Why is QM so hard to accept?

[VitalOne]

The implications of QM are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?

[so-crates]

You don't find the idea that an entity can act as either a particle or a wave, depending on how the experiment performed, to be strange?

[NonHomogenized]

I don't know. I find it pretty easy to accept anything with the kind of empirical support QM has, personally.

Although, perhaps, your perspective might be improved by considering a very similar question:

The implications of the theory of evolution are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?

Things that seem contrary to the intuition of people who have not studied a subject often seem to be rejected by said people. Perhaps it's just difficult to accept that intuition can be wrong? Again, as I said before, I don't know.

[Mizled]

Because some of the things in QM seem paradoxical and counter-intuitive to our usual line of reasonable thinking.

[untermensche]

You don't find the idea that an entity can act as either a particle or a wave, depending on how the experiment performed, to be strange?
An entity that presents itself in two ways to humans depending on the human conditions is not at all strange.

What would be strange, if it existed, would be some preconcieved notion that all things behaved as one thing under all conditions.

[Jesse]

I don't think it's merely a matter of it being counter-intuitive, since the curved four-dimensional spacetime of general relativity is quite counterintuitive as well. The central problem with QM is the measurement problem (http://en.wikipedia.org/wiki/Measurement_problem). In QM, in between measurements you just have the wavefunction of a system evolving deterministically according to the Schroedinger equation, but in order to get any definite predictions you must assume the existence of classical systems which "measure" the quantum system and "collapse" it into a definite value for whatever variable is being measured, with the probability depending on the state of the wavefunction at the moment of measurement. The problem is that the behavior of the classical measuring-system has to be put in by hand, you can't treat the measuring-systems itself as simply a very large quantum system, because then it would simply have a wavefunction evolving deterministically without collapsing into any definite state. So QM doesn't give us an objective picture of what an entire universe evolving according to quantum laws (without any external measuring-system whose choice of variables to measure is put in by hand) would look like, as in GR or any other non-quantum theory (the many-worlds interpretation does try to give a picture of what it would mean for the entire universe to evolve according to the same quantum laws, but it's not clear how to derive the actual observed probabilities we see from the MWI).

The goal of reductionist physics is to find the most basic laws of the universe, and then understand higher-level laws as emerging from the more basic ones...a reductionist would say that all the laws of chemistry should in principle be derivable from quantum physics (quantum electrodynamics might be sufficient), even though in practice it would be very difficult and only some fairly simple situations like hydrogen atoms have been "reduced" in this way. But there doesn't seem to be any fundamental problem with the idea of such a derivation, whereas the whole issue of collapse and the need for external classical measuring-devices seems to pose a fundamental problem for deriving the classical world from quantum laws. Even if you reject reductionism and imagine that the universe operates according to a sort of patchwork of different laws in different situations, surely nature must have some well-defined rules for the precise conditions where one set of laws is overridden by another set, we don't expect nature to rely on the sort of fuzzy know-it-when-we-see-it distinctions between "quantum" and "classical" systems that we do, that would seem to be a kind of anthropomorphism.

[Sven]

The implications of QM are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?
One reason certainly is that it's often badly explained. Quantum objects are neither particles nor waves, they are quantum objects - which just happen to share some characteristics with particles, and some with waves.

[I. C. Unicorns]

QM has been shoehorned into existing concepts of physics, due to the preconceived notions. The quantum world behaves according to quantum mechanics.

[Jesse]

QM has been shoehorned into existing concepts of physics, due to the preconceived notions. The quantum world behaves according to quantum mechanics. As I said, the problem is defining the boundaries of the "quantum world". Any classical measuring-device is just a large collection of particles, so why can't you treat it according to the laws of QM? Unless you have a second classical measuring-device to measure the first (with the second device's behavior being put in by hand rather than derived in any lawlike way), then trying to treat the measuring-device using quantum laws just gives you a giant superposition with no single outcome, which does not seem to agree with what we actually observe.

[epepke]

As I said, the problem is defining the boundaries of the "quantum world". Any classical measuring-device is just a large collection of particles, so why can't you treat it according to the laws of QM? Unless you have a second classical measuring-device to measure the first (with the second device's behavior being put in by hand rather than derived in any lawlike way), then trying to treat the measuring-device using quantum laws just gives you a giant superposition with no single outcome, which does not seem to agree with what we actually observe.

There is no boundary, of course.

The problem is the same as the problem of sex, or politics, or everything else.

People want to believe that particles are like billiard balls and that if you are a good person you will be happy and loved and that the purpose of government is to keep society good and that the purpose of police is to protect people and that some God is going to make things all better if you screw up and all that. It's all wrong.

If QM and Einsteinian relativity didn't exist, people would be uncomfortable with Newton and would prefer Aristotle. I know this because they did.

Don't a lot of people want to "go back" to the Family Values of the 1950s? Even though domestic violence was about three times as common? They don't care. They just imagine a better, simpler time. Woo-hoo!

[Jesse]

There is no boundary, of course. But what does that mean, exactly? Are you saying that every system of particles in the universe, even the ones that make up "classical" measuring-devices, are obeying the same quantum laws? If so, then again you have the problem that a system obeying quantum laws will not naturally "collapse" into a definite state on its own, for that you need to assume some external measuring-system whose behavior is put in by hand. People want to believe that particles are like billiard balls As I have tried to point out, the trouble with QM is more fundamental than the fact that it just doesn't conform to people's billiard-ball intuitions, it is the problem of not being able to get predictions out of QM without bringing in "external" measuring-devices whose behavior cannot themselves be modeled using only quantum laws (at least not without bringing in a secondary measuring-device to measure the first, leading to an infinite regress). All other non-quantum theories, no matter how counterintuitive in other ways (and general relativity is plenty counterintuitive), at least have the property that we can imagine what it would mean for the entire universe to be obeying these laws, but with QM there is a real problem with that (various 'interpretations' of QM try to solve this problem in various ways, but each has its own difficulties).

[Cacofonix]

In my experience, there seem to be people with classical blinkers, and those lucky folk without them, like good epepke.

My problem with classicism is the history. Newton explains God's mechanism. Then God can fade away. But unfortunately he has left his signature in the third person plural, the Omniscient Narrator or the science write up in the passive - you know, that bunsen burner supernaturally igniting itself! In contrast with strictly metaphorical storytelling techniques, there is a still belief in the Objective-Point-of-View - reality with a big R , as opposed to the approximation of the inferred objective - reality with a small r.

Well, there is, of course, by definition no Objective Point of View, merely ourselves being as 'objective' in our outlook as we can be.

It is obvious to anyone that without viewpoint there is no perception and without perception there can be no representation nor discussion of reality.

Why people find QM frightening is that when they do Wheeler's delayed choice gedanken experiment and it works perfectly they are alarmed in case cause and effect are thrown into the melting pot.

[KeithJM]

Because our brains evolved to be able to handle the day to day necessity of the world we live in and can see, and is unfit to handle the alien world of the very small, the very large, the very slow, or the very fast.

[fromdownunder]

I am not sure that QM is as much an example of "acceptance" and "rejection" , so much as "what the fuck does it all mean?"

To a layman such as I, and I would like to think that I have some sort of a brain, I simply do not understand it at all. Is there an "Idiot's Guide to QM"?

Or is it of a mathematical order that those of us who never got much past the 12 x 12 = 144 really have not, and can never have a clue as to what it actually is and what the implications of verification of the theory of QM is/are?

Norm

[premjan]

Basically quantum wave functions work at some size scale, and at a slightly larger scale, the superposition of states description changes to something with a harder edge. It ought to be possible to empirically determine at what point quantum behavior shades into classical behavior. The measurement problem could simply describe what happens when something with quantum behavior slams into something with classical (large-scale quantum) behavior. Is there a real explanatory gap? I mean there is a theoretical gap, perhaps, and even maybe a complexity gap in that what happens at an intermediate scale to change quantum behavior to classical behavior has not been yet worked out mathematically. But it ought to be possible to work out with a combination of measurement and model-building. The quantum scale is simply the scale at which the inherently fuzzy "system" tends to react to external intrusions such as measurement. At a larger scale, the system rebounds without any permanent change from measurements.

[Oxymoron]

QM's fine. It's a theory of information, see. No-one is saying that "electrons are particles" or "electrons are waves". The concepts "particle" and "wave" are simply models we apply that model aspects of whatever it is that electrons (or whatever) actually are. Viewed in this way, "Quantum Weirdness" evaporates - albeit at the expense of considering that it describes the fundamental nature of matter and fields and energy (which themselves are but models we apply to our universe). Indeed its central message seems to be that there are limits to what we can know and how precisely we can know it. This isn't such a big surprise, is it?

[Breath]

But what does that mean, exactly? Are you saying that every system of particles in the universe, even the ones that make up "classical" measuring-devices, are obeying the same quantum laws? If so, then again you have the problem that a system obeying quantum laws will not naturally "collapse" into a definite state on its own, for that you need to assume some external measuring-system whose behavior is put in by hand.

You're talking to a QM newbie here, but what determines that a particle is a member of one system, or another? What / who defines the system?

[Jesse]

You're talking to a QM newbie here, but what determines that a particle is a member of one system, or another? What / who defines the system? I don't think the word has a precise technical meaning, I was just using it in a colloquial way to mean any group of interacting particles. Usually in QM you would deal with groups that are sufficiently isolated from the external environment outside the group (between measurements, at least) that you can ignore the effects of anything outside when predicting future states from past ones.

[Jesse]

Basically quantum wave functions work at some size scale, and at a slightly larger scale, the superposition of states description changes to something with a harder edge. It ought to be possible to empirically determine at what point quantum behavior shades into classical behavior. The measurement problem could simply describe what happens when something with quantum behavior slams into something with classical (large-scale quantum) behavior. Is there a real explanatory gap? I mean there is a theoretical gap, perhaps, and even maybe a complexity gap in that what happens at an intermediate scale to change quantum behavior to classical behavior has not been yet worked out mathematically. But it ought to be possible to work out with a combination of measurement and model-building. The quantum scale is simply the scale at which the inherently fuzzy "system" tends to react to external intrusions such as measurement. At a larger scale, the system rebounds without any permanent change from measurements. I think there is a real explanatory gap. Suppose you have a quantum system interacting with a classical measuring-device, and you had access to the maximum possible information about all the particles that made up the measuring-device, enough to construct a giant wavefunction for the combined system composed of the small system being measured + the measuring-device. Without bringing in a second measuring-device to measure this large system, if you just use the ordinary laws of QM to evolve this giant wavefunction in time, it will not naturally "collapse" into a nice classical state at the time of measurement, but just evolve into a massive superposition of very different macroscopic states a la Schroedinger's cat. So what is the explanation for why that isn't what we actually see when we use a measuring-device to measure a quantum system? If the many-worlds interpretation is correct maybe this is what happens, but if not you need some explanation for the "collapse" which lies outside the system itself (which won't work if you imagine the entire universe is obeying quantum laws).

[epepke]

But what does that mean, exactly? Are you saying that every system of particles in the universe, even the ones that make up "classical" measuring-devices, are obeying the same quantum laws?

Yes. Come on, you know this.

If so, then again you have the problem that a system obeying quantum laws will not naturally "collapse" into a definite state on its own, for that you need to assume some external measuring-system whose behavior is put in by hand.

Um, no. You know this, too.

As I have tried to point out, the trouble with QM is more fundamental than the fact that it just doesn't conform to people's billiard-ball intuitions, it is the problem of not being able to get predictions out of QM without bringing in "external" measuring-devices whose behavior cannot themselves be modeled using only quantum laws (at least not without bringing in a secondary measuring-device to measure the first, leading to an infinite regress). All other non-quantum theories, no matter how counterintuitive in other ways (and general relativity is plenty counterintuitive), at least have the property that we can imagine what it would mean for the entire universe to be obeying these laws, but with QM there is a real problem with that (various 'interpretations' of QM try to solve this problem in various ways, but each has its own difficulties).

It's a matter of degree, not nature. Quantitative, not qualitative. At least as far as most people go.

[spamandham]

The implications of QM are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?

What implications are you referring to? The only implication of QM I'm aware of is that the predictions match observation extremely well.

All the philosophical musings regarding QM are presently mere speculation.

[-RRH-]

The "spooky action" thing is so counter-intuitive to me that I can't help but assume there are undiscovered variables at work. But at the same time, I am not well-versed in the subject, so assuming that my thoughts are more valid than the many people who actually study QM is the same as me going, "How come there's still monkeys?" But I can't shake the belief that hidden variables are the only logical explanation.

[premjan]

I think there is a real explanatory gap. Suppose you have a quantum system interacting with a classical measuring-device, and you had access to the maximum possible information about all the particles that made up the measuring-device, enough to construct a giant wavefunction for the combined system composed of the small system being measured + the measuring-device. Without bringing in a second measuring-device to measure this large system, if you just use the ordinary laws of QM to evolve this giant wavefunction in time, it will not naturally "collapse" into a nice classical state at the time of measurement, but just evolve into a massive superposition of very different macroscopic states a la Schroedinger's cat. So what is the explanation for why that isn't what we actually see when we use a measuring-device to measure a quantum system? If the many-worlds interpretation is correct maybe this is what happens, but if not you need some explanation for the "collapse" which lies outside the system itself (which won't work if you imagine the entire universe is obeying quantum laws).
I believe that the reason there is an explanatory gap is that noone has worked out the math for systems of size intermediate between QM and classical level. For instance IIRC the hydrogen atom is as complex as QM is able to model practically. So some other maths is needed in between. Has anyone ever successfully modelled large molecules (e.g. DNA) using QM?

[Jesse]

But what does that mean, exactly? Are you saying that every system of particles in the universe, even the ones that make up "classical" measuring-devices, are obeying the same quantum laws? Yes. Come on, you know this. I think they are, but I think it only makes sense to say so with an interpretation of QM like the many-worlds interpretation or Bohmian mechanics. If you use the standard Copenhagen interpretation, it's not at all clear what it would mean to describe the entire universe using the laws of QM, with no external measuring-device. And these other interpretations have their own problems. If so, then again you have the problem that a system obeying quantum laws will not naturally "collapse" into a definite state on its own, for that you need to assume some external measuring-system whose behavior is put in by hand. Um, no. You know this, too. "No" as in, "no, they won't collapse into a single macroscopic state on their own" (agreeing with my comment above), or "no" as in "you are wrong, the natural evolution of the wavefunction of a macroscopic system will naturally take it into a single macroscopic state rather than a superposition of totally different macroscopic states like in the Schroedinger's cat thought-experiment?" If the latter, your understanding of quantum mechanics is flawed. It's a matter of degree, not nature. Quantitative, not qualitative. I don't understand what you mean by this comment, can you elaborate?

[premjan]

Can't you just say "decoherence" and let that explain why macroscopic objects don't exhibit quantum behavior? Except possibly for consciousness / microtubules?

[spamandham]

Can't you just say "decoherence" and let that explain why macroscopic objects don't exhibit quantum behavior? Except possibly for consciousness / microtubules?

Macroscopic objects do exhibit quantum behavior, but the bahavior is too small to be noticible in everyday life.

[placebo messiah]

woop thought this was a Queer Marriage thread

[premjan]

Macroscopic objects do exhibit quantum behavior, but the bahavior is too small to be noticible in everyday life.
Do they though? Maybe if the decoherence is sufficient then macroscopic objects exhibit anti-quantum (decoherence) effects which precipitate quantum commitment on actual quantum objects.

[Jesse]

Can't you just say "decoherence" and let that explain why macroscopic objects don't exhibit quantum behavior? Except possibly for consciousness / microtubules? Decoherence doesn't explain why a quantum system would behave classically if there is nothing external interacting with it. The idea behind decoherence as I understand it is that if you have a small system A and its external environment B, and you treat them both using the laws of QM, then as A interacts with B, it becomes entangled and you can no longer see any interference effects in A on its own, they become spread throughout the combined system A+B. This is sort of analogous to the way that if you have 3 photons emitted together and thus entagled with one another, you will see no evidence of entanglement if you look at any pair of photons--the statistics of any pair will look just like that of unentangled photons--but you will see the correlations characteristic of entanglement if you measure all three together. As this page on decoherence (http://www.ipod.org.uk/reality/reality_decoherence.asp) puts it: Note that the interference components do not actually disappear - because they are out of phase we just don't notice them at the macroscopic level. In fact, they just get dissipated out into the wider environment. I always imagine them as little ripples in the ocean - we only ever notice the big (macroscopic) waves in the ocean. The little ripples get entangled with other little ripples until it is impossible to tell from which big wave each little ripple came. If you look at the "reduced state" of the smaller subsystem A, apparently interactions with the external system B will cause it to go from a "pure state" to something close to a "mixed state", where you assign different probabilities to different states in a classical way, and you can assume that the probability of getting result X when you make a measurement can be broken down into a weighted sum like P(getting X) = P(getting X if system is in state A)*P(system in state A) + P(getting X if system is in state B)*P(system in state B) + P(getting X if system in state C)*P(system in state C) + ... (in a superposition of states you can't reason this way--that would be like assuming the probability a photon in the double-slit experiment is found at position x can be broken down into the probability it lands at position x if it goes through the left slit + the probability it lands at position x if it goes through the right slit, which would fail to take into account interference effects). However, if the combined system A + B is not modeled as interacting with anything external, then it'll still be in a giant superposition of very different macroscopic states, like Schroedinger's cat, so this doesn't really help with the problem of explaining why macroscopic systems behave classically without introducting some external system to measure or interact with them (which, again, is a big problem in quantum cosmology if you want to treat the entire universe using quantum laws).

See the comment from "vanesch" on this physicsforums thread (http://www.physicsforums.com/showthread.php?t=40071): One has to be careful. When looking at the combination (system+macroscopic measurement apparatus), a superposition remains a superposition of course, because the hamiltonian is a linear evolution operator. But what decoherence shows you is that *when you restrict your attention to the system*, then what formerly was a superposition, is now best described (best described because you are neglecting part of it, namely the measurement system) by a statistical mixture of eigenstates of the operator that is measured by the measurement apparatus. Likewise, this comment (http://www.lns.cornell.edu/spr/2001-04/msg0032382.html) from sci.physics.research says: >4) "Collaps by decoherence" relies on the separation of a system from an
>environment. With respect to the supposedly all encompassing character
>of quantum mechanics, which becomes important in quantum cosmology, this
>seperation of the universe in "subject" and "object" is a defect that it
>shares with the Kopenhagen interpretation of "collaps by measuring with
>classical appartus".

Ah, but decoherence does not purport to solve the problem of quantum
cosmology. I don't think you will find any claims in that direction in the
literature. Greg Egan also has a good set of pages on decoherence starting here (http://gregegan.customer.netspace.net.au/SCHILD/Decoherence/Decoherence.html).

[premjan]

OK so some people don't think that decoherence causes wavefunction collapse - it just causes entanglement of measurer and measured's wave functions. So this raises another question - in the Schrodinger's cat experiment, why doesn't the cat's wave function (alive+dead) interfere / cohere with that of the box? Maybe it does a little bit? Maybe wavefunction collapse occurs exclusively wherever there is some barrier to full entanglement. In other words, entanglement is not automatic or simple to achieve. This must be the case because classical objects are not all entangled with one another. BTW apparently Max Tegmark's objection to neuron quantum computation in microtubules has been answered in that a more realistic computation concludes that quantum computation could occur at observed neuron speeds (i.e. long-term coherence of 10^-2 seconds is possible).

[Jesse]

OK so some people don't think that decoherence causes wavefunction collapse - it just causes entanglement of measurer and measured's wave functions. So this raises another question - in the Schrodinger's cat experiment, why doesn't the cat's wave function (alive+dead) interfere / cohere with that of the box? Maybe it does a little bit? Sure it does, and in fact, there are all kinds of thermal interactions with the environment outside the box; in practice it's impossible to keep a macroscopic system like a cat isolated. But as an idealized thought-experiment, you can imagine that everything inside the box is completely blocked from interaction with anything outside (if large quantum computers are possible, they'd have to be designed in a way that kept their internal state isolated, so perhaps we could imagine a quantum-computer simulation of a cat). Similarly, decoherence is based on assigning a single wavefunction to both the small system you're interested in and the larger environment interacting with it, and then letting this wavefunction evolve according to the normal quantum rules. Obviously this involves some idealization too, because as long as you're picking some finite-sized system as the "environment", realistically that system would not itself be isolated but would be interacting with an even larger environment around itself, and so on ad infinitum until everything in the future light cone of your starting point is involved. Still, when dealing with the measurement problem, even if in practice you model decoherence using a small system A plus the larger system B to act as its "environment", in principle you'd get the same result even if B was the entire external universe--if you assign the combined system A+B a single wavefunction and let it evolve, the linear evolution of the wavefunction means there can't be anything intrinsic that'll cause it to "collapse", instead it'll naturally evolve into a giant superposition. Maybe wavefunction collapse occurs exclusively wherever there is some barrier to full entanglement. In other words, entanglement is not automatic or simple to achieve. This must be the case because classical objects are not all entangled with one another. BTW apparently Max Tegmark's objection to neuron quantum computation in microtubules has been answered in that a more realistic computation concludes that quantum computation could occur at observed neuron speeds (i.e. long-term coherence of 10^-2 seconds is possible). As I said, if you apply the rules of QM to the small system plus the environment the whole thing becomes entangled (indeed, in the MWI where the notion of "collapse" is dropped, the whole universe would be a giant entangled system), but it then becomes impossible to detect any signs of entanglement by looking only at the small system (see my comments in the last post about 3-photon entangled systems and how you won't see entanglement if you examine just 2 of them). To do quantum computation, you need to be able to control the way the entanglement happens, keeping the components of the computer entangled in a certain way but avoiding random entanglement with the environment, this is what Tegmark's objection is based on. Where did you read that coherence would be possible for 10^-2 seconds in neurons?

[premjan]

I'm getting the feeling that coherence or entanglement of macroscopic objects is more of a dogma than a measured reality (entanglement is sort of a half step towards coherence right?). I know that lasers are quantum coherent and so are superfluids but has anyone measured the quantum coherence or entanglement of any conventional macroscopic "objects"? How much entanglement actually occurs? Why do macroscopic objects exist at all? Maybe they are constantly collapsing their own wave functions for some reason.

The paper which claimed biological feasibility of quantum computation was this one:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002PhRvE..65f1901H&db_key=PHY&data_type=HTML&format=
I believe Tegmark has some responses to this as well here.
http://space.mit.edu/home/tegmark/brain.html

[Jesse]

I'm getting the feeling that coherence or entanglement of macroscopic objects is more of a dogma than a measured reality You're missing the point, it's not claimed as a measured reality, it's a theoretical prediction based on applying the laws of QM to macroscopic systems. If you want to believe that macroscopic systems don't actually obey the laws of QM, go ahead! But that's the whole basis for the measurement problem, that either some systems don't obey quantum laws and it's completely unclear where the boundary is or what alternate laws these classical systems follow, or all systems do obey quantum laws and it's unclear how this can be reconciled with what we actually see when we look at classical systems that don't seem to be in giant superpositions (interpretations such as the MWI and Bohmian mechanics attempt to perform a reconciliation, but there are problems with each approach). So whichever way you slice it, the measurement problem is not adequately solved by decoherence, although it may be a step in the right direction. (entanglement is sort of a half step towards coherence right?). To say a system shows quantum coherence over a certain time period basically just means you can treat the whole system as being in a "pure state" (see my earlier post where I distinguished between pure states and mixed states) over that time, and entanglement just means that you need to assign a single state vector to the entire system rather than assigning separate state vectors to individual parts (in order to take into account their interactions with one another), so they're all pretty interchangeable AFAIK. As I said above, decoherence means that if you have a small system A which can interact with an "environment" system B, then if you treat the combined system A+B as being in a pure state, the smaller system A will tend to evolve from a pure state into something closely resembling a mixed state because of its interactions with B, although by assumption the combined system A+B is still in a pure state. Again, this is a theoretical prediction based on treating the combined system using the laws of QM, although you can connect the theory to experiment in terms of the behavior of just the small system A, how long it takes to stop showing interference and so forth. I know that lasers are quantum coherent and so are superfluids but has anyone measured the quantum coherence or entanglement of any conventional macroscopic "objects"? Not sure, but for conventional macroscopic objects it would probably be too difficult in practice, since the evidence of entanglement involves statistical correlations seen when you make measurements of all the components of the system, and may be invisible if you just measure a subset or make a coarse-grained measurement. Why do macroscopic objects exist at all? Why they don't appear as giant superpositions is just one version of the measurement problem, there's no agreement on how to solve it. Maybe they are constantly collapsing their own wave functions for some reason. There is a theory about modifying the laws of quantum mechanics so systems can "collapse themselves" when they reach a certain size--see objective collapse theory (http://en.wikipedia.org/wiki/Objective_collapse_theory). Roger Penrose made the suggestion that a theory of quantum gravity might say that an entangled system self-collapses when its mass goes above a certain threshold, he suggests it might be the Planck mass (which is fairly large, about the mass of a flea--it basically represents the mass of a black hole whose size is at the Planck scale). But I don't think there's any real theoretical motivation for this sort of thing aside from the fact that it could solve the measurement problem, and it seems like an inelegant solution...presumably objective collapse theories would have testable implications for what should happen if we try to create larger and larger quantum-coherent systems, like superfluids or large quantum computers.

[premjan]

OK so the question of how measurement / wave function collapse occurs and why macroscopic or nonquantum objects exist at all is an open problem. It seems to me that from a quantum perspective, measurement occurs only when collapse occurs. So all macroscopic objects are being prevented from evolving / their quantum wave functions being made to collapse constantly as we are able to observe them with substantial spatial and temporal resolution. Probably there has to be a theory which takes all that into account. Wave function collapse / measurement is a fact of macroscopic life whereas it is something exotic and hard to explain in the quantum realm. Presumably some math is missing to explain exactly why and perhaps it will come along at some point.

[barbos]

Here is another hypothesis:
Wave function collapse ( reduction of composition of states to smaller number of states) is a result of our universe being simulated. Simulations clearly have limits and can't keep WF evolving indefinetely. Collapse is just a global rounding up.

[StillDreaming]

Because our brains evolved to be able to handle the day to day necessity of the world we live in and can see, and is unfit to handle the alien world of the very small, the very large, the very slow, or the very fast.
Exactly. After all, why is it so easy to accept that an apple starts to move spontaneously as soon as you disconnect it from a tree?

[judge]

Why people find QM frightening is that when they do Wheeler's delayed choice gedanken experiment and it works perfectly they are alarmed in case cause and effect are thrown into the melting pot.

Wow, I had never heard of this experiment but looked it up here, Wheeler's Classic Delayed Choice Experiment (http://www.bottomlayer.com/bottom/basic_delayed_choice.htm).

This is what the mathematics predicted for a result, and this is exactly the result obtained in the laboratory.

They did this in a lab and changed the past? :eek:

Is that right?

[JamesBannon]

...surely nature must have some well-defined rules for the precise conditions where one set of laws is overridden by another set...
I usually don't get involved in discussions about QM because I have no knowledge of the subject, but this little bit caught my eye. Why must "nature" have some well-defined "rules"? Mankind observes the universe and then develops hypotheses in an attempt to explain its behaviour, but all of these hypotheses are man-made, "nature" really has nothing to do with it. This is like Hawking saying about one his equations (I forget which one - something about black holes) "something this beautiful and simple has to be correct". Que? What's that got to do with the price of fish? Not that I'm saying that the QM model is incorrect, it certainly seems to have plenty of empirical support like evolution, but this is no real reason to suppose that nature must have a set of rules. Am I wrong?

[naturalist.atheist]

The implications of QM are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?

Humans have evolved to be well adapted to the scales of time and space that we inhabit. So everything in that realm makes pretty good sense. It ought to. If people have trouble making sense of it they are usually considered to be ill in some way.

However science is constantly trying to explore and explain reality and has discovered scales much larger and smaller than the scale that we inhabit. And we have discovered that the dynamics of motion at those scales is not exaclty like that of the scale we inhabit. On the very small scall it is very different from our scale. So we have trouble comprehending it. It doesn't appear to make any sense. It is not how we intuitively understand motion.

But there it is.

Quantum mechanics represents our best attempt to-date to explain it. It has constructs in it that we just can't help but try to interpret in terms that we understand from motion at our scale.

But it is a foolish thing to do. There is no point in trying to force the universe to conform to how we think it ought to be. Just get on with it and try to figure out how we find it to be.

[naturalist.atheist]

They did this in a lab and changed the past? :eek:

Is that right?

The actual experiment was performed recently and the result reported in Science.

A single dollop of light, or photon, must be described by a flowing quantum wave that gives the probability of finding it at any particular place and time. At the same time, the photon acts a bit like an indivisible bullet: When observed with a particle detector, it produces a distinct signal, like a pebble pinging off a car door. And things get weirder. The quantum wave can split in two and recombine, like ripples flowing around a stump in a pond, to create striking "interference" effects that determine which way the recombined wave flows. On the other hand, it's simply impossible to split a photon at a fork in the road. If there is no way to eventually put the pieces back together, the photon acts like a particle and goes one way or the other.

Even weirder still, the choice to allow the waves to recombine or not can be made even after the photon passes the fork where it should have split--or not. Famed physicist John Archibald Wheeler realized that nearly 30 years ago and dreamed up an experiment to prove the point. Now Jean-François Roch of the Ecole Normale Supérieure de Cachan in France and colleagues have performed the experiment. The researchers shot photons one by one at a half-silvered mirror, or "beam splitter," to cleave the quantum wave describing each photon. After traveling different distances, the two halves sloshed back together at a second beam splitter 50 meters away, which could recombine them. The experimenters could randomly switch this second beam splitter on and off electronically well after the photon had passed the first one.

http://www.sciencemag.org/content/vol315/issue5814/images/large/315_966_F1.jpeg

[premjan]

Basically delayed choice means that quantum mechanical effects propagate instantaneously therefore faster than the speed of light? I remember a long thread on the delayed choice quantum eraser at one point on IIDB which is an extended form of the delayed choice experiment.
http://www.bottomlayer.com/bottom/basic_delayed_choice.htm
http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm
So there is no quantum reality to space and time.

I actually find the description of the delayed choice quantum eraser a bit confusing because the author perhaps inadvertently gives the impression of retroactive changing of a quantity. I don't think that is exactly what is meant though.

[Bomb#20]

I don't know. I find it pretty easy to accept anything with the kind of empirical support QM has, personally. ...
The implications of the theory of evolution are often very difficult and hard to accept...even though it is based chiefly off of concrete empirical experimental evidence....so why is it so hard to accept? Is it just because it "sounds strange"?
Things that seem contrary to the intuition of people who have not studied a subject often seem to be rejected by said people. Perhaps it's just difficult to accept that intuition can be wrong? Again, as I said before, I don't know.
Lots of things are hard to accept because they're counterintuitive and people just haven't studied them enough. QM isn't one of those things."Anyone who is not shocked by quantum mechanics hasn't understood it." - Niels Bohr

I am not sure that QM is as much an example of "acceptance" and "rejection" , so much as "what the f**k does it all mean?"
To a layman such as I, and I would like to think that I have some sort of a brain, I simply do not understand it at all. Is there an "Idiot's Guide to QM"?
Or is it of a mathematical order that those of us who never got much past the 12 x 12 = 144 really have not, and can never have a clue as to what it actually is and what the implications of verification of the theory of QM is/are?
There's a fantastic book that explains what it means with hardly any math: "QED", by Richard Feynman.

The "spooky action" thing is so counter-intuitive to me that I can't help but assume there are undiscovered variables at work. But at the same time, I am not well-versed in the subject, so assuming that my thoughts are more valid than the many people who actually study QM is the same as me going, "How come there's still monkeys?" But I can't shake the belief that hidden variables are the only logical explanation.
Don't feel bad about it. Plenty of people have shared that thought, including a guy so well versed in the subject he got his Nobel Prize in it -- fellow by the name of Einstein. The trouble is, it's been proven that any hidden variable theory that matches QM's predictions has to contain faster-than-light communication. Bohm found it necessary to throw out relativity and bring back the ether. A lot of smart people find this even harder to swallow.

[premjan]

Spooky effects seem to imply the transspatiality of quantum phenomena. And the delayed choice quantum eraser experiment implies that causality can appear to go backwards in time. So space and time are not as solid as they look. Or maybe that the universe as a whole is a unit in some sense. Probably another way to put it is that space and time apply only for manifested or measurable phenomena. For stuff that hasn't been measured there is no space or time.

[Jesse]

I usually don't get involved in discussions about QM because I have no knowledge of the subject, but this little bit caught my eye. Why must "nature" have some well-defined "rules"? Mankind observes the universe and then develops hypotheses in an attempt to explain its behaviour, but all of these hypotheses are man-made, "nature" really has nothing to do with it. This is like Hawking saying about one his equations (I forget which one - something about black holes) "something this beautiful and simple has to be correct". Que? What's that got to do with the price of fish? Not that I'm saying that the QM model is incorrect, it certainly seems to have plenty of empirical support like evolution, but this is no real reason to suppose that nature must have a set of rules. Am I wrong? Well, isn't that the assumption behind all of science since Newton? It has a pretty successful track record. And since statistical rules like those of QM are still a form of rules, what would be an alternative to either events following deterministic rules or events following statistical rules with a random element? As in discussions of "free will", I really have no idea what people even mean when they imagine there's some alternative to these options. Also, in the comment you were responding to it was being assumed that quantum laws work correctly for quantum systems, I was just saying that if these laws don't apply to classical systems, nature should have rules for distinguishing between classical and quantum...what would be the alternative to that, would nature judge which class a system fell into on a case-by-case, "know it when I see it" basis?

[Jesse]

Basically delayed choice means that quantum mechanical effects propagate instantaneously therefore faster than the speed of light? I remember a long thread on the delayed choice quantum eraser at one point on IIDB which is an extended form of the delayed choice experiment.
http://www.bottomlayer.com/bottom/basic_delayed_choice.htm
http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm
So there is no quantum reality to space and time.

I actually find the description of the delayed choice quantum eraser a bit confusing because the author perhaps inadvertently gives the impression of retroactive changing of a quantity. I don't think that is exactly what is meant though. There's no need to imagine anything is changed "retroactively" in the DCQE experiment. You certainly can't use it to send a message back in time, since no matter whether the which-path information in the "idler" photons is erased or not, the pattern of all the "signal" photons on the screen never shows an interference pattern. Here's a little explanation I posted on another thread which may help you follow what's going on in this experiment: Even in the case of the normal delayed choice quantum eraser setup where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the setup in fig. 1 of this paper:

http://xxx.lanl.gov/PS_cache/quant-p...03/9903047.pdf

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference. Also, I added a bit to the wikipedia page on the DCQE (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser) to make it clear that the results of the experiment can be explained without any backwards-in-time-influences by most interpretations of QM: It might initially seem that the "choice" to observe or erase the which-path information of the idler can change the position where the signal photon is recorded on the detector, even after it should have already been recorded. However, as noted above, the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics (http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics). The transactional interpretation (http://en.wikipedia.org/wiki/Transactional_interpretation) would interpret the results in terms of genuine backwards causality, but other interpretations such as the Copenhagen interpretation ( http://en.wikipedia.org/wiki/Copenhagen_interpretation), the Bohm interpretation (http://en.wikipedia.org/wiki/Bohm_interpretation) and the many-worlds interpretation (http://en.wikipedia.org/wiki/Many-worlds_interpretation) would predict the same experimental results without the need for backwards causality. For example, according to the Copenhagen interpretation the initial measurement of the position of the signal photon (whose probability distribution would not show interference if it were measured first) would discontinuously alter the wave function (http://en.wikipedia.org/wiki/Wave_function) of the combined signal/idler system, affecting the probabilities that the idler would be detected at different locations. If the signal photon was detected by detector D0 at a position near a peak of the D0/D1 joint detection graph (fig. 3) and a trough of the D0/D2 joint detection graph (fig. 4), this would increase the probability that the idler would be detected at detector D1 and decrease the probability that the idler would be detected at D2; likewise, if the signal photon was detected at a position near a peak of the D0/D2 joint detection graph and a trough of the D0/D1 joint detection graph, this would increase the probability that the idler would be detected at D2 and decrease the probability it would be detected at D1. This would ensure that both correlation graphs showed the correct interference pattern, with the interference patterns now explained in terms of the initial measurement of the signal photon affecting the probabilities of the later measurement of the idler rather than the other way around.

[spamandham]

Basically delayed choice means that quantum mechanical effects propagate instantaneously therefore faster than the speed of light?

I suppose that's one way of looking at it, but an alternative way of looking at it is that there is memory somewhere in the system, such that it 'knows' that it went through a double slit, or a gravitational lens, or whatever.

I seem to recall reading about an idea someone came up with, that what we observe, photons moving in 'straight lines', is really nothing more than the superposition of all possible paths, which is why we observe a probability distribution at the end (the central limit of all paths). I don't recall where I heard this or what the idea is called. If this turns out to be true, the implication is that the universe is able to deal with infinite postulates in a finite amount of time.

[JamesBannon]

Well, isn't that the assumption behind all of science since Newton? It has a pretty successful track record. And since statistical rules like those of QM are still a form of rules, what would be an alternative to either events following deterministic rules or events following statistical rules with a random element? As in discussions of "free will", I really have no idea what people even mean when they imagine there's some alternative to these options. Also, in the comment you were responding to it was being assumed that quantum laws work correctly for quantum systems, I was just saying that if these laws don't apply to classical systems, nature should have rules for distinguishing between classical and quantum...what would be the alternative to that, would nature judge which class a system fell into on a case-by-case, "know it when I see it" basis?
I've really no idea actually, I was just "musing" so to speak. It is one of the "unquestioned" assumptions in scientific enquiry I suppose. Certainly my own experience would suggest there is such a thing as causality, and I presume so would everybody else's, but I have this "nagging" doubt that what we are observing is mere pattern rather than something that is a genuine phenomenon of nature. Of course I'm probably wrong.

[premjan]

I suppose that's one way of looking at it, but an alternative way of looking at it is that there is memory somewhere in the system, such that it 'knows' that it went through a double slit, or a gravitational lens, or whatever.

I seem to recall reading about an idea someone came up with, that what we observe, photons moving in 'straight lines', is really nothing more than the superposition of all possible paths, which is why we observe a probability distribution at the end (the central limit of all paths). I don't recall where I heard this or what the idea is called. If this turns out to be true, the implication is that the universe is able to deal with infinite postulates in a finite amount of time.

Even if there was memory, some sort of information is propagated between the two halves of an entangled photon pair. Faster than light and backwards in time in some cases, as long as the information is not yet used or measured. It indicates that time and space are not real in the quantum realm.

[barbos]

TIQM tries to resolve delayed choice paradox by introducing waves which propagate back in time. I don't think that interpreting weird thing by introducing even more weird thing is such a good idea, unless of course it produce some testable evidence.

I think that delayed choice paradox doesn't exist CI, because it doesn't allow that kind of questions, I mean "which pass photon took" is not physical question in CI.

[epepke]

Macroscopic objects do exhibit quantum behavior, but the bahavior is too small to be noticible in everyday life.

Horseshit.

You couldn't see the screen without quantum behavior in your trans-retinal.

Your computer wouldn't even work without quantum behavior.

Liar, liar, pants on fire.

[spamandham]

It indicates that time and space are not real in the quantum realm.

Maybe they're not real even outside the quantum realm. If you think about it, spacetime is a concept we use to aid in understanding relationships between things. It need not actually exist. All that matters are the relationships.

Would the math or observations change if we gave up the idea of space and time as real entities? If not, I suppose our dear friend Ockham would tell us we should thus do it!

[spamandham]

Horseshit.

Gesundheit.

[Answerer]

Why is QM so hard to accept?

Because it has the tendency to go against laws of Causality. Hell, even Einstein and friends found that notion absurd much a less other people who are clueless about it. With its random and chaotic nature, QM destroy logic.