RE: White Rabbit vs. Tegmark

From: Hal Finney <hal.domain.name.hidden>
Date: Fri, 27 May 2005 11:19:18 -0700 (PDT)

Brent Meeker writes:
> I doubt that the concept of "logically possible" has any absolute meaning. It
> is relative to which axioms and predicates are assumed. Not long ago the
> quantum weirdness of Bell's theorem, or special relativity would have been
> declared "logically impossible". Is it logically possible that Hamlet doesn't
> kill Polonius? Is it logically possible that a surface be both red and green?

I agree. We went around on this "logically possible" stuff a few
weeks ago. A universe is not constrained by logical possibility.
Our understanding of what is or is not a possible universe is constrained
by our mental abilities, which include logic as one of their components.

If I say a universe exists where glirp glorp glurp, that is not
meaningful. But it doesn't constrain or limit any universe, it is
simply a non-meaningful description. It is a problem in my mind and my
understanding, not a problem in the nature of the multiverse.

If I say a universe exists where p and not p, that has similar problems.
It it is not a meaningful description.

Similarly if I say a universe exists where pi = 3. Saying this
demonstrates an inconsistency in my mathematical logic. It doesn't
limit any universes.

More complex descriptions, like whether green can be red, come down
to our definitions and what we mean. Maybe we are inconsistent in
our minds and failing to describe a meaningful universe; maybe not.
But again it does not limit what universes exist.

To summarize, logic is not a property of universes. It is a tool that
our minds use to understand the world, including possible universes.
We may fail to think clearly or consistently or logically about what
can and cannot exist, but that doesn't change the world out there.

Rather than expressing the AUH as the theory that all "logically possible"
universes exist, I would just say that all universes exist. And of course
as we try to understand the nature of such a multiverse, we will attempt
to be logically consistent in our reasoning. That's where logic comes in.

Hal Finney
Received on Fri May 27 2005 - 15:14:14 PDT