Any idiot can make a science error, but the best movie errors give something of real value. A good science blooper has pedagogic value. It has some superficial plausibility, and in explaining to your students why it’s wrong, they really learn something. Here are my three favorites:
- Movie: Star Trek: Generations; Mistake: Evil Dr. Sauron fires Li_3 missle into star, which immediately stops all fusion in the core. Star immediately dims and starts to collapse. Why it’s wrong: Being an astronomer, I’m partial to star mistakes. It’s true that thermal pressure from fusion is what holds stars up against gravitational collapse. However, if you turn off fusion, the thermal energy in the star won’t immediately disappear. It only loses energy as fast as it radiates it away. The timescale on which the star would lose an appreciable fraction of its heat is the Kelvin-Helmholtz timescale: about 10 million years for a star like the sun. More problems with this scenario: Even if the star started collapsing right away, it should take several minutes for light from the star to reach the planet where Sauron and Picard are fighting, so even then, they shouldn’t see it right away. Also, Sauron’s plan of moving the Nexus by changing the star’s gravitational field is flawed. The star being very nearly spherical, its collapsing–even to a black hole–wouldn’t change the exterior gravitational field. Also, since the star contracts quasi-statically, it obeys the virial theorem throughout: E = T + W = -T. So the star actually heats while contracting, even though it’s radiating energy, because it’s sinking deeper into its gravitational potential. This really is a science mistake that keeps on giving.
- Movie: Mission to Mars; Mistake: Human body exposed to vacuum immediately freezes. Why it’s wrong: This was a pretty forgettable movie, but I have a soft spot for it for all of the attention-grabbing science bloopers. (“That looks like human DNA!”) I expect one of the writers read somewhere that “space” has a temperature of 2.7K, which is really colde, so they thought “Hey, people in space should freeze.” First of all, the vacuum itself has no temperature. 2.7K is the temp. of the cosmic microwave background, which is irrelevant to a body inside the solar system. Such a body will cool (or heat) until it radiates away heat at the same rate that it absorbs it from the sun. True, in Mars orbit this would be below freezing, but once again we must ask how long it would take for a human body to reach this equilibrium state. The answer is a long time (hours) because the only way to lose heat in space is by radiation. Like all hot bodies, a human emits blackbody radiation (about 100W, mostly in the infrared). The cooling won’t be what kills you, though. One exeption: In a vacuum, the water in your eyes and mouth would boil off quickly (boiling temp. is a function of pressure; your blood, being pressurized, won’t freeze right away), and the latent heat the water carries away would quickly cool those parts.
- Movie: The Day after Tomorrow; Mistake: Air from the stratosphere descends quickly to Earth, “doesn’t have time to heat”, and so freezes the surface of the Earth. Why it’s wrong: I just love this mistake. Whoever wrote this movie should get Al Gore’s Nobel Prize. When a gas contracts or expands faster than it can heat or cool, its not the temperature that stays constant; it’s the specific entropy. Thus, rapidly descending gas from the stratosphere would adiabatically heat. Although the stratosphere has a lower temperature than the troposphere, it has a higher specific entropy. (If it didn’t, the atmosphere would be convectively unstable and would quickly adjust itself.) So descending air from it would actually heat us rather than cool us.