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Author Topic: How are classical (Newtonian) laws of physics connected to quantum mechanics?  (Read 9258 times)

Offline john ford

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Hello All      I'm in no way a scientist but I do like to learn things.  Recently I had a discussion and was told that the mechanistic laws (Newton) are now dead since QM - that they no longer apply.  What, if any, is the connection now between those classical laws and QM?
« Last Edit: 09/03/2008 10:52:28 by chris »


 

Offline DoctorBeaver

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Newtonian laws do still apply. The Apollo spacecraft were sent to the moon using calculations based on Newtonian physics. For everyday calculations of gravity & motion, Newtonian laws are more than adequate. However, when you get down to very small scales, a different theory is required.

QM applies on the atomic & particle scales where Newtonian laws go phut. Similarly, QM & Newtonian laws give way to relativity on the largest scales.
 

Offline lightarrow

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Hello All      I'm in no way a scientist but I do like to learn things.  Recently I had a discussion and was told that the mechanistic laws (Newton) are now dead since QM - that they no longer apply.  What, if any, is the connection now between those classical laws and QM?
Classical mechanics don't apply to quantum processes but it's the best choice for classical processes. That could seem a trivial sentence, but it's intended to say that there are processes (not necessarily on atomic and particle scales) which can only be described with QM, and other processes (usually on macroscopic scales but not necessarily) which can be described perfectly with classical mechanics. The last can be classified into other two kinds: newtonian and relativistic mechanics. So, comparing newtonian mechanics with QM is not appropriate; you should compare classical mechanics with quantum mechanics.
 

Offline john ford

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Thank you Doc Beaver and Lightarrow.  It seems then that different groups of laws apply to different things (?) - depending on what is being measured? - dependsing on time?   What applies here on planet Earth may not apply to something  xxx M light years away.  Am I somewhere near the right track?
 

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A commonly held opinion about the Universe involves the Cosmological Principle  which basically assumes that things behave the same everywhere and that Space is, on average, isotropic. (It's really the best we can do.)
It is different conditions and not different places that cause different 'laws' to apply. So, at high speed or under extreme gravitational fields or in very small distances the 'old' laws are not followed and different ones come into play - Relativity  or QM etc.
When we look at light from a distant star and say it has been red-shifted and so it is moving away at such and such a speed, we are basing that on the assumption that the atoms that created that light are behaving the same as atoms on Earth. That assumes the Cosmological Principle.
Without that, we could say nothing at all about any of the more distant objects and events. We would have even less idea about our circumstances and origins.
« Last Edit: 27/02/2008 22:31:48 by sophiecentaur »
 

Offline john ford

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Thank you Sophie
 

lyner

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Of course, we could be wrong. If we went somewhere else things might be different; it could be a quirk of Space that just makes them appear the same. However, it would have to be very clever because we do see a lot of 'consistencies' in our remote observations.
 

Offline Relph

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I think that probably what the discussion you read was referring is not Newton laws, but rather the deterministic view of science.
After the incredible success of Newton laws, a lot of scientist and philosophers started to think of the world as a clock, everything moves in precisely and deterministic ways. So basically, they said, if you could know the exact configuration of the universe at a certain time and all the laws that govern it, you should be able to predict any future configuration. Laplace was probably the biggest exponent of this idea.
So, comes the 20th century and a guy named Heisenberg decided to throw a bomb on this classical view. You cannot know the exact configuration of the universe, because, on the smaller scale, trying to read the configuration will change it. So basically, quantum mechanics introduces a lot of probabilistic, not just in a calculated way, but as an inherent trait of the universe. You might have heard Einstein's phrase opposing this view: "God does not play dice with the universe".
So basically, that's the idea that QM killed Newton, not in his laws that are still applicable today, but on the idea of the world as deterministic.
Now, there's also a debate regarding if this is actual the case. Some people is of the idea that the world might be deterministic, in the sense that every configuration follows in a deterministic way from previous ones; but that humans (or for that matter, any self-aware entity) is not capable of determine the evolution of the universe.
Finally, there's also another argument against determinism, but this is more of a epistemological argument. Basically, there are a lot of systems which are sensitive to initial conditions. This means that if you don't have the exact numbers, the result might give you very different. Since humans and computers can't store infinite information, you won't be able to have the exact information most of the time. Chaos Theory is the branch of mathematics that study this kind of systems.
Sorry if I made the post to long....
 

Offline DoctorBeaver

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Relph - firstly, welcome to TNS.

There was a reply to Einstein's dice quip along the lines of "I don't presume to tell God what he can do". I can't remember who said it but possibly Heisenberg or Bohr.

Quote
Now, there's also a debate regarding if this is actual the case. Some people is of the idea that the world might be deterministic, in the sense that every configuration follows in a deterministic way from previous ones...

However, it does not follow that you can reverse engineer the evolution of the universe as 2 different starting conditions could have the same outcome.
 

Offline Make it Lady

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Bohr, definitely.
 

Offline john ford

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Relph

So basically, that's the idea that QM killed Newton, not in his laws that are still applicable today, but on the idea of the world as deterministic

I think that is what I was looking for.  Thank you.
 

Offline lightarrow

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Relph

So basically, that's the idea that QM killed Newton, not in his laws that are still applicable today, but on the idea of the world as deterministic

I think that is what I was looking for.  Thank you.

Yes, this is a shared thought among physicists, but I don't agree completely. It would be true if properties as position or momentum for quantum systems had the same meaning they have for classical systems, but they don't have; first, a quantum system doesn't have at all such properties between two measurements, second, for example a quantum system is described by a wavepacket which spreads during time, while a classical system behaves differently.

In my personal opinion, such quantities are just properties of complex systems, which arise in specific conditions (as for example higher mass and number of particles, decoherence with the environment ecc.) that make the system behave classically.
« Last Edit: 29/02/2008 12:03:02 by lightarrow »
 

Offline Relph

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However, it does not follow that you can reverse engineer the evolution of the universe as 2 different starting conditions could have the same outcome.


What I meant to say is basically that you cannot reverse engineer the universe from the inside. I still think that if you use Laplace's Demon argument, you can get a deterministic world view.
Why? On the inside of the universe you cannot make any observation without change the outcome. But if you were on the "outside" of the universe, the laws of this universe wouldn't have to applied to you, as such, you might be able to know the exact configuration of the universe without changing outcomes, and things won't need to be probabilistic to you.
I know this approach will have nothing to do with science, cause after all, science regards only with this universe, but this is no different that all the multiverse ideas that are polluting academic circles nowadays. ;D
 

Offline DoctorBeaver

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...but this is no different that all the multiverse ideas that are polluting academic circles nowadays. ;D

I note with interest your use of the word "polluting". Do you, then, consider multiverse theories to have no merit?
 

Offline Make it Lady

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How the hell can ideas be polluting. Ideas are the answers of the future.
 

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How the hell can ideas be polluting. Ideas are the answers of the future.

Pollution is a subjective concept, and can be applied to anything.  To pollute is to mix something undesirable in with that which is desirable, but what you consider desirable and what you consider undesirable is dependent on the context, and your own objectives.
 

Offline JimBob

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George and Doc - have you ever heard of the principle of multiple working hypotheses? (T.C. Chaimberlin) It is the foundation of science. Until some thing is proven objectively, then any hypothesis is still in the running, no matter how far out of the mainstream they are. The have yet to be concretely discredited. At the level of QM almost everything falls into this category, just a hypothesis.
 

Offline Make it Lady

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Here, here JimBob (are we agreeing on something...EEk!) Bohr said (not an accurate quote but near enough) Your ideas are crazy, but are they crazy enough to be true?

I like Bohr.
 

Offline lightarrow

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George and Doc - have you ever heard of the principle of multiple working hypotheses?

It's the same of Many Worlds Interpretation?
http://en.wikipedia.org/wiki/Many-worlds_interpretation
 

Offline DoctorBeaver

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George and Doc - have you ever heard of the principle of multiple working hypotheses? (T.C. Chaimberlin) It is the foundation of science. Until some thing is proven objectively, then any hypothesis is still in the running, no matter how far out of the mainstream they are. The have yet to be concretely discredited. At the level of QM almost everything falls into this category, just a hypothesis.


Absolutely; which is why I queried the use of "polluting". It makes multiverse theories seem distasteful.
 

Offline Relph

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I actually meant populating, that was a small mistake.
Even though I don't like this theory, I'm in no position to discredit them on a forum. I'm not expert on the subject.
The reason I don't like them is probably that, I don't know them enough. As I see them, they are trying to take science out of it's realm. For me, science is about this universe and this universe only. Other universes will have to be outside of the realm of science, as they shouldn't have a direct or indirect contact with the causality of this universe, or otherwise, they will be part of this universe.
I want to make emphasis on the fact that I'm not expert, and this probably makes me wrong. ;D
 

Offline DoctorBeaver

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I'm glad we've cleared that up  :)
 

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