Thirty Years of ‘Against Measurement’ | Hacker News Reader

Thirty Years of ‘Against Measurement’

physicsworld.com

43 pointssohkamyung71 comments

It's not so much that we don't have a good understanding of what quantum mechanics is telling us, it is that people don't want to accept the answer. The answer people want is that the fact that measurements are repeatable is because they are giving us information about some kind of objectively real metaphysical reality, and that our measurements are a faithful reflection of that reality. But that simply turns out not to be the case. The reason measurements are repeatable is because of entanglement. When you and Bob look at the output dial of a measurement apparatus and agree that it says "spin up" it is not because there is really an electron "out there" that was spin-up in point of actual metaphysical fact, it is because you and Bob, but virtue of having interacted with the same measurement apparatus, have become mutually entangled, and so you behave as if you were classically correlated systems (because of decoherence) even though you actually aren't. That's it. Like it or not, that's The Answer. And in retrospect it is actually pretty easy to explain why that had to be the answer, why God had no choice but to construct the universe in this way in order to make it work the way it does (or the way it appears to). People resist this because they don't want to believe that the actual metaphysical foundations of our universe are so radically different from our subjective perceptions. The chair you're sitting on just has to be real. It can't be an illusion. Because if the chair is an illusion, well, then so are you. And that's right. You are. At root, you are living in the quantum Matrix, a simulation running on the cosmic quantum computer. You may not like it. Einstein certainly didn't. But that's the way it is.

I think your post is good fodder for the "quantum formalism" described in the article, because it seems like you're getting pretty carried away with interpretation, what with terms like "metaphysics" and the conclusion that chairs are illusions. Some chairs may be illusory, but in general they would reveal themselves as such because you can't, for example, sit in them.

Chairs are mostly space, but our interactions with them are solid. This is mostly known and measurable, though not always appreciated.

Models are not measurable.

All macro-sized things are mostly empty space.

Neutron stars are one of the few exceptions.

Oh right! The universe well and truly is an incredible place.

You don't actually know that. Neutrons are made of quarks, which are probably point like, i.e. the inside of a neutron is entirely empty space.

And if they're not, then eventually they're going to be made of something (that is going to be made of something and so on) that is pointlike. Very probably. If not, which will be a surprising result, then it's still going to be empty space.

Yeah, but they push the pauli exclusion principle to the extreme, meaning there's hardly anywhere within its structure for particles to not exist.

Not mostly empty space, entirely empty space. All fundamental particles are point like and have no physical size.

Except ..... that's not the right way to measure size, you measure the size of a particle by the forces interacting with it.

By that measure an electron has an infinite size (since an electric field is infinite), and a chair has exactly the size it appears to have, because that's the size measured when you interact with it electromagnetically.

Objects don't have one single size, their size varies depending on what tool you use to measure them.

What's the "size" of pressure and vibration?

All parts, remain as parts of the environment, the whole.

Is the suggestion that chairs are illusions because they appear solid but are not in fact? It would be more reasonable to say that chairs _are_ solid, and many solid objects, like chairs, are mostly empty space when considered at the atomic scale.

No, I believe the suggestion of illusion was to convey the fundamental point that the world doesn't exist in the classical objective sense as we are brought up to understand.

"Mostly empty space" isn't related, but is a valid point when considering the different matter (pun intended) of the solidity and physical character of macroscopic things like chairs.

Inquiry doesn't need to reinterpret facts.

Just pointing out two different things: chair mostly space (true) vs chair existing as an object "out there" in the clearly definable way we normally understand (false).

"Illusion" in its usage here referred to the latter.

Depending on reference, both are true, and false.

Logic is not probabilistic, but it is referential and not absolute.

It’s irrelevant to the argument and a strawman

Just because something isn’t the way we expected doesn’t imply that there isn’t an objective, underlying reality.

And that’s the heart of what nonlocal deterministic interpretations try to answer.

Given the chance.

Just because something isn’t the way we expected doesn’t imply that there isn’t an objective, underlying reality

"in the clearly definable way we normally understand"

Misquoting me and replying to that is the strawman approach, though I give you the benefit of the doubt and assume you just misinterpreted me.

I didn’t misquote you.

That bit doesn’t add to the assertion that something isn’t real, or that there isn’t an underlying objective reality.

If something doesn’t exist in the classical objective sense, then it means that something doesn’t exist or have meaning outside of an observer. It’s not empirical nor measurable.

That’s not the same as saying that the building blocks are quite different from everyday experience. It’s still measurable, real, and well-defined. Just not in the ways that are intuitive.

I didn’t misquote you. That bit doesn’t add to the assertion that something isn’t real, or that there isn’t an underlying objective reality

Of course it adds to the assertion, I was careful to add it for that reason.

Again, assuming good faith (despite your original unnecessary "strawman" tone), I will assume you were just mistaken in your omission or misreading of it.

I think the point was that there isn't an objective world in the classically understood sense, and our refusal to accept that since it was discovered a century ago has no bearing on the truth of it; I'm not suggesting the language and imagery were perfect, but the chair example was clearly accompanied by an equally illusory "you" sitting on it.

There's just no contradiction. The deepening of knowledge doesnt't contradict prior observations. All hypotheses may and should be contradicted though.

Quantum mechanics pretty much overturned our understanding of the world; it produced a qualitatively different world view to that of classical theories (including GR), more than just a deepening in that sense.

Depends on who is understanding or not, if they reject it.

However, evolving perspective is good as we see past assumptions, not facts, are always wrong to some extent.

Not sure how QM changed who though.

That everything is interconnected and complex, is not new logic at all. Maybe this brings artifacts across scales too. However we only observe effects. Period.

Maybe what is needed is humility.

That still doesn't explain why macroscopic objects behave differently from particles - the measurement problem is currently inescapable. Until you can prove that tunneling happens with chairs or find the interference pattern of bowling balls, the possibility that there is some other phenomenon that we currently don't understand remains open.

You may prefer to believe that quantum rules apply to the macroscopic universe, and that is a valid possibility. Superdeterminism is another possibility that can't be discounted. Perhaps wave function collapse is a physical phenomenon. There are other possibilities, and currently no way to choose between them, so just claiming that one of them is true doesn't make it right.

Not to mention that QM and GR can't both be true, so we know for sure that at least one of them has some flaw. Most believe that the flaw is with GR, but the possibility that QM is falwed remains there.

With the exception of gravity, the behavior of macroscopic objects is pretty well understood in a quantum sense.

Probability, when averaged over an astronomical number of cases, becomes certainty. The interference pattern of a bowling ball is a bowling ball! Any single particle in the bowling ball could end up off from the classical, but the aggregate probabilities suggested by the wave function implies classical mechanics.

> With the exception of gravity, the behavior of macroscopic objects is pretty well understood in a quantum sense.

The rules of motion of quantum mechanics say that particles move according to the Schrödinger equation, AND that once a particle is measured (i.e. it interacts with a macroscopic device) the particle can be found in some particular place with some probability that can be computed from the Schrodinger equation.

So QM has 2 rules of motion: one for particles which interact only with other particles (the completely deterministic Schrödinger equation) and a different one for particles interacting with macroscopic objects (the Born rule, which assigns probabilities based on the Schrodinger equation).

Exactly what counts as a macroscopic object, i.e. when the Born rule needs to be applied, is an open problem (or if there is some way to account for experimental results without this rule). Until we have some answer to this problem, I wouldn't make too many pronouncements about how QM applies to the macroscopic world.

The chair I'm sitting in isn't an illusion for any practical meaning of the word illusion.

Its solidity of the chair is, to a certain extent, illusory inasmuch as the odds of measuring a particle in any particular point in it at any given time are approximately zero, but my classical experience of the chair quite similar to any other human's experience of the chair, so the fact that there is an object there, it can be approximated by the word "chair" is pretty objectively true.

An illusion is a sensory stimulus that appears one way until you examine it closely, at which point it reveals itself to be other than what it initially appeared to be. Your chair absolutely fits that description.

Analogies and perceptions work in the mind only.

That is correct but I think you are talking about a different area of quantum physics than the article is. In particular the Bell paradox seems to show that there is no local interpretation of quantum physics, which seems quite unintuitive with the speed of light limitations, even under a “many universes” explanation.

The many universes interpretation is the only one that makes Quantum Mechanics even vaguely intuitive.

Every other interpretation is just nonsense, but physicists have just stubbornly refused to give in and admit that this is so.

The Bell paradox tells us that quantum mechanics is incompatible with local realism. Local referring to the idea that nothing is faster than light, and realism referring to the idea of there being a single objective world. You only have to sacrifice either locality or realism to make sense of the Bell paradox. The Many-Worlds Interpretation sacrifices realism but not locality. Faster-than-light communication isn't necessary for MWI to work.

https://en.wikipedia.org/wiki/Bell%27s_theorem says:

> In the vernacular of Einstein: locality meant no instantaneous ("spooky") action at a distance; realism meant the moon is there even when not being observed.

That's not what you said. MWI is a realist model; in fact, there's an argument that it's realism turned up to 11.

In MWI, there isn't the same moon there or even any moon in many "worlds"; its existence depends on your point of view's world. Realism in this context is about the idea that probabilistic events and measurements end up having a single objective result. In MWI, different points of view can see the same event have different results (because they're branching apart). MWI can be described as realist if you're considering all the branches real, but then you're not using the same definition of realism as the specific thing that the Bell Paradox is said to rule out.

By this argument, 3D perspective is non-realist, because a cube looks like a square from some perspectives and a hexagon from others – it depends on your point of view!

MWI is the most realist theory, because it says that everything is there when not being observed – in fact, it eliminates the concept of observation entirely. I think the tricky thing here is that MWI says that you are part of the system, which means that the intuition you've built up around collapse theories (where the “observer” has a privileged position outside and above the bits of physics they're “observing”) kind of falls over.

Wikipedia thinks it's realist.

The Bell paradox shows there's no local collapse theory (colloquially “single universe”, even though all quantum interpretations are single universe so it's a misnomer). The relative state formulation is consistent with the Bell “paradox”; in fact, the only thing it fails to explain is where the Born rule comes from (which, to be fair, is a pretty massive hole).

This is interesting. This looks big. Too big to judge from a short summary. Can you back this up with some references? and some explanatory readings?

Ideally, and since the audience is at a varying levels, a number of complementary sources, along with a few words, would be best.

Not sure if what you talk about is related to quantum decoherence, so I'll take that as an example.

    * Some formal peer-reviewed scientific paper? (Probably too dry for 99,99% of people including me, but I'll try to at least read the abstract, introduction and conclusion.)
    * Some Wikipedia article, like https://en.wikipedia.org/wiki/Quantum_decoherence ?
    * Some blog or "popular" science magazine?
    * Some video, like from PBS Space Time https://www.youtube.com/watch?v=GlOwJWJWPUs or https://www.youtube.com/watch?v=vSnq5Hs3_wI
    * Some different video, like Sabine's https://www.youtube.com/watch?v=igsuIuI_HAQ or https://www.youtube.com/watch?v=Be3HlA_9968
    * Some simpler video like, I don't know, https://www.youtube.com/watch?v=LsxJmHS0cc8 if it even make sense?

I hope there are some that explain your actual, precise point, precisely. Thanks in advance.

Just about any modern book on QM interpretation will tell you this, though not in so many words. (They won't necessarily endorse this point of view, but they will tell you about it.) Here is my take on it:

http://www.flownet.com/ron/QM.pdf

(Or the movie version: https://www.youtube.com/watch?v=dEaecUuEqfc )

(These are now a wee bit dated, but still defensible.)

For something written by an actual physicist I recommend anything by Tim Maudlin (he's a Bohmian, but his pedagogy is without peer) or David Z. Alberts, "Quantum Mechanics and Experience."

UPDATE: There's also https://arxiv.org/abs/1204.4616

Thank to you, Ron (Erann, as your site explains) for your answer.

I have read a number of texts and books, and viewed a number of videos. Too many totally ignore the point. Even among the better ones, I’m unsatisfied. There is something different in the way you tell it.

I have looked at the beginning of your video and switched to your text. It looks like you explain things differently from other people, which feels like you aren't afraid of going straight to what is important. Even the arxiv link from a different author feels promising. Thanks again!

You bet! Thanks for the kind words.

There is no single answer that may be considered an absolute truth. There are only models “all the way down.” All models are false, some are useful.

Whether a solid table exists or is it just mostly an empty space depends on the task ahead.

Models do not yet consider the co-dependent nature of the chaotic nature of the whole; stuck in brief stable zone to support advanced civilization.

The main problem is that quantum mechanics is telling us that systems evolve in two, apparently contradictory, ways:

  - isolated systems evolve through unitary time evolution; and
  - coupled systems evolve through an application of the measurement process.

Decoherence is an attempt to reconcile these two processes, but it's not clear that it works for systems with spatialy seperated entagled states. However, it all works out if you define a theory with emergent locality.

Yes, but the crucial question is: isolated from what? Isolation is realative, not absolute. If you put Schrodinger and his cat [1] (or Wigner and his friend) in a box, they are isolated from you but not from each other. (c.f. the Fruachinger-Renner paradox).

---

[1] This is intended as subtle ironic humor. Schrodinger actually hated cats, which is the reason he chose one to be the subject of his famous thought experiment.

Two bodies have simple, non-chaotic solutions.

Three+ bodies do not solve linearly. Thus reveal themselves as unsolvable probabilities and pattern structures across scales.

The individuals and environment exists co-dependently.

Unless you can imagine yourself up a chair from nothing, it's not imaginary.

Nothing changed when we discovered microscopes and cells. One day your skin is a solid piece of material and a disease is the result of a bad smell. The next day both are made up of a zillion cells.

This is no different. So what if the macroscopic chair is made out of the interaction between a bunch of different forces? It changes nothing. Molecules, atoms, and sub-atomics were already imaginary, in that you can only deduce them from tool-assisted and indirect observations, not see or feel them directly. "this iron isn't solid, it's a mass of atoms." "oh ok". "oh hey the atoms are made out of these other sub-atomic things..." "oh ok". So now the sub-atomic things are really just forces. So what? Now suddenly at this next level of theory it's all unreal?

It wasn't already unreal? Or it's suddenly no longer behaving as real? Neither of those are true. It's no more or less real than yesterday, and it behaves no more or less real than yesterday.

What's imaginary is this attempt at a new distinction.

I didn't say it was imaginary, I said it was an illusion. Illusions are not imaginary.

There is an emotional appeal here, which complains that ivory tower types are too goody-two-shoes to accept mother nature doesn't ship the wine and pate we thought we deserved. It's a Trump like whining session about the entitledness of the realist camp. It's not compelling. I too personally rather doubt the wave function is real, however, much remains to be learned.

For example (https://www.nature.com/articles/s41598-020-72817-7): where NPRF means non-preferred reference frame:

"Herein, we make progress on that front by extending NPRF to include the measurement of another fundamental constant of nature, Planck’s constant h. As Steven Weinberg points out, measuring an electron’s spin via Stern–Gerlach (SG) magnets constitutes the measurement of “a universal constant of nature, Planck’s constant”3, p. 3 (Fig. 1). So if NPRF applies equally here, everyone must measure the same value for Planck’s constant h regardless of their SG magnet orientations relative to the source, which like the light postulate is an empirical fact. By “relative to the source” of a pair of spin-entangled particles, we mean relative “to the vertical in the plane perpendicular to the line of flight of the particles”4, p. 943 (Fig. 2). Here the possible spin outcomes ±ℏ2 represent a fundamental (indivisible) unit of information per Dakic and Brukner’s first axiom in their reconstruction of quantum theory, “An elementary system has the information carrying capacity of at most one bit”5. Thus, different SG magnet orientations relative to the source constitute different “reference frames” in quantum mechanics just as different velocities relative to the source constitute different “reference frames” in special relativity. Since NPRF leads to the counterintuitive aspects (“mysteries”) of time dilation and length contraction in special relativity, it is perhaps not surprising that NPRF produces a “mystery” for quantum mechanics associated with the measurement of h as well."

There is yet room to grow in QM. I definitely agree with the the original paper's argument on how and why the focus has moved away from foundations, and why the Copenhagen interpretation gets more emphasis than it should. Since this is a very hard nut to crack, the human organizational impulse is to shut up and do. And for new comers who can't do, all they can do is verse themselves in Copenhagen so at least they can do that.

> It's a Trump like whining session about the entitledness of the realist camp.

We're going to need to coin a new version of Godwin's law.

Local realism is ruled out by the Bell inequalities and concomitant experiments. When Donald Trump provides some mathematical proof of any of his claims, then you can compare these two positions. But until then, please choose your analogies with more care.

Meanwhile, that's not what the nature article says!

I'm not sure if you actually don't understand quantum mechanics, or if you do but are expressing your point extremely poorly, but "You and Bob, by virtue of having interacted with the same measurement apparatus, have become mutually entangled, and that is what a classically correlated system is." is the objectively real physical reality.

The fact that measurements are repeatable is, in fact, because they are giving us information about that specific kind of objectively real physical reality - the fact that that reality is, at base, better described by quantum wavefunctions rather than trying to shoehorn it into a model resembling tiny billiard balls bouncing around only seems like a problem if you allow your brain's myriad evolution-induced biases and deficiencies to overwhelm your capacity for abstract reasoning. (Like, admittedly, Einstein seems to have.)

Please make your substantive points without personal swipes.

https://news.ycombinator.com/newsguidelines.html

FWIW, I didn't take offense at this. The truth is it's probably a little of both. But in my defense, I will just say that if I failed to explain it well, I would not be the first.

> if I failed to explain it well, I would not be the first.

This, at least, is definitely true. If anything, the fact that you don't posit magically violating pretty much every conservation law in the history of physics (Copenhagen interpretation/collapse postulate) or magically duplicating the entire universe (Many worlds interpretation), arguably puts you ahead of most actual quantum physicists (albeit, one hopes, more on the explanation front than the understanding one), although my estimation of that relative to the latter might just be a artifact having a easier time imputing metaphorical status to "illusion" than to "world". (Copenhagen gets no such excuse, though, so you're still ahead of a plurality of actual quantum physicists.)

I think there is more to it then that. How come we see the same value when information cannot be cloned?

I think the answer has to do with thermodynamics somehow right? Because the measurement apparatus starts as low entropy waves hands decoherence waves hands it behaves as a classical system.

That is a really excellent question! It goes straight to the heart of what is actually going on. I give a longer explanation of this in the last section of my paper, but here is the TL;DR: The way in which you get copyable classical information out of non-copyable quantum information is by discarding parts of the wave function. The mathematical description of this operation is called a trace. The classical information that comprises our classical reality is a proper subset of the quantum information in the universal wave function.

The next question people usually ask at this point is: isn't that just multiple-worlds? And the answer is: yes, it sort of is, but with one important difference: the MWI insists that all of the myriad multiple worlds are equally real. And if you take the "god's-eye point of view" and look at the universal wave function, this is true. But it is not possible to actually take the gods-eye view except in the abstract. In order to take any "point of view" at all, you have to be a classical entity. So nothing can actually take the god's-eye view, not even God! Only a classical entity can take a point of view, and that point of view is necessary bound to one particular universe, and so that universe -- your universe, and mine -- is privileged from our mortal's-eye point of view, which is the only point of view anything can ever actually take.

I feel like the question has not been fully answered. After taking the trace, how do you make enough information-copies to tell all your friends about it?

The trace is just the mathematical description of how you get from quantum to classical. The TL;DR is that a proper subset of a quantum system behaves, when considered separately from the rest of the system, as if it were a classical system even though it really isn't. To see the details, read this:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.79...

For some additional background:

https://adami.natsci.msu.edu/blog/2014/6/26/on-quantum-measu...

Just to be clear where I'm coming from, I have taken a course on quantum information theory, so I'm not completely lost here. But some things I for-got and somethings I never-got.

And in particular, the (separate) course on qft lost me at hello, so I never got the chance to incorporate that into my worldview.

In that case: if you look at an entire macroscopic system, isolated from everything else, before and after a measurement, you will find that the total entropy of the system has not changed. It can't because the evolution of an isolated system is always unitary.

If you trace over any degree of freedom of the system (i.e. separate the system into two parts, which are usually the "particle being measured" and everything else) then what you will find is that after the measurement the degree of freedom over which you traced has a negative entropy, and the rest of the system now has a positive entropy (which is the source of the apparent randomness), and that all of the degrees of freedom of the rest of the system (in the preferred basis as determined by decoherence) are now in classical correlation with each other. That is how you make "copies" of (classical) information.

Thanks for the pointers, I'll have to think about this!

I am not a physicist.

With that disclaimer.

Back in the 1990s I was excited after stumbling on some article on Bohm. Sure, the math was beyond me but determinism wasn’t actually ruled out!

Sharing this on various forums was met with outright hostility. The usual schlock like Bell’s Inequality “proving” that hidden variable theories are impossible, etc.

Bohm was definitely not well known outside of experts back then.

Things hav improved in the last decade, but I wonder how far we’ve setback physics by being so dismissive?

i.e., the failure of the scientific method is us.

The problem with Bohmian mechanics (which is widely known and taught for like one lecture in undergraduate degrees) is not that it is violates the Bell Inequalities -- it doesn't, as they only forbid local hidden variables and this is a global one, iirc -- it's that it adds an additional layer of complexity for no benefit except to ease the conscience of people who can't accept entanglement, without making any calculations any easier. Occam's razor says to not give it the time of day.

Wikipedia also tells me it's hard to make relativistic; I don't know anything about that but it sounds plausible.

I’m glad it’s taught now. But 25 to 30 years ago it was not common knowledge to the average undergrad.

The first entry on Bohmian Mechanics in Plato is from 2001, for example.

Aside from that I think the primary value of BM is as a proof of concept: Non-local, hidden-variable interpretations can work.

As for the hurdles I feel we’ve only scratched the surface; so few physicists ever considered this avenue until relatively recently.

Doesn't the "spooky action at a distance" become moot if we set aside the notion of a single forward direction of time for information? That is, when you observe one half of the entangled pair there is not a super-luminal speed transfer of information because the information was already available to the other half of the pair at the time that they were entangled; it will become the necessary state upon observation of its pair because that is what is necessary in the future.

Setting aside the single direction of time seems even spookier than action at a distance. Is it really possible that when two atoms interact with each other, the result is partially determined by my decision two minutes from now about what way I’m going to measure the result of that interaction?

Maybe? The concept is called "retrocausality", though I find the name itself to be poorly chosen, since without a single direction of time there isn't really a "before" to be retrocausal towards.

Aside from whether it’s possible, it seems inevitable. Light does not experience time. There is no time between the emission and absorption of a photon.

It's more complicated than that (in particular, you need to deal with conservation of information, which is difficult even in a classical particle physics model, and I don't know of any mathematical formalisms that make the quantum case any better), but very approximately yes (in roughly the same sense that, if you cut up a piece of cheese into smaller and smaller pieces, you eventually end up with a single cheese atom).

For example, if a photon decays (spontaneous pair production) into a electron and a positron, those two particles will have some of their information content 'entangled', and that is roughly the same information that would be 'preserved' accross a electron (say, in a atom) emitting a photon to drop into a lower energy level. Because those are the same particle interaction, rotated into different reference frames. (In practice, I think you mostly end up with entanglements like Bit_γ xor Bit_e xor Bit_e̅ = 0, which is why a electron and positron (or matter and antimatter in general) can always annihilate to form a photon (respectively photons) in yet another rotation of said particle interaction.)

Aren't you still discussing it in the form of a linear arrow of time? That initial decay is also the annihilation that forms a photon, when "reversed".

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