A very interesting post. David Deutsch spends some time in his book
about time travel, but this is a new slant on the subject. My personal
hunch is that your proposition 2 is the correct one, and that as you
say it conveniently answers Stephen Hawking's "spaceship" argument
about why time travellers are not extant now.
An interesting experiment you have proposed to test which alternative
is valid. I'm not quite convinced that it is a valid test, although
I'll buy that it potentially falsifies your proposition 2 (without
saying anything at all about proposition 1). I'll need to ruminate on
this some more.
Cheers
>
> 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
> thoughts/ideas.
>
> 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
> impossible.
>
> 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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Dr. Russell Standish Director
High Performance Computing Support Unit,
University of NSW Phone 9385 6967
Sydney 2052 Fax 9385 6965
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http://parallel.hpc.unsw.edu.au/rks
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Received on Sun Feb 20 2000 - 18:14:36 PST