Copenhagen seems to imply that it [interference] cannot occur on any scale larger than a human observer.
I’m always skeptical when the opponent of an idea informs me of something ridiculous that the idea “seems to imply”. If it were a defender of Copenhagen drawing that implication, I would be more likely to join you in hooting with disdain.
I am far from an expert on fundamental physics, but I seem to recall someone once pooh-poohing the notion that QM and Copenhagen are in any sense tied to human observers. After all, said this author, we use QM to explain the big bang and there were no observers there. Sorry, I don’t remember who it was that made that point, so I can’t give you a quote.
I say “seems to imply” because its not really clear what Copenhagen does or does not imply, because it doesn’t really make predictions in every circumstance.
Copenhagen implies that when we make a measurement, the value we measure is in fact the one true value. In particular, there are not other worlds where the measurement returned other values. This is precisely the thing that distinguishes it from many worlds, which suggests that there are other worlds where the measurement returned other values.
By accepting that only one value of the measurement actually happens, you reject the possibility of one human civilization, where one thing was measured, interfering with a different human civilization, where a different thing was measured (because you don’t believe the other human civilization even exists).
That is not a prediction, it is a postdiction, and it is a postdiction that Copenhagen and MWI agree on. And that is, when you make a measurement, the result you get is the only result you get in the world that you are in. Copenhagen and MWI agree on this.
MWI implies that you could interact with other worlds where there was a different outcome, at least in principle. This is not a postdiction because we have never engineered such an intricate situation , and MWI and Copenhagen don’t agree on it. In fact this was proposed as a test of MWi vs. Copenhagen; build a quantum computer, upload a human into it, and then run the experiment I described above (to my knowledge, this is actually the first proposed use of a quantum computer).
Actually this isn’t really an “in principle” thing. If we ignore gravity (I suspect that if we understood gravity correctly we wouldn’t have to) and assume the universe has finitely many degrees of freedom, MWI predicts that all of these world lines will eventually converge in the future. It will just be a long time in the future, after all order in the universe has eroded and entropy is decreasing again. This is clearly not what Copenhagen says. In Copenhagen, it is possible that we never again return to the state of the universe at the big bang. In MWI this is not possible, because all systems with finitely many degrees of freedom are periodic.
Copenhagen seems to imply that it [interference] cannot occur on any scale larger than a human observer.
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I am far from an expert on fundamental physics, but I seem to recall someone once pooh-poohing the notion that QM and Copenhagen are in any sense tied to human observers
Copenhagen implies that under some circumstances, interference stops. That’s all that can be meant by “collapse”. Maybe above some length scale; maybe above some critical mass; maybe above some number of interacting particles—it’s fuzzy on the details. And of course, if that scale happens to be larger than, oh, say, a person, then you are branching and then having your branches destroyed all the time.
So yes, if everything is allowed to interfere as naively implied by the Schrödinger equation, you’re not talking about Copenhagen, you’re talking about MWI.
It’s not about scale, it’s about theory of measurement and what “observer” means. If electron 1 bounces off electron 2 in Copenhagen, electron 1 sees electron 2 as “collapsed” into one eigenstate. If electrons bounce in MWI, they see the entire spectrum of each other. However, this is really just a change what we’re looking at—whether we want to “observe” all the information about the electron (MWI), or just one instance of that information (Copenhagen). The reason to look at only one instance is because this is what corresponds to what people see—it’s what quantum physics looks like from inside.
I’m not aware of any examples of interference that are not explainable by the ordinary interpretation that uses collapse—I think it’s likely that some people are interchanging the different ideas of observer and not remembering that to describe entangled states in Copenhagen you need to do more work than that. Once a state is entangled you can’t describe a single (Copenhagen) observer by a pure state, which is probably what you’re thinking when you think of “destroying the other branches.”
What you are describing is to first compute the answer using Many Worlds, and then figure out where to apply the collapse in Copenhagen to not affect anything.
What I am saying is to compute the answer using quantum mechanics.
The way to do it correctly, Copenhagen style, is to say “okay, the electron goes through the plate with two holes in it. But since, from the perspective of the electron, it can’t go through two holes, the state of the electron on the other side should be entangled something like |10> + |01>. If we fast-forward to the screen, we get an interference pattern”
The way to do it correctly, MW style, is to say “okay, the electron has equal probability of going through each hole, so let’s split into two worlds with equal phase. The detector will then observe the superposition of the two worlds, something like |10> + |01>, except fast-forwarded to the screen, so there should be an interference pattern.”
If these two approaches look similar, there’s a reason. And it’s not that one is cribbing off the other! As you can see, introducing entanglement in the Copenhagen interpretation was definitely not arbitrary, but it is conceptually trickier than thinking through the same process using the MWI.
Does your understanding of Copenhagen Quantum Mechanics reject the conclusion of Many Worlds, that the universe is in superposition of many states, many of which can contain people, which can’t observe each other?
If not, I think this has become an argument about definitions.
I’m actually pretty sure the Copenhagen Interpretation isn’t complete/coherent enough to actually be turned into a computer program. It just waves it’s hands around the Measurement Problem. The Occam’s Razor justification for people want to make for Many Worlds needs to be made in comparison to the interpretations viable competitors like de Broglie Bohm and company.
I’m actually pretty sure the Copenhagen Interpretation isn’t complete/coherent enough to actually be turned into a computer program. It just waves it’s hands around the Measurement Problem.
I can’t argue with that.
The Occam’s Razor justification for people want to make for Many Worlds needs to be made in comparison to the interpretations viable competitors like de Broglie Bohm and company.
Bohm’s theory is one of those hidden variable theories, which according to EPR must have something like faster than light signaling?
Bohm’s theory is one of those hidden variable theories, which according to EPR must have something like faster than light signaling?
It is a hidden variable theory and as such it is non-local but the non-locality doesn’t imply that we can use it for ftl signalling. The main problems are a.) you need to do weird things to it to make it Lorentz invariant and B.) it is less parsimonious than Many Worlds (as all hidden variable theories probably will be since they add more variables!). On the other hand it returns the Born probabilities (ED: which I guess I would argue makes it parsimonious in a different way since it doesn’t have this added postulate).
I don’t really know enough to make the evaluation for myself. But my sense is that we as a community have done way to much talking about why MWI is better than CI (it obviously is) and not nearly enough thinking about the other alternatives.
Does your understanding of Copenhagen Quantum Mechanics reject the conclusion [...] that the universe is in superposition of many states?
Yes, it does. The Copenhagen interpretation says that when you observe the universe, your observation becomes right, and your model of the world should make a 100% certain retrodiction about what just happened. This is mathematically equivalent to letting the a MWI modeler know which world (or set of worlds with the same eigenstate of the observable) they’re in at some time.
However, in Copenhagen, the universe you observe is all there “is.” If I observe the electron with spin up, there is no other me that observes it with spin down. The probabilities in Copenhagen are more Bayesian than frequentist. Meanwhile in MWI the probabilities are frequencies of “actual” people measuring the electron. But since there is no such thing as an outside observer of the universe (that’s the point), the difference here doesn’t necessarily mean this isn’t an argument about definitions. :P
Your Copenhagen Interpretation looks like starting with Many Worlds, and then rejecting the implied invisible worlds as an additional assumption about reality.
My Copenhagen interpretation (the one I use to demonstrate ideas about the Copenhagen interpretation, not necessarily the interpretation I use when thinking about problems) looks like the Copenhagen Interpretation. And yes, it is close to what you said. But it’s not quite that simple, since all the math is preserved because of stuff like entanglement.
Copenhagen implies that under some circumstances, a non-deterministic and non-continuous process happens. (I find the phrase “interference stops” as misleading.) The circumstances aren’t defined by some scale, but, as Manfred says in the other reply, by what “observer” means. There is a completely analogous question in MWI, where, under some circumstances, branching occurs.
Another thing is, if Copenhagen = objective collapse in the LW parlance, then MWI isn’t the only alternative to Copenhagen.
What is the “analogous question” in MWI? I can make predictions in any situation using MWI without answering any such questions, whereas in Copenhagen I make different predictions depending on what notion of “observation” I use. This is one major reason I prefer MWI.
The analogous question is “when does branching occur?” Your predictions in MWI depend on what notion of “observation” you use, it is only less apparent in the formulation. To obtain some meaningful prediction, at least you have to specify what are the observables and what subsystem of the world corresponds to the observer and his mind states.
But I am not completely sure what you are speaking about. Maybe you can give a concrete example where MWI gives a unique answer while collapse formulation doesn’t?
Branching is not a physical phenomenon in MWI, it is a way humans talk about normal unitary evolution on large scales. It is not involved in making predictions, just in talking about them.
The typical example distinguishing MWI and Copenhagen is the following. Suppose I build a quantum computer which simulates a human. I then perform the following experiment. I send an electron through a slit, and have the quantum computer measure which slit it went through (that is, I tell the human being simulated which slit it went through). I then stop the electron, and let the simulated human contemplate his observation for a while. Afterwards, it is still possible for me to “uncompute” the simulated human’s memory (just as it is in principle possible to uncompute a real human’s state) and make him forget which slit the electron went through. The electron then proceeds to the screen and hits it. Is the electron distributed according to an interference pattern, or not?
If you think the answer to that question is obvious in Copenhagen, because the simulated human is obviously not an observer, then suppose instead that I replace the simulated human with a real human, maintained in such a carefully controlled environment that I can uncompute his observation (technically unrealistic, but theoretically perfectly possible).
If the answer to that question is also obvious, suppose I replace the real human with the entire planet earth.
MWI predicts an interference pattern in all of these cases. However, Copenhagen’s prediction seems to depend on exactly which of those experiments have “observation” in them. Can a quantum computer observe? Can a single isolated human observe? Can an isolated planet observe? Does collapse occur precisely when “there is no possible way to uncompute the result”? The last would give the same predictions as MWI by design, but is really an astoundingly complex axiom and probably is never satisfied.
Oh, I’m all in favor of MWI. I just don’t think we should claim that it makes different predictions from Copenhagen based simply on our scorn for Copenhagen.
I’m always skeptical when the opponent of an idea informs me of something ridiculous that the idea “seems to imply”. If it were a defender of Copenhagen drawing that implication, I would be more likely to join you in hooting with disdain.
I am far from an expert on fundamental physics, but I seem to recall someone once pooh-poohing the notion that QM and Copenhagen are in any sense tied to human observers. After all, said this author, we use QM to explain the big bang and there were no observers there. Sorry, I don’t remember who it was that made that point, so I can’t give you a quote.
Does anyone else remember this better than me?
I say “seems to imply” because its not really clear what Copenhagen does or does not imply, because it doesn’t really make predictions in every circumstance.
Copenhagen implies that when we make a measurement, the value we measure is in fact the one true value. In particular, there are not other worlds where the measurement returned other values. This is precisely the thing that distinguishes it from many worlds, which suggests that there are other worlds where the measurement returned other values.
By accepting that only one value of the measurement actually happens, you reject the possibility of one human civilization, where one thing was measured, interfering with a different human civilization, where a different thing was measured (because you don’t believe the other human civilization even exists).
That is not a prediction, it is a postdiction, and it is a postdiction that Copenhagen and MWI agree on. And that is, when you make a measurement, the result you get is the only result you get in the world that you are in. Copenhagen and MWI agree on this.
MWI implies that you could interact with other worlds where there was a different outcome, at least in principle. This is not a postdiction because we have never engineered such an intricate situation , and MWI and Copenhagen don’t agree on it. In fact this was proposed as a test of MWi vs. Copenhagen; build a quantum computer, upload a human into it, and then run the experiment I described above (to my knowledge, this is actually the first proposed use of a quantum computer).
Actually this isn’t really an “in principle” thing. If we ignore gravity (I suspect that if we understood gravity correctly we wouldn’t have to) and assume the universe has finitely many degrees of freedom, MWI predicts that all of these world lines will eventually converge in the future. It will just be a long time in the future, after all order in the universe has eroded and entropy is decreasing again. This is clearly not what Copenhagen says. In Copenhagen, it is possible that we never again return to the state of the universe at the big bang. In MWI this is not possible, because all systems with finitely many degrees of freedom are periodic.
Copenhagen implies that under some circumstances, interference stops. That’s all that can be meant by “collapse”. Maybe above some length scale; maybe above some critical mass; maybe above some number of interacting particles—it’s fuzzy on the details. And of course, if that scale happens to be larger than, oh, say, a person, then you are branching and then having your branches destroyed all the time.
So yes, if everything is allowed to interfere as naively implied by the Schrödinger equation, you’re not talking about Copenhagen, you’re talking about MWI.
It’s not about scale, it’s about theory of measurement and what “observer” means. If electron 1 bounces off electron 2 in Copenhagen, electron 1 sees electron 2 as “collapsed” into one eigenstate. If electrons bounce in MWI, they see the entire spectrum of each other. However, this is really just a change what we’re looking at—whether we want to “observe” all the information about the electron (MWI), or just one instance of that information (Copenhagen). The reason to look at only one instance is because this is what corresponds to what people see—it’s what quantum physics looks like from inside.
I’m not aware of any examples of interference that are not explainable by the ordinary interpretation that uses collapse—I think it’s likely that some people are interchanging the different ideas of observer and not remembering that to describe entangled states in Copenhagen you need to do more work than that. Once a state is entangled you can’t describe a single (Copenhagen) observer by a pure state, which is probably what you’re thinking when you think of “destroying the other branches.”
What you are describing is to first compute the answer using Many Worlds, and then figure out where to apply the collapse in Copenhagen to not affect anything.
No.
What I am saying is to compute the answer using quantum mechanics.
The way to do it correctly, Copenhagen style, is to say “okay, the electron goes through the plate with two holes in it. But since, from the perspective of the electron, it can’t go through two holes, the state of the electron on the other side should be entangled something like |10> + |01>. If we fast-forward to the screen, we get an interference pattern”
The way to do it correctly, MW style, is to say “okay, the electron has equal probability of going through each hole, so let’s split into two worlds with equal phase. The detector will then observe the superposition of the two worlds, something like |10> + |01>, except fast-forwarded to the screen, so there should be an interference pattern.”
If these two approaches look similar, there’s a reason. And it’s not that one is cribbing off the other! As you can see, introducing entanglement in the Copenhagen interpretation was definitely not arbitrary, but it is conceptually trickier than thinking through the same process using the MWI.
Does your understanding of Copenhagen Quantum Mechanics reject the conclusion of Many Worlds, that the universe is in superposition of many states, many of which can contain people, which can’t observe each other?
If not, I think this has become an argument about definitions.
I’m actually pretty sure the Copenhagen Interpretation isn’t complete/coherent enough to actually be turned into a computer program. It just waves it’s hands around the Measurement Problem. The Occam’s Razor justification for people want to make for Many Worlds needs to be made in comparison to the interpretations viable competitors like de Broglie Bohm and company.
I can’t argue with that.
Bohm’s theory is one of those hidden variable theories, which according to EPR must have something like faster than light signaling?
It is a hidden variable theory and as such it is non-local but the non-locality doesn’t imply that we can use it for ftl signalling. The main problems are a.) you need to do weird things to it to make it Lorentz invariant and B.) it is less parsimonious than Many Worlds (as all hidden variable theories probably will be since they add more variables!). On the other hand it returns the Born probabilities (ED: which I guess I would argue makes it parsimonious in a different way since it doesn’t have this added postulate).
I don’t really know enough to make the evaluation for myself. But my sense is that we as a community have done way to much talking about why MWI is better than CI (it obviously is) and not nearly enough thinking about the other alternatives.
Yes, it does. The Copenhagen interpretation says that when you observe the universe, your observation becomes right, and your model of the world should make a 100% certain retrodiction about what just happened. This is mathematically equivalent to letting the a MWI modeler know which world (or set of worlds with the same eigenstate of the observable) they’re in at some time.
However, in Copenhagen, the universe you observe is all there “is.” If I observe the electron with spin up, there is no other me that observes it with spin down. The probabilities in Copenhagen are more Bayesian than frequentist. Meanwhile in MWI the probabilities are frequencies of “actual” people measuring the electron. But since there is no such thing as an outside observer of the universe (that’s the point), the difference here doesn’t necessarily mean this isn’t an argument about definitions. :P
Your Copenhagen Interpretation looks like starting with Many Worlds, and then rejecting the implied invisible worlds as an additional assumption about reality.
My Copenhagen interpretation (the one I use to demonstrate ideas about the Copenhagen interpretation, not necessarily the interpretation I use when thinking about problems) looks like the Copenhagen Interpretation. And yes, it is close to what you said. But it’s not quite that simple, since all the math is preserved because of stuff like entanglement.
Copenhagen implies that under some circumstances, a non-deterministic and non-continuous process happens. (I find the phrase “interference stops” as misleading.) The circumstances aren’t defined by some scale, but, as Manfred says in the other reply, by what “observer” means. There is a completely analogous question in MWI, where, under some circumstances, branching occurs.
Another thing is, if Copenhagen = objective collapse in the LW parlance, then MWI isn’t the only alternative to Copenhagen.
What is the “analogous question” in MWI? I can make predictions in any situation using MWI without answering any such questions, whereas in Copenhagen I make different predictions depending on what notion of “observation” I use. This is one major reason I prefer MWI.
The analogous question is “when does branching occur?” Your predictions in MWI depend on what notion of “observation” you use, it is only less apparent in the formulation. To obtain some meaningful prediction, at least you have to specify what are the observables and what subsystem of the world corresponds to the observer and his mind states.
But I am not completely sure what you are speaking about. Maybe you can give a concrete example where MWI gives a unique answer while collapse formulation doesn’t?
Branching is not a physical phenomenon in MWI, it is a way humans talk about normal unitary evolution on large scales. It is not involved in making predictions, just in talking about them.
The typical example distinguishing MWI and Copenhagen is the following. Suppose I build a quantum computer which simulates a human. I then perform the following experiment. I send an electron through a slit, and have the quantum computer measure which slit it went through (that is, I tell the human being simulated which slit it went through). I then stop the electron, and let the simulated human contemplate his observation for a while. Afterwards, it is still possible for me to “uncompute” the simulated human’s memory (just as it is in principle possible to uncompute a real human’s state) and make him forget which slit the electron went through. The electron then proceeds to the screen and hits it. Is the electron distributed according to an interference pattern, or not?
If you think the answer to that question is obvious in Copenhagen, because the simulated human is obviously not an observer, then suppose instead that I replace the simulated human with a real human, maintained in such a carefully controlled environment that I can uncompute his observation (technically unrealistic, but theoretically perfectly possible).
If the answer to that question is also obvious, suppose I replace the real human with the entire planet earth.
MWI predicts an interference pattern in all of these cases. However, Copenhagen’s prediction seems to depend on exactly which of those experiments have “observation” in them. Can a quantum computer observe? Can a single isolated human observe? Can an isolated planet observe? Does collapse occur precisely when “there is no possible way to uncompute the result”? The last would give the same predictions as MWI by design, but is really an astoundingly complex axiom and probably is never satisfied.
Oh, I’m all in favor of MWI. I just don’t think we should claim that it makes different predictions from Copenhagen based simply on our scorn for Copenhagen.
Surely that isn’t the reason:
Q16 Is many-worlds (just) an interpretation?
Q36 What unique predictions does many-worlds make?