It is going to enter a Stern-Gerlach device in a beam going in the positive y direction and the Stern-Gerlach device will deflect a spin up beam entirely left. And the Stern-Gerlach device will deflect a spin down beam entirely right. These directions, y , left, and right were all details that were determined by the initial setup of the beam and the device.

But the device does this not because the Stern-Gerlach device is magical, it does so because the Stern-Gerlach device has an inhomogeneous magnetic field and there is a particular Hamiltonian for magnetic fields and spin and the Hamiltonian dictates how things evolve. So anyone that agrees about how the Schrödinger equation evolves states agrees this happens. Copenhagen, Many-Worlds Interpretation (which was niwt called Many Worlds by its creator and I think its creator did not mention the words many worlds), decoherence, ddB, etc.

So the Schrödinger equation has a wave that is a beam spread in the x and z direction and with an initial probability current in the y direction for an initial condition of the wave function. And it evolves according to the Schrödinger equation. Not what if the spin of the particle is a superposition of spin up and spin down. Again we can just mathematically change the initial wavefunction and then mathematically evolve it according to the Schrödinger for then particular Hamiltonian that accurately describes the actual inhomogeneous magnetic field in the exact actual Stern-Gerlach device and then again every theory that agrees with the Schrödinger equation can evolve it and they all predict the same thing. They see two beams coming out, one with spin polarized as spin up, one with spin polarized as spin down.

And again, any theory that uses the Schrödinger equation can track the so-called probability current and see streamlines I. The initial beam, some of which end up in that left deflected beam, some which end up in that right deflected beam, they can see the beam deflect like a stream that forks when then is a hill in the way. They see that there is more beam deflected left or right (in the L2 norm sense) depending on how big a superposition you had of spin up versus spin down.

Effectively the Stern-Gerlach device has co-evolved the spin and the position so the two are now entangled you have (wave on the left and spin up) added to (wave on the right and spin down) and they are orthogonal, orthogonal for two reasons now, whereas when it was incoming beam spin is (sum of up and down) they were orthogonal for only one reason. Now that the interaction has separated the beam into two spatially separate beams, future things that interact based on where the beam is can by surrogacy be correlated with the spin.

Great. And everyone that uses the Schrödinger equation and is also willing to bother to use it on the actual experimental setup (a rarely utilized option) agrees this what happens.

Now it might help to reveal a big fact that isn't mentioned much, and again it is just about the Schrödinger equation. If you have two particles or more. The wave function is not a wave like the electric field or the magnetic field, it is not defined in actual space. It is defined on configuration space, a space that has a different x for every particle a different y for every particle and a different z for every particle. So for two particles your function is $\Psi=\Psi(x_1,y_1,z_1,x_2,y_2,z_2,t)$ so for every point in the 6d configuration space you are specifying a full configuration, you are saying where every particle is.

So this space is huge. If two beams go off in slightly different directions and bounce off separate things and go into different directions and possibly different and changing speeds when you have $10^26$ or more particles the chance that two beams ever cross each other once they get pretty far away is vanishingly small. This is key.

So the Schrödinger equation predicts that in absolutely every way the two beams evolve as if they were the only beam when they gt to a point where they never overlap again. And this happen when devices like Stern-Gerlach devices separate them and then further interactions with lots of particles have the lots of other particles evolve differently with the two beams. So far there is no mention of Many Worlds. And any theory that uses the Schrödinger equation for the actual setup will predict this.

So now let's talk about Many Worlds. These beams that never again overlap act like they are the only beam, act like they are a world unto their own. The math says so. The math everyone uses that uses the Schrödinger equation. So why not just let them them be a World unto their own? Your personal experience is generated by the state of your neurons and such, so the wave function of every particle includes the configurations of your neurons and such. So there is a beam with your neurons in some configurations (the wave is nonzero there) and there is a completely separate non overlapping beam (the beam is defined in configuration space so if just one particle has the beam separated to be non overlapping in the direction for that particle it is like separating a beam in the x direction the whole beam is separated). A separate non overlapping beam for your neurons is a possibly different collection of configurations (or possibly the same, maybe the beam has separated in other places but the dynamics of your neurons haven't changed yet, but the beams as a whole are separate so the math that every single person that uses the Schrödinger equation trusts says the beams won't affect the so called probability-current or the evolution of the other if they never overlap again).

And a die hard solipsists will sometimes go so far as to throw the the baby of science out the window with the bathwater of non-solipsism.

Why? Because by carefully arranging beam reflectors we can get those beams to bounce back and overlap and show that every beam was still there all along, every one, all along. Always, every time. So you can just find out how many beam reflectors we can make and how precisely we can arrange them (find out our current technical expertise) and then postulate that raw unobservable (because you specifically designed it to happen where we can't test it) magic happens and that the Schrödinger equation doesn't hold there because it interferes with your self centered egotistical solipsism. Which is really just a kind of sexism and racism and hating of other people and things that is so profound that you just refuse to believe that anything other than you exists (even a body just like yours that was entirely the same up until one day it dynamically evolved the way you would if you interacted with a world where a Stern-Gerlach device deflected a beam differently than the one you personally subjectively experienced) to the point were you make unscience to defend your solipsism.

Why unscience? Because drawing the lines in different places means when our technical expertise advances we get an infinite number of different predictions. So Copenhagen can't even make predictions. It, Copenhagen, isn't even science when it wants to have magical collapses to defend solipsism. And there is no reason the hold one of the splittings of the beam as special just because you experienced it.

It is going to enter a Stern-Gerlach device as a beam going in the positive y direction and the Stern-Gerlach device will deflect a spin up beam entirely left. And the Stern-Gerlach device will deflect a spin down beam entirely right. These directions, y , left, and right were all details that were determined by the initial setup of the beam and the device.

But the device does this not because the Stern-Gerlach device is magical, it does so because the Stern-Gerlach device has an inhomogeneous magnetic field and there is a particular Hamiltonian for magnetic fields and spin and the Hamiltonian dictates how things evolve. So anyone that agrees about how the Schrödinger equation evolves states agrees this happens. Copenhagen, Many-Worlds Interpretation (which was not called Many Worlds by its creator and I think its creator did not mention the words many worlds), Decoherence, Ithaca, dBB, etcetera.

So the Schrödinger equation has a wave that is a beam spread in the x and z direction and with an initial probability current pointing in the y direction for an initial condition of the wave function. And it evolves according to the Schrödinger equation. Now, what if the spin of the particle is a superposition of spin up and spin down? Again, we can just mathematically change the initial wavefunction and then mathematically evolve it according to the Schrödinger equation for the particular Hamiltonian that accurately describes the actual inhomogeneous magnetic field in the exact actual Stern-Gerlach device and then again every theory that agrees with the Schrödinger equation can evolve it and they all predict the same thing. They see two beams coming out, one with spin polarized as spin up, one with spin polarized as spin down.

And again, any theory that uses the Schrödinger equation can track the so-called probability current and see streamlines in the initial beam, some of which end up in that left deflected beam, some which end up in that right deflected beam. And they can see the beam, as a whole, mathematically split into two like a stream that forks when there is a hill in the way. They see that there is more beam deflected left or right (in the L2 norm sense) depending on how big a superposition you had of spin up versus spin down in the initial wave.

Effectively the Stern-Gerlach device has co-evolved the spin and the position so the two are now entangled you have (wave on the left and spin up) added to (wave on the right and spin down) and they are orthogonal, orthogonal for two reasons now, whereas when it was incoming beam with spin of (superposition of up and down) they were orthogonal for only one reason, the spin, which could have been written as a super position of many different orthogonal states. Now that the interaction has separated the beam into two spatially separate beams, future things that interact based on where the beam is can by surrogacy be correlated with the spin.

Great. And everyone that uses the Schrödinger equation and is also willing to bother to use it on the actual experimental setup (a rarely utilized option) agrees this is what happens.

Now it might help to reveal a big fact that isn't mentioned much, and again it is just about the Schrödinger equation. If you have two particles or more, then the wave function is not a wave like the electric field or the magnetic field, it is not defined in actual space. It is defined on configuration space, a space that has a different x for every particle a different y for every particle and a different z for every particle. So for two particles your function is $\Psi=\Psi(x_1,y_1,z_1,x_2,y_2,z_2,t)$ so for every point in the 6d configuration space you are specifying a full configuration, you are saying where each and every particle is.

So this space is huge. If two beams go off in slightly different directions and bounce off separate things and go into different directions and possibly different and changing speeds when you have $10^{26}$ or more particles the chance that two beams ever cross each other once they get pretty far away is vanishingly small. This is key.

So the Schrödinger equation predicts that in absolutely every way the two beams evolve as if they were the only beam when they get to a point in time where they will never overlap again. And this happens when devices like Stern-Gerlach devices separate them and then further interactions with lots of particles have the lots of other particles evolve differently with the two different beams. So far there is no mention of Many Worlds. And any theory that uses the Schrödinger equation for the actual setup will predict this.

So now let's talk about Many Worlds. These beams that never again overlap act like they are the only beam, they act like they are a world unto their own. The math says so. The math everyone uses that uses the Schrödinger equation. So why not just let them them be a World unto their own? Your personal experience is generated by the state of your neurons and such, so the wave function of every particle includes the configurations of your neurons and such. So there is a beam with your neurons in some configurations (the wave is nonzero there) and there is a completely separate non overlapping beam (the beam is defined in configuration space so if just one particle has the beam separated to be non overlapping in the direction for that particle it is like separating a beam in the x direction the whole beam is separated). A separate non overlapping beam for your neurons is a possibly different collection of configurations (or possibly the same, maybe the beam has separated in other places but the dynamics of your neurons haven't changed yet, but the beams as a whole are separate so the math that every single person that uses the Schrödinger equation trusts says the beams won't affect the so called probability-current or the evolution of the other if they never overlap again).

And a die hard solipsists will sometimes go so far as to throw the baby of science out the window with the bathwater of non-solipsism.

Why? Because by carefully arranging beam reflectors we can get those beams to bounce back and overlap and show that every beam was still there all along, every single one, for any length of time, so was there all along. Always, every single time. So you can just find out how many beam reflectors we can make and how precisely we can arrange them (find out our current technical expertise) and then postulate that raw unobservable (because you specifically designed it to happen where we can't test it) magic happens and that the Schrödinger equation doesn't hold there because it interferes with your self centered egotistical solipsism. Which is really just a kind of sexism and racism and hating of other people and things that is so profound that you just refuse to believe that anything other than you exists (even a body just like yours that was entirely the same up until one day it dynamically evolved the way you would if you interacted with a world where a Stern-Gerlach device deflected a beam differently than the one you personally subjectively experienced) to the point were you make unscience to defend your solipsism.

Why unscience? Because drawing the lines in different places means when our technical expertise advances we get an infinite number of different predictions. So Copenhagen can't even make predictions. It, Copenhagen, isn't even science when it wants to have magical collapses to defend solipsism. And there is no reason to hold one of the splittings of the beam as special just because you experienced it.