Yes, I think that’s a good explanation. One question it raises is ambiguity in thinking of QM via “many worlds”. What constitutes a “world”? If we put a system into a coherent superposition, does that mean there are two worlds? Then if we transform it back into a pure state, has a world gone away? What about the fact that whether it is pure or in a superposition depends arbitrarily on the chosen basis? A pure-state vertically polarized photon is in a superposition of states using the diagonal basis. How many worlds are there, two or one? This interpretation can’t be more than very metaphorical—it is “as though” there are two worlds in some sense.
Or do we only count a “world” when we have (some minimal degree of) decoherence leading to permanent separation? That way worlds never merge.
The explanation of QC in terms of MWI will vary depending on which interpretation we use. In the second one (worlds on decoherence) the explanation is pretty much the same as in any other interpretation. We put a system into a coherent state, manipulate it into a pure state, and the measurement doesn’t do anything as far as world splitting.
But in the first interpretation, we want to say that there are many different worlds, once for each possible value in the quantum registers. Then we change the amplitude of these worlds, essentially making some of them go away so that there is only one left by the time we do the measurement. It’s an odd way to think of worlds.
I agree that unitary wavefunction evolution is a MUCH better name than the misleading “Many Worlds”. Then, of course, you say that computation takes place within the evolving wavefunction and that you are part of that computation everywhere certain patterns in that computation take place. Still some handwaving here, but SO MUCH better than the standard misunderstandings of Many Worlds.
Thanks, I’ve been wanting a better name to use than ‘Many Worlds’ for some time! To be honest I would have settled for ‘Greebo’ or “Bliggle’, so long as it got away from the confusion bait.
Even though it has a technical meaning, I think “unitary wavefunction evolution” would be worse for communication than “Greebo.”
I suspect you are right. This leaves me with ‘Greebo’, ‘QM without collapse’ or, I could just say ‘quantum mechanics’ and choose not to talk to people about such topics when they persist in advocating a collapse beyond the first time they hear it isn’t necessary.
This made me chuckle. I suppose that as the intelligence and amount of knowledge held by the average member of an intellectual group goes up, their lower bounds on the amount of knowledge someone must have in order for that member to have an intellectual conversation with them goes up as well. I’m horrible at communicating clearly, so I’ll give an example.
4chan poster: You’re a scientologist!? Idiot. RationalWiki member: You’re a creationist?! I refuse to speak to you. Less Wrong member: You insist that there is such thing as waveform collapse in quantum mechanics?! I see you cannot be saved.
Creationists I can accept. Ardent ‘qualia’ and ‘philosophical zombie’ advocates on the other hand I will not speak with. Creationists often acknowledge that their beliefs are based on faith. ‘Qualia’ folks are more likely to claim philosophical or intellectual authority, invoking my eternal contempt. This ties back in with your original point: it’s about having intellectual conversations. For me at least it is those encroaching on intellectual territory in particular that must be held to higher standards.
That’s a good point. I’ve never actually interacted with someone in real life that even knew what philosophical zombies were, so my ‘intellectual’ conversations take place along the lines of ‘atheism versus theism’, sadly. Maybe there is some merit to joining Mensa after all?
Maybe there is some merit to joining Mensa after all?
Do you like debating? That is, do you like throwing about very clever non-sequiturs and a whole bunch of straw men to prove your high status amongst a tribe of high IQ monkeys?
I’ve let my membership lapse. I keep in touch with some of the guys to play games with but the MENSA around here is mostly an online thing and decidedly inferior to lesswrong as far as intellectual stimulation goes. Although I must admit some of the Mensa guys were great to get business and professional advice from.
Well, if the people spitting out the clever non-sequiturs have charming British accents, then possibly. Otherwise, no… is Mensa about ‘debating’, normally? I always figured it’d be more of a casual social meet-up. But even then I suppose it could quickly dissolve into a mere signalling competition, or a ‘debate’.
Interesting. Sounds like you’re saying that the entire process of quantum computation aims to keep the system coherent, and so avoid splitting the universe. Which make sense. They tell me the difficulty, in an engineering sense, is to stop the system de-cohering.
Yes. Except that the universe doesn’t ever split. It’s always continuous. But we’re trying to keep two blobs of amplitude in close contact rather than letting them diverge, so that some parts can overlap and add up or cancel out.
Or do we only count a “world” when we have (some minimal degree of) decoherence leading to permanent separation? That way worlds never merge.
This makes me wonder something. It seems that the many-worlds theory involves exponential branching: if there’s 1 world one moment, there are 2 the next, then 4, then 8, and so on. (To attempt to avoid the objection you just raised: if 1 pure state, defined intuitively, has significant amplitude one moment, then . . .) Since this grows exponentially, won’t it eventually grow to cover every possible state? Admittedly, the time this would take is more or less proportional to the number of particles in the universe, and so I really don’t know how long it would take for coinciding to happen, but it seems that this would produce observable consequences eventually, maybe-maybe-not while minds are still around.
Since this grows exponentially, won’t it eventually grow to cover every possible state?
Yes. The process is observable as entropy. And the extremum (equalization of most or all of configuration space) is the conjectured heat-death of the universe.
I agree that equalization of configuration space is the heat death of the universe. I’m not sure, given this, that there won’t be significant interaction until most of the decay has occurred.
Yes, I think that’s a good explanation. One question it raises is ambiguity in thinking of QM via “many worlds”. What constitutes a “world”? If we put a system into a coherent superposition, does that mean there are two worlds? Then if we transform it back into a pure state, has a world gone away? What about the fact that whether it is pure or in a superposition depends arbitrarily on the chosen basis? A pure-state vertically polarized photon is in a superposition of states using the diagonal basis. How many worlds are there, two or one? This interpretation can’t be more than very metaphorical—it is “as though” there are two worlds in some sense.
Or do we only count a “world” when we have (some minimal degree of) decoherence leading to permanent separation? That way worlds never merge.
The explanation of QC in terms of MWI will vary depending on which interpretation we use. In the second one (worlds on decoherence) the explanation is pretty much the same as in any other interpretation. We put a system into a coherent state, manipulate it into a pure state, and the measurement doesn’t do anything as far as world splitting.
But in the first interpretation, we want to say that there are many different worlds, once for each possible value in the quantum registers. Then we change the amplitude of these worlds, essentially making some of them go away so that there is only one left by the time we do the measurement. It’s an odd way to think of worlds.
I agree that unitary wavefunction evolution is a MUCH better name than the misleading “Many Worlds”.
Then, of course, you say that computation takes place within the evolving wavefunction and that you are part of that computation everywhere certain patterns in that computation take place. Still some handwaving here, but SO MUCH better than the standard misunderstandings of Many Worlds.
Thanks, I’ve been wanting a better name to use than ‘Many Worlds’ for some time! To be honest I would have settled for ‘Greebo’ or “Bliggle’, so long as it got away from the confusion bait.
I advocate “no collapse.”
Even though it has a technical meaning, I think “unitary wavefunction evolution” would be worse for communication than “Greebo.”
That works.
I suspect you are right. This leaves me with ‘Greebo’, ‘QM without collapse’ or, I could just say ‘quantum mechanics’ and choose not to talk to people about such topics when they persist in advocating a collapse beyond the first time they hear it isn’t necessary.
This made me chuckle. I suppose that as the intelligence and amount of knowledge held by the average member of an intellectual group goes up, their lower bounds on the amount of knowledge someone must have in order for that member to have an intellectual conversation with them goes up as well.
I’m horrible at communicating clearly, so I’ll give an example.
4chan poster: You’re a scientologist!? Idiot.
RationalWiki member: You’re a creationist?! I refuse to speak to you.
Less Wrong member: You insist that there is such thing as waveform collapse in quantum mechanics?! I see you cannot be saved.
Creationists I can accept. Ardent ‘qualia’ and ‘philosophical zombie’ advocates on the other hand I will not speak with. Creationists often acknowledge that their beliefs are based on faith. ‘Qualia’ folks are more likely to claim philosophical or intellectual authority, invoking my eternal contempt. This ties back in with your original point: it’s about having intellectual conversations. For me at least it is those encroaching on intellectual territory in particular that must be held to higher standards.
That’s a good point. I’ve never actually interacted with someone in real life that even knew what philosophical zombies were, so my ‘intellectual’ conversations take place along the lines of ‘atheism versus theism’, sadly. Maybe there is some merit to joining Mensa after all?
Do you like debating? That is, do you like throwing about very clever non-sequiturs and a whole bunch of straw men to prove your high status amongst a tribe of high IQ monkeys?
I’ve let my membership lapse. I keep in touch with some of the guys to play games with but the MENSA around here is mostly an online thing and decidedly inferior to lesswrong as far as intellectual stimulation goes. Although I must admit some of the Mensa guys were great to get business and professional advice from.
Well, if the people spitting out the clever non-sequiturs have charming British accents, then possibly. Otherwise, no… is Mensa about ‘debating’, normally? I always figured it’d be more of a casual social meet-up. But even then I suppose it could quickly dissolve into a mere signalling competition, or a ‘debate’.
Interesting. Sounds like you’re saying that the entire process of quantum computation aims to keep the system coherent, and so avoid splitting the universe. Which make sense. They tell me the difficulty, in an engineering sense, is to stop the system de-cohering.
Is that remotely accurate?
Yes. Except that the universe doesn’t ever split. It’s always continuous. But we’re trying to keep two blobs of amplitude in close contact rather than letting them diverge, so that some parts can overlap and add up or cancel out.
This makes me wonder something. It seems that the many-worlds theory involves exponential branching: if there’s 1 world one moment, there are 2 the next, then 4, then 8, and so on. (To attempt to avoid the objection you just raised: if 1 pure state, defined intuitively, has significant amplitude one moment, then . . .) Since this grows exponentially, won’t it eventually grow to cover every possible state? Admittedly, the time this would take is more or less proportional to the number of particles in the universe, and so I really don’t know how long it would take for coinciding to happen, but it seems that this would produce observable consequences eventually, maybe-maybe-not while minds are still around.
Yes. The process is observable as entropy. And the extremum (equalization of most or all of configuration space) is the conjectured heat-death of the universe.
I agree that equalization of configuration space is the heat death of the universe. I’m not sure, given this, that there won’t be significant interaction until most of the decay has occurred.