From: lintz@buster.cis.udel.edu (Brian Lintz)
Subject: Re: Free will and time travel
Organization: University of Delaware, Newark
Date: Mon, 17 Aug 1992 20:04:11 GMT

In article <1992Aug14.011312.13075@organpipe.uug.arizona.edu> dave@cs.arizona.edu (Dave Schaumann) writes:

>So whither time travel?  There are several possibilities:
>
>  - The past is not immutable.  I'm not sure what you could do with
>    this.  The nature of free will suggests that there would be
>    continuous spontaneous changes in the past as people made different
>    choices, and thus, there would be little logical consistency to the
>    world.
>
>  - You can travel to the past by "reversing" time, so everyone comes
>    along.  Then, the future you know of will only be one possibility.
>    The new one that is subsequently  played out will no doubt be
>    different...

I'm not sure what the difference between these two are. If the past is
changeable, and you don't have alternate universes, when you travel
back to the past, the time you travel to becomes your present, and
as you say, the future you are from is only one possibility. Maybe
my problem is that I don't know exactly what you mean by "reversing time
so that everyone comes along." Could you expand on this?

>
>  - You can travel to the past, but if you do, you lose your free will.
>
>  - You can travel to the past, but as soon as you arrive, reality
>    "branches" ala `parallel universes'.  (Or, you can only travel
>    to alternate universes, but some of them would be indistinguishable
>    from our own except for being n years behind us).
>
>  - You can't travel in time.
>
>
>That's all I can think of.  

Another possibility is that, while you have free will, you are unable
to change anything. Or it may be that you can change little things,
but not major things. Imagine time as a river. When you attempt to change
something, you are putting things in the river to try and change its
course. If you put a tree branch in the river, it will flow right around
it and there will be little or no change. If you put a big rock in it,
it will change a bit, but will very quickly go back to its original flow.
This represents some minor change but nothing significant. This is
probably the most one person could do. The more people you have trying
to change, the bigger the obstruction in the river, and the more change
is possible. With enough people and technology, you could change the course
of the river, representing a changeable past, or you could create a separate
stream branching off of the original, representing the alternate universe
idea. Sometimes when you put something into a river, it doesn't obstruct
it a bit, but it just gets washed down the river. This represents a person
that goes back to change an event, and actually ends up causing it.


>Notice that none of them really allow you to play out the grandfather 
>paradox...

Well, the first one you mentioned, a changeable past, allows the paradox.
You go back and kill your grandfather. The future you came from no longer
exists, so you couldn't have come from there. So you couldn't have
killed your grandfather, etc, etc.

Just some comments,

Brian Lintz
lintz@udel.edu