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Wait a minute. As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human interven
by jonathanstrange 5y ago
Wait a minute. As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human intervention and that this can be shown. Isn't "measurement" just an interaction of one physical system with another? If so, why would anyone include subjective degrees of belief in a formulation of QM?
Could a physicist please clarify this?
- monktastic1 5y agoWhen one system measures another, there is no collapse, only entanglement. The superposition just grows. As trillions of environmental particles entangle with the system, that superposition becomes in practice -- but not in principle -- impossible to detect or exploit. This is called decoherence, and it is often wrongly used to explain away the issue. When does "collapse" happen in principle? There's no clear answer. Yet, when you observe a system yourself, you are free to say it has one result (at least, in "your world / branch" if we use MWI lingo). So there is (possibly) a sense in which you are the only observer -- at least, w.r.t. the place you think of as "the world." (And of course when we say "you observe," we mean "you experience". If only your toe measures it, you may treat your toe as an external measuring device. This is how "consciousness" enters the picture, though nobody agrees on what exactly it means.)
- phkahler 5y ago>> When does "collapse" happen in principle? There's no clear answer. It does not happen at all. There is no measurable distinction between a particle whose wave function has collapsed and one that hasn't. If there were a discernable difference the "spooky action at a distance" when an entangled pair is collapsed could be used for faster than light communication. >> When one system measures another, there is no collapse, only entanglement. This is the first time I've seen this explanation. To me it feels like an attempt at resolving the issue, but I think not. The right answer AFAICT is that we must embrace non-locality. But that's still another sort of punt.
- monktastic1 5y agoI put "collapse" in quotes for a reason. There is an experimentally verifiable distinction between a particle in superposition (w.r.t a particular basis) and one that is not. Both theory and experiment give us every reason to believe that prior to your observation, you can exhibit the superposition through an interference experiment (modulo decoherence), while afterward you cannot (even if "God" still can). Also, this surprises me (assuming you studied QM formally): > This is the first time I've seen this explanation. Entanglement is basically the definition of measurement (in this context).
- kgwgk 5y agoI’m not sure of what is your context but that unitary interaction is just the first step (also called pre-measurement) in von Neumann’s model of measurement.
- monktastic1 5y agoIndeed. The context here is: > As a non-physicist who only knows QM from popular science texts, I always thought that wave function collapse does not require any human intervention and that this can be shown. When pop sci treatments explain that any physical system interacting with the apparatus "measures" it, they are just talking about what you call "pre-measurement." This causes a lot of confusion for students.
- kgwgk 5y agoI agree, I think. The observed “collapse” is not completely explained by decoherence (and even less satisfactory by many worlds).
- deleted 5y ago[deleted]
- rkagerer 5y agoIf only your toe measures it, you may treat your toe as an external measuring device. But from the viewpoint of the toe would it have collapsed? Extrapolating that, why isn't your consciousness just another measuring device that's become entangled with the system?
- monktastic1 5y agoYes, if you assume the toe has "a viewpoint," then from its viewpoint, things have collapsed. But as you are not your toe, this has no observable consequence for you. It's pure philosophy. Similarly, your consciousness could be considered just another measuring device from the perspective of someone outside the system. But by virtue of being "outside the system," their results have no bearing on you either. You are the point at which "the magic happens" in your world -- which is just the place you normally call "the" world. It is the point at which neither you nor anyone else you can communicate with (i.e., no one else in "the world") can demonstrate interference, even in principle.
- Jensson 5y agoExactly what is required to cause it to collapse is still not a solved problem. But no, it isn't humans causing it, since it alters the state you can see that it happened afterwards in a system and systems works the same regardless if humans watch them or not.
- TchoBeer 5y ago>since it alters the state you can see that it happened afterwards in a system What experiment shows this?
- Jensson 5y agohttps://en.wikipedia.org/wiki/Double-slit_experiment https://en.wikipedia.org/wiki/Double-slit_experiment
- simiones 5y agoThe version of the double-slit experiment with a detector on one of the slits can be taken to show this. As you know, in the classical double-slit experiment, the screen is the only detector, and launching particles even one at a time through the slits produces the interference pattern. However, if you add a second detector in one of the slits capable of measuring if a particle passes through that slit or not, the interference pattern on the screen disappears, regardless of how many particles you send through. This is normally taken to mean that the slit detector collapses the wave function (in Copenhagen parlance); or that one particular state of the particle becomes entangled with the initial detector, which is already entangled with the screen, to there is no more constructive interference to be seen (in Many Worlds terms).
- dekhn 5y agoI believe you're referring to delayed choice quantum eraser?
- simiones 5y agoNo, that's a different, more complex experiment. This is just about adding a detector to one of the slits so you can tell which slit each electron is passing through. Here is a more complete explanation: https://phys.org/news/2011-01-which-way-detector-mystery-double-slit.html https://phys.org/news/2011-01-which-way-detector-mystery-dou... The delayed choice quantum eraser is a much more complex experiment.
- dekhn 5y agoQM works even if there are no humans or other subjective "observers". Any theory of QM that involves subjective observers for wavefunction collapse is quantum woo.
- drdeca 5y agoThe former doesn’t imply the latter. I suspect one could think of QBism as a theory of rational belief for agents in a context where details of quantum mechanics are highly relevant. Just as bayesianism is a theory of rational belief more generally.
- superposeur 5y agoYes, most physicists agree that human observers are not required to collapse a wave function. Even the QBists speak of “agents” that are abstract entities, not necessarily human beings. (An agent is something like a “point of view” or a “reference frame” in that it can be thought to exist independently of a living, breathing human who inhabits it.) However, n.b. that the great Nobelist Eugene Wigner did seriously think it had to be an actual human consciousness. (Or a dog, but not an insect.) Full disclosure that I’ve never really been able to pin down the QBist notion of an agent myself and do not find QBism (or any such “instrumentalist” interpretations” of QM) appealing.
- dekhn 5y agotheir use of the terminology of agent is wrong. agents have the ability to make their own decisions independently. Reference frames are just mathematical coordinate conversions, not agents.
- kgwgk 5y ago> Even the QBists speak of “agents” that are abstract entities, Do you have a reference for that?
- cosmophany 5y agoA reference is points 17 and 18 in [https://arxiv.org/abs/1810.13401 https://arxiv.org/abs/1810.13401] Long story short, QBism inherits "agent" from Bayesian decision theory. In that context, we talk about gamblers making bets on the outcomes of dice rolls. In QBism, we talk about scientists making bets on the outcomes of measurements. What exactly can be a "gambler?" Can a dog gamble? What about a trained dog? Or a flea? The answer is: if the shoe fits, wear it. Same goes for QBism. If a dog walks into a lab, lights up a pipe, scribbles equations on the blackboard and starts aligning beamsplitters, then QBism regards him as an agent.
- kgwgk 5y agoA dog is not something like a “point of view” or a “reference frame” in that it can be thought to exist independently of a living, breathing being. A personalistic interpretation requires a "person". Would a rock or a cell phone qualify as a person? I'm not even sure the authors of that reference would accept a dog, despite saying the concept is extremely flexible (they discuss agents "social" agents but not "dumb" agents). It's not clear to me that the mathematical description can exist independently of the agent that has some knowledge, for some definition of agent. Even if it can be make to work with no-one's "abstract knowledge" what would be the physical meaning of that? (Not that the physical meaning when agents are present is clear, to be fair.)
- joeberon 5y ago> Isn't "measurement" just an interaction of one physical system with another? We don't have an accepted theory on what "measurement" is in Quantum Mechanics yet.
- wizzwizz4 5y ago> I always thought that wave function collapse does not require any human intervention and that this can be shown. It is, in principle, impossible to determine this, because eventually humans will see the output of the experiments. However, we can say that if humans are special, the only things able to cause “measurement”, then our specialness also gives us time travel powers that can only be used for “measurement”, and Occam's Razor says that's probably nonsense.
- MrYellowP 5y ago> then our specialness also gives us time travel powers Why? Because the Quantum Eraser Experiment comes out as it does?
- wizzwizz4 5y agoThe delayed choice quantum eraser experiment works too, but I was thinking of a two-channel (CHSH) Bell test where the observations are made separated by a few light-minutes, then later compared with each other (and somehow you eliminate enough noise that you don't need a coincidence monitor).
- cosmophany 5y ago(physicist here). I think monktastic1 mostly nailed it, I just want to add some further clarifications. Take the electron double-slit experiment. Suppose no human checks which slit the electron goes through, but the "which-path" information leaks out into the environment somehow. The interference fringes will be destroyed anyway (called "environmental decoherence"). This is usually framed by saying "oh look, the environment measured the system". In fact, what happened is that (1) you prepared what you believed to be an electron beam in a coherent superposition; (2) you left it open to the environment; (3) you used quantum theory to model the system-plus-environment and predicted that you would not see interference fringes when you check; (4) YOU observed the dots on the glass plate and confirmed that there were no interference fringes. The whole story is told in terms of what YOU believed, expected, and finally what you saw. That is QBism's whole point. Put it another way: in QBism wavefunctions just ARE sets of probability assignments to certain actual events that could be observed. And probabilities are human constructions: we invented them to help us reason about our expectations. So, without humans (or other beings capable of reasoning about expectations), there are no probabilities, hence no wavefunctions either. But stuff still happens, even when there's nobody around to see it or assign probabilities to it. The key conceptual leap is separating "wavefunction" completely from "reality". There's more to quantum physics than just the wavefunctions -- the reality lies in the events that happen, not in the quantum state itself.