Basic Movie Science: THE DARK KNIGHT

Please welcome back my friend, movie-loving science nerd Dr. Ray Wagner. His previous BAD installments of Basic Movie Science include Moon and 2001 and 2010. Let us know what movies you'd like to see get the Basic Science treatment in the future. -Meredith

With the impending arrival of The Dark Knight Rises, I’ve found myself reflecting more and more lately on director Christopher Nolan’s previous bat-film, The Dark Knight. Though to say that this reflection is something new really isn’t true. I’ve been wrestling with TDK since my first viewing, and recently I’ve come to see it as a fascinating case study in the times when loosey-goosey movie science offends versus when it merely entertains.

If anything, the exercise of writing this series on science in the movies has been one of deciding where to draw that line. Clearly, there’s an element of personal preference at work, but I hate to fall back on a Supreme Court “I know it when I see it” definition of bad script science. Obviously, any detail - scientific or otherwise - must be included in the service of story, and it’s often better to introduce and lightly misapply a concept that moves the narrative ball down the field compellingly than it is to interrupt the momentum for a proof from first principles*. In return for a healthy suspension of disbelief, the audience is rewarded with a richer, faster paced film. Other times, though, it deepens the story to slow things down a bit and ask the audience "into the lab," so to speak, for a bit of a more serious treatment of the science being invoked. This, in my opinion, puts the screenwriter in a very precarious position. If s/he isn’t willing to follow this scene through with the level of knowledgeable rigor that s/he’s now inviting, the whole thing can quickly devolve into a bad episode of CSI.

I find TDK interesting because it provides examples of both these techniques - the former of which works quite well for me and the later of which falls flat. And, most illuminatingly, the scene I’m inclined to forgive uses science much closer to my own area of study, and it never occurred to me to take it at anything other than face value until well into working on this article.

So, we’ll start there (spoilers ensue, so be warned). Nolan, sharing the screenwriting duty with his brother Jonathan, gives Bruce Wayne a new wonderful toy early in the film. Wayne Enterprises CEO Lucius Fox, as always serving as Bruce’s personal Sharper Image, shows him a new device R&D has cooked up - a modified cell phone capable of building a 3-D image of the space around it. A short interchange between Fox and Wayne tells us everything we need to know about how it works:

Fox: It sends out a high frequency pulse, records the response time, for mapping an environment.
Wayne:  Sonar? Just like a...
Fox: Submarine, Mr. Wayne. Like a submarine.

As the film progresses, we see Bruce take this technology and, without Fox’s knowledge or consent, develop it as (presumably) malicious code that he loads on every cell phone in Gotham. A one-line callback from Lucius later in the movie confirms as much. By eavesdropping on Gothamites’ phone calls and searching for target voice patterns, Wayne can locate a perp and map the space around him using all the nearby mobiles. It’s a nifty idea, quickly explained and then moved past so we can get into meatier ethical issues like the tensions between security and liberty... and, of course, see Bruce use the tech to kick the Joker’s ass in the film’s climax. 

But here’s the thing: from a technical standpoint, it’s pretty bogus. There’s no way Bruce could build that system using off-the-shelf cell phones modified only with software hacks. Open-air sonar usually takes the form of ultrasonic ranging, using frequencies of sound up to several MHz to derive the distance to a target of interest. This is well in excess of the range of human hearing, which is more like 20 Hz-20 kHz. Since the only component on the cell phone capable of producing sound is the speaker, that’s what Bruce’s software would have to use to generate an ultrasonic ranging pulse. But guess what? Speakers - even the high end ones - aren’t built to produce sound at frequencies much greater than what our ears can perceive. So there’s literally no way for Bruce to pull this off without issuing a product recall to every phone in Gotham and replacing their speakers.

Of course, that’s not the only thing dooming Bruce’s plan to failure - there would have to be a huge density of phones, distributed fairly uniformly around each target of interest, to achieve the imaging resolution we see through Bruce’s bat-vision. But it doesn’t matter. The Nolans tell us everything we need to know about the mapping tech in three lines, and they don’t invite us into the engineering at play. They introduce a concept that, on the surface, is interesting, plausible and has a good thematic resonance with the Bat. And even if you’re inclined to mull over this stuff to an unhealthy degree like I am, I submit that you’re unlikely to be truly bothered by the subtly wonky science that’s invoked here.

This economical handling stands in contrast to a scene bookended by the sonar bits. Near the midpoint of the film, we see Bruce in full-on Detective mode. The Joker is terrorizing Gotham and leaving clues to the next phase of his plan in order to draw Batman into the open. As part of this, he shoots and kills two unlucky guys in an apartment, guessing that their names (“Harvey” and “Dent”) will get Batman’s attention. Batman turns his attention to bullet holes in the apartment wall left by the shooter’s stray rounds, and the dialog with Commissioner Gordon explains what he’s up to:

Gordon: That’s brick underneath. Gonna take ballistics off a shattered bullet?
Batman:  No. Fingerprints.

Intriguing, Bat - tell me more.

Our hero then pulls out his bat-Dremel and starts cutting at the masonry surrounding one of the bullet holes, and in the next scene we see Bruce back in his lair with butler-turned-lab-assistant Alfred setting up an experiment. Alfred loads a number of handgun rounds into an automated Gatling gun which proceeds to shoot each bullet into a different sample block of masonry. 

Bruce scrutinizes the impact damage of each block and picks the one closest in appearance to the block he pulled from the crime scene wall. He uses a laser scanner to map the topology of the inside of both the sample bullet hole and the one from the apartment. Bruce then takes the two scans to fairy godmother Lucius, and using the map of the lab-generated hole as truth data they compute the difference between it and the crime-scene hole, which gives - wait for it - the thumbprint left by the perp on the round as he loaded it into his clip. Eureka!

It’s a fascinating bit of procedural wizardry, and it’s presented in enough detail to really get my engineer juices flowing. We start with a challenging puzzle - how to recover meaningful information from a shattered bullet - and we see Bruce set up the coolest experiment in recent movie history, complete with a control group, and employ some fun image processing on the backend to sink the putt. It’s exactly how I’d try to approach the problem were I handed it. And it makes the scientific method look damn cool in the process! 

Ever since I first saw the sequence, I haven’t been able to stop going over it in my head. You see, the thing is presented in such detail that you’re invited to think about it. You’re practically given ear protectors and a pair of safety glasses and whisked into the bat-cave (garage?) along with Bruce and Alfred. If you’re juiced by this sort of thing, it’s difficult not to keep reflecting on it.

Boy, is that a mistake.

Try as I might - and believe me, I’ve really tried - I haven’t been able to get what Bruce does to make a damn bit of sense. Now, I’m not a mechanical engineer or a materials scientist, so the specifics of this are a little outside my wheelhouse. But a little online research and back-of-the-envelope scribbling is quickly illuminating. 

Let’s start with the outcome - a reconstructed image of the fingerprint - and work backward to see if we can make Bruce’s experiment add up. We see that Bruce comes to Lucius with what appear to be scans of the various bullet fragments, which Lucius’ software then assembles back into an image of the complete bullet. Fragments of the thumbprint image come along for the ride, so putting together the bullet image rebuilds the thumbprint image. Provided we can get scans of the fingerprint fragments along with scans of the bullet fragments, this makes enough sense.

But that raises the question of how exactly Bruce gets those fragment scans. As I see it, there can be two possibilities: (1) Bruce images the fragments themselves, or (2) Bruce images the inside of the bullet hole, where the bullet fragments have transferred segments of the thumbprint in the process of penetrating the masonry.

So what about the first option? If he’s really getting images of the actual fragments, why does he need to go through an elaborate process to compare against a test impact? He knows what the surface of the bullet looks like, so as long as his scanner can see enough thumbprinted fragments that Lucius’ software can build them back into a bullet, he should be able to cut to the chase and just image the fragments in the original bullet hole. So that doesn’t quite add up, given the scene progression.

Okay, well, what about the second option? Clearly, Bruce thinks he needs a background image, which implies to me that he’s interested in finding the difference between an impact from a clean bullet and one from a thumbprinted bullet, hoping to back out the thumbprint from that difference information. Since he’s likely not scanning the fragments themselves by the previous reasoning, this further suggests that he’s looking for the thumbprint to be transferred to the masonry in the bullet hole. 

Okay, now, I want everybody to try a quick experiment. Clean all the smudges off the screen of your smartphone or a nearby window, and then lay down a nice, fat thumbprint by depressing and quickly removing your thumb (straight up!). Now, using the (non-printed) side of your thumb or finger, apply some pressure to the print and drag your finger along the glass a bit. What happens to the print? It’s unrecognizable, you say? 

Now, the mechanics aren’t exactly the same, but that’s a good illustration of how fragile the thumbprint on the bullet really is. Fingerprints are mostly deposits of sweat and oil that our fingers have secreted or picked up from elsewhere (like our skin oil). So let’s quickly review the mechanics of a bullet impact to see how we think prints would hold up. A .45 ACP round (probably the perp’s choice) travels upwards of 250 meters and rotates more than 600 times every second. It carries a fantastic amount of kinetic energy that must be dissipated on impact. Some of that energy is used to fragment the bullet. Some of it turns the masonry into dust and blows it and bullet fragments out the back of the hole (check out a cool student experiment from the Dept. of Physics at Purdue for more detail). And some of it is dissipated as heat. 

I don’t need to remind you that sweat is steadfastly a liquid at room temperature, and a slightly-creepy Google search reveals that sebum, or human skin oil, turns into a liquid at a temperature slightly higher than our skin’s surface temperature. So the heat generated from the impact alone is likely enough to liquefy the fingerprint, if not turn it into a gas. Add to that the twisting motion of the bullet as it enters the masonry, and the fact that the surface on which we’re laying down the print is itself being partially pulverized and ejected, and I don’t see that there’s any way you could recover anything resembling a thumbprint from the inside of an impact hole. 

And never mind the fact that Alfred begins the whole lab scene by loading the test magazines with his bare hands.

So it appears that we either have an experiment that isn’t self-consistent or one that defies basic mechanical analysis. Choose your poison.

Now, I’ve clearly spent way too much time trying to make this work in my head. And you can probably (at least partially) hand-wave away any of my objections over beers. But my point is that the sequence got us probing its inconsistencies by bringing us into the process. The Nolans slow down the action and take 90 seconds over three scenes - about one percent of the film’s runtime! - to ask us to marvel at the deductive science on display. So OF COURSE we’re going to be tempted to think about it. And think about it some more. And eventually realize that it’s BS.  

I don’t fault the Nolans for fudging science a bit to move their story forward in a cool way. I enjoy the phone-phreaked sonar, wonky as it is. But the economical approach to that sequence is very different from the drawn-out quest for the thumbprint. 

I love detailed science in my movies as much as the next guy - hell, more than most next guys - but when it’s brought into a script, it needs to used carefully. So I think the lesson to be learned is this: when screenwriters want to invite the audience "into the lab," they need to be prepared for the level of scientific rigor they’re courting. That invitation may not always be as literal as it is in TDK, but when the details of the science are used to drive the logical progression of the story, they need to hold up. Otherwise, it’s best to just show us a bit of plausible razzle-dazzle and move along as quickly as possible.

* Note that I’m not excusing films that ask the audience to completely turn off their brains and accept lazily bad science - I’m looking at you, G.I. Joe.