I "No objective reality" in quantum mechanics?

[PeterDonis]

what are those interactions and how do they scale with energy scale?

That's what quantum field theory and the renormalization group are for.

In the external view one naturally faces a problem of fine tuning.

I don't know what you're referring to here.

renormalzation which hardly is a physical problem but a pathology of our models

This was a common view of renormalization decades ago, but I don't think it is now, nor has it been since Wilson and others developed the modern view of renormalization group theory in the late 1960s and 1970s. The common view now is that, as (IIRC) Weinberg says in one of his articles about QFT, renormalization is something you would have to do even if everything was finite, in order to properly understand what you are actually measuring when you measure something like the mass or charge of the electron.

One idea is that the "naked actions" ie the actions relative the simple observer itself must be much simpler than the "dressed action" seen when including a part of the enviromment. Trying to explain the dressed actions from the naked actions creates a fine tuning problem on the "space of naked actions" this is why this has a low explanatory value.i want to see a learning evolutionary explanatory chain, not a reductionst explanation (that needs fine tuning)

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".

 

[kurt101]

A single qubit won't cause decoherence, so it can't be an "observer".

Why do we need such a theory when the whole point is that small, "non-classical" objects cannot cause "measurements" in the first place, because they do not cause decoherence?

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

 

[PeroK]

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

That is definitely not decoherence!

 

[PeterDonis]

if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

No. The device that measures which slit the electron went through is not "a small non-classical object". The photons are, but the photons are not all there is to the device that does the measurement. Something has to detect the photons and determine whether they indicate that an electron passed through or not. That something will be a large, classical object.

 

[gentzen]

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

The difference between environmental decoherence and more liberal interpretations of the word "decoherence" also created some puzzlement in a previous thread:

Such a phenomenon of suppression of interference is what is called decoherence.

This was somewhat surprising for me, because I expected "decoherence" to be the same as "environmental decoherence".
...
Interestingly, the first version of that SEP entry from 2003 did include "environment" as part of decoherence:

It is this phenomenon of suppression of interference through suitable interaction with the environment that we refer to by ‘suppression of interference’, and that is studied in the theory of decoherence. For completeness, we mention the overlapping but distinct concept of decoherent (or consistent) histories.

In 2012 Bacciagaluppi included sections on decoherent histories in the entry and weakened his statement to "It is this phenomenon of suppression of interference through suitable interaction with the environment that we call ‘dynamical’ or ‘environmental’ decoherence." Finally in 2020, he fully embraced that "suppression of interference" is what is meant by "decoherence" (if further qualifications are omitted).

My impression is that while the inclusion of "decoherent histories" might have been Bacciagaluppi's personal decision, there was also a real shift in the meaning of "decoherence" over time. What you can measure is the "suppression of interference" (within a well defined subsystem), so as control of decoherence became important for quantum computers and other quantum technologies, it made sense to separate the well defined "measurable" concept from the less well defined "explanatory" concept.

My personal conclusion in that previous thread was:

I guess the difference between "irreversible" and "not reversed, maybe accidental, maybe intentional, or maybe because fundamentally impossible" is less important for (most) practical purposes than was once believed. What is important for decoherence in quantum computing is whether it is actually suppressed (by error correction methods), not whether is would have been possible (or easy) to suppress it.
...
To conclude, please just continue to use the word decoherence as you did before, and rely on the context to disambiguate whether it means "environmental decoherence" or just "suppression of interference".

But you mention a similar puzzlement over "measuring which slit the electron went through". A measurement should create a permanent record. But of course, no record will be forever, it just has to be permanent for a long enough period of time. So the question arises why the state of a single qubit should not count as a measurement, in cases where it is sufficiently permanent for the purposes of the experiment. Well, I would personally rather try to avoid this, because the "sufficiently permanent" will typically be implicitly understood, and then you risk unproductive discussions (a la Frauchinger-Renner) with people who don't understand this.

 

[PeterDonis]

A measurement should create a permanent record.

No, that's too strong a criterion, since "record" implies that a human can retrieve the information. There is no such requirement for a "measurement", at least not in the sense that @kurt101 is using the term, namely "something that eliminates the interference in a double slit experiment". There is no need for anything to make a "permanent record" of which slit the electron went through in order for the interference to go away. All that is required is that something is present at the slit for the electron to interact with that causes decoherence. The decoherence does not have to lead to a human-retrievable "record" of which slit the electron went through.

the question arises why the state of a single qubit should not count as a measurement

Because, as I have already stated, a single qubit does not cause decoherence.

 

[gentzen]

No, that's too strong a criterion, since "record" implies that a human can retrieve the information. There is no such requirement for a "measurement", at least not in the sense that @kurt101 is using the term, namely "something that eliminates the interference in a double slit experiment".

Even so the main intention of my post was to help @kurt101 reconcile his understanding with yours, I didn't worry at all how he used the term "measurement" when I wrote: "A measurement should create a permanent record." My intention was to translate the obviously ambiguous term "measurement" into the seemingly less ambiguous synoymous term "permanent record". That synonymous term was supposed to be appropriate both to the everyday meaning of the word "measurement" and to how it is normally used in QM. I didn't notice the "difficulties" of reconciling the normal meaning of the word "record" to how the term "measurement" is used in QM.

On the one hand, a "record" should contain the stored information in a way that it can in principle be retrieved. And because humans are normally able to achieve things that are achievable in principle, this indeed implies that a human can retrieve the information. Now the "record" could be in a place unreachable by humans (for example, it could head away from the solar system with a sufficiently high speed such that no human born on Earth has any chance to every catch it again), but that doesn't contradict the information being "in principle" retrievable by humans.

However, there is a sense in which the information stored in the "records" in typical QM experiments like the Stern-Gerlach experiment cannot be retrieved by humans, not even in principle. The results of the spin measurements do impact where the atoms will land. But in a typical SG experiment, it will be impossible to reconstruct the exact sequence of up/down measurement results (or even a reasonable approximation to it), even in principle. For some CERN experiments, such a reconstruction of an exact sequence might exist temporarily, but even here most are discarded immediately, because the generated data volume is just too big to be stored. So the "records" are gone long before even a single run of an experiment is over, and hence using the term "record" for such a "measurement" is indeed inappropriate, even without the qualifier "permanent".

There is no need for anything to make a "permanent record" of which slit the electron went through in order for the interference to go away.

Making the interference go away is very easy. Even too easy. Calling everything which makes the interference go away a "measurement" is certainly a bad idea. I don't think that it captures the way that the word "measurement" is normally used in QM.

All that is required is that something is present at the slit for the electron to interact with that causes decoherence. The decoherence does not have to lead to a human-retrievable "record" of which slit the electron went through.

Because, as I have already stated, a single qubit does not cause decoherence.

A single qubit does not cause environmental decoherence. But it can make the interference go away. I guess I see why you think that this answers "the question ... why the state of a single qubit should not count as a measurement". But the trouble is that some people do count it as a measurement. So I tried to come up with a stronger argument for convincing kurt101 to not follow those people.

 

[PeterDonis]

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

How?

 

[Morbert]

This might be related: A photon incident on double-slit apparatus with no detectors at the slits has a wavefunction that looks like $\frac{1}{\sqrt{2}}(|\psi_A\rangle + |\psi_B\rangle)$ after the particle has passed through the slits (labelled $A$ and $B$). If an optical crystal is placed behind the slits, such that the photon is transformed into a signal and idler photon pair, we instead get a wavefunction like $\frac{1}{\sqrt{2}}(|\psi_A\rangle_s|\phi_A\rangle_i + |\psi_B\rangle_s|\phi_B\rangle_i)$ This makes the interference pattern go away, even though no correlation with the collective degrees of freedom of some macroscopic apparatus has been established. I.e. If we compute $|\langle x|\Psi\rangle|^2$, it will be the idler photon terms, and not any detector terms, that zero the interference.

 

[Delta2]

The only objective reality imho is that there seem to exist conscious observers . Everything else (not only information and conclusions regarding QM or QFT but also regarding daily life such as that we are human beings with flesh and bones) is information in the mind of observers. But we can't be sure what exactly the observers are cause that is also (self) information in the mind of the observers.

 

[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".