10-17-2003

10-17-2003 You always see people dodging laser beams in futuristic sci-fi movies, but in the interests of science, I think it should be made clear that those aren't really laser beams they're dodging.

Think about it for a second. A laser beam, which may or may not have enough energy to actually kill you by making you explode (as opposed to slicing you in half, slowly searing through you, frying your optic nerves, giving you a slight suntan, or just making a red dot appear on your forehead) is made of light. It travels, surprisingly enough, at the speed of light. In fact, it can't travel any slower. So, by the time the laser beam gets to you, or even to your general area, it will be too late.

How will you even know when and which way to dodge? Let's suppose, just for a second, that you have really good vision, and that you have an insanely quick wetware processor speed for your brain (that means your reaction time is just above zero seconds (and by just above, I mean about 1 x 10^-20 seconds (that's pretty fast))). Also, you're really talented at dodging things like nerf balls, so maybe you can actually dodge a laser beam.

Well, to dodge the right way, you'll have to know which direction the laser beam is coming. Here's the bad news: you won't know until the laser beam hits you. You see, the light which reflects off the shooter's trigger finger at the moment of shooting travels to your eye at the exact same speed that the laser travels to you. They're both light, you see. So, the information you need to dodge the laser beam doesn't even reach you until the beam does. And actually, since the beam has a six-inch (for a laser pistol, 18-inch for a laser rifle) head start on the trigger-finger light, you'll probably get hit even before you know it. Literally.

But what about anticipating it? Maybe you see the trigger finger start to bend, and can guess exactly where the beam will go. But light has a speed of about 186,000 miles per second, which is about 3E8 (that's 3 x 10^8) meters per second. If, let's say, you know by the time the trigger is half pulled, with a total pulling time of 0.2 seconds, you have an 0.1-second head start on the light. In this time, light can travel 30,000 kilometers, or about 18,600 miles.

That's a really long way. If you're at any kind of weapons range, say even the extremely long range of 1 mile, you'll have about 5.376E-6 seconds to get out of the way of your onrushing death. That's 5 and one-third microseconds.

And that isn't much time.

But you do your best to dodge it anyway. You have a reflex time of about zero.

[Consider what that means. Your eyes see the image, then your nerves fire until the signal hits your brain, and then immediately your motor nerves send another signal the opposite direction to your muscles, which causes you to get out of the way of the laser beam. Nerve signals are essentially waves of electrical discharge which propagate along your nerve cell axons, and also bursts of chemicals that jump from one nerve cell to the next one in line. For a zero reaction time, you would probably need to have the individual chemical atoms in the bursts, and the waves of discharge, travel very quickly. Since the total distance to be covered is appreciable, you would need to have both an ionic-diffusion rate and wave propagation rate well above lightspeed. This is impossible, and also impractical (most nervous impulses travel at a couple hundred miles an hour). Also, if just one nerve cell in your brain were to experience a diffusion rate or wave propagation rate of even lightspeed, your head would probably explode. Considering all the nerves involved in this whole process, an outside observer would see your head and most of your large muscle groups vaporize simultaneously. Actually, since the total energy of a moving object approaches infinity as the velocity approaches lightspeed, the entire world might well be incinerated in the explosion of your overdriven neurons. Since we've already established that the laser beam would hit you at the same time too, it would be pretty messy.]

Okay, so if we suspend the laws of physics within the confines of your body for a minute, what happens? Can you dodge the laser beam yet? Well, we've seen that you have about 5E-6 seconds to act. If the laser beam is aimed directly at your sternum, you'll have to travel approximately .28 meters to the left (or right, but left is luckiest) to dodge it. And even then you'll get your shirt singed.

So you have 5.376E-6 seconds left. To get 0.280 meters to the left in that time, you'll need a minimum acceleration of 19.4E9 meters per second squared. Even if you have an obviously superhuman nervous system, that acceleration is 1,975,151,893 gravities, and will probably rip you to shreds. Your muscle fibers will move sideways quick enough, but all the other tissue of your body will want to be left behind, and will be pulverized by the onrushing muscles from the other half of your body. It might be kind of funny to watch.

Okay, so dodging lasers is not realistic. That means, whenever you see a stormtrooper shooting at Luke Skywalker, and Luke narrowly ducking out of the way, you can safely decide that the evil stormtrooper isn't using a laser, but rather whatever the heck a blaster is (we don't know). Dodging bullets gives you the same problems as dodging laser beams, except that you're dealing with projectile speeds of only a few hundred meters per second, rather than 3E8 meters per second.

You see people dodging bullets in those old cheesy kung fu movies, but let me tell you, it's not very realistic either. Going through my above calculations, you'll see that a reaction time of about 0.167 seconds is okay for bullet-dodging, if you're talking about a pistol with a muzzle velocity of 600 meters per second (figuring a range of 100 meters). That's unlikely.

Dodging machine gun bullets is even stupider. I don't think math is necessary here. Heck, machine guns were actually designed with this exact situation in mind. Your best bet is to wait until the gunner has to reload.

But don't try this at home. Either you'll get shot, your head will explode, or the government will come and steal your cool death-ray laser.