What I never understood is: is there any reason to believe this is even possible in theory, let alone in practice? Presumably everything started at a single point in the Big Bang, so wouldn't that necessarily imply all particles are causally connected (hence superdeterminism)? How can anyone make "random" choices when everything is linked? Under what accepted theory do scientists believe this experiment to be possible?
That was not the case. The universe is probably infinite, thus it always has been infinite. Distances decrease as t goes to zero, but at t=0 we have a singularity. Known physics break down before that.
Besides, keeping particles entangled is actually quite difficult. Entanglement is lost as soon as they interact with their environment, in particular when being observed. This is called decoherence.
> is there any reason to believe this is even possible in theory, let alone in practice?
Yes. It has been done. https://en.wikipedia.org/wiki/Bell%27s_theorem#Experiments
Also, how can you "lose" entanglement to an environment that's itself already entangled? The scientists are the environment here; if they're already entangled, surely they can't make their particles unentangled?
There is a singularity in the equations. No one knows what it actually looked like or if it even makes sense to talk about the state at t=0.
> how can you "lose" entanglement to an environment that's itself already entangled?
I don't know what you mean the environment being entangled or why you think that this is the case.
I guess I don't really care about t = 0 itself necessarily. What I'm saying is that at some point near t ~= 0 (whether that's at t = 0 or some time shortly after), everything was packed so closely that all particles would've had the time to interact with each other (and thus to become entangled). Why wouldn't this have been the case?
> I don't know what you mean the environment being entangled or why you think that this is the case.
See the above paragraph.
Think of a pond with a smooth undisturbed surface. Now, define two spots as center, and for each spot in the exact same distance a point of measurement.
Here it comes: now drop two identical balls of steel into the pond at the very same time and do your measurement. You will detect that the waves on the surface of the pond are perfectly the same.
The entanglement is - simply spoken - just a syncronizing of the particles, hence the reason why they "seem" to behave the same.