A howitzer is a crude-looking weapon, essentially a small smokestack with a door at the bottom that allows the insertion of breadbox-sized shells. but operating one requires a year of specialized schooling. That's because howitzers, like most artillery guns, are "indirect fire" weapons-that is, if you aim directly at your target, you'll literally miss by a mile, and probably by several. Adjustments have to be made for distance, wind, temperature, atmospheric density, humidity, the amount of wear in the barrel and the spin of the earth (aim left in the Northern Hemisphere, right under southern skies). Even then, most of the shell's explosive force will not end up precisely where intended. Then, 30 seconds of frenzy among a crew of six sees a new shell dragged into the gun, and you can try again. For all this bother, though, howitzers remain the weapon of choice for delivering destruction at a distance.

A military expert, given a clean sheet of paper and asked to sketch out the howitzer's ideal replacement, might end up with something like this: fires weightless and unlimited ammunition, is mountable on aircraft or ground vehicles, can be aimed directly at a target, reloads instantly, tracks fast-moving targets, shoots with pinpoint precision, creates no risk of collateral damage. In the end, he or she would have essentially described a class of weapon that could play a significant role in the next major U.S. armed conflict-weapons that hurl photons instead of chunks of metal.