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Even Physicists Don’t Understand Quantum Mechanics (nytimes.com)
156 points by projectileboy 46 days ago | hide | past | web | favorite | 201 comments

Sean Carroll, the author, is a huge proponent and believer [1][1.1] in Many-Worlds Interpretation (MWI) interpretation of QM. MWI's basic idea is that the wavefunction is real and that there is no wavefunction collapse. Each time there's a decoherence, there's a universe split. Some physicists think this is pseudoscience or 'crackpottery' at best [2] since it can never be proven. A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value (if you assume it to be true, there's nothing that this belief 'buys' you).

So when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM and he's been known for trying to shame physicists for not sharing his view too. [3][3.1]

[1] http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...

[1.1] https://www.youtube.com/watch?v=HxgGYJ3OjM8

[2] https://motls.blogspot.com/2014/07/many-worlds-pseudoscience...

[3] http://www.preposterousuniverse.com/blog/2013/01/17/the-most...

[3.1] https://www.youtube.com/watch?v=ZacggH9wB7Y

> So when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM and he's been known for trying to shame physicists for not sharing his view too.

This is incorrect and bad faith representation of Carrol's position.

While Carrol is proponent of MWI, he is calling more research on foundations of quantum theory in general. If you follow his podcast it's clear what his position is. Some of these so called interpretations are "legitimately distinct physical theories, with potentially new experimental consequences". In this article he specially mentions Ghirardi–Rimini–Weber theory. https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

I don't believe any of the possible interpretations of QM have a clear path to being proven, so are they pseudoscience as well?

If Hawking Radiation can never be observed till the heat death of the universe, is that pseudoscience, or just really hard to test?

At this regime, all of physics is very hard to design an experiment for on ranges we have access to.

> I don't believe any of the possible interpretations of QM have a clear path to being proven, so are they pseudoscience as well?

The characterization is a little bit harsh, but yes. The Copenhagen interpretation is non-falsifiable, (relative to MWI) so while I don't know what it is, I know it isn't science.

There's a reason most reputable scientific journals don't accept papers dealing with interpretations of quantum mechanics. There's no "there" there.

Hawking radiation is interesting because a proof of lack of Hawking radiation is a proof of the falsehood of quantum mechanics. Which is really interesting; any time you can falsify something like quantum mechanics every physicist should be paying attention. Also, if certain models of string theory are correct, particle accelerators like the LHC should be able to produce micro black holes, which will then evaporate in detectable ways. So Hawking radiation is of observational interest as of 2010 as a possible way to provide evidence to support string theory. (spoiler alert: we didn't find any)

It feels like you are way too quick to kick things out that aren't on a path to being proven. String theory forever has had the problem that it didn't make experimentally verifiable conclusions. A quick web search finds recent articles discussing apparently discussing this. Aren't people still working on string theory?

Or why is mwi impossible to verify.

The difference is, string theorists are trying. Everyone knows that it is a major problem that we cannot currently design experiments to falsify ST or other quantum gravity theories, and everyone is working towards it in the hope that one day we may. When it comes to interpretations of QM, there is not only nothing on the horizon, people don't even speak in these terms. No one is insisting on falsifiablity, and indeed every new interpretation seems completely hell-bent on just providing a "story" around the equations in a way that does not - and fundamentally cannot - make any predictions that contradict basic QM. I can count on one hand the number of interpretations I can think of that make experimentally testable assertions, and for those I have plenty of respect[1]. But the majority of interpretations and their proponents seem to deliberately stay within the comfort zone of non-falsifiablity, writing paper after paper whose actual impact on either physics or philosophy can be summed up with "yeah well, that's just like, your opinion, man". I did my MSC in physics under a professor who is a well-known proponent of MWI (though my master's thesis was not directly related to the subject). Two years of working side by side with him proved to me that the detractors of work on interpretations are, by and large, correct in their assertions about the proponents of them.

[1] There is this one theory whose name I cannot remember, where each particle in the universe gets their wavefunction multiplied by a delta function or very thin gaussian at random intervals, and whose sudden localization also causes a sort of chain reaction and localizes all particles it is entangled with. This theory is interesting because it helps set a (statistical) limit between microscopic and macroscopic interactions; basically, when you go past a certain number of entangled particles, everything will be well-localized virtually all of the time, but for small numbers of particles we'll see quantum phenomena. Unfortunately I recall this theory having too many holes, and perhaps it has already been falsified, but the point is that at least it made an effort to address inherent problems in QM (namely, wavefunction collapse) in a way that physicists actually should; by making testable claims.

This was linked above and sound like the theory your footnote describes: https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

That's the one! Thanks.

Maybe particles and black holes are the same thing. They behave differently due to quantum gravity mechanics.


> If Hawking Radiation can never be observed till the heat death of the universe

That's a pretty big "if". As I suspect you know, how much Hawking radiation a black hole generates is inversely proportional to its size (it's probably an inverse square function to be precise, I forget).

A consequence of that is that supermassive black holes, the kind at the center of galaxies that we are most likely be able to indirectly (or even directly) observe, will generate almost none and whatever is generated will likely be outshone many orders of magnitude by the accretion disk anyway.

But... small black holes should shine extremely brightly, albeit briefly. A black hole resulting from a supernova will take a stupendous amount of time to lose enough mass (from Hawking radiation) to get this small and the window of time is so small we're unlikely to ever directly observe this.

But if we can ever figure out a way to lase gamma rays we can construct so-called Kugelblitz black holes (that some call the ultimate battery or even the ultimate computer).

Granted there's no theorized let alone practical way of lasing gamma rays, which is a problem.

But nevertheless, the theory of Hawking radiation makes testable predictions so yes, it's unlike Many Worlds (or even, honestly, string theory).

Yes that is accurate, but I feel doesn't detract from the main point, which is about whether to do theoretical physical at in these regimes.

We might decide a theory isn't testable even on principle, or discover that a theory we thought that wasn't testable actually is testable, but we'll never know until we do the work.

How would it make the ultimate computer?

I agree that deciding between MWI and other interpretations seems impossible, so it's not a problem that's considered important in physics, more of a good topic for a dinner conversation among physicists.

Our understanding of quantum measurements is much better now than it was at the advent of quantum mechanics, and it's understood that there's no such thing as an "abrupt" collapse of a wave function during measurement, it's instead a continous process involving entanglement and decoherence between two quantum systems (the system being measured and the measurement system). This understanding allows us to explain any measurement process using only "plain" quantum mechanics, i.e. unitary evolution of two coupled systems described by a Hamiltonian. If we just look at the Hamiltonian of the measured system it will seem that it shows wave function collapse because its evolution is non-unitary, but the evolution of the entire system is still unitary. From this angle I personally find any further invokation of a wave-function collapse mechanism unnecessary, and I think it overcomplicates quantum mechanics without giving us any benefits, so we might as well do away with it. Unfortunately, most courses on quantum mechanics still introduce the students to wave-function collapse when explaining measurement outcomes, which might be the reason that so many physicists have a bad understanding of the measurement process. Of course wave-function collapse is still a good shortctut for describing strong quantum measurements, I think it should be introduced as an approximation of a continuous measurement though.

Contra your first paragraph, isn't your second paragraph ultimately saying that MWI has won over Copenhagen as the more useful theory?

I'd say wave-function collapse is a good approximation for many strong quantum measurements, but isn't able to describe many experiments that involve weak quantum measurements or quantum feedback. As I said it might be good to start teaching quantum measurements using the entanglement+decoherence mechanism first and then introduce wave-function collapse as an approximation.

Motl is a zealot for his own interpretation and thinks everyone else is an idiot crackpot, including his colleagues.

Carroll does not mention his preference for many-worlds in the article because that's not what it's about. The fact that he has a preferred interpretation is not relevant to the point that physicists should pay closer attention to the lack of consensus on the interpretation of qm.

> when Carroll talks about 'physicists not understanding QM', he's just unhappy that most physicists don't share his view about QM

I don't see that in this particular article (which is not to say it isn't there in other articles Carroll has written). In this article, he is simply pointing out that QM is the only physical theory we have ever had that does not have a single generally accepted interpretation. Not believing the MWI is fine (I'm not a fan of it myself), but that just means you have to find some other interpretation that works, and nobody has done that; every interpretation of QM has issues that nobody has figured out how to resolve.

> Each time there's a decoherence, there's a universe split.

The universe is constantly splitting. Not just when there is decoherence.

Or more accurately. Every possible state of the universe has a probability amplitude right now. And those probability amplitudes are constantly evolving, diffusing into each other, interfereing with each other.

Every such state with a probability amplitude is a world of the many worlds.

In the article, there is a statement: "Empirically minded physicists have realized that the phenomenon of measurement can be directly probed by sufficiently subtle experiments."

Then please write about these probes, and not just the rant that "nobody understands."

The public doesn't have to worry (and we don't have to read) about the "interpretations" until there is a specific proposition of how to do such probe an the experiment can be constructed and performed.

He writes "students who demonstrate an interest in the topic are gently but firmly — maybe not so gently — steered away, sometimes with an admonishment to “Shut up and calculate!”."

My answer to this article would be, to theoretical physicists: "shut up and construct an experiment that can be performed." As long as there's no practical difference (in the sense that any experiment that can be imagined can demonstrate it) there's just no physics there.

So yes... "shut up and calculate."

It's either that or "shut up and meditate"

A thought experiment called Quantum Suicide [1] is an interesting read in this context.

[1] https://thoughtexperiments.net/quantum-suicide/

All these people saying many worlds and Copenhagen have identical predictions are totally ignoring this. If many worlds implies quantum immortality and Copenhagen doesn't, that's a clearly detectable physical difference.

Detectable how precisely?

By not dieing.

So if you do it and you don't die would you say that you proved something?

If someone does it and doesn't die would you think that did prove anything?

If one thousand people do it and one of them is still alive after 10 coin flips does that prove anything?

What is the detectable physical difference? How do you carry on an experiment to detect that physical difference? What is the protocol to determine that the physical difference has been detected?

Please let me know how should we do to not totally ignore this.

QI predicts you’ll still be alive in 1M years with 100% probability. Not-QI puts that probability lower.

So if you are still alive in 1M years that’s evidence of QI. Basic Bayesian reasoning.

Or you are just lucky. It all depends on the prior probability, even if you are eager to misapply Bayesian reasoning.

I'm not sure what point you are trying to make. It's still a different physical prediction and evidence of QI over non-QI.

You can always just be lucky; maybe gravity causes the Earth to move randomly and we've just been lucky to see if orbiting the sun for the past few years.

So the physical prediction is that I will never die, unlike the alternative physical prediction that I will die at some point. And therefore every second I remain alive is evidence for the MWI. If not having died before makes you think you're immortal more power to you. I don't find that reasonable.

>> Each time there's a decoherence, there's a universe split.

Or, the universe was split before and that quantum uncertainty is really us perceiving those already-existing other universes. The collapsed wave is really just us measuring the only particle, the one in our universe, capable of stronger interactions. Under that interpretation the rest of the electron shell is a cloud of other electrons existing across universes but unable to interact strongly in ours, literal ghost particles.

Every claim you are making requires justification why it explains reality better than the model you are arguing against. Without that this reply paints you as a crank.

I, and more than a few others, find the ongoing existence of multiple universes easier to swallow than the concept of new universes being created whenever a wave function collapses. The pre-existence of an infinity is of greater likelihood than the continuing creation of multiple infinities as time moves forward.

This is testable, if not practical atm. If something like an electron exists in ghost forms across multiple universes then they may be linked in those other universes more strongly than ours, a concept that may offer explanations for a variety of quantum behaviors. Proof of this would be the observation of a reliable link between particles not linked in our universe.

IMO, I have heard no explanation that is more consistent and coherent than this.


Consider the famous Schroedinger's Cat.

The decayed particle has two states: decayed and not decayed.

The cat has two states: dead and alive.

The researcher that opens the box has two states: sad and happy.

The waveform does not collaprse. The particle, the cat, and the researcher each have a uncollapsed waveform; i.e. are in both states.

However, these waveforms are not independent; they are entangled. The decayed particle is coincident with the dead cat and the unhappy researcher. And the undecayed particle with the alive cat and the happy researcher.

All states "exist" simultaneously, but not independently. This bifurcation in the wave function magnitudes is a "split" in the universe: every future waveform interaction will be in the context of decayed particle/dead cat/unhappy researcher and undecayed particle/alive cat/happy researcher.

"Many worlds" sounds nonsensical, but it is really just a colloquial expression for waveform entanglement.


Now this does present a philosophical question on how consciousness functions in light of the split waveforms.

I maintain that science -- quantum mechanics or otherwise -- has no explanation or even language to express "consciousness". It's like asking science an ethics question. And this inability applies to all science, not just quantum mechanics.

Many Worlds is not "a colloquial expression for wavefunction entanglement." Entangled states are part of the mathematics of QM, not a feature of an individual interpretation.

MWI distinguishes itself by omitting (as you say) any notion of "collapse" from its axioms. This is appealing for simplicity but the problem is how it connects to experimental results:

1. How does the Born rule arise? If the universe split into two but the measure of cat-dead is 1/pi, why do I find myself in the cat-alive universe more often?

2. More generally, how does randomness arise? MWI contains only deterministic axioms, so how can it explain the experimental observation that measurements of similarly prepared states result in different values?

The important question here is how do you know you find yourself in the cat universe more often? How can that possibly be measured? It's a difficult question when it comes to conscious experience.

Imagine instead of a single experimenter observing a billion possible outcomes, a billion experimenters observing a single possible outcome.

I propose to measure this by writing down the results on a piece of paper, and then counting them.

The hope of MWI is to produce a simpler theory by eliminating an axiom. If this forces us to axiomatize "conscious observers" then it has already failed. Though I admit it may be fun to think about.

That's the funny thing, MWI works perfectly fine without conscious observers. It's predictions match other QM interpretations. The problem comes from introducing ourselves as a conscious observer and trying to understand what it means to be consciously aware of a single outcome.

Yeah, this is indeed the key question.

My guess would be that they are all consciously experienced, some more and some less. The alternative seems to be to say that they are so called "p-zombies". Very similiar to us, evolving according to the same laws, but not concious. But it's really up in the air.

Okay, let me rephrase. Many Worlds states that there is no collapse; the entanglement continue to propagate. Many Worlds is the familiar label for saying there's entanglement, and that's it.

> 1. How does the Born rule arise?

> 2. More generally, how does randomness arise?

As you said, with Many Worlds there is no randomness. The multiverse is entirely deterministic. Randomness is a Copenhagen-like concept.

There is a "universe" with an alive cat and a "universe" with a dead cat. These universes have a certain waveform distribution.

Neither of these universe are any more real or extant than the other (they have the same magnitude).*

* Though consciousness and its observations seems to follow only one path. This is a peculiarity outside the realm of physics.

>* Though consciousness and its observations seems to follow only one path. This is a peculiarity outside the realm of physics.

Of course physics can answer this: the human minds on the different branches can't physically interact. They can't share memories. Each one is going to remember experiencing only its own branch.

Sure, but why am "I" the one and not the other?

Whether this is a problem or not depends on what your conception of consciousness/identity is. I see a conflict or unanswered problem there; you may not.

There's multiple, and each of them have their own experiences since the split. The other versions of yourself are basically other people that just happen to share a part of your history.

Wondering why you're a specific one of them and not the other seems equivalent to me to wondering why you're you and not some another person in our own world. I think it's an interesting question, but I'm not sure what an answer to it could possibly look like (since any answer that says why you're you somehow needs to work out differently for other people to explain why they're themselves too) which implies the question is malformed somehow. (If your consciousness were ... someone else, or even switched people it was tracking somehow, it's not like anyone would notice.)

> seems equivalent to me to wondering why you're you and not some another person in our own world.


Randomness is an empirical observation. Identically prepared quantum states, when measured, produce different experimental results, distributed probabilistically.

MWI's solution to this is to say the experiments are wrong, and the results are actually the same. It rejects empiricism to simplify the theory. But that's not good science.

> Neither of these universe are any more real or extant than the other (they have the same magnitude)

What is meant by "magnitude?" It can't be the measure of the wavefunction because those can easily be different, e.g. if the particle's decay probability is not 50%. This is the Born rule problem again.

If the universe is deterministic, how can there be entanglement or branching at all? What need is there for a multiverse if you have determinism?

It’s as if non-determinists aren’t sure that 2 doesn’t act like a 3 sometimes because of decoherence and so 2 + 3 = 6 as a result.

Where am I going wrong in this line of thinking? How could randomness be injected into a system without superpositioning of quarks? Is that literally the only way?

Is Hawking radiation the smoking gun we think it is?

Edit: I admit I am tired when posting but this topic is perhaps one of the most fascinating of all. I can’t stay away.

The waveform is deterministic. It does not "collapse" to a randomly chosen value; it continues to be all the values in perptuity.

It's like matrix multiplication: yes you can wind up with more elements after the operation, but the result as a whole is entirely deterministic.

If you defined matrix multiple multiplication where you collapsed the result down to a single unspecified vector, then you would have "randomness" (according to the selection).

Many worlds says "don't choose"; keep the entire matrix, and the results will get larger and larger.

The universe is not fundamentally rolling dice, though it appears that way from your perspective since you can only see a single slice of the matrix.

> This is a peculiarity outside the realm of physics.

So the “realm of physics” is now disconnected from empirical reality? Great.

> empirical reality

Unfortunately, yes. Empirical reality is limited by the empiricist.

Sure. But I wouldn't say it's outside the realm of physics.

I think more concisely, we don't understand it yet.

You will find answers to 1 and 2 in https://arxiv.org/abs/quant-ph/0503009

> A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value

David Deutsch (in many ways, the father of quantum computing) initially imagined quantum computing as a way to test MWI: if a problem could be solved that required more gates than there are particles in the universe, then it would (so the argument goes) prove that the problem was being solved in the multiverse.

Paraphrasing the title of the submission, “David Deutsch doesn’t understand Quantum Mechanics” (if what you said is true).

> 'physicists not understanding QM'

This is quite a popular claim among the Physics community. It goes even further saying: those who understand it don't understand it. ;-) Emphasizing how counter-intuitive the concepts are and also the fact the even Standard QM (with 1 or 2 particles) is still a research topic. Even Einstein didn't believe in QM as it stands now.

"he's just unhappy that most physicists don't share his view about QM" - Can you prove that? Are you just unhappy that Carroll doesn't share your view about QM?

>Each time there's a decoherence, there's a universe split.

Where does the energy come from to create the other universes?

Completely agree.

The article strikes me as crackpottery, but this statement

> A lot of physicists also don't see any value in MW's interpretation since it offers no predictive value

is not entirely correct either.

This is the probably best "explain like I am an undergraduate not in Physics" article on Everett's proposal.


To further summarize the value of Everett's proposal:

In the standard von Neumann-Dirac collapse formulation of QM, a collapse happens when we measure it. But what a measurement event looks like and conditions when it occurs are not described by the theory at all. This is one reason why QM cannot completely describe the universe, or approximate it. Note that even if we can describe what a collapse is, this+QM still fails to model parts of the world, only approximating it.

But one can try to propose a formulation of QM that agrees with the collapse formulation, that does not have that collapse, so that it is closed in the sense that for each system of observers observing a quantum system collapsing, that system can be modeled as quantum system itself. That is Everett's proposal. From the linked article,

> Everett’s solution to the problem was to drop the collapse postulate from the standard formulation of quantum mechanics then deduce the empirical predictions of the standard collapse theory as the subjective experiences of observers who were themselves modeled as physical systems in the theory. The result was his relative-state interpretation of pure wave mechanics.

The “no predictive power” is beyond what is predicted by standard QM, because the MWI is supposed to recover the same predictions. And it’s not clear how does it so, because if standard QM has an issue with “what a measurement event looks like and conditions when it occurs” then exactly the same problem remains in the MWI if the same predictions have to be recovered. Even though there is no collapse in the MWI it looks like there is a collapse, even though you don’t have to specify when the collapse happens you have to specify when does it look like a collapse does happen.

> even though you don’t have to specify when the collapse happens you have to specify when does it look like a collapse does happen.

In MWI there is no collapse event, because there are no observers in the traditional sense that may collapse the system. In MWI we resign and just say that observers are subject to QM, and when a "measurement" is made, what happens is that observers and the observed system get entangled so that certain combinations of properties of the observer-observed system are not possible. This entanglement of two projections of a composite system can be described in the language of QM.

> what is predicted by standard QM, because the MWI is supposed to recover the same predictions.

MWI recovers the same predictions. Where in standard QM if one says an observer observes that a system may collapse 40% in state a and 60% in state b, MWI says that in systems (of the observer and the observed) where the observer makes the observation in the same exact manner, the observer will record state a with the system in state a 40% and the observer will record state b with the system in state b 60%, and there are no states where the observer observes something and the subsystem is in the other state.

Of course, you can simply model the entire system of observer and observed in standard QM and recover the same relative predictions without invoking the absolute observation of an observer. In MWI we do away with the conceit that there is an absolute observation of an absolute, but ill-defined observer.

The point is that if standard QM has a problem to make predictions, then the MWI would face the same problem to make the same predictions starting from a universal wavefunction.

If standard QM has no problem to make predictions then I agree than in principle the MWI could make the same predictions (but it seems to me that in practice it can only make predictions by using standard QM and saying that those are the same predictions that could in principle be derived).

Can you clarify what you mean by standard QM? I had taken your usage of "standard QM" to mean, as you used it in context, to be collapse interpretations, but strictly speaking standard QM just concerns itself with quantum systems and is not incompatible with MWI.

Edit: it's too late to alter my original ancestor comment, but I think the way I wrote it may have yielded a misunderstanding. I didn't mean to suggest that MWI has value because it can predict phenomena. I meant to say that it has value because it is a metaphysically cleaner model of reality than the collapse interpretations.

By "standard QM" I mean the "textbook quantum mechanics" which is based in a set of postulates (like for example https://personal.utdallas.edu/~son051000/comp/postulates.pdf) including in some form von Neumann's projection postulate.

The idea that someone with equal ability to do calculations with both could understand Newtonian mechanics better than they understand quantum mechanics belies an extremely naive view of how easy it is to perceive the true metaphysics of classical mechanics. In fact, the metaphysics of classical mechanics remains unsettled even today, the only reason it seems like there isn't a lot of philosophical debates left to have is that it has fallen out of fashion. A brief conversation with any philosopher will reveal that nobody "understands" the basic essence of being or whatever you want to call it. In fact, if "understand" means "have direct sensation of the true nature of reality," or even "have some grasp of the true metaphysics," then nobody understands anything.

To give a specific example, let's say you have a classical system with only three particles in the entire universe, and further let's say they are interacting via a conservative force. So, how many things are there in the universe? Is each particle communicating with the others to organize their movements? Then, there would be three things in the universe. Is there a potential field permeating all of space, into which the particles project their influence? That would be four things. Is there a "strand of force" connecting each particle to every other particle? Then, there would be six things.

The debates in the paragraph above were alive and well in Newton's time. However they were philosophical debates, not physical debates, because the three suggestions I proposed (along with the other possibilities I didn't mention) are all mathematically equivalent and would lead to the same motion.

The thing is, since all those descriptions are mathematically equivalent, nobody cares whether, in a Platonic sense, there are three or four or six things, because it makes no difference.

Isn't the analogy to a similar situation with quantum mechanics, where people seem concerned with different mathematically equivalent interpretations?

You say "nobody cares", but people do care, and if they care in one context, why shouldn't they care in another?

Many worlds is an actual difference - one we cannot ever see or prove, but it's different. "New universes are being created" is deeply different from "they are not".

Forces at a distance vs fields? Not so much.

so why do they care about many-worlds interpretation then?

> The idea that someone with equal ability to do calculations with both could understand Newtonian mechanics better than they understand quantum mechanics belies an extremely naive view of how easy it is to perceive the true metaphysics of classical mechanics.

This entire comment is arguing against an opinion I believe nobody even stated. But why would we waste time trying to discover the metaphysics of an ultimately incorrect description of physical reality?

Ultimately all descriptions of physical reality are incorrect.

"All models are wrong, but some are useful." -- George Box

If we think our models are right, we will push them until we find out where they are wrong. That's what we do, over and over again. Pushing classical physics to its breaking point spawned quantum mechanics. We know there's a breaking point for present day physics too.

In machine learning we have many more models, all being imperfect. The ease of trying out a new model makes them cheap. Hell, we even do AutoML and search the space of billions of possible models. They are all approximations, we don't 'understand' reality. We can only do useful things with them, but there's a limit to what the models can teach us about the metaphysics of the world.

No offense, but the word "model" is used very loosely in ML to describe what is essentially a black box function that can be computer on a computer.

Models in Physics involve experimentation, validation, interpretability

There is a description that’s correct. We may never find it, but it exists.

Do you have evidence in favor of the idea that, say, there exists a finitely-axiomatized formal system which describes reality? We've been searching for a while, and evidence so far suggests that it's not going to be that easy. Tarski's Undefinability is a non-trivial barrier.

With mathematics, we can describe hypotheses of reality, but there may be details and nuances to reality which we're simply not able to observe and thus not able to empirically consider.

> finitely axiomatized formal system

I said “description.” You’re adding additional constraints that may or may not be warranted.

I'm merely helping you catch up on the past century of improvements in logic. What would your description consist of, if not a finite listing of axioms? I bet that you'll complain if my description includes a susbystem for arithmetic, in accordance with Gödel's Incompleteness.

Why would a description of physics necessarily include a formal system or arithmetic?

It necessarily includes a formal system. That much has been hammered into the ground already, and you'll discover it for yourself as soon as you attempt to formalize your description.

All existing attempts to describe physics have encountered some sort of incompleteness. Newtonian physics yields ordinary differential equations, which can encode Turing-complete problems. [0][1] Quantum mechanics yields Hamiltonian operators, which can encode Turing-complete problems too. [2][3] Both of these paradigms predate Turing's work; physics was Turing-complete a long time before you came along to look at the problem.

I can empathize with your original point: Surely reality exists! It seems so real! But reality can be real without existing. Ultimately, our models are only hypotheses about reality, and what they have shown us here is that even our models are so complex as to admit problems that are not going to be solved by mere assertion of existence.

[0] https://link.springer.com/chapter/10.1007%2F11494645_21

[1] https://sapientia.ualg.pt/bitstream/10400.1/1008/1/05-GCB-st...

[2] https://www.nature.com/articles/nature16059

[3] https://eprints.ucm.es/38062/1/spectral-gap_supplementary.pd...

> It necessarily includes a formal system. That much has been hammered into the ground already, and you'll discover it for yourself as soon as you attempt to formalize your description.

That’s a bald assertion, not an answer. And one can certainly imagine descriptions of physics that don’t require a formal system.

Here’s one: “every particle remains at rest for all t.” Now, that doesn’t describe our universe very well. But it might be a perfectly good description of a single-electron universe.

There’s no a priori reason to assume that a valid description of physics is axiomatizable in a formal system. Aruguments to the contrary are necessarily based on our own ignorance, not on anything fundamental.

Sorry, but what is a particle?

We've been searching for a while?

Interesting definition of “while”. Even relative to human civilization (which itself is a drop in the bucket) we haven’t been searching all that long.

Yes, and this isn't specific to physics. You could say the same about any other scientific field, IE:

Biologists don't fully understand the human brain or aging.

Chemists don't fully understand superconductors.

Astrophysicists don't fully understand black holes.

Computer scientists don't fully understand P vs. NP.

Ha!, yes, what does it mean to understand something? To fully understand an apple you must fully understand the entire evolution of the universe, where it came from and down the rabbit hole we go :)

Still, chasing understanding is useful as every now and then something jumps out of the rabbit hole and changes things... Trying to understand the revolutions of the celestial spheres, why black bodies don't emit endless amounts of energy or what it would be like to travel at the speed of light.

Physics is a series of models of the world. None of them is perfect. What is understanding the world when all we have is imperfect models of it?

> The idea that someone with equal ability to do calculations with both could understand Newtonian mechanics better than they understand quantum mechanics belies an extremely naive view of how easy it is to perceive the true metaphysics of classical mechanics.

I don't think that was the OP's idea. He doesn't even reference Newtonian mechanics in the article.

Agreed. Very confused by the comment

But note that your specific example of a philosophical question regarding the mediation of forces has since been resolved through advancements in physics (General Relativity, where you have to account for the energy-momentum of the fields permeating spacetime).

Do people understand causality?

Causality is probably okay. Continuity? Not so much.

The fact that we assume that the rules yesterday are the same as the rules today is one of those axioms that we simply have to accept in order to make progress.

Not sure why this is being downvoted?

> Physicists don’t understand their own theory any better than a typical smartphone user understands what’s going on inside the device.

This article seems to take a confused definition of "understand". Not admitting to a full and complete understanding is rather different from knowing nothing at all.

And to give your typical smartphone user some credit, most of them probably have some idea of the concepts of radio waves and computers, even if they don't grasp the details.

I think it's a somewhat legitimate comparison. The average person can use a smartphone but couldn't tell you much about why it works the way it does. They can accomplish all sorts of things with it but they have little insight of its internal structure. It's the same with Schroedinger's equation: I can use it just fine, and it's clearly a useful way of looking at the world, but I don't understand why reality is like that.

I think quantum mechanics is well-understood, the mathematics behind it provide us with excellent predictions.

I think because at its core it is a probabilistic (or perhaps there are many worlds, but the implications are identical) theory--there is some inherent uncertainty in the predictions. This uncertainty builds up over time if left unmeasured, due to the chaotic nature of the system. We won't be able to result this uncertainty until we have more precise and accurate measurement devices.

I was thinking, the only real way for us to progress was to get the results out of CERN we were expecting--either smaller particles or some detection method which allows us to peer even deeper into the atom.

I think certain physicists are tired of the search for a unifying theory since it probably won't help us make better predictions. We need to get out there and get new empircal data.

I did my M.Sc. in quantum physics, and I'll be the first to admit I don't understand it.

Yes, the equations do seem to predict what we observe in experiments very well. Shockingly well, given how strange some of the predictions made by them are! However, we lack an intuitive understanding of why the equations work. Matter and energy behave very differently on the quantum scale than they do on the macroscopic scales we're used to observing. If you're trying to model a pendulum, your intuition about how objects behave in the real world can be of immense help when performing sanity checks on your equations. Your intuition is worse than useless when it comes to quantum systems. It's not just flat-out wrong much of the time, it can lead you to ignore things that should not be ignored.

Quantum physics has all sorts of English language, short-hand descriptions for mathematical functions like "tunnelling", "non-local", "spooky action at a distance" or, as is pertinent to this article "observation". You may think you understand what these mean, but you're probably at least partly wrong if you're thinking about them in terms of human language, natural human intuition, or even the very specific short-hand terminology of physicists. We are continually teasing out nuances in the math that language simply does not capture. If we lack the language to adequately describe reality, can we really understand it?

Feynman was very correct to observe that people who think they understand quantum physics on a deep, intuitive level, probably do not. Doing quantum physics with a human brain evolved for bashing stones together is like trying to breathe in outer space. It's possible with considerable effort and the right tools (e.g. a space-suit), but if you ever forget that it's a deeply unnatural thing to be doing you're probably going to get yourself into trouble.

Feynman's diagrams (like the orbital model of the atom) were attempts to visualize a world we can't experience.

Those of us who aren't deaf and blind begin to make audio-visual models of our macro world in childhood. We can agree with confidence that "that's a rock" (if not its physical properties). Even in our macro world, our models have limits. (What do radio waves 'look like' as they're emitted by different antennas? Go around buildings? We draw pictures.)

But we're nearly deaf and blind to the quantum world. We have collections of indirect observations and math. Our models of it are necessarily limited by our inability to experience it directly. Those with math 'intuition' are in a better position ... but still.

> Feynman's diagrams (like the orbital model of the atom) were attempts to visualize a world we can't experience

They are a convenient notion for defining path integrals.

> However, we lack an intuitive understanding of why the equations work.

I think the word mechanism captures what you are describing. We have a set of equations we know work, but we have absolutely no idea what underling mechanism gives rise to those equations. It's like the position ancient seafarers were in: the could use the sun and stars to navigate, but because they had no idea of the mechanism behind their movements they were happy with the idea the world was flat.

It's not a good state of affairs because understanding the mechanism yields far better predictive powers than the mathematical equation describing what happens. For example, we had good equations describing rechargeable battery degradation. But until we understood the mechanism, we were just stumbling about in the dark on how to improve it.

I ask you because I definitely do not have an M. Sc. In quantum mechanics - at the lowest level of any system isn’t it inevitable that there is no deeper understanding. At a certain point you have hit the base level and things just “are.” Correct me if this is erroneous!

Consider the equation: 1+1=2

You don't have to understand the mathematical explanation for why this is so (one does exist) in order to have an intuition that serves you well in practically all situations. For most of us, there is no deeper understanding. It just is, and that's good enough. That doesn't necessarily mean there is no deeper understanding, but let's ignore that for now.

What I was trying to get across is that basic intuitions we have about the macroscopic world that we can see and touch are often flat-out wrong on the quantum scale. Likewise, human language has developed to describe macroscopic phenomena and struggles to capture the meaning of quantum phenomena. Intuition and language are crucial parts of understanding, but we find ourselves on treacherous ground when we try to apply human language or intuition to quantum systems. The confidence that comes with intuition and mastery of language are wholly counter-productive in quantum physics. You need to constantly reexamine what you think you know in the light of both new discoveries and the math. Both are sources of continual surprise.

Hence, if you think you understand quantum physics, you probably haven't given it enough thought.

Yes and because we live in the macroscopic dimension we develop the intuition this way and thinking in terms of another dimension makes out heads swirl. Perhaps we’ll never build an intuition at the quantum scale, just some tools to abstract it away and get some benefits out of it.

This is a bigger question than physics can answer. The project of hard science is essentially reductionist and based on predicting events based on hypothesis. Your question is more logical or philosophical.

Logic may have something to say about open or closed systems, but mapping that onto the real world will always be a following question.

It seems to me the limited hypothesis "We have reached a point where no further progress can be made in understanding why quantum mechanics works the way it does" is falsifiable and thus not purely logical or philosophical. (i.e. quantum mechanics is the lowest level of physics).

It is only the broad hypothesis that is only answerable in philosophy: "There is a point in physics where no further progress can be made in understanding why things work the way they do and we will be purely limited to describing how things work." (i.e. there is some lowest level of physics)

Your 'limited hypothesis' can never be proved, because you can't prove things you haven't observed i.e. the proof works only up until counterexample, which by definition of unknowns, counterexamples (or a deeper understanding) would always be a possibility.

Falsification is not end end-all-be-all, it's important, but it's not the only thing that matters.

What scientific law or rule can be proved?

what is the point shkkmo is making?

That the limited hypothesis is a scientific hypothesis because it is falsifiable and testable while the broad hypothesis is a philosophic question because it is not falsifiable.

This is relevant as it is sort of ambiguous as to which hypothesis jshaqaw was asking about.

based on the specific wording and vocabulary jshaqaw used, i assumed he was referring to the latter

I think it was actually the former I was asking about but again I recognize I am outside my zone of competence in this area.

Yes, and I assumed that was your assumption. :D

ok good night :)

> [...] quantum physics on a deep, intuitive level, probably do not.

Intuition and understanding are two different things. Our intuition behind quantum mechanics is obviously wrong--quantum mechanics doesn't even obey regular logic! Quantum logic gets rid of a rule, for example.

So it's probably impossible to develop an intuitive understanding if that is your criteria.

As we've progressed, we slowly just get used to new abstractions. The concept of zero, negative numbers, complex numbers, rationals, reals, they were all very non-intuitive to the people at the time but were required to understand the science of the time.

> quantum mechanics is well-understood

No it is not, we don't even understand what causes quantum waves to collapse. We call it "observation" but we don't know what that is. So you can't say we understand quantum mechanics until we have a theory which can accurately predict when wavefunctions will collapse.

When two quantum systems interact their wave functions either gets entangled with each other or they cause each other to collapse, it seems like the first happens in small systems and the second happens in large systems, we don't know much more than that.

From what it seems, we don't actually even know if "collapse" as such is a thing, nevermind what causes it.

We are one experiment away from placing serious doubts even on counterfactual definiteness.

Observation = interaction

But what kind of interaction? Is it a tensor field? Change of spacetime? Particle mediated? Wavelike? Nonlocal string or brane vibrations? Does it interact with other known fields and particles and in what way?

What is its range and speed? Does it work same in very low or very high energies? Does temperature affect it? Does it change at relativistic speeds?

Etc. The tool we have to probe it is currently only entanglement and secondarily tunneling. Maybe some calorimetry and charge detection.

None of that is a problem if you accept MWI.

The mental benefit of MWI is that instead of worrying about both entanglement and observation/wave function collapse, you only need to worry about entanglement because what Copenhagen describes as collapse is really just entanglement of the observed system with the observer.

A group of particles can interact without it counting as an observation which is why I mentioned entanglement.


I would bet ssijak thinks in MWI, where observation is not a separate thing from interaction and there is no such thing as "counting as an observation" in physical reality.

The "observations" that other interpretations cause endless confusion about are simply interactions between the observed system and the observer, but "observed system" and "observer" are concepts without a corresponding physically reality. They're simply labels assigned by humans.

Entangled particles share a state, they do not interact: https://en.wikipedia.org/wiki/No-communication_theorem

Particles can entangle when they interact, ie they go from not sharing a state to sharing a state. Your link states that wavefunction collapse is random and can thus not be used to communicate information, it doesn't say anything about interactions resulting in shared state.

Note that there's no way to observe a particle without some form of entanglement, though.

> I was thinking, the only real way for us to progress was to get the results out of CERN we were expecting--either smaller particles or some detection method which allows us to peer even deeper into the atom.

This cannot be more wrong.

Science advances with one unexpected experimental hypothesis failure at a time.

Quantum computers working (or not working) would be another important test for quantum mechanics. Because quantum computer with n qubits to work requires amplitudes to behave the way quantum mechanical equations predict to the precision of 1/2^n which for 256 bits is much larger than the precision amplitudes can be measured in principle (e.g. for a spin ~1/planck length)

Would higher precision of measurements help here?

Certainly. Effects can show up as qualitatively new things such as a new kind of particle, but also as tiny discrepancies between theory and experiment. There might be interesting new physics in every digit that gets added to the fundamental constants, for instance.

quantum mechanics is well-understood if you don't question the axioms and blindly calculate probabilistic outcomes. That doesn't mean we understand why those calculations work in the first place, what their limits are, what they represent, etc...

ptolemy might object, 'but we can calculate the answer with arbitrary precision!!' but his 'model' didn't capture reality in a meaningful way dispite the accuracy.

The same could be said of quantum mechanics

Ultimately you have to accept axioms of how the universe functions... we didn't question the non-probabilistic newtonian outcome metaphysically when it was presented, why is QM now being held to that standard?

We questioned non-probabilistic newtonian physics when we asked harder questions than it could answer or it gave wrong answers when pushed to a higher regime.

Something similar is going on now in QM.

And do you really think we have discovered the "axioms of the universe" already? That is quite haughty. No we don't have those.

We have models that perform well in certain regimes. Push those regimes, and the models break, then we find new and better models.. rinse and repeat. That is science. https://youtu.be/ka9KGqr5Wtw

* We have 'axioms' (I would not call them that. I would say models) that work in a limited regime e.g. quantum field theory or Maxwell's equations, but we don't know if they are truly fundamental - in that sense they are not axioms , which in math have a precise meaning.

> And do you really think we have discovered the "axioms of the universe" already? That is quite haughty. No we don't have those.

That is also haughty.

It is possible--probable, even--that our current models of physics are incorrect. We should certainly continue to probe them and look for something better. I would bet money that before my lifetime is out, some part of QM or relativity are overturned just like newtonian physics.

However, you cannot just assume that that is the case. It is no certainty. We do not have certainty. The universe is sticky; it very well may be that we can never reduce it to something as simple as rule 30. It may be that our current theorems are axiomatic--at least, as far as the universe can be said to have axioms.

(Or, as I think is more likely in the universe where we never discover any more physics, the difference between our laws and 'truth' is smaller than is possible to detect. That is, any instrument we use to measure will inherently produce too much noise to produce a statistically significantly different result.)

I agree that we should push our models to regimes where they fail. I disagree that the questions you were asking above will lead us there.

I've no idea about nuffink, but

> We need to get out there and get new empircal data.

To do this it's necessary to formulate the right questions first, as gathered data (which are in effect, answers) provide no information alone.

Edit: "provide no information alone" is a bit too strong. What I'm trying to say is more precise questions tend to produce more precise answers in normal discourse, I'm generalising that.

> excellent predictions

Sometimes QM provides us with no prediction whatsoever as to what will happen next (eg. photon splitting experiment.) I would not call this "excellent."

> chaotic nature

Chaos theory is something else.

> results out of CERN

Particle physics is also not particularly relevant to this discussion. Fundamental particles is a separate issue to interpretations (foundations) of QM.

"If you think you understand quantum mechanics, you don't understand quantum mechanics." - Richard P. Feynman

A related comment from one of my physics profs: "You don't understand quantum mechanics, you just get used to it."

Never heard that one before, but I love it.

The original version is from Von Neumann

John Von Neumann once said to Felix Smith, "Young man, in mathematics you don't understand things. You just get used to them."

I think the author (Sean Carroll - whose other books I've read and pre-prdered the one this article comes from) is trying to point out that we're at a stage where maybe we should regroup a bit and find better analogies for what's happening in the quantum world, which may lead to better understanding in the future. The mish-mash of crazy phenomena could be laid out a bit more logically than "wow, check out this insanity!" We need to kill Shrodinger's cat, stop yammering about entanglement, and stop talking about wave functions as if they're real things. There has to be better ways to describe this stuff.

Personally, I think they should start with the nomenclature. It may have been amusing at first to call things strange and up and down and colored, etc. but at this point it's a mess, especially when it's now clear that many particles are just the same thing with varying energy levels.

Also, while I'm on a rant, the double slit experiment is crap. It never shows the particle grouping pattern, just the interference pattern. They talk about measuring the photon at the slit, collapsing the wave function and creating a grouping as if by little ping-pong balls, but it's never demonstrated!

The double slit experiment has been repeated many times in single-particle (photon, etc.) mode.


Here is data showing particle grouping patterns. This forms the basis of the many-particle interference pattern.

If you put a detector near the slits to try to measure what passes through, then this pattern does not arise.

"...then this pattern does not arise."

Have you ever seen an example of that? There's not a picture on the internet of two groupings that I can find.

Here is a transition image of the double slit experiment being disturbed to skew the probability distribution function: https://www.researchgate.net/figure/Mask-movement-A-mask-is-...

Thanks. Just to be clear, I understand and believe the results of the experiment. That said, none of those pics show two distinct blobs as described by Feynman, et. al. :-)

> They talk about measuring the photon at the slit, collapsing the wave function... but it's never demonstrated!

I believe they have done real experiments, Feynman mentions doing it with electrons going through the slit and a light source to make a flash by the slit when the electron passes. If you use short enough wavelength light to tell which slit then the interference goes. But true it was done in a lab sometime, not as a popular demonstration.

Stop BSing and run it with protons, electrons or neutrons. Measure charge distribution in the sensor, track paths in a bubble chamber, call it a day.

De Broglie called from afterlife to check if people still run simple experiments.

Despite the forthright title, it is worth noting that this is an Opinion piece in the NYT and there are other physicists who would be uncomfortable with the popularising tendency to woo-ify our most successful scientific theory.

This opinion piece is written by a well-known research professor at CalTech and sits behind Feynman's old desk.

He's been extolling this exact message for years.

I think he would say he's trying to de-woo QM by asking folks to think critically about it's principles

He is not saying that the predictions that QM gives is wrong.

He's a hard materialist and rejects the Copenhagen interpretation of QM (as many do who think intently about it) and prefers Many World's Interpretation, but admits there's more work to be done. He's saying in the article that folks should work on it.


I'm well aware of the author's credentials, writings and views - and would have thought that being in a position of popular renown would have made him more cautious of playing to the crowd with titles that indicate mysticism is the acknowledged heart of the scientific project.

I'm not sure how you get from a title like "Even Physicists Don’t Understand Quantum Mechanics" to the "mysticism is the acknowledged heart of the scientific project" thing. The heart of science has been trying to figure out things we don't understand which is not the same as mysticism.

Quantum Mechanics is sold as weird and magical and unfathomable etc in popular representations. The title clearly bolsters that view and insists the author's confusion is shared by physicists generally.

Carroll says essentially this exact same thing in most of his podcast episodes and his dozens of talks and interviews. I don't think there's any sensationalism here. He is indeed saying that the public's impression of QM as inscrutable and mysterious is basically right, in this current scientific climate, and that, even worse, many physicists want to keep it that way.

I think the problem is that people have trouble shedding the illusion that particles are small balls on 3d billiard table.

If they accepted that the wave function with all it's weirdness is the particle they colud get a better feel for quantum mechanics.

Funny thing is that Schrodinger equation that describes how wave function evolves simplifies to classical movement equation if you consider "pointlike" wave function.


There's no need for thinking that there's a collapse of the wave function and then some little ball pops up that's governed by different equation. The equation is the same. Just a lot od terms can be simplified if wave fuction is small and sharp. Just like there is no collapse when you slow down from relativistic speed even though you can now skip relativistic terms in equations and still get mostly correct result.

How did you conclude that there's no need for thinking that there's a collapse of the wavefunction? How does wavefunction collapse arise from the Schrodinger equation?


Basically classical mechanics is just Schrodinger equation for sharp, small wave functions.

So you don't need to think that due to measurement classical particle pops up out of fuzzy thing that the wave function is. Instead you can think instead that due to measurement wavefunction itself reshapes to be really narrow and sharp but it still obeys same shrodinger equation. Just now thanks to sharpness you can simplify the equation a lot so it becomes classical motion equations.

An integral part of quantum mechanics is the actual discrete states, ie the eigen values which it probabilistically chooses during a wave function collapse.

Hence it doesn't behave like a wave, also it doesn't behave like a particle, it behaves like a mix of both. That is quantum mechanics and we don't fully understand the maths of it yet, ie when do these waves behave like particles and when do they behave like waves ie when do the wave function collapse?

What I'm saying is you don't have to see eigen values as something of different nature than the wave function. They represent very condensed wave function. Instead of getting rid of all probability by calculating eigenstates you could get rid of just a part of probability (sort of like by using fuzzy numbers in your calculation instead of real numbers) and get something that is close to eigenstates but still is a wave function. If you track such object in time and space you'll see it still obeys Schrodinger equation.

Nobody does that because that won't help you with predicting results of your experiments and it's way more work. But it gives you insight some people crave while others feel it's completely unnecessary.

> What I'm saying is you don't have to see eigen values as something of different nature than the wave function. They represent very condensed wave function.

They do not represent very condensed wave functions, have you taken even a single course in quantum mechanics? You should have learned about the Double slit experiment in high school at least, how do you explain that using only the schrödiner equation?


The wave function are reshaped by the slits. When it interacts with the barrier with two slits it gets reshaped to have for example very low value directly after the barier far away from the slits. And then final reshaping into something extremely sharp occurs at the screen.

The final reshaping is so precise that given photon wave function gets condensed precisely to a single atom of a particle of the photographic dye.

Anything except for those reshapings is governed by Schrodinger equation.

> [Eigenvalues] do not represent very condensed wave functions

User kgwgk said: "Note that eigenvalues are wavefunctions just like any other state"

Even if mathematically they don't. Same way as limit of a function at some point outside the domain of a fuction is not a value of that function. But if you consider something close to that limit it is still value of that function.

Limits are useful for calculations but if the function represents someting real, limits unlike the function don't actually exist. They are one step more removed from reality than the function is.

What I'm saying is that it might be useful for developing intuition for quantum mechanics to consider that eigenvalues don't exist. That they are just mathematical constructs that are useful to represent wavefunctions that are close in shape to them.

The genral tendency is exactly opposite. To consider wave function to be mathematical construct (becouse complex numbers and no macroscopic analog) and eigenvectors and states they describe as reality (just because we measured something close to their precisely calculated values).

The interaction with the slits is governed by Schroedinger’s equation because it is not a measurement (if it is a measurement the interference pattern disappears).

Sort of. Shrodinger equation doesn't so much describe interaction with the slit barrier as it describes propagation through space not occupied by the slit barrier.

The only difference between passing through a slit and the measurement at the screen is the size and location of the "hole". Measurement at the screen is just a silt of the size of single atom.

If your measurement is just ensuring that photon passed through one slit not the other you no longer have interference pattern at the screen but you still get diffraction pattern. Your measurement didn't collapse a wave to single point because it wasn't precise enough.

> Shrodinger equation doesn't so much describe interaction with the slit barrier

And what would describe the interaction with the slits according to you?

If you want to have precise description you would have to look at how interaction with wave functions of electrons bound in atoms of the barrier reshapes wave function of incoming wave reshaping it in a way that some of it bounces off, some of it dissappears to encompass the act of potentially absorbing the photon on the barrier. And part of the wave is left undisturbed to propagate through the slits.

Completely infeasible to calculate but I argue valuable to think about. :-)

You know that is all happening according to Schroedinger’s equation, right?

Culling of the the wave function due to absorbtion by the atoms of the barrier that happens or not with some probability is a consequence of Schrodinger equation? I thought it describes evolution of the wave function in spacetime empty or not (like around nucleus) but doesn't cover any kind of decay, absorptions, emmisions and such.


We reached max thread depth and I can't reply to your response. Not sure if you are going to see this.

If you are using electrons instead of photons for double slit experiment barrier still absorbs them. They get caught by atoms of the barrier or exchange virtual photons with electrons of the barrier. Shrodinger doesn't cover that I think.

In principle you can model the interaction between the electrons and the particles in the barrier and the whole system evolves according to Schroedinger’s equation and will be described by a wavefunction. This wavefunction may be a superposition of states where the electron goes through the slits and states where it doesn’t. Until there is a “measurement” - a “detection” which is “amplified” - you cannot say what has happened to the electron: every outcome remains possible and the system remains described by a wavefunction. If the detection happens first at the screen there will be a probability of finding the electron at each position and a probability of not finding it at all.

Note that the detection on the screen is done using a scintillator/photomultiplier or something like that to amplifify the detection of a single electron into a measurable electric signal.

Absorption of what? If you’re talking about photons, strictly speaking they are not described by Schroedinger’s equation at all (it applies to massive, non-relativistic particles). The double-slit experiment is often discussed in quantum mechanics in terms of matter waves (having interferences in light is not particularly surprising anyway).

Why do you think the result of the double slit experiment cannot be explained using Schroedinger's equation?

If there is a problem at the time of the measurement this is not something specific to the double slit, the same problem exists with a single slit and in any detection of any particle ever.

Note that eigenvalues are wavefunctions just like any other state (and obey Schroedinger’s equation just like any other state). The issue is with the process taking you from one state to another, not with the states themselves.

Note that you are wrong, it doesn't become classical mechanics for small sharp wave functions. It has similar movement, but the wave function would also thin out until it is smoothed out over the entire universe. We all know particles doesn't do that, hence schrödinger equation alone isn't enough to explain observations about classical mechanics.

Could you elaborate on that? How do we know particles don't do that? Being high near one spot and smooth, near zero over entire rest of universe looks exactly like a particle to me.

No, I mean it will be evenly spread out in all directions and not have any location where it is dense.

I don't see what you are saying in the description of Erhenfest theorem.

They are saying:

"For general systems, if the wave function is highly concentrated around a point [... the thing different between classical and quantum ...] will be almost the same, since both will be approximately equal to [...]. In that case, the expected position and expected momentum will approximately follow the classical trajectories, at least for as long as the wave function remains localized in position."

> at least for as long as the wave function remains localized in position."

The problem is that, as he said, “the wave function would also thin out until it is smoothed out over the entire universe”.

Is this then consequence of Schrodinger equation? How fast does that happen? Would the photon from remote galaxy be significantly spread out? How can we know it is not?

Is the effect you are talking about the one described here:


in section "Spread of a wave packet"?

See also https://en.m.wikipedia.org/wiki/Wave_packet#Gaussian_wave_pa...

“For example, if an electron wave packet is initially localized in a region of atomic dimensions (i.e., 10−10 m) then the width of the packet doubles in about 10−16 s. Clearly, particle wave packets spread out very rapidly indeed (in free space): For instance, after 1 ms, the width will have grown to about a kilometer.”

“Note that a very narrow initial wave packet instantly becomes infinitely wide, but with a phase which is more rapidly oscillatory at large values of x. This might seem strange — the solution goes from being localized at one point to being "everywhere" at all later times, but it is a reflection of the enormous momentum uncertainty of a localized particle, as explained above.”

I see but what about particles that are not that precisely localised and don't have such extreme uncertainity of momentum?

It's very logical that a particle that might have any momentum quicky spreads out to be smoothly everywhere.

Particle with not that precise position and not that uncertain momentum should stay fairly comapct for large amount of time.

What does “fairly compact” mean if not “precisely localised”? If the wave function is not localized it doesn’t make much sense to talk about a particle in the first place.

"Fairly compact" as in precission that we can achieve, instead of perfect precission and mathematical consequence of quickly spreading over entire universe.

I think it never makes much sense to talk about a particle. In the wave particle duality, the particle part doesn't have many arguments for it. You can understand and calculate everything using waves. Just not macroscopic kind of waves but waves that have few peculiar qualities that macroscopic waves don't have.

Any way... Thank you for indulging my ramblings about the things I barely grasp. :-)

You can get some intuitive understanding of “the wave” as something in space only when you have one single particle. But as soon as you have two interacting particles what you have is a single wave function which is a (complex) function of six parameters (three coordinates for each particle). Add more particles - and their spins - and pretty soon you cannot “understand” anything at all from the wavefunction on its own.

But how does the wavefunction become narrow and sharp "due to measurement?" That's the crux; that's what the measurement problem is.

Making a wavefunction "narrow and sharp" means projecting it onto an eigenvector of your measurement basis (e.g. "spin up.") Projection is non-unitary and irreversible. Whether and how this can be achieved via the Schrodinger equation is what all the fuss is about.

Schrodinger equation describes evolution, but not reshaping.

Reshaping is caused by interaction. Interaction occurs with probability defined by wave functions.

The thing is, to predict results of experiments we don't really need to know how exactly wave function reshapes. It's enough to know how it "reshapes" to a specific point, that we from that moment on call a particle that came out of wavefunction collapse. You can consider as many points in as you need to calculate everything you want to know.

Hence current state of quantum mechanics where we can calculate everythig but don't feel as we have a full picture of what happens.

Many interactions are perfectly described by Schroedinger's equation. The interactions inside an atom, for example. Or the operation of a quantum computer.

What "occurs" (that's the part we don't really understand) with probability defined by wave function is "measurement" but not every interaction is a measurement. A measurement is the macroscopic effect of interactions (which may or may not be fully described by Schroedinger's equation).

Said reshaping is the hard part, basically everybody who have studied a bit of physics understands the schrödinger equation and the wave part, the hard part is the particle like behavior which causes the wave to break down. Meaning it isn't a wave, waves doesn't break down like that, waves doesn't just gather together into some coherent state randomly. That is the hard part, if you can't explain that then don't say that QM is easy.

What I don't exactly understood is why the hostility toward theories that could lead to quantum gravity and whatnot? What are the historical reasons for it?

It's complicated, and it's going to take a long time for historians - and multiple books - to untangle the failure.

But so far as I can tell it started because Bohr was a bit of a tyrant and didn't like having the Copenhagen Interpretation questioned. So that set the tone for the first few decades. And QM was still developing, so it was more important to solve specific problems than worry about generalities.

After that, progress was still being made on the Standard Model, so there was academic selection pressure pushing graduates towards PhD specifics and away from more philosophical speculations about Quantum Origins.

And after that the String Theory people were much better at aggressive self-promotion than any of the competing QG theories. So something related happened again.

Now there's no data to allow further fundamental progress on the Standard Model. SM alternatives continue being ruled out. And String Theory has run into very serious problems, So there's more interest in Quantum Origins and alternative QG theories. The odds of a big breakthrough are very low, they're higher than they used to be.

And also it's really kind of annoying not to understand what's happening.

>And after that the String Theory people were much better at aggressive self-promotion than any of the competing QG theories.

I'm not sure if that's a fair interpretation of history. At the time string theory still looked like the best option, and some say that even today it is the closest thing we have to a consistent theory.

Was there ever a sting theory that looked plausible to explain gravity say? As far as I can tell it started off as a simple model of how the nucleus held together - to stop the protons flying apart lets model stings holding them together. And then after that it's been let's play with the maths and the aggressive self-promotion thing mostly.

In this Lex Freidman interview, Eric Weinstein (a bit kooky) explains his view on the trenches of advanced mathematics/physics:


He goes into depths about the problems right now in academia

I think a problem is the current theoretical attempts to get to quantum gravity are not very good leading to a certain amount of waffling about probably irrelevant stuff.

Vote bank politics version of academia.

You can read about the newer theory mentioned (but not explained) in the article here: https://en.m.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%8...

"If you think you understand quantum mechanics, you don't understand quantum mechanics." - Richard Feynman

> If you think you understand quantum mechanics then you don't understand quantum mechanics.

Richard Feynman

This is the third time I'm reading this quote here.

Isn't this a tautology?

It's the same with all probanalistic descriptions of a phenomenon, isnt it?

You can use probability to model all kinds of things that you cannot (yet) directly model, cannot observe, and do not understand. It seems natural that it's a way to describe a system not well understood.

Isn't many worlds what you get when you interpret the Randomized Model as Truth?

Time to grab your tinfoil hat. Of course physicists don't understand Quantum Mechanics, it has been classified in 1964. Before that time we didn't really have the computational resources to use the alternate model efficiently.

You've got to remember we are talking about the technology which make nukes.

What if the physics world was as simple as the one we teach in high school, a simple classical world with a twist. In the 1950s there were even science kits for kids with Uranium. We were on the track towards a world with clean infinite energy. Clean, yes, because the first fusion bomb had just been tested, and fusion technology was just around the corner.

Then the cold war paranoia made everything change. The Vietnam War brought what ifs ?

What if the know how was so simple that any Vietcong could build a nuke ? What if one of those damn pacifists decided to make one at home ?

They gathered a few people from Los Alamos to seek council. The tale has it that it was Von Neumann that put the final nail in the coffin of the green world utopia. With machines using its architecture, it would, before a few decades, be simple for everyone to simulate everything from chemistry to atom bombs. It was simply not acceptable. They devised a plan to make sure that was not going to happen.

The beauty of science is that once you accept a false result, you can easily disprove the true theories. Then it's just a question of smoke and mirrors (should I say vapor chambers, and single particle detectors ?). Control the experiment results by making sure the experimenters know the theory it needs to prove.

That's how in 1964, a crackpot physicist made the world non-local. It was such a success to attract young new people to the field, that those who knew better played along.

Of course since then smart people saw through the veil and are thankful for the elders' wisdom to not free the dragon.

" it has been classified in 1964"

oh, and math too? All over the world? Seriously...

QM comes from a time where computers weren't powerful enough. It's a non-local theory that's helpful to calculate by hand. With the advancement of computers, they stumbled upon ways of calculating the same results faster in a different way. It was deemed too dangerous. So they figured a way to backdoor the science.

Come on, it's not that hard of a campfire story to understand, burning-man was last week and area 51 raid is very soon, you gotta step up your game...

Presumably the "crackpot physicist" of 1964 who "made the world non-local" in this interesting narrative was John Bell.

Yep, here is the red queen.

Also famous for not winning the Nobel.

Time for the rabbit hole!

Maybe it was that refurbishing Boole's inequalities wasn't enough.

Gather a few mathematicians well-versed in probability theory, pick a wrong hypothesis, wrap with a theorem, don't get the conclusion, then take the contra-positive : It's like playing three-card Monte with physicists.

Thank you for the fun story!

I can't follow, what do you recht to say? That quantum physics doesn't exist?

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