The Real Center of the Earth

Just how realistic is Journey to the Center of the Earth in 3-D?

Hollywood, in its infinite desire to generate easy profits, has decided to do yet another remake of the Jules Verne classic Journey to the Center of the Earth -- this time in 3-D!. As we can see from the trailer, this movie is going to be a special effects extravaganza. Now, while we all know that the entire idea of traveling to the center of the Earth is pure fantasy, and any "science" represented in the movie is not to be taken seriously, we have so much scientific information about the state of the Earth's interior -- much more than Jules Verne ever could know -- that somehow the premise just falls flat.

(Probably, back in the more innocent year of 1959, when the original film version made its debut, it didn't seem so egregious.)

If you don't care, and simply want to escape into the fantastical special effects, that's fine, but for some it might be more fun to use the film trailer to highlight a few interesting principles of physics and geology that preclude a pleasant stroll down into the Earth's deep interior (and saving us eight-and-a-half dollars [a movie's $11.50 here in New York. -ed.] in the process).

Here are a few enjoyable discussion points:

1) Much of what we know about the structure inside our planet actually comes from seismic waves generated by earthquakes that travel right through the interior. Because we know the physics of these propagating waves, we can make very accurate estimations of the types and states of the material through which they travel. (This type of imaging is similar in concept to the CAT scans that we use to see inside the body).

2) Based on seismic data, we know that you don't have to go too far before it gets really hot, and the pressure gets really high. At the Earth's center the pressure is millions of atmospheres, with a temperature around 5000K. This would be an unpleasant environment for a family vacation, although not as bad as Buffalo.

3) The extreme pressure is simply a result of the massive weight of overlying rock. This pressure is so great that the idea of a system of caverns existing down there could only be possible if the gas pressure inside the cavern counterbalanced the surrounding rock pressure. While the most likely scenario is that there are no empty spaces down there, if there were, in addition to the protagonists being crushed into jelly, it would be pretty slow going through an atmosphere that thick. Maybe the body acclimates on the way down, like mountain climbers do as they slowly progress to the summit of Everest!

4) The high temperature results in part from the pressure, in part from the heat released by decaying radioactive materials, and in part from the original kinetic energy of colliding meteorites that formed our planet in the first place. Not such a good environment for finding undiscovered populations of prehistoric animals living in large bodies of liquid water -- although dinosaurs do make an adventure movie more fun.

5) Finally, it's always amusing to consider what happens to the force of gravity as one descends to the center of the planet. It gets weaker. In fact, at the very center of the Earth the net force of gravity acting on an object would be exactly zero because there is equal mass "above" the object in every direction. The total force cancels out. Anyone standing in a hole at the Earth's center would be completely weightless and floating around like an astronaut on Apollo 13. Which is the subject of a much better movie.

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.