When turned on, it emits an invisible, focused beam of radiation - similar to the microwaves in a domestic cooker - that are tuned to a precise frequency to stimulate human nerve endings.
It can throw a wave of agony nearly half a mile.
Because the beam penetrates skin only to a depth of 1/64th of an inch, it cannot, says Raytheon, cause visible, permanent injury.
But anyone in the beam's path will feel, over their entire body, the agonising sensation I've just felt on my fingertip. The prospect doesn't bear thinking about.
"I have been in front of the full-sized system and, believe me, you just run. You don't have time to think about it - you just run," says George Svitak, a Raytheon executive.
Silent Guardian is supposed to be the 21st century equivalent of tear gas or water cannon - a way of getting crowds to disperse quickly and with minimum harm. Its potential is obvious.