> 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 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.