Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does the classical world exist

  1. Jan 15, 2014 #1
    I know this has been asked before but Im still not getting it. Does anyone know why a classical deterministic world with fixed positions emerges from the statistical distributions IE superpositions of the quantum mechanical world. Why am I nor my tv or dog not in superposition. There is a bridge I need, to close this gap between the quantum weird world and the classical world. I mean the classical world does exist doesent it its all around us. Whatever interpretation of qm that will be the right one must cohere with the everyday classical world not contradict it. I mean the classical world exists doesent it. Is it a matter of accumulation of qm that creates the classical. I know some of you will invoke decoherance but decoherance doesnt collapse particles from what I understand.
    Last edited: Jan 15, 2014
  2. jcsd
  3. Jan 15, 2014 #2
    Without restating the obvious (big things cause superpositions to collapse) I don't think a good answer has been discovered.
    But consider the possibility that it is not true - that you really are in a complex superposition. How much would any one instance of you know about the other superpositions?
  4. Jan 15, 2014 #3
    Last edited: Jan 15, 2014
  5. Jan 15, 2014 #4
    In principle, macroscopic objects are subject to the laws of QM.

    Why we don't see superposition in macroscopic objects? Because "measurement" has taken place.

    When measurement takes place is unknown at this point.
  6. Jan 15, 2014 #5


    User Avatar

    Staff: Mentor

    Posing a similar question.... Why do the classical deterministic gas laws for pressure and temperature emerge from the statistical distributions of the positions and momenta of the individual molecules in a large volume of gas?

    The answer in both cases is the same: the math describes HOW the world works, and that's the result that emerges from the math. WHY this math, and not some other, isn't a question that science is set up to answer.
  7. Jan 15, 2014 #6


    User Avatar
    Science Advisor

    Why the classical world in which things definitely occur is unknown or not agreed upon in quantum mechanics.

    In the standard textbook interpretation, quantum mechanics requires a "Heisenberg cut" in which the universe is divided into classical and quantum parts. Quantum mechanics is only an operational theory that allows us to predict the probabilities for definite classical outcomes. You can find this point of view in Landau and Lifshitz.

    In the Bohmian interpretation, hidden variables are supplied together with a condition called "quantum equilibrium". The entire setup is classical (deterministic evolution from an initial condition), so here there is no problem for why a classical world exists.

    In the many-worlds interpretation (oversimplified form), every time you make a measurement, the universe splits and all possible outcomes occur. Each future you sees only one outcome, and the universe appears classical to each future you.

    Both many-worlds and the Bohmian interpretation are consistent with the standard textbook interpretation. The Bohmian interpretation is an example in which quantum mechanics is incomplete, while the many-worlds interpretation is an example in which quantum mechanics is complete. It is still not entirely clear that many-worlds works, but there are some pretty persuasive accounts. Bohmian mechanics definitely works for non-relativistic quantum mechanics, but the extension to relativistic quantum mechanics is still being researched.
    Last edited: Jan 15, 2014
  8. Jan 15, 2014 #7


    Staff: Mentor

    That's true.

    But in modern times its generally assumed to have occurred once decoherence has taken place. With that assumption many (not all - some issues still remain) of the issues about how a classical world emerges is resolved.

    You will find the detail at a non technical level in Omnes book - Understanding Quantum Mechanics:

    The book is excellent because it not only elucidates what we do know, but areas where further research is required, such as the lack of certain key theorems in how the classical world emerges - they exist but need further generalization.

  9. Jan 15, 2014 #8

    The superpositions are "within" very narrow limits....and may be intimately tied to the uncertainty principle.

    A large object is composed of smaller particles (such as photons, electrons, quarks, atoms etc).

    The smaller particles (more precisely called as fundamental particles, though the definition of fundamental particle could change in the future) are in superposition.

    The superposition of the smaller/fundamental particles are chaotic, and changing, thus the net additive effect of these "chaotic/disorganized" superposition would not be that big.

    Apart from that, the larger object, as a single entity, may have its own ....superposition (?). However this effect is again so small because of "smaller (probability) wavelength" etc.
    Last edited: Jan 16, 2014
  10. Jan 16, 2014 #9
    It may have something to do with our minds. Maybe our brains and our senses work that way so we see the world as classical. Just as we don't see ultraviolet. The limitations of our senses may give us the illusion that the world is classical.
  11. Jan 16, 2014 #10
    THIS is a load of balloni we evolved our senses to perceive the only things that mattered so our genes could survive. Perceiving the ultraviolet spectrum for example doesent help in any way a lion to catch a gazzelle.
  12. Jan 16, 2014 #11
    There may be a grain of truth in this. If we were able to see, in detail, what is happening over the course of a nanosecond as clearly as we actually see things happening over an hour, I believe we would see larger and more pronounced affects of location uncertainty.
    But more broadly, what we cannot measure directly with our senses, we measure with our instruments. The world we live in appears classical because it is nearly classical at the macro level.
  13. Jan 16, 2014 #12

    This doesn't contradict what haael said.

    Good luck explaining reality with Newton's notions.
  14. Jan 16, 2014 #13


    User Avatar
    Gold Member
    2017 Award

    Modern physics experiments are not done inside our minds. They are done with e.g. detectors, computers and other equipment. And physical/mathematical models are e.g. developed, confirmed or not confirmed from the results of these experiments.
    Last edited: Jan 16, 2014
  15. Jan 16, 2014 #14


    User Avatar
    Science Advisor
    Gold Member

    Then why did I enjoy watching "Breaking Bad"? I doubt that is doing anything for my gene propagation.

    The truth is that you can't actually provide a reasonable argument that we live in a deterministic world at the macro level. Some macroscopic things appear to operate deterministically (a billiard ball perhaps) and some things do not (human behavior for example). There is no generally accepted model of human behavior, and perhaps the human mind is a direct reflection of micro level physics. Who knows?
  16. Jan 16, 2014 #15


    User Avatar
    Gold Member
    2017 Award

    Why? :confused:
    Why does white light contain light with different wavelengths? Why do some particles have mass? Why is there gravity? Why do we ask "why questions"? :wink:

    Now, what's really interesting is how the classical world emerges from a quantum world; see this quite recent experiment (published 8 Sep 2013) probing the transition from quantum to classical behavior in a Bose gas:

    Local emergence of thermal correlations in an isolated quantum many-body system
    Tim Langen, Remi Geiger, Maximilian Kuhnert, Bernhard Rauer, Joerg Schmiedmayer
    (Submitted on 16 May 2013, Published 8 September 2013)

    "We experimentally demonstrate how thermal properties in an non-equilibrium quantum many-body system emerge locally, spread in space and time, and finally lead to then globally relaxed state. In our experiment, we quench a one-dimensional (1D) Bose gas by coherently splitting it into two parts. By monitoring the phase coherence between the two parts we observe that the thermal correlations of a prethermalized state emerge locally in their final form and propagate through the system in a light-cone-like evolution. Our results underline the close link between the propagation of correlations and relaxation processes in quantum many-body systems."
    (4 pages)

    Quote from the final section of the paper:
    "In our experiment thermal correlations emerge locally. A local observer would see thermal relaxed correlation function appear immediately after the splitting and spread through the system in a light-cone horizon-like fashion, while long-range phase coherence remains outside. This leads us to conjecture a general pathway to relaxation and the emergence of classical properties in isolated quantum many-body systems: the decay of quantum coherence starts locally and then spreads through the system to establish a globally relaxed (dephased) state. In systems where interactions manifest themselves in excitations with a linear dispersion relation the decay of quantum coherence takes the form of an effective lightcone."

    Article 1: Quantum Temperature: Scientists Study the Physics That Connects the Classical to the Quantum World (ScienceDaily)
    Article 2: Scientists manage to study the physics that connect the classical the quantum world (PhysOrg)
  17. Jan 16, 2014 #16


    User Avatar
    Science Advisor

    Our description of the world (i.e., physics) is scale-dependent. At each scale, we have different degrees of freedom and different dynamics (i.e., the world at [itex]10^{ - 18 } cm[/itex] is very different from the world at 1cm.) and, therefore, we need a different theory to describe the world at each ''fundamental'' scale of distances. Our experiences about the world at a larger scale (almost always) decouples from that at a smaller scale. This is because, a theory at a larger scale remember only finitely many parameters from the theories at smaller scales, and throws away(average over) the rest of irrelevant degrees of freedom. Mathematically this means many parameters become integration variables and thus disappear at the new scale.
  18. Jan 17, 2014 #17
  19. Jan 17, 2014 #18


    User Avatar
    Science Advisor

    I would like to comment on the relationship between my answer (#6) and Nugatory's (#5) and samalkhaiat's (#16) answers. In the naive textbook interpretation (Landau & Lifshitz), I said that the question is not answered, because when one uses quantum mechanics, one must make a cut dividing the universe into classical and quantum realms. This seems to be at odds with the view that there is some coarse graining or limiting procedure from which quantum mechanics yields classical mechanics. Both answers are correct, and as stated by Landau & Lifshitz, classical mechanics is needed to formulate quantum mechanics, and classical mechanics is also a limit of quantum mechanics.

    One very interesting mathematical limit of quantum mechanics is found in Klaus Hepp's paper in which the limit of an infinite measuring apparatus yields rigourous wave packet reduction http://dx.doi.org/10.5169/seals-114381. John Bell argued that this mathematical limit was correct, but not of physical significance for solving the measurement problem http://dx.doi.org/10.5169/seals-114661. (I found out about Bell's paper in Allahverdyan et al's http://arxiv.org/abs/1107.2138)
    Last edited: Jan 17, 2014
  20. Jan 17, 2014 #19
    What do you mean by Classical Mechanics is needed to formulate QM? You mean in terms of terminology?
  21. Jan 17, 2014 #20


    User Avatar
    Science Advisor

    No, I mean that in this interpretation, the classical world with definite outcomes is presumed to exist. Measurement which produces definite outcomes is what happens when a classical apparatus interacts with a quantum system. Quantum mechanics is simply a way to calculate the probabilities of definite classical outcomes. By assumption, when we open the box, the cat is either dead or alive, never dead and alive. In this view, quantum mechanics is not necessarily a fundamental theory. The dividing line between quantum and classical requires human judgement, and is indefinite in theory, but in practice has never been falsified. This is basically a version of shut-up-and-calculate.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook