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PROFESSOR: Imagine
holding a party

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and then sending out the
invitations after the party.

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Wait a minute.

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That doesn't make any sense.

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Are my friends going to
be able to time travel

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back just to attend my party?

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In 2009, Stephen
Hawking actually

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conducted such an experiment
to disprove time travel.

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What exactly is time travel?

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And how does it work?

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It actually has a lot
to do with-- speed.

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A lot of what we know about
time traveling actually

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comes from Einstein's
theories of

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special and general relativity.

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We are actually time
traveling right now, but not

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in the manner that sci-fi
films have depicted them.

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We are, in fact, time traveling
at a pace of one hour per hour.

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In other words, every hour I
experience, the world around me

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experiences a
singular hour, too.

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Now, it may sound simple and
trivial, but stay with me.

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This is where it
gets interesting.

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Now, the last part of Einstein's
theory of special relativity

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is actually a phenomenon
known as time dilation.

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Effectively this means
that the faster you travel,

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the slower time
passes around you.

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It potentially means that
I could travel at, maybe,

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more than one hour per hour.

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This phenomenon, however,
is only noticeable

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at really, really high speeds,
speeds near the speed of light.

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This rocket here?

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Not even close.

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Say I do have a
spaceship now that

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travels at 90% of
the speed of light,

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and I have a pair
of newborn twins.

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I bring one of them with me
on this journey for 10 years.

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When I return, one of them
will have turned 23 years old

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while the one with me will
have only turned 10 years old.

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While we have experience
10 years on a spaceship,

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23 years have actually
passed on earth.

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This amount of time actually
increases exponentially

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as we get closer to
the speed of light.

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So we could potentially
travel forward in time.

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But what about
backwards in time?

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That's a whole other issue.

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Now, remember our
previous graph.

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The time that it
took [INAUDIBLE]

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to the speed of light.

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[INAUDIBLE] may be
the secret to time

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travel lies on the
other side of the line.

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In other words, we may
have to travel faster

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than the speed of light just to
be able to travel back in time.

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Now, when you do travel at
speeds near the speed of light,

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something called relativistic
mass comes into play.

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In other words, as your speed
goes up, your mass increases.

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As your mass increases,
you require more energy

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to move the same object.

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This might mean that you need
an infinite amount of energy

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just to move an object
beyond the speed of light.

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However, the more energy you put
into an object, the more likely

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you are to increase
its mass rather than

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to increase its speed.

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Therefore, to get an object
past the speed of light

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is quite impossible
at this point in time.

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So why are we still so
obsessed with time travel?

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Time travel does come with
its own set of problems.

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Take, for example,
my party before.

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If I were to send out my
invitations after the party,

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my friends would have come
back in time to attend it,

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and if they did come back
in time to attend it,

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I wouldn't have time to
send out my invitations.

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And if I didn't send
out my invitations,

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they wouldn't be at my party.

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It doesn't make any sense at all
that they are both at my party

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and not at my party.

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This is what we call
the grandfather paradox.

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It's one of three
time travel theories

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that we currently have.

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So it seems that we may
not be able to time travel

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yet, based on what physics
tells us, at least.

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For now, if you are
planning a party,

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be sure to send out
the invites first.

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Your friends can't
time travel, yet.