On 29/05/2017 9:45 am, Pierz wrote:
WRT to this whole multi-coloured T-Rex business, there is a simpler point to be made. My original argument was in favour of MWI. Now whether, in MWI, macroscopic histories can merge is surely an interesting puzzle. But /without /MWI, there cannot be any ambiguity about the colour of T-Rexes. In a single universe interpretation of QM, T-Rexes were blue or they were red and they can't exist in a superposition of both. The past can't turn back into a mush of probabilities, because that would imply either that there is some "3p" significance to the concept of "now", or that the past consists of many worlds. In other words I am asserting that, /sans MWI,/ any ambiguity about the past is a matter of ignorance, not of quantum uncertainty. Otherwise you are saying that past moments which once were well defined are somehow dissolved back into ambiguity by the passage of time. But that would be privileging the now with some kind of absolute significance, which is untenable. All nows are equal.

Another way of stating my argument is that the following three propositions are mutually incompatible:

  * There is a single history
  * There is no objective significance to the concept of the current
    moment ("now")
  * The future is objectively uncertain

You can take your pick which of those propositions you reject, but it's logically impossible to support all three.

I would say that there is only one history leading to our present state. Whether you take an MWI view or a collapse view, the wave function branches deterministically at every point, so if you follow your current twig back down to the main trunk etc, there will be a unique path. If there are points along this path where there has been no decoherence, so the potentially separate worlds never form into distinct worlds (the potential branches recombine), then you will pass through such potential branches as though they were solid stems.

There is another question as to whether we are in a superposition, of say red and green T.Rexes, but that they are in separate decohered worlds and the overlap function is zero FAPP, as Russell says. I don't think such a question has a definitive answer. Despite what some people say, viz., that in the QM MWI, everything that is possible happens in some branch or other, I don't think that the branches of the quantum tree necessarily pass through every point in the possibility space. Quantum evolution in the Everettian picture is strictly deterministic, so if some apparently possible state of affairs is not consistent with the initial conditions, then if will never appear anywhere. Given any particular imagined possibility, one cannot say whether it occurs in some world or other, or in no world whatsoever.

Bruce



On Monday, May 29, 2017 at 8:17:53 AM UTC+10, Russell Standish wrote:

    On Sun, May 28, 2017 at 06:37:09PM +0200, Bruno Marchal wrote:
    >
    > On 28 May 2017, at 14:23, Pierz wrote:
    >
    >
    >
    > >We are merely ignorant of its state. I would argue the same
    > >applies to the colour of T. Rex. The past is not in a
    > >superposition of possible values, but we are ignorant of those
    > >values.
    >
    > For all practical purpose. But there are temporal form of Bell's
    > inequality violation which suggest that in principle (tacking track
    > of all the particles the T-Rex interacted with (!)) we could make
    > measurement showing interference of "different T-rex".
    >
    >

    There seems to be some confusion between superposition and
    interference. In crude terms, we do live in a superposition |green
    T.Rex>
    + |blue T.Rex>. It may well be that |green T.Rex> and |blue T.Rex>
    are
    orthogonal, ie <green T.Rex|blue T.Rex>=0 (at FAPP), in which case
    the
    two histories do not interfere, and there is no quantum
    interference phenomena.


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    Dr Russell Standish                    Phone 0425 253119 (mobile)
    Principal, High Performance Coders
    Visiting Senior Research Fellow hpc...@hpcoders.com.au
    Economics, Kingston University http://www.hpcoders.com.au
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