We went to the New Mexico desert to blow up a car. It was an ugly car--a white 1970s Cadillac, rescued from the junkyard. In the trunk, 300 pounds of ammonium nitrate and fuel oil, the same lethal ingredients that the Oklahoma City Bomber used to destroy the Federal Building in 1995. Our car bomb, we were told, would vaporize sections of the Cadillac and level the modular building standing next to it. And our explosives were just one 15th the size of the bomb in Oklahoma City.
This was one of our most anticipated shoots for our two-hour NOVA special, "Hunting the Elements"--and also one of the most horrifying. We had stepped into a virtual terrorist's playground--the Energetic Materials Research and Testing Center at New Mexico Tech--and our morbid curiosity grew with each demonstration. Here, bomb experts and scientists spent their days teaching emergency personnel how to respond to nightmare scenarios, and the daily menu of demos was extensive. For an appetizer, a briefcase bomb that blew the torso off a wooden dummy and a pipe bomb that obliterated a watermelon and the table it was sitting on. For the main course, a letter bomb that amputated and wedged a mannequin's foot in the ceiling and a wooden dummy suicide bomber whose improvised explosive device shot bolts through metal signs like bullets.
Above: A wooden dummy "suicide bomber" wears a vest packed with C4 explosives (a class of plastic explosives) and bolts that double as projectiles.
We filmed all four explosions with a special high-speed camera that played back the action in eerie slow-motion detail. But it was the dessert course--the grand finale car bomb--that most horrified...and fascinated...our crew. As we watched the playback of the slow-mo footage, a dome of super-heated gas--invisible in real time--appeared above the initial fiery blast, and with each frame, moved up and outwards, warping our view of the desert behind it into a wavy, trippy landscape. "That's the shock wave," one of the bomb technicians said, pointing to the gaseous dome. It's not just the heat and debris from the bomb that'll kill you, he told us. It's the shock wave that'll knock you dead.
Above: Watch the car bomb explode at 3300 frames per second. The high-speed footage reveals the deadly shock wave, invisible to the naked eye.
Suddenly everyone on the crew wanted to rewind the video. It's not that we'd never heard of a shock wave--most people can tell you it's a wave of energy released when you set off a bomb, literal or otherwise (as in...your dad's reaction to the sentence, "I just totaled the car..."). But few of us realized this was the primary source of destruction. When most of us non-scientists think of bombs, we think of the fiery blast and the lethal projectiles it sends shooting into the air. We were riveted as the ordinance technician described this dome of energy that expanded too quickly to see with the naked eye. "Often, we find victims on the scene who've died without a scratch on them," he explained. They might escape the fire, but a shock wave can literally liquefy their insides.
Now the room was alive with questions: What exactly was a shock wave? How did it work? And why did some chemical reactions create them, while others just caused a fire?
Turns out the answers lie not just in chemistry, but in physics.
Chemistry tells us how fire works. When you strike a match or introduce energy from any heat source (like friction or lightning) to a fuel (like wood or gasoline), carbon atoms in the fuel combine with the oxygen in the atmosphere. In other words, they "oxidize," or burn.
This is how explosives work too. The only difference is the speed of the reaction. Fire needs oxygen to burn, but it has to forage for it in the atmosphere. But a substance like gunpowder, for instance, packs its own oxygen. When ignited, the fuel molecules recombine with oxygen, just as they do with fire--only much more quickly since the oxygen is right there, more available. Instead of the sizzle of a flame, when a gun fires, you hear a bang. That bang is the calling card of an explosion...and it's caused...by a shock wave.
Above: Watch the briefcase bomb explode at 3300 frames per second.
Speed up the reaction even further, however, and the shock wave becomes incredibly destructive. Gunpowder is a relatively slow explosive and its destruction is fairly limited. That's why you can shoot a bullet out of a gun without destroying the barrel. But a high explosive like the ammonium nitrate fuel oil bomb in the car trunk can deliver mass calamity in an instant. Made of unstable molecules with incredible amounts of stored-up energy, these compounds react instantaneously, liberating all that energy in the form of a high-pressure wave traveling at supersonic speeds. According to scientists at New Mexico Tech, in just billionths of a second, a shock wave can produce pressure up to 500,000 times the earth's atmosphere, can travel as fast as six miles per second, and can heat the air to more than 9,000 degrees Fahrenheit.
With power like that, the shock wave pushes away everything in its path, from buildings to bodies--often without even breaking skin. This is where we can really see the role of physics. Think of how crushing a soda can displaces the liquid or air inside it. Our bodies are filled with air-containing organs (lungs, intestines, eardrums) that crush, distort, and tear under the sudden pressure of a blast. There may be no penetrating injury from debris or shrapnel, but victims may hemorrhage with massive internal bleeding.
A shock wave even displaces the air particles in the atmosphere. And this fact, the technician told us, generates the bomb's second lethal force. When the shock wave pushes away the air surrounding the detonation site, it creates a vacuum that sucks air particles and debris back towards the bomb site. When you see glass inside a structure after a bomb attack, the technician said, people tend to think the bomb detonated from outside. It's a common error. Often, the glass was just sucked back in by the vacuum created by the shock wave.
When we inspected the briefcase bomb site after detonation--the charred wooden half-torso sitting in front of the remnants of a desk in a modular building--sure enough, there was glass inside the structure from the windows shattered in the blast. An hour later, after detonating the car packed with explosives parked in front of the structure, the entire building lay in pieces on the ground. Standing miles away in a bunker, we felt a grand rush of adrenaline, followed by a mix of horror both at the destruction and our own fascination with it. These were some of the most devastating--and exciting--reactions chemistry had to offer, and as horrified as we were, we wanted to slow them down and watch them again. Not just to see the wreckage wrought by explosives, but to marvel at the invisible force that wrecked it all in the first place.