‘Observation’ is a shorthand (for historical reasons) for ‘interaction with a different system’, for example a detector or a human; but a rock will do as well.
I’m still confused. This seems to imply that there is no physical meaning to the term “observation,” only a meaning relative to whatever model we’re entertaining in a given instance. Specifically (as far as I know) there’s only one system of relevance, the Universe (or the Universe of Universes, if multiple worlds stuff means anything and we insist on ruining another perfectly clear English word), so it can’t interact with a different system except from the point of view of a particular mathematical model of a subset of that system. Edit: or is the word system a technical term too. Sigh.
Indeed, your point is well taken; it is precisely this sort of argument that makes the MWI (sorry if you dislike the phrase!) attractive. If we prepare an electron in a superposition of, say, spin-up and spin-down, then it makes good sense to say that the electron eventually interacts with the detector, or detector-plus-human, system. But hang on, how do we know that the detector doesn’t then go into a superposition of detecting-up and detecting-down, and the human into a superposition of seeing-the-detector-saying-up and seeing-the-detector-saying-down? Well, we don’t experience a superposition, but then we wouldn’t; we can only experience one brain state at a time!
Push this argument out to the whole universe and, as you rightly say, there’s no further system it can interact with; there’s no Final Observer to cause the collapse. (Although I’ve seen Christians use this as an argument for their god.) So the conclusion seems to be that there is no collapse, there’s just the point where the human’s wave function splits into two parts and we are consciously aware either of the up or down state. Now, there’s one weakness to this: It is really not clear why, if this is the explanation, we should get the Born probabilities.
So, to return to the collapse postulate, one popular theory is that ‘observation’ means “the system in superposition becomes very massive”: In other words, the electron interacts with the detector, and the detector-plus-electron system is in a superposition; but of course the detector is fantastically heavy on the scale of electrons, so this causes the collapse. (Or to put it differently, collapse is a process whose probability per unit time goes asymptotically to one as the mass increases.) In other words, ‘observation’ is taken as some process which occurs in the unification of QM with GR. This is a bit unsatisfactory in that it doesn’t account for the lack of unitarity and what-have-you, but at least it gives a physical interpretation to ‘observation’.
Indeed, your point is well taken; it is precisely this sort of argument that makes the MWI (sorry if you dislike the phrase!) attractive.
Yay! The rest of your argument seems sensible, but I’m too giddy to really understand it right now. I’ll just ask this: can you point me to a technical paper (Arxiv is fine) where they explain, in detail, exactly how they get a certain electron “in a superposition of, say, spin-up and spin-down”?
Well, I don’t know that I need to point you to arxiv, because I can describe the process in two sentences. Take a beam of electrons and pass it through a magnetic field which splits it into two beams, one going left and one going right. The ones which went left are spin-left, or to put it differently, they are spin-up with respect to the left-right axis; conversely the ones that went right have the opposite spin polarisation on that axis. Now rotate your axis ninety degrees; the electrons in both beams are in a perfect up-down superposition with respect to the new axis. If you rotate the axis less than ninety degrees you will get a different superposition.
Well, that’s helpful, but of course, I don’t know how you know that the electrons have such and such spin or what superposition has to do with anything. Neither could I reproduce the experiment (someone competent could, I’m sure). Maybe there was a first experiment where they did this and spin was discovered?
EDIT: anyway, I’m tapping out of here and will check out the sequences. Thanks All
I don’t know how you know that the electrons have such and such spin
Electrons have both electric charge and spin (which is a form of angular momentum), and in combination, these two properties create an intrinsic magnetic moment. A magnetic field exerts torque on anything with a magnetic moment, which causes the electron to precess if it is subjected to such a field. Because spin is quantized and has only two possible values for electrons (+1/2 or −1/2), they will only precess in two discrete ways. This can be used to separate the electrons by their spin values. The first experiment to do this was the Stern-Gerlach experiment, a classic in the early development of QM, and often considered to be the discovery of spin.
Four is equal-ish to two for large values of two, at least in the limit where four is small. Besides, the last sentence is a comment, not a description of the process, so it doesn’t count. :)
Thanks.
Edit:
I’m still confused. This seems to imply that there is no physical meaning to the term “observation,” only a meaning relative to whatever model we’re entertaining in a given instance. Specifically (as far as I know) there’s only one system of relevance, the Universe (or the Universe of Universes, if multiple worlds stuff means anything and we insist on ruining another perfectly clear English word), so it can’t interact with a different system except from the point of view of a particular mathematical model of a subset of that system. Edit: or is the word system a technical term too. Sigh.
Indeed, your point is well taken; it is precisely this sort of argument that makes the MWI (sorry if you dislike the phrase!) attractive. If we prepare an electron in a superposition of, say, spin-up and spin-down, then it makes good sense to say that the electron eventually interacts with the detector, or detector-plus-human, system. But hang on, how do we know that the detector doesn’t then go into a superposition of detecting-up and detecting-down, and the human into a superposition of seeing-the-detector-saying-up and seeing-the-detector-saying-down? Well, we don’t experience a superposition, but then we wouldn’t; we can only experience one brain state at a time!
Push this argument out to the whole universe and, as you rightly say, there’s no further system it can interact with; there’s no Final Observer to cause the collapse. (Although I’ve seen Christians use this as an argument for their god.) So the conclusion seems to be that there is no collapse, there’s just the point where the human’s wave function splits into two parts and we are consciously aware either of the up or down state. Now, there’s one weakness to this: It is really not clear why, if this is the explanation, we should get the Born probabilities.
So, to return to the collapse postulate, one popular theory is that ‘observation’ means “the system in superposition becomes very massive”: In other words, the electron interacts with the detector, and the detector-plus-electron system is in a superposition; but of course the detector is fantastically heavy on the scale of electrons, so this causes the collapse. (Or to put it differently, collapse is a process whose probability per unit time goes asymptotically to one as the mass increases.) In other words, ‘observation’ is taken as some process which occurs in the unification of QM with GR. This is a bit unsatisfactory in that it doesn’t account for the lack of unitarity and what-have-you, but at least it gives a physical interpretation to ‘observation’.
Yay! The rest of your argument seems sensible, but I’m too giddy to really understand it right now. I’ll just ask this: can you point me to a technical paper (Arxiv is fine) where they explain, in detail, exactly how they get a certain electron “in a superposition of, say, spin-up and spin-down”?
Well, I don’t know that I need to point you to arxiv, because I can describe the process in two sentences. Take a beam of electrons and pass it through a magnetic field which splits it into two beams, one going left and one going right. The ones which went left are spin-left, or to put it differently, they are spin-up with respect to the left-right axis; conversely the ones that went right have the opposite spin polarisation on that axis. Now rotate your axis ninety degrees; the electrons in both beams are in a perfect up-down superposition with respect to the new axis. If you rotate the axis less than ninety degrees you will get a different superposition.
Well, that’s helpful, but of course, I don’t know how you know that the electrons have such and such spin or what superposition has to do with anything. Neither could I reproduce the experiment (someone competent could, I’m sure). Maybe there was a first experiment where they did this and spin was discovered?
EDIT: anyway, I’m tapping out of here and will check out the sequences. Thanks All
Electrons have both electric charge and spin (which is a form of angular momentum), and in combination, these two properties create an intrinsic magnetic moment. A magnetic field exerts torque on anything with a magnetic moment, which causes the electron to precess if it is subjected to such a field. Because spin is quantized and has only two possible values for electrons (+1/2 or −1/2), they will only precess in two discrete ways. This can be used to separate the electrons by their spin values. The first experiment to do this was the Stern-Gerlach experiment, a classic in the early development of QM, and often considered to be the discovery of spin.
Thanks.
That was four sentences! D:
Four is equal-ish to two for large values of two, at least in the limit where four is small. Besides, the last sentence is a comment, not a description of the process, so it doesn’t count. :)