Light bullets are not the deadly things their names make them out to be. They are, however, decidedly weird. To make a light bullet, researchers have to make a pulse of laser light that is continuously re-focusing itself.

The Solitary Wave

When you turn on a flashlight, or even a laser beam, and shine it across the room, what happens to the beam as it travels? It fans out. The tendency of light to spread outwards is helpful when it comes to flashlights, not helpful when it comes to lasers, and fatal when it comes to light bullets. Light bullets, ideally, are pulses of light that keep their coherence, moving as one little packet no matter how far they go.

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Light bullets are more formally called spatiotemporal solitons. Solitons are an old concept, first described in the 1800s by engineer John Scott Russell. As he was walking by Union Canal in Edinburgh, he saw a boat move along a canal, then stop. The water underneath the front of the boat formed a single, rounded peak of water. This single wave continued along the canal, staying together, only slightly diminishing with time. Russell chased it on horseback for over a mile until the canal system got more complicated and he lost it.

Light bullets should be like that soliton wave. A spatiotemporal soliton should be a single lump of light that doesn't spread to the side or stretch out as it moves along.

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The Prism and the Light Bullet

This is not easy to create. To understand why, look at how light behaves as it hits a prism. We all know that light changes direction when it hits a glass prism. We all know that it separates out into its different colors. Let's look at the reason why.

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Light moves at different speeds through different media. It moves faster in air than it does in glass, so light changes direction when it hits glass for the same reason you change direction when you are walking and someone tugs slightly on your right elbow. When one side of you slows down and the other keeps going, you swing around and change direction. Light separates out into colors because, in a medium like glass or air or water, different wavelengths of light have different speeds, and so the "tugs" and the subsequent direction change is more radical for some wavelengths than for others.

The fact that different wavelengths of light have different speeds means that, even if the light didn't hit the prism, it would eventually lose coherence. Some wavelengths of light would be traveling slightly faster than others. What's more, though we don't see it in prisms, different intensities of light have different speeds as well. The more intense the light, the slower it goes. Again, this causes light to lose coherence.

Using Variable Speed to Build Coherence

Physicists build light bullets by using these different speeds to their advantage. To understand why, consider the intensity of a single pulse of light. If, at the center, the light is very bright and at the edges the light is very dim, it will be slower in the center than at the edges. That sounds like a recipe for decomposition, but remember that the edges and the middle affect each other. If you were to see a dim (and very thin) laser beam, and you shone a light along only the right side of it, you'd increase the intensity of light on the right side of the beam and slow that side of the beam down, which would bend the beam of light to the right. If you shone a light along the bottom of the beam, it would bend the beam down. If you shone it along the top, it would bend the beam up. And if you shone a light right along the middle of the beam, the edges would always be bending towards the middle.

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That's what the light bullet does. If the intensity is always highest in the middle of the beam, and gradually decreasing towards the outsides of the thing, the faster, outer edges of the beam will always be pulled held back by, and pulled towards, the center of the beam. The laser will re-focus itself. The same trick can be done with colors. Slow colors to the front, fast in back, and the bullet will pull together as it flies.

The deceptively variable speed of light becomes a way to make a light constantly re-focus and cohere. Although light bullets aren't an easy phenomenon to create, who knows? Maybe one day we'll have light blasters. Sure, they'll be tiny, they'll be harmless to nearly everything, and they'll be used by doctors for medical procedures instead of guards for security procedures, but a blaster is a blaster.

Soliton Image: Christophe.Finot et Kamal Hammani

[Sources: Spaciotemporal Pulse-Train Solitons, Light Bullet Home Page, Focus: Bullets of Light.]

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