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If humanity is good at one thing, it's imagining ways that aliens might show up and wipe us off the face of the Earth. Sure, we've created exceptions like the Vulcans, who arrived to welcome us into a larger galactic society and help us evolve into a better version of ourselves, but they are far from the norm. By and large, when aliens show up, they're bringing hellfire and it's going to take a highly trained group of elite soldiers or a ragtag group of underdogs to save us.
Independence Day had massive weapons capable of disintegrating entire cities, Invasion of the Body Snatchers and The Thing had doppelgangers that used our own fears against us, and Mars Attacks! had ray guns which reduced a person down to their technicolor skeleton.
There's a lot about Mars Attacks!, which turns 25 years old this week, that we could pick apart. To be fair, it's not even pretending to be realistic. Still, those ray guns prickle at the imagination and demand examination. How powerful would they have to be to turn a person into a skeleton, and would they even be safe to use?
SETTING PHASERS TO DISINTEGRATOIN
If you want to utterly destroy an object, or a person, you're going to need energy. Every weapon you can think of, from a palmed rock to a nuclear blast ultimately comes down to delivering energy to a target in a way which creates a catastrophic event. In terms of ray guns, there are basically two types: those which break the bonds in the atoms of an object, and those which heat it until it becomes gaseous. To the outside observer those two end states might look similar. In both cases the target vanishes but heating something to a gaseous state requires a lot less energy than breaking it up at an atomic level.
In the case of Mars Attacks, we have couple hints as to what sorts of ray guns we're working with. First, by looking at the ray beam coming off their weapons, it's clear they're using the same technology in their flying saucers as they are in their handheld guns. In one scene, a laser nails a fighter jet, causing it to explode in a fiery inferno. The jet doesn't vanish, it just explodes, suggesting their weapons aren't breaking atomic bonds. The other clue is the skeletons. Taking something apart atom by atom would leave you with nothing but vaporizing its more liquid bits might leave the bones behind, discolored as they may be.
Luckily for us — or perhaps luckily for the Martians and unluckily for us — humans are mostly water, and we know the energy needed to turn water into a vapor.
Turning a substance like water from a solid to a liquid is known as a phase change, and that's essentially what we're doing. We're adding energy to the water in the body until it undergoes a phase change and becomes a gas. We know that water boils at 100 degrees Celsius (212 Fahrenheit) but getting it to become a vapor requires a little more energy. It takes about 540 calories to convert one gram of liquid water into a vapor. It's not a lot of energy, about what you'd get out of two and a half candy bars. But the human body has a lot more than a gram of water in it.
The average body mass globally (they are taking over the whole planet after all) is 136 pounds, or 62 kilograms. On average, the human body is about 60 percent water, meaning that each of us holds about 37 kilograms of water inside us, give or take. That's 37,000 grams, which is, technically speaking, a lot of candy bars. In order to vaporize all that water, you'd need to hit the body with just shy of 20 million calories of energy. Lasers aren't typically measured in calories, however, so we'll have to convert it.
Twenty million calories of energy is about the same as 83 billion joules. One of the most powerful lasers in the world, the NIF at Lawrence Livermore, pumps out two million joules of energy in a nanosecond (one-billionth of a second).
We'd need about 42,000 of those pulses to vaporize a person, but that would happen in a fraction of a second, before you'd even notice. The only downside is, that laser requires infrastructure about 300 yards long, which makes it difficult to hold in the hand. Supposing you could pack the 192 individual laser beams from the NIF laser into a handheld device, and supposing you had enough juice to power it, you might not want to use it as a weapon anyway. The whole purpose of that laser is to generate heat. A lot of it.
The NIF is used in fusion experiments — you know, that thing that stars do — and even though each of its pulses only last a small fraction of a second, it heats its target to more than 3 million degrees in an instant. If you've ever accidentally placed your hand over a pressure cooker, you know the sorts of heat generated.
Now imagine tens of kilos of water all transforming to a vapor inside of a sealed container in a fraction of a second. You'll definitely eliminate your target, but you're just as likely to take yourself out in the process. That invasion should have been over before it started without needing to involve Tom Jones.
There must be easier ways to take over a planet. A fraction of the energy would get the job done. Though, admittedly, it's not as flashy. When you've got a head like a walnut in a fishbowl and a vocabulary which consists of only one syllable, I guess you have to make up for it with theatrics.