Many Worlds Interpretation (MWI) is favored by EY as having a shorter message than others.
However, the short-message version of MWI does not include a theory as to how my particular stream of consciousness winds up in one branch or another. So Copenhagen (wave function collapse) is a theory of what I will experience, MWI is not.
Further, I have always thought MWI motivated by the ideas behind Einstein’s “God does not play dice with the universe.” That is, a non-deterministic theory is no theory at all. And then, MWI, would be a theory without wave function collapse, so a theory with no randomness. But of course, it is NOT a theory of what a particular observer will experience. To go from MWI to a theory of what I will experience, it seems I still need to have a random function. I suspect some will answer, “no, there is one of you in every branch so MWI predicts you will experience it all, but in separate non-interacting branches. No randomness.” To which I would reply, we still need a theory that accounts for my subjective experiences, how did this me, the one I actually wound up as, “choose” between the various branches. To me it would seem essentially theological to say that because some me I can’t see, hear or interact with in any way experience all the other possibilities that there is no randomness in the universe. It sure seems random that I wound up experiencing this particular version, in the absence of a non-random theory of that.
Please take this as an invitation to educate me or discuss the conclusions I reach. I am interested in sorting out just what MWI really gains you when leaving Copenhagen, and as competing theories of my own personal experience, they both seem to have, essentially, a random choosing event at their core: one calls it wave function collapse, the other one tries not to talk about it.
how my particular stream of consciousness winds up in one branch or another
This assumes there is such a thing as a particular stream of consciousness, rather than your brain retconning a stream of consciousness to you when you bother to ask it (as is what appears to happen).
This assumes there is such a thing as a particular stream of consciousness, rather than your brain retconning a stream of consciousness to you when you bother to ask it (as is what appears to happen).
Yes it does assume that. However, we have plenty of evidence for this hypothesis.
My memory, and the memory of humans and higher mammals alike, has tremendous predictive power. Things like I remember a particular National Lampoon magazine cartoon with a topless boxer chanting “I am the queen of england, I like to sing and dance, and if you don’t believe me, I will punch you in the pants,” from about 40 years ago. I recently saw a DVD purporting to have all National Lampoons recorded digitally on it, I bought this and sure enough, the cartoon was there.
It seems clear to me that if conscious memory is predictive of future physical experience, it is drawn from something local to the Everett Branch my consciousness is in.
Let me design an experiment to test this. Set up a Schrodinger’s cat experiment, include a time display which will show the time at which the cat was killed if in fact the cat is killed. If I once open the lid of the box and find the cat, and look at the time it was killed, record the time on a piece of paper which I put in a box on the table next to me and then close the box. I reopen it many subsequent times and each time I record the time on a piece of paper and put it on the box, or I record “N/A” on the paper if the cat is still alive.
My prediction is that every time I open the box with the memory of seeing the dead cat, I will still see the dead cat. Further, I predict that the time on the decay timer will be the same every time I reopen the box. This in my opinion proves that memory sticks with the branch my consciousness is in. Even if we only saw the same time 99 times out of 100, it would still prove that memory sticks, but not perfectly, with the branch my consciousness is in, which would then be a fact that physics explaining what I experience of the world would have to explain.
Having not explicitly done this experiment, I cannot claim for sure that we will conclude my consciousness is “collapsing” on an Everett Branch just as in Copenhagen interpretation it was the wave function that collapsed. But I will bet $100 against $10,000 if anybody wants to do the experiment. The terms of the bet are if you have a set-up that shows the counter result, that consciousness apparently dredges up memories of different nearby Everett branches by seeing different times on the timer, then I will come to where you are with your set-up and if you can show me it working for both you and I you get the $10,000, otherwise I get the $100 to defray my travel expenses. I’ll reserve the right to pass on checking your set-up out if travel costs would be over $600, but for me that covers a good fraction of the world (I am in Sandy Eggo in this Everett Branch).
Fortunately for you cat lovers, the experiment can be done without the cat. You simply need to measure the time of radioactive decay, killing a cat with cyanide on detection of the radioactive decay is not necessary to win or lose the bet (or to prove the point.)
Note the box of papers with recorded times in it can also be used as evidence. If I open that box and all the papers have the same time written on them, and that is the time I remember, then I take this as strong evidence that my memory has been returning memories from only the current everett branch. If my memory were unhooked from this everett branch, then one would expect the physical evidence of what I had previously remembered which is in this everett branch, to include times from other everett branches. If it does not, then I think we can conclude that human consciousness, including its memories, are branch local, that a “collapse” occurs in MWI when we attempt to use it to predict what we will experience in this universe.
And indeed, I think predicting what we will experience is the hallmark of all good theories of how the universe works. We may say we want to predict “what will happen,” but I believe by this we mean “what I will see happen.”
It seems clear to me that if conscious memory is predictive of future physical experience, it is drawn from something local to the Everett Branch my consciousness is in.
Whatever makes you think that your consciousness is in only one Everett branch? (And what do you think is happening on all those other branches that look so much like this one but that lack your consciousness?)
Surely the right account of this, conditional on MWI, is not that your consciousness is on a particular branch but that each branch has its own version of your consciousness, and each branch has its own version of your memory, and each branch has its own version of what actually happened, and—not at all by coincidence—these match up with one another.
What happens to your consciousness and your memories is much more like splitting than like collapse.
(It sounds as if you think that this ought to mean that you’d have conscious memories in one branch from other branches, but I can’t see why. Am I missing something?)
(It sounds as if you think that this ought to mean that you’d have conscious memories in one branch from other branches, but I can’t see why. Am I missing something?)
I misunderstood what David Gerard was suggesting and took a long riff proposing an experiment to address something he wasn’t saying.
The tricky part for me is the extremely clear conscious experience I have of being on only one branch. That there are other consciousnesses NEARLY identical to mine on other nearby Everett branches, presumably having the same strong awareness that they are on only one Everett branch and have no direct evidence of any other branch is clearer to me. MWI seems to truly be an interpretation, not a theory, with apparently absolutely no Popperian experiments that could ever distinguish it from wave function collapse theories.
You can upload a person into a quantum computer and do Schrödinger’s cat experiments on them. If you have a computational theory of mind, this should falsify at least some informal collapse theories.
You could have that predictive power without actually having a continuous stream of awareness. Consider sleepwalkers who can do things and have conversations (if not very good ones) with no conscious awareness. You’re using philosophy to object to observed reality.
OK, i misunderstood what you were implying in your previous post. So there are multiple streams of consciousness, one on each everett branch, and the memories returned on each everett branch are the ones in the (conscious+unconscious) brain that exists on that everett branch.
So I experience my mind always returning memories consistent with my branch even as other branch-mwengler’s experience memories consistent with their branch, and like me, use that as evidence for their uniqueness.
So it really is an interpretation, predicting nothing different in experience than does copenhagen.
I haven’t seen one example of a precise definition of what constitutes an “observation” that’s supposed to collapse the wavefunction in Copenhagen interpretation. Decoherence, OTOH, seems to perfecty describe the observed effects, including the consistency of macro-scale history.
This in my opinion proves that memory sticks with the branch my consciousness is in.
Actually it just proves that memory sticks with the branch it’s consistent with. For all we know, our consciousnesses are flitting from branch to branch all the time and we just don’t remember because the memories stay put.
We may say we want to predict “what will happen,” but I believe by this we mean “what I will see happen.”
Yeah, settling these kinds of questions would be much easier if we weren’t limited to the data that manages to reach our senses.
In MWI the definition of “I” is not quite straightforward: the constant branching of the wavefunction creates multiple versions of everyone inside, creating indexical uncertainty which we experience as randomness.
Your mistake lies in using the word “I” like it means something. There is some mwengler-stuff, it has some properties, then there is a split and the mwengler-stuff is in two separate chunks. They both experience their “stream of consciousness” showing up in their particular branch, they both wonder how it is that they ended up in the one branch rather than the other.
Copenhagen is an interpretation where I have one mind, you have one mind, and each of us have one thread of experience. There are numerous places along that thread where the physics to calculate the time evolution of that thread is not deterministic, where a random choice has been made.
MWI is an interpretation where I have many minds, as opposed to the one mind I have in Copenhagen. In the MWI interpretation, each of my minds exists in a separate and non-interacting universe from all the other versions of my mind. If I wonder as I type this why this version of me is the one in THIS branch, MWI has no theory for that. MWI tries to make that question seem less interesting by pointing out that there are lots of versions of me asking that same question, so somehow obscuring the me-ness of the me in this branch with the me-ness of all these other similar but not identical me’s in these other branches would render the question meaningless.
But as an interpretation with no observable experimental differences, MWI and Copenhagen are likely to have the same number of random events dictating progress. In MWI, the randomness is isolated to just one of many me’s which of course is still quite unique and interesting to me, but which is not as bad as Copenhagen where it is the entire universe that got changed by each random waveform collapse.
In the MWI interpretation, each of my minds exists in a separate and non-interacting universe from all the other versions of my mind. If I wonder as I type this why this version of me is the one in THIS branch, MWI has no theory for that.
How is this different to wondering why you are THIS mind in THIS branch rather than THIS OTHER mind in THIS branch? Why you are you rather than someone else?
I mean that there are other minds in the world, in the sense of other people. Neither Copenhagen nor many worlds chooses a preferred mind, but people don’t notice it as strongly in Copenhagen since they’re already used to the idea of other conscious beings.
Copenhagen is not a theory of what you will experience either; there are multiple minds even in Copenhagen’s single world
If I understand correctly, Copenhagen has only one mind for me, and the reality experienced by this mind is fundamentally randomly branched through wave function collapses. MWI creates a new mind for me so their are many minds for me, one in each Everett Branch. Did I miss something?
Let’s suppose that your mind is a function of your brain, and that your brain is composed of atoms.
In MWI there are many branches with many configurations of atoms, that means many branches of your brain, that means many branches of your mind. In every branch your mind is entangled with the other atoms of the same branch. So for example in the universe with atoms of a dead cat, your mind is in the “poor kitty” state, and in the branch with the atoms of an alive cat, your mind is in the “kitty, you are so lucky, I promise I will never try this cruel experiment on you again” state.
In Copenhagen, on a tiny time scale there are many branches of atoms. But it is believed that on a larger scale it is not so. At some unspecified moment there is supposed to be a collapse where many branches of atoms become a single branch again (through a process of random selection). Nobody knows when does this happen. On a large scales, we are not able to run a precise enough experiment that would say either way. On smaller scales, where we can run the experiment, the result has always been that the collapse did not occur yet. So after the collapse, there is only one branch, and therefore one mind. Before the collapse… I would say that there is a superposition of minds (because there is a superposition of brains, because there is a superposition of atoms the brain is composed of), which should become one mind again at the moment of the collapse. But it is believed that this superposition exists only for a very small fraction of the second, so it’s not like the different minds in the superposition have enough time to really think significantly different thoughts. The neurons work at a limited speed, and sending a signal from one neuron to another requires dozens of chemical reactions.
You bounce a photon off a half-silvered mirror and don’t look at the results: no universe split.
You bounce a photon off a half-silvered mirror and look at the results: Bam! Split universe.
MWI:
You bounce a photon off a half-silvered mirror and don’t look at the results. Since the physical state of your brain is not causally dependent on the destination of the photon, you don’t branch into two mwenglers in any noticeable way.
You bounce a photon off a half-silvered mirror and look at the results. Since you’ve made the state of your brain causally dependent on an event with quantum randomness, you branch into two mwenglers which are different on a macroscopic level. Two persons, which happen to share a causal history up to looking at the experimental outcome.
… You bounce a photon off a half-silvered mirror and look at the results: Bam! Split universe.
Copenhagen Interpretation never splits universes. Instead, you have a wave function collapse in the one and only universe.
You bounce a photon off a half-silvered mirror and don’t look at the results. Since the physical state of your brain is not causally dependent on the destination of the photon, you don’t branch into two mwenglers in any noticeable way.
In MWI, you NEVER branch in to two anythings in a “noticeable” way. All the myriads of branches have no interactions, there is nothing noticeable about any of the other branches from within the branch we are in. If there is something noticeable about other branches, then an experiment could be defined to check the hypothesis of branching, and we would start to gather evidence for or against branching. Until such time as an hypothesis is created and tested and shows evidence for branches, MWI is an interpretation, and not a theory.
So why does it even matter? I am thinking it through and realizing that an interpretation is in some way a pre-theory. As we sit with the idea of MWI, maybe one of us develops hypotheses about experiments whic might show evidence for the other branches, or not. Without the interpretation of MWI, that hypothetical progress might never be available.
They do interact. This is how quantum physics was discovered.
The problem is that the magnitude of interaction is getting very small very quickly, so after a few microseconds it becomes technically impossible to measure. This is what allows people to say: “Yeah, for a few microseconds there is something mathematically equivalent to branches, but then it disappears completely” and you can’t experimentally prove them wrong.
One side believes that the interaction is getting smaller, but it never reaches exactly zero. Other side believes that the interaction is getting smaller, and then in some unspecified moment all branches except one disappear. Experimental data say that the interaction is getting smaller until it becomes too small to see… and then, well, it is too small to see what happens. So essentially both sides disagree about who has the burden of proof; about the exact meaning of “fewest assumptions” in Occam’s razor. One side says that “the extra branches disappearing” is the extra assumption. Other side says that “the extra branches not disappearing, even when their interaction becomes too small to measure” is the extra assumption.
More precisely, the magnitude of interaction depends on how much the particles in the branches are different. Therefore the branches we have measurable interaction with are those almost the same as our branch. The interaction is largest when both branches are exactly alike except for one particle. This is the famous double-slit experiment—two branches of the universe with the particle going through different slits interact with each other. The branches are there. The question is not whether multiple branches exist, but whether they disappear later when their interaction becomes very small.
maybe one of us develops hypotheses about experiments which might show evidence for the other branches, or not.
How do you prove experimentally that the other branches do not disappear, especially if your opponents refuse to specify when they should disappear. If you make an experiment that proves that “after N seconds, the branches still exist”, your opponents can say: “Yeah, but maybe after N+1 seconds they disappear.” Repeat for any value of N.
Many Worlds Interpretation (MWI) is favored by EY as having a shorter message than others.
However, the short-message version of MWI does not include a theory as to how my particular stream of consciousness winds up in one branch or another. So Copenhagen (wave function collapse) is a theory of what I will experience, MWI is not.
Further, I have always thought MWI motivated by the ideas behind Einstein’s “God does not play dice with the universe.” That is, a non-deterministic theory is no theory at all. And then, MWI, would be a theory without wave function collapse, so a theory with no randomness. But of course, it is NOT a theory of what a particular observer will experience. To go from MWI to a theory of what I will experience, it seems I still need to have a random function. I suspect some will answer, “no, there is one of you in every branch so MWI predicts you will experience it all, but in separate non-interacting branches. No randomness.” To which I would reply, we still need a theory that accounts for my subjective experiences, how did this me, the one I actually wound up as, “choose” between the various branches. To me it would seem essentially theological to say that because some me I can’t see, hear or interact with in any way experience all the other possibilities that there is no randomness in the universe. It sure seems random that I wound up experiencing this particular version, in the absence of a non-random theory of that.
Please take this as an invitation to educate me or discuss the conclusions I reach. I am interested in sorting out just what MWI really gains you when leaving Copenhagen, and as competing theories of my own personal experience, they both seem to have, essentially, a random choosing event at their core: one calls it wave function collapse, the other one tries not to talk about it.
This assumes there is such a thing as a particular stream of consciousness, rather than your brain retconning a stream of consciousness to you when you bother to ask it (as is what appears to happen).
Yes it does assume that. However, we have plenty of evidence for this hypothesis.
My memory, and the memory of humans and higher mammals alike, has tremendous predictive power. Things like I remember a particular National Lampoon magazine cartoon with a topless boxer chanting “I am the queen of england, I like to sing and dance, and if you don’t believe me, I will punch you in the pants,” from about 40 years ago. I recently saw a DVD purporting to have all National Lampoons recorded digitally on it, I bought this and sure enough, the cartoon was there.
It seems clear to me that if conscious memory is predictive of future physical experience, it is drawn from something local to the Everett Branch my consciousness is in.
Let me design an experiment to test this. Set up a Schrodinger’s cat experiment, include a time display which will show the time at which the cat was killed if in fact the cat is killed. If I once open the lid of the box and find the cat, and look at the time it was killed, record the time on a piece of paper which I put in a box on the table next to me and then close the box. I reopen it many subsequent times and each time I record the time on a piece of paper and put it on the box, or I record “N/A” on the paper if the cat is still alive.
My prediction is that every time I open the box with the memory of seeing the dead cat, I will still see the dead cat. Further, I predict that the time on the decay timer will be the same every time I reopen the box. This in my opinion proves that memory sticks with the branch my consciousness is in. Even if we only saw the same time 99 times out of 100, it would still prove that memory sticks, but not perfectly, with the branch my consciousness is in, which would then be a fact that physics explaining what I experience of the world would have to explain.
Having not explicitly done this experiment, I cannot claim for sure that we will conclude my consciousness is “collapsing” on an Everett Branch just as in Copenhagen interpretation it was the wave function that collapsed. But I will bet $100 against $10,000 if anybody wants to do the experiment. The terms of the bet are if you have a set-up that shows the counter result, that consciousness apparently dredges up memories of different nearby Everett branches by seeing different times on the timer, then I will come to where you are with your set-up and if you can show me it working for both you and I you get the $10,000, otherwise I get the $100 to defray my travel expenses. I’ll reserve the right to pass on checking your set-up out if travel costs would be over $600, but for me that covers a good fraction of the world (I am in Sandy Eggo in this Everett Branch).
Fortunately for you cat lovers, the experiment can be done without the cat. You simply need to measure the time of radioactive decay, killing a cat with cyanide on detection of the radioactive decay is not necessary to win or lose the bet (or to prove the point.)
Note the box of papers with recorded times in it can also be used as evidence. If I open that box and all the papers have the same time written on them, and that is the time I remember, then I take this as strong evidence that my memory has been returning memories from only the current everett branch. If my memory were unhooked from this everett branch, then one would expect the physical evidence of what I had previously remembered which is in this everett branch, to include times from other everett branches. If it does not, then I think we can conclude that human consciousness, including its memories, are branch local, that a “collapse” occurs in MWI when we attempt to use it to predict what we will experience in this universe.
And indeed, I think predicting what we will experience is the hallmark of all good theories of how the universe works. We may say we want to predict “what will happen,” but I believe by this we mean “what I will see happen.”
Whatever makes you think that your consciousness is in only one Everett branch? (And what do you think is happening on all those other branches that look so much like this one but that lack your consciousness?)
Surely the right account of this, conditional on MWI, is not that your consciousness is on a particular branch but that each branch has its own version of your consciousness, and each branch has its own version of your memory, and each branch has its own version of what actually happened, and—not at all by coincidence—these match up with one another.
What happens to your consciousness and your memories is much more like splitting than like collapse.
(It sounds as if you think that this ought to mean that you’d have conscious memories in one branch from other branches, but I can’t see why. Am I missing something?)
I misunderstood what David Gerard was suggesting and took a long riff proposing an experiment to address something he wasn’t saying.
The tricky part for me is the extremely clear conscious experience I have of being on only one branch. That there are other consciousnesses NEARLY identical to mine on other nearby Everett branches, presumably having the same strong awareness that they are on only one Everett branch and have no direct evidence of any other branch is clearer to me. MWI seems to truly be an interpretation, not a theory, with apparently absolutely no Popperian experiments that could ever distinguish it from wave function collapse theories.
You can upload a person into a quantum computer and do Schrödinger’s cat experiments on them. If you have a computational theory of mind, this should falsify at least some informal collapse theories.
You could have that predictive power without actually having a continuous stream of awareness. Consider sleepwalkers who can do things and have conversations (if not very good ones) with no conscious awareness. You’re using philosophy to object to observed reality.
OK, i misunderstood what you were implying in your previous post. So there are multiple streams of consciousness, one on each everett branch, and the memories returned on each everett branch are the ones in the (conscious+unconscious) brain that exists on that everett branch.
So I experience my mind always returning memories consistent with my branch even as other branch-mwengler’s experience memories consistent with their branch, and like me, use that as evidence for their uniqueness.
So it really is an interpretation, predicting nothing different in experience than does copenhagen.
https://en.wikipedia.org/wiki/Wigner%27s_friend
I haven’t seen one example of a precise definition of what constitutes an “observation” that’s supposed to collapse the wavefunction in Copenhagen interpretation. Decoherence, OTOH, seems to perfecty describe the observed effects, including the consistency of macro-scale history.
Actually it just proves that memory sticks with the branch it’s consistent with. For all we know, our consciousnesses are flitting from branch to branch all the time and we just don’t remember because the memories stay put.
Yeah, settling these kinds of questions would be much easier if we weren’t limited to the data that manages to reach our senses.
In MWI the definition of “I” is not quite straightforward: the constant branching of the wavefunction creates multiple versions of everyone inside, creating indexical uncertainty which we experience as randomness.
Your mistake lies in using the word “I” like it means something. There is some mwengler-stuff, it has some properties, then there is a split and the mwengler-stuff is in two separate chunks. They both experience their “stream of consciousness” showing up in their particular branch, they both wonder how it is that they ended up in the one branch rather than the other.
Copenhagen is not a theory of what you will experience either; there are multiple minds even in Copenhagen’s single world
Copenhagen is an interpretation where I have one mind, you have one mind, and each of us have one thread of experience. There are numerous places along that thread where the physics to calculate the time evolution of that thread is not deterministic, where a random choice has been made.
MWI is an interpretation where I have many minds, as opposed to the one mind I have in Copenhagen. In the MWI interpretation, each of my minds exists in a separate and non-interacting universe from all the other versions of my mind. If I wonder as I type this why this version of me is the one in THIS branch, MWI has no theory for that. MWI tries to make that question seem less interesting by pointing out that there are lots of versions of me asking that same question, so somehow obscuring the me-ness of the me in this branch with the me-ness of all these other similar but not identical me’s in these other branches would render the question meaningless.
But as an interpretation with no observable experimental differences, MWI and Copenhagen are likely to have the same number of random events dictating progress. In MWI, the randomness is isolated to just one of many me’s which of course is still quite unique and interesting to me, but which is not as bad as Copenhagen where it is the entire universe that got changed by each random waveform collapse.
How is this different to wondering why you are THIS mind in THIS branch rather than THIS OTHER mind in THIS branch? Why you are you rather than someone else?
Do I have multiple minds even in Copenhagen? And by I I mean flesh-and-blood me?
I mean that there are other minds in the world, in the sense of other people. Neither Copenhagen nor many worlds chooses a preferred mind, but people don’t notice it as strongly in Copenhagen since they’re already used to the idea of other conscious beings.
If I understand correctly, Copenhagen has only one mind for me, and the reality experienced by this mind is fundamentally randomly branched through wave function collapses. MWI creates a new mind for me so their are many minds for me, one in each Everett Branch. Did I miss something?
I’m not sure what you’re getting at here. Even under Copenhagen, one can duplicate an upload as it’s running.
Let’s suppose that your mind is a function of your brain, and that your brain is composed of atoms.
In MWI there are many branches with many configurations of atoms, that means many branches of your brain, that means many branches of your mind. In every branch your mind is entangled with the other atoms of the same branch. So for example in the universe with atoms of a dead cat, your mind is in the “poor kitty” state, and in the branch with the atoms of an alive cat, your mind is in the “kitty, you are so lucky, I promise I will never try this cruel experiment on you again” state.
In Copenhagen, on a tiny time scale there are many branches of atoms. But it is believed that on a larger scale it is not so. At some unspecified moment there is supposed to be a collapse where many branches of atoms become a single branch again (through a process of random selection). Nobody knows when does this happen. On a large scales, we are not able to run a precise enough experiment that would say either way. On smaller scales, where we can run the experiment, the result has always been that the collapse did not occur yet. So after the collapse, there is only one branch, and therefore one mind. Before the collapse… I would say that there is a superposition of minds (because there is a superposition of brains, because there is a superposition of atoms the brain is composed of), which should become one mind again at the moment of the collapse. But it is believed that this superposition exists only for a very small fraction of the second, so it’s not like the different minds in the superposition have enough time to really think significantly different thoughts. The neurons work at a limited speed, and sending a signal from one neuron to another requires dozens of chemical reactions.
Copenhagen:
You bounce a photon off a half-silvered mirror and don’t look at the results: no universe split.
You bounce a photon off a half-silvered mirror and look at the results: Bam! Split universe.
MWI:
You bounce a photon off a half-silvered mirror and don’t look at the results. Since the physical state of your brain is not causally dependent on the destination of the photon, you don’t branch into two mwenglers in any noticeable way.
You bounce a photon off a half-silvered mirror and look at the results. Since you’ve made the state of your brain causally dependent on an event with quantum randomness, you branch into two mwenglers which are different on a macroscopic level. Two persons, which happen to share a causal history up to looking at the experimental outcome.
Copenhagen Interpretation never splits universes. Instead, you have a wave function collapse in the one and only universe.
In MWI, you NEVER branch in to two anythings in a “noticeable” way. All the myriads of branches have no interactions, there is nothing noticeable about any of the other branches from within the branch we are in. If there is something noticeable about other branches, then an experiment could be defined to check the hypothesis of branching, and we would start to gather evidence for or against branching. Until such time as an hypothesis is created and tested and shows evidence for branches, MWI is an interpretation, and not a theory.
So why does it even matter? I am thinking it through and realizing that an interpretation is in some way a pre-theory. As we sit with the idea of MWI, maybe one of us develops hypotheses about experiments whic might show evidence for the other branches, or not. Without the interpretation of MWI, that hypothetical progress might never be available.
They do interact. This is how quantum physics was discovered.
The problem is that the magnitude of interaction is getting very small very quickly, so after a few microseconds it becomes technically impossible to measure. This is what allows people to say: “Yeah, for a few microseconds there is something mathematically equivalent to branches, but then it disappears completely” and you can’t experimentally prove them wrong.
One side believes that the interaction is getting smaller, but it never reaches exactly zero. Other side believes that the interaction is getting smaller, and then in some unspecified moment all branches except one disappear. Experimental data say that the interaction is getting smaller until it becomes too small to see… and then, well, it is too small to see what happens. So essentially both sides disagree about who has the burden of proof; about the exact meaning of “fewest assumptions” in Occam’s razor. One side says that “the extra branches disappearing” is the extra assumption. Other side says that “the extra branches not disappearing, even when their interaction becomes too small to measure” is the extra assumption.
More precisely, the magnitude of interaction depends on how much the particles in the branches are different. Therefore the branches we have measurable interaction with are those almost the same as our branch. The interaction is largest when both branches are exactly alike except for one particle. This is the famous double-slit experiment—two branches of the universe with the particle going through different slits interact with each other. The branches are there. The question is not whether multiple branches exist, but whether they disappear later when their interaction becomes very small.
How do you prove experimentally that the other branches do not disappear, especially if your opponents refuse to specify when they should disappear. If you make an experiment that proves that “after N seconds, the branches still exist”, your opponents can say: “Yeah, but maybe after N+1 seconds they disappear.” Repeat for any value of N.