equilibrium - Do airlocks in space decompress violently as they do in movies?

The standard scene follows: The good guys have finally captured the enemies and have them in an airlock chamber on a spaceship. A button is pressed and out they go, violently propelled by the blast of air moving from a pressurized zone to a zone of much less pressure (space).

Here's my question: If people don't immediately decompress, then why do airlocks do this in movies? If the air moves due to expansion to reduce pressure, then why wouldn't the human body do the same?

Answer

People don't immediately compress because the body is more or less a pressure vessel. It's not a very good pressure vessel for dealing with vacuum, but it's something. It's your body's resistance to pressure that lets you do things like spray bodily fluids (from your mouth, or from your bladder, or from your arteries). When my wife was in labor with my son, one of the sensors monitoring her reported the internal pressure during her contractions in pascals (though you'll have to find an obstetrician to tell you what a typical uterine contraction pressure is, because I was mostly paying attention to other things at the time).

Gases don't have this constriction. Nitrogen and oxygen at room temperature have mean molecular velocities following \begin{align} \frac12 mv^2 &= \frac32 kT \\ \frac vc &= \sqrt\frac{3kT}{mc^2} \approx \sqrt\frac{3\cdot 25\,\mathrm{meV}}{30\,\mathrm{GeV}} \\ v &\approx 1.5\times 10^{-6}c \approx 450\,\mathrm{m/s} \approx 1000\,\mathrm{mph} \end{align} When you depressurize a gas volume at room temperature, this is the average speed of the gas that moves into the vacuum. If you depressurize an airlock by letting the air flow through a 2 m$^2$ doorway, you have a lot of momentum to carry things along with you.

Emilio Pisanty gives a nice example in a comment: if your airlock is the size of a bathroom (~ 20 m$^3$) and it's depressurized rapidly, so that all the air is "suddenly" moving away from the door at a thermal speed, the momentum of the air is comparable to the momentum of a car. You have a good intuition for what it's like to be hit by a car (hint: it sucks, even at low speed); getting thrown out the door of the airlock is a totally plausible outcome, but not a certainty. Keep in mind, if you start to calculate things, that the kinetic energy available, $p^2/2m$, goes way up if you take the momentum of a car collision and put it into a few kilograms of air.

Note that in a real airlock, you'd change the pressure slowly. If you were on a spacecraft where air was a precious resource, you would probably even pump the air from the airlock into the spacecraft rather that throwing it away.