On 08-04-2022 19:04, Brent Meeker wrote:
On 4/7/2022 8:00 PM, smitra wrote:
On 07-04-2022 18:25, Brent Meeker wrote:
On 4/7/2022 8:25 AM, smitra wrote:
On 07-04-2022 03:05, Bruce Kellett wrote:
On Thu, Apr 7, 2022 at 10:52 AM smitra <smi...@zonnet.nl> wrote:

On 07-04-2022 02:30, Bruce Kellett wrote:

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 --
that is a fairly pointless exercise.

That's the preferred basis as used in practice. But that's useless
this context and it would amount to doing things  things backward.
Observers cannot be defined using decoherence. That you do
under decoherence allows for us as stable observers to exist. So
decoherence explains our existence.

It also explains our ability to make measurements and record results. To claim that this is not a fundamental account of measurement is just silly. Nothing is more fundamental than quantum entanglement evidenced
in decoherence.

While entanglement is a phenomenon that exists at the fundamental level, effective macroscopic concepts can never be fundamental, they have to be explained using the fundamental microscopic theory in which many macroscopic concepts do not even exist.

You need to keep in mind that there are different meanings of
"fundamental".  Those "macroscopic concepts" like measurements and
records and facts are epistemically fundamental; and remain so however
theories change.  The reductionist base of the current theory is
ontologically fundamental, but it may be replaced by a new theory with
a different ontology, as QM replaced Newtonian mechanics and
statistical mechanics replaced thermodynamics.  Being ontologically
fundamental is a precarious position.

Yes, and that means that the new theory must reduce effectively to the old theory in the macroscopic regime where the old theory makes (almost) correct predictions.

That incorrectly insinuates that the old theory only makes accurate
predictions in the macroscopic domain.  For example, I suspect that
the solution to the problem of quantum gravity will imply natural
cutoffs at the Planck scale which we now often invoke heuristically. 
Just because an ontology is microscopic doesn't make it immune to
replacement.  Strings or loop quantum gravity are microscopic too.

Yes, I agree here. Another example is statistical physics. We teach students about the equal prior probability postulate, but it's not actually true. However we do know that the predictions made by assuming this are correct. The question why statistical mechanics works, is then an interesting problem. See e.g. here:


If we then ask fundamental questions about e.g. the existence of a multiverse that can only be addressed by getting the details about the dynamics at the microscopic level correct, then it's not appropriate to fix up the theory by introducing notions from the macroscopic domain that should in principle follow from the fundamental dynamics at the micro-level.

The notion of "result" and "measurement" are not introduced, they are
fundamental to knowledge.  They are exactly where MWI gets into
trouble.  By saying there is no result of an experiment it muddles the
concept of probability.

In a Bruno-type copying experiment one can get multiple outcomes with various distributions too. If 100 Brents are created with one of them going to get $1 millions what is the probability that you'll win $1 million? If instead of identical Brents we create 100 random persons such that one of them is guaranteed to be Brent and one of them is randomly chosen to receive $ 1 million, then the probability would be 1/100. If we then tweak the random person generator so that the random persons that are not Brent start to look more like Brent, then that would not matter for the 1/100 probability. So, we can take the limit where the other persons become exact copies of Brent.

So, we can deal with probabilities in cases where all outcomes for an observer are realized.


The appearance of permanent records should follow from decoherence. But it makes sense to consider states of algorithms that process information as a more general notion of observation.



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