I don't understand quantum physics (2018)

Don't know if it's just me, but whenever I read or asked about quantum physics, I'd immediately get stuck on what it means that a photon is a particle and a wave, usually that exact phrasing. A wave of what? Some explanations even make it look like the particles move through space in a sine wave.

This doc is the first one I've seen that explains upfront what a wave means here: "The quantity which travels in this way is known as a 'disturbance'. Precisely what this disturbance is depends on the wave," and for EMR, "it is an electric field and a magnetic field which are perpendicular to each other and also perpendicular to the direction of the wave."

So many entry-level explanations of quantum mechanics start with classical physics, waves, particles, the double-slit experiment, etc. and then jump straight to infinite-dimensional Hilbert spaces. In my opinion, there's way too much conceptual baggage that comes with these concepts [0].

As a non-physicist, QM only clicked for me when I learned the linear algebra of one- and two-qubit systems and observables on those systems [1]. The math is accessible at a pre-calculus level, and the most "counter-intuitive" aspects of QM (interference, the uncertainty principle, entanglement) make perfect sense in this framework. I think it's the best starting point for understanding what QM really says.

[0] That's not to say they aren't important historically or for a complete understanding of QM. I'm talking about conceptual understanding for someone, especially an amateur, encountering QM for the first time.

[1] Chapter 2 of Nielsen and Chuang is what did it for me. It's heavygoing if you've never seen linear algebra before, but I don't know of a clearer explanation of the postulates of quantum mechanics.

Of possible interest

Basic Concepts Of Quantum Mechanics by L. V. Tarasov

https://archive.org/details/tarasov-basic-concepts-of-quantu...

The dialogues of the author engages with various personas and the reader that are interspersed as Prelude and Interludes are really enlightening.

For example:

Classical Physicist: Can one seriously speak about an object without having an idea of what it looks like? Isn’t it strange that we study, for example, the behaviour of an electron in a crystal while we don’ t even know what an electron actually is?

Author: I don’t agree that we don’t even know what an electron is. I have just indicated a number of precisely determined characteristics and properties of an electron. More detailed properties of microparticles in general and electrons in particular were considered in the preceding sections of the book (and will be considered in the following sections). In fact, we know quite a lot about the electron and know, in particular, about its behaviour in a crystal. This is evidenced by the large number of semiconducting devices fabricated and used by us in practice. As you will see, the absence of a “physically intuitive” model of the electron has in no way turned out to be a serious obstacle. We may even go a step further and state that an understanding of natural phenomena in which Planck’s constant plays an important role is possible only through a significant rejection of a graphic description. By the way, this idea was given by Heisenberg, in whose works much attention was given to questions of the use of physically intuitive methods.

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

"This rather fanciful interpretation of Quantum Physics has now become accepted by many theoretical physicists as being more satisfactory than the Copenhagen interpretation, despite the fact that the mechanism which causes the splitting of the Universe is not explained in any more detail than wavefunction collapse. As you can probably tell, I am in the minority of physicists who finds the Many Worlds interpretation dificult to accept, and not particularly illuminating."

I think these lectures are a good intro to QM. This statement feels very emotional though. Many-worlds just states there is no such thing as wave function collapse. The many worlds are a logical conclusion following. Copenhagen simply refuses to elaborate to when wave-function collapse happens, which to me is not a complete theory.

You could have some critique of many-worlds in relation to statistics, although I think many people have given quite satisfactory explanations.

Many worlds is just bare bones QM.. If any other theory is proposed, it should be falsifiable (like pilot wave theory for example). Doesn't mean it won't be difficult to measure.

Sharing in case useful for someone. Started getting into this subject in 2015, and full disclosure: I'm no physicist by any means. Without the formal background, the math and concepts were really hard to conjure and comprehend purely from the page, so I built an interactive self-study guide to "feel" the equations from Newton to holography. https://apples-to-entanglement.jasonrys.com/

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

Regarding questions of what how measurement relates to schrodinger time evolution: https://arxiv.org/abs/quant-ph/0503009 . I think it is absolutely astonishing that many physicists do not know this.

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.

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.

Unfortunately, this work perpetuates the misunderstanding of Bell's theorem and its implications with respect to hidden variable theories. See the Scholarpedia article on Bell's theorem for an in-depth discussion, including quotes from Bell himself, deeply frustrated at the misunderstanding.

https://scholarpedia.org/article/Bell's_theorem

I started to read from "3.1 The Ultraviolet Catastrophe" and this sentence

"The number of different wavelengths of electromagnetic radiation, N(λ)dλ, with wavelengths in a small interval between λ and λ + dλ, (dλ means a small interval in wavelength) can be shown to be proportional to 1/λ⁴."

doesn't make sense. It should not talk about a number of wavelengths, but the number of allowed standing-wave modes (number of eigenmodes that a mathematical solution to the electric field in the relevant situation has) in the small wavelength interval.

I used to struggle with quantum physics- in the same way many others do- in that the model is not intuitive to people who think in terms of billiard-ball physics. But, I think it's clear at this point that all the observable data from well-run experiments supports QM better than the alternatives. IN particular, the observation of matter waves and double-slit experiment on atoms were what finally made me accept it. Entanglement doesn't make any sense to me, but I'm assured it can't be used to transmit information faster than the speed of light, so I can live with it :)

How strange to see a familiar name here! I wasn't his PhD student but worked in the same department as Douglas Ross, and can only say that he was warm, extremely intelligent, with a very strong natural intuition about high energy physics, enjoyed puzzles, and wouldn't shy away from quizzing people giving talks. I remember he had a back and forth with a visiting speaker for about 20 minutes on some minutia!

I don't have much to add on the article itself except that it's almost certainly worth reading.

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

"I have used many photos and drawings which I have found on the Internet, and in many cases the source of such photos and drawings is not clear. I have assumed that images which appear on the Internet are in the public domain and do not need to be individually referenced."

That's a pretty bold assumption.

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.

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

- Richard Feynman

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.

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?

"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.

No one on earth understands it perfectly.

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.

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.

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

So do the rest of us.

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.

Wave function collapse is a misnomer. We now know that weak measurements do not collapse the wave function but rather drives the probability amplitudes of the states towards an eigenvalue. For those familiar with linear algebra recall that power iteration of a matrix with a random vector will yield the matrices eigen vector given enough iterations. Collapse is similar to this phenomenon but non deterministic.

My theory: The reason we perceive collapse as irreversible is because the computational complexity of reversing the equations is very high. Especially as the state converges towards an eigenvalue of the measurement operator, it becomes more difficult to reverse. This is similar to how a hash function is reversible in principle but not practice. My two cents.

Many worlds and such are just pseudo religious nonsense.

It’s easier the accept quantum physics if you first

1) Disregard many worlds as a purely remaining mathematical outcome of bounded probability rather than discrete reality. (No 1 thing exists just 1ish things but ultimately that doesn’t mean there are many versions of any give 1)

2) just accept you can’t find the “stuff” of reality. You simply measure increasingly small probability domains which interact with other probability domains

3) time is the sequential working out of probability of anything existing and other things that might exist co defining themselves.

4) space is just the interaction of these probabilities working themselves out into a stable projection.

Many worlds is a mathematical construct of our inability to measure specific dimensions hyper locally and an artifact of that. Which for some reason the entire scientific community mistakes as a real observation of reality.

Just because at a quantum scale everything becomes potential doesn’t mean all potential has been manifested.

It baffles me that so many serious scientists don’t understand this basic tenet.

This deserves a [pdf] in the title.

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.

Imo it's a bunch of rules and if u follow them you get accurate answers. What more is there to understand? It seems very unlikely that aspects of the universe more removed from our macro-scale existence would make sense or be comprehensible to beings whose minds evolved for hunter-gathering and social dynamics in Pleistocene Africa.

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.

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 =\

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(2018)

2018 ragebait?

>> but we have to live with this strange idea if we accept that Quantum Physics describes Nature correctly.

That depends on how you define nature, world and universe. For me nature is defined by things that can be perceived by human senses.