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You Are in a Superposition (or, Why Every Explanation of Many-Worlds Quantum Mechanics Sucks) #81

Open giladmargalit opened 2 years ago

giladmargalit commented 2 years ago

About the author

I'm a 6th-year PhD student working on the theory of quantum materials. I'm busy writing my thesis right now, but the concept for this video is so simple and elegant that it shouldn't take much time to put together. Though I study in Israel, I'm American-born and have English as a first language (I could in principle record lines, though I lack the technical skill or equipment to do a professional job of it). Excuse the clickbait title; I tried to convey my main point in a more nuanced title but it ended up too long.

Quick Summary

Every explanation I've heard of the many-worlds interpretation of quantum mechanics goes something like this: "when we measure a particle, it randomly collapses to one of its possible states in our universe, but somewhere else (??), in another universe (???), the measurement had a different outcome. The universe splits. In fact, every choice we make causes the universe to split countless times." This explanation prompts a lot of questions - where are these other universes? Why would you even suppose they exist? If we can't reach them, what explanatory power do they have at all? - on and on. That's because it's a poor explanation of a beautifully simple chain of reasoning.

It all comes down to the idea of a universal wavefunction. In short, we have to ask the question: how does it feel to be in a superposition?

For instance, how would you feel if you - yes, YOU, macroscopic you - were in a quantum superposition of being in the Sahara and being at the North Pole? Would you see both sand and ice at once? No. Because photons from the Sahara would only enter the eyes of Sahara-you and photons from the arctic would only enter the eyes of North-Pole-you, in effect entangling your wavefunction with those photons. Each of "you" would see and experience only the location you're in, despite the fact that your probability amplitude is half in one place and half in the other.

Now take the logical next step: would you know if you were in a superposition right now?

Would you know if the whole universe were one big entangled wavefunction, every particle and macroscopic object entangled in many possible states, none aware that the other exists? THAT is the true many-worlds interpretation. The beauty of this reasoning is it requires no mechanism for "collapsing the wavefunction," because the wavefunction never collapses. It doesn't require an arbitrary distinction between "quantum" objects that can be in superposition and "macroscopic" objects that can't. We're all part of the universe's quantum wavefunction, and the probabilities we see are just us finding ourselves in one part of it or another.

Target medium

I'd like the above explanation to end up as a short video aimed at people with no formal knowledge of quantum mechanics. It's a clarification that desperately needs to be made, one that made the concept crystal-clear to me and I'm sure will help many others who are interested in the subject. I've seen some YouTube videos that come so, so close to giving this intuition, like this one by PBS Space Time, but judging by the first few comments, it was more confusing than enlightening. I personally think the source of the confusion is simply using the term "many-worlds." I think sticking to "universal wavefunction" does a better job of describing what's going on, and can get the idea across in 5-8 minutes instead of over 12.

I would construct the video as follows:

  1. Explain the basic idea of a superposition, making use of a ket notation: $$\lvert \text{proton's location} \rangle = 0.6 \lvert \text{left} \rangle + 0.8 \lvert \text{right} \rangle$$
  2. Connect the idea to entanglement: $$\lvert \text{proton and detector} \rangle = 0.6 \lvert \text{proton went left and detector flashed left} \rangle$$ $$+ 0.8 \lvert \text{proton went right and detector flashed right} \rangle$$
  3. Construct a universal wavefunction to drive home the fact that you, the observer, are in the superposition: $$\lvert \text{proton and detector and you} \rangle = 0.6 \lvert \text{proton went left and detector flashed left and you saw the left flash} \rangle$$ $$+ 0.8 \lvert \text{proton went right and detector flashed right and you saw the right flash} \rangle$$

I think Manim would be ideal and relatively easy, even for people who have never used it before, seeing as this topic doesn't require any plots or simulations, just brackets and descriptions (illustrations would also help). I don't have any experience with Manim or video editing, but I'd be interested in collaborating on a script with someone who thinks they could pull it off.

More details

I find it interesting that the person who first proposed many-worlds, Hugh Everett, originally called his interpretation the "relative state" interpretation, and described it quite well. Then Bryce DeWitt renamed it to the "many-worlds" interpretation, and his popularization of it proceeded to confuse the casual masses for generations.

Contact details

The email I'm most responsive to at the moment is my university email, gilad.margalit@weizmann.ac.il. Place a comment below first if you're interested, and we'll switch to email once we've set up the collaboration.

gralnys commented 2 years ago

I wouldn't be able to help with this, but I'm so glad you posted this here. This makes so much more sense now.

twtww commented 2 years ago

Hi Gilad. Good luck! I also watched the PBS Space Time video you mentioned.

Just in case you find it interesting, I made a video on this topic, but looking at superposition from a different angle. https://youtu.be/2jOvmj7PDBs