Re: Many Fermis Revisited

From: Tim May <>
Date: Mon, 13 Jan 2003 11:47:18 -0800

On Monday, January 13, 2003, at 10:47 AM, George Levy wrote:

> Tim, Hal, Russell
> Since we have several futures ( and several pasts), time travel is
> just a particular case of many-world travel.

I somewhat agree...and we are not the first to make this point.
However, we need to be careful about saying we have "several pasts" (I
assume by "several" you mean "many").

The usual modal operators are needed. We have many possible futures,
but our possible pasts are limited by the events which are "necessary"
to produce the world we are actually in. The square operator for "that
which necessarily may be" and the diamond operator for "that which may

If you mean that "many presents" have "many pasts," yes. But the
current present only has a limited number of pasts, possibly just one.
(The origin of this asymmetry in the lattice of events is related to
our being in one present.)

...interesting theory elided...

If there is only a single sequence of events ("a past") which produces
the actual world we are in today, then your time machine will not work,
as one cannot go back to a world where the past was different from what
"actually happened."

(And if one did, then of course one would be an actor in a past that
never happened, a la the usual grandfather paradox in all of its usual
variants. So "returning to the present" would be to a different

> If this idea has any merit this is why space travelers are not
> observable either. It provides a form of cosmic censorship.  By
> reducing their measure through QS and the likes, advanced aliens just
> evolve out of existence in our world!

You ought to read "Finity," by John Barnes. He explores a very similar

--Tim May
"They played all kinds of games, kept the House in session all night,
and it was a very complicated bill. Maybe a handful of staffers
actually read it, but the bill definitely was not available to members
before the vote." --Rep. Ron Paul, TX, on how few Congresscritters saw
the USA-PATRIOT Bill before voting overwhelmingly to impose a police
Received on Mon Jan 13 2003 - 14:52:27 PST

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