X-Message-Number: 22183
Date: Fri, 11 Jul 2003 22:42:24 -0700
From: Mike Perry <>
Subject: Time Travel (again)

Alan Mole, #22178, and my responses

>There are serious and reputable physicists who think time travel is possible,
>and their ideas for ways to do it are reported from time to time in
>Scientific American and other periodicals.

I think Ettinger was on the mark to ask for a definition of time travel.

>As for the usual paradox of killing your grandfather etc., the latest answer
>I read is that you just can't.

How much *can* you do, and does this qualify as time travel?

>Of course, it's easier to observe the past (e.g., to record the state of
>Alexander's brain) than to go back and change it. There is no paradox 
>involved in
>that.  Every night we see stars as they were thousands or millions of years
>ago, when the light left.  Not that I have any idea how to see details from
>Alexander's time, but that is just a practical problem, not an innate 
>physical
>impossibility.

There is a big difference between observing the past of a distant 
astronomical object and observing the past that is far inside your own 
light cone. It is not clear that the hidden, historical past (what does not 
survive in the historical records) can be observed or recovered, in any 
usual sense.

>there is still much to discover about the fundamental
>nature of our universe. Until we understand that better, it is far to 
>early to
>say that something like time travel cannot be done.

I (for one) haven't said it *cannot* be done, but there is also no 
guarantee it *can* be done, and (I think) it looks dubious. Time travel, 
the way I think of it, backward time travel at any rate, does seem to 
involve paradoxes, thus seems impossible, unless you restrict it somehow, 
then, again, you have to ask is it true time travel. If you consider the 
more modest goal of just recovering the hidden past, determining "what 
actually happened," I think even here the odds are very long. If you cannot 
recover the hidden past, it would at least seem to put notable restrictions 
on what you might do in the way of "time travel"--however you define it. 
Anyway, the reason this discussion got started (if I understand it right) 
was because a way was sought for recovering information about deceased 
persons of the past (and maybe more than information) so they could be 
reconstructed and restored to life. Whatever you mean by "time travel" we 
are really interested in recovering that hidden past (not necessarily 
interfering with or modifying it) so we can proceed with this alternative 
to cryonics. There are certain experiments with particles that seem to show 
that, in general, recovering the hidden past is impossible in principle, 
mainly because there is in fact not just one unique past but more than one 
that coexist on a more or less equal footing, then (under appropriate 
conditions) fuse into one present. There is no unique "the way it was" so 
you might reconstruct one of the "ways it was" and another and another, but 
that is the best you could do. I deal with this in my book, and again, the 
bottom line conclusion I reach is that a kind of resurrection is possible 
"from dust," but it does not make a better-than or equal alternative to 
cryonics.

Mike Perry

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