Relational Quantum Mechanics

[Lord Jestocost]

These sorts of ideas were actually the kinds of things that Schrodinger had in mind when he proposed his cat thought experiment; as I understand it, he actually intended it to illustrate the kinds of things you are saying here--that, even though standard QM, taken literally, says it should be possible to put a cat in a superposition of being dead and alive, and Schrodinger's thought experiment describes how that could be done based on standard QM, that implication doesn't really make sense.

Regarding Schrödinger’s Cat problem:

“… that at heart the problem does not lie with the (dis)appearance of interference terms (which is a red herring) but with the inability of quantum mechanics to predict single outcomes.” (Klaas Landsman in
"Foundations of Quantum Theory From Classical Concepts to Operator Algebras)

 

[PeterDonis]

Regarding Schrödinger’s Cat problem:

“… that at heart the problem does not lie with the (dis)appearance of interference terms (which is a red herring) but with the inability of quantum mechanics to predict single outcomes.” (Klaas Landsman in
"Foundations of Quantum Theory From Classical Concepts to Operator Algebras)

While I agree that the disappearance of interference terms (which is a lot of what decoherence theory deals with) is not the same as the problem of single outcomes, I think Schrodinger's Cat has elements of both.

 

[PeterDonis]

What I think happens here is that a hidden assumption is made, that a measurement is only a measurement if some observer looks at the instrument. Such an assumption is not part of the standard QM

If you are referring to the assumption in the original Schrodinger's Cat experiment that no "observation" takes place until a human observer opens the box and looks at the cat, I agree that that assumption is not part of standard QM. However, it took decades for the development of decoherence theory to explain why that assumption is not part of standard QM; prior to the development of decoherence theory, it was by no means clear that a macroscopic object like a cat, with so many degrees of freedom, will have interactions between its degrees of freedom that count as "observations" even without the box being opened.

even single electrons interact continuously with the environment. In a 2-slit experiment the electrons are still subject to gravitational and EM interactions.

You can eliminate any effects of gravitation by doing the experiment in free fall. You can eliminate EM interactions by isolating the experiment. No such isolation will last forever, but for electrons (as opposed to cats), it is perfectly possible to isolate them for long enough to do a reasonable experiment.

In short, you are trying to make claims about microscopic systems with very small numbers of degrees of freedom, that are only valid for macroscopic systems with very large numbers of degrees of freedom.

In the case of the cat, the interactions DO give you enough information

Not the interactions that take place while the cat is inside the box, no. Those interactions decohere the "alive" and "dead" states so there is no interference between them; but they do not, by themselves, create enough information to create a single outcome that is either "alive" or "dead". Even the interaction with the human observer, when they open the box and look at the cat, doesn't do that. The single outcome has to be put in by hand as an extra assumption in the theory, and its only real content (if we look just at basic QM and not what various interpretations claim) is that the physicist should use a collapsed wave function whenever it is necessary to make the predictions come out right. That works in a practical sense, but it does not make for a satisfactory theory, which is why there is so much literature on interpretations of QM.

 

[AndreiB]

"QM" (meaning basic QM without adopting any particular interpretation) only says this as a mathematical convenience, not as a claim about what "really happens". Whether or not a collapse "really happens" is interpretation dependent.

The collapse predicts correctly the post-measurement state. The pre-collapse state does not make correct predictions. So, from this point of view the collapse is "real".

Wrong. MWI makes the same predictions as all other interpretations of QM, since all interpretations use the same or equivalent mathematical machinery to make predictions.

MWI does not make the same predictions since it doesn't have a collapse. If you prepare a spin superposition of 99% UP and 1% DOWN, MWI predicts that the measurement results in two worlds, one UP and one DOWN. Since both worlds are similar (except for the spin value) they should look to the two observers' copies just as real. The probability for the pre-measurement observer to find himself in the UP and DOWN worlds should be the same, 50%.

The MWI camp know this problem and are trying for decades to solve it. As far as I can tell, they did not succeed. The decision-theoretic approach is circular (you need to assume the existence of rational agents, but how are they supposed to exist without a notion of probability is not explained). They also try to postulate a "measure of existence" to make some worlds less likely than others, but I couldn't find what is the meaning of that measure of existence. In the end they simply postulate (not deduce) that the predictions are correct, which is fallacious.

 

[AndreiB]

However, it took decades for the development of decoherence theory to explain why that assumption is not part of standard QM; prior to the development of decoherence theory, it was by no means clear that a macroscopic object like a cat, with so many degrees of freedom, will have interactions between its degrees of freedom that count as "observations" even without the box being opened.

Let's get over this by replacing the cat with a 1Kg elementary particle (or a black hole if you want). When the atom decays the black hole is moved 1m in some direction. There are no internal degrees of freedom to speak about.

You can eliminate any effects of gravitation by doing the experiment in free fall.

Earth is in free fall, yet it surely interacts with the Sun. Free fall does not eliminate gravity, it simply means that there are no other forces/fields except gravity. If the black hole is in free fall it does not mean that it will not make my torsion balance move.

You can eliminate EM interactions by isolating the experiment.

You cannot. Let's say the 1kg black hole also has charge. Tell me how you isolate that charge so that it has no influence on charges outside the box.

No such isolation will last forever, but for electrons (as opposed to cats), it is perfectly possible to isolate them for long enough to do a reasonable experiment.

Time is not an issue here. I grant you that if something can be in principle isolated, that isolation is perfect. for example I agree that the box can be made perfectly reflective for photons of every wavelength, no photon will leak out.

In short, you are trying to make claims about microscopic systems with very small numbers of degrees of freedom, that are only valid for macroscopic systems with very large numbers of degrees of freedom.

The black hole example should solve this.

 

[Lord Jestocost]

What I think happens here is that a hidden assumption is made, that a measurement is only a measurement if some observer looks at the instrument. Such an assumption is not part of the standard QM, has no evidence in its favor, so it should be dismissed.

And what will you infer from a "measurement" if no observer looks at an instrument? Von Neumann's "quantum mechanical measurement chain" would here be the keyword.

 

[AndreiB]

And what will you infer from a "measurement" if no observer looks at an instrument? Von Neumann's "quantum mechanical measurement chain" would here be the keyword.

Once a measurement is performed, an observer has no chance not to "observe" that fact. The Moon is there even if you don't look in its direction because it produces effects at your location. For example your body would feel Moon's gravity.

 

[EPR]

Once a measurement is performed, an observer has no chance not to "observe" that fact. The Moon is there even if you don't look in its direction because it produces effects at your location. For example your body would feel Moon's gravity.

This is indeed very common-sense but sadly has little to do with QM(which doesn't mention any of the the following terms - "Moon", "tides", "body", "location", "direction", "observer"). The whole framework is only about observables, probabilities and measurements. So the whole point about the alleged persistence in time and space of those macroscopic bodies is largely moot. They are in a grey area.
You can't simply assume their 'classical' all-time existence to help yourself deal with measurements outcomes. Unless you think they are made of classical stuff which nobody has seen so far..

 

[PeterDonis]

The collapse predicts correctly the post-measurement state.

This is interpretation dependent. You keep ignoring interpretations like the MWI, in which the post-measurement state is not the "collapsed" state; in such interpretations "collapse" is just a mathematical convenience, not a reflection of an actual change in the physical state.

MWI does not make the same predictions since it doesn't have a collapse.

Collapse is not a prediction of basic QM; it's just a mathematical convenience. Please read the "7 Basic Rlues of QM" Insight article that is one of the sticky links in the QM forum.

Also, all QM interpretations make the same predictions. Please read the ground rules for this subforum; even if you don't personally agree with the statement I've just made, it is not up for discussion on this forum; by definition in this forum QM interpretations all make the same predictions; if you have some thingie that doesn't make the same predictions, it's not a QM interpretation, it's a different theory.

As far as the MWI itself is concerned, I'm not aware of anything in the literature claiming that the MWI does not make the same predictions as other QM interpretations; it uses the same 7 Basic Rules mathematical machinery as all of the other interpretations. It tells a very different story about what is "really happening", but that story is not a prediction since there is no way to test it. (The same is true for the stories that all the other QM interpretations tell.)

The MWI camp know this problem

I agree there are problems with the MWI that have not been solved, but I don't think the problem you are describing (to the extent I understand what you are describing) is one of them.

Free fall does not eliminate gravity

It eliminates gravity as a "force" (according to general relativity, the force is not "gravity" anyway). If you are going to make claims about "gravitons", then you need to say what working theory of gravitons you are using, which will be difficult since there isn't one.

Let's say the 1kg black hole also has charge.

You keep introducing special cases as if they were a basis for general arguments. They're not. Let's say the hole doesn't have charge: then your argument doesn't apply. Which means it's not a general argument.

The black hole example should solve this.

Only if you can show me a working quantum theory of black holes. Which you can't because there isn't one.

 

[AndreiB]

This is interpretation dependent. You keep ignoring interpretations like the MWI, in which the post-measurement state is not the "collapsed" state; in such interpretations "collapse" is just a mathematical convenience, not a reflection of an actual change in the physical state.

OK, never mind.

Collapse is not a prediction of basic QM; it's just a mathematical convenience. Please read the "7 Basic Rlues of QM" Insight article that is one of the sticky links in the QM forum.

I fully agree with those postulates. The problem is that MWI has a significant difficulty to accommodate the 6 postulate, Born's rule. Please take a look at this very recent paper:

Quantum fractionalism: the Born rule as a consequence of the complex Pythagorean theorem
ALG Mandolesi
Phys. Lett. A 384 (28),126725 (2020)

The abstract reads:

"Everettian Quantum Mechanics, or the Many Worlds Interpretation, lacks an explanation for quantum probabilities"

At page 7 the problem is discussed in more detail:

"Not knowing how probabilities can emerge in EQM, we can not assume they satisfy Everett’s or Gleason’s hypotheses. Other attempts [45, 2, 10] have been made to get the right probabilities, without much success."

Sure, the author proposes his new solution, including an infinite number of universes, etc. Maybe this works, I am not sure, since the paper is new and nobody seems to offer a critique of it. But in the end the truth is that the status of the Born rule in MWI is still "work in progress", so, technically, MWI cannot make any prediction at all. But since this forum postulates that MWI makes similar predictions with Copenhagen I'll leave this issue aside.

It eliminates gravity as a "force" (according to general relativity, the force is not "gravity" anyway).

It doesn't. I hope you agree that a planet is in free fall. A space probe in orbit around that planet is also in free fall. Yet, by a careful monitoring of the probe's orbit one can deduce the mass distribution inside the planet. The same approach can be used to detect the mass distribution of a cat inside the box, by observing the motion of a small object in orbit around the box. If the mass distribution changes (as a result of its heart beating for example) you know the cat is alive.

If you are going to make claims about "gravitons", then you need to say what working theory of gravitons you are using, which will be difficult since there isn't one.

No gravitons.

You keep introducing special cases as if they were a basis for general arguments. They're not. Let's say the hole doesn't have charge: then your argument doesn't apply. Which means it's not a general argument.

OK, forget the charge, let's just go with mass.

Only if you can show me a working quantum theory of black holes. Which you can't because there isn't one.

OK, no black holes either.

 

[AndreiB]

This is indeed very common-sense but sadly has little to do with QM(which doesn't mention any of the the following terms - "Moon", "tides", "body", "location", "direction", "observer"). The whole framework is only about observables, probabilities and measurements.

No theory defines the instruments used to make measurements. I do not get your point.

So the whole point about the alleged persistence in time and space of those macroscopic bodies is largely moot. They are in a grey area.

We know that the Moon persists because we can observe the consequences of that persistence, like tides. The assumption that the Moon disappears when you don't look is falsified by those tides.

You can't simply assume their 'classical' all-time existence to help yourself deal with measurements outcomes.

I'm not assuming their existence, I deduce their existence based on observations we can make. The tides point to the existence of a massive body, consistent with the Moon. So, if I see tides I know the Moon is there.

Unless you think they are made of classical stuff which nobody has seen so far..

I think that the quantum-classical correspondence is an accepted result, so I don't need to assume any classical stuff. The classical world "emerges" out of the quantum one.

 

[PeterDonis]

MWI has a significant difficulty to accommodate the 6 postulate, Born's rule

Yes, this is a well known open issue with the MWI. Some think it can be resolved, others do not.

by a careful monitoring of the probe's orbit one can deduce the mass distribution inside the planet

Not by measuring "gravitational force" (since gravity isn't a force in GR), but by careful measurements of spacetime curvature--geodesic deviation. However, making such measurements takes time; it is not something you can just read off a meter instantly; and the smaller the change in mass distribution that you want to detect, the longer it takes. The same would apply to trying to detect whether a live cat is inside a box by measuring small changes in mass distribution. One could easily set up a Schrodinger's Cat scenario in which the time for the experiment to run was much shorter than the time that would be required to detect a live vs. a dead cat from changes in mass distribution.

Also, as I have already pointed out multiple times now, the fact that one can in principle make such measurements does not mean such measurements must be made in every scenario. If there is no "mass distribution measurement" being made in a given experiment, there is no way to detect a live vs. a dead cat by that method in that experiment, no matter how long it takes.

 

[AndreiB]

Not by measuring "gravitational force" (since gravity isn't a force in GR), but by careful measurements of spacetime curvature--geodesic deviation.

OK.

However, making such measurements takes time; it is not something you can just read off a meter instantly;

LIGO makes measurements at 10 KHz and I see no reason to believe that there is some theoretical limit. Still, 10 KHz is more than enough to detect the "wobble" associated with a beating heart.

and the smaller the change in mass distribution that you want to detect, the longer it takes.

Practically, yes, but in theory this doesn't need to be the case. A small wobble shouldn't take more time to measure than a large one.

The same would apply to trying to detect whether a live cat is inside a box by measuring small changes in mass distribution. One could easily set up a Schrodinger's Cat scenario in which the time for the experiment to run was much shorter than the time that would be required to detect a live vs. a dead cat from changes in mass distribution.

Again, I don't think there is any theoretical limitation of that time.

Also, as I have already pointed out multiple times now, the fact that one can in principle make such measurements does not mean such measurements must be made in every scenario. If there is no "mass distribution measurement" being made in a given experiment, there is no way to detect a live vs. a dead cat by that method in that experiment, no matter how long it takes.

I disagree here. If you send a stream of photons to the slits in a 2-slit experiment and, by detecting those photons you could, in principle, detect which slit the particle passed, no interference would be observed regardless of the fact that you actually detect those photons or not. You don't actually need to look at the instrument, the presence of the instrument is enough to eliminate the superposition.

In the cat scenario, the "instrument" can be any object with mass outside the box. The fact that you do not look at that object changes nothing, the cat is not in a superposition anymore. So, the only way to keep the cat in a superposition is to eliminate all external objects, but this implies that the experiment cannot be done, since you need to have an outside observer.

 

[EPR]

I disagree here. If you send a stream of photons to the slits in a 2-slit experiment and, by detecting those photons you could, in principle, detect which slit the particle passed, no interference would be observed regardless of the fact that you actually detect those photons or not. You don't actually need to look at the instrument, the presence of the instrument is enough to eliminate the superposition.

In the cat scenario, the "instrument" can be any object with mass outside the box. The fact that you do not look at that object changes nothing, the cat is not in a superposition anymore. So, the only way to keep the cat in a superposition is to eliminate all external objects, but this implies that the experiment cannot be done, since you need to have an outside observer.

You implicitly assume those objects are not quantum in nature but classical(Newtonian and persisting in time and space). You justify this fallacy with an even bigger fallacy - that you naturally observe the effects of their Newtonian existence on other classical-like objects(like measuring instruments). The basic fact is you can't infer anything about the world from just quantum theory. This is why you always have to reach out and grab a 'classical" object or two and use them to justify your philosophy. The hard part is describing the classical world in purely quantum terms.

 

[AndreiB]

You implicitly assume those objects are not quantum in nature but classical(Newtonian and persisting in time and space).

Can you please define what you mean by a "quantum" object? And where exactly did I assume that the objects are not "quantum"?

The "persistence in time and space" is a consequence of mass/energy conservation that applies also in QM. Anyway, where exactly did I assume that "persistence"? Can you please quote the relevant part?

You justify this fallacy with an even bigger fallacy - that you naturally observe the effects of their Newtonian existence on other classical-like objects(like measuring instruments).

The fact that one massive object acts on other massive objects in an observable way is not a fallacy, it's an experimental fact. When you see the sea level rising, you "observe the effect of the Newtonian existence" of the Moon. It's a fact.

The basic fact is you can't infer anything about the world from just quantum theory.

This is a strange claim. A claim that has been refuted countless times by the many confirmed predictions of QM. QM predicts superconductors. Superconductors exist. Your unjustified assertion has been experimentally refuted.

This is why you always have to reach out and grab a 'classical" object or two and use them to justify your philosophy.

What philosophy?

The hard part is describing the classical world in purely quantum terms.

What do you mean by "quantum terms"?

 

[EPR]

Can you please define what you mean by a "quantum" object? And where exactly did I assume that the objects are not "quantum"?

The "persistence in time and space" is a consequence of mass/energy conservation that applies also in QM. Anyway, where exactly did I assume that "persistence"? Can you please quote the relevant part?The fact that one massive object acts on other massive objects in an observable way is not a fallacy, it's an experimental fact. When you see the sea level rising, you "observe the effect of the Newtonian existence" of the Moon. It's a fact.This is a strange claim. A claim that has been refuted countless times by the many confirmed predictions of QM. QM predicts superconductors. Superconductors exist. Your unjustified assertion has been experimentally refuted.What philosophy?What do you mean by "quantum terms"?

I specifically said that you can't infer anything about the classical world from just purely quantum terms. This is why you always insist on having observational gravity waves, tides, massive bodies and other classical items to explain the emergence of... other classical behaviour.

Specifically here:

When you see the sea level rising, you "observe the effect of the Newtonian existence" of the Moon. It's a fact.

Unless you can prove the world is Newtonian, I suggest you do not use "Newtonian" objects to explain quantum behavior because it's a logical fallacy and also a tautology. Saying the cat is either alive or dead because it's classical and Newtonian makes zero sense from a quantum mechanical point of view. Esp. when tied to a quantum process like atomic decay.

 

[AndreiB]

I specifically said that you can't infer anything about the classical world from just purely quantum terms.

I asked you to define what do you mean by "quantum terms". You didn't, so the above assertion is meaningless.

This is why you always insist on having observational gravity waves, tides, massive bodies and other classical items to explain the emergence of... other classical behaviour.

Can you give me an example of observation that is not "classical"?

Unless you can prove the world is Newtonian, I suggest you do not use "Newtonian" objects to explain quantum behavior because it's a logical fallacy and also a tautology.

Describe me an experiment that does not involve the observation of "Newtonian" objects!

Saying the cat is either alive or dead because it's classical and Newtonian makes zero sense from a quantum mechanical point of view.

I agree, but this is not what I said. You are fighting a straw man here. I said that the cat is in a well defined state all the time because its state is measured from outside the box. And QM says that the superposition ends when a measurement is done. No "Newtonian" assumption is used here.

 

[PeterDonis]

LIGO makes measurements at 10 KHz

LIGO is not the tool we use to measure the mass distribution of the Earth. It is not suitable for that purpose. Measuring gravitational waves is not the same thing.

Practically, yes, but in theory this doesn't need to be the case.

You can't just wave your hands and say what should be true "in theory". You have to actually show that the theory supports your claim.

A small wobble shouldn't take more time to measure than a large one.

Measuring the mass distribution of the Earth is not the same as measuring a single wobble.

I don't think there is any theoretical limitation of that time.

See my response about theory above.

If you send a stream of photons to the slits in a 2-slit experiment and, by detecting those photons you could, in principle, detect which slit the particle passed, no interference would be observed regardless of the fact that you actually detect those photons or not.

You are misdescribing this. If you don't actually detect the photons, you don't know whether or not there is interference: to know whether or not there is interference, you have to detect the photons.

What you don't have to "detect" (in the sense of "a human observes") is the output of the which-way detector: just the fact that the which-way detector is present is enough. So it's the presence of the which-way detector--the fact that there is an extra interaction in the path of the photons--that removes the interference.

In the case you're talking about, if there is a "mass distribution detector" present (and if it is actually capable of distinguishing mass distributions on the appropriate time scale), then you are correct that no human has to actually read its output for the "live cat" and "dead cat" alternatives to decohere (i.e., not interfere with each other). However, the detector still has to be there. If it is not there, then the fact that it could have been there is irrelevant. Just as, if a which-way detector is not there in a double slit experiment, there is interference, even though there wouldn't have been if the detector were there. And I was saying that, since it is perfectly possible to run the experiment with such a detector not there, you cannot make fully general claims based on what would have happened if it were there.

 

[EPR]

I asked you to define what do you mean by "quantum terms". You didn't, so the above assertion is meaningless.

[/QUOTE]Explain to me what you mean by massive bodies, gravitational tides, etc in terms of wavefunctions. without resorting to acts of measurements by other unexplained "classical" objects. The framework wasn't devised to explain how the world works but to make predictions. You are using it wrongly to push your circular philosophy. There is no deep knowledge why, how and what exists prior to measurements. This is entirely philosophy, unless you adopted a minimalist interpretation which you obviously did not.

Can you give me an example of observation that is not "classical"?Describe me an experiment that does not involve the observation of "Newtonian" objects!
I agree, but this is not what I said. You are fighting a straw man here. I said that the cat is in a well defined state all the time because its state is measured from outside the box. And QM says that the superposition ends when a measurement is done. No "Newtonian" assumption is used here.

That all observations appear "classical" is not an excuse to use them as an explanation for the appearance of other classical-like behavior. This is a short-coming of the theory. Not its foundation.
Your reasoning and explanations are amount to : there are massive classical bodies... because this is what we observe.

 

[AndreiB]

You are misdescribing this. If you don't actually detect the photons, you don't know whether or not there is interference: to know whether or not there is interference, you have to detect the photons.

Yes, my formulation was not clear. The particles that are subjected to interference are some atoms or molecules that can interact with photons of some frequency. The photons are used to get which-path information. Of course, the atoms/molecules need to be detected in order to observe the pattern.

What you don't have to "detect" (in the sense of "a human observes") is the output of the which-way detector: just the fact that the which-way detector is present is enough. So it's the presence of the which-way detector--the fact that there is an extra interaction in the path of the photons--that removes the interference.

OK, right. In the case of the cat in the box, the "which-way detector" is any object with mass that exists outside the box. The only difference is that this object interacts with the cat gravitationaly, while in the 2-slit experiment the interaction is electromagnetic. Since you agree that the presence of the detector is enough to suppress interference, it follows that the presence of at least one object with mass outside the box is enough to eliminate the superposed cats.

In the case you're talking about, if there is a "mass distribution detector" present (and if it is actually capable of distinguishing mass distributions on the appropriate time scale), then you are correct that no human has to actually read its output for the "live cat" and "dead cat" alternatives to decohere (i.e., not interfere with each other).

Right, and any object with mass is a "mass distribution detector" since its trajectory is correlated with the mass distribution. Since any finite segment of its trajectory contains an infinite number of points and you need a finite number of points to determine the mass distribution it follows that there is no minimal time required. Any time will do, assuming that the camera you use to determine the trajectory is fast and accurate enough. Sure, there are limits to the speed and accuracy of any camera but this can be compensated by using more cameras.

However, the detector still has to be there. If it is not there, then the fact that it could have been there is irrelevant.

Yes, but since any object with mass outside the box is a "detector", and the experiment requires that something/someone outside exists so that he/it can open the box and look inside, it follows that the "detector" is guaranteed to be there.

 

[AndreiB]

Explain to me what you mean by massive bodies, gravitational tides, etc in terms of wavefunctions.

I think you have a wrong understanding about what a theory is. It is an algorithm that takes as input some observations (initial data) and outputs some numbers that need to be related to other observations (predictions).

The observations themselves are as they are, they are neither "quantum" nor "classical".

A wavefunction cannot be observed, hence it cannot play the role of an initial data. The wavefunction can be deduced from observations. In the case of the hydrogen atom you need to know the mass/charge for the electron and proton. once you have those you can calculate the Hamiltonian (assuming the classical Coulombian force) and only then you get the wavefunction. You use the wavefunction to infer the energy levels, and then, you can check the prediction based on spectral observations.

The concept of mass/charge together with the 3D space and time are primitives. They are not explained by the theory. A massive body is an object with mass. A tide is a type of motion of water, itself composed from objects with mass and charge (water molecules)

The framework wasn't devised to explain how the world works but to make predictions.

I agree.

You are using it wrongly to push your circular philosophy.

There is nothing circular here.

There is no deep knowledge why, how and what exists prior to measurements.

So what? Did i ever pretend to know that?

That all observations appear "classical" is not an excuse to use them as an explanation for the appearance of other classical-like behavior.

There are no "classical" or "quantum" observations. You are confused.

Your reasoning and explanations are amount to : there are massive classical bodies... because this is what we observe.

My argument has nothing to do with explaining the existence of "massive classical bodies". I have no idea what made you think so.

 

[PeterDonis]

In the case of the cat in the box, the "which-way detector" is any object with mass that exists outside the box.

Most objects with mass are incapable of detecting the tiny differences in mass distribution that distinguish a live cat from a dead cat, or even the much larger differences in mass distribution inside the Earth. That's why, in order to measure the Earth's mass distribution in detail, we can't just put a bunch of "objects with mass" around; we have to build specialized equipment and launch it in spacecraft and let them orbit the Earth for quite some time.

The only difference is that this object interacts with the cat gravitationaly

Not every gravitational interaction can serve as a "mass distribution detector".

 

[EclogiteFacies]

I know this is late but Rovelli with RQM believes that yes your wife is in a defined position before opening the door. Decoherence plus billions of uncorrelated degrees of freedom wash out relationality. This produces stable facts. Interference is so unbelievably subtle that FAPP there is none. Macro objects under normal circumstances (access to the environment).
Are not in superposition.
RQM is not solipsistic.

Also, I do think Wigner's Friend experiments are disingenuous. Anthropomorphising photons is ridiculous.
Under normal circumstances (excluding MWI) people are not in superposition and these experiments are only applicable for the macroscopic under unbelievably strange idealised circumstances with the use of a magic lab that separates the friend from the entire universe.

It doesn't change day to day life.

 

[AndreiB]

Most objects with mass are incapable of detecting the tiny differences in mass distribution that distinguish a live cat from a dead cat

How "incapable"? We can measure, in principle, the position of such an object with sub-Planckian level accuracy (corresponding to the uncertainty principle applied to a maroscopic object). I'm not saying you are wrong, but I think some sort of calculation needs to be done. I would say that, based on Newton's third law, the disturbance produced on the external object by the change in the cat's mass distribution is of the same order as the change of the mass distribution itself.

Not every gravitational interaction can serve as a "mass distribution detector".

Why?

 

[PeterDonis]

We can measure, in principle, the position of such an object with sub-Planckian level accuracy

We do not know that this is possible, no. Such claims are highly dependent on what quantum gravity theory turns out to be correct.

Why?

For the same reason that, as I've already pointed out, we couldn't just put a bunch of "massive objects" around to measure the Earth's mass distribution. We had to design special equipment and put it aboard satellites with precisely determined orbits and collect data for a long period of time.

 

[AndreiB]

We do not know that this is possible, no. Such claims are highly dependent on what quantum gravity theory turns out to be correct.

Currently, QM does not impose any limit for the accuracy of a position measurement. I have no reason to believe that quantum gravity would change that.

For the same reason that, as I've already pointed out, we couldn't just put a bunch of "massive objects" around to measure the Earth's mass distribution. We had to design special equipment and put it aboard satellites with precisely determined orbits and collect data for a long period of time.

This is a thought experiment. It's not about technology, it's about principles. As a matter of principle there is no difference between a space probe and some random object. It's just not convenient to find such an object and go there to measure its position.

 

[PeterDonis]

Currently, QM does not impose any limit for the accuracy of a position measurement.

Currently, QM has only been tested down to length scales about 18 orders of magnitude larger than the Planck length.

I have no reason to believe that quantum gravity would change that.

Um, yes, you do, since quantum gravity is expected by most physicists to predict that the very concept of "length" (and "spacetime" in general) is no longer meaningful at the Planck scale--that our current concept of "spacetime" is an emergent phenomenon at scales much larger than the Planck scale.

As a matter of principle there is no difference between a space probe and some random object.

Yes, there is: a random object is not designed to make a particular very precise measurement. Just as we don't use random rocks as detectors in experiments in general; we use specially designed equipment. To claim that any random object would work just as well is ridiculous.

 

[Sunil]

A wavefunction cannot be observed, hence it cannot play the role of an initial data.

It can and does. Because it can be prepared by a preparational measurement. You measure some A, throw away all cases where $a\neq a_0$ and have prepared a state with a wave function which is an eigenstate of A with eigenvalue $a_0$.
Another way to prepare is simple relaxation. You have atoms in various possibly excited states. You isolate them from influences which can excite them and wait. After some time, the excited states will have emitted photons and returned to the ground state, and you can use the ground state for the initial data.

 

[AndreiB]

It can and does. Because it can be prepared by a preparational measurement.

What you actually see is not a wavefunction, it's the macroscopic disposition of your preparation device. Sure, you can deduce the wavefunction from "classical" observations but you can't "see" a wavefunction directly.

A "pure" version of QM without any reference to "classical" observations is just a piece of mathematics, not physics.

 

[gentzen]

A wavefunction cannot be observed, hence it cannot play the role of an initial data. The wavefunction can be deduced from observations.

It can and does. Because it can be prepared by a preparational measurement. You measure some A, throw away all cases where ...

I have to agree with AndreiB on this one. You can prepare the density matrix of a thermal state, and you can filter that density matrix to become more concentrated around some pure state you want to prepare. But the state will always stay far away from being pure. A laser achieves states whose density matrix is nearly pure. Perhaps the way how this is achieved can be interpreted as using filtering to directly influence a thermal state.

And if you want to measure the density matrix of some prepared state, then you need measurement results for a number of different measurement settings. So believing that you have in any way direct access to the wavefunction is naive.

 

[AndreiB]

Currently, QM has only been tested down to length scales about 18 orders of magnitude larger than the Planck length.

This is not about accurate position measurements, it's about the de-Broglie wavelength of the "probe" particles. LIGO can measure distances of 10^-20 m using lasers with a wavelength of 10^-6 m.

Um, yes, you do, since quantum gravity is expected by most physicists to predict that the very concept of "length" (and "spacetime" in general) is no longer meaningful at the Planck scale--that our current concept of "spacetime" is an emergent phenomenon at scales much larger than the Planck scale.

I think this is pure speculation. I am aware about Nima Arkani-Hamed's arguments that presumably show that spacetime is "doomed". I don't buy his arguments and I don't think he was able to replace spacetime with something more fundamental. But even Nima accepts that measurements of any accuracy can be performed, as long as the accuracy is not infinite. The theories we have at this moment accept a continuous spacetime, and Lorentz invariance requires that nothing special happens at Planck distance.

Yes, there is: a random object is not designed to make a particular very precise measurement. Just as we don't use random rocks as detectors in experiments in general; we use specially designed equipment. To claim that any random object would work just as well is ridiculous.

You already agreed that, in the case of the of a 2-slit experiment it's not actually necessary to detect the photons that carry the relevant which-path information. Their detection, sure, requires "specially designed equipment", like a fluorescent screen.

In our case, the rock carries the relevant information, and, by analogy, the "specially designed equipment" is not necessary to suppress the live/cat superposition.

 

[Sunil]

Um, yes, you do, since quantum gravity is expected by most physicists to predict that the very concept of "length" (and "spacetime" in general) is no longer meaningful at the Planck scale--that our current concept of "spacetime" is an emergent phenomenon at scales much larger than the Planck scale.

This maybe what "most physicists expect". But is it relevant?
The part of quantum theory which works and is well-defined is QG as an effective theory:

Donoghue, J.F. (1994). General relativity as an effective field theory: The leading quantum corrections. Phys Rev D 50(6), 3874-3888

It works on the base of the field-theoretic variant of GR, thus, has a background, and breaks full GR symmetry given that it uses harmonic coordinates (but tries to recover symmetry using ghosts).

Yes, there is: a random object is not designed to make a particular very precise measurement. Just as we don't use random rocks as detectors in experiments in general; we use specially designed equipment. To claim that any random object would work just as well is ridiculous.

Depends on the particular experiment. Many statistical experiments especially use random numbers or so to randomize the input, especially in medicine. There is no relation between such random input and precision. The precision of such experiments you can, for example, improve with larger numbers of trials. (But I agree that in this point criticizing the OP is correct.)

 

[Sunil]

I have to agree with AndreiB on this one. You can prepare the density matrix of a thermal state, and you can filter that density matrix to become more concentrated around some pure state you want to prepare. But the state will always stay far away from being pure.

That's only the trivial point that there is no ideal experiment. That it stays "far away" is simply wrong, it may be quite close to the ideal. Close enough to use the wave function as the initial datum for what is done later.

And if you want to measure the density matrix of some prepared state, then you need measurement results for a number of different measurement settings. So believing that you have in any way direct access to the wavefunction is naive.

There would be no need to measure such a density matrix.

Moreover, in the preparation you can restrict yourself to the particular measurement you are interested in. If you prepare some spin up particles, because for your experiment spin is important but position quite irrelevant, the positions will remain far from being in a pure state simply because this is nothing worth to care about. The state will be almost pure only for the relevant degrees of freedom, say, the spin. $|\psi_a(a)\rangle\langle \psi_a(a)| \times \hat{\rho}(b,\ldots)$

So, I don't have to claim some direct access. I claim that one can prepare which can be, with good enough accuracy, described by a well-known wave function for the relevant degrees of freedom.

 

[Sunil]

What you actually see is not a wavefunction, it's the macroscopic disposition of your preparation device. Sure, you can deduce the wavefunction from "classical" observations but you can't "see" a wavefunction directly.

So what? There is no need for this "seeing directly". This may be relevant only for proponents of outdated methodology of science like empiricism where "seeing directly" is somehow important.

A "pure" version of QM without any reference to "classical" observations is just a piece of mathematics, not physics.

The point being? Even the minimal interpretation uses "measurement results" and so implicitly refers to such observations of classical measurement devices.

 

[AndreiB]

So what? There is no need for this "seeing directly". This may be relevant only for proponents of outdated methodology of science like empiricism where "seeing directly" is somehow important.

In my discussion with EPR (see post #89) he asked me:

"Explain to me what you mean by massive bodies, gravitational tides, etc in terms of wavefunctions. without resorting to acts of measurements by other unexplained "classical" objects."

My point was that what EPR wants cannot be done, since the wavefunction cannot be directly observed. My reply has to be understood in this context, it's not that I have something against wavefunctions.

The point being? Even the minimal interpretation uses "measurement results" and so implicitly refers to such observations of classical measurement devices.

EPR is not satisfied with the minimal interpretation. I was replying to him.