|MadSci Network: Physics|
"Reduce a person to ashes"? Talk about overkill (pun intended)! For millenia, weapons makers have realized that a strategically-placed hole is all that is necessary to kill a human being. Firearms make the job more effective by adding the shock waves resulting from supersonic bullets.
While the military is certainly doing research into this field, I would treat it as science fiction for now. The idea of "death rays" goes back at least to H. G. Wells, and so for over 100 years they have been thought of as "weapons of the future." The basic science is now understood, but in practical terms such weapons aren't much closer to reality than when Wells wrote "The War of the Worlds."
A good starting point to estimate for how much energy would be required to reduce a person to nothing is to assume that humans are basically bags of water, which isn't far from the truth. In that case, what you need is the heat of vaporization of water--the amount of energy needed to turn liquid water into vapor.
Breaking the bonds that hold the water molecules together requires 2260 Joules of energy per gram of water (see "heat of vaporaization" at hyperphysics). The Joule is the unit of energy in the metric system. If you lift a 2.2 pound weight 40 inches (one meter) up, you have used up 10 Joules of energy (see gravitational potential energy at hyperphysics. A large-ish human weighs about 200 lbs., or about 90 kg. That means that around 200 million Joules of energy would be needed to vaporize a human.
Let's try and put that in perspective. 200 million Joules is about the amount of energy required to lift one ton 13 miles into the air. It's also the amount of energy used by 2 million 100 Watt light bulbs in one second (a Watt is a measure of how quickly energy is being used, and is equal to a rate of one Joule per second).
To completely reduce a person to ashes would actually require more energy. Bones are the most difficult, and require very high temperatures to burn. Crematoria use temperatures over 1400 F (water boils at 212 F), and even then the process takes a couple of hours, and some bones still have to be crushed ( http://www.icfa.org/cremation.htm). Given that the military is more in the business of killing the enemy, and not providing cremation services, I believe that they'd stop at vaporizing them, and leave their bones intact.
Of course, the energy needs to be delivered quickly. If you were out in the Sun all day long, you would absorb something like 60 million Joules of energy in the form of sunlight. And yet, no part of you is vaporized, except for the sweat that your body uses to cool you off. If you're going to deliver the energy needed to vaporize the water in a person in one second, then the rate of energy delivery would be 200 million Watts, or about the energy usage rate of 200,000 average homes ("Electrical Energy." The New Book of Popular Science. 2000 edition. Grolier Incorporated, 1998). In a battlefield environment, you would need to repeat that energy delivery very quickly, and very many times.
Of course, in reality you wouldn't need so much energy to kill a single person. Vaporizing a small portion would be sufficient to incapacitate them (and cause a more lingering, painful death). The reality of such weapons would, therefore, most likely be less spectacular and more gruesome than the video game stuff presented on TV shows. The energy needs would still be extreme. Even if you only vaporize 1/100th of a person, and deliver the energy in one second, you still need 2 million Watts.
It won't happen anytime soon, though. While it might be feasible someday, the equipment currently needed to generate energy at the rates needed would be far from battlefield-ready. It would be incredibly heavy and bulky, and vulnerable to attack. For the forseeable future, projectile weapons making use of the chemical energy of gunpowder are far more effective and far less expensive.
On a side note. You mentioned that being struck by lightning doesn't vaporize a person. A lightning bolt actually delivers enough energy, estimated at around 5 million Joules ( http://www.newton.dep.anl.gov/askasci/phy00/phy00876.htm). The question is how much of that is actually absorbed by the body? The salty water that makes up most of our bodies is pretty conductive, and so only a tiny fraction of the energy will be absorbed. That differs from an energy beam, which would be tuned to be strongly absorbed by the body. Lightning kills by passing current through the body, triggering heart attacks, collapse of the lungs, circulatory failure, or central nervous system damage ( http://en.wikipedia.org/wiki/Lightning).
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