At the end of 2016, I decided to focus on working through an introductory textbook in quantum mechanics, instead of trying to make progress on my paper(s) to be published. I finished that textbook, which taught me things like Clebsch-Gordan coefficients, the Aharanov-Bohm effect, perturbation theory, Fermi’s golden rule, path integrals, and the Dirac equation. Another physics book that I worked through in 2017 taught me about thermodynamics and its importance for chemistry and solid state physics.
I spend much time with other physics textbooks in 2017, but didn’t read them cover to cover. In general, I spent much time in 2017 on physics, and I was happy about that. My day job (in semiconductor manufacturing) is concerned with physics to a large part. I previously had a bad conscience, because I was spending so little time on physics, or at least I was making so little progress.
But spending so much time on physics meant that little time was left for this blog. And I submitted (with coauthors) an abstract for a conference, which got accepted as oral presentation. The conference was end of February 2018, and I spend the last two months before working day and night to still finish the paper (and presentation) in time. But even after the conference, I continued to spend most of my time on physics. So a post about physics seems like the only reasonable way to get any blog post done at all.
My love-hate relationship with physics
When I had the choice for my first job between one closely related to my thesis and one closely related to physics, I took the one related to physics. In my experience, establishing friendly relationships with physicists was easy. It did work out as expected, but I was surprised how much physics ended up on my plate. But it was fine for me, those were things like electrodynamics and optics, which were easy for me. When quantum mechanical tasks started to land on my plate, I protested that I didn’t manage to finish that course at university. But after switching jobs (Feb 2013), I sort of had to accept that the quantum mechanical tasks were part of my responsibilities.
On the one hand, this was a nice excuse for me to try to understand that part of physics which I was once unable to understand. On the other hand, the question “Why me? There are real physicists that already understood this stuff while at university!” hit me with all its practical implications. I had to spend time to read books and articles about practical and theoretical aspects of my specific tasks. And in parallel, I also had to fill-in the gaps in my understanding of the basics. But I never fully devoted my time to physics, at least not before 2017.
Given that it is my job, am I even allowed to admit that I still fill-in the basics? Or am I too hard trying to pretend being a physicist, for whatever reasons? I did wonder whether I should do a master in physics, given that it is my job. But what for, just to prove that I am able to fulfil some formal criteria for being a physicist? And I doubt that I would become a physicist in my heart, so it could turn out to be very hard to finish a master degree. But I still like to discuss physics, and I would like to be able to judge certain physical fashions.
The code of silence
What could I have said about the Lightwave Research Laboratory at Columbia University in New York? Nothing, it would have just been name dropping. What could I have said about Information ist Energie by Lienhard Pagel? Something, it might have given the impression of name dropping, and it is the reason for this silent section:
This book uses QM in normal handwaving mode. By information, the author means his definition of dynamic information, which is something like information(flow) per time. Higher energy is normally associated with shorter times. What is good about this book is that it really discusses the time dynamics, which is otherwise often neglected.
What could I have said about quantum field theory? I could have talked about my motivations to learn more about it. But most of those were pointless, because I expect silence, no matter how much I have learned. But Sean Carroll gave me a good reason to learn more about QFT with his lecture Extracting the Universe from the Wave Function. I started with Quantum Field Theory for the Gifted Amateur and the many comments in Quantum Field Theory I, Lecture Notes (HS14+) and Quantum Field Theory II, Lecture Notes (FS17). However, then I read An Interpretive Introduction to Quantum Field Theory cover to cover. It was easy to read, with a good mix of technical details, explanations, interpretations, and philosophical clarification.
… much of the interpretive work Teller undertakes is to understand the relationship and possible differences between quantum field-theory — i.e., QFT as quantization of classical fields — and quantum-field theory — i.e., a field theory of ‘quanta’ which lack radical individuation, or as Teller says, “primitive thisness.”
Not sure when I will continue to work through the more technical material. At the moment, I read How is Quantum Field Theory Possible?, just to see how far I will come, before it gets too boring or too difficult.
A conversation about the Copenhagen interpretation
I had an email conversation with BT in March 2017, where he praised Deutsch’s version of MWI, and I defended Heisenberg’s version of Copenhagen:
28 Feb 2017 BT: purpose of science is explanation
9 Mar 2017 TK: what is Copenhagen
13 Mar 2017 BT: Copenhagen is Bohr
20 Mar 2017 TK: historical defence of Copenhagen
28 Mar 2017 TK: QM as general probability
During that conversation, I did some historical investigations:
When David Bohm proposed his new interpretation in 1952, the term “Copenhagen interpretation” didn’t exist yet, he had to talk of the “usual physical interpretation of quantum theory” instead. The idea that the “Copenhagen interpretation” should be the common core of the beliefs by Bohr and Heisenberg probably goes back to Henry P. Stapp in 1972. Before, it was just another way to refer to the orthodox interpretation(s) taught by (good) text books. Heisenberg is the unfortunate soul who coined the term Copenhagen interpretation (in the 50s).
Here is how I see Heisenberg’s position, independent of whether Bohr agrees:
Max Born and Werner Heisenberg claimed in 1927 that quantum mechanics is a closed theory (abgeschlossene Theorie), just like Newton’s mechanics is a closed theory. And by this they meant that its description of reality is complete and accurate, whenever it is applicable. The limits of applicability are not part of the closed theories themselves, according to Heisenberg.
The conversation started, when I quoted Heisenberg’s own explanation:
… what one may call metaphysical realism. The world, i.e., the extended things, ‘exist’. This is to be distinguished from practical realism, and the different forms of realism may be described as follows: We ‘objectivate’ a statement if we claim that its content does not depend on the conditions under which it can be verified. Practical realism assumes that there are statements that can be objectivated and that in fact the largest part of our experience in daily life consists of such statements. Dogmatic realism claims that there are no statements concerning the material world that cannot be objectivated. Practical realism has always been and will always be an essential part of natural science. Dogmatic realism, however, …
and BT replied:
That’s all a bit technical for me! I can’t tell whether he is in favour of “explanations” in the simple(-minded?) Deutsch sense or not.
After setting the passage in context, I got very concrete:
But I disagree that the quoted passage is technical. If he adheres to this passage, then Heisenberg cannot claim that Schrödinger’s cat would be both alive and dead, or that the moon would not be there if nobody watches.
Others, like Christopher A. Fuchs and Asher Peres in Quantum Theory Needs No Interpretation”, are apparently less sure whether (neo-Copenhagen) quantum theory is so clear about that fact. Hence they try to weasel out by claiming: “If Erwin has performed no observation, then there is no reason he cannot reverse Cathy’s digestion and memories. Of course, for that he would need complete control of all the microscopic degrees of freedom of Cathy and her laboratory, but that is a practical problem, not a fundamental one.”
This is non-sense, because the description of the experiment given previously was complete enough to rule out any possibility for Erwin to reverse the situation. Note the relevance of “… a consistent interpretation of QM as applied to what we do in a physical laboratory and how practitioners experience QM in that context.” If Erwin had access to a time machine enabling him to realistically reverse the situation, then it might turn out that Cathy and Erwin indeed lived multiple times through both situations (and experienced real macroscopic superpositions), as depicted in movies like “Back to the Future”.
According to Jan Faye in the SEP article on the Copenhagen Interpretation, even Bohr does not disagree with that conclusion: “Thus, Schrödinger’s Cat did not pose any riddle to Bohr. The cat would be dead or alive long before we open the box to find out.” However, I doubt that Bohr really said this, or replied to the content of Schrödinger’s article in any other direct way. (I read that he complained to Schrödinger about that article for indirectly supporting Einstein in his crusade against QM.)
Questions about Schrödinger’s famous article
At the end of this article on Erwin Schrödinger there are links to interesting papers by Schrödinger. After reading “Die gegenwärtige Situation in der Quantenmechanik” (“The Present Status of Quantum Mechanics (“Cat” Paper)”), I had several questions:
- Did Schrödinger know the density matrix formalism? He could have formulated some points he was making more explicit by using it.
- Did Bohr ever admit that the example of the cat was spot on, and that of course the cat was either dead or alive, even if nobody cared to check?
- Did Schrödinger know about Fermions and Bosons? The last paragraph talks about how the relativistic theory would make the foregoing discussion invalid.
When I first read Schrödinger’s article in September 2014, I didn’t knew much about quantum mechanics or it history. But I had the hope that my instrumentalist ideas related to the density matrix were not too far off from the orthodox interpretation. Today, I know that Carl Friedrich von Weizsäcker denied that the Copenhagen interpretation asserted: “What cannot be observed does not exist.” Instead, it follows the principle:
What is observed certainly exists; about what is not observed we are still free to make suitable assumptions. We use that freedom to avoid paradoxes.
The interesting thing is that Schrödinger’s article is not really about the cat, and not really about attacking quantum mechanics. Instead, Schrödinger tries to explain his own understanding of it. But Bohr interpreted the article as an attack on his interpretation.
- Schrödinger at least knew the consequences that follow from the density matrix formalism, so I guess he knew it. Not sure why he does not use it, maybe it was not yet popular among physicists back then.
- I doubt Bohr ever admitted that Schrödinger was right.
- Schrödinger knew about Fermions and Bosons. The basic connection between Fermi–Dirac statistics, Bose–Einstein statistics and the spin was already known since 1926. But the first derivations of the spin-statistics theorem from relativistic quantum field theory by Markus Fierz and later by Wolfgang Pauli are from 1939.
However, I guess Schrödinger wrote that last paragraph not just because Fermions and Bosons were challenging to fit into his scheme of explanation. He also wanted to stress that relativistic quantum mechanics was not yet fully worked out, so that he could conclude: “Perhaps, the simple procedure of the nonrelativistic theory is only a convenient calculational trick, but it has at the moment attained a tremendous influence over our basic view of Nature.”
While talking about famous articles by Schrödinger, I want to quote a passage from the beginning of “Are There Quantum Jumps? Part I”:
Along with this disregard for historical linkage there is a tendency to forget that all science is bound up with human culture in general, and that scientific findings, even those which at the moment appear the most advanced and esoteric and difficult to grasp, are meaningless outside their cultural context. A theoretical science, unaware that those of its constructs considered relevant and momentous are destined eventually to be framed in concepts and words that have a grip on the educated community and become part and parcel of the general world picture – a theoretical science, I say, where this is forgotten, and where the initiated continue musing to each other in terms that are, at best, understood by a small group of close fellow travellers, will necessarily be cut off from the rest of cultural mankind; in the long run it is bound to atrophy and ossify, however virulently esoteric chat may continue within its joyfully isolated groups of experts.
I want to quote a passage from the beginning of “Are There Quantum Jumps? Part II”:
Even if this shorthand were admissible for the microevent, we have to keep in mind that physicists are not the only people interested in physics and in the outcome of theoretical research in physics. Those results that are tenable and will survive are destined, eventually, to be absorbed into our world picture and to become part and parcel thereof. A great many of our educated contemporaries, not equipped with the mathematical apparatus to follow our more technical deliveries, are yet deeply concerned with many general questions; one of the most stirring among them certainly is whether actually natura facit saltus or no. Whatever abbreviated language we physicists may find convenient to use among ourselves, we ought to be aware of the dilemmas that justly and duly interest others; we must be careful not to veil or distort them by indulging in loose speech
Why did I wrote this post? Probably because I wanted to share my email conversation about the Copenhagen interpretation. And because I finally managed to read a technical book about quantum field theory cover to cover. However, that part already touches the code of silence, and I really wrote two of the emails (the ones were silence would not hurt me too bad) for which I expected silence (and really got silence as expected). But it was a start, and I am confident that I will finish more of the small stuff in the near future, even if the silence will hurt me more. Writing about Schrödinger was just bonus, or maybe I wanted to avoid having to write about physics again in the near future. But I could do a post about my thoughts on probability, at least that would be related to logic.