There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky’s writings. <...> Coherent superpositions are small scale , down to single particles, observer dependent, reversible, and continue to interact (strictly speaking , interfere) after “splitting”. the last point is particularly problematical. because if large scale coherent superposition exist , that would create naked eye macrocsopic scale:, e.g. ghostly traces of a world where the Nazis won.
I don’t believe Yudkowsky ever supported such “branches” which could change each other’s internal content? Wherever I look, I see only that they interfere with each other’s amplitudes (thus also with probabilities). If so, I’m going to drop “coherent superpositions” at all.
I’m quite interested what labels would you use for the following experiment.
We’re talking to each other on LessWrong.
I invoke a quantum RNG to get a number between 1 and 8, and look at its response.
You do not know the generated number yet.
I say that the generator yielded ‘1’.
This random number is not very relevant for you, and you forget it over time.
I would label it so:
There is some region of world-branches, differing in some facts we do not know, but in each of those branches we’re talking to each other on LessWrong (and each of the concepts is meaningful).
The quantum RNG had some superposition of states; through measurement, this uncertainty also gets to sensors, is transmitted over the wire, so on, until I see the result. Thereon, my region of world-branches splits into eight branch-sections, equally sized and equally plausible, identified by the number I had seen.
For you, these branch-sections are not distinguishable yet; some could say that you are essentially in single branch.
Now, the decoherence (or should I say information?) is again transmitted over the wire, and you land in the eight sections with different random numbers too.
If you forget the number, your region of world-branches essentially merges the eight sections back.
I did really use an online, presumably quantum RNG.
Wherever I look, I see only that they interfere with each other’s amplitudes (thus also with probabilities)
That’s what I meant. And it’s bad enough. It means that when an observer goes into a coherent superposition with themselves, they are not split into two observers who are unaware of each others existence, as required by the standard many worlds account of observation, ie. there is not even an appearance of collapse. It important to notice that amplitudes aren’t just the probabilities of an eventual sharp, classical style observation, they also define the interference between the components of a superposed states, and the evolution of a coherently superposed state depends on the interference between all it’s components...you can’t treat them individually and separately. (You can with decoherent branches, and that’s one of the differences).
If so, I’m going to drop “coherent superpositions” at all.
The overall argument is that branching is either coherent or decoherent, and both are flawed. I’ve explained the problems with coherence. To restate the problems with decoherence:-
There is no empirical evidence of decoherence causing multiple branches.
There could be a theoretical justification for decoherent branching , but there currently isnt. Decoherence could be a single world phenomenon.
And it isn’t clearly simpler, since the mechanism isn’t fully understood.
The Yudkowsky-Deutsch claim is that there is a single MW theory, which explains everything that needed explaining, and is obviously simpler than its rivals. But coherence doesn’t save appearances , and decoherence, while more workable, is not simple.
If we are talking to each other, we are not in decoherent branches.
If I am.entangled with your eight possible observations , then I am in a quantum superposition with myself, and my previous objection applies. Why is the forgetting important? I can’t forget a single definite observation, because I haven’t made one...you haven’t introduced any collapse of decoherence. Or are you saying that remembering is decoherence?
Or are you saying that branching is subjective? Well, it is ,.for coherent “branching”.
If I am.entangled with your eight possible observations , then I am in a quantum superposition with myself, and my previous objection applies. Why is the forgetting important? I can’t forget a single definite observation, because I haven’t made one...you haven’t introduced any collapse of decoherence. Or are you saying that remembering is decoherence?
Yes, that is a superposition! I don’t think it can leave any “ghastly traces”—there are billions or more particles which moved differently (at least several angstroms away) depending on which exact number you see in my comment, and interference—even on amplitude level—fades. Presumably exponentially.
Remembering has to do with how many particles have different places now, and thus determines amount of decoherence; if the random number I generated was entirely forgotten (that is, no conscious nor unconscious nor whatever else visual memory), nor produced effects larger than thermal noise, I argue we would be exactly back to step 3 when I had not named the result yet.
Decoherence could be a single world phenomenon.
What would happen to decoherent branches which had their distinguishing features “evolve out to nothing” (become very close to “our” branch)? They will come back and influence the amplitudes and probabilities we observe once again.
Or are you saying that branching is subjective?
Not particularly. Though it is very convenient when the wavefunction factors into several world parts—say, it would be very strange if my generating a random number would influence a LW non-reader—in that sense they can subjectively just not consider that I had done anything.
Yes, that is a superposition! I don’t think it can leave any “ghastly traces”—there are billions or more particles which moved differently (at least several angstroms away) depending on which exact number you see in my comment, and interference—even on amplitude level—fades. Presumably exponentially.
Ok. That’s an argument for decoherence..but not an argument for multi branch decoherence.
And minor remnants can be left, so long as they stay minor… they are not going to affect observation much. (Although the reasons for that could be cosmological..an expanding universe with slightly negative curvature is the ideal way to get rid of unwanted information).
What would happen to decoherent branches which had their distinguishing features “evolve out to nothing” (become very close to “our” branch)? They will come back and influence the amplitudes and probabilities we observe once again.
But not much? Theres an argument that decoherent splitting can’t be completely irrevocable, because it emerges from time reversible microphysics … but there’s also an argument that the “losing” branches spread out, and become so thinly distributed in the overall mish mash that they can no longer matter in practice..and for for macrophysical reasons.
Though it is very convenient when the wavefunction factors into several world parts—say, it would be very strange if my generating a random number would influence a LW non-reader—in that sense they can subjectively just not consider that I had done anything.
That’sexactly how decoherent branching works .. if it works. It’s not a causal process that leaves causal traces.
but there’s also an argument that the “losing” branches spread out
They don’t spread much faster compared to “winning” branches I guess? World has no particular dependence on what random number I generated above, so all the splits and merges have approximately same shape in either of the eight branch regions.
That’sexactly how decoherent branching works .. if it works. It’s not a causal process that leaves causal traces.
With a remark that “decoherent branching” and “coherent branching” are presumably just one process differing in how much the information is contained or spreads out, and noting that should LW erase the random number from my comment above plus every of us to totally forget it, the branches would approximately merge,
yes I agree. Contents of worlds in those branches do not causally interact with us, but amplitudes might at some point in future. AFAIK Eliezer referenced the latter while assigning label “real” to each and every world (each point of wavefunction).
They don’t spread much faster compared to “winning” branches I guess
They don’t spread faster, they spread wider. Their low amplitude information is smeared over an environmental already containing a lot of other low amplitude information, noise in effect. So the chances of recovering it are zero for all practical purposes.
With a remark that “decoherent branching” and “coherent branching” are presumably just one process differing in how much the information is contained or spreads out
Well, no. In a typical measurement, a single particle interacts with an apparatus containing trillions, and that brings about decoherence very quickly, so quickly it can appear like collapse. Decoherent branches, being macroscopic , stable and irreversible, for all practical purposes, are the opposite to coherent ones.
I don’t believe Yudkowsky ever supported such “branches” which could change each other’s internal content? Wherever I look, I see only that they interfere with each other’s amplitudes (thus also with probabilities). If so, I’m going to drop “coherent superpositions” at all.
I’m quite interested what labels would you use for the following experiment.
We’re talking to each other on LessWrong.
I invoke a quantum RNG to get a number between 1 and 8, and look at its response.
You do not know the generated number yet.
I say that the generator yielded ‘1’.
This random number is not very relevant for you, and you forget it over time.
I would label it so:
There is some region of world-branches, differing in some facts we do not know, but in each of those branches we’re talking to each other on LessWrong (and each of the concepts is meaningful).
The quantum RNG had some superposition of states; through measurement, this uncertainty also gets to sensors, is transmitted over the wire, so on, until I see the result. Thereon, my region of world-branches splits into eight branch-sections, equally sized and equally plausible, identified by the number I had seen.
For you, these branch-sections are not distinguishable yet; some could say that you are essentially in single branch.
Now, the decoherence (or should I say information?) is again transmitted over the wire, and you land in the eight sections with different random numbers too.
If you forget the number, your region of world-branches essentially merges the eight sections back.
I did really use an online, presumably quantum RNG.
That’s what I meant. And it’s bad enough. It means that when an observer goes into a coherent superposition with themselves, they are not split into two observers who are unaware of each others existence, as required by the standard many worlds account of observation, ie. there is not even an appearance of collapse. It important to notice that amplitudes aren’t just the probabilities of an eventual sharp, classical style observation, they also define the interference between the components of a superposed states, and the evolution of a coherently superposed state depends on the interference between all it’s components...you can’t treat them individually and separately. (You can with decoherent branches, and that’s one of the differences).
The overall argument is that branching is either coherent or decoherent, and both are flawed. I’ve explained the problems with coherence. To restate the problems with decoherence:-
There is no empirical evidence of decoherence causing multiple branches.
There could be a theoretical justification for decoherent branching , but there currently isnt. Decoherence could be a single world phenomenon.
And it isn’t clearly simpler, since the mechanism isn’t fully understood.
The Yudkowsky-Deutsch claim is that there is a single MW theory, which explains everything that needed explaining, and is obviously simpler than its rivals. But coherence doesn’t save appearances , and decoherence, while more workable, is not simple.
If we are talking to each other, we are not in decoherent branches.
If I am.entangled with your eight possible observations , then I am in a quantum superposition with myself, and my previous objection applies. Why is the forgetting important? I can’t forget a single definite observation, because I haven’t made one...you haven’t introduced any collapse of decoherence. Or are you saying that remembering is decoherence?
Or are you saying that branching is subjective? Well, it is ,.for coherent “branching”.
Yes, that is a superposition! I don’t think it can leave any “ghastly traces”—there are billions or more particles which moved differently (at least several angstroms away) depending on which exact number you see in my comment, and interference—even on amplitude level—fades. Presumably exponentially.
Remembering has to do with how many particles have different places now, and thus determines amount of decoherence; if the random number I generated was entirely forgotten (that is, no conscious nor unconscious nor whatever else visual memory), nor produced effects larger than thermal noise, I argue we would be exactly back to step 3 when I had not named the result yet.
What would happen to decoherent branches which had their distinguishing features “evolve out to nothing” (become very close to “our” branch)? They will come back and influence the amplitudes and probabilities we observe once again.
Not particularly. Though it is very convenient when the wavefunction factors into several world parts—say, it would be very strange if my generating a random number would influence a LW non-reader—in that sense they can subjectively just not consider that I had done anything.
Ok. That’s an argument for decoherence..but not an argument for multi branch decoherence.
And minor remnants can be left, so long as they stay minor… they are not going to affect observation much. (Although the reasons for that could be cosmological..an expanding universe with slightly negative curvature is the ideal way to get rid of unwanted information).
But not much? Theres an argument that decoherent splitting can’t be completely irrevocable, because it emerges from time reversible microphysics … but there’s also an argument that the “losing” branches spread out, and become so thinly distributed in the overall mish mash that they can no longer matter in practice..and for for macrophysical reasons.
That’sexactly how decoherent branching works .. if it works. It’s not a causal process that leaves causal traces.
They don’t spread much faster compared to “winning” branches I guess? World has no particular dependence on what random number I generated above, so all the splits and merges have approximately same shape in either of the eight branch regions.
With a remark that “decoherent branching” and “coherent branching” are presumably just one process differing in how much the information is contained or spreads out,
and noting that should LW erase the random number from my comment above plus every of us to totally forget it, the branches would approximately merge,
yes I agree. Contents of worlds in those branches do not causally interact with us, but amplitudes might at some point in future. AFAIK Eliezer referenced the latter while assigning label “real” to each and every world (each point of wavefunction).
They don’t spread faster, they spread wider. Their low amplitude information is smeared over an environmental already containing a lot of other low amplitude information, noise in effect. So the chances of recovering it are zero for all practical purposes.
Well, no. In a typical measurement, a single particle interacts with an apparatus containing trillions, and that brings about decoherence very quickly, so quickly it can appear like collapse. Decoherent branches, being macroscopic , stable and irreversible, for all practical purposes, are the opposite to coherent ones.