Stephen Weinberg on Understanding Quantum Mechanics

[Demystifier]

Yep alphabet. What does it mean?

A step forward, two steps back.
In Croatian: Korak napred, dva koraka nazad.

 

[martinbn]

Are you a Russian? I thought no Russian likes Bourbaki style.

No, I am not Russian.

And why do you think that I understand Russian? (Which I do, I've learned it in elementary school in former Yugoslavia.)

Well, you've said before that you are Croatian, and I was guessing that you are old enough to have studied Russian in school.

 

[Demystifier]

No, I am not Russian.

Well, you've said before that you are Croatian, and I was guessing that you are old enough to have studied Russian in school.

Did you also study Russian in school? If so, where are you from?

 

[martinbn]

Did you also study Russian in school? If so, where are you from?

Just enough so I can read maths and physics in Russian, but not enough to communicate or read anything else. I stopped when it was no longer compulsory. I probably can read maths and physics in Croatian. I was born in Bulgaria, but for the most part of my life I have lived in US/UK.

 

[gmalcolm77]

The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wavefunction, the Schrödinger equation, to observers and their apparatus.

What do you think of Feynman and his vector analysis of light and matter? Does this explain the wave/particle duality?

 

[bhobba]

What do you think of Feynman and his vector analysis of light and matter? Does this explain the wave/particle duality?

I assume you mean Feynman's sum over histories approach.

OK a few points:

1. There is no such thing as wave/particle duality - it was done away with when Feynman's hero, Dirac, came up with his transformation theory in 1926 - likely sooner. It just hangs about because of the semi-historical approach most beginner and even a few intermediate textbooks take. In advanced textbooks like Ballentine it, correctly, doesn't even get a mention. We all must start somewhere and popularizations and beginner texts often start with this wrong now outdated idea.

2. The sum over history approach is logically equivalent to Diracs transformation theory and both are in many modern textbooks (eg Ballentine) but strictly speaking its a hidden variable interpretation of the QM formalism of Dirac - but of a very novel type.

So the answer to your query is - yes Feynman's approach explains quantum behavior equally as well, or not as well, as ordinary QM depending on your viewpoint.

It's much more modern variant, decoherent histories, of which the sum over histories approach is just one example of a history, is a very well respected modern interpretation that is worthwhile studying - some say its Copenhagen done right - but I won't enter into that argument. Some also say its many worlds without this weird , unnecessary, and silly, many worlds stuff. I happen to agree with that - but here is not the place to discuss it, and its not really science - just a personal opinion. As a counter argument MW is mathematically very beguiling and beautiful - on that score even more so than decoherent histories. Like all interpretations its what you are attracted to. Feynman towards the end of his life was converted to it after attending some lectures by one of its originators, Murray Gell-Mann (they were both at Cal-Tech together). In fact that was why Murray went to Cal-Tech to be with Feynman and they collaborated a lot - to start with. But after a while he became a bit disenchanted with Feynman - not for any scientific reason, just simply his personality grated Murray. Feynman had this habit of promulgating all these anecdotes about himself you can read in Surely Your Joking Mr Feynman:


I love that sort of thing myself, and love the book as well - if you haven't read it please do. But it grated Murray and they drifted apart. Still they had the greatest respect for each other scientifically and would often attend each others lectures.

If you want to find more about Decoherent Histories (also called Consistent Histories), another of its originators, Griffiths, has kindly made his textbook on it available online:
http://quantum.phys.cmu.edu/CQT/index.html

Added Later
Just refreshing my mind about Gell-Mann and his views. My god - he is good:


And so did Feynman:
https://www.math.rutgers.edu/~oldstein/papers/qtwoe/qtwoe.html
The photograph shows Richard Feynman and one of us (Gell-Mann), and the caption describes Gell-Mann as ``one of the most sensible critics of orthodox quantum theory'' and Feynman as ``one of its most sensible defenders.'' In fact, both physicists held very similar views of quantum mechanics. Some months before Feynman's death in 1988, Gell-Mann described to a class at Caltech the status of our work on decoherent histories at that time. Feynman was in attendance, and at the end of the class, he stood up, and some of the students expected an exciting argument. But his comment was, ``I agree with everything you said.''

BTW even though I like Dechoerent Histories its not my favored interpretation but that is a whole new thread. It must be emphasized however, and it is of crucial importance, no interpretation is better than any other. The reason you study interpretations is they all shed some light on what the formalism is saying. For example, and even some textbooks fall into this trap, a superficial reading of QM can easily lead one to think that collapse is part of QM. It isn't - only of some interpretations - but it only becomes clear once you study interpretations with and without it.

Thanks
Bill

 

[vanhees71]

The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wavefunction, the Schrödinger equation, to observers and their apparatus.

What do you think of Feynman and his vector analysis of light and matter? Does this explain the wave/particle duality?

Wave-particle duality is very simple to explain: It doesn't exist anymore for nearly 92 years anymore. Since modern QT has been discovered by Heisenberg, Born, Jordan, Schrödinger, and Dirac there's no need for this idea from "old QT" anymore.

How to understand the emergence of a classical world for macroscopic systems is a longer issue. The key concept is "coarse graining".

 

[gmalcolm77]

I assume you mean Feynman's sum over histories approach.

Thanks for the information and references. All good. Malcolm.

 

[vanhees71]

I've no clue what "Feynman's sum over histories approach" might be. So it's for sure not, what I meant in #147.

 

[bhobba]

I've no clue what "Feynman's sum over histories approach" might be. So it's for sure not, what I meant in #147.

Its just another name for the path integral approach eg:
https://en.wikiquote.org/wiki/Path_integral_formulation
Feynman’s so-called path-integral, or sum-over-histories approach to quantum mechanics, set this remarkable concept out as a mathematical procedure. It remained more or less a curiosity for many years, but as physicists pushed quantum mechanics to its limits— applying it to gravitation and even cosmology—so the Feynman approach turned out to offer the best calculational tool for describing a quantum universe. History may well judge that, among his many outstanding contributions to physics, the path-integral formulation of quantum mechanics is the most significant.

Thanks
Bill

 

[vanhees71]

Ah, I see. However, the path-integral formalism is also no new theory or even interpretation. It's QT (including both non-relativistic "1st quantization" and relativistic QFT) but offers alternative analytical methodology to evaluate things. It's for sure, together with the invention of Feynman diagrams, among Feynman's most significant contributions to the methodology of theoretical physics.

 

[bhobba]

Ah, I see. However, the path-integral formalism is also no new theory or even interpretation.

The formalism isn't an interpretation - just the math expressed a different way.

However when people say its taking all possible paths at once it is an interpretation - the path is a hidden variable. Its very novel because the idea of actually taking every possible path at once is, how to put it it, rather unusual.

That said I am not terribly fussed about it - its just semantics which isn't really that important - its the math that is.

Thanks
Bill

 

[vanhees71]

What you indeed do in the path integral is to evaluate probability amplitudes in a specific way, i.e., by integrating over all possible trajectories in phase space, leading to the propagator.