Trying to save the dinosaurs

Remember I mentioned that we can still ‘meddle’ with the past?
Let me put it another way: we are allowed to go back to the past
and cause things to turn out the way they have because of our
meddling. I will illustrate this with another example.
Scientists are mostly in agreement now that a large meteorite
hit the Earth over sixty million years ago and that the fall-out
from the impact caused a dramatic climatic change that killed
the dinosaurs. Some mammals did, however, survive and some
of these evolved into apes and then humans millions of years
later. In fact, we can probably say that, had the dinosaurs not
been wiped out then mammals, and hence humans, would not
have been allowed to evolve. Put another way, it is thanks to that
meteorite that we are here at all.
Suppose a palaeontologist gets hold of a time machine and
a thermonuclear missile (unlikely I know, but bear with me). He
travels back 65 million years into the past intent on destroying the
meteorite before it hits the Earth. But if he saves the dinosaurs
from extinction then not only will he ‘pop’ out of existence but so
will the rest of the human race. This is a pretty extreme form of
the grandfather paradox.
I should come clean at this point and say that if we do ever
build a time machine then the laws of physics definitely do not
allow us to travel back to a time before the time machine was
built. This is because constructing a time machine involves linking
different times together within spacetime. So the earliest time that
is linked in this way will be the moment of the time machine’s
creation. All times before this would have been lost forever and no
longer ‘available’. This rules out any possibility of us ever being able to go back to prehistoric times—unless we stumble upon a
cosmic time machine somewhere in space that has been around
for a long time.
For the sake of argument let us say that the mad
palaeontologist does manage to go back to a few hours before the
impact of the meteorite and points his missile at it. He looks up
and sees to his dismay that it is much larger than he expected. In
fact, if it collides with Earth, as it seems rather intent on doing, it
will wipe out all life, not just the dinosaurs.
“Well,” he thinks to himself, “I might as well do what I can.”
He fires the missile and scores a direct hit. The meteorite is
destroyed.
But . . . it seems a small fragment survives and is still heading
towards Earth. Not having any more missiles there is nothing
he can do now but watch the fireball as it streaks through the
atmosphere, hits the Earth and, guess what? causes the extinction
of the dinosaurs!
So you see, not only was he unable to change history,
he was actually the cause of it. Had he not gone back and
destroyed the meteorite he would never have existed. I have
turned the argument upside-down and the paradox seems to
have disappeared. But unlike the last story in which John must
keep missing however hard he tries to shoot his mother, now our
palaeontologist cannot miss, for if he does he will never have
existed. He cannot decide at the last minute not to fire the missile,
nor can anything deflect it from its course. It is easy to argue away
any hint of the no choice paradox in this case by claiming that,
unlike the previous example wherewe require something to cause
him to fail, here the time traveller is unaware of the enormity of
his task and of his lack of choice in the matter.