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Quantum Mechanics Overview 
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#1
Dec2403, 03:47 PM

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For the few of us here who use this forum for things other than physics (strange, I know...) could someone do me a favour and please give us a brief overview of what it is that quantum mechanics really concerns itself with, and how this stuff is known?
I was asked this the other day, and wqas surprised to realise that I couldn't really answer it myself. I know there is the stuff about velocity and location of a particle, but that is about all I know, and not only do I not know it well, but I have no idea how that knowledge claim is justified. So, please, looking for a quick rundown of 'QM' Thanks 


#2
Dec2403, 04:35 PM

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#3
Dec2503, 03:32 AM

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Thanks alot, I'll have a read of that, and post any questions that remain



#4
Dec2603, 10:07 PM

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Quantum Mechanics Overview
OK, that has given me a good background now, for what has happened and stuff, but it hasn't really explained any of the theoretical aspect of anything: It just says what theories were proposed and by who without really covering why or what it means. Because of that, most of it was quite meaningless to me.
So now that I have some sort of background framework, maybe someone could fill in some of the details and the meanings? One question that comes straight to mind from that paper though, is the momentum/position issue. So often in discussion on this forum I have heard people refer to that, implying that the claim is that a particle can either have momentum, or position, neith at the same time. I mean, they imply that the uncertainty principle claim is a claim about the nature of the particle. I never liked that, but accepted that i knew nothing, and so never questioned it. This paper on the other hand states it as if it is in fact our 'measurement' of the momentum and the position which are the issues, not the momentum and position of the particle itself. And the fact that it is based on us measuring either the momentum OR the position, implies to me that it may be possible to one day refine our techniques so that we can somehow measure 'the particle', and thereby gain knowledge of both momentum and position AT THE ONE MOMENT, rather than trying to measuring one, and then the other.... How far wrong am I? 


#5
Dec2703, 02:52 PM

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One way I make myself comfortable with the indeterminacy issue is by recognizing that the classical variables (position and momentum) are artifacts of our macroscopic experience. That is, we think that a particle should have a simultaneously measurable position and momentum because we are used to thinking that they should. Once one sheds the idea that position and momentum are the correct degrees of freedom in which to embed one's mindset, one can see that quantum mechanics is a deterministic theory in a different set of variables: Ψ and Ψ*, the wavefunction and its conjugate. Now a lot of people would find this distasteful, because wavefunctions cannot be measured (they are complexvalued quantities), whereas position and momentum can be measured. They would say that nature would never be so absurd that its true, fundamental degrees of freedom are purely mathematical objects. But what are you going to do? It's not as though you can tell nature how to behave. edit: typo 


#6
Dec2803, 11:40 AM

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[QUOTE]One way I make myself comfortable with the indeterminacy issue is by recognizing that the classical variables (position and momentum) are artifacts of our macroscopic experience. That is, we think that a particle should have a simultaneously measurable position and momentum because we are used to thinking that they should. Once one sheds the idea that position and momentum are the correct degrees of freedom in which to embed one's mindset, one can see that quantum mechanics is a deterministic theory in a different set of variables: Ø and Ø*, the wavefunction and its conjugate.
Now a lot of people would find this distasteful, because wavefunctions cannot be measured (they are complexvalued quantities), whereas position and momentum can be measured. They would say that nature would never be so absurd that its true, fundamental degrees of freedom are purely mathematical objects. But what are you going to do? It's not as though you can tell nature how to behave. [\QUOTE] I agree with the spirit of what you are saying, but I think you have been rather simplistic. In what sense are you regarding the wavefunction as a dynamical variable? Are you saying that it is an objective property of the real world? I am much more of the opinion that it is a state of knowledge that we have about a physical system. There is a sense in which the wavefunction can be measured  if one has a number of copies of identically prepared physical systems. This is known as quantum state tomography, and is becoming an increasingly important tool for quantum information and computation experiments. However, the fact that you need an infinite number of copies of the system to determine the wavefunction precisely leads me to regard it as being analogous to a probability distribution over dynamical variables rather than being a dynamical variable itself. Now, classical probability distributions can be interpreted in two ways: 1) They tell us about the relative frequencies of outcomes we can expect (the classical approach). 2) They encode states of knowledge, or states of belief (Bayesian approach). Analogous approaches to quantum states have been proposed, mainly in the relative frequency interpretation, but I believe that the Bayesian approach has the possibility to clear up a lot of the thorny issues in the foundations of quantum theory (See Chris Fuchs paper  "Quantum foundations as quantum information (and only a little more)" available on the quantph arXiv http://www.arXiv.org/quantph/). 


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Dec2803, 02:41 PM

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#8
Dec2803, 10:04 PM

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Most of those last two posts meant nothing > Very little to me, but I can get the idea of dropping our idea of momentum and position in terms of the macroworld.
I am reading a book atm, although it had nothing to do with me starting this thread, it i largely based on QM stuff, and so coming to understand QM stuff better will allow me to be more critical of what is being said in the book: it's called The Field by Lynne McTaggart, and it is all about the Zero Point Field. I'm only a few chapters into it, and it has so far claimed that scientists have explained Gravity, F=ma, Homeopathy, and how biological molecules communicate. And while I feel like the author isn't the best (Journalistic and quite biased), I can't deny the scientific work, so I am intrigued. 


#9
Dec2903, 05:28 AM

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I too have just started reading the same book (I got it for Xmas).
I don't pretend to understand all the theories behind the Zero Point Field (ZPF) but don't virtual particles have a corresponding virtual antiparticle and so preclude the harnessing of 'free' energy as it all equates back to zero? Are the theories of the physics of ZPF accepted or is it something that the new age spiritual groups have grasped upon as their proof for for all things psychic etc. ? 


#10
Dec2903, 05:05 PM

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Apparently, the key is to harness both the positive and the negative, thereby setting up an alternating current. The one counter claim that I have heard against the idea sofar though, is that it can't be done because the frequency is too low. You wouldn't be able to get any real power out of it.
Thats what I understood anyway.... 


#11
Jan204, 04:03 AM

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Getting back on topic, I think I have a good enough basic conception of Qm (ie: The stuff that everyone knows), but I am now interested in moving into some more practical knowledge. I am already starting to simply read the threads here (that is always a good way to learn stuff), but are there any good tutorialesque sites around that can help someone make their way into the Qm world?
I am even willing to start relearning Maths... 


#12
Jan204, 08:15 AM

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I just googled on tutorial, quantum mechanics and found several. Rather tha my choosing one for you, why don't you do the google and pick the one that fits your needs. Although a lot of them have animated apps to illustrate things, you don't want to limit yourself to tutorials that only do that. You would probably learn more than you know know, but you wouldn't improve your ability to understand the LaTex conversations on these boards. A one dimesion of space plus time simplified model done with math is your best bet.



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