What if the Earth were Hollow?


Uploaded by minutephysics on 26.08.2012

Transcript:
Flying in a 747 from one side of the earth to the exact opposite side would take about
22 hours… and while I know there's a bit of rock in the way, that's really going the
long way round.
So what if we did dig a hole all the way through the earth, through the center, and jumped
in?
Well, Michael, you probably would't make it very far - that's because of the Coriolis
effect (which is why a ball curves weirdly when you toss it while riding a merry go round
and why hurricanes always spin counterclockwise in the northern hemisphere). At the equator,
the earth (and you on it) is rotating eastwards at 1670 km/hr. As you go deeper, the bits
of earth around you are still spinning around once per day, but they don't have as far to
travel so they're going at slower and slower speeds. If you jumped into a vertical shaft,
you'd soon be traveling east faster than the rock around you so that after falling only
a few kilometers, you'd crash into the eastern wall. It might not be a disaster, but some
miners near Lake Superior tried to test this by dropping cannon balls down a mile-long
shaft - and the balls never reached the bottom.
OK, so what if the tunnel went from pole to pole, so the Coriolis effect didn't apply,
and let's also assume that there's no air resistance, or friction
Ok. Since the earth's mass is more concentrated close to the middle, gravity would pull you
down with roughly the same amount of force for the first 3000 km, or halfway to the center
of the earth - this familiar, constant force would accelerate you until you were falling
8km every second, and the trip halfway to the middle of the earth would only take 13
minutes. Soon after, you'd reach the earth's outer core, and this is the point in your
journey where the pull of gravity would be strongest - but only slightly stronger than
the force we're used to on the surface.
As you continued to fall closer to the center, so much of the earth's mass would now be above
you that it would begin to seriously cancel out the attraction of the mass below, and
the pull would weaken until you reached the center. Here, you'd experience no gravitational
pull at all - or rather, the earth would be pulling on you the same amount in all directions,
so you could float freely around with no sense of "up" or "down". Except, remember, that
you'd be speeding past at 22,000 miles per hour, or 6 miles a second.
Once you passed the center, the whole process would reverse and you'd gradually slow, pulled
down (or is it up?) weakly at first and then more strongly, until when you got to the other
side, you'd stop moving and could step out on the surface, a mere 37 minutes, or one
dryer cycle, later.
Of course, the deepest we've ever been able to dig is the Kola Superdeep borehole in Russia.
But it only went down 12km, which is only two thirds the length of Manhattan. They had
to stop because it got too hot: 180°C. And this is sort of the problem with digging a
hole through earth - earth is hot, and molten in the middle. You can't just dig a hole through
it with shovels. But here's a question: wasn't the middle of earth wasn't all "liquidy",
what if earth was hollow, but weighed the same?
Well, with its entire mass concentrated in a thin shell right under our feet, the earth
wouldn't have a magnetic field any more, because that comes from the molten iron core. So we'd
be totally vulnerable to radiation from the solar wind and storms, and this means we'd
see the aurora EVERYWHERE. Look! The NorthernSouthernEastern lights!
And if you jumped inside the hollow earth to escape the solar storm? Well, gravity from
the different parts of the spherical earth-shell would perfectly cancel out and you'd float
freely about inside as if the earth weren't there at all! Of course, you'd better bring
a space suit, because there's not nearly enough air ON earth to fill up the entire INSIDE
of earth. But what if the entire inside of the hollow earth were covered with mirrors?
Henry, that's ridiculous… for now
Ok, so back outside of the earth, we wouldn't really notice much difference from a gravitational
perspective - falling things would still accelerate at 9.8 m/s^2, a baseball would follow the
same trajectory, and the moon would follow the same orbit around the earth.
Hey Henry, let's go to outer space, right now. Bring your gun - this'll all make sense
later. C'mon.
Oh, and you're coming too. Click this annotation to head over to my channel, Vsauce, to see
the rest of our adventures. I'll see you over there.