Okay, so, so far we've covered Lewis' account of what time travel actually
involves, and we've covered Lewis' attempt at trying to diffuse the appeared
grandfather paradoxes. So, so far, the account is this: travel
into the past is logically possible provided that what the traveler does in
the past is consistent with the history whence the traveler comes.
Now at this point, you might be thinking well hang on, if the traveler's actions
already exist in the past, and some say it's before the travelers set sight, or at
least if the traveler's actions exist at external times earlier in the traveler's
departure, how could the traveler have any impact on the past.
People sometimes hear Lewis' analysis and think, well surely the traveler is just a
sort of pre-programmed robot, completely pre-determined to go through a completely
rigid set of actions. Or what still, people might think that a
traveler in the past, is doomed to be some sort of ghost, forced to witness events,
but be powerless to intervene. Well Lewis thinks that it's possible for a
traveler be really concretely present in the past, a proper functioning human agent
with intentions, and wishes, and choices, and to make a difference to the past.
We have to be careful how we imagine the impact of a traveler in the past.
Lewis distinguishes between two senses in which it could be said that a traveler
could change the past, what I'm going to call replacement change and counterfactual
change. In a nut shell, Lewis says that a traveler
in the past cannot effect replacement changes, but can effect counterfactual
changes. Well what's a replacement change?
Consider a perfectly ordinary object, like a glass.
If I were to drop a glass, from waist height, onto a concrete floor, and the
glass shatters, I would have replaced an intact glass with a set of glass
fragments. I would have affected a replacement change
in events, there was an intact glass, the intact glass is shattered, the intact
glass has gone away. And in its stead, it's been replaced by, a
set of glass fragments. Now Lewis thinks, that replacement
changes, can happen to concrete objects, but not to times.
You can't replacement change, any time, past, present, or future.
Suppose you make a plan to meet a friend for lunch at 12 o' clock at a certain
restaurant. And then, you get a text from your friend
saying, I'm sorry I can't make lunch today, can we meet tomorrow?
Well that hasn't replacement changed the future.
It's not that you did in the future meet at a certain restaurant and then that
future somehow went away. So Lewis says, yes, you can't affect the
replacement changes in the past, but you can't affect the replacement changes in
the future either. Replacement changes can only happen to
concrete objects. You can replace a concrete object, like an
intact glass, with a set of glass fragments, but concrete objects are not
the same as times. So that's replacement change.
Counterfactual change, maybe a little bit harder to get a handle on, but
counterfactual change is the impact that you have, assessed in terms of what would
have happened, counterfactually, if you hadn't been present.
One of the things that enabled me to be on time for this session this morning, was
that my alarm clock went off on time. But if the alarm clock hadn't gone off, I
would have been late, so I can assert the counterfactual, if my alarm clock had
broken I wouldn't have been on time. So when my alarm went off, clearly had an
impact on my ability to attend this session on time.
If my alarm clock had broken, I would have been late.
So, in a sense, the alarm has changed the course of my day.
But that change is not to be assessed in replacement terms.
It's not that there was an original version of events, or my alarm clock
didn't go off and I was late, and then somehow my alarm clock did go off and
history was replacement changed and I was on time.
Rather, the impact the alarm clock had can be assessed counterfactually.
This morning only happened once, and it happened with my alarm clock going off on
time. But if it had happened differently,
history would have unfolded differently. Another example.
Historians who treated the period, not least Arthur Wellesley, the Duke of
Wellington, maintained there's a crucial factor in determining the outcome of the
battle of Waterloo, was the arrival in the late afternoon at the battle Of Prussian
forces under the command of Field Marshall Blucher.
Wellington, himself, frequently said that if Blucher had been late, Napoleon would
have won. So that's clearly a counterfactual
conditional: If Blucher had been late, Napoleon would have won.
Again, it's not that Blucher made a replacement change to the battle of
Waterloo. It's not that Waterloo originally issued
in a French victory and then Blucher's forces arrived and the French victory
somehow was made such that it never was and an Allied victory took its place.
Waterloo happened only once, with a victory for the Allies.
But an important factor in that victory, was the arrival of Marshall Blucher.
So we can see that Blucher's arrival changed the course of history, but it
changed it in the counterfactual sense, not in the replacement sense.
Okay, we've got two senses of change, placement change, counterfactual change.
Lewis maintains that time travelers can have an impact on the past, in the
counterfactual sense. The presence of a traveler might make
history different from what it would have been if the traveler hadn't been there.
Going back to my attempt at assassinating Hitler.
Suppose my time machine deposits me in Vienna in 1908 with a great flash of
light. And I've arrived so close to Hitler that
Hist-, that Hitler sees a flash of light, and recoils.
He steps back in shock, out of the path of a tram that would otherwise have cut him
down. In this case, I could assert the
counterfactual, if I hadn't traveled back in time, Hitler would have died.
So in this case, I clearly had a counterfactual impact on history.
I have, albeit unwittingly, been partly responsible for Hitler's survival.