Quantum mechanics is strange. But I bet you already knew that. What’s worse, even to physicists it is really strange. And even though physicists use the theory with ease and obtain remarkably accurate predictions in most cases, when it comes to the philosophical understanding of the theory, everybody feels lost. As an example, check out the results of a poll carried at quantum foundations meeting, as mentioned in this great blog post by Sean Carroll.
As he nicely puts it, these results are a bit embarrassing in a sense that there is no agreement — leading physicists, mathematicians and philosophers seem to have totally different ideas when it comes to the interpretation of one of the most successful theories in science. The leading result is the Copenhagen interpretation, however the information based & many world interpretations are not that far away. So what is the reason of such a split? To put it simply, Copenhagen interpretation, which is often regarded as the standard view on quantum mechanics, has a number of problems, which other interpretations try to solve. The biggest problem is the so-called collapse of the wave function, which happens when a measurement is performed. So the idea is that a wave function evolves deterministically, according to Schrodinger’s equation as a linear superposition of different states, but, when a measurement is performed, we find the system in a certain single state.
One of the main ideas of Copenhagen interpretation is that it treats such questions as “where was the particle before the measurement” as meaningless. So, as Paul Davies puts it, the reality is in the observation and not the electron. As you might guess, such a position seems highly unsatisfactory to many physicists. After all, would you be satisfied with a statement that the moon might not be there when you are not looking at it at night? Or that such a question is meaningless? This, as you might imagine, is philosophically unsatisfactory to many experts, which is one of the reasons why we have all these other interpretations.
So what are these other interpretations? Let’s do a short summary of the main points of the most popular alternative interpretations of QM. Also, check out this video from the man himself, Sean Carroll, discussing different interpretations.
Many Worlds Interpretation
So this one is probably the favourite interpretation of sci-fi fans. First of all, this interpretation takes it for granted that wavefunctions are a part of objective reality. And the main idea behind this interpretation is that when a measurement is made, the universe splits into multiple universes, all of which are equally real. So, for example, if you try to perform the famous Schrodinger’s cat experiment, when you open the box, the universe splits into to universes, in one of which the cat is alive, whereas in the other one the cat’s dead.
These approaches, roughly speaking, claim that QM describes our knowledge of the world rather than the world itself. So, essentially, it’s all about gaining information about the system and exchanging information among systems. Philosophically, there are two kinds of approaches — one sharing many ideas with immaterialism and the second kind similar to instrumentalism.
Objective Collapse Theories
This interpretation claims that both wavefuctions and their collapse are ontologically objective. One of the most interesting features of such interpretation is that collapse is said to occur randomly, or due to a breach of a certain threshold, and so the observer has no special role. Two most known examples are Ghirardi–Rimini–Weber theory and Penrose interpretation.
This approach is basically a mix of quantum information theory and the Bayesian probability. Perhaps the most famous such approach is due to Caves, Fuchs, and Schack, who aim to provide an understanding of quantum mechanics and to derive modern quantum mechanics from informational and Bayesian considerations.
Relational Quantum Mechanics
Relational interpretation of QM states that what happens to quantum states is dependent on the reference frame of the observer. So, essentially, it is related to special relativity. An interesting conclusion then is that different observers may give different accounts of the same series of events. As an example, for one observer a system may be in a single, “collapsed” eigenstate, while to another observer at the same time, it may appear to be in a superposition of two or more states.
So these are the more popular interpretations. This list, of course, is not complete, as presenting all of the interpretations would take ages. However, just to wrap things up, here’s a list of the least popular interpretations:
- Calogero conjecture
- Semiotic Interpretation
- Landé Interpretation
- Prowave Interpretation
- Time-symmetric interpretations
- Pondicherry Interpretation
- Quantum Mysticism
- London (Ticker Tape) Interpretation
- Theory of Incomplete Measurements
- Montevideo Interpretation
- Synchronized Chaos Interpretation
- Vaxjo Interpretation
- Dimensional Theory
- Intrinsic Quantum State Interpretation