Quantum Physics: I need some sanity here x_x

[JanisChambers]

I was watching a cute animation called "Dr. Quantum" recently. The clips I could find talked about the amazing science of Quantum Mechanics, but then when I tried to look up more on this field of science It quickly spiraled into very shaky ground.

I don't think I have to describe some of the Woo Woo ideas that some people have tried to get away with by applying quantum physics with larger scales. My question is where can I find information about Quantum Physics that isn't polluted by crazy claims?

 

[HghrSymmetry]

Many texts are available on the subject. For a general audience introduction;

http://www.amazon.com/Quantum-World...2860154?ie=UTF8&s=books&qid=1188865356&sr=1-3

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If you don't mind a technical introduction, you may find this one interesting:

http://www.amazon.com/Introduction-...860154?ie=UTF8&s=books&qid=1188864904&sr=8-13

 

[Terry]

Feynman's QED: the strange theory of light and matter is a nice non-mathematical book.

 

[fuelair]

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...pf_rd_p=305856701&pf_rd_i=Quantum Physics BBC

http://www.amazon.com/ELegant-Unive...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-...7-7901452?ie=UTF8&s=dvd&qid=1188865804&sr=1-1

 

[Mobyseven]

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]

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]

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]

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.



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".

 

[SusanB-M1]

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]

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]

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]

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]

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]

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]

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.

 

[Ziggurat]

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]

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]

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]

(*) 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]

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.