4 ms·

This is also known as the Wigner's Friend paradox. Your explanation is not the standard understanding of Many Worlds today, however. The party line is that wor

This is also known as the Wigner's Friend paradox. Your explanation is not the standard understanding of Many Worlds today, however. The party line is that world splitting happens when decoherence has become "sufficiently irreversible" (for some arbitrary definition of "sufficiently", since there's no evidence of genuine irreversibility anywhere in physics), which in almost all real-world setups happens almost immediately.

bonoboTP 7y ago
Is there actually a "split"? I've always imagined it more like parallel strands that were always distinct but had overlaps. Like in the cat case, it's undetermined whether I'm on the cat dead or cat alive time-line because both are "overlapping" in the part of space-time where my brain is at the time of not knowing the cat's state. Then when I make a measurement, the two strands no longer overlap. With Wigners friend there are also these two branches all along, but they overlap where the friend hasn't yet looked in the box, they then go apart in the part where the friend makes an observation, overlap where Wigner hasn't looked at his friend yet, but then they are distinct again where Wigner has looked at the friend's state. Similarly to how particles and light take all possible trajectories at the same time (path integral formulation), we could think that in the space-time places where the branches overlap, all compatible pasts are equally valid, at other space-time locations the particular strand extends back through the past to just one of the cat states, so the state seems collapsed, but the stand also led through the overlap area, so you remember the time where you didn't yet have the "which branch you're on" information.
- monktastic1 7y ago
  I'm not sure I've digested your whole comment, but the basic idea is this: A "split" only means that the respective branches of the wave function cannot interfere with each other any longer. Then they will evolve completely separately from each other, and can be considered distinct. The only way you can get branches to interfere is if you have precise control over all particles involved in the entanglement, which is practically impossible for any macroscopic system. Long before you entangle with the cat system, the two branches have decohered. While technically there is still an enormous superposition there, for all practical purposes it is indistinguishable from being a classical system, with classically indeterminate branches. I should say that I'm not a fan of this explanation, possibly related to your points. Regardless of "practical purposes," QM does not allow me to say that there is a well-defined answer before it entangles with me (and that there is one afterward). I'm much more comfortable using the word "split" only there. But this skirts too close to solipsism for some, since "my world" (which I also know as "the world") only comes into being upon contact with me.

bonoboTP 7y ago

Is there actually a "split"? I've always imagined it more like parallel strands that were always distinct but had overlaps. Like in the cat case, it's undetermined whether I'm on the cat dead or cat alive time-line because both are "overlapping" in the part of space-time where my brain is at the time of not knowing the cat's state. Then when I make a measurement, the two strands no longer overlap. With Wigners friend there are also these two branches all along, but they overlap where the friend hasn't yet looked in the box, they then go apart in the part where the friend makes an observation, overlap where Wigner hasn't looked at his friend yet, but then they are distinct again where Wigner has looked at the friend's state. Similarly to how particles and light take all possible trajectories at the same time (path integral formulation), we could think that in the space-time places where the branches overlap, all compatible pasts are equally valid, at other space-time locations the particular strand extends back through the past to just one of the cat states, so the state seems collapsed, but the stand also led through the overlap area, so you remember the time where you didn't yet have the "which branch you're on" information.

monktastic1 7y ago
I'm not sure I've digested your whole comment, but the basic idea is this: A "split" only means that the respective branches of the wave function cannot interfere with each other any longer. Then they will evolve completely separately from each other, and can be considered distinct. The only way you can get branches to interfere is if you have precise control over all particles involved in the entanglement, which is practically impossible for any macroscopic system. Long before you entangle with the cat system, the two branches have decohered. While technically there is still an enormous superposition there, for all practical purposes it is indistinguishable from being a classical system, with classically indeterminate branches. I should say that I'm not a fan of this explanation, possibly related to your points. Regardless of "practical purposes," QM does not allow me to say that there is a well-defined answer before it entangles with me (and that there is one afterward). I'm much more comfortable using the word "split" only there. But this skirts too close to solipsism for some, since "my world" (which I also know as "the world") only comes into being upon contact with me.