As any child with a magnifying glass can tell you, light from the Sun can be harnessed for destruction. And for thousands of years people have touted the concept of a death ray. Archimedes thought that a death ray could be used to fight off a Roman invasion of Sicily. His plan: arm the Greek army with mirrors, turn them to the Sun, and beam parallel waves of concentrated light straight at the Roman fleet.

He didn’t get there, but 20th Century technology made people believe that the death ray was possible. In the 1930s the world was gearing up for war. There were reports that a German engineer was building a 150,000 candlepower ray for the Nazis. Meanwhile, in the US, Nikola Tesla was preparing a different type of death ray. Instead of heat, Tesla planned to use 80 million volts of electricity to catapult microscopic particles that he said were potent enough to obliterate a plane over 400km away.

Fortunately these destructive plans didn’t see the light of day. But using energy as a weapon has now become a reality. The US military has the Active Denial System, a non-lethal, directed-energy weapon used to control crowds. It zaps people with a focused wave of electromagnetic radiation, heating the water and fat molecules in the skin to up to 50C. After 3 seconds the pain is unbearable. It’s not lethal, but it’s not that far away in principle from Archimedes’ death ray. Or, indeed, a magnifying glass.

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