Say Hello to the Goodbye Weapon.

It’s a new weapon called the Active Denial System, by the US Air Force:

The crowd is getting ugly. Soldiers roll up in a Hummer. Suddenly, the whole right half of your body is screaming in agony. You feel like you’ve been dipped in molten lava. You almost faint from shock and pain, but instead you stumble backwards — and then start running. To your surprise, everyone else is running too. In a few seconds, the street is completely empty. …

The ADS shoots a beam of millimeters waves, which are longer in wavelength than x-rays but shorter than microwaves — 94 GHz (= 3 mm wavelength) compared to 2.45 GHz (= 12 cm wavelength) in a standard microwave oven. …

The beam produces what experimenters call the "Goodbye effect," or "prompt and highly motivated escape behavior." In human tests, most subjects reached their pain threshold within 3 seconds, and none of the subjects could endure more than 5 seconds.

A Wired article: Link