Plausible Technologies from Science Fiction

In H.G. Wells' The War of the Worlds, the martians that were invading England attacked with their fighting machines – massive tripods with mounted heat rays capable of incinerating people instantaneously.

Forthwith flashes of actual flame, a bright glare leaping from one to another, sprang from the scattered group of men. It was as if some invisible jet impinged upon them and flashed into white flame. It was as if each man was suddenly and momentarily turned to fire.

Then, by the light of their own destruction, I saw them staggering and falling, and their supporters turning to run.

I stood staring, not as yet realising this was death leaping from man to man in that little distant crowd. All I felt was that it was something very strange. An almost noiseless and blinding flash of light, and a man fell headlong and lay still; and as the unseen shaft of heat passed over them, pine trees burst into fire, and every dry furze bush became with one dull thud a mass of flames. And far away towards Knaphill I saw the flashes of trees and hedges and wooden buildings suddenly set alight.

So, would it be possible to create an invisible shaft of light that sets people aflame? How about using infrared radiation, invisible electromagnetic radiation commonly called “heat radiation?”

According to Dr. Channon Price, Professor of Physics at the University of Alaska Fairbanks, “it is possible. IR carries energy. If the energy is absorbed faster than it can be dissipated, it will accumulate. As energy accumulates, typically temperature goes up. If it goes up high enough, the person could 'fry'. More likely, they will start to radiate in the IR at a pretty high rate, and equilibrate at a temperature that would lead to dehydration and death due to overheating.”

If it's a matter of pumping energy into something until it bursts into flame, though, then IR radiation would not be the best mechanism, because it carries less energy even than visible light. There are other forms of electromagnetic radiation, however, which carry much more energy, such as Ultra Violet rays, X-rays, and Gamma rays (of which, Gamma rays carry the most).

Higher energy electromagnetic radiation is also a form of ionizing radiation, meaning it actually carries enough energy to detach electrons from their molecules. This has many implications, such as inducing electrical charge as well as modifying DNA and subsequent health hazards. Also, enough Gamma radiation may carry enough energy to cause a person to burst into flame.