I feel like this is missing something. How do we know they have both spins before we measure them and that they haven’t “decided” their spin beforehand?
Because this is how all properties work in quantum mechanics. This was the point of my reference to the double slit experiment, which is the classic example of this idea (called “superposition”). In the double slit experiment, you shoot a particle at a barrier that has two openings in it, and watch where it goes. If you shoot a bunch of particles through at once, then they interact with each other and produce a particular pattern. If you shoot them through one at a time, and they randomly picked one of the two holes to go through, you would expect to see them cluster in two places. This is not what you actually see. What you actually see when you shoot them through one at a time is exactly the same pattern that you saw when you shot them through all at once. Therefor, individual particles actually go through both holes at once and interact with themselves, they are in two different states simultaneously until someone observes them, and forces them to be in one. This is how all quantum mechanical properties work, including spin.
I feel like this is missing something. How do we know they have both spins before we measure them and that they haven’t “decided” their spin beforehand?
Because this is how all properties work in quantum mechanics. This was the point of my reference to the double slit experiment, which is the classic example of this idea (called “superposition”). In the double slit experiment, you shoot a particle at a barrier that has two openings in it, and watch where it goes. If you shoot a bunch of particles through at once, then they interact with each other and produce a particular pattern. If you shoot them through one at a time, and they randomly picked one of the two holes to go through, you would expect to see them cluster in two places. This is not what you actually see. What you actually see when you shoot them through one at a time is exactly the same pattern that you saw when you shot them through all at once. Therefor, individual particles actually go through both holes at once and interact with themselves, they are in two different states simultaneously until someone observes them, and forces them to be in one. This is how all quantum mechanical properties work, including spin.