Observing across the spectrum

Cast against the background of a magnificent sunset above the Chajnantor Plateau in the Atacama Desert, the antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) keep watch over the skies. ALMA, of which ESO is a partner, is the largest ground-based astronomical project in existence.

The beautiful shades of orange and red that illuminate the sky as the Sun sets behind the horizon teach us an interesting physics lesson. As it passes through the Earth’s atmosphere, sunlight with shorter, bluer wavelengths gets scattered off the molecules of gas and dust in the air more than that with longer, redder wavelengths. As the Sun approaches the horizon, light that reaches the Earth’s surface takes a longer path through its atmosphere, and most of the blue part of its visible spectrum gets scattered away. The light that reaches our eyes dyes the sky around the setting Sun in stunning hues of red.

However, the beauty of the blood-red sky is invisible to ALMA. The 66 telescopes of ALMA don’t look at the sky in the visible part of the electromagnetic spectrum, as we do — they observe it in the millimetre and submillimetre wavelength range. Electromagnetic radiation of these wavelengths gets absorbed by water vapour in the Earth’s atmosphere, which is why telescopes used for submillimetre astronomy must be built in extremely dry locations, at high altitude — where atmospheric absorption doesn’t hinder their observations. At five kilometres above sea level, ALMA’s powerful antennas allow us to see the mysteries of the Universe in an extraordinary light.