On 04-04-2022 01:38, Bruce Kellett wrote:
On Mon, Apr 4, 2022 at 12:43 AM smitra <smi...@zonnet.nl> wrote:
On 25-03-2022 23:01, Bruce Kellett wrote:
On Sat, Mar 26, 2022 at 6:37 AM smitra <smi...@zonnet.nl> wrote:
One can indeed write down non-local effective Hamiltonians, but
the
fundamental laws of physics are strictly local.
God told you this, did he? If one needs non-local physics to
describe
certain experiments, then locality is not universal.
The Standard Model is local.
The standard model, and quantum field theory, are local because they
were made that way. A fundamental assumption of QFT is
micro-causality, namely, the assumption is that all field commutators
vanish outside the light cone. This assumption imposes strict
locality. But it is an assumption -- not a God-given truth. There are
clear reasons for supposing that this assumption cannot be universally
valid, since non-local effects, as evidenced by non-separable states,
are clearly present in the world.
QFT is consistent with all of the low energy physics we know about
including bell-type experiments, because it's ultimately just QM applied
to fields.
If you can experimentally verify that for a
certain physical system the Standard Model (extended to take into
account finite neutrino masses) is not adequate, then you'll win the
Nobel prize for that discovery. We know that the Standard model has
to
fail when quantum gravity will become important. But we cannot probe
this regime experimentally, and for the sort of things we're
discussing
here, it's not relevant.
What matters is the time evolution step by step proceeds in a
manifestly
local way. The entangled spin system is created locally, the two
particles travel to the locations of Alice and Bob. Alice and Bob
and
their polarizers considered as large collections of particles
interact
locally with the spins. While you then do find correlations that
exhibit
nonlocal properties, there is then a local account of how these
correlations arise, namely the time evolution via the Schrodinger
equation with a manifestly local Hamiltonian. Such a Hamiltonian is
always available because the well established low energy physics
won't
violate the Standard Model.
The philosophical position that you adopt here is known as "Humean
supervenience". It derives from the empiricism of David Hume, and has
been given its modern form by the philosopher David Lewis.
"A broadly Humean doctrine holds that all the facts there are about
the world are particular facts...it might better be taken as a
doctrine of supervenience: if two worlds match perfectly in all
matters of particular fact, they match in all other ways too -- in
model properties, laws, causal connection, chances...." (David Lewis,
Philosophical papers, 1986)
Tim Maudlin (in 'The Metaphysics within physics', 2007) discusses this
in detail. He summarizes Humeanism as follows:
(Separability): "The complete physical world is determined by
(supervenience on) the intrinsic physical state of each spacetime
point (or each pointlike object) and the spatio-temporal relations
between these points.
Separability posits, in essence, that we can chop up space-time into
arbitrarily small bits, each of which has its own physical state... so
the world as a whole is supposed to be decomposable into small bits
laid out in space and time."
Since it has its origin in 18th century empiricism, it is not
surprising that Humean supervenience (and separability) are true for
classical Newtonian physics. The point that Maudlin is stressing is
that although true for classical physics, Humeanism is not true for
quantum physics. The crucial difference is that quantum physics allows
non-separable states, for which the locality assumption does not hold.
The phenomenon of quantum entanglement is the manifestation of this
non-separability, and it is now eminently clear that entanglement lies
at the heart of quantum physics. So QM is not Humean, and not
intrinsically local.
I still read papers in which Humean supervenience is simply assumed,
and taken to be non-negotiable -- as, indeed, you do. But these papers
are taking an outmoded philosophical position, based largely in
classical intuitions, so not appropriate for quantum physics. And it
is important to stress that this is a philosophical position (or
dogma); it is not a scientific position, because the scientist must
always remain open to the possibility that previous philosophical
assumptions might well be found to be wrong or inappropriate.
This is the case with Humean supervenience. Quantum physics, because
of the phenomenon of entanglement (non-separability), shows that the
world cannot, in fact be chopped up into arbitrarily small bits, each
of which has its own physical state, independent of all the other
bits.... Consequently, locality as the Humean would want, can no
longer simply be assumed.
States can be non-local, but interactions are local and non-local states
can be created locally from separable states.
To make this point even more strongly, I would urge you to come up
with an actual local account of the Bell correlations -- and that
means that you can no longer simply assume that it must be local
because everything is local -- you have to give the details of the
local mechanisms that make it all happen, in which "time evolution
step by step proceeds in a manifestly local way". I am completely
confident that you cannot do this -- if you could, you would have
produced this local account long ago, rather than relying on some
unspecified magic to make everything work out.
One can just put an entire Bell-type experiment in a quantum computer
create entangled qubit states using Hadamard and CNOT operations and
then let two programs representing Alice and Bob do measurements. Then
everything is always manifestly local from start to finish. Doing this
exercise will then make clear that in the MWI it's important to define
observers as algorithms and that this then defines a preferred basis.
I'll write up the details later.
Saibal
Bruce
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