# What are you reading now? (STEM only)

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Wow, you've a very different experience to me. Pauli wrote essays about Jungian psychology and so on and Born wrote long essays on the meaning of science and society in general.
Bohr however always seemed flat and sober to me. Aside from the Como essay which was a bit confused, most of his writing is short and not particularly philosophical I would have said. Maybe it's different ideas of what's "philosophy".
Yes, that could be the case. It's an ongoing discussion, especially in topics on quantum mechanics. Somehow, a lot of physicists nowadays see "ontology" as "mere philosophy", while for many physicists and especially founding fathers of quantum mechanics it was just part of the physics. Maybe it's part of the increased level of abstraction of modern physics that physicists nowadays make this distinction between "philosophy" and "physics", but in my opinion such a cut is as artificial as the one by Heisenberg in quantum mechanics :P

WernerQH and Demystifier
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I know that Pauli was also inclined to the "paranormal" and some Jungian psychology. Fortunately he kept this part of his thinking separate from his physics, which he as always presented in the no-nonsense mathematical style, which is clarifying and "demystifying" things rather than introduce obscure notions like "complementarity" like Bohr and Heisenberg. For me Bohr's writings are usually pretty obscure. Particularly his answer to the EPR paper is even more confusing than the EPR paper itself. To get Einstein's true thoughts one should rather read the Dialectica article of 1948:

A. Einstein, Dialectica 2, 320 (1948)

I'm sure that there must be somewhere an English translation of this brillant article.

Concerning philsophy and quantum theory I'm also thinking that the philosophers usually mess up the subject severely and unnecessarily. It's indeed true that to talk about quantum theory adequately you have to use pretty abstract descriptions of "physical reality", i.e., Hilbert-space vectors and operators and also, if you want to understand it from a fundamental level, Lie group and algebra-representation theory.

LittleSchwinger
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Pauli's Handbuch der Physik article is very good. I guess for me Heisenberg is pretty much the same, his Chicago lectures are excellent, mathematical and concise and the more general musings are in other essays.
Bohr and Complementarity always seemed pretty clear to me as well since he constantly talks about experiments, it's just the physical fact corresponding to the mathematical fact of operators not commuting. I never found him obscure I have to say. In fact Complementarity is a common enough word in quantum information. Perhaps a difference between fields?

Currently I'm reading the 2nd Edition of Gottfried's textbook with Yan as co-author as preparation for teaching.

vanhees71
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Pauli's "Handbuchartikel" is indeed a masterpiece, particularly such little gems like the argument, why time must be a parameter and not an observable. Gottfried and Yan is a very good textbook too.

LittleSchwinger
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I should have said Complementarity is usually mathematical stated in modern QI as two operators ##A## and ##B## whose associated eigenstates obey ##(a_{i},b_{j}) = const, \forall i,j##. That's the mathematical encoding of what Bohr spoke about.

Pauli's "Handbuchartikel" is indeed a masterpiece, particularly such little gems like the argument, why time must be a parameter and not an observable. Gottfried and Yan is a very good textbook too.
One of the best reasons for a non-Teutophone physicist to learn German is the price difference between the English translation of Pauli's article and the original!

vanhees71
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Indeed, complementarity can be simply substituted by the clear mathematical statement of the uncertainty principle,
$$\Delta A \Delta B \geq \frac{1}{2} |\langle \mathrm{i} [\hat{A},\hat{B}]|.$$
##A## and ##B## don't necessarily need to be canonically conjugated as are ##x## and ##p_x## but in the latter case it's most simply to discuss, because then ##[\hat{x},\hat{p}_x]=\mathrm{i} \hbar##, and you get
$$\Delta x \Delta p_x \geq \frac{\hbar}{2},$$
which says that if a particle is prepared in a well-localized state (i.e., ##\Delta x## "small") then necessarily ##\Delta p_x## is "large". You don't need complicated philosophical arguments about "complementarity" to understand this.

LittleSchwinger
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Indeed, complementarity can be simply substituted by the clear mathematical statement of the uncertainty principle...
..don't necessarily need to be canonically conjugated...
This is only out of historical interest, nothing you say is wrong of course.

Basically if you read Bohr's essays, whenever he says "Complementarity" he always means the case where the two quantities are canonically conjugate, i.e. Complementarity is the special case of the non-commutativity of canonically conjugate pairs. The definition I gave above is essentially a way of defining "canonically conjugate" without using Hamiltonian Mechanics.

It has turned out that Complementary observables are especially important in an information theoretic sense.

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vanhees71
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Pauli's "Handbuchartikel" is indeed a masterpiece, particularly such little gems like the argument, why time must be a parameter and not an observable.
The fun fact is that this gem is only a footnote.

vanhees71
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Born's probability interpretation is also a footnote in an article about scattering theory :-).

physicsworks and Demystifier
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Reading the mathematician Michel Talagrand's excellent "What is a quantum field theory?"

If you ever wanted to deeply understand all orders perturbative renormalization this is the text.

AndreasC, Demystifier, vanhees71 and 3 others
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I just got a copy, but I am too busy to give it much attention. I hope to give it a serious go starting in April or May.

LittleSchwinger
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I just got a copy, but I am too busy to give it much attention. I hope to give it a serious go starting in April or May.
You'll have a lot of fun. The renormalization isn't the only good thing. There's a very careful and detailed exploration of representing the Poincaré group, why we are lead to Dirac matrices and a proper walk-through of all the details of LSZ reduction.

George Jones, dextercioby and vanhees71