# Can Superman read books without opening them?

An acquaintance recently brought me a homework problem which asked roughly "True or false: If you had x-ray vision you could read a book without opening it." Phrased like this, I'm reasonably confident the answer is "False", but it's a bit complicated to analyze.

First, we have to ask what x-ray vision is. It's natural to think of it as being like ordinary vision, i.e., your eyes are sensitive to x-rays. This doesn't make any sense, though: ordinary vision works because there is ambient light in the visible spectrum (from the sun, indoor lights, etc.) and it reflects off ordinary objects. What you're seeing is that reflected light. But there's next to no ambient x-ray radiation in the ordinary environment, so even if you could detect x-rays, it would be like trying to see in the dark.

So, in order for some sort of x-ray vision to work, you'd presumably need to be emitting some sort of radiation to illuminate the subject. Assuming that the source is attached to you, then x-ray absorption/transmission measurements (like with medical or dental x-rays) won't work, but x-ray reflection or x-ray fluorescence (XRF) will. The other possibility is that you're emitting some other sort of radition that the subject absorbs and then fluoresces in the x-ray region (really, it could be in any region, but remember we're calling it x-ray vision, not hard radiation fluorescing in the visible vision).

There are actually at least two aspects to this question:

1. Can you use x-rays to distinguish ink from paper?
2. Can you resolve the letters on individual pages?

I suspect that the answer to (1) is "Yes". Older inks, at least, often contained metals (e.g., iron gall ink), and so should have fairly different spectra from paper. In fact, this article describes the use of just such a technique: XRF to find iron gall inks in the Archimedes Palimpsest. It's less clear how modern inks would respond. I've seen some articles suggesting that x-ray fluorescence spectra of modern inks also are distinguishable from the background paper spectrum (see this article on cyan ink and this paper that mentions the use of XRF on postmark inks).

This only answers half the question, because books aren't just one page: they're a bunch of pages on top of each other and so even if the ink does fluoresce differently from the paper, you still need to read the book. There are a bunch of possible problems here: first, if you're using XRF possible your fluorescence isn't in the x-ray, it may get blocked by the paper on top of it. But let's assume you're irradiating in the hard x-ray and your fluorescence is in the soft x-ray and makes it out of the paper; we still have to ask whether you can spatially resolve the letters. This partly depends on the characteristics of your x-ray source.

If it's an uncollimated point source like a lightbulb, then you'll irradiate the whole book and each letter will be an independent emissions source, each of which will also be radially symmetrical, plus all the background noise from each sheet of paper. It's not at all clear you can disambiguate these sources. If you have a collimated source, the situation may be a little better, since then you just get a bunch of point sources along the line the beam is taking through the book). That's probably still not enough though, since the fluorescence from multiple radially symmetrical point sources still overlap on your eyes, and you need to distinguish light which is coming from incredibly small angular displacements (the thickness of a piece of paper apart at a distance of 10s of centimeters away). If you were able to take measurements from multiple independent angles like in a CT scanner, then you would have enough information to resolve things, though with an extremely large amount of computation. There are also multiple beam techniques like four-wave mixing, but that also seems biologically unlikely.

Bottom line, then, I think the answer is "False". I.e., it probably is possible to spectroscopically read books without opening them, but I doubt you could do it with any plausible (or even implausible) biological process.

Extra Reading: Man of Steel, Woman of Kleenex

Confocal microscopy, which we use in the semiconductor world, may help. But focusing x-rays is a challenge...

In the overthinking of my youth, when the "Superman" comic books I collected[1] cost a dime each (I remember when they raised the price to 12 cents), I figured that "X-ray vision" was a metaphor, and didn't really refer to any specific mechanism. His vision served a similar function to that of X-rays in medicine, but how it worked was a mystery of alien biology alternative physics. Who could say that there might not be some means by which he could focus on each individual page within a closed book? Not by using actual X-rays, of course, but somehow.

It makes as much sense as his skin being skinlike for Lois to touch, but impenetrable. As much as his ability to fly, or to survive without air even though he breathes when there's air available. As much as the likelihood that anything more than the tiniest pebble of Kryptonite would have made its way to Earth, considering the vastness through which it was dispersed.

So my bottom line is, "True, but not by any means we can explain, because we don't understand the mechanism of 'X-ray vision'."

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[1] When I was 9 or 10, my parents gave my entire collection of comic books away to the local hospital, that I might spend less time reading them and more time studying. Whether this was a good thing or not I leave to the opinions of others.

yeah, to get an idea of how hard focusing xrays is, look at some of the xray space telescope designs like Chandra: http://en.wikipedia.org/wiki/Chandra_X-ray_Observatory#Technical_description

Fiction suggests a few examples:

Consider the "Sense of Perception" of the entire circumambient sphere common to the Rigelians and Wheelmen of Doc Smith's "Lensman" setting. The Gray Lensman later gains this ability, and we see him learning to use it. He in particular is aided by the Lens, a device which renders meaning out of any channel perceived. He's shown using it to break otherwise unbreakable codes and to understand "BEEP" as "Begin the plan now as we discussed." Anyway, he seems to have the ability to put an invisible eyeball anywhere nearby, and to then ignore whichever objects he likes. That does have a super-tech pseudolife device assisting him.

You're assuming a single emitter (collimated or uncollimated), which, like you say, obviously prevents XRF from working because you end up lighting up either a line or a diffuse area of space. However, if Superman were emitting from muliple points with appropriate phase shifts, as in an RF phased array, then he could light up a single point in space. Given that X-rays have wavelengths on the order of nanometers and paper has thickness on the order of microns, if he had enough emitters, Superman would be able to resolve pages. He would need at least many emitters, and the phase shifts involved would be tiny, so perhaps this strains biological plausibility. However perhaps, if the surface of his body radiated X-rays with phase shifts controlled by some central nervous function related to his sight, then he would have pretty good resolution even if the phase control wasn't spot-on. And in any case, "biological plausibility" seems like kind of a silly metric for this problem.

Approaching this problem with an inferior knowledge of physics, I'd just like to know how Superman can tell the ink on page 2 apart from the ink on page 3, assuming those two are facing each other? Since the ink on each page is touching the ink on the adjacent page, it seems like it would be like reading a single sheet of paper with two separate text pages printed on it.