Why does wave-function collapse occur?

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In summary, the term "collapse" is a little over-exaggerated, but why is it that we measure things as points and not waves even though particles exist as waves?First, as Feynman points out, only particles exist, no detector has detected waves - what always appears is particles.Secondly, the issue of wave function collapse is interpretation dependent. It only exists if you think a quantum state has an external existence like say an electric field. You can simply view it as a device to calculate probabilities. You will find a discussion of this in Chapter 9 of Ballentine - Quantum Mechanics. Bottom line here is the assumption it has that kind of existence leads to all sorts of issues so
  • #1
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As I understand it, the term "collapse" is a little over-exaggerated, but why is it that we measure things as points and not waves even though particles exist as waves?
 
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  • #2
First, as Feynman points out, only particles exist, no detector has detected waves - what always appears is particles.

Secondly the issue of wave function collapse is interpretation dependent. It only exists if you think a quantum state has an external existence like say an electric field. You can simply view it as a device to calculate probabilities. You will find a discussion of this in Chapter 9 of Ballentine - Quantum Mechanics. Bottom line here is the assumption it has that kind of existence leads to all sorts of issues so its best not to interpret it that way. What is thought of as waves is simply that states sometimes have wave like solutions - but if you think of a state as a calculational device only then the so called wave-particle duality is rather moot.

Look at it this way. Suppose you have a possibly biased dice then you would describe it by 6 positive numbers that add up to one - that would be its state. It doesn't have an existence 'out there' - it simply is a way of describing the likely occurrence of a certain face of the dice lying up. The same with a quantum state. When you observe the outcome of throwing the dice the state does not collapse - you simply make an observation. Looked at this way Shrodenger Cat, and other contrivances, is rather trivial - it has no more a mystery than tossing a dice.

For my personal view of QM check out:
http://arxiv.org/pdf/quant-ph/0101012v4.pdf

Basically in a stochastic theory you have two choices - a theory where you have continuous transformations of so called pure states - and ones where that is not allowed. The former (basically) leads to QM - the latter standard probability theory.

This is not to say QM does not have deep mysteries (eg non local behavior and what properties it has between observations), but by viewing the theory this way they can be kept under control without being bogged down with foundational issues.

Thanks
Bill
 
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  • #3
questionpost said:
As I understand it, the term "collapse" is a little over-exaggerated, but why is it that we measure things as points and not waves even though particles exist as waves?

Sometimes the motion of particles resembles that of a wave. However, particles do not exist as waves, but as... particles.

Essentially you are asking why particles behave as particles.
 
  • #4
Ugh. There seems to be no consensus on this subject.
 
  • #5
If particles actually existed as particles and did not oscillate like waves, shouldn't they lose all their energy by traveling real distance over time and accelerating?
 
  • #6
questionpost said:
If particles actually existed as particles and did not oscillate like waves, shouldn't they lose all their energy by traveling real distance over time and accelerating?

Come again - can't follow that one. In QM you can actually derive the dynamics from Galilean invariance - you can see the details in Chapter 3 of Ballentine that I gave before. In the classical limit they behave exactly like Newtonian mechanics says they should and do not accelerate by themselves.

As far as interpretation goes - those that posted there is no consensus are correct - the view I gave is basically the shut up and calculate view - but other interpretations have a different take.

Thanks
Bill
 
  • #7
bhobba said:
Come again - can't follow that one. In QM you can actually derive the dynamics from Galilean invariance - you can see the details in Chapter 3 of Ballentine that I gave before. In the classical limit they behave exactly like Newtonian mechanics says they should and do not accelerate by themselves.

As far as interpretation goes - those that posted there is no consensus are correct - the view I gave is basically the shut up and calculate view - but other interpretations have a different take.

Thanks
Bill

So in otherwords, your saying that because we measure or calculate particles as particles and not waves, that they exist only as particles and now waves?
 
  • #8
I am saying they are only ever detected as particles - never as waves so the most reasonable thing to do is model them as particles. But they obey the rules of QM which is described by a quantum state that has, in some circumstances, wave-like solutions. However whether a state has a real existence is open to question - I view it purely as a device for calculating probabilities.

Thanks
Bill
 
  • #9
bhobba said:
I am saying they are only ever detected as particles - never as waves so the most reasonable thing to do is model them as particles. But they obey the rules of QM which is described by a quantum state that has, in some circumstances, wave-like solutions. However whether a state has a real existence is open to question - I view it purely as a device for calculating probabilities.

Thanks
Bill

And I assume you already know about the double-slit experiment (just to be sure)? Because I do not know how electrons could me measured in those locations they are at in that experiment without the electrons themselves following discrete wave mechanics.
Unless by "solutions" to you mean somehow working backwards from results?
Because I don't think this is just a basic pop-science mis-understanding, but at the same time, we don't actually see particles themselves as waves even though they seem to have to travel as waves to end up in the locations they do.
 
  • #10
questionpost said:
And I assume you already know about the double-slit experiment (just to be sure)? Because I do not know how electrons could me measured in those locations they are at in that experiment without the electrons themselves following discrete wave mechanics.
Unless by "solutions" to you mean somehow working backwards from results?
Because I don't think this is just a basic pop-science mis-understanding, but at the same time, we don't actually see particles themselves as waves even though they seem to have to travel as waves to end up in the locations they do.

The problem here is you are ascribing a classical view to a quantum experiment. The state has wavelike properties and that is why the particles display an interference effect typical of waves when measured by the screen in the double slit experiment - which only ever shows particles. What the particle does when it is not measured is anyone's guess. Feynman describes it as in some sense particles go through both slits. My view is you can't say anything other than the state, which may or may not have an external existence, displays an interference effect typical of waves.

Thanks
Bill
 
  • #11
questionpost said:
I guess it might be safer to say that a sub-atomic particle is actually neither a particle nor a wave, but its own thing.

It has wave-like properties. The quantum system is probably neither a particle or wave, thought of in the classical sense.
 
  • #12
questionpost said:
So I suppose it can't be agreed upon what particles actually are, even though if particles were actually just particles they should radiate their energy away by constantly accelerating around the nucleus whereas with a wave they would simply oscillate which is not the same as accelerating? I guess it might be safer to say that a sub-atomic particle is actually neither a particle nor a wave, but its own thing.

It is most certainly agreed what atomic sized particles are - they are quantum objects. What can't be agreed on is how to interpret QM.

Your second statement is correct - it is not a wave nor classical particle - but a quantum particle which is something entirely different and definitely weird - although with some acquaintance you get used to it and get an idea of why it must be like that - check out:
http://arxiv.org/pdf/quant-ph/0111068v1.pdf

'The usual formulation of quantum theory is very obscure employing complex Hilbert spaces, Hermitean operators and so on. While many of us, as professional quantum theorists, have become very familiar with the theory, we should not mistake this familiarity for a sense that the formulation is physically reasonable. Quantum theory, when stripped of all its incidental structure, is simply a new type of probability theory. Its predecessor, classical probability theory, is very intuitive. It can be developed almost by pure thought alone employing only some very basic intuitions about the nature of the physical world. This prompts the question of whether quantum theory could have been developed in a similar way. Put another way, could a nineteenth century physicist have developed quantum theory without any particular reference to experimental data? In a recent paper I have shown that the basic structure of quantum theory and countably infinite dimensional Hilbert spaces follows from a set of five reasonable axioms. Four of these axioms are obviously consistent with both classical probability theory and with quantum theory. The remaining axiom states that there exists a continuous reversible transformation between any two pure states. This axiom rules out classical probability theory and gives us quantum theory. The key word in this axiom is the word “continuous”. If it is dropped then we get classical probability theory instead.'

Basically QM is necessary in a stochastic theory if you want to model continuous transformations - for the exact meaning of that see the link above.

Thanks
Bill
 
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  • #13
bhobba said:
It is most certainly agreed what atomic sized particles are - they are quantum objects. What can't be agreed on is how to interpret QM.

Your second statement is correct - it is not a wave nor classical particle - but a quantum particle which is something entirely different and definitely weird - although with some acquaintance you get used to it and get an idea of why it must be like that - check out:
http://arxiv.org/pdf/quant-ph/0111068v1.pdf

'The usual formulation of quantum theory is very obscure employing complex Hilbert spaces, Hermitean operators and so on. While many of us, as professional quantum theorists, have become very familiar with the theory, we should not mistake this familiarity for a sense that the formulation is physically reasonable. Quantum theory, when stripped of all its incidental structure, is simply a new type of probability theory. Its predecessor, classical probability theory, is very intuitive. It can be developed almost by pure thought alone employing only some very basic intuitions about the nature of the physical world. This prompts the question of whether quantum theory could have been developed in a similar way. Put another way, could a nineteenth century physicist have developed quantum theory without any particular reference to experimental data? In a recent paper I have shown that the basic structure of quantum theory and countably infinite dimensional Hilbert spaces follows from a set of five reasonable axioms. Four of these axioms are obviously consistent with both classical probability theory and with quantum theory. The remaining axiom states that there exists a continuous reversible transformation between any two pure states. This axiom rules out classical probability theory and gives us quantum theory. The key word in this axiom is the word “continuous”. If it is dropped then we get classical probability theory instead.'

Basically QM is necessary in a stochastic theory if you want to model continuous transformations - for the exact meaning of that see the link above.

Thanks
Bill

Remember a molecule composing of 430 atoms called buckyball can still interfere with itself in the double slit. These buckyballs obviously stay as particles in between (as it's hard to imagine the 430 atoms with their protons and neutrons just dissolving into waves in between). But what propel them into certain regions to form inteference patterns using your reasoning above?
 
  • #14
@bhobba: I'd very much like to hear how you feel about multiverse concepts. Your descriptions here are very straightforward, "down to earth". I'd also like to see if I have a grip on your view by restating it:

From the ground up. There is field. Fluctuations occur in the field. Excitations are particles. When unviewed particles are doing something they are treated as waves - the wave function being the probability spread (is that misleading of me?) It may be possible they are actually moving like waves. When a particle/wave is interacted with (including measurement/observation) it has a definite particle form.

One more thing: what is the field? I understand the mathematical concept of scalar and vector fields - numbers assigned to points in spacetime - but what does the field record in this situation? Energy fluctuations? Or is this not the idea?
 
  • #15
rodsika said:
Remember a molecule composing of 430 atoms called buckyball can still interfere with itself in the double slit. These buckyballs obviously stay as particles in between (as it's hard to imagine the 430 atoms with their protons and neutrons just dissolving into waves in between). But what propel them into certain regions to form inteference patterns using your reasoning above?

Wow I didn't know they called it a Bucky-Ball outside of Wisconsin
 
  • #16
One way to understand it is to rephrase the loaded terminology: Wave functions don't collapse, they get updated. That corresponds to CI where the wave function is understood as a symbolic representation of possible quantum behavior and not as a direct representation of the physical reality of the quantum.

In that understanding updating the wave function given a change of knowledge is the same as updating say the probability distribution of where to find a lost sailboat given an observation that it was not in sector X.

The update is qualitatively the same as with a classical probability distribution though the way of representing probabilities of observations is distinct.
 
  • #17
questionpost said:
Wow I didn't know they called it a Bucky-Ball outside of Wisconsin

It is named after an inventor called Buckminster Fuller, from Massachusets, for its resembelance to a geodesic dome he invented. The ball part comes from its similarity to the association football ball. So says wikipedia, bless them.
 
  • #18
rodsika said:
Remember a molecule composing of 430 atoms called buckyball can still interfere with itself in the double slit. These buckyballs obviously stay as particles in between (as it's hard to imagine the 430 atoms with their protons and neutrons just dissolving into waves in between). But what propel them into certain regions to form inteference patterns using your reasoning above?

Pretty sure there are only 60 atoms, but your question still stands, since that's still 720 protons and neutrons and I have no idea how many electrons.
 
  • #19
rodsika said:
Remember a molecule composing of 430 atoms called buckyball can still interfere with itself in the double slit. These buckyballs obviously stay as particles in between (as it's hard to imagine the 430 atoms with their protons and neutrons just dissolving into waves in between). But what propel them into certain regions to form inteference patterns using your reasoning above?

To me its not so obvious they remain as particles between observations or have any property at all when not observed. QM does not say they dissolve into waves etc between observations - in fact it says noting at all about what properties they have independent of an observation. Weird - of course - but if you have a stochastic theory without an underlying cause of the randomness that's what's forced on you.

Thanks
Bill
 
  • #20
bhobba said:
QM does not say they dissolve into waves etc between observations - in fact it says noting at all about what properties they have independent of an observation. Weird - of course - but if you have a stochastic theory without an underlying cause of the randomness that's what's forced on you.
And maybe it's not so weird after all-- maybe what was weird was the way we got away with imagining that there was an underlying cause of classical stochasticity. Maybe it was actually more weird to think of reality like an "answer man" that had an answer to any question, even questions that no apparatus was present to answer-- as if answers were somehow built into reality independently of the means to answering them. In my view, it is actually more natural, and so in a way less weird, to imagine that it is quite a fundamental aspect of reality to be utterly ambivalent to any question that the reality itself is not set up to answer. Seen in that light, indeterminism seems both inevitable and natural.
 
  • #21
salvestrom said:
@bhobba: I'd very much like to hear how you feel about multiverse concepts. Your descriptions here are very straightforward, "down to earth".

You mean the many world interpretation? To me its simply too weird to be correct and amounts to a sort of mysticism. But opinions are like bums - everyone has one - it does not make t correct - just because I find it unappealing does not make it incorrect.

salvestrom said:
I'd also like to see if I have a grip on your view by restating it:

From the ground up. There is field. Fluctuations occur in the field. Excitations are particles. When unviewed particles are doing something they are treated as waves - the wave function being the probability spread (is that misleading of me?) It may be possible they are actually moving like waves. When a particle/wave is interacted with (including measurement/observation) it has a definite particle form.

One more thing: what is the field? I understand the mathematical concept of scalar and vector fields - numbers assigned to points in spacetime - but what does the field record in this situation? Energy fluctuations? Or is this not the idea?

That's not my view of QM - what you wrote is more like a description of QFT. QFT is based on fields because being a relativistic theory it treats time and space on the same footing. Standard QM has time as a parameter and position as an observable. To treat them on equal footing you make position a parameter (which leads to having a field) or you promote time to an observable - which evidently also works but is mathematically more difficult and leads to basically the same theory.

My view on QM is its basically a theory based on a new type of probability calculus that is forced on us because you want to be able to continuously go from whatever describes it at one instant to the next. In standard probability theory you can't do that - you find instead it tends to a stable limit or cycles between states - this is well known from the theory of Markov Chains. You need to go to complex numbers for this behavior not to occur, but then you have the problem of defining probabilities on complex numbers - that's where Gleasons Theorem comes into it showing there is only one way to define probabilities - the standard way QM does it. Check out:
http://www.scottaaronson.com/democritus/lec9.html
'The second way to teach quantum mechanics leaves a blow-by-blow account of its discovery to the historians, and instead starts directly from the conceptual core -- namely, a certain generalization of probability theory to allow minus signs. Once you know what the theory is actually about, you can then sprinkle in physics to taste, and calculate the spectrum of whatever atom you want. This second approach is the one I'll be following here.'

Thanks
Bill
 
  • #23
Ken G said:
And maybe it's not so weird after all-- maybe what was weird was the way we got away with imagining that there was an underlying cause of classical stochasticity. Maybe it was actually more weird to think of reality like an "answer man" that had an answer to any question, even questions that no apparatus was present to answer-- as if answers were somehow built into reality independently of the means to answering them. In my view, it is actually more natural, and so in a way less weird, to imagine that it is quite a fundamental aspect of reality to be utterly ambivalent to any question that the reality itself is not set up to answer. Seen in that light, indeterminism seems both inevitable and natural.

+1. Abso-friggen-lutely. Reality is what realty is - not what we might like to think it is.

Thanks
Bill
 
  • #24
Bingo. Is the goal of physics to get reality to fit into our templates, or to keep an open mind and just let it tell us what it is? This is the dark side of Occam's Razor-- it's fine to simplify things, but we mustn't take our simplifications too seriously, or we fall into self-delusion, which is what science is supposed to cure!
 
  • #25
Occams razor has us accepting, for now, the simplest models we can get away with.
The goal for physics is to let the Universe tell us what is real or not. But surely this is not controversial... after all, there are whole shelves written on the subject of the philosophy of science.

I suspect that the original question has been answered?
 
  • #26
In the buckyballs experiment, Zeilinger and co. mention no cooling of the fullerene molecule. Instead they heat them up to 900K and separate them with rotating discs and send them at the slits:

http://www.google.com/url?sa=t&rct=...7pStCA&usg=AFQjCNH5quntpOPVJgVTo6Tkw--isU1BUA


How are decoherence effects dealt with?

They conclude and show at the end of the paper that which-path information destroys the interference. This is direct confirmation that reality cannot be(entirely) mind-independent, as information is property of mind(if information can affect the behavior of matter at the micro level, then mind-independent theories of the world are untenable)?
 
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  • #27
Hi, I'm reviewing how they did the interference experiment and knew how but one thing puzzled me.

http://www.nature.com/news/2011/110405/full/news.2011.210.html

"In the team's experiment, the beams of molecules are passed through three sets of slits. The first slit, made from a slice of silicon nitride patterned with a grating consisting of slits 90 nanometres wide, forces the molecular beam into a coherent state, in which the matter waves are all in step."

What is the difference between coherent state and coherence? For example referring to the buckyball (in isolation for sake of discussions), how do its internal parts of 60 atoms differ when it is in "coherent state" versus when it is in "coherence"?
 
  • #28
Ken G said:
Bingo. Is the goal of physics to get reality to fit into our templates, or to keep an open mind and just let it tell us what it is? This is the dark side of Occam's Razor-- it's fine to simplify things, but we mustn't take our simplifications too seriously, or we fall into self-delusion, which is what science is supposed to cure!

Again - abso-friggen-lutely

Thanks
Bill
 
  • #29
Simon Bridge said:
Occams razor has us accepting, for now, the simplest models we can get away with.
The goal for physics is to let the Universe tell us what is real or not. But surely this is not controversial...
You wouldn't think so, but actually, much of the debate surrounding wavefunction collapse, and its interpretations, exist expressly because of not following that rule. If we don't take our simplifications seriously (simplifications like "unitary evolution", "wavefunction reality", and "collapse"), much of the problem goes away, and we can simply treat interpretations like what they are: interpretations of simplifications.
 
  • #30
salvestrom said:
It is named after an inventor called Buckminster Fuller, from Massachusets, for its resembelance to a geodesic dome he invented. The ball part comes from its similarity to the association football ball. So says wikipedia, bless them.

That's... weird, because I could have sworn I remembered that the University of Madison Wisconsin released that they had invented that, and "Buckey" is a mascot for the basketball team there.
 
  • #31
Ken G said:
... Is the goal of physics to get reality to fit into our templates, or to keep an open mind and just let it tell us what it is?

I'm reminded of the 1970's observation of the rotation of galaxies. I don't know exactly why they were conducting these measurements but as is well known these days they found it didn't add up right. Fourty years on they've got some pretty good modelling of the dark matter regions. This would seem to be a really good example of the universe telling us something about itself. And yet, it would seem in order to learn more we do need a template, one that might give us an idea where else to look for additional information. I've encountered a few times on these forums a tendency to push aside the conceptual beginnings of templates as philosophy. But a concept can be testable, without a mathematical support structure. (Stay in school kids, not saying you can do without the numbers!) But I find myself alternatingly concerned and relieved by various posts. One says; "it's just maths. Another; "the maths works, we don't need to know the underlying reality". Others have even said we can't know the underlying reality because it's taking place on an unobservable scale, which is actually not as terrible as it first sounds. But perhaps a little disappointing.

I'm rambling. /hug
 
  • #32
There is a speculative/creative side to scientific investigation - but if we want to know what to call "real" we have to check with reality.

Debating possible complications as human beings with feelings and biases is not the same as accepting any of them as scientists.

I think this is a useful distinction - there is a model of the ideal scientists which none of us ever live up to...
 
  • #33
salvestrom said:
And yet, it would seem in order to learn more we do need a template, one that might give us an idea where else to look for additional information.
I agree, the issue is how seriously to take the template. If our template is a circle, we then go out into the world and look for circles, because we understand circles. However, this does not mean there are actually circles out there, it means we learn something by entering into a kind of provisional pretense that there are circles out there. We must still "interpret the circles", but we needn't debate what is the "correct interpretation" of the existence of circles, because there is no existence of circles, there is only the existence of the interpretations and how we use them. The relevance here is if we substitute "circle" with "wavefunction collapse."
Others have even said we can't know the underlying reality because it's taking place on an unobservable scale, which is actually not as terrible as it first sounds. But perhaps a little disappointing.
And still others would say that there's no such thing as something "happening on an unobservable scale", because all we can say about what happens is what we can observe to happen, and that is completely provisional to what we do in fact observe to happen. The rest is interpretations-- and templates.
 

1. What is wave-function collapse?

Wave-function collapse is a phenomenon in quantum mechanics where the superposition of multiple possible states of a particle or system collapses into a single definite state when it is observed or measured. This means that the particle or system is no longer in multiple states simultaneously, but rather in one specific state.

2. Why does wave-function collapse occur?

The exact reason for wave-function collapse is still a topic of debate among scientists. Some theories suggest that it is caused by the interaction between the observer and the system being observed, while others suggest it is due to the limitations of our current understanding of quantum mechanics. However, it is widely accepted that wave-function collapse is an inherent property of quantum systems.

3. Can wave-function collapse be predicted?

No, wave-function collapse cannot be predicted. The collapse of the wave-function is a random and unpredictable event, and the exact outcome of the collapse cannot be determined beforehand. This is one of the fundamental principles of quantum mechanics known as the uncertainty principle.

4. Does wave-function collapse violate the laws of physics?

No, wave-function collapse does not violate the laws of physics. While it may seem counterintuitive, it is a natural consequence of the probabilistic nature of quantum mechanics. The collapse of the wave-function is a fundamental aspect of the quantum world and has been observed and verified through numerous experiments.

5. Is there a way to prevent or control wave-function collapse?

Currently, there is no known way to prevent or control wave-function collapse. This is because it is an inherent property of quantum systems and is not influenced by external factors. However, scientists continue to research and explore ways to better understand and potentially manipulate this phenomenon.

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