Quantum mechanics and our part in creating reality
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Rovelli is a proponent of what he calls the "relational interpretation" of QM, which basically states that particles only have properties when they interact with other systems (and may be better thought of as events than persistent objects). Entanglement is a general phenomenon that describes the relationship between two interacting systems and a third which hasn't interacted yet. Put another way, something can be real in relation to A but not to B. However, the correlation that entanglement guarantees ensures that previously isolated systems will see the same things if they do eventually intersect.
It's different from QBism because there is no special privileging of human observers; all physical systems exist in direct interaction with some systems, are in superpositions in relation to others, etc.
[1]: https://bookshop.org/books/helgoland-making-sense-of-the-qua...
[1]: https://blog.theangry.dev/2017/01/05/ron-garrets-the-quantum...
I'm not familiar with Garret, but Rovelli's book contains a great discussion of the "quantum eraser" experiment in that blog post, where quantum interference causes a split laser beam to recombine in different ways whether or not a detector is present. This is something you can observe with tabletop equipment apparently, a pretty visceral demonstration that quantum phenomena are a real part of our world.
A good physics undergrad could assemble this and demonstrate it successfully in a day. It's really a Mach Zender interferometer (https://en.wikipedia.org/wiki/Mach%E2%80%93Zehnder_interfero...) which is one of the nicest designs for doing quantum information experiments.
The laser they use isn't anything super-special, although it isn't like what you'd get if you bought a laser pointer or a cheap laser engraver. The beam size is very fine and the beam shape is round. Most of the other parts are fairly generic but precisely made. The most exciting part is the beamsplitters, https://en.wikipedia.org/wiki/Beam_splitter since they are key to assembling the interferometer itself.
Even so, an effect that can be demonstrated with tabletop equipment in a lab seems more intuitively real to me than something that requires a giant particle accelerator...
100 or 200 years ago a more or less average person could set up cutting-edge scientific experiments with basic equipment and patient observation – this was the modus operandi of Michael Faraday[1] for example. Sadly it seems like those days are mostly behind us now.
My friend did the experiment in an afternoon Physics lab (princeton), almost all of of this is due to innovations in precision manufacturing and material science.
First, the stuff we do with quantum mechanics just seems sort of... really obviously objective? It's not like Bell tests work for me but not for you, or quantum computers factor numbers for me but not for you. If I sell you a machine to produce a specific superposition, and it produces the wrong one, you can tell. Yes you need to run it multiple times and do statistics, but ultimately anyone can check that the machine is doing the wrong thing.
Second, two agents can't actually disagree on what the superposition of a state is, to any appreciable degree, without one of them being provably irrational (modulo some weak-sauce assumptions we take for absolute granted in classical mechanics). [1]
Third, note that (w.l.o.g) diagonally polarized photons are superpositions of horizontally and vertically polarized photons. This suggests that, in an interpretation where superpositions are less real than classical states, slightly rotating a polarizing filter radically changes the conceptual machinery used to understand a photon passing through the filter. That seems... silly? There is a rotational symmetry in the system that should be natural in the model (e.g. see how special relativity reifies Lorentz boosts).
Fourth, although this is ironically subjective, I've never read an epistemic explanation of some quantum phenomenon and thought "Ah, that makes it clearer!". But that has happened with collapse interpretations, with path integral interpretations, and with many-worlds.
Superpositions are statistical distributions. Only statistics are wholly predictable. Individual events are wholly unpredictable.
You can have two statistical views of the same experiment in which each individual event is different but the statistics agree.
You can have a distribution of views in which events are somewhat different to varying degrees and the statistics still agree.
Each view will experience itself as unique in specifics while agreeing that the distribution of events is identical.
That doesn't solve the problem.
It's hard to get random processes to define an identical distribution without having classical mechanics as a foundation.
Basic QM hand-waves this away with "Here's some math that defines the distribution and..." But that's like "Starting with this universe we can..."
It so happens that the statistics correlate spatially with various field potentials. Which is interesting, but... Why is there a distribution at all? Why are there creation/destruction operators? Where does the "code" that defines this mechanism live? How do particles know they should follow it?
Do particles even exist, or are they just something random and constrained a field does every so often? How do fields know the physical configuration of an experiment has changed and sometimes that information appears to travel FTL? (Even though it can't be used for signalling.)
How does a particle keep track of how many other entities it's entangled with and in what ways? Where does that information live? (Bell suggests it's not inside the particle itself. So where is it?)
Is all of this shaped by some kind of hidden causal propagation mechanism which also defines how relativity works?
And so on. A complete explanation would answer all of these questions - and others - with ease. Clearly we're nowhere near that.
I'm aware of the distinction.
> How does a particle keep track of how many other entities it's entangled with and in what ways? Where does that information live? [...] A complete explanation would answer all of these questions - and others - with ease.
I disagree with this framing. Fundamental laws of physics are always going to have unanswered questions of this type. Given any set of rules you can ask "But what explains those rules? What are they built out of? How are they enforced?". Sometimes those questions will have answers and lead you deeper, but for truly foundational laws you'll be wasting your time. It'd be like asking "Where is the true platonic number 2 located? Is it in Canada?".
You can ask the same questions of classical mechanics, of course. We tend not to because it agrees with our intuitions, but you can. For example, where would a classical particle store its velocity? For that matter, where would it store its position? It's circular to say it stores its position where it is! Clearly a "true" theory of classical mechanics would answer these very important questions.
Concretely, there are a variety of ways of writing programs that act like quantum mechanics, that differ wildly on how the state is represented. This detail is simply not pinned down by the postulates of QM. That being said, what all these programs do have in common is that they are actually tracking information related the state; that the state is ontic.
So pick your favorite state representation: state vector, density matrices, Feynman paths, whatever, they all work! That doesn't mean they're describing things that aren't real, it just means there's many ways of correctly describing reality; such convenience!
Every once in awhile I'll run into someone pointing out the philosophically fraught underpinnings of assuming reality is real or whatever, and I basically won't care because my goal is to be effective; not to be Descartes-level-certain about everything.
An example of something that would make me care is if QBism contained some key conceptual trick that made problems easier, and gradually many papers started using it because of this advantage. Or, of course, if there was an experiment distinguishing between interpretations.
1. In practice, experimental physicists can and do disagree all the time about what their apparatuses are doing, what the results mean, and so on -- at least, they do at the beginning of the experiment. By the end, they have gone through a process that results in agreement, which allows them to write a paper together declaring things in an objective manner. So it is important to distinguish between the "inputs" to scientific practice (our individual, subjective initial guesses about what is going on) from the products of that practice (effectively objective statements). QBism interprets wavefunctions not as products but as inputs: we need to start with a prior "best guess" about the probabilities, and it its the interactive process of updating our subjective priors in light of data that leads us to converge on a single wavefunction, which then attains an objective status in the sense of being the same for everyone. QBism's point is that this whole process of "objectification" is itself a process to be analyzed, not taken for granted as if the objectivity were there from the beginning.
2. The PBR theorem (which you cite) doesn't bite QBism, and PBR themselves admit as much in their paper. See point 8 in [https://arxiv.org/abs/1810.13401].
3. What you say would be silly, if QBism treated classical states as "more real" than quantum states. But they don't.
QBism treats quantum and classical states on the same footing: both are sets of subjective probabilities about the outcomes of possible measurements you could do. So classical states are probability distributions, and probabilities in QBism always represent subjective degrees of belief of an agent (following the de Finetti/ Ramsey school of probability theory). The difference between classical and quantum states in QBism does not lie in what the states ARE, it lies in the rules that tell you how to compute one state given your subjective beliefs about another state. And those rules are objective in QBism; as objective as the rule that says probabilities have to add up to one.
4. Doubly ironically then, this is probably the one point raised by critics that hits the QBists hardest. QBism can bring clarity to many issues (I find it handily resolves the measurement problem and non-locality of "collapse" - see eg.[https://arxiv.org/abs/1311.5253]) but there are still plenty of physical phenomena that don't yet have a neat QBist explanation.
What do you mean by “classical states”?
In your example diagonally polarized photons are not more “a superposition” than any other photon. For a pure state like that one being or not “a superposition” depends on the basis, it’s not a property of the state.
Analogy: if simultaneity is absolute, why is it so easy to confuse people about what happened at the same time by changing my speed?
Superpositions are not “less real” than basis states (if such a distinction makes sense) within a theory (psi-epistemic or psi-ontological) as far as I understand.
Core positions of QBism:
1. All probabilities, including those equal to zero or one, are valuations that an agent ascribes to his or her degrees of belief in possible outcomes. As they define and update probabilities, quantum states (density operators), channels (completely positive trace-preserving maps), and measurements (positive operator-valued measures) are also the personal judgements of an agent.
2. The Born rule is normative, not descriptive. It is a relation to which an agent should strive to adhere in his or her probability and quantum state assignments.
3. Quantum measurement outcomes are personal experiences for the agent gambling on them. Different agents may confer and agree upon the consequences of a measurement, but the outcome is the experience each of them individually has.
4. A measurement apparatus is conceptually an extension of the agent. It should be considered analogous to a sense organ or prosthetic limb—simultaneously a tool and a part of the individual.
https://en.wikipedia.org/wiki/Quantum_Bayesianism#Core_posit...
I'd recommend reading the wiki page since this article seems to say a lot of words yet not say much.
But i think physicists need to open up a little more. If QBism or other interpretations where reality is subjective were really true, it would probably lead to some "we got it right initially" by some religious organisations, but honestly, that shouldn't stop physicists from pursuing such theories. I think it can be a win-win scenario in the end.
Except they don't - the science of quantum mechanics with its practical probabilistic "interpretation" has worked pretty well so far. As far as science is concerned, that is.
Most importantly, nothing I can see in the QBist pages has anything to do with actual experimental work. It's a framework for viewing what the mathematics of human theories of QM "mean" and how to interpret that. However, nothing of what they propose is required to explain what we observe, experimentally.
More generally, no "interpretation" of QM is required to apply the theory in generalizable, predictive ways, so all this work isn't really helping us move science or engineering forward.
>> Which proposals are you referring to exactly?
> All of its interpretations of QBist "Core Position" on the wikipedia page.
You're making very broad claims here. Whether you find QBism plausible or not, it's certainly not "scientific garbage" and also fully compatible with experimental data (as is any other interpretation of QM).
> However, nothing of what they propose is required to explain what we observe, experimentally.
So you would say that orthodox views of quantum mechanics (Copenhagen, shut up & calculate, many worlds (as it's becoming increasingly popular)) explain what we observe?
> More generally, no "interpretation" of QM is required to apply the theory in generalizable, predictive ways, so all this work isn't really helping us move science or engineering forward.
I disagree, once again. Have you actually deeply looked into the issues with QM? Many of them are linked to questions that come up in quantum gravity, so I would argue that getting QM right would be a very good first step in making scientific progress.
That seems inadequate to explain Bell nonlocality.
I hardly think that's a fair description of what he's claiming. This sounds almost like the textbook example of the motte and bailey fallacy.
Could a physicist please clarify this?
When does "collapse" happen in principle? There's no clear answer. Yet, when you observe a system yourself, you are free to say it has one result (at least, in "your world / branch" if we use MWI lingo). So there is (possibly) a sense in which you are the only observer -- at least, w.r.t. the place you think of as "the world."
(And of course when we say "you observe," we mean "you experience". If only your toe measures it, you may treat your toe as an external measuring device. This is how "consciousness" enters the picture, though nobody agrees on what exactly it means.)
But from the viewpoint of the toe would it have collapsed?
Extrapolating that, why isn't your consciousness just another measuring device that's become entangled with the system?
Similarly, your consciousness could be considered just another measuring device from the perspective of someone outside the system. But by virtue of being "outside the system," their results have no bearing on you either.
You are the point at which "the magic happens" in your world -- which is just the place you normally call "the" world. It is the point at which neither you nor anyone else you can communicate with (i.e., no one else in "the world") can demonstrate interference, even in principle.
It does not happen at all. There is no measurable distinction between a particle whose wave function has collapsed and one that hasn't. If there were a discernable difference the "spooky action at a distance" when an entangled pair is collapsed could be used for faster than light communication.
>> When one system measures another, there is no collapse, only entanglement.
This is the first time I've seen this explanation. To me it feels like an attempt at resolving the issue, but I think not.
The right answer AFAICT is that we must embrace non-locality. But that's still another sort of punt.
Also, this surprises me (assuming you studied QM formally):
> This is the first time I've seen this explanation.
Entanglement is basically the definition of measurement (in this context).
> As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human intervention and that this can be shown.
When pop sci treatments explain that any physical system interacting with the apparatus "measures" it, they are just talking about what you call "pre-measurement." This causes a lot of confusion for students.
Suppose you are making a universe. You want to ensure that the universe you make is interesting, i.e. contains consciousness. So you set up the rules like QM: everything exists in probabilistic superposition until a consciousness observes it. One of the superpositions contains a consciousness, and that collapses the wavefunction guaranteeing consciousness in your universe. It's a way of using survivor bias in your favor.
Disclaimer: not to be taken seriously.
Disclaimer #2: Also, coincidentally, not related to poster drdeca. Unless he is me from another superposition and we haven't collapsed yet because you all are not as conscious as you think.
I suspect one could think of QBism as a theory of rational belief for agents in a context where details of quantum mechanics are highly relevant.
Just as bayesianism is a theory of rational belief more generally.
Perhaps we are using “theory of” in different ways?
How do you know?
I agree! However:
> because the vast majority of data we have excludes it
No it doesn't. In fact, there is not a single experimental result that rules this out and I doubt one could ever devise such an experiment in the first place.
All experimental results are consistent with the observer modeling the universe with the Schrödinger equation (i.e. with unitary time evolution / without collapse) and assuming that a measurement by said observer makes the wave function collapse to the appropriate eigenstate.
What experiment shows this?
As you know, in the classical double-slit experiment, the screen is the only detector, and launching particles even one at a time through the slits produces the interference pattern.
However, if you add a second detector in one of the slits capable of measuring if a particle passes through that slit or not, the interference pattern on the screen disappears, regardless of how many particles you send through.
This is normally taken to mean that the slit detector collapses the wave function (in Copenhagen parlance); or that one particular state of the particle becomes entangled with the initial detector, which is already entangled with the screen, to there is no more constructive interference to be seen (in Many Worlds terms).
Here is a more complete explanation:
https://phys.org/news/2011-01-which-way-detector-mystery-dou...
The delayed choice quantum eraser is a much more complex experiment.
From what I've read, the existence of "collapse" of the wave-function is not established, and not even widely accepted as more than a useful computational tool by practicing physicists.
Full disclosure that I’ve never really been able to pin down the QBist notion of an agent myself and do not find QBism (or any such “instrumentalist” interpretations” of QM) appealing.
Do you have a reference for that?
Long story short, QBism inherits "agent" from Bayesian decision theory. In that context, we talk about gamblers making bets on the outcomes of dice rolls. In QBism, we talk about scientists making bets on the outcomes of measurements. What exactly can be a "gambler?" Can a dog gamble? What about a trained dog? Or a flea? The answer is: if the shoe fits, wear it. Same goes for QBism. If a dog walks into a lab, lights up a pipe, scribbles equations on the blackboard and starts aligning beamsplitters, then QBism regards him as an agent.
A personalistic interpretation requires a "person". Would a rock or a cell phone qualify as a person? I'm not even sure the authors of that reference would accept a dog, despite saying the concept is extremely flexible (they discuss agents "social" agents but not "dumb" agents).
It's not clear to me that the mathematical description can exist independently of the agent that has some knowledge, for some definition of agent. Even if it can be make to work with no-one's "abstract knowledge" what would be the physical meaning of that? (Not that the physical meaning when agents are present is clear, to be fair.)
Take the electron double-slit experiment. Suppose no human checks which slit the electron goes through, but the "which-path" information leaks out into the environment somehow. The interference fringes will be destroyed anyway (called "environmental decoherence").
This is usually framed by saying "oh look, the environment measured the system". In fact, what happened is that (1) you prepared what you believed to be an electron beam in a coherent superposition; (2) you left it open to the environment; (3) you used quantum theory to model the system-plus-environment and predicted that you would not see interference fringes when you check; (4) YOU observed the dots on the glass plate and confirmed that there were no interference fringes. The whole story is told in terms of what YOU believed, expected, and finally what you saw. That is QBism's whole point.
Put it another way: in QBism wavefunctions just ARE sets of probability assignments to certain actual events that could be observed. And probabilities are human constructions: we invented them to help us reason about our expectations. So, without humans (or other beings capable of reasoning about expectations), there are no probabilities, hence no wavefunctions either. But stuff still happens, even when there's nobody around to see it or assign probabilities to it.
The key conceptual leap is separating "wavefunction" completely from "reality". There's more to quantum physics than just the wavefunctions -- the reality lies in the events that happen, not in the quantum state itself.
It is, in principle, impossible to determine this, because eventually humans will see the output of the experiments. However, we can say that if humans are special, the only things able to cause “measurement”, then our specialness also gives us time travel powers that can only be used for “measurement”, and Occam's Razor says that's probably nonsense.
Why? Because the Quantum Eraser Experiment comes out as it does?
We don't have an accepted theory on what "measurement" is in Quantum Mechanics yet.
I like that the writer is up front in how all we really know right know is ‘shut up and calculate’ and wish that it had been taught with such honesty when I was taught it in school, rather than basically doing everything from the copenhagen interpretation and pretending that that isn’t fundamentally philosophically weird. The observer and measurement in the ‘shut up and calculate’ school is much more believable than in the copenhagen school where people (or at least things people do, like taking measurements and observing observables) are suddenly jarringly special for the first time in a physicist’s training.
Basically the initial approach was ontological: science is about what can be measured, when quantum properties are measured we need to apply the Born rule to predict outcomes, it makes no sense to discuss the scientific properties of what is by definition not measured, so shut up and calculate.
Later, the approach was more metaphysical: since our math doesn't work if we assume that particles have definite properties, we conclude that particles don't have definite properties; instead, when we measure their properties in a particular basis, the particles randomly acquire some definite property with a probability corresponding to the amplitude of the wave function.
I guess all I'm saying is that, in hindsight, I would have done so much better with a clearer delineation of 'this part over here is an interpretation question and isn't settled, but this other part over there is math that is settled empirically' rather than being taught 'here's an interpretation that's true and here's the math for it.'
Since Wigner was a Wise One (https://en.wikipedia.org/wiki/Wigner%27s_theorem) people took the idea seriously, but these days, most physicists will simply ask you to show them a comprehensible and reproducible experiment that demonstrates the requirement of a human observer and reject your belief based on the absence of data and occam's razor.
Even more, there are many interpretations of QM where the classical world is only an illusion of the observer essentially.
https://philarchive.org/rec/ELLWDC
“Why Delayed Choice Experiments do NOT imply Retrocausality” - David Ellerman
“There is a fallacy that is often involved in the interpretation of quantum experiments involving a certain type of separation such as the: double-slit experiments, which-way interferometer experiments, polarization analyzer experiments, Stern-Gerlach experiments, and quantum eraser experiments. The fallacy leads not only to flawed textbook accounts of these experiments but to flawed inferences about retrocausality in the context of delayed choice versions of separation experiments.”
I'm gonna trust the experimentalists on this one. Not also I didn't say retrocausality, I said intuition about temporal causality, which is distinct.
I don't think there is any mystery.
Another reference: https://arxiv.org/abs/1905.03137
"The 'Delayed Choice Quantum Eraser' Neither Erases Nor Delays" - R. E.Kastner
She has published books on the foundations of quantum mechanics (not a experimentalist, though...).
You're watching a bunch of people with strong opinions about what QM theory implies for the real world. I come from a different direction: I'm recognizing that most of my intuitions about physics, which come from large particles (classically modelled atoms), are just completely and totally useless. And that ncludes the temporal ordering of events.