X-Message-Number: 22207
Date: Wed, 16 Jul 2003 00:59:33 -0700
From: Mike Perry <>
Subject: Time Travel: A Few More Comments

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Alan Mole, #22188:

>As I understand the SciAmer. articles, you could go into the past and
>influence it, up to a point.  And it would seem you could observe at any 
>time in the
>past, so long as you did not interfere.

I will need to read these articles (when I can get ahold of them). But 
"influencing the past" seems again to involve an inevitable paradox, if you 
mean "recorded past." If you stopped Lincoln's assassination, then -- 
contradiction! Perhaps it is only the unrecorded past that can be 
"influenced"--this means the influence was there all along, but it would 
seem you have a form of reverse causality. I won't rule this out--time will 
tell (in more ways than one!). But I'm highly skeptical that you could 
observe just anything you want in the past (though I'm not sure that is 
what the authors are saying), because, as I've said before, there are 
experiments that seem to show it would be impossible.

One such experiment is described in Philip Yam,  Rubbed Out with the 
Quantum Eraser,  Scientific American (January 1996): 30 31. A photon takes 
one of two paths unpredictably. On one path it is tagged by a change in its 
polarization, on the other it is left alone. At this point information has 
been recorded on the photon, and you could read back the information by 
testing it. So you could then determine which path the photon took, that is 
to say, the one path or the other. On the other hand, it is possible to 
erase the polarized information, so the photon ends up exactly as it 
started out. After this, when you detect the photon, you get an 
interference pattern reflecting the two possible paths the photon could 
have taken. Which path did the photon actually take? The fact that 
information was imprinted suggests that it "must" have taken one path *or* 
the other (though naturally not both). So, if you could get in your time 
machine and just go back, you should be able to test and see which path was 
taken, right? But the interference pattern suggests that the two paths are 
on a totally equal footing, one is not more real or "historical" than the 
other. Both, in a sense, "really" happened. Or in other words, the loss of 
information has made the past ambiguous.

I happen to think that the historical timeline is not unique, as suggested 
by this experiment and others; this is a major reason I am skeptical about 
recovery of the hidden past.

Mike Perry


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