#interpretations-of-quantum-mechanics — Public Fediverse posts

Everything is a quantum wave?

In the last post, I discussed Amanda Gefter critique of Vlatko Vedral’s view that observers have no special role in reality. Conveniently, Vedral published an article at IAI discussing his view: Everything in the universe is a quantum wave. (Warning: possible paywall.) Vedral puts his view forward as a radical new interpretation of quantum mechanics.

As a quick reminder, the central mystery of quantum mechanics is that quantum particles seem to act like waves, including portions of the wave interfering with itself, but when measured, behave like tiny localized balls. This is known as the measurement problem.

There are numerous interpretations of what’s happening here. But they seem to take one of three broad strategies. The first simply rejects that the waves are real, instead insisting that they are only probabilities, albeit probabilities which evolve deterministically and interfere with each other. In other words, it’s all happening in our mind. In its stronger incarnations, this has idealist or semi-idealist aspects, claiming that observation or interaction creates reality. These are the approaches in the epistemic versions of the Copenhagen Interpretation and its descendants, like QBism and RQM (relational quantum mechanics).

The second strategy is to add new structure to wave mechanics. Due to Bell’s theorem, these additions must be non-local in nature, that is, they must involve “spooky” action at a distance. The ontic version of Copenhagen takes this approach when it adds a physical collapse, as do its variations and descendants like consciousness-causing-the-collapse and other objective collapse theories. Another version of the second strategy is what are historically called “hidden variable” approaches, like Bohmian Mechanics (pilot-wave theory), where there is both a wave and a particle the entire time, with the wave guiding the particle.

The third strategy is to accept the mathematical structure of quantum theory as a full account, or one only requiring a few ancillary assumptions. This became easier with the development of decoherence theory in the 1970s, an extrapolation of quantum wave mechanics, in essence quantum entanglement en masse, that explains why quantum interference disappears at larger scales. It’s the approach Hugh Everett proposed, which eventually became known as the many-worlds interpretation.

And it’s the strategy Vedral uses for his interpretation, which he characterizes as “many-worlds on steroids.” Although he dislikes talking in terms of other worlds, noting that the classical worlds are only a small slice of the possibilities. He prefers to talk in terms of one world but with quantum mechanics being universal, applying at all scales.

Vedral makes a point I made in the last post, that under this universal quantum waves approach, an observation is just two quantum systems becoming entangled, that is, becoming correlated in certain ways. A reminder: entanglement is when two quantum systems have each of their states in superposition become correlated with each of the states in the other system. In other words, for each state in the first system, there is a correlated state in the second. The two systems are now part of the same wave function.

Vedral notes this could be characterized as the quantum particle observing the measuring device as much as the device is observing it. In this view, entanglement is what the apparent collapse looks like from the outside, and collapse is what entanglement looks like from the inside. So contra Gefter’s stance, there’s no special role for observers, at least unless by “observer” we mean everything.

As I noted in the last post, I like Vedral’s approach here of focusing on the physics rather than getting into multiverse language, which as I’ve noted before, often ends up being a distraction. But it’s hard for me to see how his view is radically different from the standard Everettian one. It’s worth noting that Everett’s original proposal was a theory of the universal wave function, essentially the “everything is a quantum wave” view Vedral is advocating. Everett didn’t talk in terms of a multiverse. It was Bryce DeWitt in the 1970s who characterized that way, although Everett saw it as just an alternate way of describing his view.

One difference from contemporary many-worlds views, which Vedral shares with Everett, is that the quantum nature of macroscopic objects is not beyond testability. Everett reportedly maintained that the quantum states of macroscopic objects were in principle detectable. I haven’t read Vedral’s book, but it sounds like a large part of it is finding ways to test his view.

This seems resonant with the progress being made in experimental research, where tiny macroscopic objects can now be held in a quantum superposition, which is putting increasing pressure on ontic collapse theories. And Vedral mentions the ongoing efforts in quantum computing, which is stress-testing quantum theory in ways scientists of earlier decades could only dream of. In the end, we need data, and these efforts are providing more of it.

As a minimalist Everettian myself, I find a lot in Vedral’s discussion compelling. But as he notes in his article, the various interpretation camps are like entrenched armies in World War I, unlikely to be moved except by the strongest experimental results. Even then, I suspect Max Planck’s observation that science moves forward “one funeral at a time” will likely be true here as it always has.

What do you think of Vedral’s views? Does the idea of everything being a quantum wave make sense? Or are there difficulties both he and I are overlooking with this approach?

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Does reality require observers?

Amanda Gefter has an article at Nautilus a couple of you have asked me about: Reality Exists Without Observers? Boooo! The title is an accurate summary of her thesis. Gefter is responding to a book by Vlatko Vedral, where he reportedly argues for a reality that doesn’t require observers. In terms of quantum mechanics, Vedral is an Everettian, although he seems to downplay the many-worlds aspect, focusing on the physics. He touts what’s often taken to be an advantage of the interpretation over Copenhagen, that it doesn’t require any special role for the observer.

Gefter’s stance is that this can’t be done, that any attempt to do it inevitably sneaks the observer back in. She also implies that most discussions about Neils Bohr, Werner Heisenberg, and the other thinkers behind Copenhagen, strawman their positions. Since “the Copenhagen interpretation” is actually a diversity of views held by many early quantum scientists, she’s probably not entirely wrong. But arguing about what the Copenhagen interpretation is or isn’t strikes me as just as problematic.

Even if we restrict ourselves to just Bohr’s ideas, the man’s writings are infamously opaque. When trying to decipher his views, it seems possible to come away with a variety of ideas. When I’ve read about them, he comes across to me as an instrumentalist, although one who doesn’t want to admit it. Many science historians argue he was a Kantian, or maybe a neo-Kantian. Within the bounds of this discussion, I’m not sure how significant these distinctions are. What seems clear is he doesn’t take quantum theory to be telling us about reality, only our interactions with it.

When someone starts arguing for an observer centric reality, they can mean at least a couple of different things. One is epistemic, that our knowledge of reality only comes through observers. Bohr reportedly included non-conscious observers in this. But with that stipulation, the view doesn’t seem particularly radical. Another way of saying it is that all information must come through information gathering systems, which doesn’t sound nearly as profound.

But an observer centric reality can also be a stronger claim, an ontic one, that says conscious observers construct reality. The strongest claim of all is that reality doesn’t exist until we consciously observe it, that the observation itself brings it into existence. This is either idealist or solipsist territory.

It’s important to understand that even under the epistemic view, observation has effects on quantum systems. We can observe a supernova twelve million light years away without affecting it. Or we can observe the flow of a river without meaningfully affecting it. In these cases, there’s already enough interaction happening between the system and its environment that we can learn from the effects of those interactions that reach us.

However, we can’t observe a quantum system without interacting with it, and interaction entangles the observer and the observed, changing both. For the quantum system, that typically results in at least decoherence, and in many interpretations, all but one of the possible outcomes disappearing.

Which one of the above is Gefter claiming? I’m not sure, and that, unfortunately, is all too common when trying to parse arguments for this view. If forced to guess, I’d say she’s agnostic on the distinction, as this discussion about the moon seems to show.

Let’s put this moon thing to rest. It’s true. We can’t say the moon is there if no one’s observing it. Neither can we say that the moon’s not there if no one’s observing it. It’s not as if the sky is empty until someone gazes upward and a moon suddenly pops into existence. It’s that we can’t say anything about the moon as an independent object, because quantum theory doesn’t grant us independent objects, only measurements that we can slice into moons.

Not that Copenhagen is Gefter’s preferred interpretation. She actually favors QBism. Historically the name meant quantum Beysianism, in the sense that quantum theory provides degrees credence in various outcomes. It’s a view which focuses on the subjective experience of the experimenter. Or at least that’s how I understand it.

But again, the question becomes, is this just an epistemic view, or an ontic one? It seems to depend on which QBist you ask. On the one hand, it could be seen as a straight instrumentalist approach to quantum theory, a reification of the “Shut up and calculate!” attitude. Interestingly, the originator of that phrase, David Mermin (not Richard Feynman), signed on to QBism at some point.

Many QBist proponents resist the instrumentalist label. Which in turn often leads to accusations of solipsism, which they also resist. As I noted above, this starts to sound a lot like idealism to me, although it’s not clear that’s what they mean either. In the end, most physicists seem to regard QBism as an epistemic interpretation. (I’ve already done a post on why epistemic approaches don’t work for me.)

But what about the ontic view of an observer centric universe? If you already lean toward some form of ontological idealism, then this may well be a natural conclusion. But I don’t think there’s anything in the physics that drive it. A lot of this type of discussion seems to ignore the lessons from quantum computing, where engineers struggle with systems that decohere all the time, much earlier than they would prefer, and with nothing we’d normally call an “observer” driving it. (Unless we say the environment is the observer, but then “observer” seems to lose all distinctive meaning.)

If you think about it, this is no different from the classic double-slit experiment. If we put a polarizing filter at one of the slits, no conscious agent gets the information on which slit the particle goes through any sooner. What conscious agents do see is a change in the results on the back screen, and then infer what happened at the slits.

So the epistemic point about observers seems valid enough. I haven’t read Vedral at length, but I’d be surprised if he disagreed. But the ontic one doesn’t seem particularly well motivated, at least unless your metaphysics already push you in that direction.

But maybe I’m missing something? Is there an in-between ground between the options I listed? Or evidence for the ontic version I’m overlooking?

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What physicists believe about quantum mechanics

A few years ago David Bourget and David Chalmers did a follow up survey to the 2009 one polling philosophers on what they believe about various questions. One of them was quantum mechanics, particularly the measurement problem and its various interpretations. Over the decades there have been surveys of physicists themselves on this question, but most, if not all, were with a very small sample size, usually only the attendees at a particular conference.

As part of the Quantum Centennial (the celebration of 100 years of quantum mechanics) Nature has done a fairly large survey of the community of quantum researchers with over 1100 respondents. The results are interesting, although not particularly surprising.

Copenhagen still comes out on top with 36%. It’s interesting that it’s stronger with experimentalists than with theorists (half vs a third). I suspect the experimentalists are hewing to a very pragmatic version of the interpretation. Which highlights a concern that the term “Copenhagen interpretation” means different things to different people. The article acknowledges this, noting that 29% of those who selected Copenhagen favored an ontic version of the wave function vs 63% who came down epistemic.

15% are Everettians (or “consistent-history” advocates, who I suspect object to being lumped in with the many-worlders), 7% Pilot-wave, 4% Spontaneous collapse, 4% Relational Quantum Mechanics, and a smattering in other views.

Overall 47% of respondents see the wave function as just a mathematical tool, with 36% taking a partial or complete realist take (my view), and 8% taking it to only represent subjective beliefs about experimental outcomes.

45% see a boundary between classical and quantum objects (5% see it as sharp) while 45% don’t (my view).

Just before the paywall, there is a question about the observer in quantum mechanics, with 9% saying it must be conscious. Another 56% said there had to be an observer, but that “observer” can just be interaction with a macroscopic environment, and 28% arguing that no observer at all is needed. (I think interaction with the macroscopic environment and the resulting decoherence is key, but it seems misleading to call that environment an “observer”.)

All interesting. Of course, how popular or unpopular a view is has no real bearing on whether it’s reality. Prior to Galileo’s telescopic observations in 1609, an Earth-centered universe was the most popular cosmology. Only a miniscule handful of astronomers accepted Copernicus’ view about the Earth orbiting the sun. Until the quantum-measurement equivalent of the telescope comes along, all we can do is reason as best as possible with the current data.

The results here are interesting to compare with what the philosophers thought on the Bourget-Chalmers survey. On quantum mechanics, philosophers were 24% agnostic, 22% hidden variable theories, 19% many-worlds, 17% collapse, and 13% epistemic. Once we take into account all the various forms of “Copenhagen interpretation”, these seem in a similar ballpark, except that philosophers are more open to hidden variable approaches. (It may be easier to favor hidden variables if you’re not the one who has to find them.)

My own view comes down to a preference for structural completeness (or at least more structurally complete models), which to me currently favors a cautious and minimalist take on the Everettian approach (as I described a few months ago). However, my credence in this conclusion is only 75-80%. That the survey indicates most physicists aren’t super confident in their own conclusions here makes me feel better.

This reminds me of a new approach that Jacob Barandes has been promoting on various podcasts (see this recent Sean Carroll episode as an example). Barandes calls it Indivisible Stochastic Quantum Mechanics. I won’t pretend to understand exactly what he’s trying to accomplish with it, but it involves rejecting the wave function completely, and replacing it with something more stochastic from the beginning. Which strikes me as less structurally complete than the wave function, and so a move in the wrong direction. But maybe I’ll turn out to be wrong.

Anyway, now we have a firmer idea of where the physics community currently stands on quantum interpretations, or at least a firmer one than we did before. How would you have answered the survey questions? (There’s actually a small quiz in the article which is worth taking to see the logic leading to particular interpretations.)

#InterpretationsOfQuantumMechanics #Philosophy #PhilosophyOfScience #Physics #QM #QuantumMechanics #Science

Is quantum immortality a real thing?

In discussions about the Everett interpretation of quantum mechanics, one of the concerns I often see expressed is for the perverse low probability outcomes that would exist in the quantum multiverse. For example, if every quantum outcome is reality, then in some branches of the wave function, entropy has never increased. In some branches, quantum computing doesn’t work because every attempt at it has produced the wrong result and people have concluded it doesn’t work. In other branches, you as a macroscopic object might quantum tunnel through a wall.

Of course, for enthusiasts, this comes with a hopeful aspect. Because in some branches, you would go on living indefinitely, no matter how improbable it might be. Hugh Everett himself was reportedly a believer in quantum immortality and so had little concern about the unhealthy lifestyle that led to his early demise in this branch. The idea is that if every outcome happens, then there are versions of you reading this that will live until the heat death of the universe.

This is vividly illustrated in the infamous quantum suicide thought experiment. One version described by Max Tegmark involves rigging up a gun to fire if a certain quantum event happens. Say the quantum event has a 50% chance of happening in any one second. You then put your head in front of the gun and begin the experiment. In half of all worlds where you begin the experiment, you die in the first second, but you go on living in the other half. In half of that remaining half you die in the next second, but go on living in the other half.

For you as the experimenter this goes on indefinitely with increasingly improbable outcomes leading to your survival. Of course, in virtually all worlds you leave behind grieving friends and family who are less convinced. But for you subjectively, if many-worlds is reality, you continue living until the experiment ends.

(Before getting too comforted by the possibility of quantum immortality, it’s important to remember that this is more of a side-life than an afterlife. Most of the versions of you will still experience an approaching death. It’s also worth noting that a you a million years from now would likely have evolved into something utterly strangle and unrecognizable to the you of today. And there’s no guarantee this ongoing existence would be pleasant. Indeed, under many-worlds, some would inevitably be hellish.)

One question that often comes up in discussions about this is whether reality allows for these infinitesimally low probability outcomes, or whether there is some inherent minimal discreteness at the base of reality that prevents it. There’s nothing in the math to indicate it, but of course the math, at least the math we have today, is a description of reality that is likely only an approximation.

However in a recent interview with Curt Jaimungal, David Wallace, a proponent of the many-worlds interpretation, may have provided another reason to doubt these outcomes: quantum interference. (Note: if the embed doesn’t work right, the relevant remarks are at around the 1:21 mark. Also you don’t have to watch the interview to understand this post, but it is an interesting discussion.)

https://www.youtube.com/watch?v=4MjNuJK5RzM&t=4901s

To understand Wallace’s point, it helps to realize some important points about how quantum decoherence works. Decoherence is the process of the quantum particle losing its wave like nature and becoming more particle like. This happens because as it interacts with the environment, the phase relations which keep the wave coherent become disrupted. The wave becomes fragmented. We call the fragments “particles”. This leads to the famous (infamous?) quantum interference effects disappearing. (As shown by the double slit experiment.)

But the word “disappearing” here in reference to the interference effects should be understood to mean “become undetectable”, not that they cease to exist entirely. Under decoherence the interference never goes away entirely. Like the wave overall, it becomes fragmented, and settles into an underlying “noise”. (Note: this is actually a difference in predictions between collapse and non-collapse interpretations that should, in principle, be testable. Of course, figuring out a way to do the test is another matter.)

Wallace’s point is that infinitesimally low probability outcomes should be swamped out by this remnant interference from higher probability outcomes, meaning that they should be prevented from existing. If so the branches where entropy never increased, where quantum computing never works, or to use his example, where he as a macroscopic object quantum tunnels through a wall, shouldn’t exist.

What does this mean for quantum immortality? I don’t know that it wipes it out entirely. Many of the initial survival scenarios may be very low probability, but not profoundly low ones, and so may not be swamped by interference from the other branches. But it does seem like it shortens the duration and overall makes it less certain, even once someone accepts the existence of the other worlds. So there may be versions of you reading this that live for decades or centuries beyond the normal lifespan, maybe even millenia, but probably not until the end of the universe.

Still, the implications are interesting and fun to speculate about. If there is a version of me alive in the far future, I wonder if he (it?) will remember these speculations.

What do you think of Wallace’s point? If we assume many-worlds is reality, does the idea of quantum immortality seem plausible? Or are there other reasons to doubt it?

#InterpretationsOfQuantumMechanics #ManyWorldsInterpretation #Philosophy #Physics #QuantumImmortality #QuantumMechanics #Science

Are quantum states and the overall wave function real? Or merely a useful prediction tool?

The mystery of quantum mechanics is that quantum objects, like electrons and photons, seem to move like waves, until they’re measured, then appear as localized particles. This is known as the measurement problem.

The wave function is a mathematical tool for modeling, well, something related to this situation. Exactly what that something is, is a matter of long standing debate. Erwin Schrödinger, developed the wave function after being inspired by Louis de Broglie’s hypothesis that matter has a wave like nature, similar to light. Schrödinger’s original intention was to model the electron itself. 

But Max Born took his equation and discovered that squaring the amplitude of the wave at any particular location gave the probability of finding the particle in that spot. Which converted Schrödinger’s equation about matter waves into a straight calculation tool. This might seem like a natural move. No one actually ever measures a quantum wave function, only particles. And the wave function describes a high dimensional abstract configuration space, which makes its relation to reality unclear. 

Still, Schrödinger wasn’t happy about the move, and continued arguing for some version of wave function realism. Which began the long standing debate between seeing the wave function and its quantum states as modeling something real, or just calculating the probabilities of future measurements.

In a recent conversation, someone compared my reasoning on this and consciousness, where I largely see the limitations of introspection as dissolving the hard problem of consciousness and the need for exotic solutions to it. They wondered why I don’t make a similar move for quantum mechanics, and just go epistemic, a stance epistemists see as dissolving the measurement problem.

I do occasionally review the arguments to see if I’ve overlooked anything about the epistemic view. Certainly it would appear to make the need for things like a physical collapse, non-local causality, a quantum multiverse, or other metaphysical “costs” unnecessary. The only “collapse” would be an informational one, an update in our state of knowledge. Definitely a grounded option to be taken if feasible.

But my block on this remains the whole reason we talk about wave-particle duality in the first place, the wave interference patterns revealed in the double slit experiment or the Mach–Zehnder interferometer. Crucially, in these experiments, the apparatus can be set to send one particle at a time and the landing location of each particle recorded, with the result that the interference pattern still accumulates. 

In other words, the one particle (which can be photons, electrons, or even large molecules) seems to interfere with itself. The only way that seems possible is if the particle goes through both slits at the same time. The question for people who assert the epistemic view is, how can they account for this evidence?

A frequent response over the years has been Robert Spekkens’ toy model, a hidden variable model showing a different physical reality that the wave function could model statistically, but not be an accurate description of. The argument is that this alternate model, and similar efforts, can successfully account for interference effects.

Hidden variable theories, which either expand the structure of quantum theory or propose alternative structures, are constrained by various no-go theorems, the most famous being Bell’s theorem, which requires that they be causally non-local. This seems to complicate any efforts to reconcile them with special relativity, something that was done with straight quantum theory by 1930. An alternative theory with a smaller scope of usefulness doesn’t strike me as likely to be the more accurate description of reality.

But the nail in the coffin for the toy model and similar approaches is the PBR theorem by Matthew Pusey, Jonathan Barrett, and Terry Rudolph. In short, this theorem demonstrates that pure quantum states in the wave function, if they are referencing any objective reality at all, must describe something real. Any different reality would lead to predictions incompatible with quantum theory.

The PBR theorem does have a couple of assumptions. One is that the preparation of two separate quantum systems can be independent of each other. This seems similar to the “free will” assumption in Bell’s theorem (which is actually about independent preparation of measurement choices). It seems like this assumption can be violated in the same way the Bell one can, with some version of retrocausality. So superdeterminism remains an option, albeit a long shot one in most physicist’s eyes.

But the second assumption is more basic, that the wave function is referring to something physical and objective, even if it’s not an accurate description of it. When I was an instrumentalist toward quantum mechanics, this was largely my view. I never doubted that there was some physical reality there, just not the straightforward one described by quantum states with all its bonkers implications.

The second assumption can be violated by going explicitly anti-realist, and simply asserting that there is nothing objective happening at all, that the measurement outcomes just happen, that they are fundamental interactions, brute facts of the world. In this view, the wave function is nothing but a prediction related to future measurements. Since the outcome is something fundamental, there’s nothing left to investigate. We just need to get used to it and stop asking questions.

Of course, there are a lot of people who are willing and eager to bite this bullet. It’s one of the postulates of neo-Copenhagen interpretations like RQM (relational quantum mechanics) and QBism (quantum Bayesianism). It’s worth noting that these stances come with their own metaphysical “costs”, whether it be the semi-idealism of QBism’s participatory reality, or the sparse “flash” ontology of RQM.

However, while the hidden objective reality view might have at least aspired to provide an answer to my interference question, the anti-real stance seems to outright ignore it, or assert that the question is meaningless. When you hear about the “shut up and calculate” phrase, this is where it comes from.

I find the incuriosity laden in this position difficult to understand. My interference question remains. But I also now want to know why the wave function is so useful, particularly when it comes to something as complex as quantum computing. If there’s nothing going on prior to the measurement outcome, then why are there detectable patterns at all? It seems like the “no miracles” argument for scientific realism, or at least structural realism, applies here. 

So, at least for now, I remain in the quantum state realist camp.

But maybe I’m missing something. Are there explanations for the interference effects in the epistemic view I’m overlooking? Or reasons to just dismiss the concern?

Featured image source

https://selfawarepatterns.com/2024/01/06/those-inconvenient-quantum-interference-patterns/

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