On Sun, Apr 10, 2022 at 10:25 AM smitra <smi...@zonnet.nl> wrote:

> On 09-04-2022 07:03, Bruce Kellett wrote:
> > On Sat, Apr 9, 2022 at 11:28 AM smitra <smi...@zonnet.nl> wrote:
> >
> >> On 08-04-2022 07:19, Bruce Kellett wrote:
> >>>
> >>> Permanent records do follow from entanglement and decoherence. There
> >>> is no reason to suppose that algorithms processing information are
> >>> going to produce permanent records. Unless they do, they are useless
> >>> as a model of observation. In the words of David Albert (paraphrased):
> >>> "The task of fundamental physics is to explain the manifest image
> >> ."
> >>>
> >>
> >> Decoherence is never complete it can in principle be reversed.
> >
> > No. Measurements are in principle irreversible. Whenever you have a
> > result entangled with the environment (by decoherence) you inevitably
> > have low energy IR photons that escape into space. Since these vanish
> > at the speed of light, they cannot be reversed. The records of results
> > are permanent, not just FAPP, but in principle, according to the laws
> > of physics.
> These IR photons have nothing whatsoever to do with observation.

But the have everything to do with the fact that the formation of records
is irreversible.

> it's FAPP, not in principle, anyway.

In principle means that the laws of physics forbid any violation.

If I observe something then that's
> due to my brain processing information, so, it's the processing of
> information by a particular algorithm that's the key thing here, not
> that decoherence happens and that IR photons would make this
> irreversible.

Your brain can only process external records of the result. The making of
these records is irreversible because the laws of physics do not allow you
to reverse them.

That this is FAPP and not in principle, follows from the fact that IR
> photons can be reflected back.

You are ignoring some important laws of physics here. Even if one could put
an appropriate reflecting mirror in the path of the photons so that they
are reflected back, that mirror would impart a change of momentum, so would
be slightly heated, and would emit its own IR photons in response. No
escaping those pesky IR photons.

> So, one can consider a closed system
> within which an observer exists. If we impose reflecting boundary
> conditions at some distance, then the number of modes of the
> electromagnetic field below a certain energy E is finite.

There is no such reflecting boundary. And no such boundary is relevant to
the measurement. It would still not make the process reversible. The
emission of the IR photons is a probabilistic process, so merely reflecting
them back does not ensure that they will be exactly reabsorbed -- that is a
process of zero probability measure.

And the amount
> of information carried by photons with wavelength much larger than the
> size of the experimental system is small, as for those photons that are
> emitted, there are only a small number of distinct photon states.

Not relevant, since there is no enclosing reflecting boundary.


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