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Lasers in SF, Part IV: Can laser beams even bend like that?

February 20, 2012
Note: this post is the fourth in a series about lasers in science fiction that I’m writing in preparation for a talk at Madison NerdNite on February 22.  If you have any suggestions on improving these posts before I give the talk, feedback is much appreciated! The first post on the Death Star can be found here. ~J
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Last time, I started writing about the Death Star’s superlaser. One of the things that I think is particularly cool about this weapon is the way the eight beams come out, pause for a moment, and then join together to shoot a single super powerful burst of energy toward Alderaan. This gives the weapon a lot of visual impact, and makes it look way more impressive than if it just shot a single beam straight at the planet.

But this also seems like a pretty strange behavior for light. In my mind, light always travels in straight lines, and it can’t randomly pause for a moment before it keeps going. So is there any way to make a laser beam do this? Can laser beams actually bend like that?

If you remember your introductory physics course, you might be saying, “ Of course they can! Don’t you know about Snell’s law?” And yes, Snell’s law is a great place to start when talking about bending light, because it tells us how the direction of light changes as it moves from one material (say, air) to the next (say, glass).

Snell’s law says that if light hits the interface between two materials straight on, then its direction doesn’t change at all. But if light hits the interface from an angle, then its path bends. The steeper the angle – and the more the speed of light differs in the two materials – the more the path bends.

This principle is what makes lenses work, like the ones in your eyeglasses or in your digital camera. If light hits a curved surface, light hitting the center of the curve hits it straight on and goes through without bending. Light hitting farther away from the center of the curve hits it from an angle and gets deflected. This makes the different rays of light come together together at one point, at the focus of the lens.

So in theory, if there were a giant lens between the Death Star and Alderaan, it might be able to bend the beams together and focus them onto the planet.  However, this would take a massive lens, and I don’t know about you, but I don’t see a lens between the Death Star and the planet.

But as it turns out, that’s not the end of the story – there are other ways to bend light. Let’s dig a little deeper and see if we can help the Death Star out.

First, let’s consider thermal lensing.

In some materials, the speed of light changes as the material heats up or cools down. This changes the amount that the material bends light, and can lead to effects like mirages. Because laser beams can dump a lot of energy into a very small area, they can heat some materials up enough that they begin to behave like a lens and focus light.

This is a phenomenon called thermal lensing.  Thermal lensing is actually a big problem in laser systems, because you can start to heat up optics in the laser, which starts to focus the laser’s beams.  This starts to heat up the optics even more, which makes the beams focus even tighter, and so on and so forth.  If this process gets out of control, things heat up enough to damage critical parts of the laser. But the problem isn’t limited to laser optics – you can also observe thermal lensing in materials like water and air.

The important thing, though, is that all of these examples require having some sort of material to heat up. But in the vacuum of outer space, there isn’t anything to absorb the light, and there isn’t anything to heat. So thermal lensing probably isn’t a good way to explain the bending of the beams in the Death Star’s superlaser.

How about gravitational lensing?

A second way that it might be possible to bend light is by gravitational lensing. Einstein’s theory of general relativity says that gravity can distort space-time and cause light to follow curved paths. This isn’t quite the same as saying that gravity “pulls” on the photons in light the same way that it pulls on, say, a bowling ball, but the effect is similar.

This is kind of weird to think about, because it’s not an effect that we notice in everyday life. On the grand scheme of things, the earth doesn’t weigh very much, so it doesn’t bend the path of light enough for us to really notice it (though the earth does weigh enough that things like GPS have to correct for relativistic effects).

Much more massive things like galaxies or black holes, however, do have enough gravitational pull to noticeably deflect light, and can even focus it like a lens.  Gravitational lensing can cause “double images” of distant stars and galaxies to show up in telescope images when the light gets bent around a black hole or a galaxy somewhere along the way.

So if there were a massive black hole sitting between Alderaan and the Death Star, it might be able to focus the beams of light like we see in the movie. However, this black hole would have to be very small, or it would start exerting a noticeable gravitational pull on the Death Star and on Alderaan. Even a black hole a millimeter in diameter would have more mass (and exert more gravitational pull) than earth’s moon.

But a black hole this small would be unable to bend the superlaser’s beams the way they bend in the movie. This is because the amount that light’s path bends drops off very quickly as it passes farther from the black hole. If light passes more than a few centimeters away from the center of a 1 mm black hole, its path would be deflected by only a few degrees.

Yet if we look at the width of the superlaser’s beam relative to the size of the Death Star or of Alderaan, its diameter looks much larger than that. So, we find ourselves with a bit of a Catch-22: a small black hole wouldn’t bend the beams enough, but a large black hole would exert a noticeable gravitational pull on the Death Star and the planet. At any rate, it doesn’t seem like this is a very good option for bending the superlaser’s beams.

Could it be nonlinear optics?

Most of the time, when we send light into a material, we get out the “same” light that we put in. We get out the same color of light, and unless the material absorbs that color, we also get out the same amount of light.

There are some materials, however, in which this isn’t true. For example, in some materials, it is possible to put in two photons of one color of light, and get out one photon of another color of light (that has twice the energy). Materials that can do these kinds of processes are called “nonlinear” optical materials.

One of the weird things about adding two photons like this is that you not only add their energies, but you also add up their directions. So if you have two beams of light adding together in a nonlinear material, the “summed” beam comes out at a different angle than either of the input beams. In fact, it splits the difference!

This means that if we had the right material, we could add together 8 beams of light and get out one beam travelling right down the middle, just like we see with the Death Star.

However, there’s a catch: if we add together 8 beams of light, we have to add both their directions and their energies. So if we put in 8 beams of visible light, we should get out a beam of light that’s in the extreme ultraviolet – almost as energetic as x-rays! That clearly doesn’t match what happens in the movie, because in the movie, the incoming and outgoing beams are all the same color.

Even if we ignore this issue, though, there’s still a problem. When we do experiments with nonlinear optics in the lab, we use materials (usually special types of crystals) that we know behave in very nonlinear ways. If we don’t, then the process of adding photons together is very inefficient and we get very little of the “summed” light out. But again, there isn’t a crystal in front of the Death Star. Is space itself nonlinear enough to add the superlaser’s beams together?

As it turns out, the vacuum of space might have some nonlinear behaviors, but they get into the realm of really, really weird physics. It might, for example, be possible for extremely high power laser light to spontaneously create an electron and a positron, and then interact with these two particles in a nonlinear way.

Nobody’s ever actually seen this happening, though, and doing it would require an incredible amount of energy. For example, scientists estimate that you might be able to see nonlinearities in the vacuum if you had power densities higher than 10^24 W/cm^2.

If the Death Star has to put out 10^32 W to blow up Alderaan, and if its beam measures less than a tenth of a kilometer across, it might actually be able to achieve this power density. However, the efficiency of the nonlinear process would be so low that very little of the light would actually add together and get redirected toward Alderaan.

So unfortunately, it seems like none of these methods of bending light can really explain why the Death Star’s superlaser behaves the way it does. For now, I think we’ll have to put this one more in the realm of science fiction rather than science fact.

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3 Comments
  1. Awesome post. I have a question. You said, “you can also observe thermal lensing in materials like water and air.” This sounds cool. What are some examples of this???

  2. Hi Lee! Yeah, I didn’t go into detail on that, so thanks for asking.

    Most of the examples I know of where you actually get lensing behavior (i.e. focusing the light) in air or water require using lasers. For example, a student in one of the research groups I worked in as an undergrad was using thermal lensing effects to detect minute amounts of molecules in solution. The idea is that molecules absorb light from a laser and cause the solution to heat up. This very small heated part of the solution then starts to act like a lens, which changes the focusing of a second laser beam and changes how much light hits the detector. The more molecules in the solution, the more the solution heats up and the more it changes the amount of light hitting the detector.

    However, getting this sort of lensing/focusing behavior usually requires heating a very small bit of the water/air/what have you while the rest remains cold. This is easy to do with a laser, because it can deliver a lot of energy to a very small spot. But unfortunately, this sort of heating doesn’t happen a lot in everyday life.

    Instead, in everyday life you are more likely heat a large volume of your water (or air) at once, and you are likely to heat it unevenly (some parts heat up faster than others). You do see some parts of your sample deflect light more than others, but not necessarily in a nicely defined “lens”. I mentioned in the post that this is what leads to mirages, but it is also what causes the ripples in a pot of water or oil heating on the stove.

    BTW, I ran across some cool stuff while I was scouting around on the internet about this. For example, some researchers at UT Dallas used electrically-excited carbon nanotubes to rapidly heat air and deflect light to momentarily “cloak” an object. You might also be interested in Schlieren photography, which uses a related phenomenon (changes in the refractive index at different pressures) to image airflow around airplane wings, etc. Cool stuff!

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