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Problem about quantum environmental decoherence

  1. Jul 1, 2012 #1
    "The characteristic feature of the first (often called ‘dynamical’ or ‘environmental’ decoherence) is the study of concrete models of (spontaneous) interactions between a system and its environment that lead to suppression of interference effects"

    http://plato.stanford.edu/entries/qm-decoherence/

    This seems to raise a problem since environmental surroundings are a fact even before an observer looks on the electron.(it is located in a room e.t.c). so were do we draw the line when decoherence comes into the picture? has it to do with the <distance> of the environmental influence and the electron?

    Decoherence seems to have a problem since there always exists an environment surrounding regardless of a human observer or a camera filming it. And yet the suppresion only occurs when we actually look at it closely(or put a camera in the room)

    How is this solved?
     
    Last edited: Jul 1, 2012
  2. jcsd
  3. Jul 1, 2012 #2
    short answer: the environment, in general, does not interact, optically, as much as, a camera does .....with a photon

    Any interaction that causes a phase difference between the "waves/path", is the way I see it, causes de-coherence.

    A camera is suppressing interference because it is changing the phase between the waves, coming out, from the two slits.

    The surrounding environment, in general, is not interacting, in such as way as to cause (an appreciable/noticable) phase difference between the "waves".

    However a camera is.

    if you notice - the edges of the fringes are not sharply defined. some degree of de-coherence is being triggered from the surroundings ..........(as well as minor perturbations/initial conditions within the "waves")

    also there are degrees of de-coherence, i.e. there are degrees of phase difference and we can come up with interesting patterns by manipulating the phase difference between the "waves/paths"
     
    Last edited: Jul 2, 2012
  4. Jul 1, 2012 #3
    Why do you say that?

    I would rather say that the environment typically does that.

    We should be clear about what we mean by environment here: something physical which you know fundamentally interacts with your system but which you are neglecting nonetheless.

    And rocket, indeed, whether there is a human observer or a camera is irrelevant. An environment will induce decoherence, no matter what that environment is exactly. It's just that a camera is a good example of such an environment, but indeed even if the system is contact with the atmosphere, i.e. interacting with the air, well that's a huge source of decoherence right there.
     
  5. Jul 1, 2012 #4
    there are various kinds of interaction....for example gravitational, mechanical, electrical, em, optical, nuclear etc

    we are dealing with a specific kind of interaction....an interaction that causes phase difference between the "waves/paths"........in case of photons its called "optical interference"...i think

    air does not interact, as much, optically with the "waves/paths" as does a camera..... air causes a smaller degree of phase difference than a camera does....
     
  6. Jul 1, 2012 #5
    Now that we know the effects of environmental interaction and how it subjigate the particle to suppression, what mystery is there left to solve of the observer effect and quantum mechanics?

    You might still ask why this phenomenon of transition to begin with ?

    However that would just be responded with that it's simply the way our our universe is.

    The "why" question seems to be meaningless once we have established a coherent explanation of the aspects involved(gravitational effects or others to make the suppresion). There is no missing link.

    Only weirdness would be the actual behavior of the electron but that's only because it's contrary to our everyday experience. We know why the changes occur from crazy to normal behavior and that's all we can ask for:)
     
    Last edited: Jul 1, 2012
  7. Jul 1, 2012 #6
    agree rocket1to6, good point. part of the mystery is solved and recent experiments like two-photon interference support the above idea.

    quantum mechanics is complete, though in a weak sense.

    however there are mysteries, left to solve, in other, closely related, areas of QM.

    for example -

    - what is it that is coming-out of the slits that we cannot detect (but talk about phase changes) but only infer? i.e. a deeper/better understanding of "matter waves"
    - how do these waves cause entanglement? how does entanglement work?

    we understand many things about QM conceptually, and even better mathematically, however we don't understand them at a human (mind/pyschology) level
     
    Last edited: Jul 2, 2012
  8. Jul 2, 2012 #7
    From https://sites.google.com/site/dwhallwood/research-interests

    What I don't get is why people ask 'why don't we see macroscopic superpositions in everyday life?' Clearly because some form of measurement has occured. Don't ask that question to entertain the idea that QM does't apply to the macroscopic level.
     
  9. Jul 2, 2012 #8
    Experiments are performed when there are minimial interactions with the environment.
     
  10. Jul 2, 2012 #9

    bhobba

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    The mystery is in deriving decoherence one must use the density matrix formalism which is a generalization of the projection postulate that in its very statement involves an observation. In the everyday world it IMHO easily solves the measurement problem but at the quantum level this 'bootstrapping' is an issue. Not an insurmountable one IMHO (eg consistent histories can resolve it) - but it is a blemish on what would otherwise be an extremely elegant solution.

    Thanks
    Bill
     
  11. Jul 2, 2012 #10
    I agree. Phase information is lost both in inelastic collisions that alter the populations (and necessarily the coherences at the same time) and elastic collisions which leave the populations but disrupt phase relationships.


    Is this the case? Once you assume a basis then the density matrix is no different from the wave function apart from that you can treat populations and phase relationships more transparently. The system as described by the density matrix of a pure state is no more "observed" than that of a wave function. For a mixed state, perhaps I can see your point in that the system (beginning at thermal equilibrium) has already been "collapsed" into it's statistically weighted populations. Am I missing something here?
     
  12. Jul 3, 2012 #11

    Then why is not the particle suppresed instantaneously to classical behavior? The reason we don't see an effect of this before an "observer" surely is because that the air does not interact enough to cause a fundamental change in the particles manifestation.
     
  13. Jul 3, 2012 #12

    bhobba

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    The issue is that prior to decoherence, before the off diagonal elements very quickly go to zero, superposition holds and in principle the projection postulate is required to make sense of it. Consistent Histories resolves it by saying the setup precludes, until decoherence enforces the consistency condition imposed by the observational set-up, it being looked at in such a manner because it is nonsense to do so - your setup gives some precise answer like say a specific position - it is nonsense to consider states other than those consistent with your set-up that are partly in more than one position simultaneously - they call an interpretation of such states a pre-probability rather than a probability. You can also resolve it by refusing to interpret such states at all - but to me that seems a bit defeatist. Like I say decoherence, strictly speaking, does not solve the measurement problem - but does FOR ALL PRACTICAL PURPOSES. The other issue is all interpretations suck in their own way - you simply pick the one that sucks least for you. Consistent Histories (also known as Decoherent Histories) sucks by defining your way out of problems. If your observational setup precludes states inconsistent with it then they are rejected as a valid way of looking at it - sure it solves the issue - but by sweeping it under the carpet.

    Thanks
    Bill
     
    Last edited: Jul 3, 2012
  14. Jul 3, 2012 #13

    f95toli

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    Not quite. There is a whole "subfield" of experimental physics where the goal is to engineer the environment in such as way that the coherence times of the quantum system you are interested are extended. How this is done will obviously depend on the system, but from a mathematical point of view you can actually get pretty far by parameterzing the environment as a bath of two-level system (TLS); in order to increase the coherence time you "simply" have to reduce the coupling to the TLS that are resonant with your system.

    This is obviously a simplification (it does not take non-resonant interactions into account), but it is usually a good starting point. Note also that sometimes these TLS are "real" in the sense that they correspond to something physical, in solid state systems the TLS can be unpaired spins, rotational states of molecules or even the level splitting of certain atoms/ions that are embedded in the lattice (if you e..g are operating your system at 10 GHz, you don't want any chromium or iron ions there since they will "suck" energy from your system and severely reduce T1 and T2)
     
  15. Jul 3, 2012 #14
    This quote "The mystery is in deriving decoherence one must use the density matrix formalism which is a generalization of the projection postulate that in its very statement involves an observation" is about as close to the heart of the matter that you've gotten with this thread.

    In defining decoherence theory, you assume a basis from the projection. This is necessarily based in Von Neumann measurement/coupling. There are many examples of experiments in quantum mechanics which inhierently have no measurement/coupling present. For example, pusle shaping/cavity dumping which is the process of trapping a continuous beam in an optical cavity. If you use a beam with a single mode(short uncertainty in momentum) and a long coherence length(uncertainty in posaition), if the cavity has an internal length shorter than the coherence length, then the beam "overlapps". When you open the cavity it all comes out in one pulse and the coherence length(unc. in pos.) is now the same as the cavity length/pulse length.

    This is a change in state, and note that it involves absolutely no Von Neumann coupling. Decoherence theory has absolutely no explaination of what is happenning here. Therefore decoherence theorists should be looking for a more fundamental approach to interpreting quantum mechanics, and it exists, and it is consistent with decoherence, its called the information interpretation.

    Note the info interpretation does explain the above example; when the pulse is released it is predictable that the pulse will be as short as the internal cavity. Knowability is information.

    Decoherence theory is about information implied by coupling, but a proper first principles theory of decoherence should not state that the collapse occurs due to coupling, but due to a threshhold amount of information processing being met by the changes in the systems internal phase management. A system with many wavefunctions being managed can become "too complex for God to handle all the processing" and it must decohere due to a natural limit on the information allowance of the system.
     
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