Even worse, many-worlds doesn't really solve the problem anyway - it still doesn't explain WHY you only observe one result, when the Schrodinger equation predicts several. That is, why can't you see the other worlds?
Don't you have the same problem in classical mechanics? Let's say you're standing at the edge of a pond, and you see waves rippling across the surface. The deviation in height of the surface of the water is described by h = cos(r + t) where r is the distance from the centre of the pond and t is the current time.
Why can you see the solution of the equation for the entire surface of the pond at once, but only for a single instant of time at any given moment?
It's not the same thing, because classical mechanics explicitly models the time - it can predict that at time T the system is in one state, and indeed when I look at a the system at time T, I see it in a single state.
Conversely, the Schrodinger equation gives an amplitude to the same particle/wave at many locations at time T. However, when you look for it at time T at all of those locations at once, you only find it in one of them. If you perform the experiment many times, you will find it at all of those locations some amount of the time. But then, if you try to use the Schrodinger equation to model movement before AND after interaction with the detector, you will not be able to find the particle at any position that doesn't match what the detector initially saw.
That is, say the Schrodinger equation predicts the particle has the same amplitude at locations X and Y. Then, after interacting with something at locations X and Y at time T1, it will have some amplitude at locations X1, X2, Y1, Y2 at time T2.
Now, if we try an experiment where the interaction at time T1 happens with a particle, and you have detectors at positions X1, X2, Y1, Y2, you will find it with equal probabilities at any of the 4 locations. However, if at X and Y there is a detector, and you detect the particle at X, it will never be found at positions Y1 or Y2. You have to update the Schrodinger equation after you find out that the particle is found at X, which is never how classical mechanics work.
Isn't the problem that you're only looking at the system in a single world W, when viewing all solutions requires viewing it in multiple worlds?
I mean, I get that time is a little different in that you will eventually experience and remember all possible solutions as you stand there watching the system, because classical time is a linear chain of events. In the multi-world case, it's a branching chain, and your experience and memories of the different solutions are stuck in their own branches.
That does make worlds weird and different from the other dimensions, but we accepted time as being weird and different from space for a very long time.
> Now, if we try an experiment where the interaction at time T1 happens with a particle, and you have detectors at positions X1, X2, Y1, Y2, you will find it with equal probabilities at any of the 4 locations. However, if at X and Y there is a detector, and you detect the particle at X, it will never be found at positions Y1 or Y2. You have to update the Schrodinger equation after you find out that the particle is found at X, which is never how classical mechanics work.
This makes total sense if it's actually a wave and the particle is merely a solution for a particular world W. The detector didn't change anything about the wave. It just coupled you to the wave system earlier, so now your branch of the many-world tree can only see the subset of solutions that correspond with whatever you detected. The only thing that has changed, though, is your ability to see the other solutions. You branched earlier, so now each branch you exist in only sees a subset of the full solution.
That said, I am not a physicist. The many worlds explanation was just the first thing that actually made sense to me about quantum mechanics. It's so conceptually simple.
> This makes total sense if it's actually a wave and the particle is merely a solution for a particular world W. The detector didn't change anything about the wave. It just coupled you to the wave system earlier, so now your branch of the many-world tree can only see the subset of solutions that correspond with whatever you detected. The only thing that has changed, though, is your ability to see the other solutions. You branched earlier, so now each branch you exist in only sees a subset of the full solution.
This explanation only works if either the detector is not itself made of particles, or if there is a detector wave that you could become entangled with by observing.
But the first one can essentially be discarded, and the second one is not experimentally confirmed. The equations happen the way I described whether you observe the detector or not. The detector could be hundreds of light years away from you, but you would still be able to predict what happened after the particle hit it with classical mechanics. So one particle's interaction with a detector instantly branches at least its entire future light-cone, but two particles interacting doesn't have the same effect. So at what scale does this happen? Or in what conditions?
That doesn't seem like a question that can be answered mathematically, does it? That's like asking, why do electrons have a spin of 1/2? Why is the speed of light 299,792,458 m/s? These are just properties of the universe.
Not really. It's the same question as the measurement problem: Schrodinger's equation predicts that a particle can exist in many places at the same time, with different amplitudes, and interact with particles in all those places. However, if we want to predict the particle's movement after it encounters a detector, we need to update the wave function to set its probability to 1 at the position of the detector and 0 everywhere else - otherwise, our predictions are measurably wrong.
Now, the question is: what causes this discontinuity in the equations of motion? Why is interaction with a detector different than interaction with another particle? Many Worlds simply reframes this problem, but doesn't get rid of it. In MWI, you would say 'the particle moves in all universes according to the wave function, until it interacts with a detector, possibly interfering with versions of itself in other universes. Then, when it encounters the detector, the world line of the detector splits - in some universes it passes the detector, in others it doesn't. However, it no longer interacts with other versions of itself,so we must update the wave function inside the universe where it passed the detector'.
> Now, the question is: what causes this discontinuity in the equations of motion? Why is interaction with a detector different than interaction with another particle?
> Many Worlds simply reframes this problem, but doesn't get rid of it.
Maybe I'm misunderstanding. It's like asking "why is there a difference between me jumping in a swimming pool and someone else jumping in it? I don't get wet when someone else is swimming." The difference is... one of you is in the pool. It's not going to spontaneously make the other person wet.
In MWI the difference is that if it interacts with a particle, you're not entangled, the particle is. If it interacts with a detector then you're entangled. So, there is no difference except for what gets entangled.
What that means is the wave function can only appear to collapse when you entangle. If some particle entangles, it will collapse for that particle and branch into a new world, but you're not in that world; for you it's still a waveform.
> In MWI the difference is that if it interacts with a particle, you're not entangled, the particle is. If it interacts with a detector then you're entangled. So, there is no difference except for what gets entangled.
I don't think that is the whole story. If you want to predict the motion of a particle correctly, you still need to update the Schrodinger equation after interaction with the detector, but not after interaction with another particle. And this is independent of whether you personally look at the detector or not, even if the detection occurs outside your light-cone. This is evident from the fact that MWI still needs both the Schrodinger equation and the Born rule to accurately predict experimental results.
> What that means is the wave function can only appear to collapse when you entangle. If some particle entangles, it will collapse for that particle and branch into a new world, but you're not in that world; for you it's still a waveform.
But this is not true for macroscopic objects. The motion of a detector, and indeed even the motion of a particle after it interacts with a detector, does not behave like a wave, regardless of whether I have ever interacted it. Even if the interactions are space-like separated from myself, I can still predict them with classical mechanics, and confirm when the data finally reaches me. For example, I can predict the location of a particle in a double slit experiment if I know that there is a detector at one of the slits, regardless of where in the universe that experiment happens. How can I be entangled to a detector that exists outside my past light-cone? But then, I can't predict the outcome of a double slit experiment without a detector near the slits, regardless of how close I am to the experiment.
This still shows to me that there is an observer-independent collapse happening when a particle interacts with a detector, where we don't have a physical description of what a detector actually is.
I'm not sure cross-universe communicating is necessary in the multiverse model. The splitting just resembles communication from our perspective in that it makes the probabilities look "rigged".
That still doesn't explain interference patterns in double-slit experiments, especially in double-slit experiments with a single photon/electron at a time. Those can only be explained by the particle/wave traveling through both slits and then the two versions interacting with each other.
