Dense said:
But if they don't yet have defined spins, how could their molecules have been able to join up in the first place? Don't they need to be defined as opposites first?
Your not forcing things to be entangled you must find testable entangements. In the same manner you don’t need to force match spin H atoms to make an H molecule. The do that work by themselves. You just want to take advantage of the molecule so that when you take them apart the original “spin” must be conserved.
An analogy can help BUT remember this is an analogy, NOT A DESCRIPTION of how an election “spins” in an atom.
View the two H atom electrons like gears that turn - clockwise or counter-clockwise. By picking some reference side and view you can call that up or down. When they come together they need no help in becoming aligned it is there combining that aligns them such that they fit together on their own what ever it takes.
Now that they are together what is the total spin?
Based on how gears fit together one turning clockwise means the other is turning the other way i.e. the net spin is Zero!
Now although we might not now the details of how they came together we do know where we carefully take them apart if one is up the other will have to be down to add up to the original zero we started with.
NOW COMES the problem of detailing when and where you can “define” what the spin is for each of the two electrons as they depart – that is when you have something testable you can build data on to attempt to understand (or deny as some still try) entanglement.
Working with particles it can be very hard to visualize the spin measurements.
Most find looking DrC’s example of taking one photon though a crystal easier to follow than particle spins. The PDC crystal reduces each photon frequency in half, since energy etc. must be conserved the only way a crystal can do that is to spit out two photons for every one that comes in. Here the “spin” entanglement shows up in the alignment of polarization.
