On 1/21/2014 4:01 PM, Russell Standish wrote:
On Tue, Jan 21, 2014 at 02:41:46PM +1300, LizR wrote:
On 21 January 2014 14:18, Pierz <[email protected]> wrote:
I have been thinking about this and it occurs to me that firstly, the
single history is only partially true. Since quantum interference patterns
occur in MWI due to interference between universes, which can only occur if
universes can merge again after splitting, then at least at this level, the
past is not well defined. If a universe merges back with another from which
it had temporarily diverged, then an observer within that universe cannot
say which path he followed to get there. She followed all possible paths.
Of course those divergent universes were only trivially different, or else
decoherence would have made the merging impossible. But of course in any
real universe, there will be a vast number of such "nanohistories", because
of the immense number of quantum interactions where merging occurs. So at
this very short time/space scale level at least, it is impossible to define
a single history. Correct?
Imho, that is correct. The reason universes tend to diverge more than they
merge would be that the multiverse is far from thermodynamic equilibrium
this close to the Big Bang.
However at a macroscopic scale, it appears difficult for history to be
intrinsically ambiguous. In other words the network of "nodes" of the
multiverse is like a tree not a net. There may be tiny branches that rejoin
one another at the smallest scale, but the limbs of the tree cannot merge
back together. I can always define a single route back to the trunk, though
if I go further up the tree, I will be forced to decide repeatedly which
way to go. This branching is defined by time, so doesn't this effectively
give an "arrow of time"? Yet the laws of physics are not supposed to be
directional in time except through aggregation of effects as entropy. Are
these two "arrows" related? How?
See above. I didn't realise I was answering your later question when I
wrote that! To expand slightly...
My opinion is that branching exceeds merging for the same reason that there
is a thermodynamic arrow of time. To see this, imagine a universe at
thermodynamic equilibrium. All processes can play out equally in either
time direction in such a universe (every googolplex years a Boltzman brain
pops up for a split second - but its time sense could go either way along
the time axis, they're now equally (un)likely). There is no reason why the
quantum processes involved in the MWI would not be similarly balanced once
there was no thermodynamic arrow of time.
In my "Many Minds" interpretation, splitting corresponds with learning
some fact about the world, and merging corresponds with forgetting
it. This is also roughly the view Saibal Mitra has been arguing for
too. Throughout most of our lives, we tend to remember more than we
forget, which leads to the above imbalance, although once alzheimers
sets in, I guess the imbalance will run the other way.
The problem with that is that it make mysterious all the intersubjective agreement we
found in naive and pre-quantum physics. You have the paradox of Wigner's friend. Instead
of trying to explain that directly from the wave function it seems much more perspicuous
to explain decoherence which in turn explains both out observation and the fact that
others agree with our observation and that there are reliable records of it.
Another example I have put to this list (but didn't mention in my
book, AFAICR) is to say that we exist in a superposition of worlds
where T. Rexes are blue and T. Rexes are green. There is always a
possibility of a measurement that actually decides the issue (perhaps
they were actually brown), in which case the universe differentiates,
but contra the Deutsch interpretation, its is not a matter of fact,
albeit unknown, what colour a T. Rex is.
But when the record is found it will imply that the universe differentiated long ago and
that dinosaurs already observed that were green.
Brent
Ms Schroedinger: What happened to that poor cat? It looks half dead.
Erwin: I don't know. Ask Wigner.
Eugene: I just looked in and it collapsed.
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