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Interpretations of Quantum Mechanics

Very little is known about quantum mechanics besides its formalism and that it works (it describes the world around us). For many, that is more than enough, which is known as the shut and calculate school. Others are not satisfied and would like to have, at least, answers such as:

• Is the world local or nonlocal?
• Is the world deterministic or probabilistic?
• Is there even a "world"? (elements of reality)
• Is the observer special? Are we special?
• Where is the "shifty split" and what does it split?

Providing an alternative starting point, or theory, to address satisfactorily such questions pauses the problem of quantum foundations. The main difficulty of this delicate question is that all competing interpretations, even when they differ wildly, cannot be discriminated through the prism of experiments, thus placing the point out of reach of the scientific method.

One possible approach to such problems which present no way towards their resolution, is to envision all possible alternatives. We do not know which one is correct, but we know that—being imaginative and systematic enough—we will have come to toy with the actual one. Sherlock Holmes would proceed to remove what is impossible to single out what is true, but his quantum counterpart would oppose to that that while

Sherlock Holmes observed that once you have eliminated the impossible then whatever remains, however improbable, must be the answer. I, however, do not like to eliminate the impossible.

—Douglas Adams, Dirk Gently's Holistic Detective Agency.

A recurrent clue from the various formulations is that something which is not measured, needs not have a value anyway.

“spooky action at a distance.”

Path integrals

Feynman's path integral formalism.

Hugh Everett III's many-words

This is by far the most exotic interpretation despite also being the simplest and most devoid of superfluous assumptions, thus passing Occasm's razor test. It postulates indeed that there is a full (total, universal) wavefunction of the universe, which obeys Schrödinger's equation, thus being irreversible and deterministic and not involving any measurement, let alone collapse. This comes at the price, though, that all the possible outcomes of experiments are realized as part of this giant wavefunction of all possibilities, in which we (as observers) coexist in as many alternate realities (or parallel universes) as needed to satisfy unitary evolution.

A typical QM measurement, such as that of a spin component in the Stern–Gerlach experiment, is a local and very low energy event. It is not credible that the measurement could have the huge cosmological effect of bifurcating the universe.

—Reviews of quantum foundations. D. Ballentine in Physics Today 73:11 (2020).

When I first heard of the world-splitting assumed in the MWI, I went back to Hugh Everett’s paper to see if he had really said anything so absurd.

—Reviews of quantum foundations. D. Ballentine in Physics Today 73:11 (2020).

Pilot wave

This theory, first proposed by De Broglie and then later extended by Bohm (it is also popularly known as Bohm's pilot wave or Bohmian mechanics) posits an underlying wave (which follows Schrödinger's equation) that guides the particles, the latter being well-defined in terms of their positions in 3D space, like classical particles, and thus being of the hidden-variable type, but given that their behavior is piloted by the wave, which is itself nonlocal and, this time, in the configuration space (of very high dimensions), albeit deterministic, thus have all the wonders of quantum mechanics. A dialogue covers the basic in Ref. .

Quantum Bayesianism (QBism)

This extends to the quantum realm the Bayesian school of probabilities. Jaynes being both an ardent proponent of the latter as well as a proficient quantum physicist, .

quantum states consistently as (subjective) information

One of the things that sets QBism apart from the other interpretations is its reliance on the technical details of quantum information to amplify Feynman’s point—that the modification of the probability calculus in quantum theory indicates that something new is created in the universe with each quantum measurement.

—Quantum foundations. D. P. DiVincenzo and C. A. Fuchs in Physics Today 72:50 (2019).

An advocate of QBism is David Mermin. An ennemy is Ballentine.

Enlightened

Besides or beyond the "agnostic" approach of not worrying about the interpretation because this is either futile or unnecessary, one can also find the view that it is actually not needed at all, since there is no problem of interpretation in the first place and everything is accounted for by the theory, since it works in producing what it aims to describe.

Still others

• Time-symmetric theories: putting past and future on an equal footing. Proposed by Schottky in 1921.
• Transactional interpretation: both the field $\psi$ and its complex conjugage $\psi^*$ are real (physical) objects which describe a "possibility wave" from the source to the receiver and vice-versa, whose mutual interaction realizes a transaction.
• Quantum logic: positing that logic as we know it is not suitable to describe the quantum regime. Possibly related to Feynman's negative probabilities.
• Quantum Darwinism: a well-packaged and wrapped-up version of decoherence.

Still, still others

Modal interpretations of quantum theory, Many-minds interpretation, Quantum mysticism, magic, a prank by God, etc.

Copenhagen Interpretation

We put it last while it ranks first from historical, practical and didactic points of view. Impulsed by Bohr (mainly) and Heisenberg (in his shadow, though not without disagreements), it is also the oldest formulation of some form of interpretation (in the mid-20s). It relies chiefly on the principle of complementarity, which states that not everything can be known about an object. The observation is irreversible in that it causes a collapse of the wavefunction.

Links

• From the Wikipedia.
• Feynman speaking about it (and part 2)