The Future Is Here

# How Many Laser Pointers Would It Take to Kill a Human?

Everybody knows not to shine laser pointers in your eye. They could damage your vision! (Emphasis on could, since your eye’s blink reflex would probably protect your retina.) But what if you were an evil villain determined to make a death ray? How many laser pointers would it take to create a deadly weapon?

This sounds like a crazy question because it is. Laser pointers are intentionally designed to be safe for everyday use. It’s common knowledge that staring at a laser can be dangerous. But the question of how many laser pointers it would take to strike a fatal blow is inevitably rooted in the question of how powerful any given laser pointer is.

### A Quick Primer on Lasers

Before we dig into the grisly mystery of how to kill a human with a laser point (or a lot of them) let’s revisit a question from high school physics: What is a laser?

The word laser is actually an acronym for “light amplification by stimulated emission of radiation.” In plain English, a laser is a single beam of light, a bunch of photons moving in the same direction. (Normally, light sprays out in all directions) High-powered lasers deposit large amounts of energy on a focused point producing heat and, in extreme cases, vicious burns. The United States Navy, for instance, has a laser that’s powerful enough to blast a drone right out of the sky.

The strength of any laser—and therefore danger—is determined by the amount of power it outputs, typically measured in milliwatts. In the U.S., the Food and Drug Administration (FDA) requires all products to be 5 milliwatts or less, and some cheap laser pointers have an output power of less than 1 milliwatt. However, you can also Google “cheap laser pointer” and find a 5-milliwatt device for less than \$2. Since these laser pointers originate from far off foreign lands, they’re often missing the safety hazard stickers required by the FDA. Sometimes, they’re even missing a critical shield to protect dangerous infrared light from escaping the pointer.

The dangers of infrared light alone are hardly deadly at that level, though. Infrared light is most perilous because it’s invisible so your eye’s blink reflex won’t kick in before your retina starts to burn. Even still, the burn would damage your vision, but it wouldn’t kill you. You’d need a laser much more powerful than 5 milliwatts to do that.

Or would you?

### Now Let’s Get Down to Business

The crazy thing about lasers is that you can focus the beams of several low-powered lasers and create a single high-powered beam. This is no easy task, however. Multiple laser pointer beams would need to be perfectly focused by a lens that would send the combined energy in the direction of the target. It’s sort of like the “Don’t cross the beams” trick from Ghostbusters but much more scientific.

If—as our morbid goal requires—you wanted to kill a human, it would be easiest to shoot them right in the eye. A powerful enough laser could cut through the retina and burn through the person’s brain tissue. However, it would take a while, and you’d have to focus the laser on a specific point. So basically, you’d have to strap the person down or immobilize them in some fashion.

This grisly scene is hardly the death ray sort of solution you see in cartoons. The time factor is especially tricky given the simple physics of the situation. James Kakalios, a physics professor at the University of Minnesota and the author of The Physics of Superheroes, says that building a death ray out of lasers—and even a death ray in general—is pretty darn tough.

“In order to blast a hole in something, you have to deposit more energy in a short amount of time faster than the energy can be dissipated away,” Kakalios explains. “If you can do this, then you can overwhelm the bonds holding the material together, and the atoms will fly away. But if the excess energy from the lasers can be carried away as heat, then all you’ll have done is warm up the material.”

But what happens when you warm up brain matter? It melts. Considering the fact that we’re talking about cobbling together a high-powered laser out of several laser pointers, it would still probably take a decent amount of time to burn through someone’s brain.

Blasting brains with lasers is a burgeoning technique in the field of neuroscience. In small doses, beams of laser light can actually trigger brain activity and even cure pain. Tim Spellman, a postdoctoral fellow in neuroscience at Columbia, spends his days shooting lasers at mouse brains for the sake of medical research. However, he’s also acutely aware that the sustained presence of high-powered lasers could do significant damage to the brain.

“Basically you’re going to have to heat the tissue enough to combust fat, since the brain is mostly fat—that’s anywhere from 300 to 500 degrees,” Spellman said in an email. “But how fast that will happen in an actual human brain is something you won’t know until you try. But my guess is tens of seconds or even minutes.”

### So How Many Laser Pointers Would It Take?

So we’ve determined that laser pointers could be focused through a lens to create a more powerful beam given the right circumstances. We’ve also determined that a high-powered laser could burn through a persons eye and melt their brain given a long enough period of time. But just how big would this laser pointer rig have to be?

Pretty big, according to Rebecca Thompson of the American Physical Society. When we asked Thompson, who holds a Ph.D. in physics from the University of Texas, the morbid question about death and laser pointers, she crunched some numbers and double-checked her work. It’s worth quoting her response at length:

For a laser to kill someone it essentially needs to be hot enough to boil or burn the tissue. It would take about a 1kW laser to boil through someone’s eye to their brain in a reasonable amount of time, meaning they probably couldn’t get away. Each cat toy level laser pointer is roughly 5mW. It would take 200,000 laser pointers all focusing through a lens to an exact point to create a laser powerful enough to kill. Each laser pointer (assuming the ones from laser pens since the pen tip is smaller than the cat toy laser) is 5mm across. So if you had 200,000 of them they would be in a grid about 7’ x7’. To get them to focus on a single spot, though, they would need to be in a semi circle with a radius of about 5.5’. So if you wanted to create a death ray with laser pointers, you could buy 200,000 laser pointers, mount them on a piece of a sphere with radius 5.5’, aim them all through a lens and ask your victim to sit very, very still.