In Superman Returns, the latest incarnation of the Man of Steel uses his "x-ray vision" to look through walls at his beloved Lois Lane. Is this really possible?
I work in a national facility dedicated to producing high-flux x-rays. These x-ray beams are used in a variety of experiments, some of which include shooting x-rays at a sample to "see" through it. In this article I'll try to explain what is and isn't possible with x-rays and the current scientific uses of x-rays, and in the process I'll examine the feasibility of Superman's x-ray vision.
Why did Superman's original writers choose "x-ray vision" as one of their superhero's powers? My guess is that x-rays were considered exotic and powerful during the late 1930s (when Superman first appeared), and the writers were inspired by the ability of the doctor's x-ray photograph to see though the soft tissue of the body to the teeth and bones. Today we are familiar with the x-ray detectors used in airports, so x-rays are no longer considered exotic, and the possibility of x-ray vision does not seem so far-fetched. (Unlike, say, the ability to fly with a red cape.)
The discovery of x-rays is attributed to Wilhelm Roentgen, who produced them quite by accident in 1895 while working with electrical discharges in vacuum tubes. After noticing that objects placed in the path of the x-rays showed some amount of transparency as seen by the images on a photographic plate, Roentgen shot his poor wife's hand with an unhealthy dose of x-rays, capturing that famous first x-ray photograph of the bones of a human hand .
X-rays are more similar to visible light than one might think. In fact, x-rays, visible light, radio waves, microwaves, and gamma rays are all very similar. They are all composed of electromagnetic fields—that is to say, a combination of both electric and magnetic fields—traveling through a medium (or through vacuum). They can be thought of as waves in the same way that you think of waves on the ocean . The key difference between all these different manifestations of light is the wavelength, or the distance between the tips of the waves.
The wavelength of radio waves is on the order of tens of meters. Change that wavelength to centimeters and you've got microwaves. Go all the way down to hundreds of nanometers (nine orders of magnitude smaller than a meter) and you have visible light. Decrease that a couple more orders of magnitude and you have x-rays. But they are all traveling electric and magnetic fields; they are all fundamentally the same "stuff" .
Some Problems with X-ray Vision
First, x-rays happen to be at just the right frequency to a) get through your skin, and b) break the oxygen-hydrogen bonds in water and produce radicals in your body. Radicals in turn wreak havoc on your DNA. The result? Cell growth out of control (i.e. cancer). Sorry, Superman! Maybe using x-rays to see through the clothing of your beloved isn't such a great idea. The x-rays used at the dentist and doctor's office are used very sparingly, and even so it still isn't a good idea to get too much exposure. There's a good reason they drape that lead apron over you and then leave the room while you are being irradiated.
Next, there are problems with reflection and detection. In most applications utilizing x-rays, such as the baggage screening machines at the airport, the x-rays pass through a sample and are detected on the other side by a specialized camera or photographic plate. The x-rays are absorbed to a greater or lesser extent depending on the nature of the material. In the case of the airport baggage screener, the resulting image is then digitally displayed on a screen for the surly airport security worker to view.
So how does Superman view an x-ray image? In the classic comic book scene, Superman shoots x-rays from his eyes at the wall, and then the wall sort-of dissolves and he sees what's happening behind it. But since the wall hasn't really dissolved, how exactly is he "seeing"? The underlying assumption is that the x-rays he shot through the wall are reflected off of the people in the building and head back to his eyes. We then have to assume that his eyes can not only produce x-rays, but can also detect x-rays. I'm willing to give him that—after all, his eyes can stop bullets.
The fundamental problem, however, is that x-rays are incredibly hard to reflect off of surfaces. They tend to just go all the way through. There is an entire field of study devoted to developing x-ray reflecting surfaces . It turns out if you hit the surface of certain materials at extremely glancing angles, you can get a reasonable amount of reflection. But at any angle that does more than skim the surface, reflection is highly improbable. So even if Superman could detect x-rays with his eyes, there's just no way that the x-rays are ever going to pass through the wall, bounce off of Lois and her new boyfriend, then pass back through the wall to reach Superman's eyes. He'll shoot his x-rays out at the wall, and they will just keep going. Some amount will get absorbed or scatter off of Lois and her boyfriend, but none will be reflected directly back to Superman.
X-rays like to just go through stuff rather than reflecting back to their source. Therefore, I see two possibilities for how Superman might get an x-ray image of Lois inside her apartment. Maybe Superman has secretly placed a giant photographic plate on the opposite side of the building to detect the x-rays that emerge out the other side. Then he can develop those images later to find out what Lois was up to. Or, we can reverse the locations of the source and the detector—maybe Superman has secretly placed an x-ray source on the opposite of the building. In this case, the source would shoot x-rays into the building, where they would get partially absorbed by Lois and the walls before continuing on into Superman's x-ray detector eyes.
Barring a lot of imagination entailing secretly placed x-ray sources or detectors, we have to conclude that Superman really can't see into Lois's apartment, and that the way x-ray vision is presented in the recent movie is just not possible.
But what about x-ray scatter? X-rays don't reflect well, but they can scatter. It is possible that Superman could use the phenomenon known as the Compton effect. In this effect, x-rays scatter in all directions off an object, losing energy in the process. Technically, the wavelength of the x-rays changes in the process. Also, you can't let the x-rays can't get very far into an object, otherwise they will be absorbed or transmitted.
This technology is actually being developed, especially for use in screening people at the airport . The x-rays only penetrate about a millimeter through clothing or skin before being scattered back and detected. The result is an image of the surface of the person. Of course, in this case, the dose has to be kept low and the wavelength of the x-rays adjusted to the least damaging levels for humans.
So Superman, floating in front of Lois Lane's house, attempts to use Compton scattered x-rays to see Lois, and instead sees through several layers of paint to the very interesting surface of the fiber cement that clads Lois's house. Okay, maybe not so interesting after all.
X-rays in the Real World
What are x-rays really used for? Besides airport security, the dentist, and the doctor, scientists make good use of x-rays in a variety of ways.
The same thing that makes x-rays dangerous also makes them extremely useful for studying materials. They can penetrate inside a sample, exciting certain reactions or scattering off of electron clouds. The wavelength of x-rays is the same order of magnitude as the distance between atoms, making them ideal for determining molecular structure.
X-rays are so useful, in fact, that several national facilities have been built specifically for the purpose of producing x-rays . Originally the offshoots of particle-smashing cyclotrons, these "synchrotron" facilities are now dedicated solely to x-ray experiments. They are large circular machines—the one in Berkeley, CA, for example, has a 200-meter-circumference ring. X-rays are produced tangent to the ring, and are captured by "beampipes." The synchrotron in Chicago, one of the world's largest, supports about 55 beampipes, each dedicated to a different type of experiment.
The difference between the x-ray machine in your dentist's office and the synchrotron comes down to brightness and focus. The dentist's machine spews x-rays fairly indiscriminately. It's only somewhat focused. It isn't anywhere near the flux (photons per second) of one of these synchrotron sources. That means it gives you a pretty good image of your gross anatomy: it can offer resolution down to about a millimeter, and you can see the difference between dense material and not-so-dense material (i.e. bone vs gums).
With a synchrotron x-ray source you can see features a thousand times smaller than that. And you can tune the x-rays to look at particular materials. It's used to find where the proteins in a cell are localized , or where a plant sequesters zinc compounds in its leaves. X-rays are used in contaminant analysis for detecting parts-per-million metal contaminants in soil, water, or air samples. X-rays can be tuned not just for locating specific elements, but for finding certain electronic transitions. For example, x-rays are used to determine how hydrogen might be stored within carbon nanotubes by looking at the subtleties of the carbon-hydrogen bond in the material . They can be used to see tiny areas of magnetic or ferromagnetic material, as in studies of electronic storage devices. In a recent exciting experiment, x-rays were used to show that the spin and charge of an electron can be separated.
One of the most widely known uses of x-rays is in molecular structure determination. Pharmaceutical companies use the technique of "x-ray crystallography" to determine a protein structure in order to better design drugs to target that protein.
In all these examples, x-rays are used to "see" materials at a very small scale.
X-rays are also used to study objects on a very large scale—objects such as individual stars, entire galaxies, and strange phenomena such as black holes. In the field of x-ray astronomy, x-rays that have been emitted by astronomical objects are collected by detectors in Earth's orbit. (Most x-rays emitted by these objects are in an energy range that is easily absorbed by air, so they would never make it through the Earth's atmosphere.) The resulting x-ray images give information on the location, type and characteristics (such as temperature) of the material producing the x-rays, giving a map of the sky that is impossible to see any other way.
Superman's x-ray vision is not very realistic in the way it is presented in the movie and the comics, yet it's not very far from what is actually possible with x-rays. Superman really only needs either an x-ray detector or an x-ray source on the other side of the building. And if his eyes are sensitive to x-rays as well as visible light, then he need only fly above the earth to see the x-ray map of the universe.
Will there come a day when powerful x-rays can be produced in small packages—say, for example, eyeball size? "Desktop" synchrotrons have already been developed, and are just starting to be used . Looking forward, it is conceivable that someday these machines will become even smaller, easy to use, and ubiquitous. Maybe everyone will have a mini synchrotron in their kitchen next to their microwave. Need an image of that splinter under your skin so you can better remove it? Want to see the magnetic pattern on that DVD to see where the tiny scratch is?
The day may yet come when we all have access to "x-ray vision."
 Roentgen's famous first x-ray photograph can be found here.
It's also possible to produce your own home-built x-ray source and x-ray photographs.
 Electromagnetic radiation can also be thought of as "particles of light," or "photons." Depending on the type of experiment, scientists will use either the particle model of light or the wave model of light, both of which are approximations of the true nature of light. More on "wave-particle duality" can be found here.
 A nice picture of the electromagnetic spectrum, from gamma rays, to radio can be found here.
 The Center for X-ray Optics at Lawrence Berkeley Laboratory is a facility devoted to this study.
 See, for example: "Backscatter" technology and the Rapiscan Secure 1000.
 lightsources.org has a list of synchrotrons world wide.
In the United States, there are four nationally funded x-ray facilities: Advanced Light Source (Berkeley, California), Advanced Photon Source (Argonne, Illinois), National Synchrotron Light Source (Islip, New York), and Stanford Synchrotron Radiation Lab (Menlo Park, CA).
 The Center for X-ray Tomography, named after a technique used for whole-cell imaging, is based at the Berkeley synchrotron.
 The Advanced Light Source maintains a page on this and many other specific applications for x-rays.
 One company developing these "table-top" synchrotrons is Lyncean Technologies. The first one is currently being installed at the Scripps Research Institute.