## Question about QM using classical and quantum elements

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Spen
Posts: 70
Joined: Sat Sep 12, 2009 8:28 pm UTC

### Question about QM using classical and quantum elements

I was reading this earlier:
http://gravityandlevity.wordpress.com/2 ... s-quantum/
When I was first introduced to quantum and particle physics I thought that from there you must be able to eventually get to the mechanical models which Newton etc developed. Then the heisenberg uncertainty principle was introduced which obviously put enormous holes in my idea before I realised that quantum was all about probabilities rather than anything being definite.
Anyway, this article got me thinking along those lines again, surely you could use quantum physics to describe the probabilities of certain things occurring to each particle, when you put all of this together for the greatest probabilities you'd get something similar to the more basic classical model. Obviously you'd also get the very unlikely things left over as well which aren't accounted for by the ordinary model (like the idea of all of the air in a room moving to one end of it without any outside influence on it). So surely quantum physics does not require classical physics to be used and it's only for simplicity that you identify elements as classical and quantum so that you don't have to do the working for every single particle in the system.
tl;dr Why does quantum mechanics require you to use both classical and quantum elements, rather than just quantum to describe the entire system? As the author describes in the article you can describe why you feel pain in a physical manner.
I'm only doing A-level physics, this guy's got a post grad degree so I'm sure there's something wrong with this idea but I can't see where I've gone wrong (other than the enormous computational power which would be required to use the above method).

collegestudent22
Posts: 256
Joined: Wed Feb 25, 2009 5:36 am UTC

### Re: Question about QM using classical and quantum elements

I'm not an expert here or anything, but it was my understanding that classical phenomenon, such as Newton's laws, were derviable from QM. It has to do with quantum statistics, and is extremely computationally intensive, as even a "small" object is made up of extremely massive numbers of particles, but even with the uncertainty principle, I believe it still works.

As for the article, I think the author misses a bit. For instance:

QM is decidedly agnostic about any situation involving only quantum objects.

It was my understanding that Feynman diagrams are specifically about the interactions between quantum particles.

As he describes the problems with QM, he also fails to recognize that QM, as a theory, is not complete. It is vastly a "particle of the gaps" theory; one that is constantly changing to adapt to new discoveries and fit them all into a vast framework (as opposed to more classical theories that tend to define a specific phenomenon). Hence, you have the Standard Model, various string theories, QED, QCD, etc. All of which have flaws where certain phenomenon are not explained (or only explained by different hypotheses). In addition, there are no extensions out to large classical objects, but it is my understanding that if you want to spend a few years or so, you could extend it to even a "classical" object such as the metal plate in the article.

Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

### Re: Question about QM using classical and quantum elements

Yeah, that guy has it wrong. The idea that large-scale objects aren't quantum-mechanical is simply unnecessary, and so should be ditched. When two quantum-mechanical systems interact, quantum mechanics is quite clear on what happens. There is no scale at which quantum mechanics stops working, it's just that the probability almost all gets shoved into high-entropy "classical" states that look similar from the outside. It's like how if you flip a coin once it behaves randomly, but if you look at the number of heads and tails for 10000000000000000000 coinflips it looks very "classical."
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BlackSails
Posts: 5315
Joined: Thu Dec 20, 2007 5:48 am UTC

### Re: Question about QM using classical and quantum elements

collegestudent22 wrote:I'm not an expert here or anything, but it was my understanding that classical phenomenon, such as Newton's laws, were derviable from QM. It has to do with quantum statistics, and is extremely computationally intensive, as even a "small" object is made up of extremely massive numbers of particles, but even with the uncertainty principle, I believe it still works.

Yeah, you can derive newton's law by path integrals.

Spen
Posts: 70
Joined: Sat Sep 12, 2009 8:28 pm UTC

### Re: Question about QM using classical and quantum elements

Ah right, thanks. Just wanted to make sure I wasn't missing something glaringly obvious in the article, the only quantum stuff I've really looked at so far are the uncertainty principle, Photo electric effect and Young's single/ double slit experiments so I thought there was some extra layer of complexity.