In Chile's Atacama Desert, at an altitude of nearly five kilometers, stands a radio telescope that catches millimeter waves from the depths of space. These waves carry the heat of distant galaxies, but first they must pass through a window — a thin plate of silicon that separates the cold receiver from the warm outside world.
Ordinary glass is no good for millimeter waves: it reflects too much. So engineers took silicon, the same material used for computer chips, and carved millions of tiny cones onto its surface with a laser. These cones work like a coat: light enters them gradually, without hitting a sharp boundary, and almost all of it passes through. The window transmits 99% of the light, reflecting only 1%.
The plate, 124 millimeters in diameter and just 4 millimeters thick, has been working for a year in the DESHIMA v2.0 instrument on the ASTE telescope. During that time, it has not lost a drop of transparency, and its losses were so small that they could not be measured.
The millimeter waves that this window lets through are the same as those used in airport security scanners, only they come from galaxies born billions of years ago.
In Carl Sagan's novel 'Contact,' radio telescopes pick up a signal from an alien civilization — windows like this one help real telescopes listen to ever more distant space.