[HN Gopher] Quantum mechanics and our part in creating reality
___________________________________________________________________
Quantum mechanics and our part in creating reality
Author : cosmophany
Score : 43 points
Date : 2021-09-08 16:15 UTC (1 days ago)
(HTM) web link (iai.tv)
(TXT) w3m dump (iai.tv)
| katabasis wrote:
| Carlo Rovelli's book Helgoland[1] is a short and accessible
| account of the various philosophical interpretations of quantum
| phenomena, along with their shortcomings. Highly recommended to
| anyone interested in the topic.
|
| Rovelli is a proponent of what he calls the "relational
| interpretation" of QM, which basically states that particles only
| have properties when they interact with other systems (and may be
| better thought of as events than persistent objects).
| Entanglement is a general phenomenon that describes the
| relationship between two interacting systems and a third which
| hasn't interacted yet. Put another way, something can be real in
| relation to A but not to B. However, the correlation that
| entanglement guarantees ensures that previously isolated systems
| will see the same things if they do eventually intersect.
|
| It's different from QBism because there is no special privileging
| of human observers; all physical systems exist in direct
| interaction with some systems, are in superpositions in relation
| to others, etc.
|
| [1]: https://bookshop.org/books/helgoland-making-sense-of-the-
| qua...
| raphlinus wrote:
| This sounds to me not unlike the "zero worlds interpretation"
| in Ron Garret's talk. Are you familiar with it and would you
| happen to know if it's roughly the same or whether there are
| substantial differences?
|
| [1]: https://blog.theangry.dev/2017/01/05/ron-garrets-the-
| quantum...
| katabasis wrote:
| If "zero worlds" means there is no unified single reality
| that all physical systems relate to (just like there is no
| universal frame of reference for motion according to
| relativity) then I think we're in similar territory.
|
| I'm not familiar with Garret, but Rovelli's book contains a
| great discussion of the "quantum eraser" experiment in that
| blog post, where quantum interference causes a split laser
| beam to recombine in different ways whether or not a detector
| is present. This is something you can observe with tabletop
| equipment apparently, a pretty visceral demonstration that
| quantum phenomena are a real part of our world.
| smallerfish wrote:
| I read Helgoland at the weekend (in a hammock at a glamping
| lodge in the jungle) and have been meaning since to buy
| some prisms to see if I can replicate that experiment. Do
| you happen to know if it's as simple as he describes it, or
| is there a trick to making it work?
| katabasis wrote:
| Unless you have a light source that can emit a beam
| consisting of only a few photons (and precise enough
| detectors to see them on the other side) you won't see
| the effect. Otherwise, the quantum interference
| demonstrated by the experiment will break down due to
| what is called "decoherence" - too many things
| interacting with one another will cancel out the
| phenomenon you want to observe. This is why we don't see
| quantum weirdness in our every day experience.
| smallerfish wrote:
| Interesting. How precise a laser would you need? And
| typing "photon detector" into amazon is not exactly
| yielding anything promising - is there equipment
| obtainable by somebody who's casually interested, or is
| this prohibitively expensive?
| dekhn wrote:
| If you want to do a basic quantum eraser, the simplest
| path is this: https://www.thorlabs.com/newgrouppage9.cfm?
| objectgroup_id=69....
|
| A good physics undergrad could assemble this and
| demonstrate it successfully in a day. It's really a Mach
| Zender interferometer (https://en.wikipedia.org/wiki/Mach
| %E2%80%93Zehnder_interfero...) which is one of the nicest
| designs for doing quantum information experiments.
|
| The laser they use isn't anything super-special, although
| it isn't like what you'd get if you bought a laser
| pointer or a cheap laser engraver. The beam size is very
| fine and the beam shape is round. Most of the other parts
| are fairly generic but precisely made. The most exciting
| part is the beamsplitters,
| https://en.wikipedia.org/wiki/Beam_splitter since they
| are key to assembling the interferometer itself.
| katabasis wrote:
| In Rovelli's book he talks about going to Anton
| Zeilinger's laboratory to see the experiment in person. I
| assume you'd need a proper lab with carefully calibrated
| equipment for something like this, but I'm not a
| physicist.
|
| Even so, an effect that can be demonstrated with tabletop
| equipment in a lab seems more intuitively real to me than
| something that requires a giant particle accelerator...
|
| 100 or 200 years ago a more or less average person could
| set up cutting-edge scientific experiments with basic
| equipment and patient observation - this was the modus
| operandi of Michael Faraday[1] for example. Sadly it
| seems like those days are mostly behind us now.
|
| [1]: https://en.wikipedia.org/wiki/Michael_Faraday
| dekhn wrote:
| 100 years ago, to do the Michelson Morely experiment
| (imho the most important scientific experiment done yet,
| except perhaps LIGO) required heroics and could not have
| been done by mere mortals. It required floating a massive
| appartus on a bed of mercury in a subbasement and was
| still hard to run during the day due to horse-based
| deliveries a few buildings over.
|
| My friend did the experiment in an afternoon Physics lab
| (princeton), almost all of of this is due to innovations
| in precision manufacturing and material science.
| naasking wrote:
| https://en.wikipedia.org/wiki/Relational_quantum_mechanics
| katabasis wrote:
| This is also good: https://plato.stanford.edu/entries/qm-
| relational/
| neonate wrote:
| https://archive.is/1DagJ
| 6gvONxR4sf7o wrote:
| I don't understand QBism well enough to really comment, but from
| the outside it seems like QM interpretations are taking the
| measurement problem increasingly seriously.
|
| I like that the writer is up front in how all we really know
| right know is 'shut up and calculate' and wish that it had been
| taught with such honesty when I was taught it in school, rather
| than basically doing everything from the copenhagen
| interpretation and pretending that that isn't fundamentally
| philosophically weird. The observer and measurement in the 'shut
| up and calculate' school is much more believable than in the
| copenhagen school where people (or at least things people do,
| like taking measurements and observing observables) are suddenly
| jarringly special for the first time in a physicist's training.
| simiones wrote:
| Modern Copenhagen is basically an evolution from "shut up and
| calculate". It also doesn't given (human/conscious) observers
| any special role in QM - it is the measurement apparatus that
| has a special role instead.
|
| Basically the initial approach was ontological: science is
| about what can be measured, when quantum properties are
| measured we need to apply the Born rule to predict outcomes, it
| makes no sense to discuss the scientific properties of what is
| by definition not measured, so shut up and calculate.
|
| Later, the approach was more metaphysical: since our math
| doesn't work if we assume that particles have definite
| properties, we conclude that particles don't have definite
| properties; instead, when we measure their properties in a
| particular basis, the particles randomly acquire some definite
| property with a probability corresponding to the amplitude of
| the wave function.
| 6gvONxR4sf7o wrote:
| Sure, that's probably a more precise way to say my complaint.
| The special role of taking a measurement/observing an
| observable. Unpacking what is and isn't measurement was what
| drove me nuts in undergrad and I never got an answer that
| satisfied me. We learned how to calculate the probabilities
| of each thing that would be measured, but not what counts as
| measurement.
|
| I guess all I'm saying is that, in hindsight, I would have
| done so much better with a clearer delineation of 'this part
| over here is an interpretation question and isn't settled,
| but this other part over there is math that is settled
| empirically' rather than being taught 'here's an
| interpretation that's true and here's the math for it.'
| aeternum wrote:
| Yes, some schools are still teaching that the delayed
| choice quantum eraser involves time-travel. That probably
| does more harm than good.
| spywaregorilla wrote:
| Was the human conscious part ever a serious belief among
| actual physicists historically? I'm no physicist but it
| seemed obvious to me immediately that that could not possibly
| be the case.
| dekhn wrote:
| Yes. Through grad school the physics PhDs kept bringing it
| up. I believe Wigner was the only Wise One who actually
| pursued it
| (https://en.wikipedia.org/wiki/Wigner%27s_friend)
|
| Since Wigner was a Wise One
| (https://en.wikipedia.org/wiki/Wigner%27s_theorem) people
| took the idea seriously, but these days, most physicists
| will simply ask you to show them a comprehensible and
| reproducible experiment that demonstrates the requirement
| of a human observer and reject your belief based on the
| absence of data and occam's razor.
| codethief wrote:
| Not a belief I would say, but it was seriously considered.
| Many physicists, including some of the founding fathers of
| QM, thought about it, see e.g. https://en.wikipedia.org/wik
| i/Von_Neumann%E2%80%93Wigner_int...
| Strilanc wrote:
| I've never found epistemic interpretations compelling (like
| QBism, which is what the article is about). I'll try to explain
| why.
|
| First, the stuff we do with quantum mechanics just seems sort
| of... really obviously objective? It's not like Bell tests work
| for me but not for you, or quantum computers factor numbers for
| me but not for you. If I sell you a machine to produce a specific
| superposition, and it produces the wrong one, you can tell. Yes
| you need to run it multiple times and do statistics, but
| _ultimately anyone can check that the machine is doing the wrong
| thing_.
|
| Second, two agents can't actually disagree on what the
| superposition of a state is, to any appreciable degree, without
| one of them being provably irrational (modulo some weak-sauce
| assumptions we take for absolute granted in classical mechanics).
| [1]
|
| Third, note that (w.l.o.g) diagonally polarized photons are
| superpositions of horizontally and vertically polarized photons.
| This suggests that, in an interpretation where superpositions are
| less real than classical states, slightly rotating a polarizing
| filter radically changes the conceptual machinery used to
| understand a photon passing through the filter. That seems...
| silly? There is a rotational symmetry in the system that should
| be natural in the model (e.g. see how special relativity reifies
| Lorentz boosts).
|
| Fourth, although this is ironically subjective, I've never read
| an epistemic explanation of some quantum phenomenon and thought
| "Ah, that makes it clearer!". But that _has_ happened with
| collapse interpretations, with path integral interpretations, and
| with many-worlds.
|
| [1]: https://en.wikipedia.org/wiki/PBR_theorem
| kgwgk wrote:
| > in an interpretation where superpositions are less real than
| classical states
|
| What do you mean by "classical states"?
|
| In your example diagonally polarized photons are not more "a
| superposition" than any other photon. For a pure state like
| that one being or not "a superposition" depends on the basis,
| it's not a property of the state.
| Strilanc wrote:
| In this case the "classical state" would just be a preferred
| basis for the polarization. And basically what I'm saying is
| "If superpositions of polarizations aren't real, why is there
| such a natural way to change the basis so that any given
| polarization isn't in superposition?".
|
| Analogy: if simultaneity is absolute, why is it so easy to
| confuse people about what happened at the same time by
| changing my speed?
| kgwgk wrote:
| I don't see how does it relate with the interpretations of
| QM and QBism in particular.
|
| Superpositions are not "less real" than basis states (if
| such a distinction makes sense) within a theory (psi-
| epistemic or psi-ontological) as far as I understand.
| TheOtherHobbes wrote:
| You're confusing statistics with events.
|
| Superpositions are statistical distributions. Only statistics
| are wholly predictable. Individual events are wholly
| unpredictable.
|
| You can have two statistical views of the same experiment in
| which each individual event is different but the statistics
| agree.
|
| You can have a distribution of views in which events are
| somewhat different to varying degrees and the statistics still
| agree.
|
| Each view will experience itself as unique in specifics while
| agreeing that the distribution of events is identical.
|
| That doesn't solve the problem.
|
| It's _hard_ to get random processes to define an identical
| distribution without having classical mechanics as a
| foundation.
|
| Basic QM hand-waves this away with "Here's some math that
| defines the distribution and..." But that's like "Starting with
| this universe we can..."
|
| It so happens that the statistics correlate spatially with
| various field potentials. Which is interesting, but... Why is
| there a distribution at all? Why are there creation/destruction
| operators? Where does the "code" that defines this mechanism
| live? How do particles know they should follow it?
|
| Do particles even exist, or are they just something random and
| constrained a field does every so often? How do fields know the
| physical configuration of an experiment has changed and
| sometimes that information appears to travel FTL? (Even though
| it can't be used for signalling.)
|
| How does a particle keep track of how many other entities it's
| entangled with and in what ways? Where does _that_ information
| live? (Bell suggests it 's not inside the particle itself. So
| where is it?)
|
| Is all of this shaped by some kind of hidden causal propagation
| mechanism which also defines how relativity works?
|
| And so on. A complete explanation would answer all of these
| questions - and others - with ease. Clearly we're nowhere near
| that.
| kgwgk wrote:
| I think Strilanc's second point is that apparently according
| to QBism two agents may have different quantum descriptions
| for a system and we cannot say that one is more correct than
| the other because the theory rejects that there is a
| "correct" description.
| Strilanc wrote:
| That's actually the opposite of what the PBR theorem shows
| (follow the wikipedia reference I included).
| Strilanc wrote:
| > You're confusing statistics with events.
|
| I'm aware of the distinction.
|
| > How does a particle keep track of how many other entities
| it's entangled with and in what ways? Where does that
| information live? [...] A complete explanation would answer
| all of these questions - and others - with ease.
|
| I disagree with this framing. Fundamental laws of physics are
| always going to have unanswered questions of this type. Given
| any set of rules you can ask "But what explains those rules?
| What are they built out of? How are they enforced?".
| Sometimes those questions will have answers and lead you
| deeper, but for truly foundational laws you'll be wasting
| your time. It'd be like asking "Where is the true platonic
| number 2 located? Is it in Canada?".
|
| You can ask the same questions of classical mechanics, of
| course. We tend not to because it agrees with our intuitions,
| but you can. For example, where would a classical particle
| store its velocity? For that matter, where would it store its
| _position_? It 's circular to say it stores its position
| where it is! Clearly a "true" theory of classical mechanics
| would answer these very important questions.
|
| Concretely, there are a variety of ways of writing programs
| that act like quantum mechanics, that differ wildly on how
| the state is represented. This detail is simply not pinned
| down by the postulates of QM. That being said, what all these
| programs do have in common is that they are actually tracking
| information related the state; that the state is ontic.
|
| So pick your favorite state representation: state vector,
| density matrices, Feynman paths, whatever, they all work!
| That doesn't mean they're describing things that _aren 't
| real_, it just means there's many ways of correctly
| describing reality; such convenience!
| Strilanc wrote:
| I should probably add that I know everything I've said
| isn't _convincing_ to a skeptic. Ultimately it comes down
| to: I know I can think of the quantum state as being really
| real, and that will work totally fine. I find that style of
| thinking is effective for me, and intuitively compelling,
| so I do it.
|
| Every once in awhile I'll run into someone pointing out the
| philosophically fraught underpinnings of assuming reality
| is real or whatever, and I basically won't care because my
| goal is to be effective; not to be Descartes-level-certain
| about everything.
|
| An example of something that would make me care is if QBism
| contained some key conceptual trick that made problems
| easier, and gradually many papers started using it because
| of this advantage. Or, of course, if there was an
| experiment distinguishing between interpretations.
| dbsmith83 wrote:
| I'm no physicist, but the QBism interpretation of quantum
| mechanics smells a lot like "The Secret" type of pseudoscience.
|
| Core positions of QBism:
|
| 1. All probabilities, including those equal to zero or one, are
| valuations that an agent ascribes to his or her degrees of belief
| in possible outcomes. As they define and update probabilities,
| quantum states (density operators), channels (completely positive
| trace-preserving maps), and measurements (positive operator-
| valued measures) are also the personal judgements of an agent.
|
| 2. The Born rule is normative, not descriptive. It is a relation
| to which an agent should strive to adhere in his or her
| probability and quantum state assignments.
|
| 3. Quantum measurement outcomes are personal experiences for the
| agent gambling on them. Different agents may confer and agree
| upon the consequences of a measurement, but the outcome is the
| experience each of them individually has.
|
| 4. A measurement apparatus is conceptually an extension of the
| agent. It should be considered analogous to a sense organ or
| prosthetic limb--simultaneously a tool and a part of the
| individual.
|
| https://en.wikipedia.org/wiki/Quantum_Bayesianism#Core_posit...
|
| I'd recommend reading the wiki page since this article seems to
| say a lot of words yet not say much.
| Saptarishi wrote:
| I think lots of physicists have a tendency to outrightly reject
| interpretations which seemingly put humans and other
| "conscious"(whatever that means) entities on a higher pedestal
| . It is most probably due to their general dislike of
| advocation of humans being a superior being than other
| "animals" by religious organisations, which does inculcate some
| unhealthy arrogance in people.
|
| But i think physicists need to open up a little more. If QBism
| or other interpretations where reality is subjective were
| really true, it would probably lead to some "we got it right
| initially" by some religious organisations, but honestly, that
| shouldn't stop physicists from pursuing such theories. I think
| it can be a win-win scenario in the end.
| Koshkin wrote:
| > _physicists need_
|
| Except they don't - the science of quantum mechanics with its
| practical probabilistic "interpretation" has worked pretty
| well so far. As far as _science_ is concerned, that is.
| oxymoran wrote:
| Typical that a non physicist is calling the work of actual
| physicists pseudoscience just because it "smells" funny to
| them. What exactly is so absurd about two physical entities
| interacting with each other to produce an outcome?
| [deleted]
| dekhn wrote:
| Hi. My phd is in biophysics. This is scientific garbage (the
| proposals are absolutely not supported by any experimental
| data).
| codethief wrote:
| Which proposals are you referring to exactly?
| dekhn wrote:
| All of its interpretations of QBist "Core Position" on
| the wikipedia page.
|
| Most importantly, nothing I can see in the QBist pages
| has anything to do with actual experimental work. It's a
| framework for viewing what the mathematics of human
| theories of QM "mean" and how to interpret that. However,
| nothing of what they propose is required to explain what
| we observe, experimentally.
|
| More generally, no "interpretation" of QM is required to
| apply the theory in generalizable, predictive ways, so
| all this work isn't really helping us move science or
| engineering forward.
| meroes wrote:
| > What exactly is so absurd about two physical entities
| interacting with each other to produce an outcome?
|
| That seems inadequate to explain Bell nonlocality.
| knodi123 wrote:
| > What exactly is so absurd about two physical entities
| interacting with each other to produce an outcome?
|
| I hardly think that's a fair description of what he's
| claiming. This sounds almost like the textbook example of the
| motte and bailey fallacy.
| dekhn wrote:
| Interpreting QM is a waste of time. The current human brain
| cannot understand QM well enough to mke a physical
| interpretation. Stop wasting our time with pseudoscience.
| Koshkin wrote:
| Giving interpretation of things is a big part of what
| philosophy is all about, and philosophy is not exactly
| pseudoscience.
| joeberon wrote:
| How do you know that?
| dekhn wrote:
| Because after 100 years of arguing about this, we have a
| theory which explains everything, but nobody has come up with
| a reasonable explanation of how the delayed choice quantum
| eraser could possibly work unless causality works different
| from human intuition.
| Koshkin wrote:
| Isn't this logic some sort of survivorship bias? (OK, the
| "problem" persisted for a hundred years... Let's just wait
| for another hundred, and then we'll see?)
| dekhn wrote:
| I think the dual choice quantum eraser experiment
| basically shows that to have a reasonable understand of
| quantum mechanics, we have to suspend much of our common
| belief and intuition about temporal causality, and it's
| quite clear at this point humans are not good at
| reasoning about temporal causality, as anybody who has
| been in a journal club session about relativity or EPR or
| paxos will know.
| kgwgk wrote:
| Quantum mechanics may go against common sense in some
| aspects but temporal causality is not one of them.
|
| https://philarchive.org/rec/ELLWDC
|
| "Why Delayed Choice Experiments do NOT imply
| Retrocausality" - David Ellerman
|
| "There is a fallacy that is often involved in the
| interpretation of quantum experiments involving a certain
| type of separation such as the: double-slit experiments,
| which-way interferometer experiments, polarization
| analyzer experiments, Stern-Gerlach experiments, and
| quantum eraser experiments. The fallacy leads not only to
| flawed textbook accounts of these experiments but to
| flawed inferences about retrocausality in the context of
| delayed choice versions of separation experiments."
| jonathanstrange wrote:
| 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?
| dekhn wrote:
| QM 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.
| codethief wrote:
| > QM works even if there are no humans or other subjective
| "observers".
|
| How do you know?
| [deleted]
| dekhn wrote:
| because the vast majority of data we have excludes it, and
| it seems really implausible, and gives humans (which are
| just meatbags) special status.
| drdec wrote:
| Maybe it's a clever hack.
|
| Suppose you are making a universe. You want to ensure that
| the universe you make is interesting, i.e. contains
| consciousness. So you set up the rules like QM: everything
| exists in probabilistic superposition until a consciousness
| observes it. One of the superpositions contains a
| consciousness, and that collapses the wavefunction
| guaranteeing consciousness in your universe. It's a way of
| using survivor bias in your favor.
|
| Disclaimer: not to be taken seriously.
|
| Disclaimer #2: Also, coincidentally, not related to poster
| drdeca. Unless he is me from another superposition and we
| haven't collapsed yet because you all are not as conscious as
| you think.
| dekhn wrote:
| I see no value in speculating about how the universe was
| made, nor relating that to quantum mechanics, without have
| extensive data, and none of the data we have today supports
| making any of these kinds of speculations.
| drdeca wrote:
| The 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.
| dekhn wrote:
| the former implies the latter through occam's razor.
| drdeca wrote:
| I don't follow.
|
| Perhaps we are using "theory of" in different ways?
| dekhn wrote:
| Probably. I'm using the technical, scientific definition.
| drdeca wrote:
| What I mean is, "technical account/description of"
| dekhn wrote:
| Scientific theories are explanations for ("why"), not
| passive descriptions.
| vankessel wrote:
| I don't disagree with you, but Occam's razor is a
| principle; a guideline. Not a hard law. From the
| panpsychic perspective all matter is conscious making
| everything a "subjective observer," which solves the same
| problem of humans being arbitrary special observers.
| Unprovable philosophical dead-end? Sure. Woo? Nah.
| dekhn wrote:
| this is why I said "implies". I fully acknowledge that in
| principle QM and human minds might be intimately boundm
| but that, like panpsychism is effectively ipossible to
| prove or disprove in our current scientific framework.
| superposeur wrote:
| Yes, 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.
| kgwgk wrote:
| > Even the QBists speak of "agents" that are abstract
| entities,
|
| Do you have a reference for that?
| dekhn wrote:
| their 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.
| monktastic1 wrote:
| When 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.)
| rkagerer wrote:
| _If 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 wrote:
| Yes, 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.
| phkahler wrote:
| >> 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.
| [deleted]
| monktastic1 wrote:
| I 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 wrote:
| I'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 wrote:
| Indeed. 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 wrote:
| I agree, I think. The observed "collapse" is not
| completely explained by decoherence (and even less
| satisfactory by many worlds).
| Jensson wrote:
| Exactly 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.
| gbrown wrote:
| > what is required to cause it to collapse is still not a
| solved problem
|
| From what I've read, the existence of "collapse" of the wave-
| function is not established, and not even widely accepted as
| more than a useful computational tool by practicing
| physicists.
| TchoBeer wrote:
| >since it alters the state you can see that it happened
| afterwards in a system
|
| What experiment shows this?
| Jensson wrote:
| https://en.wikipedia.org/wiki/Double-slit_experiment
| simiones wrote:
| The 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 wrote:
| I believe you're referring to delayed choice quantum
| eraser?
| joeberon wrote:
| > 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 wrote:
| > _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 wrote:
| > then our specialness also gives us time travel powers
|
| Why? Because the Quantum Eraser Experiment comes out as it
| does?
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