which causation runs from earlier to later stages in the
order of personal time. But the orders of personal and
external time disagree at some point, and there we have
causation that runs from later to earlier stages in the or-
der of external time. Elsewhere I have given an analysis
of causation in terms of chains of counterfactual depend-
ence, and I took care that my analysis would not rule
out casual reversal
a priori
.
5
I think I can argue (but not
here) that under my analysis the direction of counterfac-
tual dependence and causation is governed by the direc-
tion of other
de facto
asymmetries of time. If so, then
reversed causation and time travel are not excluded al-
together, but can occur only where there are local excep-
tions to these asymmetries. As I said at the outset, the
time traveler’s world would be a most strange one.
Stranger still, if there are local—but only local—causal
reversals, then there may also be causal loops: closed
causal chains in which some of the causal links are nor-
mal in direction and others are reversed. (Perhaps there
must be loops if there is reversal: I am not sure.) Each
event on the loop has a causal explanation, being caused
by events elsewhere on the loop. That is not to say that
the loop as a whole is caused or explicable. It may not
be. Its inexplicability is especially remarkable if it is
made up of the sort of causal processes that transmit
information. Recall the time traveler who talked to him-
self. He talked to himself about time travel, and in the
course of the conversation his older self told his younger
self how to build a time machine. That information was
available in no other way. His older self knew how be-
cause his younger self had been told and the information
had been preserved by the causal processes that consti-
tute recording, storage, and retrieval of memory traces.
His younger self knew, after the conversation, because
his older self had known and the information had been
preserved by the causal processes that constitute telling.
But where did the information come from in the first
place? Why did the whole affair happen? There is simply
no answer. The parts of the loop are explicable, the whole
of it is not. Strange! But not impossible, and not too dif-
ferent from inexplicabilities we are already inured to. Al-
most everyone agrees that God, or the Big Bang, or the
entire infinite past of the universe, or the decay of a trit-
ium atom, is uncaused and inexplicable. Then if these
are possible, why not also the inexplicable causal loops
that arise in the time travel?
I have committed a circularity in order not to talk
about too much at once, and this is a good place to set
it right. In explaining personal time, I presupposed that
we were entitled to regard certain stages as comprising
a single person. Then in explaining what united the
stages into a single person, I presupposed that we were
given a personal time order for them. The proper way
to proceed is to define personhood and personal time
simultaneously, as follows. Suppose given a pair of an
aggregate of persona-stages, regarded as a candidate for
personhood, and an assignment of coordinates to those
stages, regarded as a candidate for his personal time. If
the stages satisfy the conditions given in my circular ex-
planation with respect to the assignment of coordinates,
then both candidates succeed: the stages do comprise a
person and the assignment is his personal time.
I have argued so far that what goes on in a time travel
story may be a possible pattern of events in four-dimen-
sional space-time with no extra time dimension; that it
may be correct to regard the scattered stages of the al-
leged time traveler as comprising a single person; and
that we may legitimately assign to those stages and their
surroundings a personal time order that disagrees some-
times with their order in external time. Some might con-
cede all this, but protest that the impossibility of time
travel is revealed after all when we ask not what the
time traveler
does,
but what he
could do.
Could a time
traveler change the past? It seems not: the events of a
past moment could no more change than numbers could.
Yet it seems that he would be as able as anyone to do
things that would change the past if he did them. If a
time traveler visiting the past both could and couldn’t
do something that would change it, then there cannot
possibly be such a time traveler.
Consider Tim. He detests his grandfather, whose suc-
cess in the munitions trade built the family fortune that
paid for Tim’s time machine. Tim would like nothing so
much as to kill Grandfather, but alas he is too late.
Grandfather died in his bed in 1957, while Tim was a
young boy. But when Tim has built his time machine and
traveled to 1920, suddenly he realizes that he is not too
late after all. He buys a rifle; he spends long hours in
target practice; he shadows Grandfather to learn the
route of his daily walk to the munitions works; he rents
a room along the route; and there he lurks, one winter
day in 1921, rifle loaded, hate in his heart, as Grandfather
walks closer, closer,. . . .
Tim can kill Grandfather. He has what it takes. Con-
ditions are perfect in every way: the best rifle money
could buy, Grandfather an easy target only twenty yards
away, not a breeze, door securely locked against intrud-
ers. Tim a good shot to begin with and now at the peak
of training, and so on. What’s to stop him? The forces
of logic will not stay his hand! No powerful chaperone
stands by to defend the past from interference. (To imag-
ine such a chaperone, as some authors do, is a boring
evasion, not needed to make Tim’s story consistent.) In
short, Tim is as much able to kill Grandfather as anyone
ever is to kill anyone. Suppose that down the street an-
other sniper, Tom, lurks waiting for another victim,
Grandfather’s partner. Tom is not a time traveler, but oth-
erwise he is just like Tim: same make of rifle, same mur-
derous intent, same everything. We can even suppose
that Tom, like Tim, believes himself to be a time traveler.
Someone has gone to a lot of trouble to deceive Tom into
thinking so. There’s no doubt that Tom can kill his vic-
tim; and Tim has everything going for him that Tom