Interpretations of Quantumania

Anytime that physics finds a rare opportunity in the spotlight, it’s an exciting time for me. Whether it’s an actual scientific discovery, the announcement of the Nobel prize, or an astoundingly rare occurence of people enjoying physics in popular culture, this seems to happen at least once annually. The holy grail of the latter, to me, was 2014’s Interstellar. Interstellar not only managed to capture some of the most interesting and complex consequences of Einstein’s General Theory of Relativity in a way that was captivating for the viewer, but it was also just a phenomenal film.

The supermassive black hole from Interstellar. Once upon a time I wrote two papers on weighing these things.

We’re now amidst another such opportunity for physics in the mainstream media with the new Ant-man and the Wasp: Quantumania film. I haven’t seen it – and I dont know if I plan on seeing it – as my ability to enjoy such things has largely been ruined by my life choices. However, I do think this would be an interseting time to reflect on and discuss one of the more interesting aspects of quantum theory that’s often overlooked in the typical undergraduate and graduate instruction of the subject.

What I’m referring to are the many schools of thought in interpreting the very odd and unsettling consequences of quantum theory. Quantum theory at its core is a statistical theory of reality. Prior to the quantum revolution of the early 20th century, the most succesful physical theories coming from Newton (Mechanics) and Maxwell (Electromagnetism) were entirely deterministic. These theories typically yielded some beautiful differential equations that once solved, told the physicist exactly what they wanted to know. When experiments by Bohr and other’s started suggesting that these classical descriptions start failing at the atomic scale, physicists like Planck, Heisenberg, and Schrödinger used this time and many experimental results to start developing a theory of atomic and subatomic physics. The resulting theory is known as quantum mechanics. It should be noted that quantum mechanics is not a complete theory of subatomic physics, as it is not compatible with Einstein’s Special Theory of Relativity. The most succesful quantum theory (for now) that gave humanity the structure of the proton, the particle zoo, and a complete description of antimatter, is known as quantum field theory. Quantum field theory has not yet made its way into the mainstream media.

Quantum mechanics is not determinisitic. In quantum mechanics, you can only derive probabilities for what would happen when observing a quantum state, and this is a direct consequence of the unsettling wave-particle duality observed by Davisson & Germer and formalized by de Broglie in roughly 1923 and 1924, respectively (among others). Wave-particle duality is the concept that matter can be described as both a particle and a wave. This could have actually been discovered roughly a century earlier by Thomas Young in his original double-slit experiment, but humanity was not ready for this yet. The culmination of these results, as well as many others, was a Pandora’s box in the history of physics. Was reality not deterministic? Is free will a scam? Am I making a conscious choice right now in writing this, or is this simply the most probable result of some higher being observing my quantum state? This may sound like nonsense, but in order to be self-consistent, quantum mechanics must find a way to coexist with other physics and philosophy. In fact, quantum mechanics caused a severe reconstruction of our existing interpretations of reality and metaphysics, as phenomena such as wave-particle duality have been extensively tested and verified through numerous experiments. The quantum realm is here to stay.

The nature of quantum mechanics is most famously illustrated in a thought experiment from Schrödinger. A quantum state is described by its wave function, and when an observer observes a quantum state (I really don’t know how to word that any better), the wave function collapses and reality is achieved. Schrödinger imagined he put a cat in a box with some poison, and let it be for some time. I’m not sure if Schrödinger had a particular vendetta against cats, but this would suggest he did. While the box is closed, the cat’s wave function is in a superposition of being dead and alive; that is, it is simulataneously dead and alive. At some point, he opens the box and witnesses whether the cat is alive (didn’t eat the poison) or dead (ate the poison), thereby collapsing its wave function. Of course, this is just an analogy to get a feel for how odd quantum mechanics is. This is actually an abridged version of what was originally imagined; here’s a quote from a letter Einstein wrote to Schrödinger praising his original thought experiment:

• “You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. Their interpretation is, however, refuted most elegantly by your system of radioactive atom + amplifier + charge of gun powder + cat in a box, in which the psi-function of the system contains both the cat alive and blown to bits. Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.”

Quantumania.

Here is where we arrive at the philosophical implications of quantum mechanics. The so-called Copenhagen interpretation is the most widely-accepted school of thought in quantum theory. The Copenhagen interpretation asserts that: 1) the wave function fully describes a quantum state, 2) observation of a quantum state immediately collapses the wavefunction, and 3) that wave-particle duality is complementary. The latter likens the observed wave and particle nature of matter as two sides of the same coin. More severely, the Copenhagen interpretation demands that you relinquish any need of knowing how the quantum state is behaving outside of measurement. That is a hard pill to swallow. I think the only people who can truly comprehend this are babies before they develop object permanence.

DALLE-2 prompt: An artistic visualization of wave function collapse in quantum mechanics.

There are other succesful interpretations as well – many others in fact – but I would like to discuss my favorite which is the Many-Worlds Interpretation (MWI). The MWI was introduced by Hugh Everett in 1957. The MWI, in short, is a multiverse hypothesis. The MWI hinges on an idea called quantum decoherence. What quantum decoherence states is that information is lost when a physical system (for example, an electron I observe) interacts with its environment (me). In this picture, the previous Copenhagen interpretation of the collapse of the wave function is viewed as the physical system transitioning from a coherent to decoherent state. This, surface level, just sounds like a qualitative difference in descriptions – but do not fear, there is plenty of mathematics backing this that I will not bore the reader with.

The MWI suggests that the Universe constantly branches into multiple parallel Universes to account for the different possible outcomes of quantum events. In this sense, the wavefunction is not subject to collapse, but rather the quantum decoherence that is introduced by the observer is what elicits the branching of parallel Universes. This also crucially relies on a phenomena known as quantum entanglement, but I am too tired to think about entanglement right now. Entanglement is confusing.

There is also what’s known as Superdeterminism, and Quantum Darwinism, and Quantum Bayesianism, and so on. There is not enough time nor brain power (at least for me) to visit every exhibit in the Museum of Interpretations of Quantum Mechanics (MIQM, est. TBD). At the moment, I am comfortably Copenhagen when I’m sober and maniacally many-worlds, otherwise.