In the 'War of the Worlds' the Martians used them to incinerate pesky humans more than a century ago.
The Pentagon plans are more modest: crowd control of pesky humans.
Still, after more than a century, the 'heat ray' has made the leap from fiction to reality with the U.S. military demonstrating the so-called 'goodbye effect' of directing electromagnetic waves at people.
It delivers sudden, unbearable heat, like the invisible wave when a hot oven door is opened but far more powerful – an intense, enveloping but non-lethal blast.
"You're not gonna see it, you're not gonna hear it, you're not gonna smell it: you're gonna feel it," said Marine Colonel Tracy Taffola, at the public unveiling of the quaintly-dubbed Active Denial System at the U.S. Marines base near Quantico, south of Washington, D.C.
From a kilometre away, the invisible, high-energy, beam, concentrated (just like the Martians did) by a big concave mirror mounted atop a military vehicle can blast a crowd. Instantly, everyone wants to be somewhere else. They flee. Hence the "goodbye effect."
The Pentagon has been experimenting with killer beams for decades. A laser so powerful that it can destroy nuclear-tipped missiles shortly after launch has been mounted in a much-modified Boeing 747 and is being tested.
The heat ray on display this week is at the other end of the spectrum, at least in terms of impact. It's the "safest non-lethal capability" developed to date, Col. Taffola said.
Various development versions of the heat ray have been tested for years. One was sent to Afghanistan – but never used – in 2010. Police departments have shown interest.
The demonstration was intended to allay fears about the heat ray. In thousands of tests on more than 700 volunteers, using varying power levels, only two injuries requiring hospital treatment have occurred, according to officials.
Unlike microwave in household ovens, which penetrate deeply, heating from within, the beam in the "active denial system" is of a frequency that is absorbed at the surface of the skin. So there's an unbearable feeling of intense heat but no real danger.
No one discussed whether a little meddling with the frequencies could field a beam capable of frying crowds rather than dispersing them. Opponents of heat rays have already questioned whether those incapable of instant flight – the aged, infirm, pregnant – might suffer serious injury from prolonged exposure. Others have suggested that mass panic among those exposed to the unseen beam could result in trampling and mayhem.
Proponents claim the heat ray is safer, poses less risk and is more humane that other non-lethal means of crowd dispersal such as tear gas or pepper sprays.