What is probability? (Quantum Theoretical View)

One answer views probability as an "emergent" property of Quantum Theory, the Many Worlds interpretation, and the meaning of Identity of an Observer.

We think of probability on the macro scale -- the probability of a coin landing "heads", or of a die landing on 3.  But this macro view of probability is the culmination of an enormous (but finite) number of tiny events such as electrons and other subatomic particles bouncing off one another, popping into or out of existence, etc.  These smallest events are described by quantum theory, so the thing we think of as probability is grounded in quantum theory.


Sean Carroll gives a really good overview in this 14 minute video of the current state of interpreting Quantum Mechanics.  In 2011, Quantum Mechanics experts were asked "What is your favored interpretation of Quantum Mechanics?"  The result was that less than half favored the "textbook" interpretation, which is called the Copenhagen Interpretation, although this was still the most popular interpretation.  Sean Carroll favors Hugh Everett's "Many Worlds" interpretation, because it doesn't have the kind of problems faced by the Copenhagen interpretation, particularly the special status of the observer.  He is careful, however, to point out that there are several other interpretations that solve the Copenhagen interpretation's problems, so the Many Words interpretation is not necessarily the correct or best one.  It's just better than the Copenhagen interpretation.

I'm personally persuaded by the Many Worlds interpretation of quantum mechanics because it avoids the main pitfall of the Copenhagen interpretation, which is the special status of the "observer".  However, it introduces a new problem, which I call the "Identity Problem", namely whether "I" exist in all the so-called "pocket universes" that are created as a result of each quantum event.  I'll set that aside, for the moment, but it might come up again later!


A new paper by physics professor Andreas Albrecht and graduate student Dan Phillips at the University of California, Davis, makes the case that these quantum fluctuations actually are responsible for the probability of all actions, with far-reaching implications for theories of the universe.

Taking these interpretations together -- the "Many Worlds" interpretation of quantum mechanics, and the "Quantum" interpretation of probability, then the probability of an event occurring, is the "emergent" probability that the "observer" will be in a pocket universe in which the quantum events (that aggregate up to that event occurring) happened.

This Quantum Probability view brings us back to the existence of a single "observer" who exists in a subset of all the pocket universes that split off from the one universe extant just before the event occurred.  Here is where the "Identity Problem" rears its head, and in even if this question is answered,  Quantum Probability raises the question of whether it makes sense to ask what is the probability that the "observer" will exist in this subset of pocket universes.