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PROFESSOR: Everyone has pretty
much seen a science fiction

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movie in this day
and age, and I'm

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pretty sure everyone
has seen at least one

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that involves time traveling,
maybe Star Trek, Interstellar,

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or more recently,
Predestination.

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And it's such a widely-explored
topic in science fiction--

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the whole genre
of time traveling.

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But many times,
Hollywood films always

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have these little, little
loopholes in their story

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line and their plot lines.

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And they often use weird
theories and science to explain

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themselves, which are just like
god-from-the-machine solutions.

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Totally not efficient.

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However, scientists
have actually

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put forward three
main theories when

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it comes to time traveling.

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The first theory-- the
fixed timeline theory.

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Say, for example, you're
trying to prevent World War II,

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and you travel back in time
after inventing a time machine.

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And after killing the baby
that was supposed to be Hitler,

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you put, I don't
know, some other baby

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you found in an orphanage
and put it in the same cot.

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However, when you travel
back in the future,

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you realize that the baby
that you had replaced it with

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was still one that
turned out to be Hitler.

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And there is no changing
of the past effectively,

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or the future or the present--
it's all on a fixed timeline.

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And these particular motions
are actually often seen

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in movies such as Harry Potter.

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It was seen in
Interstellar as well.

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It was also seen
in Predestination.

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It's a good film--
you have to catch it.

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The second theory is
the multiverse theory,

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which says that every
time you go back in time

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and you alter
history-- for example,

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you step on a butterfly and that
butterfly could actually maybe

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be the prevention
of, I don't know,

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a hurricane that sweeps
through Orlando right

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now-- you spawn a new
set of events which

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happens in a separate universe.

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Which, in other words,
means that you have actually

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got infinite number
of universes after you

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have all these different
decisions and different chain

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of events occurring.

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And this particular
timeline concept

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has actually been explored
many times as well.

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It's been featured in the recent
reboot of Star Trek movies.

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It's also been featured
in the recent Fringe

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series, which kind of died
because J.J. Abrams ran out

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of money or something.

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And, yeah.

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This multiverse theory
is actually quite popular

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because it's the easiest
for producers to just throw

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this illusion, and then
they say, oh, we actually

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spawned off a new timeline.

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So that's when you
get sloppy and lazy.

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The third theory we have is what
we call the dynamic timeline

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theory.

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It's also a theory which
induces all the paradoxes, which

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you often hear about
as [INAUDIBLE],

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such as the grandfather paradox,
where you go back in time,

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and you kill your grandfather.

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And because you killed your
grandfather, you won't exist.

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And because you won't exist,
you can't go back in time

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to kill your grandfather.

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And that's where the
whole paradox comes in.

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Who kills who?

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God knows.

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The third theory we have is
called the dynamic timeline

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theory, which is also where
all your grandfather paradoxes

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come in.

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Let's say your grandfather is
a really, really evil man--

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Hitler-- and you have to go
back in time to kill him.

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And you go back,
you do the deed,

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and then you realize that,
oh no, after I killed him,

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I can't exist.

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And because you can't exist,
you can't go back in time

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to kill him.

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And because of
that, he comes back,

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and then you have to
come back, and you

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have to go back in time
again and kill him.

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And that's where
the paradox starts.

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It's an infinite circle
of never-ending events.

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And that's how the
grandfather paradox came

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about to be in the first place.

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And I think you may have seen
it in, like, Back to the Future.

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And I can't really think of
any other films right now.

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So this is a brief
history of time travel

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and the three theories
associated with it,

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as well as a quick touch on the
paradox of time-- grandfather

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paradox.