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Author Topic: How can Newtonian mechanics and quantum mechanics coexist?  (Read 15952 times)

Offline annie123

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Can anyone explain to a layman why the Newtonian view of the world - laws etc.- still work n an everyday level when quantum discoveries/theories show that things work quite differently and sometimes contrarily at the quantum level?
« Last Edit: 11/02/2012 21:30:26 by JP »


 

Offline Geezer

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Re: Newton versus Bohr
« Reply #1 on: 06/02/2012 03:27:58 »
What! Are you trying to put all the scientists out of a job?

I'm not really qualified to answer this, but I don't think there is any dividing line. The quantum level is not about absolutes. It's about probabilities. As you move towards a larger scale, those probabilities create the illusion of absolutes, and, at some point, the probabilities are so constrained that they might as well be absolutes.
 

Offline Soul Surfer

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Re: Newton versus Bohr
« Reply #2 on: 06/02/2012 08:37:09 »
It is simply that the Newtonian view is an excellent approximation of what things are like for the precisions and velocities involved in everyday life.  Relativity and quantum effects are only corrections for this view when you look at things in a great deal more detail precision and under extreme conditions.

It annoys me greatly when people and textbooks say that Newton was wrong, the later discoveries just proved him to be almost right, they just added corrections to the model.
 

Offline JP

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Re: Newton versus Bohr
« Reply #3 on: 06/02/2012 13:12:04 »
I'm not really qualified to answer this, but I don't think there is any dividing line. The quantum level is not about absolutes. It's about probabilities. As you move towards a larger scale, those probabilities create the illusion of absolutes, and, at some point, the probabilities are so constrained that they might as well be absolutes.

Exactly right.  It's impossible to generalize to all cases, but for most cases quantum effects are very very very small.  When we look at everyday objects, they're big.  They also consist of many particles.  Averaging out tiny quantum effects over large sizes and over billions and billions of particles makes the total quantum effect too tiny to notice with in everyday life.
 

Offline annie123

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Re: Newton versus Bohr
« Reply #4 on: 06/02/2012 21:16:22 »
Thanks for the input. But when I read that things can exist as a wave or a particle, and that what things are is influenced by whether they are being observed or not, (and the cat thought expt. didn't help much) this doesn't help when I'm in a plane and think is this a collection of particles that could change into a lot of waves? And what about entanglement? If this can happen to a quantum particle why not to a collection of them?
 

Offline Geezer

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Re: Newton versus Bohr
« Reply #5 on: 06/02/2012 22:07:53 »
I think one of those cats gets into my drier and entangles my socks with my shorts.
 

Offline Soul Surfer

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Re: Newton versus Bohr
« Reply #6 on: 06/02/2012 22:12:35 »
Everything is particles AND waves all the time and there is nothing weird about it because stuff just behaves the way it does.  What we know about it has no effect on everyday life and the way things behave.  For big things the waves are so tiny that you never really see them.  As for the observation.  It does not have to be a person that makes this observation.  One particle can "observe" another just by bumping into it and this is happening for the atoms all around us millions of times every second.
 

Offline wolfekeeper

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Re: Newton versus Bohr
« Reply #7 on: 08/02/2012 07:39:24 »
Can anyone explain to a layman why the Newtonian view of the world - laws etc.- still work n an everyday level when quantum discoveries/theories show that things work quite differently and sometimes contrarily at the quantum level?
This has no completely agreed upon answer on this.

To some extent, the answer is that a lot of the time, the different things that can happen don't make any difference to what happens in the end, and then the quantum effects (which are sometimes described as being in different universes) join back up (we can see the joining back together again in many quantum mechanical experiments).

But in some cases the effect of the different things that can happen get multiplied up (the classic being Schroedinger's cat) and then the cat dies (and you get very sad) or it lives. Then the universe (possibly) splits into two at least locally, and the effects of that dead/live cat spread out through every transaction that that matters for. However, whether that dead or live cat matters to an alien on alpha centauri or not or even the other side of the Earth in most cases- probably not, so the quantum 'ripples' stop spreading at some point, the split universe thing really only happens locally.

And make no mistake, quantum mechanics really matters and has observable consequences, whether you're looking at laser diffraction patterns or superconductive levitation or simply using a computer (transistors work by quantum mechanics) it really does make a big difference to the real world, we don't live in a Newtonian world.
« Last Edit: 08/02/2012 07:41:32 by wolfekeeper »
 

Offline syhprum

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Re: Newton versus Bohr
« Reply #8 on: 08/02/2012 12:07:20 »
Do transistors really require quantum mechanics to describe their operation, please give some references.
 

Offline wolfekeeper

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Re: Newton versus Bohr
« Reply #9 on: 08/02/2012 15:01:43 »
Yeah, Solid-state physics

http://en.wikipedia.org/wiki/Solid-state_physics

Basically the flow of electrons and holes in transistors and semiconductors came out of research into quantum mechanics applied to crystalline matrixes like silicon. (I think they were trying to work out how cat's whiskers worked, and why they weren't very reliable, which lead to study of doping of crystal structures which was then analysed using QM theory, and this lead directly to transistors.)
« Last Edit: 08/02/2012 15:03:54 by wolfekeeper »
 

Offline annie123

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Hello
Thanks for more replies. But I do ask why the topic title was changed. Hasn't the change made a difference to the meaning? EInsteins theories and Bohrs - I didn't think they were the same.
Anyway, another reason i asked is because in the discussion about teleportation I have read here and there a particle can be reconstituted in a completely different place, annihilated at point of origin, and I heard on the infinite Monkey show (another science podcast) that there is no reason in theory why a package of particles can't do this. But Newton's schemata  wouldn't support this.
And if something can be a wave and a particle at the same time (is this the twin slits experiment?) how can anything remain solid and relied upon to stay so?
 

Offline imatfaal

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Hi Annie - I guess the title was changed because for many people Bohr is associated with his model of the atom and the "old form" of quantum theory which are a bit outdated.   I think newtonian v quantum is the best way to describe the differences - its very wooly but most people will get the idea that it is the difference between the deterministic "pseudo-clockwork" world and the stochastic random ideas of quantum mechanics.  This is the same difference as classical and non-classical physics.  Not really sure about using Einstein as a name for the non-deterministic side of physics - he never really settled with that, although his work on photoelectric effect was massively important

On your two questions - there is no reason that the bunch of particles shouldn't move, but it is mindbogglingly unlikely that they will.  I seem to remember on IMC they were also talking about recreation of particles with the same properties (and as fundamental particles are completely alike this basically amounts to teleportation) - my worry in this would be the necessary uncertainty through heisenburg.  The double slit experiment does show the wave characteristics of light - and it still works when using individual particulate photons - which shows the duality.  Things remain everyday normal and solid because the probability for them to do so is so overwhelmingly high
 

Offline JP

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Re: How are Newtonian physics and quantum mechanics different?
« Reply #12 on: 10/02/2012 23:44:38 »
Hi Annie, 

I see my earlier post didn't take.  I think you're right about the post title--it shouldn't be Newton vs. Einstein, so I changed it to Newton vs. quantum mechanics.  Einstein's most famous theories were relativity (special and general) which are still called classical theories because they don't involve quantum mechanics.  While Bohr was one of the founders of modern quantum mechanics, he was just one of many involved in working out the theory, so its probably most accurate to say Newton vs. quantum rather than Newton vs. Bohr, which is what I've done.
 

Offline annie123

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Re: How are Newtonian physics and quantum mechanics different?
« Reply #13 on: 11/02/2012 20:14:57 »
HI
I'm not sure the new title is any better. I know how the theories are different. I wanted to know how they can coexist since they often seem to contradict each other.
 

Offline JP

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Re: How are Newtonian physics and quantum mechanics different?
« Reply #14 on: 11/02/2012 21:30:16 »
HI
I'm not sure the new title is any better. I know how the theories are different. I wanted to know how they can coexist since they often seem to contradict each other.

I think this is a case of the moderators trying to clear it up and ending up mucking it up.  I'll change it to "How can Newtonian mechanics and quantum mechanics coexist?"  If you can think of a better title, go ahead and change it, if you can edit your first post.  If not, shoot me a PM and I'll change it for you.

(I didn't make the original edit--Chris did--so I can't speak to his motivations.  However, using the term Newtonian mechanics is pretty clear, but Bohr's theories aren't really synonymous with modern quantum theory, so I kept his name out of both my edits.  :p )
 

Offline imatfaal

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #15 on: 14/02/2012 12:32:54 »
HI
I'm not sure the new title is any better. I know how the theories are different. I wanted to know how they can coexist since they often seem to contradict each other.

They do contradict each other to an extent - but we know in what realms to use one and where to use the other.  These theories are all models - and a useful way to think of them is as analytical toolboxes.  They are not final descriptions of underlying reality - they are mathematical models that allow us to predict outcomes and explain behaviour.

When you want to calculate where a field gun shell will land you use Newtonian mechanics.  To plot the course of a satellite you use Newtonian mechanics with a few modifications (and bear in mind the time dilations from einstein's relativity) - or at least you used to, maybe they use more Einstein than Newton now.  To calculate a planetary orbit to max precision you use General Relativity.  These are all classical theories - so which is right and which is wrong; the answer is neither really - they are varying toolboxes with strengths and weaknesses.  GR is most precise and most unwieldy, Newtonian mechanics is less precise (although pretty amazing) but is usable without an extra brain.

Your question of classical physics v quantum mechanics is similar.  On the very small scale QM is a much better model for predicting the outcome of experiments - but it rapidly becomes impossible to use when dealing with more complex systems.

QM does make some underpinning claims that are completely at odds with classical physics - the ultimately stochastic/random nature of nature, the fact that some(all) quantities are quantised (ie come in small lumps), the uncertainty principle etc; but when we are doing a job we can ignore the philosophical problems and use the kit that will best give us an answer.  The theoretical physicists though are massively concerned with trying to pull all the strands together - in reality it means getting gravity to fit into a quantum model (not the other way around).  To an extent a very famous way this is being attempted exemplifies the model/toolbox idea - this is string theory; rather than work piecemeal, this is an attempt to set up an overarching framework of maths and modelling that could possibly explain many differing areas of physics, make it mathematical self-consistent and ONLY THEN see if we can get it to work in the real world.

Its complicated and fascinating - in the end you choose the best tool for the job and try not to worry about theoretical problems
 

Offline annie123

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #16 on: 18/02/2012 23:09:56 »
Thanks imatfaal. I guess without the training in theoretical physics and the math. brain of a Feynman  the lowly plebs don't have much chance of really getting a handle on all this. But it is frustrating to get a glimpse of the implications of the different theories and not be able to relate them to the realities of daily life. IN the end, who will know what reality is? Only a very few highly evolved packets of grey matter? Seems rather hard on most of the human race.
 

Offline yor_on

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #17 on: 18/02/2012 23:19:58 »
See them as evolving, like building a stair. Newtons definitions works very well on earth although becomes modified at relativistic speeds. The biggest difference is the one about time, Newton saw time as a unchanging river same for all. Einstein introduced 'frames of reference' that, when compared to each other, delivered different 'clock beats'. But from a strictly local 'frame of reference', meaning the one that we all have relative our local clocks (wrist watch) time never change its pace. so Newton can, with some modification, be said to have got that one right too :) Although it now becomes a result of the fact of lights unvarying speed in a vacuum, no matter what 'frame of reference' you measure that speed from.
 

Offline yor_on

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #18 on: 18/02/2012 23:31:54 »
As for QM versus Newton?

That's a big jump, Newton looked at macroscopic phenomena, and light of course :) QM has a very different outlook on how macroscopic phenomena comes to be, which still is a hypothesis. We do not know how the microscopic transits into macroscopic phenomena.
 

Offline wolfekeeper

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #19 on: 19/02/2012 01:03:34 »
Thanks for the input. But when I read that things can exist as a wave or a particle,
Not really... things are both a wave and a particle. And not really A wave, more like every possible wave, simultaneously.

Quote
and that what things are is influenced by whether they are being observed or not, (and the cat thought expt. didn't help much)
No, not according to whether they are observed, unless you include the particles (which are waves/particles) observing each other.

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this doesn't help when I'm in a plane and think is this a collection of particles that could change into a lot of waves?
The best I can explain it, it's kinda like a sea wave made up of lots of ripples and swirls all sloshing along, it's all waves all the time. Each wave represents all possible positions and movements, of the particle, which varies bearing in mind the positions and movements of all the other particles that have interacted with it.

Quote
And what about entanglement? If this can happen to a quantum particle why not to a collection of them?
Yup. It can. That's what the Schroedinger's cat is, the cat ends up entangled with the radioactive decay event (and dies) AND doesn't, and lives, and then when you open the box, so do you get entangled with the dead moggie/radioactive decayed or live moggie/no radioactive decay.

The thing is, once the different waves have deviated enough, you've got no access to the opposite state, you either have a dead cat, or a live cat; the only way you could see the other one is if these two universes interacted in some way, but they're actually too far apart in quantum mechanics terms to do that, whereas the dual slit experiment, the two different universes can join back together so you can see the join from the interference fringes.

The thing is, Schroedinger's cat isn't a theoretical thing, it happens all the time, if you listen to a geiger counter, it's happening to you.

But the waves still, more or less obey Newton, because the ensemble of particles follow Fermat's principle, which pretty much explains things like momentum, so things take the quickest way from a->b in most situations, given their initial speed, so Newton's laws pretty much fall out.
« Last Edit: 19/02/2012 01:09:17 by wolfekeeper »
 

Offline annie123

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #20 on: 21/02/2012 01:12:56 »
Curiouser and curiouser and through a glass darkly etc etc. My comment remains - re the average /reasonably educated person has no chance of ever making sense of it all and only the very favoured few are able to get even a glimpse of what it's all about. The Jest of God . . .(not that I believe in him/her . . .) I wonder sometimes how the elite physicist can even function in an everyday life, eating his cornflakes and taking out the garbage when reality is ....well, only they know. Thanks everyone anyway for trying . . .
 

Offline Geezer

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #21 on: 21/02/2012 02:41:21 »

My comment remains - re the average /reasonably educated person has no chance of ever making sense of it all and only the very favoured few are able to get even a glimpse of what it's all about.


Annie,

I don't think that's true at all. It's simply about preconceived ideas.

Most people are smart enough to understand that preconceived ideas are very often completely wrong. You don't need to have a pointy head to get a grip on the basics of quantum mechanics. It's certainly all counter-intuitive, but our inate intuition is entirely limited by the resolution of our senses. They are simply not designed to let us detect the quantum level.

What I think is remarkable is that humans have been able to create a set of extended senses that allow us to observe what's really going on "behind the scenes", and we are nowhere near done yet!
 

Offline yor_on

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #22 on: 22/02/2012 16:00:31 »
Most things in life are 'through a glass darkly' Annie :)
Subtly altered as you learn new things and become experienced. And I think that goes for everything, from emotions and relations to physics.
Physics is about how and why we exist at all. It's our try for making some sense out of it. And I doubt that you will find anyone that never wondered about that.
 

Offline JP

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #23 on: 23/02/2012 14:26:31 »
Physics is about how and why we exist at all. It's our try for making some sense out of it.

I disagree.  Physics is about coming up with scientific models to describe nature.  It might happen to answer questions about how and why we exist along the way, but directly answering questions about how and why we exist is more in the realm of metaphysics or religion.  One of the reason questions like this one come up (why are Newton's laws still around if we have quantum mechanics?) is that there's a misconception that physics is about answering fundamental questions, which would require throwing out all previous models when a more fundamental one comes along.  But the aim of physics is to provide useful models, so we keep and use Newton's laws because they work well for many problems, even though we know that quantum mechanics is more fundamental.
 

Offline wolfekeeper

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #24 on: 23/02/2012 15:41:00 »
I wonder sometimes how the elite physicist can even function in an everyday life, eating his cornflakes and taking out the garbage when reality is ....well, only they know. Thanks everyone anyway for trying . . .
Pretty much like this:

 

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Re: How can Newtonian mechanics and quantum mechanics coexist?
« Reply #24 on: 23/02/2012 15:41:00 »

 

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