Quantum Physics: I need some sanity here x_x [Archive]

fuelair

3rd September 2007, 05:31 PM

Not knowing you, can't be sure what you mean by shaky ground - Einstein would not move to studying quanta because he found it to be that way. A couple of examples :
WOO (false): people cannot see things they have no experience with due to quantum effects so natives in the Americas could not physically see Spanish ships when they first came to the "New World" though they could see waves generated by them.
Real: Schroedinger's cat (really an explanation of particle states) - until you open the box the cat is dead or not-dead (particle has left-spin or right-spin). Only on opening the box is it actually in one of the two states (dead or not-dead for the cat, left-spin or right-spin for the particle).

If you provide more info on what the points are that elude, one or more of us will, I am sure, respond. Sometimes even helpfully!!

Try:
http://www.amazon.com/dp/1851683690/ref=s9_asin_image_0-1967_p/103-1487367-7901452?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=top-1&pf_rd_r=0EB3BKVCYQ86FCGVHQ2N&pf_rd_t=301&pf_rd_p=305856701&pf_rd_i=Quantum%20Physics%20BBC

http://www.amazon.com/ELegant-Universe-Brian-Greene/dp/009928992X/ref=pd_bbs_3/103-1487367-7901452?ie=UTF8&s=books&qid=1188865678&sr=1-3
and, if a/v is your preferred, from 2003:http://www.amazon.com/NOVA-Elegant-Universe-Maria-Spiropulu/dp/B0000ZG0TA/ref=pd_bbs_sr_1/103-1487367-7901452?ie=UTF8&s=dvd&qid=1188865804&sr=1-1

Mobyseven

3rd September 2007, 05:39 PM

Real: Schroedinger's cat (really an explanation of particle states) - until you open the box the cat is dead or not-dead (particle has left-spin or right-spin). Only on opening the box is it actually in one of the two states (dead or not-dead for the cat, left-spin or right-spin for the particle).

Just to clarify - this is 'real' in the sense that it is really one interpretation of quantum mechanics (the 'Copenhagen Interpretation').

Quantum mechanics is a strange but interesting thing. In popular books about the subject, you'll learn quite a lot, but it's still hard to grasp a lot of the concepts without understanding the maths behind it. At least that's what I've found.

fuelair

3rd September 2007, 05:42 PM

Just to clarify - this is 'real' in the sense that it is really one interpretation of quantum mechanics (the 'Copenhagen Interpretation').

Quantum mechanics is a strange but interesting thing. In popular books about the subject, you'll learn quite a lot, but it's still hard to grasp a lot of the concepts without understanding the maths behind it. At least that's what I've found.
Not just you!! For any truly advanced physics, true understanding is mathmatical understanding. We civilians just do the best we can!!!

wuschel

3rd September 2007, 07:31 PM

Since your question appears to have been answered in a useful and serious way already, I feel this sick kind of an urge to add that you are either to find something here, but that would not be sanity - or you are going to find sanity but you cannot be sure it was here.

Fredrik

3rd September 2007, 08:07 PM

SusanB-M1

4th September 2007, 12:05 AM

On this forum I suppose I should say this in a whisper, but may I suggest 'Bluff Your Way In The Quantum Universe' by Jack Klaff, ISBN 1-902825-57-8, factual and fun.

(Well, I enjoyed it!)

Tez

4th September 2007, 02:04 AM

Just to clarify - this is 'real' in the sense that it is really one interpretation of quantum mechanics (the 'Copenhagen Interpretation').

Not really - this is simply what the mathematics says - the wavefunction of the cat is entangled with that of the particle, and every "interpretation" agrees with this. Certain supra-quantum theories, i.e theories which add something to quantum mechanics, (like collapse models or decoherence theories), may say the quantum state of the cat and the particle is not entangled, but then they are more than simply an interpretation of the math. We say the cat is both alive and dead as a loose jargon for "the quantum state is neither that of an alive cat nor that of a dead cat". If one believes that quantum states are "real" then you have to take that loose jargon as a serious description of what is "really" going on - and thats hard for most people to stomach. Unfortunately, so are the alternatives people have come up with.

3point14

4th September 2007, 03:30 AM

Not really - this is simply what the mathematics says - the wavefunction of the cat is entangled with that of the particle, and every "interpretation" agrees with this. Certain supra-quantum theories, i.e theories which add something to quantum mechanics, (like collapse models or decoherence theories), may say the quantum state of the cat and the particle is not entangled, but then they are more than simply an interpretation of the math. We say the cat is both alive and dead as a loose jargon for "the quantum state is neither that of an alive cat nor that of a dead cat". If one believes that quantum states are "real" then you have to take that loose jargon as a serious description of what is "really" going on - and thats hard for most people to stomach. Unfortunately, so are the alternatives people have come up with.

I always understood that the uncertain state of the cat was completely fictional, the decay (or otherwise) of the particle having been observed by the equipment. (no-one ever told me it had to be a human observer)

Of course, I know very nearly nothing about this (apart form going 'Wow! That's amazing' when a friend (post-grad physics student) explained the double slit experiment to me.

Tez

4th September 2007, 03:41 AM

I always understood that the uncertain state of the cat was completely fictional, the decay (or otherwise) of the particle having been observed by the equipment. (no-one ever told me it had to be a human observer.

Well that would be much less interesting (except for the question of how intelligent a detector must be to collapse a wavefunction!). As far as the math of standard QM goes, it has to be a human observer to collapse the wavefunction - and so until you look in the box the wavefunction isn't collapsed. There are many attempts to design proper physical descriptions of "non-human" collapse (eg decoherence models or spontaneous collapse models) but these go beyond standard QM (ie they must add some postulates of new physical processes, not just interpret the math of QM as we have it).

But its much stranger than simply uncertainty - the issue is not just the cat having an uncertain state (since a tossed coin has an uncertain state before you observe it too). Its the nature of the state - that it cannot be understood as the same type of uncertainty as the tossed coin which makes it weird. The state of the cat is inextricably intertwined with that of the particle. What Einstein pointed out later was worse - if, without looking at the cat you then choose different measurements to make on the particle, you can collapse the cat into different states. Macroscopic entanglement really would be weird.

3point14

4th September 2007, 04:01 AM

Well that would be much less interesting (except for the question of how intelligent a detector must be to collapse a wavefunction!). As far as the math of standard QM goes, it has to be a human observer to collapse the wavefunction - and so until you look in the box the wavefunction isn't collapsed. There are many attempts to design proper physical descriptions of "non-human" collapse (eg decoherence models or spontaneous collapse models) but these go beyond standard QM (ie they must add some postulates of new physical processes, not just interpret the math of QM as we have it).

But its much stranger than simply uncertainty - the issue is not just the cat having an uncertain state (since a tossed coin has an uncertain state before you observe it too). Its the nature of the state - that it cannot be understood as the same type of uncertainty as the tossed coin which makes it weird. The state of the cat is inextricably intertwined with that of the particle. What Einstein pointed out later was worse - if, without looking at the cat you then choose different measurements to make on the particle, you can collapse the cat into different states. Macroscopic entanglement really would be weird.

Thanks for that, I was pretty sure I'd got it wrong
(only cos those who seem to know (like yourself) were saying something completely different)

I really must go and read up on this. does anyone have any handy ways of increasing my IQ by about 100 points so I can understand it?

wipeout

4th September 2007, 05:08 AM

If there are any books, articles or anything else written by anyone knowledgeable in the interpretation of quantum mechanics which are up to date, comprehensive and comprehensible, I'd sure like to know about it. ;)

Dumbledore

4th September 2007, 07:06 AM

Both "In Search of Schroedinger's Cat," and "Schroedinger's Kittens" by John Gribbin I have found very useful in my non-mathematical exploration of quantum physics. Good luck, quantum physics is strange and fascinating, and has been stated before we can only get a limited understanding of it without understanding the mathematics behind quantum physics.:D

Ziggurat

4th September 2007, 07:44 AM

Well that would be much less interesting (except for the question of how intelligent a detector must be to collapse a wavefunction!). As far as the math of standard QM goes, it has to be a human observer to collapse the wavefunction - and so until you look in the box the wavefunction isn't collapsed.

The problem is, "collapse" of the wave function isn't really part of the theory at all. It's what we do when we want to stop using the theory, usually because we hit a point where further calculations aren't possible because of incomplete knowlege. In particular, it's only when a system which we are modelling with quantum mechanics interacts with a system which we either choose not to or cannot model with quantum mechanics (usually a measurement device, but if you could trace it far enough, maybe a person) that we ever get collapse. Collapse never happens (and in fact cannot happen) with a system which we can fully model. So for the cat problem, the point at which the collapse happens (is it when we open the box? Is it when the cat senses or doesn't sense the lethal injection? Is it when the mechanical device detects a decay?) is completely ambiguous. And it will remain so as long as you can't accurately model any of the steps beyond the decay itself. So we can't actually tell whether collapse is a distinct physical process (in which case it's something we do not actually have a theory to describe) or whether it's just an artifact of processes which don't involve any collapse at all but which are complex enough (due to the massive number of interactions) and seemingly random enough (because of our lack of knowlege of the state or the measurement aparatus) that they look like collapse.

wipeout

4th September 2007, 07:45 AM

I always understood that the uncertain state of the cat was completely fictional, the decay (or otherwise) of the particle having been observed by the equipment. (no-one ever told me it had to be a human observer)

What you always understood is believed to be correct by people who have won the Nobel prize in physics.

Human consciousness being somehow "special" was the hand-waving argument von Neumann suggested in desperation to end the infinite "chain" he discovered of what you called uncertain states spreading from the initial uncertain state to all that observed or measured it. Like the "collapse" of the wave function, it is not in the mathematics of the quantum mechanics but was added to actually try and avoid it.

Tez

6th September 2007, 10:11 AM

What you always understood is believed to be correct by people who have won the Nobel prize in physics.

Human consciousness being somehow "special" was the hand-waving argument von Neumann suggested in desperation to end the infinite "chain" he discovered of what you called uncertain states spreading from the initial uncertain state to all that observed or measured it. Like the "collapse" of the wave function, it is not in the mathematics of the quantum mechanics but was added to actually try and avoid it.

Look wipeout, pick up any text on standard QM and collapse is a *necessary* mathematical postulate. It can be as well defined (or otherwise) as "collapse" of a classical probablity distribution when we make a measurement of a classical random variable. That also only occurs when a human performs the collapse, precisely because its something *we* do (i.e on paper) - as Ziggurat pointed out its actual ontological status and dynamical origins are very dubious and much disputed. Despite that, the reason we *do* need to describe the system by the collapsed state is that it the correct state for predicting probabilities of any future measurements you make on the system. Those predictions are not handwaving and we do this all the time. There is a whole "measurement based" model of universal quantum computing, for example, in which there is no dynamics whatsover other than the nonlinear jumps of collapses(*). It is likely this will be how we actually build quantum computers.

The "nobel prizewinners" argument is idiotic. For a start read Schroedingers paper and his correspondence with Einstein at the time - many historians as well as physicists and philosophers have pointed out what I'm saying. In fact I cannot think of a modern historian or philosopher of physics who claims otherwise.The fact that a nobel prizewinner gets it wrong is not surprising - e.g. 't Hooft didn't even understand Bell's theorem until after his nobel prize, and has admitted as such. One cannot be an expert on everything.

(*) In fact something I realised a year or so ago, because of thinking about this model, is that one can provide a set of measurement operators from which unitary evolution is emergent on long timescales - i.e. one can imagine a specially designed collapse mechanism as the fundamental evolution process which is happening on such short timescales we cannot observe it, and on longer timescales it provably yields unitary evolution. So the standard intuition about unitary evolution being nice, smooth and the "good part" of quantum mechanics could just be an historical artifact. I'm not claiming it is - I can simply proivide a model of such a thing which cannot be ruled out by any current experiments.

69dodge

6th September 2007, 01:57 PM

(*) In fact something I realised a year or so ago, because of thinking about this model, is that one can provide a set of measurement operators from which unitary evolution is emergent on long timescales - i.e. one can imagine a specially designed collapse mechanism as the fundamental evolution process which is happening on such short timescales we cannot observe it, and on longer timescales it provably yields unitary evolution. So the standard intuition about unitary evolution being nice, smooth and the "good part" of quantum mechanics could just be an historical artifact. I'm not claiming it is - I can simply provide a model of such a thing which cannot be ruled out by any current experiments.

I guess sometimes the little collapses do need to look like a big collapse, rather than looking unitary, right? Does your model explain how that works too?

wipeout

7th September 2007, 04:50 AM

Look wipeout, pick up any text on standard QM and collapse is a *necessary* mathematical postulate.

Here's just some quick questions that only need one-word answers. Please don't spend time typing up much more than that. This kind of discussion can time-consuming and we won't change each other's viewpoints, I am sure of that, as we didn't the times before.

So, doesn't the collapse approach add nonsensical negative probabilities to the theory? Now that'd be some good, necessary mathematics right there if it does. It certainly adds the paradoxes of faster-than-light and backwards-in-time effects to violate special relativity and apparent causality.

It can be as well defined (or otherwise) as "collapse" of a classical probablity distribution when we make a measurement of a classical random variable. That also only occurs when a human performs the collapse, precisely because its something *we* do (i.e on paper) - as Ziggurat pointed out its actual ontological status and dynamical origins are very dubious and much disputed.

Ok, here's any isolated system or even the universe. Can the collapse approach predict anything using collapse by an observation without getting into the paradox of outside observation of something you can't be outside of?

Despite that, the reason we *do* need to describe the system by the collapsed state is that it the correct state for predicting probabilities of any future measurements you make on the system. Those predictions are not handwaving and we do this all the time.

But that's easy for the collapse approach. So let's go the other way. You start with a collapsed state of any system, maybe even the universe. Can the collapse approach now retrodict anything that happened before the collapse? Anything in the prior history of system or even the universe? Or is it entirely unable to "uncollapse" and say anything about the past without adding human knowledge of that past external to the theory?

There is a whole "measurement based" model of universal quantum computing, for example, in which there is no dynamics whatsover other than the nonlinear jumps of collapses(*). It is likely this will be how we actually build quantum computers.

I have to assume these quantum computer scientists are aware that what you throw away with a collapse approach can come back to haunt what is kept if something is small enough and isolated enough? The collapsed state would then fail to be "the correct state" if the mathematics don't describe all that can cause later effects. Von Neumann wasn't waving his hands around with that one.

The "nobel prizewinners" argument is idiotic. For a start read Schroedingers paper and his correspondence with Einstein at the time - many historians as well as physicists and philosophers have pointed out what I'm saying. In fact I cannot think of a modern historian or philosopher of physics who claims otherwise.The fact that a nobel prizewinner gets it wrong is not surprising - e.g. 't Hooft didn't even understand Bell's theorem until after his nobel prize, and has admitted as such. One cannot be an expert on everything. One cannot be an expert on everything.

Yeah, and Bohr never understood EPR either, Einstein forgot general relativity in an argument against QM, Feynman misunderstood the fundamentals of calculus, and so on. Who knows, maybe even von Neumann did hand-waving consciousness arguments? Of course, someone pointing that out would be well aware that even the most brilliant can screw up.

I was pointing out that 3point14's understanding was more than fine.

I hope you make progress with your ideas, by the way. I am a little surprised you would risk anyone "borrowing" any idea by talking of it in a forum.

Tez

24th September 2007, 04:02 PM

Sorry, was in Iran the last couple of weeks, and had a complete laptop crash while there. Spent the last couple of days I've been back recovering. Before I did lose the laptop I found JREF wasnt blocked from there which was good (in fact no western news source semed to be either).

Here's just some quick questions that only need one-word answers. Please don't spend time typing up much more than that. This kind of discussion can time-consuming and we won't change each other's viewpoints, I am sure of that, as we didn't the times before.

So, doesn't the collapse approach add nonsensical negative probabilities to the theory?

No. This tells me you dont know the math at all behind what youre saying. Which makes answering anything else you say probably pointless. Care to back up your claim with an equation?

Now that'd be some good, necessary mathematics right there if it does. It certainly adds the paradoxes of faster-than-light and backwards-in-time effects to violate special relativity and apparent causality.

Where the hell are you getting this nonsense? One of the most amazing things of non-relativistic QM is that it doesn't violate these things despite its nonlocality. Understanding why is presumably a deep insight we've failed to achieve. Care to explicitly back up your claim with an equation?

I have no clue what you were trying to get at with the next bit of your examples/questions. If you can phrase a clearer question I will try and answer.

I have to assume these quantum computer scientists are aware that what you throw away with a collapse approach can come back to haunt what is kept if something is small enough and isolated enough? The collapsed state would then fail to be "the correct state" if the mathematics don't describe all that can cause later effects. Von Neumann wasn't waving his hands around with that one.

Unbelievable. I'm through arguing with you until you demonstrate you actually understand some of the math behind QM. Everything in the above paragraph is based on a complete misconception of how collapse works in quantum mechanics.

Yeah, and Bohr never understood EPR either, Einstein forgot general relativity in an argument against QM, Feynman misunderstood the fundamentals of calculus, and so on. Who knows, maybe even von Neumann did hand-waving consciousness arguments?

For the record, in case anyone is actually interested, I think it is clear Bohr misunderstood Einstein's steering arguments (which pre and post dated EPR), but did understand the EPR paper. His response to EPR does not work with the other arguments, as many, many people have pointed out. Somehow there is this mythology built around EPR that Bohr answered everything Einstein came up with. But with a lot of these discussions, such as the photon in a box thing was written many years later by Bohr, and do not concord with Einsteins correspondence of the time.

e.g For the photon in a box, at that time (similar time to EPR paper) in letters to people what he was saying was "look, after the photon has gone I can choose either to weigh the box, and therefore determine the color of the photon (which is by now far away) or I can measure its time of exit (which determines its spatial location - so for instance when it would hit a remote mirror). That seemed like a paradox to Einstein, for whom the color of a photon and its temporal location should both have been physically real, and therefore how could his observations on the box suddenly seemed to affect the physical properties of the remote photon. (Note this has nothing to do with challenging the uncertainty principle, nor did he "forget" general relativity, which is not actually needed anyway because gravitational time dilation is derivable without full GR from Newtonian potential considerations).

Its a beautiful "paradox" from which anyone can learn a lot about quantum mechanics.

Read some of Don Howard's (historian/philosopher at Pittsburg) papers for several nice pedagogical discussion of such things.

Regarding people borrowing my idea: Its certainly true someone could. In fact I've discussed it with several well known people, and made some notes which have been emailed around, which is some protection I guess (though also more of a risk - I certainly lost one semi-important idea this way). At the moment I see it as simply a mathematical curiosity I'll eventually flesh out (when I find an unwary student!) into something maybe even useful.

wipeout

25th September 2007, 12:53 AM

I knew your response would be aggressive and dismissive.

After all, you were dismissive of Murray Gell-Mann, a Nobel laureate, for his lack of understanding of EPR, the famous spin version of which was thought up by his close friend and discussion partner of that time, David Bohm.

You, of course, know better than him about that so you would, of course, obviously dismiss me too on similar issues.

I didn't understand what Gell-Mann meant either at first. But I didn't dismiss it. I studied it.

And now I understand.

He sure could do a much better job of explaining it, I'd agree, if that was raised.

But yes, I can find the equations, draw diagrams to explain the experimental set-up, point to the books, and so forth.

I was actually surprised you even asked, I had genuinely assumed you would recognise many of the points, perhaps all. The aim of my post was for you to agree on the flaws, not to point them out.

But since you don't recognise what I refer to, I'd rather have you not know.

Dismiss me. You know you are right to. Oh wait, you already did. Good.

wipeout

25th September 2007, 12:56 AM

[Edit: double post]

Dancing David

25th September 2007, 10:55 AM

Here's just some quick questions that only need one-word answers. Please don't spend time typing up much more than that. This kind of discussion can time-consuming and we won't change each other's viewpoints, I am sure of that, as we didn't the times before.

So, doesn't the collapse approach add nonsensical negative probabilities to the theory? Now that'd be some good, necessary mathematics right there if it does. It certainly adds the paradoxes of faster-than-light and backwards-in-time effects to violate special relativity and apparent causality.

The equations remain wave forms before and after the collapse, the 'collapse' is a semantic issue not a reality issue. Particles are waves and remain waves even after the intersectional 'collapse'.

The violations I am not so sure of, the faster than light issue is usually a product of HIP, at least in recent experiments. The apparent causality I couldn't answer.

Ok, here's any isolated system or even the universe. Can the collapse approach predict anything using collapse by an observation without getting into the paradox of outside observation of something you can't be outside of?

'Collapse' is the intersection of wave sets. the collapse is an artifact of semantics. At large scales the quantum effect is not a problem, HIP applies at very small scales. The waveforms of large objects are very predictable?

But that's easy for the collapse approach. So let's go the other way. You start with a collapsed state of any system, maybe even the universe.

the waves do not suddenly become particles, they remain waves throughout, they can be constrained somewhat but they don't 'collapse'. they intersect as a data point fuzzed up by HIP.

Can the collapse approach now retrodict anything that happened before the collapse? Anything in the prior history of system or even the universe? Or is it entirely unable to "uncollapse" and say anything about the past without adding human knowledge of that past external to the theory?

I am not sure that systems the size you are discussing are governed by QM weirdness. I don't believe that waveforms carry out a propagation that would have meaning without knowing the intersectional points and parameters.

I have to assume these quantum computer scientists are aware that what you throw away with a collapse approach can come back to haunt what is kept if something is small enough and isolated enough? The collapsed state would then fail to be "the correct state" if the mathematics don't describe all that can cause later effects. Von Neumann wasn't waving his hands around with that one.

Yeah, and Bohr never understood EPR either, Einstein forgot general relativity in an argument against QM, Feynman misunderstood the fundamentals of calculus, and so on. Who knows, maybe even von Neumann did hand-waving consciousness arguments? Of course, someone pointing that out would be well aware that even the most brilliant can screw up.

I was pointing out that 3point14's understanding was more than fine.

I hope you make progress with your ideas, by the way. I am a little surprised you would risk anyone "borrowing" any idea by talking of it in a forum.

wipeout

26th September 2007, 12:17 PM

The equations remain wave forms before and after the collapse, the 'collapse' is a semantic issue not a reality issue. Particles are waves and remain waves even after the intersectional 'collapse'.

The violations I am not so sure of, the faster than light issue is usually a product of HIP, at least in recent experiments. The apparent causality I couldn't answer.

People, quite naturally, try apply classical thinking to the EPR result at some point up to and including detection and this application leads then to the seeming appearance of nonlocal effects, negative probabilities or both combined. Talk turns to these faster-than-light effects, the future deciding the past and so on. Others may differ, but I don't think this is a good thing if we can avoid thinking along those lines.

'Collapse' is the intersection of wave sets. the collapse is an artifact of semantics. At large scales the quantum effect is not a problem, HIP applies at very small scales. The waveforms of large objects are very predictable?

I am sure you see like many that needing an observer performing measurements outside the system being studied is never more clearly shown to be inadequate as an interpretation of quantum mechanics than when that system being studied is the universe itself, particularly since the problem that observation was added to solve then spreads to the observer and so remains unsolved.

the waves do not suddenly become particles, they remain waves throughout, they can be constrained somewhat but they don't 'collapse'. they intersect as a data point fuzzed up by HIP.

Talking of the past with a collapse approach that throws away unrealised alternatives will be somewhat harder than another interpretation in which they are always present but hidden.

I am not sure that systems the size you are discussing are governed by QM weirdness. I don't believe that waveforms carry out a propagation that would have meaning without knowing the intersectional points and parameters.

Systems on the borderline of the weirdness.

Almo

26th September 2007, 12:48 PM

Which Dr Quantum animation did you watch? I think the one about the double slit experiment is actually very good. When I first saw it, it made me wonder why there aren't any explanations as good as that one in documentaries made by actual physicists.

DfPeprQ7oGc

The only thing I don't like about this presentation is that they don't mention that an "observation" is actually an interaction between the physical system (the electron) and the measuring device. Just like any other interaction process, it disturbs both of the interacting systems, so it's a little bit misleading to use expressions like "simply by observing".

I consider this Dr. Quantum video to be evil, because by glossing over the "simply by observing" point, the floodgates are open to all sorts of idiocy. Lots of the What the Bleep business is based on misinterpreting this.

It REALLY needs to make the point about observation being an interaction. Without this, Quantum really does seem like magic.

Dancing David

27th September 2007, 10:14 AM

People, quite naturally, try apply classical thinking to the EPR result at some point up to and including detection and this application leads then to the seeming appearance of nonlocal effects, negative probabilities or both combined. Talk turns to these faster-than-light effects, the future deciding the past and so on. Others may differ, but I don't think this is a good thing if we can avoid thinking along those lines.

I am sure you see like many that needing an observer performing measurements outside the system being studied is never more clearly shown to be inadequate as an interpretation of quantum mechanics than when that system being studied is the universe itself, particularly since the problem that observation was added to solve then spreads to the observer and so remains unsolved.

Huh, there is no observer in quantum wierdness there is an interaction like the double slit or there is a photon bouncing off a particle. The quantum things are interactions. Observers are large objects that are not subject to the wierd interactions, except at the microcosmic scale.

The universe is a large system as well, quantum wierdness is constrained when applied to larger objects. Or it averages out, like in the Feynman diagrams.

[quote]

Talking of the past with a collapse approach that throws away unrealised alternatives will be somewhat harder than another interpretation in which they are always present but hidden.

I think one can confuse an interpretation like the Copenhagen with reality. One could also confuse the many worlds interpretation with reality.

Systems on the borderline of the weirdness.

I am not sure where you are headed here.

Soapy Sam

27th September 2007, 02:00 PM

I kind of like Everett , wheeler and Graham's multiverse model. No collapsing anything, just infinite possibilities. It may be wrong, but at least it's understandable.
(Or less incomprehensible than the alternatives. )

I do wonder though- If EWG are right, must there also be an infinity of universes in which they are wrong and the Copenhagen Interpretation is right?

And if so, which are we in?

tracer

27th September 2007, 05:01 PM

Shameless plug of the SkepticWiki's page on quantum physics:

http://skepticwiki.org/index.php/Quantum_Mechanics

wipeout

27th September 2007, 09:51 PM

I kind of like Everett , wheeler and Graham's multiverse model. No collapsing anything, just infinite possibilities. It may be wrong, but at least it's understandable.
(Or less incomprehensible than the alternatives. )

You can also take the view that these "many worlds" are correct theoretical possibilities but only one actually becomes real for an unknown reason not yet in the theory. This is perhaps a form of "many worlds" a lot of people would be quite happy with.

Soapy Sam

28th September 2007, 12:08 AM

Yes- It's certainly cheaper than the other. But maybe both are real, eh?;)

At the end of the causal chain, there has to be some level where things just happen.
I tend to suspect that if the quantum level is that level, then each quantum event occurs in every way it possibly can , as in Feynman's "Sum over paths" view, rather than for any specific single reason and by any specific single mechanism. The electron arrives at the detector and the click happens. At that point we are in one of two universes which existed in potentia only, until now. But now we are in the only one that actually exists. The other is now a past potential universe- a hypothetical construct.
My gut response to the Copenhagen Interpretation has always balked at the need for an observer. That just seems profoundly silly.

Still, that's me arguing from personal incredulity- and we all know what that's worth.

schlitt

2nd October 2007, 12:19 PM

phaed

2nd October 2007, 12:32 PM

The thing about QM is that it is rife with "crazy" (ie, non-intuitive) claims. The difference between QM and woo-woo is that QM is backed up by data.

ben m

2nd October 2007, 01:20 PM

Need some advice here if possible :)

Came across a book yesterday "The quantum enigma" decided to buy it as a starting point for learning about QM.
I think this may have been a mistake... as only a few pages in they are already discussing their bias towards QM's meaning being something extraordinary to do with human conciousness.
At this stage i know NOTHING about QM, but the validity of the book already seems dubious to me.

Does anyone have experience with this book? Is it worth reading, or is it a load of what the bleep do we know type rubbish?

This book has been quite well reviewed, as far as I can tell. Quantum physics *does* require us to think critically about consciousness. Every account of quantum mechanics must describe some things happening when a phenomenon is "observed", which in some sense means that you have to build a quantum picture of the observer. This is obvious in the Schrodinger's Cat example---certainly you should be able to ask "Doesn't the cat count as an observer?" and the answer depends on your ability to describe the whole cat (including its brain) as a quantum wavefunction. From what I can tell from reviews, "The Quantum Enigma" discusses this side of quantum mechanics, which is quite serious stuff, and carefully dismisses the woo.

Ziggurat

2nd October 2007, 02:30 PM

Quantum physics *does* require us to think critically about consciousness. Every account of quantum mechanics must describe some things happening when a phenomenon is "observed", which in some sense means that you have to build a quantum picture of the observer.

This need have nothing to do with conciousness. The measurement problem in quantum mechanics always and only happens when a system we can model with quantum mechanics interacts with a system which we cannot model with quantum mechanics (because it's too complex and/or we cannot know its initial state). The result of this looks like probabilistic collapse of the wave function, but that may be just an artifact of not being able to track the dynamics anymore. One explanation for what happens is to privilege conciousness on the part of the observer, and say that conciousness is responsible for the collapse, but there's no actual need to do so, and in my opinion no good reason to either. That doesn't mean that this book may not be serious (I don't know the book), but it does mean take anything it says with a grain of salt.

schlitt

2nd October 2007, 03:12 PM

This need have nothing to do with conciousness. The measurement problem in quantum mechanics always and only happens when a system we can model with quantum mechanics interacts with a system which we cannot model with quantum mechanics (because it's too complex and/or we cannot know its initial state). The result of this looks like probabilistic collapse of the wave function, but that may be just an artifact of not being able to track the dynamics anymore. One explanation for what happens is to privilege conciousness on the part of the observer, and say that conciousness is responsible for the collapse, but there's no actual need to do so, and in my opinion no good reason to either. That doesn't mean that this book may not be serious (I don't know the book), but it does mean take anything it says with a grain of salt.

I think i understand what you are saying.
From what VERY little i know of the subject, it seems to me the phenomena is more a problem with measurement, than relating to a concious observer.
If it truly is conciousness, the obvious questions to ask would be does it need be be observed visually, or would just being aware the expirment is happening be enough to collapse the wave function?
It would defineatly not make sense if it is the former, because then it would be linked purely to sight.
But then if the latter is true, you could never conduct a real experiment.
If the conclusion is that a concious observer is what is making this happen, then it begs the question, does this observer have to be human? if the answer is yes, then the rammifiactions are obvious, and you can see where the woo springs from.
Again, it seems as though this phenomena is purely a problem or result of the measurement technique.

I am completely ignorant of this at the moment, so if i am completely off the mark, please dont mock me :)

RecoveringYuppy

2nd October 2007, 03:54 PM

One explanation for what happens is to privilege conciousness on the part of the observer, and say that conciousness is responsible for the collapse, but there's no actual need to do so, and in my opinion no good reason to either.
And this question of consciousness being responsible didn't start with quantum mechanics. It's at least as old as the tree falling in the forest.

Dancing David

3rd October 2007, 05:05 AM

Dancing David

3rd October 2007, 05:15 AM

This book has been quite well reviewed, as far as I can tell. Quantum physics *does* require us to think critically about consciousness. Every account of quantum mechanics must describe some things happening when a phenomenon is "observed",

But is electron interaction with itself (as a sum over histories) in the double slit experiment really observation in that sense, it bouncing a photon off a particle (as in Heisenberg Indetermanancy) really observation?

which in some sense means that you have to build a quantum picture of the observer.

Quantum effects are essential to chemical ineractions, however in macroscopic object the effects become statistical and the wierdeness falls off the screen.

This is obvious in the Schrodinger's Cat example

Not really the cat does not exist in a state of superposition accoding to the Copehagen interpretation. the cats is either alive or dead. the state of the cat is determined by the emission of radiation from a single atom of a radioactive substance. Scroedinger made the error of comparing a macrocosmic object to an interpretation of QM.

---certainly you should be able to ask "Doesn't the cat count as an observer?" and the answer depends on your ability to describe the whole cat (including its brain) as a quantum wavefunction.

Again with macrocosmic objects the effect of QM become statistical to an amazing degree, otherwise we would fall through our chairs and the like.

From what I can tell from reviews, "The Quantum Enigma" discusses this side of quantum mechanics, which is quite serious stuff, and carefully dismisses the woo.

But the quatum weirdness does not exist at macro scales so how does it apply? Granted a partcular cehmical interaction is govered by QM, but that does not mean that water will sudenly act in a weird way.

Dancing David

3rd October 2007, 05:22 AM

I think i understand what you are saying.
From what VERY little i know of the subject, it seems to me the phenomena is more a problem with measurement, than relating to a concious observer.
If it truly is conciousness, the obvious questions to ask would be does it need be be observed visually, or would just being aware the expirment is happening be enough to collapse the wave function?
It would defineatly not make sense if it is the former, because then it would be linked purely to sight.
But then if the latter is true, you could never conduct a real experiment.
If the conclusion is that a concious observer is what is making this happen, then it begs the question, does this observer have to be human? if the answer is yes, then the rammifiactions are obvious, and you can see where the woo springs from.
Again, it seems as though this phenomena is purely a problem or result of the measurement technique.

I am completely ignorant of this at the moment, so if i am completely off the mark, please dont mock me :)

That is amazingly accurate from my layman's POV. If a photon interacts with another very small partcile then the wave nature of a partcles kind of fuzzes up things for the interaction. So one can know the time of an event but not the location of the event with precision governed by the Heisenberg Indeterminancy Principle and versa visa. It is the place where our classical conception of how large objects behave runs into the reality of how very small things behave. The wave/partcile duality is a human concept for sure, the actual objects will interact as they interact and don't have to meet our expectations. This really bothered Bohr even as he and a bunch of other people struggled to make sense of QM.

69dodge

3rd October 2007, 05:49 AM

This need have nothing to do with conciousness. The measurement problem in quantum mechanics always and only happens when a system we can model with quantum mechanics interacts with a system which we cannot model with quantum mechanics (because it's too complex and/or we cannot know its initial state). The result of this looks like probabilistic collapse of the wave function, but that may be just an artifact of not being able to track the dynamics anymore. One explanation for what happens is to privilege conciousness on the part of the observer, and say that conciousness is responsible for the collapse, but there's no actual need to do so, and in my opinion no good reason to either.

Ok. Your theory says there's never a collapse. Rather, an experimenter and his experimental apparatus become entangled, and the wave function describing both continues to evolve unitarily. When then, according to your theory, does the experimenter become aware of the result of his experiment, and which result does he become aware of, out of the many that are possible? It is undeniable that experimenters do become aware of experimental results. Shouldn't any theory that purports to solve the measurement problem be able to explain how this happens?

I don't mean "how" in any deep philosophical sense. Just that the theory should at least acknowledge that it does happen, and should tell us when. A theory that involves only wave functions evolving says nothing about conscious observers, whose observations it supposedly is trying to explain. How could that possibly work?

Collapse serves two purposes in the current theory. It tells us what wave function to use for predicting future results, namely the collapsed wave function rather than what it was before the collapse. But it also tells us when the experimenter observes this result, namely when the collapse occurs.

You can get around the first point by saying that, to predict a future result, a second experimenter can use an uncollapsed wave function describing both the first experimenter and his apparatus, instead of using a collapsed wave function describing just the apparatus. But how do you explain why the first experimenter observed the result that he did, or any result at all for that matter?

A theory can't explain conscious observation if it refuses even to talk about it.

stevea

3rd October 2007, 07:21 AM

So one can know the time of an event but not the location of the event with precision governed by the Heisenberg Indeterminancy Principle and versa visa.

Well you can know the time and location of an event according to the Uncertainy principle, what is limited is the location*momentum product or any combination of variables that reduce to this.

I have a background in physics, but I cannot suggest any layman's books on the topic. I find that any abstract topic leads many popular authors into the arena of wholesale unsupported extrapolation. Books that try to relate QM to consciousness (without any serious supportive evidence) should be recycled immediately. I recall years ago scanning a book on holography that also included the authors pet unsupported theory of the human thinking operating by a similar mathematics - all hooey !

Personally I'd set aside any book that uses Schoedinger's Cat as a primary model. It's a cute joke, but as you can see on this forum it misleads one into many silly questions about intelligent observers and consciousness. One can readily observe photon diffraction with a sheet of film and an automatic analyser and this is done every day. To speculate that human observation changes these results is magical thinking (and that's exact where the 'cat problem' entices the reader).

The wave/particle cartoon was a good little snippet, but as already mentioned it becomes shakey at the point where they discuss observation at the double slit. They have of course ignored the discussion of observation it the back screen, and the necessary consideration of Heisenberg's uncertainty principle wrt observation. The problem is that 2 minute cartoon cannot follow all the topics to their ends.

I wish it wasn't true, but a mathematical conception of QM is really necessary. Everything else is a cartoonish view. Sadly most undergrad physics texts give a rather poor presentation of QM IMO (historical development approach).

It is the place where our classical conception of how large objects behave runs into the reality of how very small things behave. The wave/partcile duality is a human concept for sure, the actual objects will interact as they interact and don't have to meet our expectations. This really bothered Bohr even as he and a bunch of other people struggled to make sense of QM.

Let me suggest that the 'particle' viewpoint is the anthropomorphic view, and so there is no paradox to puzzle over here. The QM wavefunction descriptions are consistent without any difficulty and we can even measure single particle diffraction and make other 'wave' properties of things we normally consider to be 'particles'. Particles in macroscopic ensembles just have very tightly confined wavefunctions. The valence electrons wavefunction in a penny may extend over the entire volume of the coin, but it drop off at a high exponential rate beyond the edges. These same valence electons are repulsed by the electons of the molecules in your palm - thus giving the 'particle' sensibility for this ensemble. Still, it's simple enough in our moden era to observe the coin's electron wavefunction extend through a classically 'forbidden' potential and tunnel into a nearby detector where the small but non-zero wavefunction penetrates. 'Particles' are merely an unwarranted extrapolation from our macroscopic observations to a more microscopic domain. We were not expecting such short wavelengths.

I am not aware that N.Bohr was particualy puzzled by the duality as you suggest. "Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems". (Niels Bohr, Atomic Physics and the Description of Nature, 1934).

Dancing David

3rd October 2007, 08:27 AM

My comment on Bohr comes from readin a number of biographies about him, I cae away feeling that he wished that the equations would conform to expectations and was frustrated by it, but he was very open to accepting that they didn't meet his expectation as well. I should not have expressed it as being directly related to the alleged wave/particle duality but QM in general.

Ziggurat

3rd October 2007, 11:08 AM

Ok. Your theory says there's never a collapse. Rather, an experimenter and his experimental apparatus become entangled, and the wave function describing both continues to evolve unitarily. When then, according to your theory, does the experimenter become aware of the result of his experiment, and which result does he become aware of, out of the many that are possible? It is undeniable that experimenters do become aware of experimental results. Shouldn't any theory that purports to solve the measurement problem be able to explain how this happens?

How do you know multiple results are possible? You don't actually know that, except to the extent that multiple initial states are possible. But if you somehow could know the initial state of the entire apparatus and model its time evolution, how do you know that you wouldn't end up with only one deterministic result for any single initial state? Quantum mechanics suggests that's exactly what should happen: there is no equation anywhere in quantum mechanics that describes a discontinuous time evolution for a wave function. Collapse only happens when we stop using quantum mechanics, it is not what quantum mechanics itself describes.

We can't test whether or not collapse is actually happening because we don't know the initial state of our entire setup, we couldn't model its time evolution even if we did, we can't prepare the apparatus in specific quantum states even if we could know what that state was, and the number of quantum states for a macroscopic measurement system is astronomically larger than the number of "result" states. These practical problems prevent us from testing the theory fully, and they leave a hole in which collapse might indeed be some discrete, real process. But that's only a guess. It might be correct, but we most certainly do not know that it is correct.

I don't mean "how" in any deep philosophical sense. Just that the theory should at least acknowledge that it does happen, and should tell us when.

That's easy. "Collapse" happens whenever our quantum system interacts with a system with an unknown quantum state but with a large number of possible states. In other words, it happens when we find ourselves unable to use quantum mechanics to describe the dynamics, not because it's wrong but because we can't do the calculations. Find any example of collapse, and you'll find that such an interaction is occuring. So there's simply no need to posit that anything more.

A theory that involves only wave functions evolving says nothing about conscious observers, whose observations it supposedly is trying to explain. How could that possibly work?

That last question is exactly my question about "collapse": it either leaves unanswerable questions (what exactly are the criteria for collapse to occur? What counts as "counscious"?), or else it just becomes a sophisticated version of solipsism.

Collapse serves two purposes in the current theory. It tells us what wave function to use for predicting future results, namely the collapsed wave function rather than what it was before the collapse. But it also tells us when the experimenter observes this result, namely when the collapse occurs.

Except that it can't answer the question of what counts as an experimenter, which makes all the other answers pretty much just hand-waving.

A theory can't explain conscious observation if it refuses even to talk about it.

Sure. But that assumes that conscious observation is a critical component. But we cannot conclude that it is.