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Quantum mechanics vs Classical mechanics

  1. May 29, 2010 #1
    Just a few questions that are really confusing me...

    1. Where is the cut off boundary between the existance of Quantum mechanics and Classical mechanics, is there a specific point where classical takes over from quantum and if so where is that point?

    2. Is the micro quantum world indeterministic and the macro classical world deterministic or is the macro classical world inherently and fundementally indeterministic as well?

    3. If the quantum world works on probability and without causation then wouldn't the classical world also work on probability and without absolute causation, or in order words there would be no way to make 100% accurate predictions on things in the macro world.

    4. How does the randomness of the quantum world create structure and order in the classical world?

    Any help or info is massively appreciated, thanks.
  2. jcsd
  3. May 29, 2010 #2
    Nice questions!!!

    Hope you get some suggested answers.
  4. May 29, 2010 #3
    1. There is no well defined boundary between quatum and classical. As we scale things up, the quantum effects kind of 'fade out' and the classical ones 'fade in'. This is because there realls is no quantum mechanics and classical mechanics. There is just the universe and its laws. It obeys a single set of rules that we haven't figured out completely yet. The laws we know are only approximations that work at certain scales and energy levels (within certain constraints). I think in another thread someone posted a quote about how our laws only cover certain ranges and break down when they get out of them. That is an appropriate comment here too.

    2. We haven't quite figured out whether the quantum world is deterministic or not. For deterministic interpretations, I suggest you look into the many worlds interpretation (MWI) and the Bohm De Broglie interpretation (DBB). There's plenty of disccusion about that in the quantum physics forum. But assuming the quantum world is not deterministic, then no, the macro world isn't quite deterministic either.

    3. Going along with what I said above, yes, this statement is true if the quantum world is not determinisitic.

    4. That being said, the classical world is much more predictable than the quantum world because the extremely large number of particles involved in many cases 'locks down' the quantum nature. if something is interacting with many other particles, its observables keep getting observed, which constrains the possibilities in which it can exist. There's also what I like to call the 'soup effect', which is that the bigger you get, the more similar all the states become. for example, 11 and 10 are pretty different (binary), but 11010101010101111011011011110101011 and 10101111101101010110101001010100101 we treat as essentially the same, like a string of random numbers. That part is a little hard to explain. If I wasn't clear, I can try agian.
  5. May 30, 2010 #4


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    1, Nope there's no specific point, it's a smooth transition.
    Which also means that the study of objects in the intermediate area mixes the two. For instance, in quantum chemistry/chemical physics. The nucleus of atoms (viewed by us as a single particle, because the internal affairs of the nucleus has no effect on chemistry) behaves largely classically, because it's so heavy. Electrons, on the other hand, behave almost entirely quantum-mechanically.

    So typically, when one studies atoms and molecules, the motion of the electrons is treated quantum-mechanically, whereas the nuclei are treated as classical objects. In short, not only is there a smooth transition, but there's also a transition in how you model these things physically - quantum mechanics and classical mechanics can be mixed.

    2. Personally, I don't view this as a question that can be answered. See, whether or not something is truly deterministic or not is a philosophical question, it's a matter of interpretation.
    I.e. right now, the simplest interpretation is that QM is indeterministic. But if you advocate a hidden-variable interpretation, it would mean it only appears to be indeterministic, but is in fact deterministic. Basically, whatever answer you have, you can always add another 'layer' of complexity and say that things only appear that way.
    Occam's razor says you should prefer the simpler answer - and I do, but it doesn't mean the simpler answer is necessarily the true one. So I don't see how you can solve the issue - it's fundamentally philosophical.

    3. Problem is, we can't measure things accurately enough for this to matter. The uncertainty in the position of a single atom is so small it can barely be measured, to say nothing about macroscopic objects!

    4. Well, we still have "randomness" in the classical world - say the motion of particles in a gas. This was only philosophically different from QM (in that QM is often believed to be truly indeterministic, whereas in classical mechanics this was an 'apparent indeterminism'). In both cases the answer is that you have to use a statistical description instead. The fact that something is random doesn't preclude you from knowing quite a bit about it. In fact, it hasn't turned out to be much of an issue in practice, because measurements are usually statistical anyway. You measure the pressure and temperature of a gas; you're not typically interested in knowing where a certain particle is at a certain moment, and determining where it'll be a minute later.
  6. May 30, 2010 #5

    Vanadium 50

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    I think I disagree with the answers to #1. QM is always valid, and Classical mechanics is an approximation. For large systems, this is a pretty good approximation - so good that only a crazy person would try and solve problems using QM - and for small systems it's a bad one.
  7. May 30, 2010 #6
    Vanadium has the right idea....also QM reflects quanta, discrete values, for variables that are not present in classical formulations. For example a free electron does not have quantized energy levels, but an atomic bound electron does.

    The formulations of both classical and QM fail near singularities....big bang and black holes, so we seem to be missing something in our mathematical formulations so far.

    Try searching Wikipedia for 'quantum mechanics' and read at least the introductory sections for,say, a half dozen entries to get a feel for it.

    here is one excerpt:
    "Quantum mechanics is essential to understand the behavior of systems at atomic length scales and smaller. For example, if classical mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus, making stable atoms impossible. However, in the natural world the electrons normally remain in an uncertain, non-deterministic "smeared" (wave–particle wave function) orbital path around or through the nucleus, defying classical electromagnetism.[4] "

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