Physicists want to rebuild quantum theory from scratch | Hacker News Reader

Physicists want to rebuild quantum theory from scratch

wired.com

90 pointscgmg41 comments

All I ask is the headline not be "Physicists want..." but rather "Some physicists want..." A couple of people I've never heard of don't speak of all physicists, in fact most

>[...] just shrug and say we have to live with the fact that quantum mechanics is weird.

In fact, I don't think it's weird, I think it's reality.

But they are right, it is totally weird.

When people say that they usually mean counterintuitive with respect to everyday experience.

It makes it progressively harder for lay people to grasp bits of it, from classical mechanics, to general relativity, to QM. Maybe it’s even a factor for theoreticians, but that’s just speculation.

I guess my perspective is that a lot of physics is weird. When I learned Newton's First Law in undergrad, it was counterintuitive, because I based my knowledge on experience. The idea that throwing a ball out of the window of a moving car would have a velocity component in the direction of the car contradicted naive experience of putting your hand outside the window and feeling the wind. Not just GR, but even Newtonian gravity is weird, how is the force making me fall the same as the force that makes planets move in circles[0]?

May be "weird" isn't the word but "confusing/unexpected" is. I feel like a lot of the "weirdness" of QM is really due to physicists who were wedded to classical ways of thinking are confounded when x and p don't commute, and what that means. That plus not knowing about the delayed choice quantum eraser (not their fault).

There is a "sermon" that is given at the beginning of almost every QM intro lecture, and as it was given by Feynman more than a half century ago, it went something like:

> "Quantum mechanics is weird. Particles are not like baseballs; leave your classical intuitions behind."

Which is all well and good. But it has subtly shifted to:

> "Quantum mechanics is weird. Our human brains are not wired to understand it intuitively, so don't try, because you will fail and get the wrong answer. Just turn the math crank and get results."

And IMHO that attitude is both (a) flatly wrong and (b) mind poison.

With regards to (a), the human brain is a turning machine-- it can simulate any rules. Learn the rules and work with them enough, and they will become intuitive. If you can learn DiffEQ or Fourier transforms, you can learn QM. Actually, the rules for the "spooky bits" are probably simpler than both of those two things.

For (b), all these undergraduates grow up into research physicists and still believe the creed they were solemnly lectured on Day One about how to think about quantum mechanics (which is to NOT think about it). As such, they are all very bad scientists because they have stopped wondering about what makes it all tick. Case in point: A good fraction of scientists still believe in "wavefunction collapse", which blows my mind, because it's so self-evidently rejectable[1].

Science went from a Plum Pudding atom to quantum field theory in about 30 years. It's been nearly a century since then, and we don't really understand things much more deeply, and I think this is because we are too busy banging particles together and not asking "why" enough. I (boldly) posit that if undergrads didn't get the "don't ask" lecture, quantum physics would be decades ahead of where it is today. We can understand, and we should try, and I would love it if undergrads were told that instead. The kind of thing in this article is what more serious physicists should be doing.

[1] This is a controversial statement, but that's kind of my point. Explaining Why Not Copenhagen is probably a topic for a separate diatribe.

I'm certain that human brains aren't Turing machines; we know that the entire set of human brains halts! Also human brains do not have infinite tapes or an approximate equivalent. So it can't be said that it can simulate any rules.

> ackchyually.jpg

;)

I think you got my point, though? The rules of QM are not a busy beaver function. They are simple and they can be learned because the human mind is flexible.

Yes, manipulating the algebra can be very complicated indeed, but that is because we are analyzing how the simple rules carry out and looking for an exact answer. It's like playing checkers vs. solving checkers; or learning the rules of Conway's Life vs. actually running a simulation. We can understand the rules and gain meaningful intuitions about both. If we want an exact answer it gets complicated.

If I were to be bold enough to rephrase what you so elegantly said.

We're looking at the universe as the pre-copricornians were. Seeing chaos and logging the data, believing that the order behind the masterpiece was all locked away in God's noggin, hidden from man's grasp.

Instead we should be looking at the universe as Copernicus did. Re-frame our base assumptions. Among all this chaotic data is a simpler pattern. It may make no sense whatsoever to our human assumptions about reality, but if we are to know reality, then we must evolve within reality.

I looked the jpg up, and you are a bit mean!!! I am trying not to cry real tears ;)

I get the busy beavers thing, humans can't do that, nor computers - it says nothing much!

But... are there things that are not computable that aren't formalised either - for example the choosing to determine when shifting system is needed to prove something that may or may not be proved in a particular system?

Sorry, maybe that was a bit harsh. :)

[Recurse Center](https://www.recurse.com/manual) (explained in the section 'social rules') has a social rule against "well-actuallys" which is intended to discourage nitpicking. If someone does it, you just point out "hey, that's a 'well-actually'", and they go 'oh sorry' and you move on. That was my way of pointing out a well-actually. :)

> the choosing to determine when shifting system is needed to prove something that may or may not be proved in a particular system?

Sorry, I am having a hard time parsing this. What do you mean?

A very long time ago, I went to university and took CS. I found it too easy, so I switched to Physics. I found that too hard, so I switched back to CS ;-). I didn't get "the lecture". In fact, just the opposite. We were taught physics and then told that it was up to us to learn whatever math we needed to help us. Not being particularly fluent in math, I just couldn't keep up at the time. One of the best things about my brief foray into Physics was that it taught me the importance of learning math -- not as a prerequisite for something, but as a tool.

Having said that, I think you are wrong in your supposition. QM is hard to reason about because it doesn't manifest itself particularly visibly in our everyday lives. The framework that you have to work with comes from your own imagination (and mathematical models). There is no real problem with that, except that if you get the axioms wrong, you will necessarily get the conclusions wrong as well. There are an infinite number of ways that it could work and virtually all of them are incorrect.

With classical physics, you can reason based on your lifetime of observations of everyday objects. Of course, we still get it wrong. The more we abstract away from the ordinary, the easier it is to jump to strange conclusions. QM is abstracted so far away that it is incredibly hard to test anything intuitively. That's why math is attractive. It gives you a logical system for testing your ideas.

Math is just a language, though. It's a way of expressing an idea. If you have an idea and express it in English, it is no better or worse than expressing it in Math, and no better or worse than expressing it in pictures (Expressing it as short presentation with hand puppets, is obviously the best way, but that should go without saying).

Similarly, if you are working through a problem by talking to yourself, or doing math, or drawing diagrams, or imagining scenarios with hand puppets -- it's all equivalent. The representation of the thing is not the thing.

And therein lies the problem. Many people do not really understand things well. They think they do because they can picture a representation of it. Let me give you an example. I once showed a colleague one of those ring-within-a-ring magic puzzles where you have to unlink the rings. I showed him how to do it. I got him to do it 4 or 5 times and then I asked him, "Do you understand how this works now"? He said, "Yes, it is very simple". Then I took the magic rings away and gave him some straight wires. "Please build the magic rings", I said. Of course, he couldn't because he didn't understand them.

We see this problem all the time in programming. People think they understand the problem because they can write code. The code that they write gives them an illusion that they understand the problem. But the code is not the problem. They start focusing on the code and forgetting the problem. Eventually they have written a huge amount of code and they have not solved the problem.

You're just noticing the same phenomenon in Physics. It doesn't matter if a person builds a model with math, or diagrams, or English description, or hand puppets -- most people will start to focus on their language and their tools and become dissociated from the problem. They create their own religion around the model into which they have poured their creative energy. That becomes their reality and they have no idea that it doesn't match a more objective reality. The more abstract the system, the easier it is to tumble into fantasy land (which is why programmers are so susceptible IMHO).

The lecture I wish people would get is: "You are wrong. Now tell me why. If you don't know why, find out." That would help people a lot better, I think ;-)

I agree with a lot of this. But I still disagree with:

> With classical physics, you can reason based on your lifetime of observations of everyday objects. [...] QM is abstracted so far away that it is incredibly hard to test anything intuitively.

Most intuition that people have about physics which comes from everyday life is wrong. Even momentum is not intuitive; that was not figured out for thousands of years, and children and laypeople still get it wrong all the time. Gravity is not innately intuitive, especially with orbits. Electricity is not intuitive. Magnetism is not intuitive. But you learn the rules and you work through problems, and you start to see how the rules play out. And you develop an intuition for all of these phenomena that arises from your deep, high-level understanding of the rules, not from everyday experience.

An electrical engineer could look at a circuit diagram and tell me the gist of what it will do, even though she has no direct perception of electricity. An aerospace engineer could tell me without solving any equations what would happen if we gave this orbiting spacecraft a little push in this direction, even though he's never been to orbit. And so on.

QM is just another branch of physics like all the others in which to learn the rules and gain a conceptual understanding beyond merely cranking equations. It is not special in that way.

> In fact, I don't think it's weird, I think it's reality.

At the risk of sounding snarky, I think it's useful to point out that we call it the Standard Model and not the Standard Reality.

Model ≠ Reality, yadda yadda.

We have an immensely useful model of fundamental physics as long as the energies are low enough. However, I feel that we lose a lot by letting that utility stop us from asking epistemic questions of the model itself.

Historically, math has gained a lot by taking such questions seriously, especially in the areas of logic and computation.

> Model ≠ Reality, yadda yadda.

I think im a fan of what is described as quantum bayesianism in the article, that quantum mechanics is basically a probabiltiy theory, and this probability theory is required to express our expectations from measurements (quantum mechanics encodes our beliefs about what will be observed given some knowledge about a system) and this is fundamentally a "retreat" because we dont know how to describe the underlying reality. even though calculating the probabilitoes may be very precise.

but i am havig trouble distinguising quantum bayesianism from the standard "model vs reality" fare as the motovation seems very similar

The thing I say is "what underlying reality?" If you can't measure it, how can you make a theory out of it or research it (or publish papers on it unless you're a string theorist)? If something is not measurable, then how can you claim it exists or not?

Whenever people ask me these "deep questions" after I tell them I'm a physicist, I tell them that. Science is all about the reality you can measure, not about meaning and what lies "deeper".

I take it you are a fan of the copenhagen interpretation of quantum mechanics. its the right answer for making calculations. but how do come up with a new theory? you have to tell a story or envision an underlying reason to justify it. new theories dont come out of nowhere.

this is also like saying the universe in the time of newton consisted of only newtons laws, which is untrue. if the universe is science, then you have to say the universe is changing as science changes. unless the universe really is just made of published journal papers.

"model vs reality" implies a very clear delineation between theory and data. Bayesian models muddle that up - particularly with (based on my understanding of) the standard model. (Insert the classic statistical decision theory debate about classic/Bayesian models here.)

What I really wonder about is how, if ever, we will refine our abilities to observe the finer grains - and to what extent that's possible. I remember physicists discussing the observations possible given particle accelerator size from lectures in undergrad. The future was not very bright.

Sure, but string theory shows what happens if you can't come up with a testable theory in a reasonable timeframe.

It is the ultimate "version 2.0" product. Missing features in the original, vastly over-engineered, and never ready to ship.

I wouldn't be so quick to judge. It's pure research, and String Theory has pioneered a lot of new ideas that have successfully cross-pollinated into other fields. Even the LHC employs String Theorists with good reason.

Seems a mistake to write it off as "not delivering" simply because it hasn't given us exactly what we originally expected.

On a more social level, a focus on "delivering" and "impact" in pure science seems like it can be taken to the point of shooting ourselves in the foot. Think of all the discoveries from pure exploratory research that we recognized as useful only after the fact.

so quick? it's not as if string theory is just getting started.

That string theory is already compatible with everything we know about reality is a titanic achievement.

If you think it's not, go ahead and list all of the other alternatives that also have a hope of unifying gravity?

Except that string theory requires supersymmetry, and the latest LHC results are not happy for supersymmetry.

As for other unifying theories, loop quantum gravity is somewhat popular. And Wikipedia has a nice list of alternative theories: https://en.wikipedia.org/wiki/Quantum_gravity#Other_approach.... String theory is far from the only game in town, despite what string theorists would have you believe.

String theory is in fact compatible with a large number of possible realities. That's the problem.

When I say "reality" as a scientist, I mean experiment, and the standard model does a pretty decent job of fitting experiment/observation.

I just had this conversation last week actually, where a group mate and I were discussing "determinism" and basically, we agreed scientists have weaker definitions for words like "reality" and "determined" and such that essentially mean "within the error bars of repeatable experiment."

Well the original article headline doesn't talk about anyone "wanting" anything at all -- I don't know, why would Wired repost articles from Quanta, changing only the headline?

https://www.quantamagazine.org/quantum-theory-rebuilt-from-s...

From Quanta Magazine's 'about' page:

To reach an even wider audience, we have syndication partnerships with several publications, which reprint our articles. https://www.quantamagazine.org/about/

And I presume what's in it for Wired is that it was well written - thanks for pointing out it came from Quanta, I was surprised reading something this detailed from Wired (with links to all the original papers, no less). Sadly the headline became their Battlechess Duck (https://blog.codinghorror.com/new-programming-jargon/)

Okay but it's not reality, it's a mathematical model and if you don't look for other mathematical models you could be missing a much cleaner simpler model that is just as accurate in predicting outcomes.

Problem is, all these redefinitions of quantum mechanics via "quantum logic" or "quantum probability theory", while looking more fundamental and formal than the usual theory based on Schrödinger's equation and Born's rule, do not make it any more intuitive and do not allow anyone to say, "aha! now I get it!". This is because these new foundations themselves are just as unintuitive as quantum mechanics...

[Edit] On the other hand, I find the famous derivation of Schrödinger's equation by Feynman illuminating: http://fermatslibrary.com/s/feynmans-derivation-of-the-schro....

It will be interesting to see if theoretical physics and computer science converge at all over the next century. Intuitively there’s much to suggest that our understanding of computational limits on things (not to mention the idea that the world itself is a simulation of some kind) has analogs in fundamental physical limits (speed of light, quantum noise).

I would be interested if anyone knows about the connection between information theoretic entropy and physical entropy.

I think thermodynamics and statistical physics are really interesting. In Parallel Distributed Processing they discuss an extension of the entropy metaphor to learning and neural networks

https://mitpress.mit.edu/books/parallel-distributed-processi...

> the connection between information theoretic entropy and physical entropy

Has been a subject of interest for the last six decades, from http://compbio.biosci.uq.edu.au/mediawiki/upload/b/b3/Jaynes... to https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.117.260... or https://arxiv.org/abs/1209.5500 to give some examples.

I'm an idiot in this area but you might find this relevant[0].

[0] https://en.wikipedia.org/wiki/Entropic_gravity

> I would be interested if anyone knows about the connection between information theoretic entropy and physical entropy.

I'm pretty sure they're the same, with minor differences in notation and units, unless of course I misunderstand what you mean by "physical entropy". Entropy in physics only got a precise definition once physicists understood that the concept really comes from information theory.

erm, I think boltzman figured out entropy for thermo like 100 years before shannon's theory of communication

could be totally wrong though

entropy, complexity, and evolution are words that come to mind

Yes, Shannon explicitly stated in Mathematical Theory of Communication that the general form was the same as that in statistical mechanics:

"The form of H will be recognized as that of entropy as defined in certain formulations of statistical mechanics where p_i is the probability of a system being in cell i of its phase space. H is then, for example, the H in Boltzmann’s famous H theorem."

where the general form was H = -K\sum p_i * log(p_i)

IIRC, this form is the only kind with the following two properties:

- chain rule -- H(X,Y) = H(X) + H(Y|X)

- maximized for the uniform distribution.

Could you explain KL divergence and/or cross entropy to me?

from wikipedia: "In mathematical statistics, the Kullback–Leibler divergence is a measure of how one probability distribution diverges from a second expected probability distribution"

Kullback-Leibler divergence is used a lot in the reinforcement learning setting

"In information theory, the cross entropy between two probability distributions p and q over the same underlying set of events measures the average number of bits needed to identify an event drawn from the set"

Cross Entropy is used as a cost function in neural networks rather than least squares http://neuralnetworksanddeeplearning.com/chap3.html#the_cros...

Also, have you read Norbert Weiner's Cybernetics? I think he had parts of information theory there before Shannon

I want to understand his work on random processes too, have just kind of scanned the book and have tried to upgrade my calculus

I found this article's explanation of the basic state of physics to be one of the best I've read. I know squat about the field, and so can't say if it was accurate or not, but the layman's explanation of the core problems of quantum theory, the probability and whatnot, was easy to grasp, and made sense.

Information theory will hopefully give us deeper insight into the why. If we derive QM from information theory it might come at it from another angle and provide different insights. I had read about people trying this in the past, but not for a while...maybe it proved fruitless or uninteresting.

I'm surprised time and time again by the removal of causal ordering from quantum mechanics. It seems to be throwing away so much.

So this is a project to create the "Maxwell equations of Quantum Physics"?

What if quantum mechanics are the causal basis for spacetime. Then to the observer it might appear as random, because spacetime is a prerequisite for observation. Also for causation, this must imply time moves sideways in the quantum domain. Clearly!

I clearly had one to many to drink. My head hurts.

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