Are you ready for the trip of a lifetime?
Because this not so little machine
is about to take you on one.
A trip back in time.
Theoretically, building a time traveling device
is possible.
Or almost possible.
All you'd need to do
is to assemble an enormous cylinder,
and get yourself a spaceship.
Then you'd need to figure out
how to make the cylinder spin
at a few billion rotations per minute.
And remember when I said it was enormous?
For this cylinder to work,
its length would need to be infinite.
Before I get into the explanation
of how this time machine works,
let me give you a bit of a back story.
In 1915, Albert Einstein came up
with his theory of general relativity.
He claimed that our Universe
can be described by four dimensions.
The first three would be
the dimensions of space,
and the fourth would be time.
Together, they're space-time.
Gravity can bend space-time
like a bowling ball would bend a rubber sheet.
The bending of space-time
makes time slow down.
The passage of time also changes
depending on how fast you move.
The theory of general relativity
blew people's minds. 
But in 1974, Frank Tipler realized
that solutions to equations of general relativity
could be used to build a time-traveling device.
Now that you're all caught up,
let's start building this machine already.
First, you'd need to find material
with the total mass of about ten Suns.
Where you'd find all that matter
is a big question, of course.
But let's assume you managed to strip
all the neighboring star systems
and assemble them into a cylinder
somewhere above the Earth.
Ten solar masses worth of matter is no joke.
It would take up a lot of space.
And you would have to figure out
how to compress all that matter.
It would have to be so dense,
that it would turn into an elongated black hole.
There's no time to worry
about getting sucked into it. 
Now that you finally
assembled a Tipler cylinder,
you'd need to make it rotate
at an unimaginably fast rate,
Theoretically, instead of condensing
a whole lot of matter into a black hole,
and then having to deal
with the cylinder rotation,
you could just find a few neutron stars.
Those are leftovers from gigantic stars
that didn't have enough mass
to collapse into black holes.
And they rotate pretty fast.
You could line a few of them up
and synchronize their rotation.
But even if you chose the fastest ones,
their rotation wouldn't come close
to a few billion times per minute.
Besides, we did an earlier video about
trying to bring a spoonful
of neutron star back to Earth.
And it didn't go that well.
Anyway, regardless of
how you achieve this effect,
the massive gravitational pull of the cylinder
combined with the fast rotation
would create a frame-dragging effect.
In other words, the cylinder
would drag space-time along with it.
That means that you
could hop on a spacecraft
and follow the spiral course of the cylinder.
And that would take you back in time.
Time would fold upon itself,
and you'd find yourself
in a closed timelike curve.
It's a loop, in which your traveling into the past
has always been a part of that past.
So you wouldn't create any time paradoxes.
If you changed the direction of your travel,
you'd go back to the future.
Or rather, back to the time
you started your journey from.
The Tipler cylinder's specialty
is travel into the past, not the future.
But hey, you could hang out with
your favorite people from the past,
go see a real-life Jurassic world.
You could even go back to the time
when the Earth didn't have any life at all.
But, of course, there's a drawback.
All that would only be possible
if the Tipler cylinder had an infinite length. 
It would have to go on forever.
But that's just impossible.
You can't build something
that has no end at all.
If you tried to make a finite cylinder,
then you'd have to deal
with some exotic matter
and its negative energy.
But we haven't discovered it yet,
and it doesn't look like
we'll be getting our hands on
this kind of matter any time soon.
I haven't even mentioned the effects
of the black hole on your body.
You'd instantly get sucked
into the cylinder's black hole
if you got close enough.
But hopefully, there are other ways
that would allow you to travel through time.
And when you do, we'll be there for you,
and help you survive this kind of trip, 
on our new show How to Survive.
