Interpretations

An artist's representation of the multiverse as suggested by the Many World's interpretation.

The Copenhagen Interpretation

The Copenhagen interpretation of quantum mechanics is probably the most popularly taught interpretation to this day. Developed by Niels Bohr and Werner Heisenburg between 1925 and 1925, it proposes that physical systems generally have no definite properties until they are measured, at which point the wave function collapses into a single physical state, with all other states having never existed in the first place. It fits in somewhat nicely with classical mechanics because it describes the waves as purely a mathematical tool for calculating possible physical trajectories, rather than physical entities. So when a measurement is taken and the wave function collapses into a classical particle, no real instantaneous physical interactions occur. Additionally, the probability wave can only be said to pass through space in that the probabilities themselves are functions of time.

The Copenhagen interpretation also asserts that it is meaningless to discuss where the particle is between when it is emitted and when it is detected because to refer to the position of a particle, it must first be detected. However, the Copenhagen interpretation has fallen out of favor in recent years in part because it is non-deterministic, and in part because it uses a mathematical function that is undefined to convert from probability functions to non-probabilistic measurements. The Copenhagen interpretation also leaves open the notion that until a particle is observed, it does not exist at all (at least in any classical sense). This creates questions about what actually constitutes observation and whether there is an issue of subjectivity involved.

Einstein notoriously rejected the notion of quantum mechanics and the Copenhagen interpretation. Speaking in regards to them Einstein once made the comments "I, at any rate, am convinced that He [God] does not throw dice." and "Do you really think the moon isn't there if you aren't looking at it?" prompting Bohr to reply "Einstein, don't tell God what to do."

The De Broglie-Bohm Pilot Wave interpretation

The De Broglie-Bohm interpretation immediately differentiates itself from the Copenhagen interpretation in that it postulates that there are indeed physical waves. Named after Louis De Broglie who first hypothesized the theory in the 1920s and David Bohm who greatly expanded upon it in the 1950s and 60s, the interpretation is explicitly non-local, but distinct from the Copenhagen interpretation in that it is completely deterministic.

The principle behind the theory is that particles ride on physical "pilot waves" and that from emission to detection, the particle always exists and takes a single distinct path from the very beginning. The velocity of the particle is determined by the shape of the pilot wave, wherein things like interference and observation can affect the shape of the wave, and thereby the path of the particle. It also solves the Measurement Problem, which is left unsatisfied by the Copenhagen interpretation. The Measurement Problem is the issue of how (or if) the collapse of the wave function occurs. The Copenhagen interpretation suggests that "something" in the "observation" of the wave function causes the collapse, but is unable to explain exactly what "something" or "observation" mean. The De Broglie-Bohm interpretation solves the measurement problem by suggesting that there are two equations. There is the Schrödinger equation that every interpretation relies on to define the wave function, but the Pilot Wave interpretation also has an equation representing the "trajectory" or velocities of the particle, and the equation is such that the probability distribution for the particle remains consistent with other interpretations. With the wave function generally referring to a single wave for the entire universe, there is no wave function collapse in this interpretation, and thus no Measurement Problem. It is also a very satisfying theory because while Heisenberg's Uncertainty Principle prohibits us from knowing precisely the initial location and velocity of the particle, if we could accurately know that information, we could predict the entire future path of the particle, meaning it is indeed completely deterministic.

The interpretation is quite possibly the most classically and physically rooted fully self-contained theory, but that certainly does not mean it is without problems. One location in which it veers from the classical mechanics is that it requires that observation at any point along the pilot wave must instantaneously affect the entire wave, violating the principle of non-locality, but given that every other theory relies on things like superposition, it doesn't make this theory any more challenged than any other. However, it has received a bad name for many years because it does rely on so called "hidden variables" that are details about the state of the particle that must be encoded in the wave function itself. And because local hidden variables don't work, the entire wave function must know the spin, location, and velocity of every particle. All that having been said, the De Broglie-Bohm Pilot Wave interpretation is at the most completely wrong and at the least drastically incomplete because it does not allow for any form of relativity. It also has an intrinsic classical bias in that De Broglie and Bohm were both explicitely trying to use classical thinking to discribe quantum theory.

The Relational interpretation

The Relational interpretation to quantum mechanics (or RQM) was first put forth by Italian Theoretical Physicist Carlo Rovelli in 1994. RQM is a stark contrast to De Broglie-Bohm in that while the Pilot Wave theory is completely incompatible with special relativity, RQM was exclusively inspired by relativity. The idea behind RQM is the core principle of Einstein's Theory of Special Relativity: that an observation is based on the frame of reference of the observer. This is extended into a theory on quantum mechanics by saying that rather than describe the state of the system as simply including the particles that we're observing, rather the system refers to both the object being observed and the observer (be it a person, or any other physical object).

The theory is very similar to the Copenhagen interpretation, except it suggests that ultimately all interactions are also quantum interactions and therefore an interaction between a particle and an observer is effectively the same as an interaction between two particles. Because of this, RQM solves the Measurement Problem as well by again having no collapse of the wave function. While in the Copenhagen interpretation, the collapse of the wave function is said to occur when a quantum wave interacts with a macroscopic object which is, according to theory, exclusively governed by classical mechanics, in RQM the act of measurement is just like any other physical interaction and since the state is relative to the observer, one person could feasably measure a system to be in superposition at the same time another is measuring the same system to be in an eigenstate. This results in a very elegant theory that is similar to the Copenhagen interpretation in principle, but while eliminating a lot of the messy problems and paradoxes it creates. Surprisingly however, it is one of the lesser known and least popular interpretations being studied today.

The Many Worlds interpretation

The wildist, but also one of the most popular interpretations today is the Everett and DeWitt Many Worlds interpretation (MWI). First proposed in 1957 by Hugh Everett and expanded upon in the 60s and 70s by Bryce DeWitt, MWI is a deterministic interpretation that is based on avoiding the collapse of the wave function thanks to quantum decoherence. The basis of MWI is that unlike superposition in the Copenhagen interpretation, where when the particle was detected all of the possible states (or the wave) collapsed down into a single state, in MWI, each of the possible states persist, but in their own unique timelines.

The theory suggests that since the Big Bang, every quantum decision has effectively created a parallel universe for each possible outcome of the superposition, and our universe, because of shear probability, happens to generally end up in with the most likely outcome. For example with the dual slit experiment, every possible path the particle could take to the screen is accounted for, but because there are more possible outcomes that result in the particle being within one of the bands of the interference pattern, there are more universes where one of those outcomes is what happens, and thus we are more likely to be within one of those universes.

It may seem messy to create a universe for every possible branch of every quantum decision ever, but it actually resolves a lot of problems. For example, it doesn't have any correlation paradoxes (the EPR paradox, schrödinger's cat, etc.) like exist in most interpretations. It also doesn't have any hidden variables or conditions, and it is fully compatible with relativity. It also makes fewer assumptions than the Copenhagen interpretation. While the Copenhagen interpretation effectively creates many realities, when for example the particle is in super position, it collapses them all down into the one reality when it is measured. The math doesn't require the collapse of the wave function, so that's effectively just an additional assumption. In fact, when Everett was doing his PHD research into what would eventually become the MWI, he specifically noted that while it merely appeared that the wave function collapsed, there was no reason to assume it had actually collapsed, so using Occam's Razor he eliminated the collapse from the theory entirely. While it may seem counterintuitive, the MWI appears to be the most plausible candidate for a theory that makes the fewest assumptions outside of the strict mathematics. But then again, they're all just guesses...