I think the situation with regard to MWI ‘copies’ is different from that with regard to multiple copies existing in the same Everett branch, or in a classical universe.
I can’t fully explain why, but I think that when, through decoherence, the universe splits into two or more ‘worlds’, each of which are assigned probability weights that (as nearly as makes no difference) behave like classical probabilities, it’s rational to act as though the Copenhagen interpretation was (as nearly as makes no difference) true. Strictly speaking the Copenhagen interpretation is incoherent, but still you should act as though, in a “quantum suicide” scenario, there is probability p that you will ‘cease to exist’, rather than a probability 1 that a copy of you will go on existing but ‘tagged’ with the information ‘norm square amplitude of this copy is [1-p times that of the pre-existing person]’.
My rationale is roughly as follows: Suppose the universe were governed by laws of physics that were ‘indeterministic’ in the sense of describing the evolution over time of a classical probability distribution. Then if we want to, we can still pretend that there is a multiverse, that physics is deterministic, that all possible worlds exist with a certain probability density etc. And clearly the difference between a ‘single universe’ and a ‘multiverse’ view is ‘metaphysical’ in the sense that no experiment can tell them apart. Here I want to be a verificationist and say that there is no ‘fact of the matter’ as to which interpretation is true. Therefore, the question of how to act rationally shouldn’t depend on this.
When we move from classical indeterminism to quantum indeterminism the probabilities get replaced by complex-valued ‘amplitudes’ but for reasons I struggle to articulate, I think the fact that the universe is ‘nearly’ classical means that our prescriptions for rational action must be ‘nearly’ the same as they would have been in a classical universe.
As for sly, he should act as though he has luckily survived an event that had a 1⁄2 chance of killing him (‘once and for all’, ‘irrevocably’ etc). Presumably he would charge you with something, though I’m not sure whether ‘attempted murder’ is what you’d be guilty of.
And clearly the difference between a ‘single universe’ and a ‘multiverse’ view is ‘metaphysical’ in the sense that no experiment can tell them apart.
No, sorry, there are in-principle experiments that tell the two apart. For example, with good enough apparatus, you could do the double-slit experiment with people. (Currently they are doing it with bacteria I believe). You would be able to interfere with yourself in other branches in a wave-like way.
For example, with good enough apparatus, you could do the double-slit experiment with people. (Currently they are doing it with bacteria I believe). You would be able to interfere with yourself in other branches in a wave-like way.
Wait, what? How would you do it with people or bacteria? Do you have a link to the bacteria experiment? I thought that the different worlds couldn’t interact; I’m very confused by this comment.
MWI doesn’t strictly say that the worlds don’t interact. It just says that they are mostly approximately independent if they have decohered (itself a continuous process).
Experiments in controlled conditions show that small sub-branches can, in fact, interfere with each other like waves, hence the double-slit experiment with electrons. But the size of the object merely contributes to the difficulty of the experiment, it seems. So far, large molecules have been used, but in the future it is planned to use viruses and bacteria. See Toward Quantum Superposition of Living Organisms. Also note that a quantum computer is essentially using computation across the multiverse (though no-one has built a particularly large one of those).
Is there any reason to use viruses and bacteria as opposed to, say, bacterium-sized salt crystals? Is it to refute people who say: “But if it’s alive then perhaps it has magical quantum properties. Because life is magical.”
Human consciousness specifically, not just life. Would different interpretations give different predictions for an experiment with a human interfering with himself in other branches?
Are you asking about what this would look like to observers on the side, or about the subjective experience of the person undergoing interference?
Regarding the first question, I don’t think it would be different in principle from any other hypothetical experiment with macroscopic quantum interference; how much different interpretations manage to account for those is a complex question, but I don’t think proponents of either of them would accept the mere fact of experimentally observed macroscopic interference as falsifying their favored interpretation. (Though arguably, collapse-based interpretations run into ever greater difficulties as the largest scales of detected quantum phenomena increase.)
As for the second one, I think answering that question would require more knowledge about the exact nature of human consciousness than we presently have. Scott Aaronson presents some interesting discussion along these lines in one of his lectures: http://www.scottaaronson.com/democritus/lec11.html
Nope. The copenhagen interpretation says that the wavefunction collapses when it interacts with anything.
But the thing it interacts with is still part of a larger wavefunction until that collapses etc.
Schroedingers Cat is about this fact, the fact that a cat (an observer) can be superposed.
I have yet to work out what the difference is between the Copenhagen Interpretation and the Many Worlds Interpretation. The physical reality they describe is identical.
Thanks for the link. I’m still not clear on exactly what it would mean to be able to interfere with myself in other branches in a wave-like way. Also, I thought a non-reversible process forced decoherence: is this not correct, or is there a way to force living organisms to be reversible?
If different worlds didn’t interact, you wouldn’t even get the ordinary double-slit result. With ordinary probability, you can split off branches without a problem, but quantum amplitudes can be negative or complex, they can cancel out, etc. You just don’t typically see this macroscopically due to decoherence.
Interesting. You seem to be saying that if the laws of physics appeared to be nondeterministic, there would be no way to be sure they don’t actually create copies instead.
I think this is correct, with the caveat that the laws of physics for this universe do not appear to work that way. However, the map is not the territory—not knowing how the laws work does not mean there is no fact of how they work. Even so, assuming that the rational course action differs between these situations, the best you can do is assign a prior to both (Solomonoff prior will do, I suppose), and average your actions in some way between them.
It’s possible you’re also correct that, in that case, there would be no fact of the matter about it—I’m not quite sure what you meant. If there are multiple universes (with different laws of physics), and clones in other universes count, then indexical uncertainty about which universe you’re in translates directly into, in effect, existing in both. I think.
I think the situation with regard to MWI ‘copies’ is different from that with regard to multiple copies existing in the same Everett branch, or in a classical universe.
I can’t fully explain why, but I think that when, through decoherence, the universe splits into two or more ‘worlds’, each of which are assigned probability weights that (as nearly as makes no difference) behave like classical probabilities, it’s rational to act as though the Copenhagen interpretation was (as nearly as makes no difference) true. Strictly speaking the Copenhagen interpretation is incoherent, but still you should act as though, in a “quantum suicide” scenario, there is probability p that you will ‘cease to exist’, rather than a probability 1 that a copy of you will go on existing but ‘tagged’ with the information ‘norm square amplitude of this copy is [1-p times that of the pre-existing person]’.
My rationale is roughly as follows: Suppose the universe were governed by laws of physics that were ‘indeterministic’ in the sense of describing the evolution over time of a classical probability distribution. Then if we want to, we can still pretend that there is a multiverse, that physics is deterministic, that all possible worlds exist with a certain probability density etc. And clearly the difference between a ‘single universe’ and a ‘multiverse’ view is ‘metaphysical’ in the sense that no experiment can tell them apart. Here I want to be a verificationist and say that there is no ‘fact of the matter’ as to which interpretation is true. Therefore, the question of how to act rationally shouldn’t depend on this.
When we move from classical indeterminism to quantum indeterminism the probabilities get replaced by complex-valued ‘amplitudes’ but for reasons I struggle to articulate, I think the fact that the universe is ‘nearly’ classical means that our prescriptions for rational action must be ‘nearly’ the same as they would have been in a classical universe.
As for sly, he should act as though he has luckily survived an event that had a 1⁄2 chance of killing him (‘once and for all’, ‘irrevocably’ etc). Presumably he would charge you with something, though I’m not sure whether ‘attempted murder’ is what you’d be guilty of.
No, sorry, there are in-principle experiments that tell the two apart. For example, with good enough apparatus, you could do the double-slit experiment with people. (Currently they are doing it with bacteria I believe). You would be able to interfere with yourself in other branches in a wave-like way.
Wait, what? How would you do it with people or bacteria? Do you have a link to the bacteria experiment? I thought that the different worlds couldn’t interact; I’m very confused by this comment.
It would seem to rely on diffracting people around corners. Sounds tricky. Must be very good equipment!
MWI doesn’t strictly say that the worlds don’t interact. It just says that they are mostly approximately independent if they have decohered (itself a continuous process).
Experiments in controlled conditions show that small sub-branches can, in fact, interfere with each other like waves, hence the double-slit experiment with electrons. But the size of the object merely contributes to the difficulty of the experiment, it seems. So far, large molecules have been used, but in the future it is planned to use viruses and bacteria. See Toward Quantum Superposition of Living Organisms. Also note that a quantum computer is essentially using computation across the multiverse (though no-one has built a particularly large one of those).
Is there any reason to use viruses and bacteria as opposed to, say, bacterium-sized salt crystals? Is it to refute people who say: “But if it’s alive then perhaps it has magical quantum properties. Because life is magical.”
Yes. It is way cooler. Kind of like levitating frogs with superconducting magnets.
Well, isn’t the copenhagen interpretation the statement that life has magic effects on physics, by causing the wavefunction to collapse?
Human consciousness specifically, not just life. Would different interpretations give different predictions for an experiment with a human interfering with himself in other branches?
Are you asking about what this would look like to observers on the side, or about the subjective experience of the person undergoing interference?
Regarding the first question, I don’t think it would be different in principle from any other hypothetical experiment with macroscopic quantum interference; how much different interpretations manage to account for those is a complex question, but I don’t think proponents of either of them would accept the mere fact of experimentally observed macroscopic interference as falsifying their favored interpretation. (Though arguably, collapse-based interpretations run into ever greater difficulties as the largest scales of detected quantum phenomena increase.)
As for the second one, I think answering that question would require more knowledge about the exact nature of human consciousness than we presently have. Scott Aaronson presents some interesting discussion along these lines in one of his lectures:
http://www.scottaaronson.com/democritus/lec11.html
Nope. The copenhagen interpretation says that the wavefunction collapses when it interacts with anything.
But the thing it interacts with is still part of a larger wavefunction until that collapses etc.
Schroedingers Cat is about this fact, the fact that a cat (an observer) can be superposed.
I have yet to work out what the difference is between the Copenhagen Interpretation and the Many Worlds Interpretation. The physical reality they describe is identical.
Thanks for the link. I’m still not clear on exactly what it would mean to be able to interfere with myself in other branches in a wave-like way. Also, I thought a non-reversible process forced decoherence: is this not correct, or is there a way to force living organisms to be reversible?
If different worlds didn’t interact, you wouldn’t even get the ordinary double-slit result. With ordinary probability, you can split off branches without a problem, but quantum amplitudes can be negative or complex, they can cancel out, etc. You just don’t typically see this macroscopically due to decoherence.
Eliezer will go mad if he reads this ;-0
Interesting. You seem to be saying that if the laws of physics appeared to be nondeterministic, there would be no way to be sure they don’t actually create copies instead.
I think this is correct, with the caveat that the laws of physics for this universe do not appear to work that way. However, the map is not the territory—not knowing how the laws work does not mean there is no fact of how they work. Even so, assuming that the rational course action differs between these situations, the best you can do is assign a prior to both (Solomonoff prior will do, I suppose), and average your actions in some way between them.
It’s possible you’re also correct that, in that case, there would be no fact of the matter about it—I’m not quite sure what you meant. If there are multiple universes (with different laws of physics), and clones in other universes count, then indexical uncertainty about which universe you’re in translates directly into, in effect, existing in both. I think.
I’m still confused about this, I’ll admit.