Schrodinger's Cat and the thermal interpretation

[A. Neumaier]

The uncertainty in QM must be ontic because we observe interference between the different possibilities the state of a single subsystem ranges over

What interference are you talking about?

In the TI, beables are q-expectations, not wave functions. The reduced density matrix is only a tool to compute these. In case of a 2-state system, one can instead use the Bloch vector to encode the q-expectations, ranging over a unit ball. Interference does not apply to this representation.

We observe approximations to the ontic values. The same observed highly peaked bimodal distribution of observations in a large sample of trials can be interpreted either as the observation of a tiny random true binary value $\pm\hbar/2$ of the system measured, with small bell-shaped measurement error - this is the TI interpretation of the observations regarded as measuring the detector beable. Or it can be interpreted as the observation of a single true value of the order of $O(\hbar)$ with a bimodal measurement error - this is the TI interpretation of the observations regarded as measuring the system beable.

Only the convention used - for which beable the measurement is and what is regarded as its true value - can decide between the two possibilities, and say anything about the relation between the measurement of the detector beable and what it means for the system beable.

The possibilities in an epistemic probability distribution cannot exhibit interference, as only one of them actually exists.

A single observation gives a value approximating the corresponding beable of the detector, which approximates the corresponding beable of the measured system, with an unknown error (if one does not assume to know beables).

This is independent of any epistemic analysis. The latter is about what an observer knowing only approximate observations is rationally entitled to imagine about success rates of future predictions.

 

[charters]

Having thought over this some more, I want to share where I've landed:

So to know with certainty which exit port of my open MZI will click, I have to know whether it is raining on a planet in the Andromeda galaxy

In principle, yes, that's the inevitable consequence of nonlocal deterministic dynamics. You have the same in Bohmian mechanics. To predict with certainty the position of the Bohmian pointer variable at one point in the future you need to know now the positions of all particles in the universe, and the details of the wave function.

So then it isn't the case that

the cat is definitely dead or alive except during a short moment where the decay happens and nothing definite can be said.

Rather, like in Bohmian mech, the cat's fate is already, definitively decided at ALL times, before, during, and after the interaction/measurement. This informationnis encoded in these cross-universe correlations. In any interpretation of this variety, we do not conventionally say the cat is "alive AND dead." This is all I was trying to point out back in #70. I think what you really mean here is that there is a finite window during which the evolution of the *local* q-expectations from alive-to-alive and alive-to-dead will look the same, so as a local matter, the fact briefly seems ontologically undecided. This is what you are calling "alive AND dead" or "not definitely one or the other." That's a valid detail to highlight, but this is not the type of uncertainty that is at issue in Schrodingers cat as it is generally understood, and it isn't going to be productive to try to redefine a deeply embedded idea/term like this.

I also think some of our confusion has been related a lack of clarity/terminology agreement over the following idea: the TI universe is described by q-expectations, which are (using the definitions familiar to me) states of non-zero ontic uncertainty. But since we can't know all the exact q-expectations, the true state is embedded inside a proper mixture, but one whose elements are themselves improper mixtures rather than exact, classical states. In this way the TI beables are unlike in Bohm of T Hooft's interpretation, and require a "lighter touch," so to speak.

As a concrete example, consider an electron that we know has been prepared in the ground state of either Box A or Box B. The TI sees the electron as a fundamentally extended object, as a fieldlike object filling one box or the other, with a variable ontological density in different subregions of a single box. This can be seen as an ontically uncertain, improper mixture of classical positions within a single box. But, given the preparation constraint, there is *also* a proper mixture (epistemic ignorance) in respect of whether it is A or B that is filled at all.

For Bohm, however, the electron is instead a single classical point, so there is a proper mixture in respect of whether the electron is in A or B, but even when it is known box A is occupied, there is *another* proper mixture in respect of whether it is in subregion A1, A2, etc. The TI does away with the need for this secondary inquiry by treating this uncertainty as ontic rather than epistemic. Equivalently, the TI does not think it is a reasonable question to ask where in an atomic orbital the electron is. The electron is the entire orbital.

Now let us suppose Box A and Box B are connected. Here, the TI electron would unitarily disperse between the two boxes, becoming even more ontologically extended.

Finally, keep the boxes connected and replace the electron with a baseball. In this case, while there is always some small uncertainty/ontic extendedness in the center of mass of the baseball, even after infinite time, in the TI, the baseball never evolves to be extended over macroscopically distinct positions. Rather, nonlocal variables/correlations will steer the baseball's center of mass q-expectation so that it is non-dispersive on macro scales, so there is no need to worry about many worlds branching.

In sum, the TI uses "mild" hidden variables to the degree necessary to prevent many worlds, but without over-classicalizing the micro-ontology.

So, to tie this together, I now think the root of the confusion/dispute has been that when you say there is a window of time when the cat is alive AND dead, I hear something tantamount to "the baseball is ontologically extended over box A and B when connected, just like the electron." Once you open the door to this degree of ontic uncertainty, you have gone too far and will have many worlds. But I don't think this is what you meant to claim after all in the 2nd quote up above.

 

[PeterDonis]

the TI universe is described by q-expectations, which are (using the definitions familiar to me) states of non-zero ontic uncertainty

It doesn't matter how familiar that definition is to you; it's not the definition used in the TI, and in fact in the TI it's obviously false--in the TI the q-expectations are the beables so by definition they have zero ontic uncertainty. So you can't just help yourself to this definition when talking about the TI; it makes everything you say about the TI logically inconsistent.

 

[PeterDonis]

The TI sees the electron as a fundamentally extended object, as a fieldlike object filling one box or the other, with a variable ontological density in different subregions of a single box.

Yes.

This can be seen as an ontically uncertain, improper mixture of classical positions within a single box.

Not according to the TI, it can't. According to the TI, the "fieldlike object filling one box or the other" is the ontic state. All this talk of "classical positions" is simply irrelevant.

The TI does away with the need for this secondary inquiry by treating this uncertainty as ontic rather than epistemic.

No, it does away with the need for the secondary inquiry by not even using the concept of classical position at all.

 

[PeterDonis]

when you say there is a window of time when the cat is alive AND dead, I hear something tantamount to "the baseball is ontologically extended over box A and B when connected, just like the electron.

Suppose the baseball is in the connecting tunnel between A and B. Is it in A or B? There is no well-defined answer. Even if you draw an arbitrary line down the middle of the tunnel and say that there is the boundary between A and B, there will be some period of time when part of the baseball is on one side of the line and part is on the other side, and during that period of time, the question "is the baseball in A or B?" has no well-defined answer.

The "alive to dead" transition for the cat is something like the connecting tunnel in the above; while it is happening, there is no well-defined answer to the question "is the cat alive or dead?". You can try to draw arbitrary boundaries, but they're arbitrary, just as in the tunnel above, and there is no reason to think that the entire cat must always be on one side or the other of any boundary you draw.

Btw, I don't think the above has much to do with the TI in particular; it's just a general feature of macroscopic systems when you try to draw exact, hard-edged categories for them. Thinking that the categories we find it useful to draw for our human understanding must always be embedded somehow in the physics of the objects themselves is a simple map-territory confusion, like thinking that the Earth's prime meridian must somehow be physically set apart, at the level of individual molecules, from the rest of the Earth.

 

[charters]

It doesn't matter how familiar that definition is to you; it's not the definition used in the TI, and in fact in the TI it's obviously false--in the TI the q-expectations are the beables so by definition they have zero ontic uncertainty. So you can't just help yourself to this definition when talking about the TI; it makes everything you say about the TI logically inconsistent.

It matters that we understand each other's terms to have an effective conversation.

According to the TI, the "fieldlike object filling one box or the other" is the ontic state. All this talk of "classical positions" is simply irrelevant.

Yes. I am simply trying to explain that when I use the term "ontic uncertainty" I am talking about precisely this type of entity, which is extended over a range of classical states, and is not definitely in any single classical state.
If you don't like this word choice, that is your prerogative, but at least we can realize we're arguing labels rather than substance. So I am trying to clarify, so at least you understand my meaning, and we don't get hung up on something that is ultimately a non-issue between us.

The "alive to dead" transition for the cat is something like the connecting tunnel in the above; while it is happening, there is no well-defined answer to the question "is the cat alive or dead?". You can try to draw arbitrary boundaries, but they're arbitrary, just as in the tunnel above, and there is no reason to think that the entire cat must always be on one side or the other of any boundary you draw.

Btw, I don't think the above has much to do with the TI in particular;

Agreed fully. But this is not what the notion of "in Box A AND in Box B" means in standard discussions of the measurement problem. It is not limited to a case of a system straddling an arbitrary boundary, but also, say, a case where the baseball is only in the opposite far corners of Box A and B, never in the intervening space, yet not definitely in either corner. Unlike MWI, the TI doesn't allow macro objects to exist simultaneously in multiple macroscopically distinct states like this, it only allows the type of straddling you describe.

I am basically just saying that as a reader, I got confused when a very familiar idea like Schrodinger' cat was used to make a different point from what it normally is about. You can blame me for not immediately understanding this subtle shift from my preexisting experience if you want. But I think if you hope to effectively communicate a new interpretation of QM, you should plan on taking your audience as you find them.

 

[PeterDonis]

I am talking about precisely this type of entity, which is extended over a range of classical states

But in the TI, it doesn't. That's the point. The very definition "extended over a range of classical states" implies a particular class of interpretations, to which the TI does not belong.

Unlike MWI, the TI doesn't allow macro objects to exist simultaneously in multiple macroscopically distinct states

Agreed. You could set up a situation where, for example, a baseball was sent to the far corner of one box or the other using a gate controlled by a quantum event like a radioactive decay (the gate flips to divert to box B instead of A only if the decay occurs during the experimental time period), and the MWI and the TI would place a different ontic interpretation on the resulting mathematical description of the state.

as a reader, I got confused when a very familiar idea like Schrodinger' cat was used to make a different point from what it normally is about

That's understandable. But I don't think it helps to clear up such confusion if assumptions are made that depend on a particular interpretation or set of interpretations, even if the assumptions are only (ostensibly) made for the purpose of choosing terminology. As I think I've said before about the MWI in other PF threads, it's very important in discussing interpretations to be extremely clear and precise about what each interpretation actually says, even if (or perhaps especially if) you are going to end up disagreeing with the interpretation.

 

[charters]

That's understandable. But I don't think it helps to clear up such confusion if assumptions are made that depend on a particular interpretation or set of interpretations, even if the assumptions are only (ostensibly) made for the purpose of choosing terminology.

But I think the sociological context is important. I think the meanings I assume are quite standard across pretty much every other interpretation. I've read a good deal on this topic and never have I felt before that I had to re-tailor the meaning of "alive OR dead" versus "alive AND dead" for any particular interpretation. The TI does not have macroscopic "AND" states under the standard meaning, and life will be easier if proponents stick to this.

But just one reader's perspective in the end, so grain of salt, I guess.

 

[A. Neumaier]

like in Bohmian mech, the cat's fate is already, definitively decided at ALL times, before, during, and after the interaction/measurement.

Yes, but the fate isn't strictly 2-valued 'dead' or 'alive' but there is an intermediate grey zone where the criteria to decide aliveness cannot be reliably checked, so that during this time one can neither say that the cat is dead nor that it is alive. It is this grey zone that is present in all binary measurements.

we do not conventionally say the cat is "alive AND dead."

the root of the confusion/dispute has been that when you say there is a window of time when the cat is alive AND dead

Where did I say this? If I said this at all it must have been a slip of the pen.

Your argument sounds like we were discussing the first goal in a football match and you were claiming that as long as the score is still 0:0, both sides have achieved a goal...

I think what you really mean here is that there is a finite window during which the evolution of the *local* q-expectations from alive-to-alive and alive-to-dead will look the same, so as a local matter, the fact briefly seems ontologically undecided.

No; ontologically everything is determined at all the time. But the pointer upon which deadness or aliveness is evaluated has a continuum of values, and only its (reliably readable) asymptotic sign decides upon the epistemic (in physical terms not precisely specified) notion of being dead or alive.

The same issues appear already classically in the analogy of the football match, and indeed whenever one defines a decision based upon an alternative that requires a pointer to move to a particular position to read the result. While the pointer moves (and perhaps oscillates) no decision is possible although the pointer always has a definite position - but it need not have the one that counts as a reliable measurement.

All this has nothing to do with locality issues.

It matters that we understand each other's terms to have an effective conversation.

I defined precisely all terms I use in the description and explanation of the thermal interpretation. If you wish to use other terms, then please define precisely the meaning of your terms (or link to such a precise definition), in particular the term "ontic uncertainty" on which your arguments rest, so that we can have an effective conversation. (Needed only if the above does not yet settle the issue.)

 

[A. Neumaier]

Schrödinger' cat is defined by the following original statement:

Eine Katze wird in eine Stahlkammer gesperrt, zusammen mit folgender Höllenmaschine (die man gegen den direkten Zugriff der Katze sichern muß): in einem Geigerschen Zählrohr befindet sich eine winzige Menge radioaktiver Substanz, so wenig, daß I am Laufe einer Stunde vielleicht eines von den Atomen zerfällt, ebenso wahrscheinlich aber auch keines; geschieht es, so spricht das Zählrohr an und betätigt über ein Relais ein Hämmerchen, das ein Kölbchen mit Blausäure zertrümmert. Hat man dieses ganze System eine Stunde lang sich selbst überlassen, so wird man sich sagen, daß die Katze noch lebt, wenn inzwischen kein Atom zerfallen ist. Der erste Atomzerfall würde sie vergiftet haben.

(English translation: A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. )

as a reader, I got confused when a very familiar idea like Schrodinger' cat was used to make a different point from what it normally is about

No matter which points are normally made using this setting, it is completely legitimate to make the different point that it takes an uncertain amount of time for the poison to definitely have killed the cat, and that there is therefore a time interval where even a Laplacian quantum demon (adhering to a deterministic interpretation of quantum mechanics) cannot say objectively whether the cat is still alive or already dead. I cannot understand what is confusing about this observation.

A detailed discussion of how Born's rule follows from the evolution of the state of the universe is given in the analysis in Section 3 of my Part IV. (Please discuss details in that thread.) The point is that one only needs to consider a binary pointer variable for the property ''atom decayed'' (or not), and that this decision is definitely made in a macroscopically noticeable way within a finite (macroscopically short) time, using the standard approximations used everywhere in statistical mechanics.

Note that what I discussed in Section 3 of Part IV was not Schrödinger' cat (which is a very poor measurement device for particle decay) but the observation of an arbitrary qubit state, which is a mathematically precisely formulated problem and its solution is also mathematically unassailable - apart from having not proved the precise probability distribution, which would need the discussion of a particular, mathematically well-defined model system for the measurement process.

This applies in particular to the observation of so-called Schrödinger cat states, but without all the complications of having to consider real cats (which in the TI cannot be modeled by pure states).

 

[charters]

the root of the confusion/dispute has been that when you say there is a window of time when the cat is alive AND dead

Where did I say this? If I said this at all it must have been a slip of the pen.

When you said:

Thus the cat is definitely dead or alive except during a short moment where the decay happens and nothing definite can be said

I read "except" as meaning that the cat is alive AND dead during the "short moment."

No; ontologically everything is determined at all the time. But the pointer upon which deadness or aliveness is evaluated has a continuum of values, and only its (reliably readable) asymptotic sign decides upon the epistemic (in physical terms not precisely specified) notion of being dead or alive.

Right - I was trying to say that, after thinking some more, I realized this is what you meant and I was misunderstanding what you wrote. The "short moment" refers only to the fact that the pointer takes a finite time to shift and resolve a readable measurement. Its not an ontological claim (which would be problematic and bring you into MWI territory).

I think you'll be better off saying: Thus the cat is definitely dead or alive *at all times*, however during a short moment where the decay happens *this outcome cannot be known* by a local, finite observer/witness to the process. Schrodinger's cat, at least here in the 21st century, is broadly understood as a question of the ontological status of the cat. See, eg, page 35 here: https://arxiv.org/abs/quant-ph/0112148.

The upshot is I now don't have a concern that the TI is violating the basic constraints that I expect any viable interpretation to have to respect, whereas under my initial reading, I did.