Making excuses for an incomplete theory is not my idea of how to deal with reality. You can’t just assert that a theory adds up to normality, you have to show that it does.
What sorts of explanations should MWI provide in order to be complete, and in what sense are you worried that MWI does not add up to normality?
My point above was that it does add up to normality. When worlds are not splitting, we just have standard QM that all the interpretations agree upon. And when worlds are splitting, no valuation you make about a wave function actually depends on the exact moment that two worlds split. But you write:
saying that you don’t care about the details has no bearing on the logical need for such details to exist in a complete theory.
I think this analogy illustrates my dissatisfaction with this objection:
Imagine for a moment a classical universe with the following interesting physics: instead of point particles matter is composed of extended blobs. The blobs have no definite boundary; they just have exponential tails that trail off as you move away from the center of the blob. The nontrivial physics arises from the fact that the blobs can collide and merge with each other, or fission into separate blobs, or bounce off each other, etc., according to some underlying field equation (the blobs are bound configurations of field energy). But note that because the blobs have no definite boundaries, any merging/fissioning/scattering process proceeds over a finite length of time, instead of happening instantaneously.
Physicists in this universe develop the theory of blobs and eventually discover the underlying field equation governing all aspects of blob physics. But some insist that the interpretation of this equation is incomplete, for it doesn’t give a mathematically exact answer to the question “when does a fissioning blob become two blobs?”
In my opinion, these objectors are misguided. Yes, the interpretation does not answer this. For one thing, the question has no definite answer, because blobs have no definite boundaries and fission over a finite period of time in a continuous fashion. But more importantly, it’s not a question the theory should answer. Blobs are not fundamental objects: the fundamental object is the underlying field. The theory rightly speaks only of the underlying field, and does not answer the question of “when one blob becomes two” any more than it answers the question of “when one biological cell becomes two.” Blobs turned out to be merely a useful organizing principle for understanding the behavior of the underlying field.
The analogy to QM is fairly exact, I think. In QM we also have found the Schrodinger equation that describes in full detail the evolution of the wave function, the underlying field. We find that blobs of probability amplitude in the wave function tend to fission into separate blobs. We call the blobs “worlds.” It would be a mistake to want the theory to answer the question of “when one world becomes two” in exactly the same way it would be a mistake to expect the theory in the analogy to answer “when one blob becomes two.”
“Copenhagen wavefunction realism”—the theory that the wavefunction is the physical state and that it is caused to collapse by “observation”—is a later development, possibly due to von Neumann.
The problem is that there is a copy of you inside each “blob”. So if there is no objective moment of splitting, then during the period of fission, there is no objective answer to the question, is there one copy of you in existence, or are there two copies of you in existence? That is absurd because any instance of your consciousness is “inside” exactly one copy, and there is an instance of consciousness inside each copy that exists, and so saying that the number of copies is not an objective fact implies that whether or not a particular conscious being exists is not an objective fact, which implies that whether or not you exist is a question without an objective answer, which is absurd.
So if there is no objective moment of splitting, then during the period of fission, there is no objective answer to the question, is there one copy of you in existence, or are there two copies of you in existence?
What’s wrong with saying that there are (e.g.) 1½ half copies of me in existence? That’s an objective answer.
which implies that whether or not you exist is a question without an objective answer, which is absurd.
Suppose that somebody is hooked up to a life-support device so that they can continue to live even if the (e.g.) breath-regulating areas in their brain cease to function. Now start selectively deleting their brain cells one by one, until there are none left. At which point do they cease to exist?
As far as I know, there isn’t any such a point: their consciousness will just gradually fade away, and there isn’t an objective answer to when exactly that will happen.
What sorts of explanations should MWI provide in order to be complete, and in what sense are you worried that MWI does not add up to normality?
My point above was that it does add up to normality. When worlds are not splitting, we just have standard QM that all the interpretations agree upon. And when worlds are splitting, no valuation you make about a wave function actually depends on the exact moment that two worlds split. But you write:
I think this analogy illustrates my dissatisfaction with this objection:
Imagine for a moment a classical universe with the following interesting physics: instead of point particles matter is composed of extended blobs. The blobs have no definite boundary; they just have exponential tails that trail off as you move away from the center of the blob. The nontrivial physics arises from the fact that the blobs can collide and merge with each other, or fission into separate blobs, or bounce off each other, etc., according to some underlying field equation (the blobs are bound configurations of field energy). But note that because the blobs have no definite boundaries, any merging/fissioning/scattering process proceeds over a finite length of time, instead of happening instantaneously.
Physicists in this universe develop the theory of blobs and eventually discover the underlying field equation governing all aspects of blob physics. But some insist that the interpretation of this equation is incomplete, for it doesn’t give a mathematically exact answer to the question “when does a fissioning blob become two blobs?”
In my opinion, these objectors are misguided. Yes, the interpretation does not answer this. For one thing, the question has no definite answer, because blobs have no definite boundaries and fission over a finite period of time in a continuous fashion. But more importantly, it’s not a question the theory should answer. Blobs are not fundamental objects: the fundamental object is the underlying field. The theory rightly speaks only of the underlying field, and does not answer the question of “when one blob becomes two” any more than it answers the question of “when one biological cell becomes two.” Blobs turned out to be merely a useful organizing principle for understanding the behavior of the underlying field.
The analogy to QM is fairly exact, I think. In QM we also have found the Schrodinger equation that describes in full detail the evolution of the wave function, the underlying field. We find that blobs of probability amplitude in the wave function tend to fission into separate blobs. We call the blobs “worlds.” It would be a mistake to want the theory to answer the question of “when one world becomes two” in exactly the same way it would be a mistake to expect the theory in the analogy to answer “when one blob becomes two.”
OK—I accept this.
The problem is that there is a copy of you inside each “blob”. So if there is no objective moment of splitting, then during the period of fission, there is no objective answer to the question, is there one copy of you in existence, or are there two copies of you in existence? That is absurd because any instance of your consciousness is “inside” exactly one copy, and there is an instance of consciousness inside each copy that exists, and so saying that the number of copies is not an objective fact implies that whether or not a particular conscious being exists is not an objective fact, which implies that whether or not you exist is a question without an objective answer, which is absurd.
What’s wrong with saying that there are (e.g.) 1½ half copies of me in existence? That’s an objective answer.
Suppose that somebody is hooked up to a life-support device so that they can continue to live even if the (e.g.) breath-regulating areas in their brain cease to function. Now start selectively deleting their brain cells one by one, until there are none left. At which point do they cease to exist?
As far as I know, there isn’t any such a point: their consciousness will just gradually fade away, and there isn’t an objective answer to when exactly that will happen.