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: 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 :
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:
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:

See also

Some discussions on the nature of randomness, in Physics Forums
The use of probability in QM
Question on the "probabilistic" nature of QM

I analyze randomness in other interpretations

Before reviewing these interpretations in details, see the description of the EPR paradox, a crucial aspect among the paradoxical predictions of quantum physics, and verified by diverse experiments, that interpretations have to deal with.

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:


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


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

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:

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

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) between proponents of 4 interpretations :
The same video followed by a discussion

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).


a 1997 poll

A Snapshot of Foundational Attitudes Toward Quantum Mechanics (from 2011, published 2013)
(33 people from the conference "Quantum Physics and the Nature of Reality," held in July 2011 at the International Academy Traunkirchen, Austria; but nobody for Bohm's interpretation)
(same article posted in a blog)

Another Survey of Foundational Attitudes Towards Quantum Mechanics

Yet Another Snapshot of Foundational Attitudes Toward Quantum Mechanics
(with a majority for Bohm's interpretation)

Maximilian Schlosshauer : The Quantum Interviews (2011)

Physics Forums polls :
What is your favored interpretation of quantum mechanics? (2006)
Your favourite Quantum Interpretation? (2008)
Quantum Interpretation Poll (2011)
Interpretation of Q.M. (with more options!) (2012)

Foundations of physics