The interpretation problem of quantum physics

See my introduction to quantum physics, that provides a quick and clear initiation, useful for those not familiar with it yet.

The mystery of randomness

Identical particles - absence of individuality

One problem with the idea of an electron being a thing, is that when there are several electrons, they are the same thing. This is a necessity in the formalism of quantum physics:
Namely, if we initially have a system of two electrons, one in position A, the other in position B, then the system moves and finally observed as a system of two electrons in positions C and D, then quantum physics proves that it is exactly the same final state (as testified by interference between paths) whether (A moved to C and B moved to D), or (A moved to D and B moved to C).
And this absence of individuality between particles of a same kind is even more clearly absolutely necessary in Quantum Field Theory.

Randomness of radioactivity

In particular, any two radioactive atoms of the same kind, are the same. They have no internal clock to determine when they will break. This is the reason why radioactivity is exponentially decreasing in time: no matter when they were created, radioactive atoms still have no age and thus each one that did not explode yet keeps the same probability of exploding in any given time interval.

Richard Feynman, in Lectures on Physics, volume I, wrote:

We cannot, however, predict when [an atom] is going to emit the light or, with several atoms, which one is going to. You may say that this is because there are some internal “wheels” which we have not looked at closely enough. No, there are no internal wheels; nature, as we understand it today, behaves in such a way that it is fundamentally impossible to make a precise prediction of exactly what will happen in a given experiment. This is a horrible thing; in fact, philosophers have said before that one of the fundamental requisites of science is that whenever you set up the same conditions, the same thing must happen. This is simply not true, it is not a fundamental condition of science. The fact is that the same thing does not happen, that we can find only an average, statistically, as to what happens. Nevertheless, science has not completely collapsed. Philosophers, incidentally, say a great deal about what is absolutely necessary for science, and it is always, so far as one can see, rather naive, and probably wrong.

(A similar quotation, also from Feynman, is given there)

Einstein wrote:

God does not play dice with the universe.

Bohr wrote:

Einstein, stop telling God what to do!

On the idea that ontology = determinism

I will assume everyone knows about Schrödinger 's cat experiment ;)

Someone in a forum raised this metaphysical argument for determinism: "any being that is at all a being must be a definite being, for otherwise it should fail to be a being at all", "randomness requires a being to not have a nature".
To say it in other words, everything must either be totally real, or totally unreal, and if something is totally real then it must have a deterministic behavior.
However, the very idea that physics (or science in general) should deal with real things, is unclear : see Feynman's story about "essential objects", I quoted there.

We can admit at least the following kinds of reality, despite their lack of absolute clarity:

The universal laws of physics (mathematical "realities" independent of space and time).
The practical, approximate kind of "reality" of things as they usually appear to us.
Our perceptions (feelings), and the usual ways in which both (the perceptions and the physical world) appear to be connected.

But the question is whether any reasonable way can be found to attribute an ontology (a quality of "physical reality") to anything more: any exact (fundamental) but contingent data (physical objects), that would hopefully keep all (or part) of the qualities we might expect from a "physical reality". Namely, being altogether :

Specific to "what there is here and now".
Independent of our perceptions, which are not themselves described by physics (as they are lively, too complex, or... involving consciousness)
Friendly to the structure of theories (found to best describe the behavior of things). Namely, one of the cornerstones of modern physics is relativistic invariance ; another is the locality of laws (expression of laws by differential equations, that anyway comes as a consequence of relativistic invariance : no faster-than-light communications).

The behavior of physical systems is non-deterministic in practice : when measured, only one possibility becomes (or appears as) real, but which one ? In the expression of known physical laws, nothing can be found to describe the "collapse process", this transition from indetermination to the (seemingly random) specification of a measurement result:

When does it happen : what exactly is a measurement (it will only be approximately understood by decoherence but without answering the next points) ;
What "chooses" to give it any specific result rather than another
How does it "happen" in space-time (if it "happens" at all); any idea we might introduce for it would break "physical friendliness" : it would have to both be global (not localized in space-time), and break relativistic invariance (as shown by the EPR paradox).

So this "collapse process" seems to "not have a nature", or to not be part of physics. Admitting it as part of physics would make physical objects "unreal": as they behave in a mysterious random way, they "do not have a nature".

In order to "be about something" that is definite, it seems physical laws have to miss that "collapse process", and thus to be deterministic. This may happen in different ways depending on interpretations:

In the absence of all measurement, the state of any physical system (what is called "wavefunction" in the popularized vocabulary, though it is actually more subtle than this) has a deterministic evolution (described by Schrödinger's equation). Then:

In the Many-Worlds interpretation, this state is the physical reality, that is deterministic : in the Schrödinger's cat experiment, the system deterministically evolves into a specific final state, that is some quantum superposition [dead cat]+[living cat] whose component possibilities belong to parallel worlds.
In the mind makes collapse interpretation, this physical state is only that of how the universe is at a given time of consciousness, specified by the data of the past perceptions and until a new perception happens.

In the Bohm interpretation, one more object (the system of hidden variables) completes the description of reality by a supposedly "specific" determination of its evolution, between the dead and the living cat.
But another class of interpretations (Spontaneous collapse theories) tries to add a collapse process to the physics, forming non-deterministic physical laws.

List of interpretations

From some official lists of "interpretations" I think not all should legitimately be seen as such. Let us start by those which I see as indeed deserving the title of interpretations (follow the links for developments) hat title:bedo not consider it clear for them all to be interpretations:

I would classify interpretations in the following categories :

Interpretation name	When does the wavefunction collapse ?
De Broglie-Bohm (Bohmian mechanics) and other hidden variables theories	Already done since the birth of the Universe, but a non-collapsed version must subsist for guidance
Many Worlds	Never ; it comes as an illusion
Spontaneous collapse theories (GRW, Penrose, Transactional)	Randomly for roughly when the result is observable, but independently of observer (cannot formally wait for decoherence, but in hopefully good approximation)
Mind Makes Collapse	At the first observer's perception. But 2 possible versions : specifically: only after decoherence (selection of a world), or even before (use of the projection postulate).

(I also included another ordering of these interpretations, by ontological views in another page)

List of non-interpretations

I would not include the following in the list of interpretations:

One may like the many interacting worlds combining aspects of the many-worlds and Bohmian mechanics. I'd rather call it a rewording of many-worlds in the language of Bohmian mechanics, thus patching many-worlds with some defects from Bohmian mechanical conventions. Yet, if it wants to mean not a continuous infinity of worlds but only a large-but-finite number of worlds (roughly partitioned into classes of same-looking worlds), then it has an effective difference with other interpretations (though its author did not pay attention to this): it logically implies the prediction that the universe is going to disintegrate as soon as its total entropy is going to reach a certain maximal value...
Copenhagen/Shut-up-and-calculate is just the brute expression of how things appear, the prediction tool, remaining agnostic about reality (says "the collapse is found done when looking at the result ; any further question is unscientific"). Others in that list have more or less the same defects, being hardly more than reformulations of the same not-so-coherent principles.
Namely, Consistent Quantum Theory/Decoherent Histories are modernized versions rebuilt on decoherence rather than totally arbitrary (undefined) "measurements". Measurements are less arbitrary but still so, as decoherence is a fuzzily defined concept, that keeps a margin of under-determination.
QBism (link to another site) similar to Copenhagen (solipsism), completed by remembering the plurality of observers (relative solipsism) to describe an unclear choice of interpretation, maybe Many-worlds.
Transactional does not seem to make any sense (or does it ?) - might also be classified in spontaneous collapse theories.
Some pseudo-interpretations may be more or less the same under different names:
- Relational may either seem to fit here or as a synonymous for Qbism, depending on where I read about it...
- Ensemble/Statistical (which is not really an interpretation, but can rather be read as a refusal to interpret, like Qbism)
- A sort of local deterministic realism believed by some people in terms of "the measurement result depends on the exact state of the measurement device". See such a view presented in this debate, and criticized especially at minute 57.
- Superdeterminism, which is somewhat clearer, so :
Superdeterminism, also called conspiracy theories : a class of interpretations with 2 big problems:
- Like many other non-interpretations, it fails to specify what is the form of the reality it deals with (which it qualifies as super-determined): does the real state of a system at a given time consist in its wave-function or not ? and if the results of measurement are super-determined, then, what does a measurement consist in, how can it be precisely described, when does the transition from superposition to the unique result actually happen ? this "interpretation" fails to answer.
- That is a roughly mathematically well-defined range of theories which is actually the empty set: this class of theories has no known representatives, not even any toy model beyond the most trivial cases, and the chances to find one (specifying a form of "reality" and achieving the same predictions as quantum theory, especially the violation of Bell's inequalities) are likely to be null, by mathematical necessity rather than any limit of our imagination. Further comments
Lawless eternalism (I did not see anyone clearly propose this nor call it that way, I just need to introduce this to complete the list of possible and impossible ideas) : a copy of superdeterminism that solves the problem of impossibility to specify a law by denying the need of one. Takes eternalism, dismissing time as an illusion, to claim that nondeterminism (random outcomes) neither occurs at any time nor needs a law, it "just is". However this begs the question (leaves as a complete mystery) why from such a universe there seems to emerge any appearance of obeying, so precisely and successfully, any specific (probabilistic) physical law at all.
Quantum logic and modal interpretations sound like "there is no reality but general abstract nonsense",
Many Minds is a rather fringe theory by isolated authors mixing ideas of many-worlds with a fundamental role of consciousness conceived in a quite strange and particular way

References

Introduction to quantum physics and its paradoxes

My own introduction to quantum physics

A book presenting quantum physics in the form of some paradoxical experimental results :
Quantum Physics: A First Encounter - Interference, Entanglement, and Reality by Valerio Scarani and Rachael Thew
(in French : Initiation à la physique quantique : La matière et ses phénomènes)
Six Quantum Pieces : A First Course in Quantum Physics

In that article, Valerio Scarani explains a theorem stating some dilemma between different metaphysical hypothesis for compatibility with the experimental verifications of quantum physics; his personal choice is that we have pure randomness because the available alternatives seem less plausible to him.

The Information Philosopher gives elementary introductions to the issues, and discusses free will

“Eating the Whole Thing”: Philosophy, Science, and Anxiety; A Supplemental Podcast for Social Research with David Albert

Bringing Schrödinger's Cat to Life article in the Scientific American (2012)

Consistent Histories and Operational Quantum Physics (1995)

John Boccio : Teaching Quantum Mechanics - Notes and Papers of Interest - QM on the Web

John Baez week post on quantum foundations (2007)

Simon Kochen and the Free Will Theorem
Kochen–Specker theorem
Leggett-Garg Inequalities

An experimental test of non-local realism (but that does not rule out Bohmian mechanics)

A primer on quantum mechanics and its interpretations by Casey Blood
Is the moon there when nobody looks? Reality and the quantum theory, N. David Mermin (1985)

Measurements in Quantum Mechanics online book

What Quantum Measurements Measure

Decoherence

Decoherence and its Role in the Modern Measurement Problem (David Wallace, Nov. 2011)

Decoherence, the measurement problem, and interpretations of quantum mechanics (Maximilian Schlosshauer, Dec 2003)

Relative States and the Environment: Einselection, Envariance, Quantum Darwinism, and the Existential Interpretation (2007)

A Web site dedicated to decoherence

What is "Orthodox" Quantum Mechanics? David Wallace ALS, Nov. 18, 2016 video

The Unreasonable Effectiveness of Decoherence

Notes on decoherence and interpretations : reply to the naive misunderstandings of "Why Current Interpretations of Quantum Mechanics are Deficient"

General references on the debate

The Nine Lives of Schroedinger's Cat (Zvi Schreiber, October 1994)

The Quantum Measurement Problem: State of Play (David Wallace, 2007)

Interpretations of Quantum Mechanics and the measurement problem (M. Genovese, 2010)

Our quantum problem essay by Adrian Kent (Jan. 2014)

A list of interpretations classified by categories, inside a discussion

Courses at University of Waterloo:

A list of links at MIT

Long list of articles

The Most Embarrassing Graph in Modern Physics
(the fact there is still no consensus in the interpretation of quantum physics)

A debate on quantum mechanics interpretations (video), presented by Brian Greene, between proponents of 4 interpretations :

Many worlds (Sean Carroll),
Bohm (Sheldon Goldstein),
Spontaneous collapse (David Z. Albert) ,
QBism (Ruediger Schack - other page)

The same video followed by a discussion
Works on the philosophy of quantum mechanics by Charles "Chip" T. Sebens
The Bohr/Einstein debate over QM: who won?

A proponent of the transactional interpretation reviews other interpretations

Physics forums : Interpretations of QM? What is nature really like? - QM interpretations - Quantum interpretations

Mike Towler (who favors Bohm's interpretation) : Foundations of Quantum Mechanics

Jeremy Butterfield's old site - new site - The Oxford Questions on the foundations of quantum physics

"Interpretations" tagged articles in Matt Leifer's blog

On the reality of the quantum state : an important recent theorem, commented here and there ; Does the quantum wave function represent reality?

Guido Bacciagaluppi (old page archive) : Insolubility Theorems and EPR Argument - Collapse Theories as Beable Theories

Physics Stackexchange: Is the universe fundamentally deterministic?

Luboš Motl's blog: The Reference Frame (light loading version), defends the Copenhagen interpretation (has some very controversial political opinions and is too easily insulting people with different opinions, though).

Bottomlayer.com : The Notebook of Philosophy & Physics by Ross Rhodes
Quora threads of questions : Philosophy of Quantum Mechanics - Interpretations of Quantum Mechanics