Review: The Core

I have been waiting a long time for this movie. The trailers started running in theaters many months before the movie came out, and I knew right away that "The Core" was ripe for a BA review. I remember distinctly that one line in the trailer said that the reversal of the Earth's magnetic field wipes out all life every 700,000 years. Um. If that's true, then why are we here? Life's been around for a few billion years!

The trailers changed as the script was revised and scenes reshot. The end product, as we are assured from the director, the writer, and even various reviewers, is pretty accurate.

Perhaps I saw a different movie.

Yeah, the science of the movie was pretty bad. No shocker there, and in a moment I'll go into detail. The movie itself wasn't terrible. It just wasn't great. The characters aren't nearly as bad as I thought they'd be; in particular, a vain and overstuffed scientist (clearly based on a very exaggerated Carl Sagan, right down to the clothes) turns out to be not nearly as annoying as I expected. The lead hunky-guy-scientist actually has a decent sense of humor, and had me laughing a lot in the movie. The special effects would have been ground-breaking ten years ago, but are ho-hum now; just about every science-fiction movie coming out these days has comparable effects, and I think the effects on "Enterprise" are better!

The pacing was erratic, with sudden jumps followed by dragging scenes. A lot of the movie was telegraphed. The death of the commander was so obvious from the very first five minutes of the movie that he should have had a toe tag sticking out of his shoe in his first scene.

Still and all, as long as there are copies of "Armageddon" intact, no worse scifi movie will ever be made. This movie is probably a good renter; I prefer to see big action movies in the theater, but the effects in "The Core" just don't quite cut it.

And what the effects depict, well, let's get to the science, shall we?

Bad:
We see the Space Shuttle in orbit. Small jets fire to turn it around to the correct position for landing. Pilot Major Rebecca "Beck" Childs wants to land it, but the Commander tells her that "the commander lands the bird". But there's a problem: in a dramatic series of scenes, a failure in the ground tracking information puts the Space Shuttle off-course as it lands. They have to land in downtown Los Angeles! They fly over the Dodgers stadium, and we hear the sonic booms from the Shuttle as it passes overhead. They wind up landing in the extensive LA drainage basins.

Good:
There was some good stuff in that scene. The commander does land the Shuttle, not the pilot. This scene makes it clear that at some point in the movie, the commander will die and Beck will have to take over and make the tough decisions, which of course is exactly what happens.

There were problems, though. In the movie, when the jets (called the reaction control system) fire in orbit, the Shuttle spins like a car going around a turn! It doesn't move that quickly in real life. The rockets don't apply all that much thrust, so that the astronauts can control the turn better. That's a Hollywood compromise, of course: I don't want to sit and watch the Shuttle taking several minutes to come about! From what I can find in the Shuttle reference manual (linked above), the rate of rotation about an axis is from 1 to as much as 7 degrees per second, so it would take from 20 second to nearly two minutes to rotate the Shuttle 180 degrees.

I know this sounds nitpicky of me, but there are UFO people who claim that there are UFOs seen in Shuttle footage. What they are actually seeing is ice and debris near the orbiter, which gets blown around by the RCS units. One UFO proponent says that's silly, because the rockets are powerful and the Shuttle would move noticeably if the rockets fire. But in reality (with which this guy seems to have lost touch) the RCS units are pretty gentle. Ron Parise, a Shuttle astronaut, says that you can hear them firing (it sounds like a clunk) but you can hardly feel them when they are in automatic attitude control. So I am making the point that Shuttle maneuvers are generally pretty subtle.

Finally for that scene, the sonic booms heard do happen on the Shuttle; they are the shockwaves from the nose and tail of the Shuttle. However, they only happen when the Shuttle is moving faster than sound. By the time it's at a few hundred meters height like in that scene, it's traveling well under Mach 1, so there would be no sonic boom.

Oh, one more thing: if the Shuttle were off course by 130 miles, NASA would have known about it long before the Shuttle was already in the atmosphere. It was a faulty ground transponder (or some such line in the movie) that caused the problem, but the position of the Shuttle is determined in several ways, not just one.

Still, it was an amusing scene, though touched with a bit of sadness given recent events with Columbia. In some ways, I suppose, it could be interpreted as a tribute to the astronauts.

Bad:
So what's the premise of the movie? The Earth's magnetic field is collapsing, and the field is what's protecting us from deadly cosmic rays and microwave radiation from space.

Good:
Close, but no cigar. The Earth's magnetic field does indeed protect us from cosmic rays, or at least some forms of them. "Cosmic rays" is a catch-all term for any kind of high-energy particle whizzing around in space. A fast proton is a cosmic ray, as is a helium nucleus (two protons and two neutrons, sometimes called an alpha particle).

The Sun emits these particles as part of the solar wind (there are other sources as well, from outside the solar system). They hit the Earth's magnetic field, and in a complicated process they eventually slam into our atmosphere. If there are enough of these particles hitting us, like after a flare from the Sun, then we can get an aurora. A big dose of cosmic rays would be bad, but not deadly, because the atmosphere would block most of them, just as it does now. I also doubt that a bigger influx of cosmic rays would cause "superstorms" as they did in the movie, too. I have read that storms may be affected by cosmic rays, but this research is pretty tenuous; more of an idea than an actual hypothesis. So cosmic rays are fairly safe.

But in the movie, it's the deadly microwaves that are the problem. The real problem, at least for the movie, is that the microwaves aren't deadly. They aren't even annoying.

Microwaves are electromagnetic radiation; that is, light. They have a lower energy than the light we see by, and in fact are not terribly different from radio waves. They can heat metal and food, because water and certain metals absorb microwaves pretty well. When you use your microwave oven to cook food, you are actually heating the water in the food, which is what really does the cooking. The microwaves also need to be pretty well focused to be usable for cooking.

But that's the problem! Well, two problems: Microwaves are unaffected by magnetic fields, and the microwaves from space are far, far too weak to affect us on the ground. Charged particles, like protons and electrons, are strongly affected by the Earth's magnetic field, but microwaves are not charged particles! They zip right through a magnetic field without even breaking a sweat.

And even if the disruption of the Earth's magnetic field did let all the microwaves from space through, you wouldn't even notice! The Sun just doesn't put out much energy in that form. Think of it this way: radio stations and the like broadcast using microwave repeater antennae; they are all over the U.S. If the Sun were that much more powerful a transmitter of microwaves, it would overwhelm those repeater stations and you wouldn't be able to watch TV, listen to the radio or use your cell phone. So the movie blew it twice in one shot with this topic. But there's worse to come...

Bad:
Why is the Earth's magnetic field collapsing? Because the iron core of the planet has stopped rotating.

Good:
Oh, yikes. That's just plain old silly. They say it themselves in the movie: it's a ball of iron the size of Mars spinning at 1000 miles per hour (in the picture on the left (from http://zebu.uoregon.edu/1996/ph123/l11.html), the inner core is the red sphere, and the outer core the yellow one; the brownish stuff is the Earth's mantle, and the crust is the thin reddish-purple layer on the outside).

It takes a lot of energy to stop a ball that size and that mass spinning that quickly. In fact, the spinning core (if I have done my math correctly) has about 10³⁵ ergs of energy stored up in its rotation, or about the amount of energy the Sun emits in 100 seconds. That's a whole lot of energy. If it doesn't sound like much, let's convert it to megatons: it's the equivalent energy of five trillion one megaton bombs going off. That is comfortably more than the entire arsenal of the planet; in fact, it's about 500 million times the explosive energy of every single nuclear weapon on the Earth (my thanks to regular reader and Bad Astronomy Bulletin Board member Bob Johnston for correcting me on my estimate of the total yield of the global nuclear arsenal).

If the core were to stop rotating, all that energy would have to go somewhere, and where it would go is into the surrounding mantle. Again, if I have done my math correctly, that's enough energy to vaporize all the oceans on the Earth, ten times over. I guess the microwaves would be the least of our worries.

The point here is that something with that much rotational energy can't just stop. Period. Now, what's ironic here is that the writers didn't need to have the core stop. How, you ask?

[Note added July 10, 2004: Yikes! In the original part of the review to follow, I said that the Earth's core was ionized. That's wrong! The heat inside the Earth isn't nearly enough to ionize the metal. However, in a metal the electrons are free to move on their own, and it is this which sets up the moving charges needed for a magnetic field to be generated. So that was my mistake, and I have corrected it here. Also, I gloss over complications in creating the Earth's magnetic field, such as the flow of matter in giant convection cells in the core; but these details are not necessary to understand where the movie goes wrong.]

Again, in the movie, they got something right: they said that a spinning ball of hot metal will create a magnetic field. This is a very basic property of magnetism: it's caused by moving charges. An electron zipping past you has a magnetic field associated with it, because it is charged and moving. The core of the Earth is tremendously hot, so hot that the electrons in the iron have been stripped from the nuclei of the atoms. Since it's spinning, the charges are moving, so you get a magnetic field.

The real situation is far more complicated, as life, in general, is messier than an oversimplified movie review. The Earth's core isn't just spinning; there are currents inside it, and other factors which influence the Earth's magnetic field. If the writers had been clever, instead of saying the Earth's core had stopped, they could have said that the currents of molten and ionized iron inside the core had become chaotic. It takes a relatively stable flow to make a magnetic field, so the chaotic motion could collapse the Earth's magnetic field.

Not only is this plausible, it appears to be true: the magnetic field of the Earth is not constant. In fact, for reasons still not well-understood, the magnetic polarity of the field sometimes reverses, with the north magnetic pole becoming the south, and vice-versa. This happens every few hundreds of thousands of years (and note that the Earth's surface doesn't boil when the field drops to zero during a reversal!).

So they really blew it in the movie; they could have used real science to collapse the magnetic field, instead of the silliness of stopping the core. And even if the core stops, and the magnetic field collapses, the effects wouldn't be anything like what was shown in the movie!

Oh, and by the way, that "1000 miles per hour" line in the movie is wrong too. The outer core is about 2200 miles in radius, and spins once a day. That means the outer edge at the equator has a velocity of roughly 550 miles per hour. Just picking nits while I'm here.

Bad:
The ship slams into a giant geode, and snags itself on a giant crystal of amethyst. The crew put on suits, go outside, and free the ship.

Good:
They are deep inside the Earth's mantle when this happens. The pressure of all the mass above them would be huge; they say in the movie that it's about 800,000 pounds per square inch, or more than 50,000 times the atmospheric pressure at sea level. Needless to say, that's a lot. I have a real problem with a suit that can not only take that kind of pressure, but actually able to resist it enough to move. Sure, it's Hollywood science, and I accept that for the sake of the plot (really!), but I just wanted to point this out. Also, that they were in a huge geode, which was pretty silly too. New! Added June 20, 2003: I don't have any other source for this, but the writer of The Core himself says geodes this big are possible. Warning: that link has plenty of explicit language. Not for the faint of heart.

Incidentally, it was in this scene that the Commander gets it. A crack in the top of the geode lets in magma, which falls on his head, burns through his suit and kills him. He falls backward and sinks into the pool of magma.

But he wouldn't sink! Objects sink in liquid if they are denser than that liquid. Rocks sink in water, a cork floats. A human is roughly as dense as water, but liquid rock is far denser than water. A person in a spacesuit might sink in water, but would be far less dense than rock. So in reality the Commander would float in the pool of liquid rock, and not sink dramatically, slowly sliding out of sight under the sad gaze of the hunky hero scientist guy. My thanks to David Seidel of JPL for pointing out this to me; boy, did I feel dense for missing this!

Bad:
A microwave beam from space breaks through the weakened magnetic field of the Earth! It happens, of course, right over San Francisco. The beam destroys the central span of the Golden Gate Bridge. Afterwards, we see the south tower of the bridge leaning toward the north.

Good:
I happen to live not too far from the Golden Gate Bridge, and it's a poorly kept secret that Northern Californians and Southern Californians have some sort of feud going on that recent transplants like me don't really get (I think all Californians are nuts). This scene couldn't have helped.

[Note added April 4, 2003: in my original writing, I incorrectly described how a suspension bridge works. Keeping the original text would be confusing, so I just replaced it here. My thanks to several Bad Readers who pointed this out to me.]

It's also wrong. A suspension bridge is actually pretty simple in theory. In this case, there are two towers. Cables are anch