Seeking clarity on the physical definition of an observer

[okabe rintarou]

TL;DR

Seeking clarity for a high school student on the physical definition of an observer, decoherence in MWI, and the mathematical laws preventing cross-timeline traversal.

I am an 18-year-old student waiting for my Class 12 results, and I've been doing some conceptual reading into Quantum Foundations (specifically referencing 1926 wave mechanics and 1957 Relative State formulation). I have three specific questions I'd love to ask a professional:

1) In the Schrödinger equation, how is the 'observer' strictly defined physically? Why did the von Neumann information-based interpretation lose its mainstream status?

2) Regarding the Many-Worlds Interpretation (MWI), how does the math of decoherence physically ensure that branches become orthogonal/separated?

3) From a purely theoretical standpoint, which specific physical laws (Thermodynamics, Causality, etc.) mathematically prevent information or matter from reaching or traveling to a different decohered timeline?

Thank you for your time and guidance!

 

[PeroK]

Regarding questions 2 and 3, one suggestion is to read Feynman's book on QED:The Strange Theory of Light and Matter. It doesn't cover decoherence, but the way light behaves quantum mechanically to produce the classical wave phenomena of reflection, refraction and diffraction gives a valuable insight into how the mathematics of decoherence works.

 

[gentzen]

1) In the Schrödinger equation, how is the 'observer' strictly defined physically?

Good answers to this question can be found in "Quantum Theory: Concepts and Methods" by Asher Peres (https://en.wikipedia.org/wiki/Quantum_Theory:_Concepts_and_Methods)
and "Quantum Mechanics: A Modern Development" by Leslie E. Ballentine (https://en.wikipedia.org/wiki/Ensemble_interpretation#cite_note-BallentineBook-11)
Both are in tradition of Niels Bohr, quite direct in case of Asher Peres, and what you get if you take Bohr's positions to a logical conclusion in case of Ballentine.

Why did the von Neumann information-based interpretation lose its mainstream status?

???
Why do people ascribe all sorts of things to von Neumann?
Anton Zeilinger is the famous person advocating for information-based interpretations. No idea what you mean by "mainstream status".
Perhaps you are thinking about the version of Copenhagen defended by Werner Heisenberg and Rudolf Peierls?

 

[PeterDonis]

Moderator's note: Thread moved to the QM interpretations subforum.

 

[bhobba]

As an 18-year-old, QED has been suggested as a good place to start (and I agree).

The following video suggests the next books to read (but are above what I would expect of the usual 18-year-old, although certainly not beyond their capability if you want to go deeper):



The next would be Ballentine, as mentioned in a previous answer, but it is graduate-level and should be read after the recommendations in the above video. Perhaps a bit too advanced for an 18-year-old - but giving it a look and seeing what you can glean would be OK.

One thing sometimes not appreciated about QM is that, as you progress to more advanced material concepts at the less advanced level, how to put it, are 'tweaked' a bit. The great Richard Feynman, well known as a great teacher, lamented this, but try as he might, he could not figure out how to avoid it.

Von Neumann's classic Mathematical Foundations of QM (not to be read until you have done a course in functional analysis) is where the idea of 'consciousness' causing collapse originated. If you do look at it, understand his (scathing) comments on the Dirac Delta function have all now been resolved. At your level, the book to get on how it was done, and also highly recommended as something every mathematician and physicist should know, is:

https://www.amazon.com.au/Theory-Distributions-Nontechnical-Introduction-ebook/dp/B01DM26TPW.

His analysis showed that the Quantum Classical cut could be placed anywhere, and, for reasons he explained, he placed it at the level of human consciousness. This led to the consciousness-causes-collapse interpretation, which popular books still flirt with but is well out of favour these days, with most considering it mystical nonsense (as do I). It never really caught on, and prominent adherents like Wigner changed their mind after early work on decoherence. These days, the term "observation" is usually synonymous with decoherence.

In many worlds, the assumption is that after decoherence, each possible outcome is a separate world. However, as Murray Gell-Mann explains, the issue of their reality is to some extent just a semantic difference between real and potentially real, with each treated on equal footing:



If you think they are real, you are led to Many Worlds. If you think they are potentially real, you are led to Decoherent Histories.

However, I need to mention that we now know that ordinary QM is wrong and has been replaced by Quantum Field Theory (QFT), which incorporates relativity. Most people think QM is the limiting case of QFT, but, to my surprise, a recent paper I read shows that this is not the case. That has thrown the cat really amongst the pigeons and suggests the issues with QM are not quite what is usually thought. This is just by the by; it really is at the graduate level, and the details go beyond the level of this thread. Plus, it is only of any value to nuts like me interested in such things - no need to worry about it at your level. Remember what I mentioned, as you become more advanced, things get 'tweaked' a bit.

If you believe in the reality of many worlds, the assumption is that they are always separate; there is no law involved.

My view is, once you have read the books in the YouTube video I posted, the best book about quantum interpretations is:
https://www.amazon.com.au/Fields-Their-Quanta-Quantum-Foundations-ebook/dp/B0DLNLLG7Y

A bit pricey and a minority interpretation (which is a bit strange, as it's basically just a literal interpretation of QFT), but it has the advantage of making usual issues like wave-particle duality and what is a particle, trivial.

Thanks
Bill

 

[PeterDonis]

a semantic difference between real and potentially real

Is that like the difference between all dead and mostly dead?

 

[bhobba]

Is that like the difference between all dead and mostly dead?

It is surprising how much of what we discuss is, to a large extent, semantics. I know we don't discuss philosophy here, but many of Wittgenstein's ideas centred on this (yes, I know he started as an aeronautical engineer). His mentor Bertrand Russell was 7th wrangler. Those were the days when philosophers knew at least some science (many still do).

However, some philosophers went down a 'peculiar' path best read about in the book by the mathematical physicist Alan Sokal: 'Beyond the Hoax: Science, Philosophy and Culture'

Thanks
Bill

 

[gentzen]

Why did the von Neumann information-based interpretation lose its mainstream status?

???
Why do people ascribe all sorts of things to von Neumann?

OK, I guess I know how your mistake happened. At the end of the The Information Philosopher page for John von Neumann, Bob Doyle had explained the connection to his information physics, but without a clear separation from the extracts of John von Neumann thoughts:

Information physics places the cut or boundary at the place and time of information creation. It is only after information is created that an observer could make an observation. Beforehand, there is no information to be observed.

And Gemini (google's AI) misinterpreted Bob Doyle's writing beyond any basis in reality:

Legacy​

Today, von Neumann is celebrated by movements like Information Philosophy, which views the universe not just as a collection of matter and energy, but as fundamentally built from information structures. [1, 2]

• Dive deeper into his conceptual framework on the Information Philosopher Profile.
• Read about his evolving intellectual journey toward computation theory in this arXiv Study on Von Neumann's Philosophical Shift.

Bob Doyle (The Information Philosopher) is not bad, he is a useful source of information. It is unfortunate that minor document structuring and formating mistakes get ridiculously amplified by AI, but not really his fault.

 

[gentzen]

1) In the Schrödinger equation, how is the 'observer' strictly defined physically?

On the mathematical level of the Schrödinger equation, 'preparation' (~= initial values) and 'measurement' are defined as mathematical operations. It could also make sense to explicitly define 'control' as a combination of 'measurement' and 'classical action taken in response to measured result' (~= time dependent Hamiltonian) on that mathematical level.

The more controversial question is how to interpret 'observer' in our actual physical world. It is relatively uncontroversial that an 'observer' doesn't need to be a "human" or a "conscious being". But beyond that minimal consensus, opinions vary widely:

It is not the "human" part which upsets me about the word "observer". It is the suggested absence of "interaction" and "altering"/"influencing" capabilities. The word "observer" suggest a passive viewer of some "TV show" or "cinematic movie", not an active participant in some "massively multiplayer online game".

Or maybe more aptly, it suggests an old time astronomer like in Newton's times watching the planets, moons, and stars without any possibility to alter their course. It does not suggests a modern NASA scientist designing swing-by (gravity assist) maneuvers to steal a tiny amount of the energy of some planet or moon to let his spacecraft save fuel.

Of course, the course of the planet or moon is not significantly altered, but the course of the spacecraft is. You could model the spacecraft itself as an agent, because it has thrusters which are effectively controlled by intentions (i.e. final causes). It can also make sense to only model the ground station as an agent, because the communication delay to the spacecraft might be important. But beyond that, the usefullness and explanatory value of the model would rather decrease if you try to remove agents even further.

 

[martinbn]

In the Schrödinger equation, how is the 'observer' strictly defined physically?

What exactly do you mean by that?!