This is the third part of a series examining the Many Worlds Interpretation of Quantum Mechanics (the MWI of QM). The popularity of the MWI in books, blogs, and science videos, especially among the science-minded, tends to keep in present in some corner of my mind. Blog posts are a way to shoo it out.
The first part introduced the topic and talked about cats. The second part discussed the Schrödinger equation, wavefunctions, decoherence, and the question of how multiple instances of matter can coincide. That question, to me, is a central issue I have with MWI.
This time I dig into quantum superposition and touch on a few other topics.
Last time I started exploring questions I have about the Many Worlds Interpretation of Quantum Mechanics (the MWI of QM). Obviously I’m not a fan; quite the opposite. It presents as parsimonious, hung on the single hook of a universal wavefunction, but I think it gets more complicated and cumbersome when examined. I can’t say it’s broken, but I don’t find it very attractive.
I suspect most people, even in physics, don’t care. A few have invested themselves in books or papers, but these interpretations don’t matter to real physics work. The math is the math. But among the philosophical, especially the ontological, it’s food for debate.
Being both philosophical and ontological, I do smell what’s cooking!
Back in January, in a post about unanswered questions in physics, I included the Many Worlds Interpretation of Quantum Mechanics (the MWI of QM). I wish I hadn’t. Including it, and a few other more metaphysical topics, took space away from the physical topics.
I did it because I’ve had notes for an MWI: Questions post for a long time, but shoehorning it in like that was a mistake. Ever since, I’ve wanted to return and give it the attention of a full post. I’m reminded about it constantly; the concept of “many worlds” has become such a part of our culture that I encounter it frequently in fiction and in fact (and in other blog posts).
Its appeal is based on a simplicity, but to me it doesn’t seem at all that simple.
Lately I’ve been playing a little game of What’s the Wavelength? The question is certainly a bit evocative. Wavelength could refer to many things: a favorite radio station or, metaphorically extended, a favorite anything. It might even evoke an old news meme, although the supposed question posed that time was about frequency (which is just the inverse of wavelength).
Wavelength might even apply to one’s political, social, sexual, musical, or whatever, alignment, but in this case I mean it literally and physically. Under quantum mechanics — our best description of small-scale physical reality — everything manifests as a wave. That means everything has a wavelength — the de Broglie wavelength.
I’ve been curious about it for a couple of reasons.
The notion of emergence — because it is so fundamental — pops up in a lot of physics related discussions. (Emergence itself emerges!) A couple of years ago I posted about it explicitly (see: What Emerges?), but I’ve also invoked it many times in other posts. It’s the very basic idea that combining parts in a certain way creates something not found in the parts themselves. A canonical example is how a color image emerges from red, green, and blue, pixels.
Also often discussed is reductionism, the Yin to the Yang of emergence. One is the opposite of the other. The color image can be reduced to its red, green, and blue, pixels. (The camera in your phone does exactly that.)
Recently I’ve been thinking about the asymmetry of those two, particularly with regard to why (in my opinion) determinism must be false.
particles & their momenta
Over the decades I’ve seen various thinkers assert that entropy causes something — usually it’s said that entropy causes time. Alternately that entropy causes time to only run in one direction. I think this is flat-out wrong and puts the trailer before the tractor. (Perhaps due to a jack-knife in logic.)
The problem I have is that I don’t understand how entropy can be viewed as anything but a consequence of the dynamical properties of a system evolving over time according to the laws of physics. Entropy is the result of physical law plus time.
It’s a “law” only in virtue of the laws of physics.
When I was in high school, bras were of great interest to me — mostly in regards to trying to remove them from my girlfriends. That was my errant youth and it slightly tickles my sense of the absurd that they’ve once again become a topic of interest, although in this case it’s a whole other kind of bra.
These days it’s all about Paul Dirac’s useful Bra-Ket notation, which is used throughout quantum mechanics. I’ve used it a bit in this series, and I thought it was high time to dig into the details.
Understanding them is one of the many important steps to climb.
One small hill I had to climb involved the object I’ve been using as the header image in these posts. It’s called the Bloch sphere, and it depicts a two-level quantum system. It’s heavily used in quantum computing because qubits typically are two-level systems.
So is quantum spin, which I wrote about last time. The sphere idea dates back to 1892 when Henri Poincaré defined the Poincaré sphere to describe light polarization (which is the quantum spin of photons).
All in all, it’s a handy device for visualizing these quantum states.
Popular treatments of quantum mechanics often treat quantum spin lightly. It reminds me of the weak force, which science writers often mention only in passing as ‘related to radioactive decay’ (true enough). There’s an implication it’s too complicated to explain.
With quantum spin, the handwave is that it is ‘similar to classical angular momentum’ (similar to actual physical spinning objects), but different in mysterious quantum ways too complicated to explain.
Ironically, it’s one of the simpler quantum systems, mathematically.
Unless one has a strong mathematical background, one new and perhaps puzzling concept in quantum mechanics is all the talk of eigenvalues and eigenvectors.
Making it even more confusing is that physicists tend to call eigenvectors eigenstates or eigenfunctions, and sometimes even refer to an eigenbasis.
So the obvious first question is, “What (or who) is an eigen?” (It turns out to be a what. In this case there was no famous physicist named Eigen.)