seeing the light

AvatarThe rare breed of comic book lovers/physics nerds will no doubt roll their eyes at me for this, but yesterday I was thinking about Superman's X-ray vision (like you do…) and realized the whole thing was a sham. It is a fictional comic, you'll say, but I guess I did get my hopes up a bit because "X-ray vision" sounds like it took inspiration from real science, as much as a comic book glazed in delicious fictional syrup can. But it skips over one important detail: if Superman hoped to see anything with X-ray vision he'd have to EMIT X-rays as well as be able to see them. We cannot see things with our lowly human eyes unless they are illuminated by a source of visible light; most often the sun (or the glow of a computer screen if you're living in a writers cave like me). The sun does not emit X-rays, so superman would have to have another source. Underwear detector BUSTED.

 This goes for the 1950's X-ray Glasses as well. I know these were meant as some sort of party gag, but they equally dropped the physics ball. X-rays pass through clothing but they also pass right through flesh. Did men in the 50's find skeletal structures attractive? More importantly, who's idea was it to turn X-rays into a vehicle for SUPER CREEPINESS? These things turn a guy from clean cut to creeptacular faster than you can say "This packaging is really degrading to women."

It's true though, that the Superman pic above suggests superman DID, in fact, strike his subjects with self-generated X-rays. But I didn't automatically assume this because such vision would also leave behind a suspicious trail of tumors and hair loss. If superman were real. Which he's not. I do know this.

 When Superman debuted in 1938, the public was generally familiar with X-rays (discovered around 1875) and understood that these invisible beams could pass through materials that visible light could not. They have a high enough frequency that they interact with matter, and interact they do. X-rays are a form of ionizing radiation (thanks Lee for the recent post) and as they pass through your flesh they release electrons that careen through your body and damage cells. The X-rays are only stopped by the density of your bones. So I guess if I were searching for a fictional means of looking through walls, X-rays would be a good first pick. You couldn't use radiowaves, which are another form of light, because they are so large they don't really collide with the atoms in most materials, and instead pass right through them harmlessly. There's infrared light, but that frequency doesn't penetrate much more than visible light.

But that was in 1938. Since Superman's entrance into the world, we've really jumped ahead in our understanding and manipulation of light. And if Superman had been created sometime after the year 2000 I guarantee he would have teravision instead. What's teravision? I'm so glad you asked. Because I really do want to help Superman get a useful and scientifically accurate super power. I really do. So lets explore, shall we?

 Terahertz refers to light with frequencies lower than visible light (and much lower than X-rays) somewhere around the microwave range. It's a magical portion of the EM spectrum that when finely tuned can pass through clothing, cardboard, wood, plastic and ceramics, but not metal or skin. BUT HOW?!

Think, for a moment, about the sort of things that block visible light: clothing, skin, bricks. Now, consider things that do not: glass, water, air. Density has a lot to do with this fact. Even in the clearest water, you cannot see as far as you can through air because the water is ultimately denser. But the glass in greenhouses tinted glass (lets say green) absorbs all visible light except green light frequencies, which it reflects and sends back out (which is why we see it) pass through it like clear glass. So there's more at work than just density. The atomic structure of a material makes it possible to absorb some frequencies of light and not others. Terahertz radiation passes through clothing and not skin because this specific range of radiation is absorbed by water, which of course makes up a good portion of your body. How convenient. By finely tuning the frequency, scientists can even determine the water density of tissue.

Scientists have only figured out how to generate these very specific frequencies in the past decade, so they're only just now becoming available for a wide array of applications. And as you can see from the almost-not-safe-for-work-image below, it is already rolling into an airport near you.

Thanks to a mad rush of technology, terahertz radiation is now available in compact, wholesale scanners that have many potential applications like, for example, making sure folks getting on planes aren't hiding plastic guns in their pants. The scanners are so good that they reveal what those fictional x-ray specs always wanted to: the curves, crevices and potential weapons beneath your clothes. While a politically correct pat-down can take a precious five minutes, a terahertz scanner can swing over a randomly selected traveler in less than thirty seconds. And the TSA reports that many people prefer being looked at naked by a stranger than touched by one. Did I mention terahertz do all this without the health threats of X-rays? Seems almost too good to be true.

 It probably is. Last year the TSA decided to adopt the machines, (which have a variety of names: "Fully Body Scans" "Whole Body Imaging" or "submillimeter imaging") in a handful of airports. But of course they had to put in place regulations to make sure the images weren't abused. The people who look at the images are in another room far away from the passengers. They cannot bring any recording devices into the room, the computers cannot save, print or send data, and the person's face is blurred on the screen. But really, somewhere, something is going to slip. Patrick J Kiger just wrote an awesome post on this topic over at his "Is This A Good Idea?" blog at Discovery.

The agency is juggling the decision to place them in more airports or not, while Congressman Jason Chaffetz, representing the third district of Utah, introduced legislation to have them outlawed. In regard to this issue he has maybe my favorite quote of the year on his website:

"No body needs to see my wife and kids naked to secure an airplane." -Congressman Jason Chaffetz.

You have a point, Chaffetz, but remember that just like every other safety precaution in the airport, there are some people who we may want to see naked…er…search in order to secure the plane for your wife and children. 


So while uses for security are probably the hottest issues being discussed right now, there are medical applications for terahertz technology as well. For example, because the light can pass through the top layer of your skin, doctors think they can use it to detect skin cancer, or even breast cancer. Apparently about 85% of cancers lie in the skin, but are too small to detect. So you know, Superman could help cure cancer and not cause it.

He would of course be on his own in figuring out how to generate and detect terahertz radiation using only his eyes. I don't think we're on the way to compact, glasses-sized devices of that kind. But you know, there's only so much I can do for the guy.

14 thoughts on “seeing the light”

  1. Of course those x-ray specs were a rip-off!
    And just try getting your money back from those people!
    Nice article. Though comic book lovers/physics nerds are not quite such a rare breed!

  2. But the glass in greenhouses absorbs all visible light except green light, which it reflects and sends back out (which is why we see it).
    There is a mistake here somewhere. Glass is transparent to all visible light, or else we wouldn’t see it as transparent, but as tinted. A band-gap filter absorbing all visible light except green looks purple! Maybe you meant the plants in the greenhouse, which absorb all light except green?

  3. Don’t forget that there are a variety of x-ray wavelengths, some of which are vastly more penetrating than others. Soft (long wavelength) x-rays are quite easily stopped by just about anything, while harder (shorter) wavelength x-rays can penetrate quite a many inches of dense materials.
    There’s also the issue of x-ray scattering/diffraction, which is used to measure crystals.
    As for the glass in greenhouses, early glass commonly had quite a bit of Iron as an impurity, which gives glass a green colour. Modern production techniques have mostly eliminated this effect, though.
    A more important aspect, though, for greenhouses, is that glass is mostly impervious to infrared radiation, thus helping to contain the heat produced by the incoming sunlight.

  4. Boris – thank you I realize I misspoke. Or rather, was thinking of my neighbors lean-in greenhouse that was covered in green sheeting to keep the heat in. In my mind I was thinking of green tinted glass. But I did mix up the absorption/reflection there. It gets confusing.
    Green tinted glass does appear green because it absorbs all frequencies in the visible spectrum except green, which passes through it like clear glass. This is the general principle behind any color. Of course, no tinted glass is perfect and some light in the other frequencies gets through.

  5. Boris,
    I have no idea what a band gap filter is – although presumably it relates to the band gaps in condensed matter physics and how transitions across the gaps(s) selectively absorb radiation.
    How does the purple appearance work? Surely looking at it from either side you only see light coming from the other direction (since no light is reflected – green is passed through and the rest is “absorbed”) and since only green gets through it should look green?
    So, perhaps the absorption you mention is just temporary and the radiation is then spontaneously emitted (otherwise it would get hot and radiate anyway)? In that case half of the non-green is reflected and half passed on? But since this applies to light falling on the thing from both directions, wouldn’t the net effect be zero? Or perhaps the spontaneous emission process uses different transitions to the absorption and so the emitted spectrum differs from the absorbed – but then where’s the purple (white – green) gone?
    Yours, confused…

  6. I guess we are talking about reflection after all. The photon from spontaneous emission might be random for one electron falling back down the gap, but with zillions of electrons involved at the same time, the probability amplitudes must combine/interfere to preferentially give reflection rather than transmission, but I can’t remember how this works on a photon basis. (Also why you get 180 degree phase change with reflection from light material to heavier…)

  7. Calla,
    I think you have again miss stated the situation when you said ” Green tinted glass does appear green because it absorbs all frequencies in the visible spectrum except green, which passes through it like clear glass. This is the general principle behind any color.” in the comment above. What you said applies to opaque objects.
    It seems to me that the situation for a tranlucent object like glass is just the opposite of what you state. All wavelengths are passing through EXCEPT green which is being scattered thus it is the color we see. And for opaque objects, the color we see is the complement of those wavelengths which are absorbed.
    I own tachyon glasses so I will see the future and I have seen the past. 🙂

  8. Superman has “heat vision” right? Why in the world would emitting x-rays be any less plausible for the Man of Steel? =)

  9. X-Ray Spex (and those by-mail Amazing Sea Monkeys) were invented by Harold von Braunhut. Because real life is queerer than we can imagine, von Braunhut, who was Jewish, was also a member and financial supporter of the Aryan Nations.

  10. I am working on an article which includes terahertz metamaterials. Of course this involves terahertz radiation. Your article is really well written and makes the concepts very clear. It is very informative. Also, I like your prose style. I wish I could use this article as a source, but guidelines don’t allow me to use blogs for sources, no matter how good or accurate. Well, here is the article that I have been writing for almost a month. I hope you don’t mind me providing you with a link:
    However, you have given me some good leads for useable sources. And again, your prose style is excellent. Good job. – Ti-30X
    Hey maybe you can give me your opinion on the article if you have time. There is a page for comments here:

  11. Ti-30x – this post actually needs some edits/clarifications that I am meaning to get around to. Until then you might want to hold off directing people this way; still, thank you for the great comments!

Comments are closed.