Built on Facts

Have you seen The Dark Knight yet? No? What’s wrong with you, go add to its box-office gross now. It deserves it. No, I’m not kidding. Also, stop reading since there’s about to be some minor spoilers.

This post was inspired by my mom, who wants to know how plausible the various two major cell phone schemes are. Let’s see.

1. Lucius Fox develops a very snazzy phone which can emit sound pulses. The reflections of those pulses are received by the phone and reconstructed into a sonar image of whatever room the phone happens to be in. The idea is not inherently absurd. Submarines use it to find other subs and ultrasound machines use it to examine unborn children, to take just two of the most familiar examples. Could this work to characterize a room? Sure, to an extent. But as with any wave, there’s tradeoffs. Low frequencies can’t see detail, but high frequencies can’t see around corners. If you approach a high school band from a distance, you’ll hear the drums from the greatest distance not because they are louder but because they are lower frequency and can more easily bend around obstacles. Thus without a direct and unobstructed path to whatever surface it’s supposed to measure (as in fetal ultrasound), the phone is going to be hard pressed to see anything at all.

To make matters worse, air is not a continuous fluid. For a lot of purposes it can be treated as one, but as frequency becomes very high the wavelength begins to become comparable to the intermolecular spacing. This tends to cause severe attenuation at high frequencies, limiting the range badly.

Don’t even get me started on the signal processing problems. Real-time inverse Fourier transforms of gigabits per second per phone? Not happening. And how in the world is the phone supposed to pick up direction and phase information in the incoming wave with adequate resolution in the first place?

It’s just not happening. The concept is not absurd, but it is impossible in practice.

2. Batman arranges for his own program code to be installed in nearly every cell phone in Gotham City.  On activation, it turns every phone into both a continuous bug, listening in on its surroundings even with the phone turned off.  He hooks the millions of signals into a computer and scans for the Joker’s voice in order to find his location.

Now this one is not merely plausible, it’s a reality.  Law enforcement has already used cell phones as bugs and tracking devices.  I’m not sure if the processing power of the installed phone company hardware is enough to simultaneously monitor every phone to look for a specific voice, but it’s certainly not beyond the realm of possibility.  Anybody with Bruce Wayne’s wealth could certainly afford it.  Lucius Fox believed this is much too great of a power for Batman to have since it invades the privacy of the good people of Gotham; Batman agreed and told Fox to destroy the hardware once the Joker was found.  I think it’s unlikely the FBI would feel the same way.

So if you’re doing something (criminal or not) that requires privacy, don’t bring your cell phone.  Or at the very least remove its battery.