# Tom’s Top 10 interpretations of quantum mechanics

Dozens of interpretations of quantum mechanics have been developed over the years. Most of them attempt to address what happens when an observation or measurement is made on a quantum system. The mathematical formula known as the wave function (or state vector) describing the state of a system gets reset when a measurement is made, and the multiple possibilities that the math describes appear to “collapse” into one tangible result. A quantum “interpretation” tries to explain why this collapse happens — or whether it happens at all. And some interpretations concern themselves with whether the wave function itself is physically real or merely something mathematical.

Warning: Summaries below do not reflect all the subtleties of the various interpretations, which have often been modified over time by supporters or even the original authors. I’m just conveying some of the flavor. As cosmologist Max Tegmark writes in his new book *Our Mathematical Universe*: “There isn’t even consensus on which ones should be called interpretations.” (Note to advocates of various views: do not be concerned about the order in which these are listed. There is some quantum randomness here. And it’s not the BCS, after all — although some kind of championship playoff competition for quantum interpretations might be fun.)

#### 10. Bohmian Mechanics (David Bohm)

I don’t really like this one very much, but it has many fans and deserves to be mentioned. Developed in the 1950s by Bohm, based on earlier views from Louis de Broglie, Bohmian mechanics describes particles flying around as guided by “pilot waves.” Those waves tell particles where to go. Supposedly this approach turns physics back to determinism, avoiding the probabilities that Einstein condemned by saying “God does not play dice.” Since experiments have ruled out “hidden variables” for enforcing determinism, Bohmian mechanics requires a form of action at a distance (or “nonlocality”). Einstein didn’t like that either. It’s also hard to see how Bohmian mechanics would predict any experimental difference from the predictions of standard quantum mechanics. Shortly before he died, Einstein said he wasn’t impressed with the Bohmian interpretation. “That way seems too cheap to me,” Einstein wrote in a letter to physicist Max Born.

#### 9. Stochastic evolution interpretation (many versions)

This one perhaps isn’t strictly an interpretation of quantum mechanics itself, because it changes the math. In ordinary quantum mechanics, the wave function (or state vector) “evolves,” changing over time in a perfectly predictable way. In other words, the odds of different results can change, and you can predict exactly how they will change, up until the time a measurement is made. But several physicists have suggested over the years that the evolution itself can change in a random (or stochastic) way causing it to collapse all by itself. Presumably this collapse process would occur very rapidly for large (macroscopic) objects and slowly for subatomic particles. Nobel laureate Steven Weinberg recently examined this approach in a paper available at arXiv.org.

#### 8. Quantum Bayesianism (Christopher Fuchs, Carlton Caves, Rüdiger Schack)

This one, sometimes called “QBism,” adopts ideas from a particular school of Bayesian statistics holding that probabilities reflect a personal belief in how to bet on possible outcomes. Consequently in this view the wave function is “personal,” a measurement of an individual’s knowledge of the state of a system that can be put to use to predict its future. I blogged about it in more detail here.

#### 7. Many Worlds Interpretation (Hugh Everett III)

Ignored for years after its appearance in 1957, the many worlds interpretation has gained in popularity in recent decades. Sometimes called the “many universes” interpretation, it postulates that every time a measurement is made, all the possible outcomes actually occur in different branches of reality, creating a multitude of parallel universes. Actually, Everett thought of it as more like the observer splitting into different clones who follow the different possible measurement outcomes. In any case, it’s weird.

#### 6. Cosmological Interpretation (Anthony Aguirre and Max Tegmark)

A relatively new one. The original paper describing it was posted online in 2010. Basically, Aguirre and Tegmark contend that the many worlds interpretation is trivially true if the universe is infinite, since there would be an infinite number of parallel universes in which all the outcomes allowed by quantum mechanics do in fact occur. Aguirre and Tegmark calculate that the outcomes would occur in just the proportions predicted by probabilities calculated from the quantum math. So in this view, they write, “the wave function describes the actual spatial collection of identical quantum systems, and quantum uncertainty is attributable to the observer's inability to self-locate in this collection.”

#### 5. Copenhagen interpretation (Niels Bohr)

I’m old enough to remember when few physicists challenged the Copenhagen interpretation, as articulated by Bohr in the late 1920s, during the early days of quantum mechanics (and later embellished by Werner Heisenberg). Bohr believed that measurements produced results that could be described only in the ordinary language of classical physics, so it made no sense to ask what was going in in some invisible “quantum” realm. You had to specify an experimental arrangement for asking a question of nature, and the question you asked played a role in the answer you got. This view incorporated the Heisenberg Uncertainty principle as a statement not about the limits of measurement, but about the nature of reality — simultaneous positions and velocities simply do not exist for fundamental particles before a measurement is made. Measurements select from among the many possibilities (or potential realities, in Heisenberg’s language). Bohr explained supposed paradoxes, such as particles behaving as waves and waves behaving as particles, as mutually exclusive but “complementary” aspects of nature.

#### 4. Consistent Histories (Robert Griffiths)

First proposed by Griffiths in 1984, the consistent histories interpretation treats classical physics as a mere approximation to quantum mechanics, and the quantum math can be used to compute probabilities for large-scale phenomena as well as subatomic phenomena. Probabilities don’t refer to the results of measurements, but to physical states within a system. Griffiths emphasizes “incompatibility” of the multiple possible realities in quantum physics. You can choose to take pictures of a mountain from different sides, he points out, but the photos could be combined to make one picture completely consistent with the reality of the mountain. In quantum physics, though, you can choose which property to measure (say the velocity of a particle or its position), but you can’t combine two measurements to give a consistent picture of the particle’s premeasurement reality. There is no simultaneously real position and momentum before you made the measurement. Similarly, there is no real physical state where Schrodinger’s cat is simultaneously alive and dead. The fact that a wave function can describe such a state merely means that the wave function is simply a mathematical construct for computing probabilities of sequences of events, or histories. In real life those event sequences will tell a consistent story.

#### 3. Quantum Darwinism (Wojciech Zurek)

Similar in some respects to consistent histories, Zurek’s quantum Darwinism emphasizes the role of decoherence. That’s the process by which multiple possible quantum realities are eliminated when a system interacts with its environment. As air molecules or photons bounce off an object, their trajectories record the object’s position; very rapidly only one position remains consistent with the information recorded in the environment. Thus natural interactions produce a sort of “natural selection” of properties that are recorded in the environment in multiple copies accessible to observers. That way observers can agree on specific locations for macroscopic objects instead of multiple locations at once.

#### 2. Decoherent Histories (Murray Gell-Mann and James Hartle)

A variation of Griffiths’ consistent histories, Gell-Mann and Hartle’s interpretation (proposed in 1989) emphasizes decoherence, as does Zurek’s quantum Darwinism. But Gell-Mann and Hartle argue that the whole universe can be considered a quantum system with no external environment. So the decoherence occurs internally, producing what they call “quasiclassical domains” — sets of consistent histories that can’t be distinguished at the level of coarse graining imposed by decoherence. I discussed it in more detail here.

#### 1. My Interpretation (Me)

No summary available. I’m still working on it. (It would help if some publisher would like to offer a lucrative book contract.) I think I’ll call it the hermeneutical interpretation of quantum mechanics. Perhaps I’ll conclude that rather than interpreting quantum mechanics itself, it’s the interpretations of quantum mechanics that need interpreting.

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