Quantum "vs" Classical ?

[CraigS]

Is there really a fundamental difference between a quantum mechanical system and a classical physics system ?

When someone says that a system is 'fundamentally a quantum system', does this mean that it is somehow different from a classical system ?

If so, what is the fundamental difference ?

Cheers

[xelasnave]

Still looking at this and have yet to form an opinion.

alex

[avandonk]

When the mathematical functions describing the system are no longer piecewise continuous.

There is another region when functions asymtote to infinity.

Apart from that simple linear equations will get you by if you believe in a simple Universe! If you really want to be a smartarse you can invoke higher order polynomials.

Bert

[traveller]

If you haven't read it, I highly recommend StepheN Hawkings a brief history of time. Even I managed to understand most of the concepts there :)

[CraigS]

Thanks Bert and Bo;

Very interesting … I really don't know the answer to this question myself, so I really appreciate everyone's views on it.

It just seems to me that it is frequently said that a system is 'fundamentally behaving in a quantum mechanical way', because we cannot understand it with a classical model.

But perhaps QM kind of soaked up what might have been missing from the assumptions about single particle behaviours, which were kind of 'missed out' when classical mechanics was developed ?

There seems to be an inordinate amount of time and energy expended trying to "unite the two worlds" into a common 'framework' and yet, it kind of occurs that maybe the 'gap' (or discontinuities) which everyone is focussing on, may actually simply be more about, (for want of a better term), 'finger-pointing', aimed at highlighting the errors originally made by expecting single particles to behave in ways, they basically never did in the first place … and, perhaps it is simply 'expectations' that are in error.

For example, the double slit experiments seem to end up being described as 'fundamentally quantum mechanical', because the interference patterns can't be explained when we 'imagine' the way we think a single particle could possibly behave. Perhaps this is purely because our 'imaginings' about how particles behave, simply weren't accurate in the first place, and we were missing something fundamental in the classical descriptions of how particles in nature really behave (??)

Is any of this saying anything fundamental about nature/the system, or is it saying something about our models ? :question:

Are there any physical behaviours of a quantum system which don't depend on an expectation of how we think classical particles behave ? :question:

Cheers

[sjastro]

A classical physics system doesn't provide the nuts and bolts description of a quantum mechanical system.

Classical physics primarily involves the use of phenomenological theories.
By phenomenological it means that classical physics is built on explaining observation and experimental data without necessarily providing a mechanism.

For example the apple falling from a tree is explained by gravity, yet Newton's theory of gravitation provides no clues as to what causes gravity.

A classical physics system is an approximation of a quantum mechanical system.

Physicists empirically determined the Balmer series for the spectral lines of the hydrogen spectrum in the 19th century and explained it as atomic emissions.
This is classical phenomenological physics at work.

Quantum mechanics developed in the early 20th century on the other hand provided the explanation for the atomic emissions.

Regards

Steven

[CraigS]

Hi Steven;

Thanks for your post.
Yes, I agree … its all very interesting, and the differences between the two approaches is more often than not, portrayed in terms of how QM evolved in history, as primarily a challenge to classical physics.

As far as the historical significance is concerned, perhaps the development of QM serves more as a kind of reminder that the phenomenological approach can, and does, leave gaps in the explanations and those gaps are likely to be filled by someone ... given an 'alternate name', and forever after will be seen as representing something fundamentally different in nature ? :question:

Whilst I think following the historical development of QM might help in learning about QM, I’m not so sure this approach necessarily aids in re-inforcing the accuracy of QM, in so far as it more closely representing reality (whatever ‘reality’ is, eh ... might leave that one aside for the moment ;) ). Sometimes the QM arguments are used in this sense over classically based arguments. I suppose at the end of the day, QMs evidence based, empirical track record speaks more directly to this point.

However, I wonder what might have happened if incomplete classical physics, simply proceeded on its normal course .. would it have eventually encompassed everything QM does, but in its own descriptive language and the ‘gaps’ we now see wouldn’t appear, and lead us to the conclusion that there seems to be a fundamental discontinuity in nature (along the lines suggested by the mathematical approach .. as in Bert’s post) ?

Steven mentioned the Balmer Series for hydrogen. Is it now possible to generate a discrete spectrum using classical wave principles (I wonder) ?

Cheers

[sjastro]

There is no "conflict" between the two theories. QM is an evolution of classical physics. It hasn't replaced classical physics but takes it to a more fundamental level.
The slit experiment is explained classically by treating light as a wave. QM extends this to wave/particle duality.



Bohr came up with a "semi classical" or "classical/QM hybrid" which explained the spectral lines. This was however part of the evolution of classical physics to QM.

Regards

Steven

[CraigS]

Yes .. again, I can whole heartedly see this also.
(A first for CraigS, eh ? :clap: … woo hoo ! …)
.. However, I'm afraid quite a few others don't, though.


So then, perhaps the discontinuities made obvious when we compare 'the fundamentally quantum system' with the 'fundamentally classical system', were always there in the classical model, but were unknown and unrecognised ?… And when QM came along, it did expose the discontinuities .. but these were always present because of the ways Classical was attempting to explain wave and particle dynamics ?

I know its a subtle difference, but I hope others might see that there's been a huge amount of effort expended to unify 'two different worlds' by asking very deep questions like: "why the behaviours of the big and the small don't match up ?", when it could simply have appeared this way because the descriptions of wave and particle dynamics in classical physics were incomplete somehow ?
(I think we're on the same tracks here, also).


Fair enough, too … and its interesting to note that we still refer to the differences, (discontinuities really), as somehow a function of some different behaviours, encompassed by the labels: "QM" and "Classical".

I mean, how often is "QM behaviour" referred to as "spooky" or "weird", seemingly to evoke an alluring perception of something which QM provides us with over classical ? (This is a perfect breeding ground for nutters who for example, hijack QM terminologies to suit their own malicious intents).

Cheers

[xelasnave]

It is difficult to form an opinion on two matters one knows very little about and that is indeed part of my difficulty however the ideas outlined in this thread I have found more helpful in moving forward than any other material I have read. I thank you all.

This thread identified an undefined concern I have had for some time.

I find Steven's statement to be most helpful.

...There is no "conflict" between the two theories. QM is an evolution of classical physics. It has hasn't replaced classical physics but takes it to a more fundamental level.

I have felt that GR had closed the door on gravity (in so far as it is seemed to me to be presented that way) and whilst being presented with a feeling such to be reality I felt gravity must have a mechanical explanation.

I now dont feel guilty to suggest that possibly GR maps the activity of something smaller and that the fabric of space may be woven from particles or quanta that gives it a tangibility that seems to confilct with the fundamentals of GR. I never saw there need be a conflict.



alex:):)

[sjastro]

First of all my comment.



Remove the "has" term and it makes sense, "has hasn't" implies quantum duality in itself. It's what happens when your brain is in QM mode.:lol:

GR is a phenomenological theory so it doesn't explain the mechanism of gravity despite Brian Cox in a recent SBS documentary suggesting otherwise.

Regards

Steven

[CraigS]

Hmm … I didn't even notice this .. and if I had, my brain would have seen it as a typo !
I'm left wondering about the explanation, however … ;) :lol:
(This is fun .. and pretty 'geekish' ..).
:)

Hmm .. once again, it seems that the label "phenomenological theory" could also be interpreted as a way of confining Classical .. and could also be seen to preserve what we've established as only a perception/historically evolved matter.

I think the language is a big culprit in creating this perception. :question:

Very interesting .. and thank you to all, for such an enlightening discussion!

Cheers

[renormalised]

Just goes to show you, Steven, that's how the brain works....it's quantum:):P

Guys...to put it in plain and simple terms, what Steven is saying, is that QM is an "extension" of Classical Physics. It takes what is already known and extends it to a new level of understanding. Instead of just explaining what is happening, it is looking at why it happens as well. Instead of just looking at the forest, it's also looking at how the trees create the forest.

[xelasnave]

Thanks Steven:thumbsup:

mmmm Interesting I interpreted things to suit myself:eyepop:... curious.

I think words fit their context... but it seems we both knew what you were saying.

alex:):):)

[xelasnave]

and String theory is looking at the leaves:rolleyes:?

I like the way you put that Carl:thumbsup:.


alex:):):)

[CraigS]

String theory, I think, has also evolved to bridge the gaps, (which have always existed), in the way Classical attempts to explain wave and particle dynamics.

By replacing particles and waves by another meta-'object', a string, an integrated view emerged. Only problem is that the unexplained particle and wave 'objects' remain (over to 'QM' to sort this out). I think the string theorists recognised the integrating power of creating a new view .. but this doesn't necessarily mean that the fundamental objects in the universe are strings.

String theory, I think, carries another message for us .. which is not necessarily related to the reality of nature. :question:

Cheers

[xelasnave]

My impression is a string is a way of visualizing (quantifying etc) the movement of a particle which can be shown as a wave function...and in that form it is something we can work with... I dont know if that is valid but that is where I am up to Craig.

I was comptemplating the energy available at a point in space coming from a place where I would use only the smallest quanta of energy..what ever that may be...energy at a point would be probable as the number of trajectoies less a number that did not convey energy times the smallest quanta...this is my approach out of respect for maths Craig..using only the little I have to work with...anyways a goggle turned up some fella who had worked out the smallest quantum (by his system) only to go on to expound the push LeSage approach and tie it in with GR.. new theory of everything..he had the maths etc but the net has let me off pursuit of the matter because as with most things not only has it been done but done at a level one never could have reached... but I had to laugh.. the guy may be a crack pot..that is not the issue really... so its been done to death...and he has a book;)

http://www.rostra.dk/louis/

Does string theory treat the photon as a fundamental particle??? must check...and what is a fundamental particle really made from...the russian doll spiral of my imagination knows no bounds.

alex:):):)

[bojan]

Yes.. phenomenological... or, maybe the better term might be behavioural..

However, QM - while cutting much, much deeper into mechanisms of nature - still doesn't provide explanation as to what causes gravity, or electrical charge, what indeed is elementary particle... and why they are elementary (IF they are).
So, we can say that even QM is an approximation of "higher level" physics (strings? or something entirely different..), without conflicts again of course.

[sjastro]

All theories are phenomenological to various degrees and can change according to the physical processes they describe.

For example GR is phenomenological with respect to gravity, it treats gravity as an example of Newton's Third Law in operation.
With regards to the effect of gravity on light, GR is non phenomenological. The gravitational bending of light was predicted by the theory, rather than observation used to formulate the theory.

Regards

Steven

[CraigS]

I think that even though String Theory may not have started out with the goal of unification, (which it is now recognised as providing), the lesson here may be one of how to go about unification in theoretical physics.

QM also seems to have unified other problem parts of classical physics like the gaps between particles and fields, and a favourite of mine: deterministic and random behaviours (which doesn't appear to have been really happening prior to its conception).

The problem seems to be that it was seen as a competing idea. Nowadays, the driving 'trend' is towards unification (of just about everything) and String Theory is more 'acceptable', and is hence met with more open arms.

I guess it could also be said that QM came about because experimental/observational science got way ahead of theory, too?

Interestingly though, String Theory seems to be around the other way!

Cheers

[shane.mcneil]

Probably a bit off topic but I recently read this book. (http://www.betterworldbooks.com/three-roads-to-quantum-gravity-id-9780465078363.aspx) It is an interesting overview. From memory his big issue is that whichever theory prevails it can't be formulated against a fixed background, which GR and String include. (Please note: I really do not know what I am talking about :))

C ya.

Shane

[CraigS]

Hi Shane;
We had a thread about the latest on Smolin's work recently here. (http://www.iceinspace.com.au/forum/showthread.php?p=752269&highlight=Smolin#post752269)
Whilst I wouldn't make any call one way or the other, I kind of like his approach.

In the context of this thread however, notice the language in the 'About the Book' description:

We need to recognise the religious imagery projected by this kind of language for what it is designed to do … in this case, to sell books and promote the illusion of conflict between QM and Classical Physics .. which does not appear, once one researches the topic in more depth. If there is, I'd like to know where, how and why this supposed 'conflict' occurs (as would others in this thread) … please don't feel that anyone other than someone highly skilled in theoretical physics would know how to do this, but I do challenge (and object somewhat) to the unsubstantiated connotation this view represents.

There's a lot more scientific value in Smolin's ideas, than is portrayed in the language used in this description.

As I have presented, the two theories, whilst having discontinuities and asymptotic behaviours when united, doesn't necessarily mean that nature is working this way.

Thanks for the heads-up … I'll bet its an interesting read.

Cheers

[renormalised]

It doesn't take a whiz in theoretical physics to be able to tell you why the conflict between QM and Classical Physics occurs, Craig. That quoted bit in your reply hits the nail on the head. It's all to do with making an impression upon a generally clueless audience....the wider population. From what little they do know, they see a conflict between QM and Classical Physics...even though most couldn't tell the difference between the two. However, they do know that, on the surface of things, there appears to be a big difference between the two. You have to admit, one seems to be concerned with a nice, ordered, fairly easy to understand view of things (Classical), whilst the other just doesn't make any common sense at all (Quantum) and is completely weird. In the public's mind, they just don't meet anywhere in the middle. And so, the scientists sort of promote this themselves....it makes physics "more exciting", more "real". Plus, it helps sell books (in Lee's case) and promotes other things like TV programs etc.

Like you said, once you care to really take a closer look at it, it's nothing more than smoke and mirrors where the physics is concerned. But most people wouldn't even bother to do that, so they go with what they know. They know the science is "sexy" these days, but in reality if they knew it was actually a case of long hours of brain wracking, hard slog with very few moments of enlightenment thrown in here and there, they'd completely lose interest in the subject altogether and physicists would be back to being weird geeks in lab coats (which many still think physicists are).

[CraigS]

As I said:

… this is the topic of this thread.

At the moment, and until some rational evidence/example is produced, I refute any claim that there is any 'conflict' between the theories.

I see no need to admit anything, where terms or arguments implying divisiveness are present anywhere other than in perceptions, which thus excludes such discussions from the scope of science.

Cheers

[Dave2042]

Here's my recollection from studying it 20 years ago.

The difference between classical and quantum physics is that classical is deterministic. Given starting conditions and enough computing power, once you specify the initial conditions of a classical system, its evolution is entirely determined (this is even true of chaotic classical systems).

In quantum physics, outcomes are only probabilistic. This is a result of (a) superimposition of complex wavefunctions (Schrodinger formalism) or equivalently (b) non-commutativity of operators (Heisenberg formalism).

It is often not appreciated that relativity is strictly in the classical physics category (ie deterministic).

As to conflict, yes as mentioned below in a sense there is none - it's just that quantum extends classical to regimes in which classical breaks down. However in another sense there is. Physics currently models the universe with two 'big' theories, quantum mechanics and general relativity. Each works wonderfully well in its range of applicability, however you wind up with problems where they both should apply (big bang, inside black holes).

The simple version is that GR always treats things as having definite values (including position) where QM says that plenty of things generally can't have definite values (position being one). The full version is a blizzard of maths.

Trying to extend QM to encompass GR (gravitons etc) is not something anyone has ever really managed to do in a mathematically coherent and testable way. String theory has some promise, but suffers from lack of testability given current equipment, but also the more serious problem that it looks suspiciously like an attempt to simply chuck more and more parameters at the problem so that, of course, you'll get a fit eventually, though not one that tells you anything physical.

In short, it's a real headscratcher.

Always happy to hear a contrary view.

[CraigS]

Hi Dave;

Good to have you aboard !

Thanks for having a go at distinguishing between the two 'domains' for us … I would agree, that they are classic 'textbook' distinctions.

Where I was originally coming from, is not so much the aspects of the models, which are "quantum mechanical" because they exclusively appear in 'quantum mechanics textbooks', but rather the aspects of the actual observed phenomena which result in those models (eg: like diffraction, discrete spectra etc). I'm fine with the respective 'domain labels' providing the distinctions .. we gotta have 'em in order to communicate clearly.

I appreciate what you have said, and this is not intended to counter any of it … rather I just want to reinforce the possibility that just because the models define and label things in these ways, it is still possible, and perfectly legitimate, to view these things in Classical ways. This wasn't really done conclusively in the past although as Steven mentioned, Bohr came up with a "classical/QM hybrid" which explained spectral lines (for example).

Thesedays, I suspect no-one bothers to attempt to do such (even though nowadays, it may be more fashionable to try) .. one is kind of left to ponder what might have happened had Classical models been reworked in the light of explaining these phenomena … the conclusion might well have been that there may not be as much disconnect in nature, as we are presently led to think.

The driving question is:
When do we know we are dealing with a "fundamentally quantum mechanical system"? .. observationally that is, from the phenomena themselves ?

Your words I underlined are interesting, as they seem to re-inforce our deterministic, Classically motivated expectations, that the domains we originally invented ourselves 'should' apply externally to the ranges we originally specified for them. I can see that this expectation may simply be of our own invention, and may not necessarily be the way nature works .. why should it be ?

Cheers

[CraigS]

As an example of relevance, this article might be a couple of years old, but the research probes the size boundaries (another common 'boundary' used widely to distinguish a 'Quantum system' from a 'Classical' one):

Physicists working up from atoms to Schrodinger's cat (http://www.physorg.com/news152369994.html)

(PS (aside): Physorg crashed straight after I published this link .. hope it comes back shortly ..)



So, it would seem that if 'entanglement' can be observed at millimetre-scales, it would seem that 'scale' may not necessarily be a distinguishing feature of a 'quantum system', either ? :question:

This research shows me the conjugate view to where I've been coming from so far in this thread .. ie: perhaps because Classical systems (millimeter-scale) can be viewed as 'quantum mechanical', this may speak as much about the models as it does about the nature of anything actually behaving fundamentally differently.

(Ie: a view which seems to be already supported and confirmed by others here, earlier in this thread).

Cheers

[Dave2042]

Hi Craig

I understand and agree the dangers of simply labelling stuff 'classical' and 'quantum' or anything else and then blindly hacking away.

Nevertheless, it seems to me that there is a profound difference between QM and non-QM and that the difference is based in physical reality, corresponding to operator noncommutativity. Further, it seems that the world is quantum mechanical, but that in many situations we are simply able to approximate it away. I'd put up the failure of hidden variables, plus Bell's theorem in support of this view.

Of course, exploring and even shifting the boundary between where can / can't make classical approximations is useful and interesting, but I don't think it means that QM isn't real in some way, or that it doesn't still underpin what we see as the classical world.

Finally, my 'should work' was really only about observing that a model has passed the limit of its applicability, and that's where things get interesting, which seems to be what you're saying.

I do confess to the position that there is a reality, that elements of it are possible to understand, that maths is a reliable way of representing it and that we currently understand some of it. I understand and cheerfully accept that this is regarded as a quaint view in some quarters.

[CraigS]

Come to think of it, that research also suggests that even entanglement may not be the exclusive domain of the very small (or necessarily exclusive to the 'QM world').

Cheers

[Dave2042]

Craig

Your last two posts are exactly in line with what I was saying in my last one. I think that the universe is QM at all scales, just that we are afforded the luxury at our scale of approximating it as classical. Historically though, much effort has been expended trying to show the opposite without success.

[CraigS]

Which I think is fine, also .. it would be crazy to ignore what QM has to say .. and if I have ventured in that direction, this definitely wasn't my intent … and I'm happy to retract anything I may have said on my part, in that regard (have I said such ?? .. oh well .. I didn't mean to anyway). The evidence supporting the theory is pretty well conclusive, as far as I'm concerned.

The apparent 'gaps' between what we call Classical and QM may simply be because of the approximations made in Classical … and nature does what it does, and is unlikely to behave differently.

Yep .. agreed.

Yep .. 'interesting' is a good word for it .. there is no 'conflict' between the models, either .. the 'gaps' we search to explain, could be simply buried in the Classical approximations (or in assumptions about the real fundamental behaviours originally unrecognised in Classical).

'Quaint' ? …. maybe … but I very much live in those quarters as well !
:)
Cheers

[Max Vondel]

Craig, I think of Quantum Mechanical as a probability function, uncollapsed waveform
In Classic the wave form has collapsed and the probability is lost to a definite state. Unsure if that helps
PB
:)

[CraigS]

Hi Peter;
Thanks for your post .. interesting .. exploring wavefunctions for a bit might be an interesting exercise ..

Correct me (anyone) if I’ve got any of the following wrong .. I’m also learning about QM with every step here, so here goes ...

I haven’t found anywhere in any of the main QM interpretations where it actually states that quantum interactions are probability waves. I also think that the main famous ‘Interpretations’, like deBroglie-Bohm and Many-Worlds, contain A LOT of Classical determinism, actually.

At best, ‘Many Worlds’ seems to explain non-observations of ‘pure states‘ evolving in a determinable way, purely because our observations can’t detect the full ‘wavefunction’ of many worlds. DeBroglie-Bohm seems to start out interpreting quantum interactions as ‘fundamentally probability waves’ .. and a lack of history info about the system of interest, is caused by something unknown, but external to it (suspiciously very Godel-like and Classical Chaos Theory-like). This hence, seems to be the ultimate expression of Classical determinism at work, to me.

The Copenhagan interpretation seems to have fallen somewhat out of favour in QM Physics circles thesedays (from snooping around a bit ... something disliked about the ‘Heisenberg Gap’ approach ?)

Anyway, if three of the main QM interpretations don’t strongly bestow purely probabilistic behaviours as the exclusive domain of ‘QM’, then even these traditionally QM behaviours, don’t necessarily distinguish QM over Classical (as is commonly inferred). Also, as mentioned above, these interpretations also seem to be coming from very Classical, hence deterministic principles.

Another area usually used to distinguish QM, is Wavefunction Superposition. So, Classical Mechanics treats ‘particles’ as particles. It would seem illogical to superimpose one particle on another in Classical. But I still think it could be done (??) Superposition is kind of a ‘wave’ concept, so how does one get a wave out of a particle. :question:
Perhaps the path of a particle could be seen to be described by using superposition ?... I know in electrical field theory, superposition is all over the place (eg: Maxwell’s equations, from Classical). Fourier analysis is a good ‘flow-on’ example of this, too.

It also seems that QM talks a lot about wave amplitudes (there’s not much ‘non-classical’ about that).

Surprisingly, its tough to find anything which definitively says that QM is based on ‘probability waves’, also.

So it seems, that perhaps even ‘wavefunctions’, ‘wavefunction superposition’ and ‘probability waves’, may not necessarily be exclusively ‘owned’ by QM.

Cheers

[Dave2042]

Hi Craig et al.

Another 2 cents' worth from me.

While a lot of mathematical techniques (waves, superimposition, uncertainty, complex numbers) used in QM are also used in other (classical) contexts, I think that this can mask fundamental differences, rather than signal fundamental similarities.

As I touched on below, the fundamental difference, as I see it, is that in QM (prior to wavefunction collapse - thanks Max), physical quantities generally do not have well-defined values. As I understand it both Copenhagen and many-worlds agree this. In the language of the Heisenberg Uncertainty Principle, it is not simply that we can't measure both a particle's momentum and position exactly, its that a particle doesn't have both an exact momentum and position.

This can be intuitively thought of as a result of wavefunctions, but alternatively you can simply treat it as a mathematical framework that maps to measurements (I find the former easier).

There is no analogue of this in classical physics as I see it. For example, in classical chaos there is unpredictability which can look similar to QM, however it is only a result of our inability to precisely exactly boundary conditions. All the physical quantities concerned still have well-defined values at all times, even if we don't know what they are.

[johnnyb]

Hi All,

Sorry, coming in late to this discussion, but I'm looking for a distraction from a rather boring task at work today :thumbsup:

I just wanted to highlight a few points from the last few posts which may or may not help. The words "entanglement" and "uncollapsed waveform" were used and I think whese concepts are the key to understanding the "fundamental" difference between classical and quantum approaches to modelling "reality". I'm using lots of quotes here because I think different people interpret these words in different ways when talking about quantum stuff - for me the only way to reallly grok quantum physics is to do the maths (which I know is not practical for most people who have a real life to get on with http://www.iceinspace.com.au/forum/../vbiis/images/smilies/lol.gif) ; if you have to rely on words to explain quantum stuff it will be really hard to get a common or correct understanding of what's going on.

Bell's theorem (http://en.wikipedia.org/wiki/Bell%27s_theorem) show us that "classical" theories really cannot explain how the world works at quantum scales, due to the uniquely quantum phenonenon of entanglement. It is only when you introduce an "observer" or "measurement" into the system, which causes entanglement to be lost (i.e, "collapses the wavefunctions") that the system will start to follow classical laws, i.e., it has lost the thing that made it quantum. (As an aside, this is the reason why quantum computers are so hard to construct - you need the entanglement between the "bits" to enable the magic quantum calculations to be done, but to build anything on a scale that has enoughs bits to do a useful calcualtions exposes you to the "measuring effect" of the enviroment.)

Now how you define an observer and measurement will open up another can of worms, and is sort of off topic. These articles will get you thinking though:
http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics
http://en.wikipedia.org/wiki/Quantum_decoherence

HTH.

John.

P.S. Before anyone complains, I know the experimental tests of Bell's theorem are not conclusive, and even if they were interpreting Bell's theorem is still a difficult thing to do for our classical brains using normal language.

[sjastro]

Craig,

The mathematics is the same for each interpretation. Mathematically the solutions are spherical harmonics or "wave-like" for the various QM operators such as the Hamiltonian (energy) or angular momentum.
The term wavefunction and probability waves are a reflection on the mathematics.
The square of a wavefuction gives the probability of obtaining a certain measurement. If we knew the superimposed wavefunction, the square of this wavefunction has a probability of one, which is not surprising as all the possible outcomes are included in the superimposed wavefunction.:thumbsup:



The key here is the role of the observer. In all interpretations of QM the observer and outcome are part of the measurement process. In classical physics the observer is independent of the measurement.
This is what distinguishes classical physics from QM.



The properties of a particle such as energy, position, momentum, angular momentum, intrinsic spin etc can exist in superimposed states.
The concept of a particle existing as a wave is a direct consequence of the Heisenberg uncertainty principle where we are unable to precisely measure the position and momentum of particle at the same time.

Regards

Steven

[CraigS]

Steven;
Thanks for that … I might take some time to absorb and mull that over for a while, before commenting further on the 'hints' contained within.
Thanks kindly.
:)

Dave and John:

It seems we're all very strong on distinguishing QM from Classical .. and using all of the textbook QM concepts to do that, which, as I mentioned, is fine, and I agree, is a very traditional and effective way of learning what QM is all about. (I'm not sure I'm a traditional kind of bloke, mind you. :) )

I guess I'll have to admit that my quest is more than just learning. I suppose that has become clearer, as this thread goes on … so I'll be perfectly honest ...

I am deliberately attempting to find ways of avoiding distinguishing QM over classical at the fundamental levels, as I think it should be perfectly feasible and legitimate to do so. (My learning goal will still be achieved by following either approach, I think).

Others .. (Steven, Carl, etc) seem to agree that QM sits nicely nestled inside Classical and I agree (at the moment), so I'm trying to see that perspective and then reflect on whether the 'gaping chasm', which seemingly results when the 'worlds' come together can hint at anything about our historical prejudices, by virtue of these gaps perhaps being more a function of 'missing' bits in both theories (moreso in Classical, I think). Does this make sense ? .. (I'm trying hard to explain where/why I'm coming from here .. and I thank you all for your patience and contributions .. it seems like an interesting thread, so far).

I feel that using the QM Theories as a basis for doing this, seems to lead right back to the conclusion that the perceived QM world is somehow disjoint from the Classical World, which isn't particularly big news, I guess. QM also has some challenges in explaining things like entanglement, etc .. so I don't necessarily see QM being closer to reality than classical is, either.

This is an opportunity to break free from the old perspectives. Its a bit like trying to dwell on the differences between the sexes, as opposed to looking at the similarities of being human (for example).
:)
Cheers

[Dave2042]

Craig

Your pursuit is a noble one of which I approve, however I feel obliged to observe the steepness of the road you have set out on (for others interested as much as anything).

Firstly, the reason the textbooks take particular approaches to QM is because decades of attempts to do it other ways by very smart people (eg hidden variables) have come up largely empty. Conversely, decades of analysis and experiment support these approaches. Now, of course that doesn't totally preclude some fundamentally new understanding, however the opening is very narrow. Anything new has to (a) agree with all confirmed experiment to date, and (b) offer up a meaningful difference which can then be confirmed in some objective way. Certainly at the outer edges of particle physics this is not only possible, but arguably inevitable. On the core of QM, though, it's a very tight squeeze. As per comments on Schrodinger/Heisenberg equivalence, simply re-arranging the maths doesn't count, as we already have that.

Finally, I thought I'd clarify the issue of correspondence between QM and classical. This is in fact a directional issue. QM taken to a large-scale limit confirms and agrees with classical physics (or we'd have trouble explaining why civil engineering works) - no conflict. Not the same the other way, though. Classical applied to small-scale phenomena (eg atoms) fails spectacularly, and while semi-classical approaches can sometimes be useful, they clearly fail to capture something essential at that scale, and not for want of trying.

Anyway, enjoy exploring.

[johnnyb]

Hi again,


Just wanted to emphasise Dave's point here because I think it is important. Another simplistic way you could say it is that classical physics is a special case of quantum physics (e.g., a quantum system minus entangelment/coherence = classical).

And as Steve points out (and I sort of brushed over) how you deal with the "observer" and her "measurements" in a particular situation largely determines which approach (quantum or classical) will be more useful.

Craig, I think your quest is doomed to failure (no offence intended!). Quantum and classical essentially describe how the world works but they cover different domains - they are only useful and "correct" in their particular domain. Classical physics will never be able to properly describe a quantum system, and it would be very difficult to describe a classical system using quantum physics (not becuase it's wrong, but because the mathematics would become intractable).

This may not be a good analogy (becasue they are both classical models) if you look at Newton's laws of motion (e.g., F=m.a) and special relativity, we know that Newton's laws of motion are "wrong" becuase they don't take into account relativistic effects, but in 99.9999% of cases you'd get by with Newton's laws. We also know that special relativity is "wrong" becasue it only holds in inertial reference frames (hence the need for general relativity), but again, how often are you going to need GR when calculating the trajectory of a cricket ball? OK, if you are going to play cricket on horizon of a black hole you would, but then you'd have more important things to worry about than hitting the ball :eyepop:

I guess the point I'm trying to make is that mathematical theories always have a domain of applicability - as you cross the domains one theory may be able to morph into another (GR -> SR -> Newton), or maybe not. You just have to know which one to use in a particular situation. For me whether "reality" corresponds to the mathematics becomes a bit of a philosophical question (and therefore there may be no right or wrong answer), but one thing we do know is that quantum physics is an excellent theory for describing quantum systems, and has features that can never be derived from a classical model.

John.

[CraigS]

Hmmm … interesting .. lotsa 'words of warning' .. which sucks me in further (a major character flaw, I'm afraid) .. ;) :)


Is this is along the lines of D’Espagnat’s work ? If so, this then leads to the universality of QM .. and it is one way .. from the bottom to the top, eh ?
I think I can get this … :)

I think it is acknowledged that QM definitely brought together the problem parts of particles and fields .. and the deterministic and the random in classical, and it was kind of ignored at the time, and then QM came along and addressed it. This then would lead to QM seeming ‘strange’, but the problem was always there in classical, I think. :question:

When it comes to the scale issue, classical still allows for big, solid things to be made of atoms. Lots of atoms make a solid body, and this seems to be legitimate to say this in classical. There is a big difference between the big and the small, but once again, using 'scale' as a distinction also re-inforces the differences between QM and Classical, at the cost of de-emphasising the commonality.


...

So this is about macroscopic reality emerging with decoherence, (the probability wavefunction collapses), when the observer measures or observes, the system.

This then, gets related to proper and improper mixes, but the proper mixes are beyond our ability to measure, so the model of QM and decoherence, at all times, is referred to the observer. This is pretty philosophical stuff though.

This would seem to be the ‘crux’ of the gap between QM decoherence and classical ...its not really physics .. it involves a big dollop of, all-be-it … very well-founded, (in my view), philosophy.

That being said though, QM has let us see all this in retrospect, but it could still be said that all these phenomena existed all along.

Please note (again), I am not seeking to put down QM or Classical .. I'm just trying to get a different perspective on the reality, or otherwise, of the highly publicised 'discontinuities' ... and I'm sure you guys are going to keep me honest in the process. I'm happy to stop posting if we all weary over this … I'm not out to prove too much here, and its probably not worthy of getting too heated about. (I've had enough heat for a while).
:)

Cheers & Best Regards

[sjastro]

This has turned out to be infamous quote Craig.:lol:

One only has to look the treatment of black body radiation at a fundamental level.
Classical physics turned black body radiation into an ultraviolet catastrophe (http://en.wikipedia.org/wiki/Ultraviolet_catastrophe).

It was the first step towards quantum mechanics.

Regards

Steven

PS. The dreaded ultraviolet catastrophe turned up in Quantum Field theory as well which led to the concept of renormalization.

[CraigS]

Ouch !
:P :)

PS: Well .. there ya go … classical thinking gave birth to QM ! .. More history … :)

[janoskiss]

It just means it's a system where quantum effects make significant contribution to the system's behaviour. Classical mechanics follows from quantum mechanics but not vice versa. In other words, classical mechanics is a special case of quantum mechanics. If a system does not fall into that special case then I guess it's fair to say that it's a fundamentally quantum system.

[CraigS]

It seems that my last communications on this thread, have left others with the perception of 'wrongness' on my part. This would be a misinterpretation of where I have been coming from throughout this thread, so I'm happy to return to it, to clarify. I personally, would never be sure that a pure perspective, deliberately adopted as a way of visualising a path towards unification, could ever be viewed as ‘wrong’, myself. Perspective adjustment is a perfectly legitimate method which often leads to major steps forward … particularly when the individual components of the discussion are not in dispute. It is a quite legitimate technique.

I’m also very happy to admit that the issues raised, (in history), by the UV Catastrophe .. have caused me to think deeply and research widely, and if I were to take the easy path, I’d surely be following in the footsteps of many others, (and also in the wake of many great scientists), who possess, (and possessed), far greater brain-power, insights and scientific skills than myself. If I were to do this, I fully recognise that I might then be fortunate enough to catch what I’m sure will be, extraordinarily clear glimpses of others’ profound insights. That being said, I will nonetheless attempt to stand behind a perhaps massively feeble, (by comparison), and probably already, well-trodden path, only as an attempt at perhaps, glimpsing at a possible pathway towards potential unification of the presently, but apparently separate and overlapping, QM and classical ‘worlds’.

So here goes ..

I have stated throughout this thread, many times in fact, that classical could be viewed as having holes, discontinuities and inaccuracies (in the past), which were in need of further development during the same era as the emergence of QM thinking. I think it is probably fair to say that QM 'filled in' those holes, by revising the basic elements of classical, and in this particular case, through the development of the packetised view of particles, given the name ‘photons’.

From the historical perspective, as I understand it, all this emerged from Planck's thinking on the Equipartition Theorem .. ie: derived from classical statistical mechanics.

So, QM could perhaps, also be viewed as having unified thermodynamics (described in statistical mechanics), with particle and wave mechanics. Personally, from the perspective I’ve chosen to adopt in this thread, I can also see that it might be the unification of these domains, which eventually sorted out the UV catastrophe problem. Ie: not so much the particle we call a ‘photon’, but more importantly, the unifying aspects of the concepts behind it.

In this sense, and as it seems that there might not be anything specifically ‘non-classical’ about these three areas of physics, there also appears to be nothing particularly ‘non-classical’ about using these to describe a photon, (which seems to be the way a photon is usually described, anyway). Admittedly QM recombined these three areas of classical, in a very unexpected way, but at the end of the day, the fundamental principles still have classical descriptions at their core.

Photons might be recognised as a ‘quantum concept’, but so are particles of all types. The concepts of photon oscillation, or the energy emitted being in discrete packets proportional to the frequency, all seem to be very classical descriptions of behaviours at the core of the blackbody spectrum.

The description of the photon itself, also seems to involve fairly classical particle terms such as mass and charge. The QM interpretation of ‘spin’ may be unique to QM (fair enough), but it also comes from the concept of angular momentum. It is clear that spin is where QM does start to show unique aspects, so this may the fundamental aspect which answers my initial question. Whether spin manifests itself in some macroscopically observable behaviours, I’m unclear about and as such, I’m happy to concede that it probably exists. (Perhaps the Pauli exclusion principle leads to other examples ?).

Spin is the kind of thing I’m looking for .. as opposed to the usually cited, ‘quantum weirdness’.

So, if there is a way of describing fundamental photon properties directly resulting in the phenomenon apparent in a blackbody spectrum, and these can be described in classical terms, are these photon behaviours really an example of a distinguishing QM concept ? If they can be described in classical terms, is the UV Catastrophe really then evidence of the ‘failure’ of classical particle physics, or does it represent the successful unification of fundamental classical principles which then went on to achieve the precision needed, to accurately predict the spectrum ?

Folks, I’ll be perfectly candid by saying that I really don’t know the answers to these questions. This being said, I invite more knowledgeable others, to participate in commentary to help me continue this line of enquiry, (... or otherwise - which is also a perfectly legitimate outcome).

I’m happy to leave this thread with an understanding which goes in either direction.
:)
Cheers and best regards.

[sjastro]

Here is a family tree of the various branches of physics.
It provides a picture of the evolution of physics.

http://users.westconnect.com.au/~sjastro/Tutorial/physics_family_tree.jpg

Regards

Steven

[renormalised]

Seems that just like actual family trees lines cross...cousins marrying cousins, uncles/nieces, aunts/nephews etc etc etc. With all that "inbreeding" it's a wonder we don't have "imbeciles" running around:):P

[Dave2042]

It is an interesting attempt to make order out of a tangled mess. One of the difficulties is that modern physics is extremely coherent, by which I mean the same three basic theories (which I'd count as SR/GR, QM and SM) underpin fields that are labelled as entirely distinct, because they focus on different phenomena. That, indeed, is the power of modern physics.

The example that leapt out at me from this diagram was plasma physics. It has been placed as a branch off nuclear physics. In a way that makes sense, since its main practical application is attempting to generate power through nuclear fusion. However the behaviour of plasmas is dominated by the electromagnetic forces generated by its constituent charged particles, and the biggest practical problem in the field (as far as I can tell) is containment (through EM means), so shouldn't it branch off EM? On the other hand, any attempt to study plasma behaviour pretty rapidly winds up in statistical mechanics (in a sense its just a very complicated gas), so shouldn't it branch off SM?

This is not to knock what I think is a nice diagram, by the way.

[sjastro]

One of the problems with the diagram is determining whether the branches are based on experimental physics, theoretical physics or a combination of both.

From an experimental physics perspective it is perfectly sound to have particle physics branch off from nuclear physics. The early particle accelerators were designed to demonstrate nuclear physics as these became more powerful particle physics was born.

From a theoretical viewpoint however, one should have QM and the SR portion of relativity physics merge to form QFT. Particles physics then branches off QFT.
Nuclear physics would then be hard to classify as it is composed of both QM and non QM models as is solid state physics.

At best the diagram is subjective.

Regards

Steven

[renormalised]

The best way to put it is that most fields of physics are "incestuous" and have a relationship to one another in various ways. They've "cross pollinated" to a great extent.