no reality, no point in discussing local realism !
"Measurements happen". See how it kills inspiration ?

I think you've misunderstood my orientation. I'm very much a
local realist ... when possible. Very much interested in
the big picture of *out there*. :-)

 

[vanesch]

So you think you need constraints to have inspiration (a kind of causality or masochism?)?

Yes, of course ! That's the whole point. If "anything goes" that's, as you point out, too much choice, hard to choose from. The "blank paper" experience.

Why do you seem to think an epistemic approach does not allow to build beautifull grand principles to describe reality?

It allows for it, but there's no reason for it. It could all be "a big lookup table" too.

Look at the power of the approach: it contains surely the ontological view (while i am not sure for the converse) and there may be not a single principle, but a collection of powerfull principles or no grand principle at all.

Indeed. So when looking for grand principles, it is disturbing to be in a mindset where there might very well not be such a grand principle. You'll be less good at your job. A bit like as if you are a suicide bomber, and you go for the 74 virgins, but you're not really sure about it. You'll be a less motivated bomber, than the one that is convinced those ladies are waiting for him :-)

cheers,
Patrick.

 

[gptejms]

No, because the measurement in ONE spacetime point doesn't fix all of the field configuration, there is still a lot of degeneracy. You only project upon the eigenspace of field configurations that have this result at (x1,t1). It is like if you measured, say, the x-component of a particle in NR QM. This doesn't mean that the y and z component are now completely established !
If the x-coordinate is not entangled with the y coordinate (the wave function is a product of a function of x and of y), then the measurement of x will not influence the measurement at y (they will be statistically uncorrelated). If the wavefunction is not a product, then the "x-coordinate" is entangled with "the y-coordinate" and the measurement of x will be correlated with the measurement of y. It doesn't have to FIX it. It might. In that case, we have 100% entanglement, and we are using compatible observables (x and y).
All this being an example in NR QM of course.

You are the one,I thought,who believed in instantaneous establishment of the entire field configuration upon measurement(see post no. 146).If it dosen't happen generally(except in a limited way in cases of entanglement),it's even better for my case!I think the disturbance model is perfectly sound,it explains a lot of things and does not forbid EPR correlations(even though it doesen't explain it).Mind you,disturbance model is not something that I am proposing or propounding--it is a natural interpretation of the time-like part of the field commutator.

 

[seratend]

Yes, of course ! That's the whole point. If "anything goes" that's, as you point out, too much choice, hard to choose from. The "blank paper" experience. .

If anything
Come on patrick . The blank paper view is as beautifull as the catholic paradise before the snake: it just illustrates the set of possibilities we have before and the difficulties we have after when we choose one of them .

It allows for it, but there's no reason for it. It could all be "a big lookup table" too..

Interesting. As long as we use the set theoric approach, yes we can say it is a big look up table.


Seratend

 

[vanesch]

You are the one,I thought,who believed in instantaneous establishment of the entire field configuration upon measurement(see post no. 146).

Hum, that was indeed a particular case in which the projection of the state upon the space spanned by all field configurations which are compatible with the measurement result, resulted in a single base vector (= a specific configuration).

If it dosen't happen generally(except in a limited way in cases of entanglement),it's even better for my case!I think the disturbance model is perfectly sound,it explains a lot of things and does not forbid EPR correlations(even though it doesen't explain it).Mind you,disturbance model is not something that I am proposing or propounding--it is a natural interpretation of the time-like part of the field commutator.

I still have the impression we're talking next to each other. The disturbances and so on, ruled by unitary QM, tell us how field configurations evolve in other field configurations, or in superpositions of other field configurations.
The measurement, on the other hand, projects instantaneously out those field configurations that are compatible with the measurement and eliminates all other terms from the wavefunction. How can you consider the latter to be a special case of the former ?
Now, in many cases, this "projecting out" doesn't correlate measurements. But in some cases it does. The very fact that there are SOME cases when the "propagation of the effect" technique doesn't work, should make you accept that this is NOT the explanation in general, no ?

I mean, it is as if you said that gravity propagates through sound waves in air, and point to many experiments involving gravity, that happened in air. I point out to a few experiments where gravity was observed in vacuum, and you say, ok, then, "I think the air gravity model is perfectly sound, it explains a lot of things and does not forbid gravity working in vacuum (even though it doesn't explain it). Mind you,the air gravity model is not something that I am proposing or propounding--it is a natural interpretation of how gravity works around us."

 

[vanesch]

The blank paper view is as beautifull as the catholic paradise before the snake

This must be the quote of the day

 

[gptejms]

Hum, that was indeed a particular case in which the projection of the state upon the space spanned by all field configurations which are compatible with the measurement result, resulted in a single base vector (= a specific configuration).

In which particular case of measurement does it become a single base vector and when not?

The disturbances and so on, ruled by unitary QM, tell us how field configurations evolve in other field configurations, or in superpositions of other field configurations.

other field configurations--are you introducing interactions here?

The measurement, on the other hand, projects instantaneously out those field configurations that are compatible with the measurement and eliminates all other terms from the wavefunction.

I put this question to you.Why is the measurement so different from interactions?What special quality does it have,which other interactions don't have?

Now, in many cases, this "projecting out" doesn't correlate measurements. But in some cases it does. The very fact that there are SOME cases when the "propagation of the effect" technique doesn't work, should make you accept that this is NOT the explanation in general, no ?

I didn't offer the disturbance model as a panacea for all measurement problems.All I can say is it does not forbid EPR correlations.

I mean, it is as if you said that gravity propagates through sound waves in air, and point to many experiments involving gravity, that happened in air. I point out to a few experiments where gravity was observed in vacuum, and you say, ok, then, "I think the air gravity model is perfectly sound, it explains a lot of things and does not forbid gravity working in vacuum (even though it doesn't explain it). Mind you,the air gravity model is not something that I am proposing or propounding--it is a natural interpretation of how gravity works around us."

I think this is making light of the whole thing.BTW what interpretation do you give to the TFC(time-like part of the field commutator!)--do you agree with my disturbance model?If yes,how do you reconcile it with EPR?If you don't agree,give me your interpretation of TFC.

 

[vanesch]

In which particular case of measurement does it become a single base vector and when not?

When, in the superposition of field configurations (the quantum state written out in the "field configuration" basis), there is only ONE SINGLE term that satisfies the measured value of the field at the given event.

Consider the quantum state:

|psi(t1)> = a |fieldconf1> + b |fieldconf2> + c |fieldconf3>

Imagine that we measure F = 15.3 at (x1,t1). Now, fieldconf1(x1,t1) = 15.3, fieldconf2(x1,t1) = 15.3 and fieldconf3(x1,t1) = 18.8

That means that after measurement (using the projection postulate - so I'm NOT doing some MWI here), we have the state:

|psi(t1+epsilon)> = Normfactor (a |fieldconf1> + b |fieldconf2> )

However, imagine that we measured F = 18.8 at (x1,t1). Then it means that we now have the state:

|psi(t1+epsilon)> = |fieldconf3>

because there was only one single fieldconfiguration term in |psi(t1)> that satisfied the measurement result, namely fieldconf3.

I put this question to you.Why is the measurement so different from interactions?What special quality does it have,which other interactions don't have?

Ha, well, it ISN'T any different (except for the fact that YOU are taking part in it). But to recognize this in quantum theory, you need to put the projection postulate in the dustbin, and you end up in a Many Worlds interpretation.
All collapse models do at least one of both things: set "measurements" apart as interactions, and/or violate special relativity in their workings.

All I can say is it does not forbid EPR correlations.

This is what I don't understand. If effects of measurements ("effective collapse", say) are to propagate within the light cone, how can you then have Bell-violating correlations at space-like intervals ??

BTW what interpretation do you give to the TFC(time-like part of the field commutator!)--do you agree with my disturbance model?If yes,how do you reconcile it with EPR?If you don't agree,give me your interpretation of TFC.

I have to say I don't really know how to interpret the TFC. I'd say that a priori, a generic couple of operators DOES NOT commute, and it is rather the exception that they DO. So the thing I find interesting is that SPACELIKE field operators DO commute, which means that explicit dynamical influence (capable, say, of sending information) over spacelike intervals is prohibited ; a necessary condition to satisfy special relativity.
Why timelike commutators vanish or not doesn't have a deep message, to me.
But quantum superposition by itself, and the "effect of collapse" is not something that has to do with the specific dynamics of the quantized model at hand ; it is build into quantization itself, and independent of the underlying dynamics. It becomes 'spectacular' when something forbids any underlying dynamics to do the correlations for us ; we then suddenly realize the difference between the dynamics of the underlying model, and the strangeness of quantum theory by itself. But the strangeness was there alright from the start, only, we could trick ourselves into believing we saw "dynamical links".


cheers,
Patrick.

 

[gptejms]

Consider the following:-

There is a classical field(deterministic) superimposed on a random noise field(say white noise) in a certain region of space.My measuring device is such that I can not resolve field strength differences less than a certain value(call it a 'quantum')--so my measurements of field strengths are n quanta,m quanta etc.(in an approximate sense).Now on this mixture field,I impose the condition from outside(i.e. hypothetical condition) that whenever I measure it or introduce an interaction at x1,t1,the same affects the entire field at the velocity of light.The entire field gets readjusted.Now I can also introduce the notion of a wavefunction and say that the wavefunction is a functional of the field and at any given time,there could be field-config1 or field-config2 or field-config3 etc.The field satisfies commutator relations too,in a way.

The above is not a complete representation of the quantum field.But it's a good analogy.I wonder if the wavefunction of quantum mechanics has a very different role than the above.Feeling quite sleepy,will write more tomorrow(err..today morning).

 

[vanesch]

There is a classical field(deterministic) superimposed on a random noise field(say white noise) in a certain region of space.My measuring device is such that I can not resolve field strength differences less than a certain value(call it a 'quantum')--so my measurements of field strengths are n quanta,m quanta etc.(in an approximate sense).Now on this mixture field,I impose the condition from outside(i.e. hypothetical condition) that whenever I measure it or introduce an interaction at x1,t1,the same affects the entire field at the velocity of light.The entire field gets readjusted.Now I can also introduce the notion of a wavefunction and say that the wavefunction is a functional of the field and at any given time,there could be field-config1 or field-config2 or field-config3 etc.The field satisfies commutator relations too,in a way.

The above is not a complete representation of the quantum field.But it's a good analogy.I wonder if the wavefunction of quantum mechanics has a very different role than the above.Feeling quite sleepy,will write more tomorrow(err..today morning).

What you are describing is very close to Stochastic Electrodynamics, an attempt to modify classical electromagnetics by adding "universal noise" so as to mimick quantum effects. I don't know all of it (I think its originator is De Santos or Dos Santos or something). What I know is that it DOES mimick quite a lot of quantum effects of QED. However, on true EPR situations, it breaks its neck. It does, however, have the potential to explain "inefficient" EPR experiments, as are in fact all EPR experiments today. When I say "has the potential to explain", it still means you have to build awkward photon detector models made on purpose to trick us into believing EPR situations. This is the difference with standard QM, which predicts EPR results without any ambiguity or "fiddling on purpose", and which explains also without fiddling how a photodetector works.
There are other, deeper problems with Stochastic Electrodynamics, such as the appearance of particle-like properties of electrons and so on ; but nevertheless it is a nice attempt, and what you describe makes me think of it.

 

[gptejms]

Imagine a photon detector at one of the slits.Further imagine that it's conscious.Now the photon detector has to describe its interaction with the particle(better imagine yourself to be the photon detector).When the observer is part of the observed,can it write down the wavefunction,is the evolution still unitary?See,as long as you give yourself all the importance and write down the wavefunction,whatever interactions I may introduce,whatever picture(field/particle) I may use,if you come in in the last and write down the wavefunction,evolution is going to be unitary.I want to change that.A part interacting with the whole,and the part describing its own dynamics.I feel this evolution would not be unitary.

 

[gptejms]

What you are describing is very close to Stochastic Electrodynamics, an attempt to modify classical electromagnetics by adding "universal noise" so as to mimick quantum effects. I don't know all of it (I think its originator is De Santos or Dos Santos or something). What I know is that it DOES mimick quite a lot of quantum effects of QED. However, on true EPR situations, it breaks its neck. It does, however, have the potential to explain "inefficient" EPR experiments, as are in fact all EPR experiments today.

I am surprised,whatever I have come up with during this thread,whether potential or stochastic potential,the role of TFC/microscopic causality in measurements or the classical + random noise field picture,someone has thought of it and worked on it.I wonder what Bohr & Rosenfeld have written in their paper on TFC.
I found the following abstract when I googled for 'stochastic electrodynamics santos' :-

Stochastic optics: A local realistic analysis of optical tests of Bell inequalities
Trevor W. Marshall
Department of Mathematics, University of Manchester, Manchester M13 9PL, United Kingdom

Emilio Santos

Departamento de Física Moderna, Universidad de Cantabria, Santander, Spain

Received 22 November 1988
Stochastic optics may be considered as simply a local realistic interpretation of quantum optics and, in this sense, it is a first step in the reinterpretation of the whole of quantum theory. However, as it is not possible to interpret all the details of quantum theory in a local realistic manner, as shown by Bell's theorem, minor changes are introduced in the formalism with the consequence that the new theory makes different predictions in some special cases. In stochastic optics, the quantum-operator formalism is simply considered a formal way of dealing with stochastic fields. In particular, the quantum zero point is taken as a real random electromagnetic radiation filling the whole of space. This radiation noise has the same nature as light signals, the only difference being the greater intensity of the latter. We assume that photon detectors have an intensity threshold just above the level of the noise, thus detecting only signals. Transmission of radiation through polarizers follows Malus's law, but the interplay of signal and noise leads quite naturally to the prediction that the detection probability of some signals is enhanced, which is known to be a necessary condition for the violation of the empirically tested Bell inequalities. In our view, correlated photon pairs are pairs of light signals supercorrelated in polarization, in the sense that, as well as the signal, the accompanying noise is also correlated. Thus stochastic optics allows predictions for the empirical correlations very close, but not identical, to the quantum ones. The theory is applied to the analysis of all experiments designed to test the Bell inequalities by measuring polarization correlations of photon pairs. The predictions agree with quantum optics and experiments within statistical errors, except for the Holt-Pipkin experiment. In this case, the experimental results agree with stochastic optical predictions within two standard deviations while violating quantum optics by four.

©1989 The American Physical Society

 

[vanesch]

I am surprised,whatever I have come up with during this thread,whether potential or stochastic potential,the role of TFC/microscopic causality in measurements or the classical + random noise field picture,someone has thought of it and worked on it.

This has been my problem too during all of my career ! It's not so much that I cannot solve the problems I'm presented with, it's that the way I solve them, I discover usually afterwards, has been done already... It's so difficult to have a truly NEW idea, isn't it !

 

[DaTario]

Stochastic Electrodynamics

The Group in Spain is one of the few that sustain this idea. One remarkable result of this theory, I should say fantastic result, is that if you take the quantum vacuum inside a very large box, its energy can be cast in the form:
[tex]
E = \Sum_{{i = 0}^infty \frac{1}{2}\hbar \omega_i
[\tex]

which contains all possible frequencies for the radiation field. Notice that this expression allows one to obtain the vacuum spectrum.

Now, if you look for a classical field which must be invariant under a Lorentz transformation, i.e., which looks the same whether you are at rest relatively to some intertial referential or at constant velocity, the spectrum is the same as the one obtained from de expression above. Therefore, according to those spanishes of stochastic electrodynamics, what we quantum mechanically call vacuum, they call "the state of the EM field which doesn't change with your moving perspective". This is a very beautiful definition of vacuum in my opinion.

 

[gptejms]

This has been my problem too during all of my career ! It's not so much that I cannot solve the problems I'm presented with, it's that the way I solve them, I discover usually afterwards, has been done already... It's so difficult to have a truly NEW idea, isn't it !

Right,but keep at it.May be,you come up with something truly original one day! I think you've got it in you to make it.

 

[gptejms]

The Group in Spain is one of the few that sustain this idea. One remarkable result of this theory, I should say fantastic result, is that if you take the quantum vacuum inside a very large box, its energy can be cast in the form:
$$E = \sum_{i = 0}^\infty \frac{1}{2}\hbar \omega_i$$

which contains all possible frequencies for the radiation field. Notice that this expression allows one to obtain the vacuum spectrum.

Now, if you look for a classical field which must be invariant under a Lorentz transformation, i.e., which looks the same whether you are at rest relatively to some intertial referential or at constant velocity, the spectrum is the same as the one obtained from de expression above. Therefore, according to those spanishes of stochastic electrodynamics, what we quantum mechanically call vacuum, they call "the state of the EM field which doesn't change with your moving perspective". This is a very beautiful definition of vacuum in my opinion.

Just to correct your tex!

 

[gptejms]

The usage of terms like photon and electron isn't just
a matter of tasete. The terms, photon and electron, have
specific meanings. Photons and electrons aren't what
produce measurement values. They *are* the measurement
values. It isn't known what (in terms of modeled behavior in
some quantum or submicroscopic 'realm') produces certain
measurement values. It's only known that certain experimental
setups yield certain results at certain rates. There might
be *lots* of 'models' for a given setup, and which model
one chooses to use is what is due to taste or convenience.

If we say that it's "as if" the photon or electron is created
via measurement, then it's "as if" the terms photon or electron
refer to something other than formal constructs related to
detection attributes. But photons and electrons don't have
any objective (ie., verifiable) existence other than as formal
constructs related to detection attributes -- and this is why the
photon and the electron are *neither* particle nor wave in
any classically analogous sense (and why, by definition, they
*can't* exist, as physical phenomena differentiated from yet
associated with formal constructs, prior to detection).

In 'realistic' terms, the medium that is transmitting disturbances from one
oscillator (an emitting medium) to another (a detecting medium) is "just a field" unless
set in motion by an interaction of some sort. If the oscillations of the emitter are quantized,
then so must the associated oscillations of the transmitting medium be quantized. And, all
of this is inferred from the quantized oscillations of the detecting medium.

I think you have contradicted yourself here.You say 'photons and electrons don't have any objective (ie., verifiable) existence other than as formal
constructs related to detection attributes' and then you conclude that 'oscillations of the transmitting medium are quantized'.I think the resolution lies in understanding that quantization aspect is revealed to you only upon measurement--and then to avoid the contradiction say 'before that the photons/electrons don't exist'(though I prefer the 'as if').

Entanglement *refers to* ( at least wrt certain setups) a "higher correlation than expected
on classical grounds", doesn't it? That is, this is one way to test for the presence of
entanglement. However, if, as you seem to indicate, entanglement is the *cause* of the
higher correlations, then what is entanglement?

The disturbance model does not explain EPR,but why should it?It's just an interpretation that I have given to TFC(time-like part of field commutator).It gives you a new way of looking at some of the things--when you introduce a double slit in between a source of electrons and a screen,you've given rise to a disturbance in the field,which propagates(at vel. of light) and readjusts the field at the screen to 'interference pattern'.When you make a measurement at one of the slits,it again gives rise to a disturbance that propagates and causes the field at the other slit to correspond to 'no electron' and at the screen to 'no interference effects'.I think it's a beautiful way of looking at things.I can only tell you that this disturbance model does not contradict EPR results--EPR results are tested or arrived at at space-like intervals.Your measurement at '1' has not had sufficient time to travel to '2' and affect the measurement there.

If you think my disturbance interpretation of TFC is wrong,you must tell me why and give me an alternate interpretation.

 

[gptejms]

No one is writing any more.So I think I must sum up,at least, what my thinking is at the moment(the same has developed/evolved from points 1 to 4 mentioned in a couple of replies/posts to tdunc above to the one below,call it point 5):-

5.I think what I wrote in post #168 is not very different from the decoherence approach:-after writing it,I think decoherence is a good approach to the measurement problem.One can't do much more than that:-when the 'observer' is also a part of the observed,if you write down a wavefunction you are introducing a 3rd person in the picture(in whose frame of reference the evolution is unitary)--so the wavefunction is not a way for handling things from the perspective(or the frame of reference) of the 'observer'.In order to do that,it's ok to introduce a 3rd person,write down a wavefunction,sum over the reservoir('observer') degrees of freedom to describe the particle/system being observed.In summing up over these degrees of freedom(which can not be described by a unitary operator),you are introducing the necessary non-unitarity.
The disturbance interpretation of TFC describes the Heisenberg picture of measurement,rather what happens upon a measurement.It does not explain measurement,it tells you what happens upon measurement.It does not explain EPR results(but does not violate it either).I wish somebody could put EPR in the Heisenberg picture--i.e. show how entanglement arises from the Heisenberg picture.
Regarding the classical field + random noise field picture in post #166--looks good but need to think more.