(swords clang)
(swords clang)
(swords clang)
(screaming)
(dramatic music)
- That's old Skeletor
taking a dramatic fall
into the bottomless pit
beneath Castle Grayskull
in 1987's "Masters of the Universe."
- So what if Earth had
this same sort of chasm,
what would it be like to
fall straight through it?
Let's explore, Earthling.
(upbeat techno theme music)
- [Man] The bottomless pit is
always a fun bit of fantasy,
regardless if it's the bottomless pit
of the Bible's Book of Revelation,
or just the one beneath
Castle Grayskull here.
I mean, this was a major
feature of the franchise, too.
The original '80s He-Man cartoon
devoted an entire episode to it.
- [Woman] But what actually
constitutes a bottomless pit?
What about a single vertical
shaft that begins on one side
of a planet, and extends all
the way out the other side?
On Earth, such a shaft would
stretch a total of 7,900 miles,
or 12,700 kilometers.
Sounds like a bottomless pit to me.
- [Man] It may seem ridiculous,
but physicists love
this thought experiment.
Yet like anything involving
crazy tunnels through the Earth,
you have to suspend a
bunch of basic realities.
Forget about Earth's
outer liquid core of iron,
and just pretend the planet's a cold,
uniform, inert ball of rock.
- [Woman] Someone jumps in
the hole, perhaps in Chile,
looking to emerge in China.
What happens? Well, you start falling.
Gravity pulls on us at 32 feet
or 9.8 meters per second squared.
That means that for each second you fall,
you speed up by 32 feet per second,
but only near Earth's surface.
- [Man] Now, gravity
is a function of mass,
and mass is a property of matter.
When you're on the surface of the Earth,
all of the planet's matter
is beneath your feet.
But as you fall through
this imagined hole,
more and more of it surrounds
you, exerting its own gravity.
- [Woman] These horizontal
tugs cancel each other out,
but the increasing proportion
of mass above your head
exerts a growing counter-force
to the decreasing mass below.
So your crazy acceleration
actually slows a bit
as you near the core.
- [Man] And when you reach the core,
or the section of the
tunnel extending through it,
your acceleration due
to gravity reaches zero.
The mass of an entire
planet surrounds you,
canceling out gravity and
leaving you weightless.
- [Woman] Oh, but you're not
stopping here to float around.
You're going way too fast.
Halfway to the center your speed
hits 15,000 miles per hour,
or 24,000 kilometers per hour.
- [Man] You reach the
core segment of the tunnel
a mere 21 minutes after jumping in,
and you blast right through
at 18,000 miles per hour,
or 29,000 kilometers per hour.
We're talking re-entry speeds, here.
- [Woman] Another 21 minutes pass,
with gravity slowing the whole way,
you reach the far side of the Earth
and stop briefly in midair.
This is where it pays to have a pen pal
on the other side of the world,
because, unless someone catches
you, you'll fall back down
and enjoy the entire 42-minute,
12-second fall again,
and again, and again.
- [Man] In theory, you could keep falling
back and forth indefinitely,
falling forever in this
never-ending bottomless pit.
Again, for this thought
experiment to work,
you have to dismiss a
load of other concerns,
including the Coriolis
effect and angular momentum,
that would certainly smash
you into the tunnel walls
on the way down, and back up.
- [Woman] Again, physicists
love this thought experiment.
And in 2015, Alexander Klotz,
a graduate student in physics
at McGill University in Montreal,
tackled the problem
anew using seismic data
from the Preliminary
Reference Earth Model.
Factoring in updated
Earth density numbers,
Klotz's imagined fall through the planet
came in at 38 minutes and
11 seconds, a new record!
- [Man] Either way you
shake it, 42 minutes or 38,
that's just enough time to
watch two back-to-back episodes
of the old He-Man cartoon on your phone
as you fall through the Earth.
You just might want to skip the intros.
Whoa.
- This is a lot more relaxing
than I thought it would be.
- Oh yeah. I can't wait
'til we hit the core,
for that weightlessness.
- It's gonna be awesome.
- Gigantic pits are awesome!
They combine all of my favorite things:
mystery, danger, attractive
nuisance, and scary geology.
Here are some of the coolest.
- [Announcer] I always thought
Earth was solid. But is it?
Some people don't think so.
In fact, Hollow Earth theorists
think the Earth's surface
is only 800 to several
thousand miles thick.
- Have you ever heard
that if you're in a free-falling elevator,
and you jump up right before
the elevator hits the ground,
you can save your own life?
Well, that's probably not true.
