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Lasers in SF, Part VI: What about those pulsed weapons?

April 2, 2012

Well, I seem to have gotten a bit behind with this one. But I promise I haven’t forgotten. So in the interest of getting right to the good stuff, how about a clip from Austin Powers? I hope you like sharks with frickin’ laser beams attached to their frickin’ heads:

These are some pretty bad-ass sharks. The lasers on their heads are pretty small, but still unleash enough energy to create a nice fireball when they hit.(*)

Any time I see a laser weapon fired like this, though, I get a little tetchy because the physics doesn’t quite add up. Notice how there’s a fraction of a second delay between when the laser fires and when it hits its target? The pulse travels fast, but we still get to “watch” it travel from the weapon to the wall.

I don’t know about you, but when I walk into a room and flip the light switch, the lights look like they come on instantly.(+) I don’t see a noticeable lag as I wait for the light to mosey its way over from the bulb to my eyes. Light doesn’t mosey, it races.

So today’s question is, how far off are the movies? Do laser beams actually travel that slow?

To answer this question, we need to know a little bit about the speed of light. Light travels about 300 million meters per second, or nearly 700 million miles per hour. Those numbers are kind of hard to think about, but the point is that light travels fast. Really, really fast.

To put these speeds in perspective, consider this: if you take the freeway to work and drive 60 miles per hour, you’re going only 27 meters per second. That’s 11 million times slower than the speed of light.

Passenger airplanes travel about 10 times faster than a car on a freeway, and the fastest jets in the world are another 10 times faster. But even at these tremendous speeds, our fastest planes are still more than 100 thousand times slower than light.

Sometimes it’s easier to think about the speed of light in terms of distances. We might ask, for example, how long it takes light to travel from the sun to the earth. As it turns out, it takes about eight minutes. Or, if you prefer thinking about shorter distances, light only takes about 80 trillionths of a second (80 picoseconds) to travel across the diameter of a quarter.

We might also turn this question on its head and ask how far light can light go in a particular amount of time. Light travels nearly a billion feet every second, so in a billionth of a second (a nanosecond), it travels just one foot. And in a trillionth of a second (a picosecond), light travels just under a third of a millimeter.

If you’re into science fiction, you might have some experience thinking about the speed of light and how time converts into equivalent distances. You’re probably familiar with the term “light-year,” which describes how far light travels in a full year.

A light-year is a vast distance – equal to about six trillion miles. But even so, it’s tiny on an astronomical scale. The star closest to our solar system, Proxima Centauri, is still more than four light-years away.

So, now that we know something about how fast light travels, what can we say about the sharks with frickin’ laser beams attached to their frickin’ heads?

It looks like the laser pulse travels maybe 50 or 100 feet from the shark’s head to the wall. This means that it should only take a tenth of a millionth of a second for the pulse to reach its target. Yet in the movie clip, it looks like the pulse takes at least a tenth of a full second to get there. So someone in special effects clearly fudged the physics, and made the light move a lot slower than it really does.

The upshot of all of this is that we really shouldn’t be able to see the laser pulse travel from the shark to the wall. Instead, it should look pretty much instantaneous, like this:

Admittedly, in movies, it looks a lot cooler when you can see the laser pulse travel. So, is there any way to watch this happen in real life, or to film it and replay it at a speed we can handle?

If we’re just watching the laser pulse with our eyes, we probably can’t. Humans can only perceive about 10 to 15 separate images a second.(^) Any faster than that, and things start to blur together. So if we fire a laser pulse and it takes a millionth of a second to travel to its target, we might see one long blur along its path, but we won’t actually perceive motion as the light travels.

Do things fare any better if we try to catch the laser pulse on film? Unfortunately, not really. Movies are usually filmed at about 24 images (frames) per second, which is slightly faster than we can perceive individual images. This helps us see continuous motion when we watch the film.

However, in a 24th of a second, light still travels almost 8000 miles. So, filming at 24 frames per second still isn’t fast enough to watch a laser pulse move over a distance of a few hundred feet.  In order to watch a laser pulse travel over a short distance, you need a camera that can capture images much, much faster than 24 times per second.

This is really difficult to do – but, as a team of scientists at MIT showed last year, it’s not impossible. The group at MIT developed a camera that can make movies of light bouncing around with trillionth-of-a-second time resolution.

Remember that we said that light travels less than a third of a millimeter in a trillionth of a second. That means that with a trillion-frame-per-second camera, we can watch light travel in slow-mo over distances of just a few inches.

Here’s a video to demonstrate. In this video, the researchers shot a laser pulse into a coke bottle and made a movie of its travels:

However, you’re not going to be able to use this camera to watch a sci-fi laser battle anytime soon. That’s because this camera doesn’t work quite like a normal video camera.

A normal video camera takes in an entire image at once. The image is focused onto a digital sensor (or a piece of film, if you’re old-school). Then the camera reads and records the entire image before it takes the next one.

It’s easy to make a camera that can capture a complete image and read it out ten or even a hundred times a second. However, there’s absolutely no way to do this at a trillion images a second. The electronics just aren’t fast enough.

To capture a video at a trillion frames per second, the researchers at MIT used something called a “streak camera.” A streak camera watches a single slice of the image, uses the incoming photons to create free electrons, and rapidly deflects these electrons so that light arriving at different times is recorded on different parts of the camera sensor. This lets you get trillionth-of-a-second time-resolution, but only lets you see a very small slice of the video.

If, however, you send the laser pulse into the coke bottle again and again and again, and you watch a different slice of the video each time, you can make a video of the entire process at a trillion-frame-per-second rate.

In order to get a good video, the laser pulse has to behave exactly the same every time you shoot it into the coke bottle. That’s easy to do in the lab, but out on a sci-fi battlefield, it’s not going to happen. To make a video of a sci-fi battle and be able to watch the laser pulses, you’d have to get everyone to repeat the battle over and over and over again – and do it exactly the same every time.

All in all, it’s probably just easier to add the laser pulses in using special effects 🙂


(*) Wondering where the sharks are storing the power packs for these lasers? Maybe they have implanted fuel cells like these cockroaches!

(+) YMMV if you’re using compact fluorescents. Sometimes they have a noticeable lag time when you turn them on, though that has way more to do with the way the bulbs work than with the speed of light.

(^) Though we can perceive flashes of light – but not full, individual images – that go on and off at about three times this rate.


From → Cool Stuff

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