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Quantum Mechanics Without Maths or Philosophy (articlesbyaphysicist.com)
76 points by nabla9 on June 15, 2017 | hide | past | favorite | 20 comments


archive link: http://web.archive.org/web/20170615010943/http://www.article... (w/o images)

Skipping from the complex-valued wave function to its real-valued magnitude is a good simplification for an introductory article, but I think the need for complex numbers (the concept of phase) is very important and fundamental. That's what I would really liked explained? Why do cmplx numbers come up, and is there a QM-equivalent model using only real-valued functions?


Complex numbers aren't strictly necessary. You could even expand every complex dimension to a pair or real dimensions and get the same results (roughly speaking). However this is a bit unwieldy, so the question becomes "why do complex numbers provide such a convenient representation of quantum theory"?

There is work being done on this sort of thing. If essentially all you care about are the probabilities of certain observations occurring, then you surely can represent any theory with real numbers. The field of "generalised probabilistic/physical theories" attempts to analyse quantum theory without assuming the baggage of Hilbert spaces etc... [0]. Chris Fuchs also has done a lot of work on representing quantum state spaces in terms of outcome distributions over special types of measurements called sic-povms[1,2].

0. https://arxiv.org/abs/quant-ph/0508211

1. http://perimeterinstitute.ca/personal/cfuchs/

2.https://en.m.wikipedia.org/wiki/SIC-POVM


This work (QBism) is certainly intriguing and the promise of deriving the Schrödinger equation from the Born rule using SIC-POVMs could really change how QM is understood.

But I wonder why it gets so little attention...


The "radical Bayesian" stance is very difficult to grasp, coming from an ordinary scientific background. At least it was for me - took months, maybe years of sinking-in to figure out what Fuchs was trying to say. I think a lot of people start reading about it, quickly get the (false) impression that it's solipsistic, and just lose interest.


To be honest, I still can't understand how it's not solipsistic. If you use QBism from two different perspectives, like Alice and Bob in a Bell test experiment, then you'll still either need many worlds or get incompatible results.


Perhaps it would help to note that in QBism, reality is not denied. Rather, the objective reality of the quantum state is denied.

Not sure what you mean by incompatible results. Certainly QBism is incompatible with Many Worlds! The latter states that the quantum state is real and objective, the former states the opposite.

The QBist horsemen have devoted some attention to the question of compatible quantum state assignments. See: https://arxiv.org/abs/quant-ph/0206110

However, Chris Fuchs does not see any problem in someone assigning a probability-1 belief to a measurement outcome and being proven wrong. See section 2.2 of https://arxiv.org/pdf/1705.03483.pdf

This last point in particular should bring home the point that Fuchs does think there is an ontological theory underlying physics, but that we are fooling ourselves about how much of it is objective/observer-independent.



> is there a QM-equivalent model using only real-valued functions?

Yes, the Wigner-Weyl transformation produces a completely equivalent way to formulate quantum mechanics without reference to complex numbers. The wavefunction over real space is replaced with a (generically not positive) real-valued pseudo-probability distribution on phase space called the Wigner function.

https://en.m.wikipedia.org/wiki/Wigner–Weyl_transform


Sorry pet peeve, but title is not a grammar error, a lot of the world says maths not math.

Some dictators abuse power for material gain. If woke up dictator of Earth I think my tyranny would lean more towards standards bodies. Forcing holdouts to the metric system would actually cost the economy more? Too bad, this is the cruelty of oppression. I at least would promise not to kill or torture anyone.


Maths is short for Mathematics. Quantum Mechanics Without Mathematics or Philosophy is fine.


Website appears to be refusing connections.


Sadly, same here. We need a show HN for replicating a static website via whatever the currently best value proxy/cache/CDN is.


How about if HN itself takes a snapshot of the submitted article as soon as it hits the HN front page? For example, HN could cache the article or generate a PDF, then if the site doesn't load, the cached version or PDF would be served.

This might be good as a historical resource because even good stuff disappears off the Internet, and if it made it to the HN front page, it's probably good. (I know, can't do this because copyright. But the Wayback Machine has survived by deleting content on request.)


there is something deeply ironic about that...


i.e., you can't explain QM w/o math and philosophy?


That is my first reaction.

"Mozart without sound of music notation."


I'm not sure what you mean, because that sounds easy to do.


I suspect that was a typo, and mablap meant:

> "Mozart without sound OR music notation."


Haha yes, it was a typo.


Maybe. Maybe not.




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