"No objective reality" in quantum mechanics?

[PeterDonis]

This makes the interference pattern go away, even though no correlation with the collective degrees of freedom of some macroscopic apparatus has been established.

Yes, it has: the optical crystals are macroscopic objects and they are correlated with the photon pairs that exit them.

 

[Morbert]

Yes, it has: the optical crystals are macroscopic objects and they are correlated with the photon pairs that exit them.

I can perform a measurement on the idler photon complementary to a "which way" measurement to erase the information in a delayed choice quantum eraser experiment. Are you saying that, in fact, the information is not really erased from the universe, and is merely inaccessible in the thermal or otherwise degrees of freedom of the optical crystal? [edit] - Since the crystal correlation and the complementary measurement would be joint events on the same photon, his sounds like it would violate complementarity no?

 

[PeterDonis]

I can perform a measurement on the idler photon complementary to a "which way" measurement to erase the information

If the interference pattern can be restored by this type of "quantum eraser" measurement, then decoherence has not occurred and the interference pattern has not "gone away" in the sense in which that term is being used in this thread. Once decoherence has occurred, it is irreversible and there is no way to restore the interference.

Are you saying that, in fact, the information is not really erased from the universe, and is merely inaccessible in the thermal or otherwise degrees of freedom of the optical crystal?

Not in the case where a "quantum eraser" experiment is possible, no. In that case the crystal has not caused any decoherence and the information has not been dispersed into the untrackable degrees of freedom of the crystal.

 

[PeterDonis]

A single qubit does not cause environmental decoherence. But it can make the interference go away.

As the exchange I have been having with @Morbert should make clear, "make the interference go away" in itself is not sufficient for a measurement. The interference has to be made to go away irreversibly. If the interference can be restored by a "quantum eraser" or similar procedure, then whatever made it go away in the first place was not a measurement.

 

[gentzen]

As the exchange I have been having with @Morbert should make clear, "make the interference go away" in itself is not sufficient for a measurement.

I totally agree. However, I didn't want to bring up this double-slit with polarizers experiment, because it doesn't feel quantum enough (or "single qubit" enough) to answer your question "How?" a single qubit can destroy interference.

The interference has to be made to go away irreversibly. If the interference can be restored by a "quantum eraser" or similar procedure, then whatever made it go away in the first place was not a measurement.

My "personal" trouble when trying to explain how the term "measurement" is used appropriately is that the Stern-Gerlach experiment is a typical paradigmatic quantum measurement, but the double slit with polarizers is not. Now people will try to argue that the Stern-Gelach measurement can in principle be reverted too, and use this for a variety of clarifications, illustrations, and thought experiments. And those are indeed "appropriate clarifications", from my POV. But when you analyse it in detail, you learn that reversing the Stern-Gerlach measurement in a real experiment simply won't work. But I "hope" that the Stern-Gerlach experiment being a paradigmatic quantum measurement is unrelated to those subtle details which prevent reversing it in a real experiment.

A single qubit ... can make the interference go away.

How?

Sorry for not answering. The "simple" example I had in mind when I wrote "Making the interference go away is very easy. Even too easy." were the "garbage qubits" in a quantum computation, which have to be brought back to their initial state by uncomputation to prevent them destroying the intended interference. I didn't answer, because I realized that what is "simple" for me might be uncomprehensible and feel totally unrelated to you. In this short clip, Chris Ferrie explains uncomputation (and why it is needed) with a simple example:

But it isn't simple (even for me), because the crucial part for our discussion is not explicit demonstrated, but hidden in Chris' remark at 4:00:

We don't care what state that qubit is in. However, there is a problem in that if I want to perform this circuit in superposition, then my data and my output is going to be entangled with that garbage, so I can't just throw out that garbage, cause I am throwing out information.

Additionally, the wikipedia article on Uncomputation contains the remark

The process is primarily motivated by the principle of implicit measurement.[3], which states that ignoring a register during computation is physically equivalent to measuring it.

which might generate even more confusion with respect to the question of the appropriate use of the term "measurement".

 

[CoolMint]

Summary: After having read some headlines, I'm curious if what they say (there is no objective reality, i.e no reality in the absence of an observer) could be true or not.

As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff.

Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an amateur so I'm not sure at all on how things work in this field so that is why this thread was created.

Are there some knowledgeable members in this forum that can shed some light on this?

Objective - very likely not. Reality exists though, whatever it is. Whether it's a game, an experience you are born into or a cosmic happenstance. The interpretations are almost as many as the opinions. In general, if the MWI is true, reality could be objectively existing at all times. This is the expense to have an objective reality compatible with QT.

 

[PeterDonis]

when you analyse it in detail, you learn that reversing the Stern-Gerlach measurement in a real experiment simply won't work.

Why not?

 

[PeterDonis]

the "garbage qubits" in a quantum computation, which have to be brought back to their initial state by uncomputation to prevent them destroying the intended interference.

Actually, it's not just that: as the diagram in the Wikipedia article makes clear, you have to "uncompute" the ancilla bits in order to transfer the operations involving them to the target bit. In other words, the "uncomputation" is actually part of the computation.

In the Wikipedia diagram, the desired "computation" involves five control bits that produce a result to be stored in the target bit. (Here "bit" really means "qubit".) But you can't implement that action in a single operation that only involves the control bits and the target bit, because quantum logic operations can't act on that many bits in a single operation. So you need three ancilla bits to implement the operation. But if you leave out the "uncomputation", then part of your desired result is stored in the ancilla bits instead of the target bit; the "uncomputation" steps transfer the result information in the ancilla bits into the target bit.

I haven't watched the video you linked to so I don't know if the above is discussed there. (I generally don't want to look at videos: if what is being said in the video is valid, it should be in a published peer-reviewed paper somewhere, and that's what I want to read.)

 

[PeterDonis]

hidden in Chris' remark at 4:00

...is another clue that points to what I said in post #78. He doesn't want anything to be "entangled with that garbage", by which he means the ancilla bits. He only wants the target bit to be entangled with the control bits, since that's the desired end state of the computation ("end state" meaning just before a measurement is made to "read out" the result). So he needs to do the "uncomputation" operations to transfer the entanglement from the ancilla bits back to the control bits and target bit.

Again, this is part of the computation; the definition of the full computation is that the desired "result" information is stored entirely in entanglements between the control bits and the target bit. So calling the ancilla bits "garbage" at the point where they still store entanglement is a misnomer: actually they're not "garbage" because they are storing part of the desired result information. So of course throwing them away (i.e., not doing the "uncomputation" operations) is going to give the wrong results.

 

[PeterDonis]

which might generate even more confusion with respect to the question of the appropriate use of the term "measurement".

Yes, the "implicit measurement" thing is poorly stated. The key question left out there is: when does the "implicit measurement" take place? The answer is actually simple: "reading out" the result of the computation involves a measurement on the control bits and the target bit. If the "uncomputation" operations have not been done, the control bits/target bit subsystem is still entangled with the ancilla bits subsystem. So a measurement on one subsystem amounts to an "implicit measurement" on the other subsystem. (Whereas if the "uncomputation" operations had been done, the ancilla bits would no longer be entangled with the control bits or the target bit, so a measurement on the latter would not be an "implicit measurement" on the former.)

In light of the above, the Wikipedia article appears to me to be in error when it states that the "implicit measurement" "happens before computation finishes".

 

[vanhees71]

Why not?

Simply because of complexity. You cannot prepare an exactly equal but opposite magnetic field.

 

[PeterDonis]

Simply because of complexity. You cannot prepare an exactly equal but opposite magnetic field.

A similar objection would apply to experiments with photons, such as Mach-Zehnder interferometers: you can't orient a second beam splitter exactly the same way as the first. Yet such experiments work, within reasonable error bars. Similarly, I would think a second S-G apparatus could be given a magnetic field that was "close enough" to that required to recombine the beams from the first S-G apparatus.

The answer I was expecting to get was more along the lines of the difficulty of redirecting electron beams without changing their state. With photons you can just use mirrors, as in the M-Z interferometer.

 

[WWGD]

On the issue of "objective reality" (see https://en.wikiversity.org/wiki/Does_objective_reality_exist?)

Physics cannot answer such a questions because it is beyond its scope. The philosopher David J. Chalmers puts it in a nutshell in “Ontological Anti-Realism”:

“The basic question of ontology is ‘What exists?’. The basic question of metaontology is: are there objective answers to the basic question of ontology? Here ontological realists say yes, and ontological anti-realists say no.” [bold by LJ]

Hope this is not too much of a detour. But would we ultimately end up with an infinite chain of ontology, meta-ontology, meta-meta-ontology.., etc?

 

[gentzen]

Why not?

Simply because of complexity. You cannot prepare an exactly equal but opposite magnetic field.

Indeed, I was thinking of the following references provided by vanhees71 in another thread:

Are you referring to the "humpty-dumpty setup"? Then have a look at the marvelous papers by Schwinger et al:

https://link.springer.com/article/10.1007/BF01909939
https://link.springer.com/article/10.1007/BF01384847
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.40.1775

When Humpty-Dumpty had his great fall nobody could put him together again. A vastly more moderate challenge is to reunite the two partial beams of a Stern-Gerlach apparatus with such precision that the original spin state is recovered. Nevertheless, as we demonstrate, a substantial loss of spin coherence always occurs, unless the experimenter is able to control the magnetic field's inhomogeneity with an accuracy of at least one part in 10⁵.

 

[PeterDonis]

I was thinking of the following references provided by vanhees71 in another thread

Ok, so it looks like the accuracy required is 1 part in ##10^5## for controlling the inhomogeneity of the magnetic field. I'm actually a bit surprised that that doesn't appear to be within our current technological capabilities. I wonder if ovecoming that technical challenge is harder or easier than overcoming the challenge of redirecting the electron beams (the way photon beams are redirected by mirrors in an ordinary interferometer).

 

[vanhees71]

A similar objection would apply to experiments with photons, such as Mach-Zehnder interferometers: you can't orient a second beam splitter exactly the same way as the first. Yet such experiments work, within reasonable error bars. Similarly, I would think a second S-G apparatus could be given a magnetic field that was "close enough" to that required to recombine the beams from the first S-G apparatus.

The answer I was expecting to get was more along the lines of the difficulty of redirecting electron beams without changing their state. With photons you can just use mirrors, as in the M-Z interferometer.

Sure, to experiment with photons (optics) is much simpler than with electrons. Of course, the high-precision version of the SGE is the Penning trap!

 

[CoolMint]

A good summary of the current state of affairs regarding the question raised in the OP

https://phys.org/news/2021-06-reality-game-quantum-mirrors-theory.html

 

[haushofer]

It basically matches the original highly hand-waving definition of "measurement" that was used by the original developers of QM.

With the crucial caveat that decoherence doesn't account for what they called "the collapse of the wavefunction" ;)

 

[PeterDonis]

With the crucial caveat that decoherence doesn't account for what they called "the collapse of the wavefunction" ;)

Yes, agreed, decoherence is interpretation neutral, so it doesn't resolve any issues regarding different interpretations of what "collapse" means.

 

[vanhees71]

I find this very simple to resolve. It's obvious that you cannot say qua theoretical dictum what happens to a system in the measurement. It all depends on how the measured system interacts with the measurement device. It can be as what most textbooks call "collapse", i.e., you have a filter in connection with the measurement outcome filtering out systems only with a certain value (or a certain value with some measurement uncertainty) of the measured observable. Then collapse makes sense, and it's explained just by local interactions with the measurement device, including the filter. It can also happen that the measurment destroys the system entirely. E.g., if you have a photon detector based on the photoelectric effect, the photon (system) is entirely absorbed in the measurement process. It's not making any sense to talk about the photon's state anymore, because it's simply absorbed. So there collapse makes no sense at all. Taking the collapse literally as a physical process is also in bold contradiction at least with local relativistic QFT, because there cannot be a causal influence over space-like separated events.

My personal conclusion is that there simply is nothing missing when just taking quantum states and the entire formalism just as a description of the probabilities for the outcome of measurements, given the preparation of the measured system. Indeed as far as I know, there's nothing more to be described than these probabilities, and in this sense the QT description of Nature is "complete".

 

[Morbert]

My personal conclusion is that there simply is nothing missing when just taking quantum states and the entire formalism just as a description of the probabilities for the outcome of measurements, given the preparation of the measured system. Indeed as far as I know, there's nothing more to be described than these probabilities, and in this sense the QT description of Nature is "complete".

What you said above is true if superobservers are unphysical (a reasonable assumption imo).

Irreversibility guarantees an objective character to reality, in the sense that the outcome of a measurement of a quantum system by an apparatus will be reproduced if that apparatus is in turn measured by a 2nd apparatus. This is because there will be a logical equivalence* between i) and ii):

i) Using QM to compute a reduced density matrix ρA for the first measurement, converting it to a Liouville density fA, and then using classical mechanics to compute a reduced Liouville density fB for the 2nd measurement.

ii) Using QM to compute a reduced density matrix ρA for the first measurement outcome, and then using QM again to compute a reduced density matrix ρB for the 2nd measurement.

However, this will not hold if the 2nd observation is a "superobservation" of the first, as irreversibility will not hold.

*See chapter 12 of Asher Peres's "Quantum Theory: Concepts and Methods"

 

[physika]

Without measurements/observers

measurements of WHAT ?

 

[CoolMint]

measurements of WHAT ?

of probabilities.

PS. If reality is virtual, why am I not getting any cheat codes?

 

[physika]

of probabilities.

probabilities of what ?

Fra
1. Reality (i.e what's inside the "back box")

...Without measurements/observers
...That doesn't mean there is nothing...

 

[Morbert]

measurements of WHAT ?

This is, of course, interpretation dependent.

On one end of the spectrum, you have instrumentalist interpretations which say that all QM does is predict the likelihood of stochastic events, given a preparation. Measurements of quantum systems are therefore not true measurements in the sense that the measurement apparatus are not revealing real properties of the measured physical system.

On the other end, you have Griffiths's Consistent Histories which says a measurement is the establishment of a correlation between a physical property of a measured system, and a physical property of the measurement apparatus, even if the measurement apparatus is microscopic. E.g. We can say the centre of mass of a particle passing through a magnetic field measures the spin of that same particle, since a correlation between the two is established (see e.g. a Stern-Gerlach experiment). Measurements are therefore not only true measurements, but they are highly general, and happening all the time.

 

[CoolMint]

probabilities of what ?

Fra
1. Reality (i.e what's inside the "back box")

...Without measurements/observers
...That doesn't mean there is nothing...

Of field strength at a certain location. Fields will generate single outcomes based on probabilities and field strength. Reality is whatever it is.
It's not even a physics question.

 

[Fra]

That's what quantum field theory and the renormalization group are for.
...
Some references for where you are getting your understanding from would be helpful here. What you are saying does not look like anything in actual QFT. In particular, your description of "naked" vs. "dressed" actions seems wrong: that distinction has nothing to do with "the simple observer itself" vs. "including a part of the environment".

Sorry for the slow follow up. I was trying to describe the conceptual motivation for a more general measurement theory starting form post#58.

Yes my arguments and my points for why QFT paradigm is limiting can not be described from within QFT. My points are washed away from step 1, once you adopt the QFT paradigm as there is no such thing as inside observers/agents in QFT from which the full inference takes place.

Conceptually I see the "scaling" going on in renormalisation in QFT, is the observational "resolution" while keeping the observer/agent complexity itself essentially large enough to essentially by unlimiting. The physical observer situation this corresponds to, seems to be to the typical situation in high energy physics where you have a massive classical lab, and you just increase the energy of whatever you use to probe the target with. I suppose this is reasonable for it's original purpose but I think not for a background independent agent based model.

But the theory itself is always encoded in the essentially unlimiting environment which represents "the observer". So the context that provides the encoding capacity of the theory itself, is not scaled.

My objection to this scheme is that it fails to capture the inside view of a more general agent/observer becauase it would require scaling the complexity where the whole expectation is encoded, not just the "probe".

And it's from this inside view that I hope (from my interpretation and agent stance) that simplicity and more naturalness will be found. By naked or bare action, I meant the action as seen from the simple observer itself. The same "action" as described by an external observer, will necessarily come with an embedding that also will require more tuning, but which my be a fictional freedom. But we do not understand how to remove the fiction. The relation between this and the description of this agents interactin with other agents, as inferred from the perspective of third agent is I think necessarily more complicated than trying to scale the same mathematical model by only scaling the parameters. The theory will necessarily in the general caes involve new physics that can't be described jusy by scaling a fixed parameter set. Also gravity seems hard to renormalize anyway, so I think new physics is needed. The idea and motivation is that I think this will constrain theory space and reduce the level of fine tuning.

Mathematically the standard paradigm of the theory is based on a theory space which defines differential equations, and there is an initial value tuning required to explain the present. One "computer" does model everything as an initial value or boundary value problem. But can such a model learn and evolve and be applied to an inside agent? I think clearly not. I think the physica of the "computer" must be part of the game.

The different thinking tool for the foundations, that causes some of the difficulty may be that I think in terms of agent based models, instead of equation based models. A random reference on the notions, See https://arxiv.org/abs/2107.03619 but these models are not that common in physics.

Many problems can be modeled both as system dynamics and as agent interacations, with pros and cons, but if one tries to understand QM as a theory of inference, the agent based model has an angle to this that seems better suited. Just like some people like "geometrization" of physics and has had tremendous success with it, one can see this agent-inference stuff as another trick. The end result will still be system dynamics, but as theory builders one needs some thinking tools to constrain the mathematics. There many inspirations about "physics from inference", but the earlist ones are more like entropic methods, but the more ai-style agent interactions are not very popular. There is https://arxiv.org/abs/0808.1260 and there are various attempts to derive GR from entropic methods https://arxiv.org/abs/gr-qc/9504004, which are of the former type and there are other idea like this https://arxiv.org/abs/1712.01826. None are anywhere near the goal, but have common ideas. I'm not claiming anything here, just trying to add a perspective to the discussion on objectivit. I seem to be one of few here that represent this interpretation.

/Fredrik

 

[physika]

Physics cannot answer such a questions because it is beyond its scope.

The basic question of ontology is ‘What exists?’.

Agree 100%.

 

[Fra]

measurements of WHAT ?

I think the process of answering that question, is a physical process. And the important thing is not the final answer/state, the important thing is the they the process itself self-organizes and learns. One of the reasons for this is also that the statespace in which the answer is encoded, is changing with time. This is why I find the intercommunication and emergence of relations in-between "obsevers" to be important to understand.
https://arxiv.org/abs/1201.2632

/Fredrik

 

[martinbn]

measurements of WHAT ?

Observables.

 

[physika]

statespace /Fredrik

It is good to know that look, that possible option, among the many that are available.

 

[physika]

On the issue of "Reality"

Physics cannot answer such a questions because it is beyond its scope.

“The basic question of ontology is ‘What exists?’.

https://www.science.org/content/art...-you-measure-it-quantum-parlor-trick-confirms

"A quantum particle can exist in two mutually exclusive conditions at once. For example, a photon can be polarized so that the electric field in it wriggles vertically, horizontally, or both ways at the same time—at least until it’s measured. The two-way state then collapses randomly to either vertical or horizontal. Crucially, no matter how the two-way state collapses, an observer can’t assume the measurement merely reveals how the photon was already polarized. The polarization emerges only with the measurement."

Regardless of psychotic existence; horizontal or vertical, it, exists.

...