Random Rambling: Physics

We have... let's say an eclectic mix of interests, and we feel our patrons do too.  With that thought in mind, we present a feature we call "Random Rambling."  It isn't quite random, but it's close:  we made a little spinner with some of our favorite subjects.  Every Friday, we give the thing a spin, and then you, lucky people that you are, get to learn a fact related to the subject the spinner landed on.

This week's topic is:

Physics!

Since we're covering a similar topic over on Worldbuilding Wednesdays, let's talk about the weirdness that is the Hollow Earth theory.

The Hollow Earth in/is a Nutshell

The Hollow Earth theory isn't very popular nowadays, but it is technically a way to explain the geophysical composition of Earth.  The theory basically boils down to the idea that, from the surface at least, a Hollow Earth with a fairly dense crust would have many of the same features as a solid Earth with a less dense interior.  Also technically, the theory hasn't been disproven... but it has been mostly discarded, because our measurements support a solid Earth.

Well, not quite solid.  The measurements we've made indicate that Earth probably has a thin but firm outer layer surrounding an extremely hot inner layer of liquids and solids, making it, by some definitions, a giant ravioli.

Pictured:  the birth of planets, apparently.

However, let's suppose that the Hollow Earth theory is true.  It turns out that the world actually consists of a dense crust surrounding a hollow "core."  Furthermore, you have learned that Jules Verne's Journey to the Center of the Earth (and the 2008 masterpiece of the same name starring Brendan Fraser) is based on truth:  you can indeed find a volcanic tube leading from the outer surface of the Earth to its hollow core.  What could you expect to find?

The Land That Time and Dinosaurs Forgot

For starters, you probably won't be finding a prehistoric world.  While it is true that Earth generally gets warmer as you go down into the crust, warmth (or the lack thereof) isn't what killed the dinosaurs.  Neither was the asteroid that struck some 65 million years ago, although it was the catalyst.  What actually killed the dinosaurs was an ecosystem collapse:  so much dust was kicked up by the asteroid impact that the entire world went dark, potentially for years.  Most plant life died off, as did the animals that mostly depended on that plant life, followed by the animals that depended on those herbivores to live.  It getting cold without the presence of sunlight certainly didn't help matters, but large terrestrial animals were doomed either way.  Scurrying closer to the core would have just made the dietary problems they were experiencing even worse... and that's even assuming that there was anything to eat at all.  While deep caves do have actual biomes with actual, weird life, the available food levels past the first, say, 100 meters or so, drop to near zero.  One of the largest cave predators in the world is Microcreagris grandis Muchmore, the Great Basin Cave Psuedoscorpion.  It's about an inch long.  The chances of, say, a single allosaurus surviving on the available life in the depths of a cave are nil, let alone a line of allosaurs stretching back 60+ million years without any evolutionary changes.

This isn't to say that there can't be life within a Hollow Earth, for the record.  What this means is that there needs to be an alternative energy source for the bottom of the food chain to persist on.  Perhaps extremophiles living near volcanic vents could serve, or, as is proposed in many Hollow Earth stories, maybe inside the Hollow Earth is a tiny little star that serves as a sun for the creatures living inside the core.

Even if the latter were true, however, you still wouldn't find an allosaurus roaming the core, for another, more interesting reason.

The Land That Gravity Forgot

If you thought that we forgot about the whole "physics" aspect of today's article, don't worry, we were working up to it.  If we have a Hollow Earth, then the way gravity works for that Earth would be dictated by the Shell Theorem, proven by Sir Isaac Newton.  We won't bore you with how the math works.  The important part of the Theorem is that, from the outside of the Hollow Earth, gravity works the same way as it does on a solid Earth.  On the inside, however, there would be no gravity at all.

That isn't to say that someone entering the Hollow Earth's core would instantly start floating.  Earth is rotating, which will generate a centrifugal force that pushes anyone in the Hollow Core toward the surface.  This will feel an awful lot like a very weak gravity, in much the same way that science fiction and actual real-life scientists posit we can simulate gravity in space by rotating whatever spaceship or space station we're in.

How weak will the gravity be?  Well, say that you're a physically fit adventurer who looks suspiciously like 2008-era Brendan Fraser.  On the surface, you likely weigh around 200 pounds.  In the center of the Earth, your effective weight will be -10 ounces.  That negative sign is accurate, by the way; your weight is going to be pointed toward the surface, not the center.

Micro-pseudo-gravity like that experienced in a Hollow Earth would be, to put it mildly, weird.  Because of how much hang-time you would have, it becomes difficult to figure out just how far a typical 2008-era Brendan Fraser could jump, but 200 feet vertically isn't unreasonable.  For the record, even with this low level of "gravity," you still couldn't fly.  It turns out human arms aren't designed to generate lift, so flapping them doesn't actually push you off the ground at all.  You could, however, glide, for potentially quite some distance (we're talking miles) just by leaping toward the center of the Earth and opening up your jacket at the top of your leap.

That might not be the best idea, however, as cool as it sounds.

The Real Answer to "Where Did All The Dinosaurs Go?"

Remember how we said that, for anything larger than a tiny insect to survive, there would need to be an alternative energy source to the sun?  And then we suggested that maybe there was a tiny little star inside the Earth?

Yeah, that's really dangerous.

It takes a surprisingly small star to overcome the gravitational pull toward the outside of the Earth.  And by surprisingly small, we mean "a star about 25 km wide."  Of course, you wouldn't be able to measure the star from the inner "surface" of the core, so how about this:  if it appeared to be any bigger than about 1/4 the size of the moon from where you were standing when you first popped into the core of the Hollow Earth, we really hope you were tied down, because otherwise you're doomed.

At that size or larger, the gravitational pull of the tiny little star is more than the roughly 10 ounces of pull toward the surface that you'll experience.  Once you are no longer attached to the Earth's crust, you will inevitably fall toward this star.  And once you reach the star, you'll do the same thing that everything else does when it reaches a star:  burst, very briefly, into flames before becoming a fine ash.

So, to round out the reasons why, even if Hollow Earth were real, you wouldn't run into any dinosaurs beneath the surface:

• Going beneath the surface wouldn't have saved the dinosaurs from starving to death
• Even if it did, the dinosaurs wouldn't have stayed the same for 65+ million years
• Even if they did, there would need to be a miniature sun providing light to sustain all the plant life the dinosaur food chain requires
• Even if there were a miniature sun inside the Hollow Earth providing enough light to sustain plant life for all the dinosaurs, the dinosaurs themselves probably fell onto said sun and died

Fun Little Aside

When we get to really big numbers, such as the numbers found in planetary physics, weird coincidences start popping up.  For example, the amount of light produced by a sun that was the same as our sun but shrunk down to a diameter of 25 km?  Almost exactly the same amount as the light that reaches the Earth's surface from the real sun.  It's for this reason, among others, that once you get to the really big numbers, you stop worrying about anything that isn't a power of 10.  While we did the actual math for fun, an astrophysicist would have immediately rounded everything to the nearest zero, and the tiny little sun inside the Hollow Earth would either have a diameter of 10 km (and thus be safe) or 100 km (and Brendan Fraser would be doomed to fall into it, had his wonderful movie been a documentary instead of a work of fiction)

Conclusion

The dinosaurs were doomed, no matter where they hid.

Brendan Fraser is, of course, awesome at any point in his career, but he was never in more danger than in 2008.

Last but not least, if the world were hollow, we could play absolutely epic games of basketball.  We're talking playing in a football stadium, jumping from one endzone and dunking the ball in the other endzone, levels of awesome.

But we could only do so in the dark.