On 07-04-2022 02:30, Bruce Kellett wrote:
On Thu, Apr 7, 2022 at 10:24 AM smitra <smi...@zonnet.nl> wrote:
On 07-04-2022 00:58, Bruce Kellett wrote:
But then you have the problem of whether "observers" simulated by
a
quantum computer can actually make measurements. The essence of a
measurement is the formation of permanent records in the
environment.
It's perfectly possible for an observer to make an observation
without
there being any permanent record. The physical processes that makes
someone be able to see and feel something has noting to do with the
formation of permanent records.
One word. Decoherence.
Decoherence is just entanglement with a large number of degrees of
freedom.
Quantum computers cannot do this unless they stop and print out a
result. Your quantum computer simulation requires a redefinition
of
the concept of measurement so that it becomes essentially
meaningless.
It forces one to come up with a more reasonable definition of
measurement.
There is a perfectly reasonable definition of measurement that
involves decoherence and the formation of multiple records in the
environment -- quantum Darwinism of Zurek.
That's an effectively macroscopic description of measurements. It cannot
possibly be a fundamental description of measurement. If you perform a
measurement of the z-component of a spin of a particle polarized in the
x-direction and the measurement process takes one minute, then the
physical degrees of freedom that are entangled with the final state are
located inside a radius of one lightminute. That patch is (assuming MWI)
in a superposition of the different measurement outcomes
If I observe something, then that's because there is a
brain that's running the algorithm that corresponds to that
observation.
In the context of the MWI one then needs to assume that there exists
algorithms for observations made by Alice and Bob which then defines
the
required preferred basis.
The preferred basis is not determined by algorithms -- it is
determined by robustness under decoherence. You can redefine
everything so that your theory is no longer quantum mechanics -- but
that is a fairly pointless exercise.
That's the preferred basis as used in practice. But that's useless in
this context and it would amount to doing things things backward.
Observers cannot be defined using decoherence. That you do robustness
under decoherence allows for us as stable observers to exist. So
decoherence explains our existence.
Saibal
Bruce
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