"DAVID WILLIAMS" bravely wrote to "MILES MAXTED" (25
Mar 05  21:08:14)
 --- on the heady topic of "Re: Pnu 722"

 -> And that precludes interaction from within it, `cos for `now' to
 -> stay unchanged nothing could emanate from it - not even one pesky
 -> photon.
 DW>
 DW> What's "unchanged"? Someone went back to the past and did
something,
 DW> and what exists "now" is, to some extent, influenced by
what he did.
 DW> The non-existent situation is the "now" which *would
have* existed had
 DW> the time-traveller never gone. But that situation is entirely
 DW> imaginary. In reality, he did go, and "now" reflects the effects of
 DW> that. Reality is reality. It isn't changed.

About changing the past, I think the Pauli Exclusion Principle would
apply to your self's double. This would entail that the only past you
could visit would be an alternate past in which you did not exist.
There are theories of alternate timelines where for example the 3rd
Reich won the war, and we had conquered faster than light spaceflight
etc, etc. The past one could visit would have clear differences from
the one which is our history and of course the time traveller couldn't
exist in it. Thus he couldn't go back in time to change his own future
and neatly sidesteps all the paradoxes. Then again quantum theory
allows the superposition of similar states such as Bose/Einstein
condensates. Thus one might be able to travel back in time but only on
a parallel dimension that wouldn't physically interact with the other.

 M*i*k*e

... "God is subtle but he is not malicious" - A. Einstein
--- Blue Wave/DOS v2.30