Countertime beings: in search of a paradox

Mitchell Porter (
Thu, 13 Mar 1997 10:11:37 +1000 (EST)

I've recently been thinking about time-reversal symmetry in physics.
This is one of the basic symmetries of existing theories, and John
Cramer's transactional interpretation of quantum mechanics seeks to
explain nonlocal correlations by back-and-forth-in-time ("zigzag")

If time-reversal symmetry is real, it ought to be possible to have
beings like ourselves whose "arrow of time" points the other way.

Is there something paradoxical about the notion of *communication*
between ourselves and countertime beings? Or could we build a
time-reversed robot that reports on the future?

In the case of time paradox via wormhole, the most popular idea
is that just when two wormholes comes within range of causing a
closed timelike curve, they will be destroyed by vacuum
fluctuations which will repeatedly traverse the neighborhood of
the CTC until the energy density is sufficient to destabilize the

In the case of countertime communication I see no such convenient
physical mechanism to avoid paradoxes of the form: at time t,
countertime machine A tells "forward-time" machine B that B sent a
signal at time t+dt, and B is programmed not to send the signal if
it receives such a message from A. So perhaps countertime entities
aren't possible, or communication with them is not possible, but
if so, why not?

(Time reversal has been explored a bit in science fiction:
in a short story by Greg Egan; in Brian Aldiss' *Cryptozoic*;
in Ian Watson's "The Very Slow Time Machine"; probably in
Philip K. Dick, and possibly in Stephen Baxter. No doubt
Damien B can suggest a few more...?)