Illustration: if we use the example of the experiment here, the Copenhagenite would just point to the steps between measurement 2 and measurement 3 that reverse measurement 2 and say “look, to do this you need to put your subject in either |+> or |-> according to what’s still in your memory, and so the collapse at measurement 2 is entirely separate from what the result of measurement 3 will be.”
The only difference between the two physicists will be their vocabulary- one will have the unfortunate word “collapse” and the other will have the unfortunate word “multiverse”- but they’ll agree on the final result.
OK, the example linked is defective, in that there are two different operations that get the same result when the machine reverses its x-axis measurement. The first is the time-reversal of the measurement operation; the second is the recreation of the state created by measurement 1. You seem to be saying that the Copenhagenite would assume the latter.
Here is a modification of the experiment that tests the idea of collapse more severely. Instead of preparing an electron in a |+z> or |-z> state, I prepare an entangled pair of electrons with opposite z-axis spin (a spin anti-correlated pair). I now give one electron to the machine intelligence, which measures its spin in the x-axis, and then applies the time-reversal of the measurement, restoring the electron’s original state and erasing its memory of the x-axis state. It then passes the electron back to me, and I measure the two electrons’ z-axis spins.
If the machine intelligence’s measurement had caused a collapse, the anti-correlation would be erased. But in fact everything we know about quantum mechanics says that the electrons should remain anti-correlated.
I now give one electron to the machine intelligence, which measures its spin in the x-axis, and then applies the time-reversal of the measurement, restoring the electron’s original state and erasing its memory of the x-axis state.
I don’t see why the Copenhagenite can’t make the exact same objection here. Perhaps it would be clearer if you gave an example of how one would perform the time reversal of a measurement? If I have a z spin up electron, and I put it through a Stern-Gerlach device and find it is now a x spin up electron, how do I go back to a z spin up electron?
The link doesn’t make it explicit, but a reversible machine intelligence which can actually reverse a measurement is a quantum computer. In this context, a measurement occurs when the AI purposefully entangles its computing elements with the electron. The AI can now choose whether to let the information it gains leak out of it or not. Provided it does not allow the entanglement between the electron and the outside world to increase, it can choose to unentangle its state from that of the electron. In the simplest case, where it does not allow the rest of its mind to become entangled with the part of itself that it is using as a measurement apparatus, all it need do is run the inverse of the unitary transform that it used to entangle the apparatus with the electron. However, it can theoretically do quite a bit more. It can use the information in other computations, and then carefully carry out an operation that restores the original state of the electron and turns the results it obtains into superpositions.
Humans don’t have such fine-grained control over where they shuffle quantum information, nor can they keep themselves from becoming entangled with their environment. Using macroscopic devices to register phosphorescence is right out.
It seems to me that this makes assumptions about entanglement and disentanglement which I find suspect (but I am not an expert on entanglement, so they may hold). It doesn’t appear to be “choosing” to unentangle its state from the electron- we’re assuming that the information it generates through entanglement is not leaked to the outside world, and that the information can be thrown away and the system returned to where it was before. If it’s making a choice, it seems that that choice would cause information leak.
If those assumptions hold, I don’t see why they hold for just MWI. That is, I believe it may be possible to get to a final situation where you have your initial configuration despite the fact that your apparatus poked the system- but I don’t think that gives you any meaningful information differentiating the flavors of QM.
Quantum Information cannot be thrown away. Nor can it be copied. Information is conserved. *Apart, perhaps, from Copenhagen collapse). Information can be made difficult to retrieve by e.g. entanglement with the environment, specifically propagating modes that take it beyond your control, but it’s still “in principle” there.
Is there a meaningful difference between “propagating modes that take it beyond your control” and “throwing it away”? In my mind, the first is a much longer restatement of the second, but I apologize that it was unclear. (Here, you’re throwing it back into the electron, not the outside world, but the idea is the same from the computer’s point of view.)
Yes, they have very different effects. Throwing it into the electron allows recoherence in principal. Throwing it into the environment makes that impossible.
As stated you can’t. In the MWI picture, you are split into two one who has measured it x+, the other x-. Both must send it back to have it recohere, and they must at the same time erase their measurement of which way it went—really anything that distinguishes those two branches. They can record the fact that they did measure it, as this is the same in the two branches.
A person obviously can’t just “forget”, and will at best leak information into the environment, encoded as correlations in the noise of heat. A (reversible) computer, on the other hand, works quite well for doing this.
Illustration: if we use the example of the experiment here, the Copenhagenite would just point to the steps between measurement 2 and measurement 3 that reverse measurement 2 and say “look, to do this you need to put your subject in either |+> or |-> according to what’s still in your memory, and so the collapse at measurement 2 is entirely separate from what the result of measurement 3 will be.”
The only difference between the two physicists will be their vocabulary- one will have the unfortunate word “collapse” and the other will have the unfortunate word “multiverse”- but they’ll agree on the final result.
OK, the example linked is defective, in that there are two different operations that get the same result when the machine reverses its x-axis measurement. The first is the time-reversal of the measurement operation; the second is the recreation of the state created by measurement 1. You seem to be saying that the Copenhagenite would assume the latter.
Here is a modification of the experiment that tests the idea of collapse more severely. Instead of preparing an electron in a |+z> or |-z> state, I prepare an entangled pair of electrons with opposite z-axis spin (a spin anti-correlated pair). I now give one electron to the machine intelligence, which measures its spin in the x-axis, and then applies the time-reversal of the measurement, restoring the electron’s original state and erasing its memory of the x-axis state. It then passes the electron back to me, and I measure the two electrons’ z-axis spins.
If the machine intelligence’s measurement had caused a collapse, the anti-correlation would be erased. But in fact everything we know about quantum mechanics says that the electrons should remain anti-correlated.
I don’t see why the Copenhagenite can’t make the exact same objection here. Perhaps it would be clearer if you gave an example of how one would perform the time reversal of a measurement? If I have a z spin up electron, and I put it through a Stern-Gerlach device and find it is now a x spin up electron, how do I go back to a z spin up electron?
The link doesn’t make it explicit, but a reversible machine intelligence which can actually reverse a measurement is a quantum computer. In this context, a measurement occurs when the AI purposefully entangles its computing elements with the electron. The AI can now choose whether to let the information it gains leak out of it or not. Provided it does not allow the entanglement between the electron and the outside world to increase, it can choose to unentangle its state from that of the electron. In the simplest case, where it does not allow the rest of its mind to become entangled with the part of itself that it is using as a measurement apparatus, all it need do is run the inverse of the unitary transform that it used to entangle the apparatus with the electron. However, it can theoretically do quite a bit more. It can use the information in other computations, and then carefully carry out an operation that restores the original state of the electron and turns the results it obtains into superpositions.
Humans don’t have such fine-grained control over where they shuffle quantum information, nor can they keep themselves from becoming entangled with their environment. Using macroscopic devices to register phosphorescence is right out.
It seems to me that this makes assumptions about entanglement and disentanglement which I find suspect (but I am not an expert on entanglement, so they may hold). It doesn’t appear to be “choosing” to unentangle its state from the electron- we’re assuming that the information it generates through entanglement is not leaked to the outside world, and that the information can be thrown away and the system returned to where it was before. If it’s making a choice, it seems that that choice would cause information leak.
If those assumptions hold, I don’t see why they hold for just MWI. That is, I believe it may be possible to get to a final situation where you have your initial configuration despite the fact that your apparatus poked the system- but I don’t think that gives you any meaningful information differentiating the flavors of QM.
Quantum Information cannot be thrown away. Nor can it be copied. Information is conserved. *Apart, perhaps, from Copenhagen collapse). Information can be made difficult to retrieve by e.g. entanglement with the environment, specifically propagating modes that take it beyond your control, but it’s still “in principle” there.
Is there a meaningful difference between “propagating modes that take it beyond your control” and “throwing it away”? In my mind, the first is a much longer restatement of the second, but I apologize that it was unclear. (Here, you’re throwing it back into the electron, not the outside world, but the idea is the same from the computer’s point of view.)
Yes, they have very different effects. Throwing it into the electron allows recoherence in principal. Throwing it into the environment makes that impossible.
As stated you can’t. In the MWI picture, you are split into two one who has measured it x+, the other x-. Both must send it back to have it recohere, and they must at the same time erase their measurement of which way it went—really anything that distinguishes those two branches. They can record the fact that they did measure it, as this is the same in the two branches.
A person obviously can’t just “forget”, and will at best leak information into the environment, encoded as correlations in the noise of heat. A (reversible) computer, on the other hand, works quite well for doing this.