I propose that all writers of time travel fiction should be required to read certain
articles, and this is the first. Deutsch and Lockwood do an admirable job of describing
the well-known Grandfather Paradox and the lesser known paradox of the causal loop (in
which, for example, an art critic brings a book of famous paintings back to the artist
before the time when the paintings were painted, and this book then inspires those very
paintings, leaving the question of who created the paintings).
The article then tries
to unwind these paradoxes in classical physics, where there is but one universe. In this
universe, a time traveler who returns to the past can do nothing except that which was
already done. For example, the traveler simply cannot kill his or her own grandfather
before Grandpa meets Grandma because we know (by the birth of the traveler) that that
didn’t happen. So, something in the universe must stop the murder. Things must happen
as they happened.
But, say Deutsch and Lockwood, this conspiracy of the universe to
preserve consistency violates the Autonomy Principle, according to which “it is
possible to create in our immediate environment any configuration of matter that the laws
of physics permit locally, without reference to what the rest of the universe may be
doing.” In other words, if it’s physically possible for the traveler to point a gun
at Grandpa, then the fact that elsewhen in the universe Grandpa must knock up Grandma
cannot interfere with the traveler’s ability to pull the trigger.
Deutsch and
Lockwood use the Autonomy Principle to reject something, but it’s classical physics
they reject, not time travel. In a similar way, for stories that rely on a Causal Loop
Paradox, Deutsch and Lockwood ask: Just where did the original idea of the paintings
come from? They reject that the paintings might have come from nowhere (TANSTAAFL!),
and again they reject classical physics.
Personally, I hope that time travel writers
don’t fully embrace the Autonomy Principle and TANSTAAFL, because I want more wonderful
stories where, in fact, there is but one history of events, the future and past may both
be fixed, free will is an illusion, and free lunches exist. Hooray for “—All You
Zombies—”!
But with classical physics banned, what else is there? Deutsch and
Lockwood turn to Everett’s Many Worlds model wherein each collapse of the quantum wave
function results in a new universe. When a time traveler goes to the past, they say, the
arrival of the traveler creates a new multiverse, and this multiverse does not need to
act the same as the original. Grandpa can die! The artist can be given inspiration from
an artist doppelgänger in the original universe!
Notably, though, Deutsch and Lockwood
never discuss how time travel might cause the same kind of universe splitting as the
collapse of the wave function, but never mind. What they do discuss is how the new
universe must respond to changes, and many stories where changing the past is possible
fall down on this account. For example, if you change the past so that the reason for
your trip to the past no longer exists, then when you return to the present you should
find a new version of yourself who never considered traveling to the past. Multiverse
time travelers should read this article just to understand that the present they return
to may very well have another version of themselves. Two Marties McFly!
One final note:
Of course we don’t live in a classical physics universe. That's clear from the many
experiments that support quantum physics. But living in a quantum world doesn’t
immediately imply Many Worlds. Could time travel exist in a single quantum universe? Or
does it? For thoughts on that, check out the online Scientific American article
“Time
Travel Simulation Resolves Grandfather Paradox” by Lee Billings.
In the art critic story, quantum mechanics allows events, from the participants’
perspective, to occur much as Dummett describes. The universe that the critic comes from
must have been one in which the artist did, eventually, learn to paint well. In that
universe, the pictures were produced by creative effort, and reproductions were later
taken to the past of another universe. There the paintings were Indeed plagiarized—if
one can be said to plagiarize the work of another version of oneself—and the painter
did get “some- thing for nothing.” But there is no para- dox, because now the
existence of the pictures was caused by genuine creative effort, albeit in another
universe.
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