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Lasers in SF, Part V: Can you really see a phaser beam when it’s fired?

March 4, 2012

Note: Thank you to those of you who asked questions and gave feedback as I prepared my NerdNite talk! The talk went well, and you can find a video of it here if you missed it. This post goes into detail on my third point from the presentation (the bit about Star Trek). The next two posts after this, however, will be online “exclusives” – new stuff even for those of you who have watched the talk. Enjoy! ~ J

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In my previous four posts, I’ve touched on James Bond and Star Wars, two of my favorite science fiction series. I don’t think I can really call myself a science fiction nerd, however, if I don’t also talk about Star Trek.

My memories of Star Trek go back about as far as I can remember. I remember lounging around on the beanbag in our TV room with my dad, watching whichever new episodes and reruns happened to be airing. I know we watched a lot of Voyager and Deep Space Nine, but in my opinion, nothing really beats Next Generation.

So since we’re on the topic of lasers, how about a clip of Picard and Riker using their phasers to blow an alien’s head to gooey smithereens?

There are two questions that come to mind when I watch this clip. First, can you really make a hand-held laser weapon that powerful? And second, can you really see a laser beam when it’s fired?

The answer to the first question is “no, not really.” There are two major reasons for this. First, we don’t yet have good enough batteries to pack this much power into a weapon you can carry around with you. At a minimum, you’d need to plug your phaser into a wall socket, and maybe lug around a huge power supply everywhere you go. Can you imagine Picard and Riker walking in on that scene, turning to the alien, and saying “hang on, can you wait a moment while we find somewhere to plug in our phasers so we can blow you up?” I don’t think so.

The other problem is that high-power lasers tend to be large. They need to have a lot of the material that amplifies the laser light (called a “gain medium”) to generate high-power beams. They also often generate a lot of waste heat, and so you need some sort of fans or cooling system to prevent the laser from heating up and destroying itself.

Both of these things add a lot of bulk. So, while lasers have been getting smaller and smaller, super-powerful handheld laser weapons are still a ways from reality.

Being able to see a laser beam when it’s fired, however, is most certainly in the realm of science fact. Here, for example, are two photos in which the path of a laser beam is clearly visible:

The photo on the left is one that I took in a dance club in Madison. On the right is a photo of the inside of one of the laser systems in my lab. In both pictures, you can clearly see the path of the laser beam.

But light usually travels in a straight line, right? If you aren’t standing directly in the light’s path, none of the light will hit your eyes and you won’t see anything but darkness.


(That little black triangle thing is supposed to represent your eye. None of the photons reach it.)

So if none of the photons reach your eyes, how can you see the lasers in the dance club and in the lab?

The key here is that light travels in a straight line until it hits something.  Then it can either bounce off or be absorbed. So, if there are things like dust particles in the laser’s path, then some of the photons can scatter off of these particles and reach your eye. If enough photons bounce toward your eye as the light travels forward, then you see the entire path of the beam.


(Now, some of the photons are bouncing off particles in the air and reaching your eye.)

This is what’s happening in the photo at the dance club. There’s some sort of artificial smoke in the air over the dance floor, and the laser beam scatters off of the particles in the smoke. This makes it easy to see the laser’s path.

In the photo I took in my lab, however, there’s no artificial smoke. Instead, the laser light is actually scattering off of individual molecules in the air!

This is a phenomenon called Rayleigh Scattering. Light consists of oscillating electric and magnetic fields, and these fields can push around the electrons in the molecules in the air. Once the electrons start moving, they create their own oscillating electric fields and emit light. This light doesn’t need to have exactly the same direction as the incoming light, so the result is that the laser beam is scattered away from its original direction.

It’s cool that Rayleigh scattering lets us see laser beams, but it’s also cool for another reason: Rayleigh scattering is one of the main reasons that the sky appears blue.

Higher-energy photons (like blue light) scatter off of air molecules better than lower-energy photons (like red light). So as sunlight travels through the atmosphere, more of the blue light scatters toward your eyes, while more of the red light just keeps going.  At sunset, when the sun’s light has to travel through more of the atmosphere to reach your eyes, most of the blue light scatters away before it gets to you, and you just see the leftover red.

Rayleigh scattering isn’t the only reason that the sky is blue, because the sun also doesn’t put out equal amounts of blue light and red light, and our eyes aren’t equally sensitive to all of these colors. However, Rayleigh scattering certainly plays a large role.

At any rate, if we can see relatively low-power lasers like these, then we certainly ought to be able to see phasers set to “kill.” The more photons that the laser puts out, the more chances there are for some of these photons to scatter toward your eyes.

But here’s an interesting caveat: you may be able to see a phaser beam when it’s fired, but only if you’re on a planet. If you’re in outer space, things are a lot different.

Remember, we said that in order to see a laser beam as it travels through the air, there has to be stuff for it to scatter off of. On earth, our atmosphere contains roughly 10^24 molecules in every cubic meter of air, so there are plenty of things for light to scatter from.

In outer space, however, there are many, many fewer particles. In the space between planets in a solar system, there might be ten million (10^7) atoms in every cubic meter. In the interstellar space between stars, this drops by a factor of 10. And in the no-man’s land between galaxies, there can be as few as 5 or 10 protons in a cubic meter.

So a million particles in a cubic meter sounds like a lot, but even in the best case, this number of particles is about 10^17 (or a hundred million billion) times smaller than in a planet’s atmosphere. That means that there are 10^17 times fewer particles for light to scatter off of! If you had a powerful enough laser, you might still get enough photons to scatter that you could see the beam, but it would take a laser nearly a trillion times more powerful than the best ones we have today.

And that’s not even all. The “stuff” in outer space is usually hydrogen atoms or bare protons. These don’t scatter light as well as the larger molecules in a planetary atmosphere. A hydrogen atom might scatter ten times less light than a nitrogen molecule. And a proton – a hydrogen atom stripped of its only electron – is about a million times worse.

So if you ever hear someone say “you can’t see a laser beam fired in space because there’s nothing there for it to scatter off of,” they’re essentially correct, but they’re glossing over a ton of interesting science. There actually is stuff in outer space – but there’s not much of it, and what is there doesn’t scatter light very well.

With that in mind, the video I posted at the top of this post might be okay. We probably could see a phaser beam fired in a planet’s atmosphere. But this?

Not so much. You probably can’t see a phaser fired in outer space.

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