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B Decoherence branches

  1. Sep 29, 2016 #1
    After decoherence, is there something wrong if the branch chosen would be selected by consciousness?

    So there is ambient consciousness that select default branches and human consciousness can bias which branches chosen? Is there any experiment that has excluded this possibility?
  2. jcsd
  3. Sep 29, 2016 #2


    Staff: Mentor

    No. Its just has zero experimental support and is far too new age weird for most peoples taste. That of course means nothing - but the experimental thing is another matter.

    And it really must be asked - why do you want QM to be weirder than it already is? Just what exactly do you gain?

  4. Sep 30, 2016 #3


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    Science Advisor

    I have no idea what is ambient consciousness. :wideeyed:
  5. Sep 30, 2016 #4
    I'm thinking of the branches before and after decoherence.
    If the branches all exist before decoherence.. then only one branch is selected by consciousness after decoherence?
    So it's like Many worlds before decoherence and one world after decoherence.
    But then. Is it possible that the branches prior to decoherence already reduced to one even before decoherence.. or should the branches reduction to one only occur after decoherence in the general concept of decoherence?

    This is just to get versatile in the thinking about it all.
  6. Sep 30, 2016 #5


    Staff: Mentor

    Its an improper mixed state, how it becomes a proper one in the issue with consciousness being a very fringe explanation - so fringe I really know anyone that adheres to it.

    To understand the issue see the following:

  7. Sep 30, 2016 #6
    There is portion inside page 31: "The problem of Outcome: Why does one perceive a single outcome among the many possible ones in equation (2.1)?"
    Outcome means the branches?

    But this is not related to decoherence directly.. because decoherence just explains why everything is not in superposition... even without the concept of decoherence, there is still the problem of outcome prior to decoherence or after it, correct? In other words, is the problem of outcome independent of decoherence?
  8. Sep 30, 2016 #7


    Staff: Mentor

    That is by definition decohrerene.

    The deep dark issue is how it becomes a proper mixed state ie only one output with certain probability.

    MW resolves it by each output being a separate world.

  9. Sep 30, 2016 #8


    Staff: Mentor

    Last edited by a moderator: May 8, 2017
  10. Sep 30, 2016 #9
    Before spending 6 months with your book below. Just some general ideas. Let's use actual example. In the double slit experiment.. decoherence means the electron or photon has collided with those from the environment hence the screen detector no longer shows interference. So you were saying the environmental decoherence has made the electron choose either left or right path.. so why is there the deep dark issue of how it becomes a proper mixed state.. or only one output with certain probability? Isn't it the environment photons have decohere it and therefore obviously it would produce only one output with certain probability?

    Or how would the double slit experiment look like if it doesn't become proper mixed state (ie only one output with certain probability) even though environmental photons have decohere the electron? What detection pattern would result?
  11. Sep 30, 2016 #10


    Staff: Mentor

    That's not decoherence.

    In the double slit it interacts with the screen, becomes entangled with it so its a mixed state of position. The question is which 'element' of the mixed state is realised.

    Do you know the difference between a pure and a mixed state? Do you know states are not really elements of a vector space but operators?

  12. Sep 30, 2016 #11
    I think you understood me. I got the idea from the following that when the photons or electrons from the double slit experiments collided with the photons from the environment or "the reason why the interference term is not observed is because the electron has become entangled with the stray particles......It is this phenomenon of suppression of interference through suitable interaction with the environment that we call ‘dynamical’ or ‘environmental’ decoherence".. are you saying the following description is wrong?? If it's not wrong.. why did you say what I wrote which is based on the following was wrong (or not decoherence)?? http://plato.stanford.edu/entries/qm-decoherence/#DynDec

    "There are, however, situations in which this interference term (for detections at the screen) is not observed, i.e. in which the classical probability formula applies. This happens for instance when we perform a detection at the slits, whether or not we believe that measurements are related to a ‘true’ collapse of the wave function (i.e. that only one of the components survives the measurement and proceeds to hit the screen). The disappearence of the interference term, however, can happen also spontaneously, when no collapse (true or otherwise) is presumed to happen. Namely, if some other systems (say, sufficiently many stray cosmic particles scattering off the electron) suitably interact with the wave between the slits and the screen. In this case, the reason why the interference term is not observed is because the electron has become entangled with the stray particles.[3] The phase relation between the two components of the wave function, which is responsible for interference, is well-defined only at the level of the larger system composed of electron and stray particles, and can produce interference only in a suitable experiment including the larger system. Probabilities for results of measurements performed only on the electron are calculated as if the wave function had collapsed to one or the other of its two components, but in fact the phase relations have merely been distributed over a larger system.[4] It is this phenomenon of suppression of interference through suitable interaction with the environment that we call ‘dynamical’ or ‘environmental’ decoherence."
  13. Sep 30, 2016 #12


    Staff: Mentor

    I think you first need to understand the double slit before discussing decoherence implications:

    Once you do we can discuss how decoherence applies to it.

    For example wavefunction collapse has nothing to do with it - its from the uncertainty principle and superposition principle. Its got nothing to do with interactions other than the slits between siource and screen. The reason detecting at the slits destroys interference is the superposition principle no longer appllies.

    Last edited: Oct 1, 2016
  14. Oct 1, 2016 #13
    The paper has so many dense math. I'm a layman. This thread has a B label to it. B means Beginner. Therefore I hope others who are more verbal can answer my question whether the branches are still there prior to decoherence or after it. This is because decoherence doesn't solve the measurement problem. So I'm thinking if consciousness chooses the outcome.. then it is independent of decoherence. I read that if objective collapse occurs faster than decoherence.. then there is no decoherence but only objective collapse... i'm thinking if consciousness choosing outcome is faster than decoherence or slower. (Demystifer. for sake of discussion. Ambient consciousness that choose outcomes without human consciousness simply means background consciousness that let's say is part of the universe. So it can chooses branches in default mode.. hence even without humans present.. it works.. but ignore this.. I simply want to understand about the branches before and after decoherence and whether the outcome is chosen before or after and how to do it. So those who were laymen friendly advisers.. please share your views. Bill is super super advance and I can't read 6 months of his books and papers before jus knowing a certain specific question).
  15. Oct 1, 2016 #14


    Staff: Mentor


    I will write a easier to understand version a bit later.

  16. Oct 1, 2016 #15


    Staff: Mentor

    Ok - a few things.

    A pure state is written as Iu>. The principle of superposition says given any two states |u1> and |u2> then |u1> + |u2> is also a possible state.

    Now any pure state can be decomposed into the sum of a lot of other pure states - usually though is broken down into superposition's of position so |u> = |x1> + |x2> +++++ |xn> wjere each |xi> is a state of definite position.

    Now remember I mentioned states really aren't vectors, they are operators. The operator for a pure state is written as |u><u|. In general operators are of the form ∑pi |xi><xi| where the |xi><xi| are states of exact posiotion. It turns out the probability of it having position |xi> is pi. There are a number of ways of preparing mixed states. One way is simply to take a state and randomly present it with probability pi. Such are called proper mixed states. With proper mixed states everything is sweet - objective reality exists before observation - much of quantum wierdness disappears. But that is just one way of going it. Another way is to take a state and subject it to the process of decoherence - you get exactly the same mixed state and their is no way to tell the difference - no way at all. But because its prepared differently than a proper mixed state its called an improper one. The trouble is you cant say its in the state prior to opservation - there is simply no way to tell. If not quantum wierdness remains. This is the modern version of the measurement problem. What causes an improper mixture to become a proper one. Colloquially its why we get any outcomes at all. With different interpretations like MW and BM its trivial, with others like ensemble its much more controversial - even to the point of its a problem at all.

    Now to the double slit. The electron interacts with the screen and via decoherence becomes a mixed state. It registers one of the positions - the question is - why did it register a particular one?

    How conciousness would come into it is anyones quess.

    Last edited: Oct 1, 2016
  17. Oct 1, 2016 #16
    Thanks. Since in consciousness based interpretation.. ambient or background consciousness can automatically collapse wave function or else how can the universe exist before humans appear, then we may as well just talk about Objective collapse to avoid much weirdness. They say if Objective Collapse occur faster than decoherence, then there is no decoherence.. in this case.. improper mixed state turns to proper mixed state by objective collapse. But is this very statement true at all that that objective collapse can occur faster than decoherence? Is decoherence proven 100%? If true, then objective collapse can't occur faster than decoherence?
  18. Oct 2, 2016 #17


    Staff: Mentor

    Decoherenve is proven 100% and follows from the formalism. What it means is the issue.

    Objective collapse doesn't exist in the theory - only interpretations - collapse is not part of QM or the QM formalism.

    There is an interpretation called GRW where it exists:

  19. Oct 2, 2016 #18
    I comprehended that the electron interacts with the screen and via decoherence becomes a mixed state.. but if the electron were hit from photons from one of the slits or from the environment, there is no longer any interference... and from the paper "Decoherence of matter waves by thermal emission of radiation"
    https://arxiv.org/pdf/quant-ph/0402146.pdf It is also called decoherence or environmental decoherence.. why did you state "That's not decoherence"? Is it not environmental decoherence effect?
  20. Oct 2, 2016 #19


    Staff: Mentor

    Decoherence is a particular type of interaction:
    The original system's wavefunction can be expanded in many different ways as a sum of elements in a quantum superposition. Each expansion corresponds to a projection of the wave vector onto a basis. The basis can be chosen at will. Let us choose an expansion where the resulting basis elements interact with the environment in an element-specific way. Such elements will—with overwhelming probability—be rapidly separated from each other by their natural unitary time evolution along their own independent paths. After a very short interaction, there is almost no chance of any further interference. The process is effectively irreversible. The different elements effectively become "lost" from each other in the expanded phase space created by coupling with the environment; in phase space, this decoupling is monitored through the Wigner quasi-probability distribution. The original elements are said to have decohered. The environment has effectively selected out those expansions or decompositions of the original state vector that decohere (or lose phase coherence) with each other. This is called "environmentally-induced-superselection", or einselection.[4] The decohered elements of the system no longer exhibit quantum interference between each other, as in a double-slit experiment. Any elements that decohere from each other via environmental interactions are said to be quantum entangledwith the environment. The converse is not true: not all entangled states are decohered from each other.

    Although it must be said people often speak a bit loosely about it. For example decoherence is usually considered to be irreversible, but in explaining the delayed choice experiment I, and others say, in simple cases dechorence can be undone. Strictly speaking if it can be undone its not irreversible so wasn't decoherence to begin with. An in the double slit what happens to the electron as it travels from slit to screen is irrelevant - in fact its usually a vacuum.

    What I was trying to get across in the double slit is the key decoherence occurs at the screen.

    Last edited: Oct 2, 2016
  21. Oct 2, 2016 #20
    In the "environment" in the above description.. can you considered the screen some kind of "environment" that cause decoherence? Or is "environment" everything else except the screen?
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