Re: Time travel in multiple universes

From: Hal Finney <>
Date: Sun, 19 Jun 2005 17:11:30 -0700 (PDT)

Ben Goertzel writes:
> I recently wrote a blog entry on time travel
> and Tom Buckner followed up with an interesting comment on the potential
> for time travel in Tegmarkian multiple universes.

Those are interesting speculations, but I don't think it really makes
sense to imagine travelling between the worlds of the Tegmark multiverse.
There are no causal connections between them of the type that would be
necessary for an information packet to travel in the way we normally
think of it happening.

I think David Deutsch had some ideas about time travel in the MWI
going between parallel worlds, but again I didn't think that could work,
physically. Once worlds have decohered, there are no physical mechanisms
for them to interact to any measurable degree.

However I do think there are connections between time travel and the
MWI, different from Deutsch's rather simplistic picture of travel to
parallel worlds.

The big problem with time travel is not so much the kill-your-father
paradox, because as Ben writes this can be easily dealt with by
postulating that only consistent time travel works. The bigger puzzle
then is the apparent necessity of the universe to be intelligent, for the
natural laws to engage in strategic reasoning at least as advanced and
sophisticated as the intelligent beings whose free will it is thwarting.

When a time traveller tries to do something, there has to be the potential
for a sort of back-reaction from the universe which can interfere with
his actions if they would lead to a paradox. Let's suppose he goes
to do something, make a change in the past which it turns out will be
inconsistent with his memories in the future. Something's going to
stop him. But how does the universe know that this has to be stopped?
It seems that there has to be at least a potential or virtual universe
created in which his actions play out, their consequences extend through
time into the future where the time traveller departed from, and the
inconsistency with his mental state is detected.

This can be represented as a quantum wave function which is travelling
around a causal loop, such that only consistent conditions can exist.
But for this to work I think we have to imagine universes existing where
the other things happen, the ones which will get cancelled out in the end.

This is not unlike a commonplace phenomenon in QM, where the classical-
physics paths of particles arise from what is actually a much more muddled
condition at the quantum level. The seemingly straight path of a beam of
light is actually the result of constructive interference much like what
we are postulating to prevent time paradoxes. Light actually takes every
possible path from A to B. Most of them cancel out, and only the paths
that are straight interfere constructively rather than destructively.

The thing is, though, that the other paths do exist and are represented
in the wave function. They are not usually considered, even by believers
in the MWI, to represent "worlds". This is because they do not decohere
and so do not acquire independent existence. They all get folded back
together much like the two paths through a double-slit experiment.

We can imagine a model of the MWI which does treat the two paths through
a double slit experiment as separate worlds, which then recombine.
I think this is actually the perspective that Deutsch prefers. In that
view, we could say that the crooked light beams also exist in worlds
of their own, which recombine and cancel each other out.

In the case of time travel, if we apply the same concepts, then the
decohered-out worlds would also be said to have existence in the MWI.
They don't have lasting existence or effects, but they exist in some
sense for some period of time.

It is in these "shadow" worlds that the apparent intelligence of nature
arises in the case of time travel. Any action that would produce a
paradox, no matter how complex the effect or how long the chain of
causation that it requires, gets simulated and all its effects get
determined in shadow universes. Real people live there whose lives
are affected by the changes in history, and whose actions may play a
part in making the paradox arise. It's almost like a super-powerful
quantum computer running to check everything the time traveller does
for consistency.

In the movie Back to the Future, Marty travels back to the past and meets
his mother when she is a teenager. She falls in love with him instead of
his father, so Marty won't be born. This is a paradox which nature would
prevent in a real situation. Nature might stop Marty from meeting his
mother, so things will still go as they were supposed to. But how does
nature know that he shouldn't be allowed to meet her? It seems that there
must exist a shadow world where they do meet, where she falls in love,
and all the consequences play out. Yet that world ends up having no
real existence. It gets cancelled out just like the crooked photon paths.

What of the version of Marty's mother who fell in love with him? What is
it like to be her? I think QM would predict that she is of measure zero.
It's not clear what that means, but under the ASSA it would seem to mean
that there is essentially zero chance that anyone will find themself
in such a situation. Yet, she had to live, she had to breathe, her
heart had to beat and fall in love, for the paradox to be recognized.
Was she a zombie? Did she act as an automaton, without consciousness?
How could she fall in love if so?

These are tough questions and I don't really have an answer for them.
On the one hand, it seems that we will never have to face the prospect
of living in a paradox. On the other, it seems that someone must live
there or else there would be no way of recognizing that a paradox would
arise out of certain actions.

A related manifestation of this paradox in time travel stories is the
creation of information out of nothing. The classical example is the man
who is given a gift by his older self of a time machine from the future.
He presents it to the world and is acclaimed as the inventor of time
travel. All future time machines are based on his model. There's no
paradox here, but who invented the time machine? Well, the universe did.
How did the universe get that intelligence? From the shadow worlds.

Imagine, for example, if the time machine were actually invented by
someone, and they wanted to go back in time and give it to their younger
self. That would be a paradox and would be prevented. But maybe the
simplest way to prevent it is to just skip the invention part and let the
younger man receive the time machine as a gift. This solves the problem,
avoids a paradox, achieves local consistency, and perhaps requires less
work on nature's part.

Deutsch, or maybe it was Moravec, wrote an article years ago about
how you could coerce nature into doing computation for you, if you had
a time machine. Basically you would set things up so that a paradox
would arise unless a randomness circuit got lucky and came up with a
solution to a hard-to-solve problem. Where does the solution come from?
Again you are getting nature to act as a quantum computer for you,
in the shadow branches.

Isaac Asimov had a story about a chemical which would dissolve *before*
it was placed in water. He wrote about how scientists learned to extend
the time span, and then the military found a way to use this effect
for a bomb. When a sample of the chemical was observed to dissolve, it
was sealed in a dry, water-tight container and dropped in an unfriendly
country. Now, in so many hours, it was certain that the interior of the
bomb would get wet. But how could that happen? The only force strong
enough was a disastrous flood, of such magnitude that the container
would be carried off and torn open, so that the chemical could dissolve.
And of course this would do tremendous damage to the enemy's facilities.

Once again, nature is forced to act intelligently, even strategically,
in order to prevent paradox. The substance had to get wet, it had, in
a sense, already gotten wet, and nature had to find the best way to do it.

One final point, which is that there is a related phenomenon in some of
our multiverse models. It seems that to avoid a paradox, nature must
in some sense try different things to see which ones work. But "when"
does it do this? How much time does it take? Well, it can't happen in
the regular time coordinate. We already know what happens there, the
non-paradoxical time line occurs and no paradoxes exist. I suggested
that there are parallel universes where the paradoxes occur, and this
provides a mechanism and place and time for the extra work, the trial
and error, to happen.

In some multiverse models, we allow for a certain amount of trial and
error to create a universe. We don't always stick to just a simple case
where we set up the initial conditions and natural laws, and evolve them
forward, time-step by time-step. We have discussed more complex rules
which would work in part by trial and error, setting things first one
way and then another, until various consistency criteria are satisfied.
Such an optimization or satisficing machine might be a better model
for our own universe than the simple Cartesian clockwork program we may
naively imagine as our first conception.

The point is that there is a time coordinate within the universe, but it
is not necessarily the same as the time coordinate of the computer that
is creating the universe. That computer may be going back and forth,
tweaking here, changing there, taking a long time just to set up a small
patch of space-time in its output tape. This is another way to think of
"where" and "when" the alternatives for paradox free time travel could
be considered and rejected.

Hal Finney
Received on Sun Jun 19 2005 - 21:07:47 PDT

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