Re: Infinite universe

Date: Fri Apr 25 2003 - 19:37:10 MDT

"Mike Williams" <> said:
> If it were possible for our universe to interact with the other
> universes, then it would have already happened. Since it would be
> possible that one of the universes would have the desire and ability to
> consume the others, that would have already happened.

If I'm interpreting you correctly, you're stating the multiverse version
of the Fermi paradox. I don't think it applies in the way you're arguing.

Using Tegmark's classification of levels, the first level of multiverse
is, in my opinion, the most likely to cause Fermi problems. If faster
than light travel is not possible, then we need only account for the
observable universe. If faster than light travel is possible, then the
paradox becomes much worse, since there are then many more civilizations
that could have reached us. If instantaneous faster than light travel
(with respect to the coordinates defined by cosmic expansion) is
possible, it becomes impossible to argue "no one has visited us because
no one is there" -- if nothing else, you would have expected a complete
civilization to have accidentally come forth from random particle motions
just after the Big Bang, or to have Hawking-radiated out of a black hole
somewhere. This civilization could then get to any place in essentially
zero time. Of course, it's not quite this simple -- for one thing, it
takes resources. It does mean you have to throw away either an infinite
universe, or unlimited FTL, or most attempted Fermi paradox solutions.
(I'd easily pick unlimited FTL)

The second level is much more Fermi-friendly. In one sense, the false
vacuum and other thermalized regions are more than infinitely far away
-- if you kept flying, you could never get there, because in the
"external" time your thermalized region is expanding too quickly. This
may be what they mean by "to infinity... and beyond!". In another,
probably more accurate sense, these events are in your past. This means
that you can't get there, unless you know of a way to travel back in
time through the Big Bang.

Even if you could do that, I'm not sure it would be possible to exist
there. The false vacuum is in principle just ordinary space, but with
weird fields in it. IIUC, the kinds of particles we know couldn't
exist there.

The dimensionality of the false vacuum is not known; it depends on what
theory of quantum gravity is correct. In the context where different
thermalized regions have different dimensions because different
compactifications occur (in string theory), I'm pretty sure it's
(10+1)-dimensional. In loop quantum gravity, it would probably be
(3+1)-dimensional. In other theories, it would be some other number
that I don't know; probably also 3+1. Also, I have no idea what
theories of quantum gravity are compatible with the chaotic eternal
inflation picture.

One thing I'm sure of is that the false vacuum is physical space, not
quantum-mechanical infinite-dimensional Hilbert space. Hilbert space
contains quantum states, not places. It's not the sort of space you could
fly through in a space ship. (I hope this answers Paul Hughes's question)

Even though it seems impossible to travel between different thermalized
regions, there can apparently be some one-way communication between them.
This is one of the subjects discussed in the paper I linked in my
previous post -- the authors claim that if an inflating false vacuum can
be created within our thermalized region, it may be possible to send
"messages tightly packed in durable containers" to them. They then argue
that this is pointless, because only finitely many civilizations would
receive them, so that this wouldn't increase the probability. I don't
think I agree with this argument; I'm not sure it wouldn't also prove
it's pointless to get up in the morning.

I also remember reading that there could theoretically be ways to set the
values of the fundamental constants in future thermalized regions. If
this is possible, it's likely that this is true in our case (only very
few universes would have constants compatible with the existence of life
by accident). Could this be used to send messages -- say, an AI design?
The values are constrained somewhat by the need to make the universe
suitable for life; it seems to me the decimal places beyond that could be
used. At our present knowledge, it can't yet be much information, though
-- not that many fundamental constants, and only a few decimal places per
constant. Still, it seems to me to have a greater chance of working than
SETI. Does anyone know of attempts to systematically look for messages in
the values of physical constants? (not Pi -- that's as silly as hiding a
message deep in the number 19)

Returning to the multiversal Fermi paradox, could different level 3
universes (quantum worlds) communicate? If the correct quantum mechanical
theory is linear (which is true as far as we can tell experimentally),
then the answer is no -- "linear" means that different terms superposed
in a wave function ("worlds") evolve independently. If physics is not
exactly linear, then it may or may not be possible to make use of this to
communicate with or travel to other quantum worlds. This has been used to
argue physics is exactly linear (see because if it wasn't, we'd have
noticed the inter-world travellers by now.

For worlds to influence each other across the fourth level of multiverse,
there would have to be a causal interaction between mathematical
structures out there in Platospace. This doesn't happen. Complex numbers
don't suddenly leak into the integers; the same is true for structures
complicated enough to have observers in them.

In conclusion, we will probably not have any interaction with the
inhabitants of other universes. I don't think this makes the subject
moot, though -- the questions whether they exist and what they are like
can still affect what we should expect to find here, as well as help us
think about various other subjects.

Much of the above is either an informed guess or pure speculation; please
keep taking it with an infinitely large grain of salt and an "IIUC"


"To infinity, and beyond, and even further than that"
-- - Access all of your messages and folders
                          wherever you are

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