Why does wave-function collapse occur?


by questionpost
Tags: collapse, occur, wavefunction
bhobba
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#19
Jan28-12, 01:50 AM
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Quote Quote by rodsika View Post
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 whats forced on you.

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Ken G
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Jan28-12, 02:16 AM
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Quote Quote by bhobba View Post
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 whats 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.
bhobba
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Jan28-12, 02:28 AM
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Quote Quote by salvestrom View Post
@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.

Quote Quote by salvestrom View Post
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 cant 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.'

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rodsika
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#22
Jan28-12, 02:31 AM
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Quote Quote by salvestrom View Post
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.
No, it has been upgraded to 430 atoms buckyball

http://physicsandphysicists.blogspot...atoms-fat.html
bhobba
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Jan28-12, 02:34 AM
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Quote Quote by Ken G View Post
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.

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Ken G
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#24
Jan28-12, 02:37 AM
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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!
Simon Bridge
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#25
Jan28-12, 02:52 AM
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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?
Maui
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#26
Jan28-12, 03:03 AM
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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=j...o6Tkw--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)?
rodsika
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#27
Jan28-12, 03:38 AM
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Hi, I'm reviewing how they did the interference experiment and knew how but one thing puzzled me.

http://www.nature.com/news/2011/1104....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"?
bhobba
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#28
Jan28-12, 03:49 AM
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Quote Quote by Ken G View Post
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

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Bill
Ken G
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Jan28-12, 09:14 AM
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Quote Quote by Simon Bridge View Post
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.
questionpost
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#30
Jan28-12, 09:37 AM
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Quote Quote by salvestrom View Post
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.
salvestrom
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Jan28-12, 03:06 PM
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Quote Quote by Ken G View Post
... 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
Simon Bridge
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#32
Jan28-12, 10:47 PM
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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...
Ken G
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#33
Jan29-12, 01:14 AM
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Quote Quote by salvestrom View Post
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.


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