Say hello to the U.S. military's "pain ray," a crowd control device that zaps targets with an invisible electromagnetic wave that incapacitates without leaving permanent damage. After 15 years of development and testing, the weapon is finally ready to be deployed. Here's what you should know: 

How exactly does it work?
Although the Pain Ray has been described as a "microwave" weapon, "that's not really correct," says Spencer Ackerman at Wired. Instead, it uses a different wavelength called the millimeter wave to generate heat. Millimeter waves don't go as deeply as microwaves; they only penetrate 1/64 of an inch. (The military has tried and failed to use the The Active Denial System (ADS)   to cook a turkey.)

What kind of pain do targets feel?
The ADS, though non-lethal, can cause a burning sensation on the skin from more than 3,000 feet away. There's "no flash, no smell, no sound, [and] no round," says Ackerman, who experienced the Pain Ray firsthand. "Suddenly my chest and neck feel like they've been exposed to a blast furnace, with a sting thrown in for good measure." 

How long does the pain last? 
The burning sensation disappears after a few seconds. And of the 11,000 people the military tested the weapon on, only two required additional medical treatment for second-degree burns. Both have since made full recoveries. The Department of Defense has spent $120 million testing the system's long-term safety, and reports no instances of radiation-related side effects.

Has it ever been used in the real world?
The weapon was deployed briefly in Afghanistan in 2010, but was sent back by General Stanley McChyrstal, who anticipated "that the Taliban might see the weapon as a propaganda opportunity" against the United States, says Jacey Fortin at the International Business Times. In December 2005, Colonel James Brown, commander of the 18th Military Police Brigade in Iraq, requested the ADS system "to supress insurgent attacks and prison uprisings," but the weapon never saw the light of day. But now, "it could be used across the military spectrum of operations, perimeter security, crowd control, entry control points. You name it," says Marine Colonel Tracy Tafolla of the Department of Defense.

Does it have any drawbacks?
Rain, snow, and dust can all weaken the weapon's potency. The crowd control device also requires 16 hours to boot up. Transporting the device is an issue as well, as it must be mounted on a military vehicle, which itself requires expensive fuel tanks to power it.

Sources: Daily Mail, Daily Tech, International Business Times, Wired