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I Don't Understand Quantum Physics (2018)

91 points rramadass 2 hours ago 55 comments

rramadass 2 hours ago | parent

Author: Prof. Douglas Ross of Southampton University - https://www.southampton.ac.uk/~doug/

polotics 2 hours ago | parent

me neither! How does the Einstein Rosen Podosky Paradox work? can the unknown that's being collapsed not be a pre-determined value that therefore is not traveling faster than the speed of light?

superluserdo 1 hour ago | parent

>can the unknown that's being collapsed not be a pre-determined value that therefore is not traveling faster than the speed of light?

This is known as a "hidden variable theory". People will tell you that a local (things can only affect their immediate surroundings) version of the theory is impossible because it would be a violation of Bell's inequality, but there is a loophole that requires accepting the idea of "superdeterminism". This is the idea that where what measurement a scientist chooses to measure is predetermined and correlated with what the particle being measured is going to do.

So if a particle lands on a screen according to distribution p when going through a double slit, but with distribution p' when one of the slits is measured at, conventional QM says that the measurement "collapses the wave function". But a superdeterministic local hidden variable theory would imply that the choice of whether to measure at the slit or not was already set in stone at the same time that the particle "decided" which path to take to land on the screen, more or less.

lkm0 58 minutes ago | parent

One of the "coping mechanisms" is to say that information cannot travel faster than light, even if entanglement is propagated instantaneously and measurement acts faster than light. If Alice and Bob share an entangled pair of spins and Alice measures it, Bob has gained no information until Alice communicates (sublight speed) her result. No new information is shared at measurement time. This is known as the no-communication theorem.

beloch 47 minutes ago | parent

If you split up an entangled pair of particles and measure them in a the same 50/50 basis simultaneously (sort of like a coin flip) at distantly removed places, the results will be individually random but perfectly correlated when compared with each other. It seems natural to assume that something here was pre-determined. i.e. There's was some kind of hidden information that we haven't found a way to measure yet. The way in which the coin-tosses were going to pan out was determined when the pair of particles were entangled.

Quantum physics makes predictions that suggest this isn't so. You can design experiments (e.g. A Bell inequality test) in which quantum physics predicts correlations that go beyond what would be possible with some hidden local information attached to the entangled particles that we just don't know about. Experiments don't prove theories to be correct. That's not how science works. However, experimental tests of Bell inequalities have pretty conclusively shown that local hidden variable theories are inadequate to explain what's going on.

Yes, it appears what Einstein called "spooky action at a distance" seems to propagate close to instantly. It's important to appreciate that you can't transmit information this way though. We can only make sense of the quantum correlations after communicating other information at classical speeds. Sometimes you see quantum entanglement devices allowing FTL communication in sci-fi, but that's pure horse hockey.

That all being said, I don't understand quantum physics either.

atemerev 1 hour ago | parent

When I learned about causal sets program and algebraic quantum field theory, I finally (somewhat) understood quantum physics.

Tl;dr spacetime does not exist as a global background where things are "placed", it is not ontic. "Things" are first, and spacetime is derived from interaction statistics of "things". This eliminates nearly all naive paradoxes like double slit etc. Now, these "things" are somewhat complicated and non-intuitive (e.g. there are no individual "particles", they are a limit of discrete distributions), but that's a good start.

cgio 1 hour ago | parent

Sorkin is great. I think causal sets make some things simpler but other more mysterious. I also found t’ Hooft’s automata and Spekkens toy illuminating in different ways.

atemerev 1 hour ago | parent

"Was", unfortunately. He died this September. What a loss, he earned a Nobel long ago.

isolli 1 hour ago | parent

In a podcast, Alain Aspect (Nobel Prize winner) explained that he does use the power of visualization and his imagination to generate new ideas, but that he always needs to check them with actual calculations because no mental picture can be an accurate representation of quantum physics.

chistev 1 hour ago | parent

"I think I can safely say that nobody understands quantum mechanics."

- Richard Feynman

ahartmetz 1 hour ago | parent

I also like "If you think you understand quantum mechanics, you don't understand quantum mechanics" which is supposedly also from Feynman.

atemerev 1 hour ago | parent

Quantum _mechanics_ cannot be understood, it is full of paradoxes. It contradicts classical EM and special relativity, among other things. A toy theory that was an important step to what came later.

Feynman's own QED is a part of QFT, but this is no longer quantum mechanics. Special relativity and quantum physics are reconciled since then. Now, we only need to reconcile GR and QFT, but this leap was so far beyond our attempts to scale it. Yet, QED/QFT/SM can be "understood" as there are fewer obvious paradoxes that plagued QM.

chistev 1 hour ago | parent

I think it's one of those famous quotes that was never said. The one I quoted was what he apparently said.

stratos123 1 hour ago | parent

Whether this was even true in Feynman's time, it's much less true now.

tim333 14 minutes ago | parent

With the meaning that Feynman meant it was true then and now. From QED:

>The next reason that you might think you do not understand what I am telling you is, while I am describing to you how Nature works, you won't understand why Nature works that way. But you see, nobody understands that. I can't explain why Nature behaves in this peculiar way.

quietFerry94 1 hour ago | parent

That pre-determined value idea is exactly local hidden variables, and Bell's inequality rules it out, Aspect's 1982 CHSH experiment measured the violation.

caturopath 1 hour ago | parent

To be precise, Bell's inequality rules out a local hidden variable theory. Pilot wave theories/Bohmian mechanics is an attempt to describe a nonlocal hidden variable theory of quantum mechanics.

antonvs 1 hour ago | parent

To be even more precise, it rules out hidden variable theories that satisfy Bell locality, i.e. the assumptions that the theorem makes.

Superdeterminism and retrocausality both allow for local hidden variables - it’s just that they don’t satisfy Bell’s assumptions.

antonvs 1 hour ago | parent

If you saw that in section 15, the very next paragraph covers Bell.

mono442 30 minutes ago | parent

it doesn't if you assume superdeterminism

salsa_catsup 14 minutes ago | parent

I don't understand the superdeterminism idea. Wasn’t that already the classical/Newtonian view - that everything was predetermined, a universe of billiard balls following their destiny exactly?

MattPalmer1086 6 minutes ago | parent

Newtonian is just plain determinism. One thing causes another, but does not imply things are fixed in any particular way.

Superdeterminism implies that things are fixed in advance in such a way that the things we "choose" to measure always looks as if there are no local hidden variables (e.g. Bell's inequality) even though there are.

asystole 1 hour ago | parent

This piece looks great, adding to my ever-growing "to read" collection on Zotero.

If anyone is interested in self-studying quantum physics, QFT for the Gifted Amateur [1] is very well regarded.

1: https://academic.oup.com/book/36442?login=false

ktallett 55 minutes ago | parent

Second this book, but you need a source before this to study QP successfully. This requires a lot of UG maths and physics knowledge that I think most would not have.

rramadass 18 minutes ago | parent

See also, Atomic Physics for Everyone: An Introduction to Atomic Physics, Quantum Mechanics, and Precision Spectroscopy with No College-Level Prerequisites. - https://news.ycombinator.com/item?id=46961595

frumiousirc 1 hour ago | parent

I take issue with "understand". I agree QM can not be understood in terms of everyday experience. One must build experience, language and proper metaphor for the understanding. For example, in the intro Ross uses the example of explaining a waterfall to someone who never saw it. He says that can be done but he relies on implicitly the listener knowing what people on Earth may take for granted. That understanding of a waterfall can only happen if the listener already has some understanding of fluid flow and gravity. We do, but say a newly found alien friend from a dry environment that had never experienced fluid flow would not have the mental building blocks to understand it. Feynman gave a famous interview where he said, essentially, "no, I can't explain it to you because you are too ignorant" (he said it more politely). Going the other direction, Physics steeped enough in quantum theory do understand QM enough to have meaningful discussions free (at times) of rigorous math. They do this because they have a shared language, vocabulary, metaphor. Sure, they must rely on math and experiment to ground that understanding in truth but I guarantee they do not communicate understanding (solely) in math and measurement data.

charcircuit 1 hour ago | parent

Why can AI generate pictures of waterfalls if it has never lived on Earth?

bozdemir 1 hour ago | parent

So do the rest of us.

dtj1123 1 hour ago | parent

We understand quantum mechanics well enough to build machines that depend almost entirely on quantum principles.

It sounds all superior and mysterious to quote Feynman here, but the fact is many people absolutely do understand quantum mechanics.

Razengan 1 hour ago | parent

Or we understand just enough to make things and instruments that only work on (and up to) the layer that we understand.

Ancient humans could build boats 5000 years ago and expand throughout the planet, but they didn't know about water being made up of hydrogen+oxygen, or the tides being caused by the moon, etc.

Their calculations served them on the layer they operated on.

lloeki 57 minutes ago | parent

> the tides being caused by the moon

Mostly because people around the mediterranean sea didn't have to care, tides being so small there. Yet the Greek had fairly advanced models of tide schedules.

Pretty sure peoples that were experiencing bigger tides and depended on it e.g for fishing figured out the relationship with the moon pretty quickly, whatever esoteric reason they may have plastered upon it.

uecker 44 minutes ago | parent

If one does understand quantum mechanics well enough to use it, then one also understands that there are aspects of it we do not fully understand.

dtj1123 27 minutes ago | parent

You can make an identical argument with respect to classical physics, e.g. outstanding issues in fluid dynamics or statistical mechanics.

Nobody takes this to mean that classical physics is generally incomprehensible.

mettamage 31 minutes ago | parent

Can’t we say the same about LLMs? Yet interpretability of them is rough.

We can build black boxes and exploit the behavior of what we see.

But we can’t always go to deeper and deeper layers of understanding.

QM is not a black boxes we built but one could perhaps say it’s a black box that we see.

jasode 24 minutes ago | parent

>It sounds all superior and mysterious to quote Feynman here,

It isn't about being superior, mysterious, or clever. The following "nobody understands" type of quips and thought experiments are different ways of saying our intuitions don't help us understand it :

- "God does not play dice with the universe.", Albert Einstein

- "Shut up and calculate", N. David Mermin

- "Schrödinger's cat" being both alive and dead

- various interpretations/analogies explaining quantum physics leading to absurdities or contradictions

Yes, humanity has harnessed quantum mechanics with mathematic frameworks and repeatable experiments to make useful things... but that's a different layer of "understanding" than the deeper levels in the above quotes.

"Nobody understands what gravity is." ... Wait, if nobody "understands" gravity, how did we send people to the moon?!?

Because those are different usages of the word "understand".

tim333 17 minutes ago | parent

We understand part of it but not how it works.

bobvanluijt 1 hour ago | parent

Somewhat related: a book that really helped me is Picturing Quantum Processes by Bob Coecke. It's a visual representation of quantum processes, which really helped me understand the concept

noduerme 1 hour ago | parent

My question is, if I just refuse to believe in quantum physics, that makes it go away and/or be inapplicable to me, right?

Uh oh, some people have no sense of humor =\

mkesper 1 hour ago | parent

(2018)

lkm0 1 hour ago | parent

"IS QUANTUM PHYSICS CORRECT AND COMPLETE?" is reductive, there are more quantum denominations: Copenhagen, QBism, RQM, consistent histories, Many-worlds, Bohmian mechanics, and various spontaneous collapse people. I might've missed a few.

Synaesthesia 56 minutes ago | parent

Yes this is a particular interest of mine, the various interpretations of QM. Most physicists subscribe to the Copenhagen interpretation, but I still find it has philosophical quandaries. He mentioned many-worlds. Bohmian mechanics has been shown to be inadequate.

Blokhinstev's ensemble interpretation, which I like, is unattractive to many.

phpnodejspwn 1 hour ago | parent

2 things to understand about science:

It's useful, not metaphysically real. (Hot take btw) This is called Instrumentalism. The belief that it's real is called Scientific Realism. However over and over again, science can't cover edge cases, and we find better fitting models. The Copernician revolution is a good example of this dichotomy.

2. Academia basically forces you to believe the doctrine or you fail the class. From philosophy to physics to sociology. If you read the philosopher survey, you will find trendy things being quite high. It's a religion in itself.

Bonus 3: Philosophical Pragmatism and Instrumentalism generally solves this, but is intellectually a dead end. There is a reason analytic philosophy continues despite gaping holes in linguistics (late Wittgenstein) that ruin it. But at the same time, best fits might be missing a new paradigm. I wish I could tell people to ask AI to explain Hegel to understand, but I think people need to read it themselves.

Tl;dr: quantum mechanics is the best we have. It's definitely wrong, but it's the most useful we have.

gus_massa 46 minutes ago | parent

> 2. Academia basically forces you to believe the doctrine or you fail the class.

Yes, but if you can make a reliable experiment that disagree with the doctrine, it will change, even if everyone hates the idea. In particular https://en.wikipedia.org/wiki/Wu_experiment

ktallett 56 minutes ago | parent

The key aspects of quantum physics can be explained very simply. The issue is going in depth without those basics. I often find if someone doesn't get Quantum Physics, it's that source or you need the steps explaining more in-depth. Often sources assume your base knowledge and that is where many fall flat.

immmmmm 21 minutes ago | parent

I loved the book QED: The Strange Theory of Light and Matter by Feynman as a child. Funny enough I always saw QM from a path integral PoV, a decade before I got to learn the formalism

_heimdall 20 minutes ago | parent

Unpopular opinion I think, but I still believe the physics world was wrong to chase the Copenhagen interpretation.

The math does indeed work, and many experiments and practical uses seem to show that there is something to quantum broadly. But to throw determinism out the window and believe that quantum is fundamentally probabilistic, and that one can never really run scientific experiments because observing state collapses superposition, has always seemed asinine to me.

I could be completely wrong, but it's always seemed obvious to me that the need for probabilities in quantum is a clear indicator that something is missing in the theory. I'm never surprised when a person can't understand quantum as its described today.

karussell 8 minutes ago | parent

It has been shown that you can keep determinism (Bohm etc), but what follows is not necessary something you'll find more intuitive or cleaner or more logical.

boltzmann64 19 minutes ago | parent

Saying I don't understand Quantum Mechanics is like saying, "I don't understand how attention/transformers explain the human brain."

It doesn't. It's model. All models are wrong (including quantum mechanics and attention) but some are useful. You understand the domain of the model, the problems it can solve, the math behind the models and use the model to make predictions. If you are competent at linear algebra and functional analysis, you can do quantum mechanics.

If you make the error of believing that quantum mechanics is ontological god's honest truth of nature, you will end up with blunders like using it to explain consciousness like Prof Penrose. His math was impeccable, but he was making a category error.

DiogenesKynikos 16 minutes ago | parent

There are only a few things that I think are exactly correct in physics, and quantum mechanics is one of them, alongside Relativity.

Quantum mechanics is a framework for how theories are built, not a specific theory, and it's really all or nothing. There's not much between the quantum and classical ways of viewing the world.

kergonath 13 minutes ago | parent

Both cannot be “exactly correct” at the same time. And most likely, neither are.

DiogenesKynikos 8 minutes ago | parent

Indeed. The classical view is wrong, and the quantum view is correct.

But it just turns out that there's a limit in which the quantum world looks approximately classical.

truculent 15 minutes ago | parent

> It doesn't. It's model. All models are wrong (including quantum mechanics and attention) but some are useful.

Sure, but what if you don’t understand the model?

lefra 7 minutes ago | parent

Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".

Models are simplifications/approximations made on purpose, as in "I know this is wrong on some level, but it's close enough for my use case", or "this makes the maths solvable".