Quantum Time Travel

From: Fritz Griffith <fritzgriffith.domain.name.hidden>
Date: Fri, 18 Feb 2000 21:16:14 MST

I've been considering time travel in the context of quantum mechanics and
parallel universes, and have come up with some thoughts and ideas about the
subject. This might be a little off-topic, but it does have to do with the
theory of parallel universes, which seems to be a large part of the
discussion on this newsgroup. I'm interested to know what you think of my

Most scientists agree that time travel is theoretically possible. Although
the task seems monumentous, many scientists also agree that it will
eventually be possible. Using MW theory, the paradoxes are resolved. Here
are some of my observations/ideas/theories behind quantum time travel.

I know from some of the research I did (and some reasonable assumptions)
that travelling back in time would put you back to the branch in which you
set your "time machine" to travel back to, and from there you would branch
into a universe in which you suddenly appear from the future. From there,
probability would remain as you expect it to using the SE, but since things
are initially different in this branch (you are there), things would turn
out differently in that universe than in one in which you didn't appear.
But how exactly is probability affected during time travel? I assume one of
two possibilities:

1. Travelling back in time actually affects the probability of the people
from that time suddenly seeing you appear from the future. So, if normally
the probability of you appearing would be nil, then because in the future
you decided to travel back to that time, you actually made their probability
of seeing you appear significant. Perhaps those people actually have a
50/50 chance of seeing you appear. Those who end up in the universe in
which you didn't appear would eventually end up at the point in which you
decide to go back in time. Those who end up in the universe in which you
did appear would probably follow an entirely different path.

2. Travelling back in time does not affect the probability of the people
from that time suddenly seeing you appear from the future. The only way
they would see you appearing is if molecules from their universe suddenly,
by pure chance, arranged themselves to create you. Of course, you know that
this universe was very likely to be YOUR universe, but at the same time you
know that everyone else you see in it is in a universe that they had an
absolutely nil chance of being in. They are justified in claiming that you
appearing in their universe was an absolute miracle and fluke of nature,
while at the same time you are justified in claiming that this universe was
very likely for you, and was not a miracle or a fluke of nature at all. In
other words, by time travelling you have managed to put yourself with
virtual certainty into a virtually impossible universe. If this was the
case, then for us to see beings from the future visiting us would be no more
special, and in fact no different, than me suddenly appearing on the moon by
pure chance. This would give an answer to Stephen Hawking's argument
against time travel, which was that if time travel were possible, we should
be seeing visitors from the future.

If proposition #1 was true, there would have to be some sort of relationship
between the measure of a certain universe and the probability of seeing
visitors that went back in time from that universe, otherwise we'd see
visitors from every possible future universe that sent visitors back in
time, and we'd constantly be seeing an infinite number of visitors coming
from the future. It is most reasonable to assume that we will see visitors
from a likely future universe, so the relationship would probably be the
greater the measure of a future universe, the more likely we are to see
visitors from that universe.

This brings me to one potential downfall of proposition #1. Let us suppose
this scenario: we continue through time (without any visitors from the
future) and eventually achieve the ability to time travel. We travel back
to a certain point in our past, not once, but many, many hundreds of
millions of times. Because travelling back in time changes the probability
of what happened, then because we travelled back in time hundreds of
millions of times, the probability that the people from that time won't see
visitors from the future is very, very small. The chance of not seeing
visitors from the future becomes insignificant compared to the probability
of seeing visitors from the future. Because it is so insignificant, it is
unlikely that the people will see visitors from that universe due to the
relationship I mentioned earlier. But if we are unlikely to see visitors
from the future, then the original universe becomes very likely again. So,
we have a paradox. For this reason I believe that proposition #1 is

This leaves proposition #2, which is not very attractive to people who
believe we are being visited by beings from the future, or people who dream
of one day being visited. However, just in case I have erred in my logic of
proving proposition #1 wrong, I have come up with some interesting
possibilities that proposition #1 provides, besides a paradox. Below I
present a way in which to test, right now, whether time travel - and,
specifically, proposition #1, exists.

The experiment is simple: organize an international decree that when we
discover time travel, we should travel back to this very day, month, and
year. The hope is that when we do discover time travel, we will go back to
the year 2000 because of the decree, and show everyone in this time that
time travel is indeed possible, and hopefully even give these people
advanced technology. If part of the decree is that we go back many, many
times, then the chance the people from this time don't see any visitors from
the future is very small (we are pretending that the original universe is
still signinficant). In other words, by simply issuing a decree right now,
we can possibly discover whether time travel is possible, right now. Of
course, proposition #1 would have to be correct, and even if it was, there
are plenty of reasons that our future selves wouldn't visit us. They might
not do it for moral or economic reasons, or they might have forgotten or
just don't care about the decree anymore. Or, time travel might be
theoretically possible, but not practical in any way. A disaster might have
even befallen our descendants, wiping them out before they could develop
time travel. However, all of these potential problems aside, I believe that
if proposition #1 was a possibility, then by issuing this decree we would be
siginificantly increasing the chances of seeing visitors from the future
right now. Of course, the sooner we do the experiment, the more likely it
is that we won't see visitors from the future, because there is more time
for something to go wrong before we develop time travel technology.
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Received on Fri Feb 18 2000 - 20:24:26 PST

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