From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
On 11 Jun 2018, at 14:48, Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
Only in the sense that the biological brain has evolved through
decoherence with respect to some base, but as you say, that process
lead to the same result from the 1p perspective of those who have
chosen the base, or have the base imposed through decoherence and
evolution, say.
If not then QM would be inconsistent, and had to different laws of
physics for different observers.
[BK] That would be the result absent decoherence to the stable basis
for any measurement.
OK. We don’t differ on the fundamentals. I would need to revise Zurek
and Zeh to assures myself that some base are more stable than other
for physical reason (and not simply Everett-anthropic one),
I don't think an anthropic approach is adequate here. That relies on
some unexplained magic to account for the fact that evolution has
selected our brains to perceive in some preferred basis. But that is not
an explanation -- that is an excuse. I don't know if Zeh has actually
explored this in more detail. He seems content with decoherence as
delocalization of the phases into the environment. But Zurek is quite
critical of ending one's account there: "Popular accounts of decoherence
... often start from the observation that when a quantum system S
interacts with some environment E 'phase relations in S are lost'. This
is a caricature, at best incomplete if not misleading: It begs the
question: 'Phases between what?'. This in turn leads directly to the
main issue addressed by einselection: 'What is the preferred basis?'.
This question is often muddled in 'folklore' accounts of decoherence."
(arXiv:0707.2832)
Zurek here hits the nail on the head: loss of phase relations does not
explain what are the basis states between which phase relations are
lost. Any more complete account must explain how preferred basis states
emerge. Anthropic accounts just appeal to magic for this -- evolution is
not a dynamical account of the emergence of a preferred basis. According
to Zurek, preferred states of quantum systems emerge from the dynamics.
In that he cannot be faulted -- there is no other non-magical way in
which this could happen.
Schlosshauer gives a more complete account of the these dynamics than is
given by Zurek. (Schlosshauer, arXiv:1404.2635) In summary, Schlosshauer
points to the fact that basis states that are stable against decoherence
are those for which the corresponding quantum operator commutes with the
interaction Hamiltonian. I think that this is both a necessary and
sufficient condition for states to be stable against decoherence. But
even then, it is not an entirely satisfactory dynamical account. The
problem is that for position measurements, for example, the stable
states are eigenstates of the position operator, but the interaction
Hamiltonian is generally given by point particle interactions, and those
are themselves defined in terms of the same position operator. This is
circular -- the same position operator defines both the basis states and
the interaction Hamiltonian so they necessarily commute: But that would
be true no matter what the position operator was -- provided one used
the same operator to define both the states and the Hamiltonian. So this
does not rule out alternative position operators which would have
different sets of basis states, given by superpositions of our usual
basis states. This does not, therefore, explain why we do not see
superpositions of dots on the screen in position measurements, or
superpositions of classical pointer readings.
I do not know the answer to this problem. I do not think that it has
really been addressed by either Zurek or Schlosshauer. Clearly, the
dynamics must be central to the emergence of preferred basis states, but
I do not see a my way to a non-circular account of this. We might,
unfortunately, be up against a 'brute fact' that has no more fundamental
explanation. Alternatively, the answer might lie in a full quantum
understanding of the nature of space itself -- somehow the underlying
nature of spacetime determines that the interaction dynamics will be in
terms of a position operator whose eigenstates are delta functions on
the real line. Some such explanation is required.
but when I do that I eventually put myself on a slope leading to the
problem of marrying the quantum and gravity, a nightmare from which I
come back to mechanism rather quickly :)
I seem to have ended up with the problem of giving a quantum account of
gravity as well. But I do not think that mechanism is going to gave you
an answer to this.
Bruce
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