How does ALMA see ‘invisible light’?
Look around you. You see the text you are reading right now. You see the room where you’re sitting in. Outside the window, you may see the street where you live, and the Sun in the sky. Or, if it’s already dark, you may see the Moon and the stars. But how does this all work?
What you actually see is visible light. The Sun and the stars emit light of their own – they really shine in the sky. The Moon does not produce its own light. In stead, it reflects light from the Sun, like a mirror. And all the things around you – tables, chairs, cars, houses – also reflect sunlight. If it’s dark, and if there are no lamps and streetlights, you wouldn’t see them!
So you see the world around you by catching visible light. You do that with your two eyes. Light enters your eye through the pupil – the dark, round opening in the center of your eyeball. The bigger your pupil is, the more light you catch. That’s why your pupils get wider in the dark – your eyes want to catch more light!
At the back of your eyeball is your retina. Your retina actually detects the light that enters your eye. Tiny cells in your retina, called rods and cones, produce a small signal when light falls upon them. The signal is transferred to your brain. Your brain makes sense of all the signals. It tells you what you’re looking at.
ALMA has 66 antennas – 66 ‘eyes’. Most of the antennas are 12 meters across – much larger than the pupils of your eyes! So ALMA catches a lot more light. But it’s a form of ‘invisible light’ that our eyes cannot see. ALMA is designed to detect longer light waves, called millimeter and submillimeter waves.
At the back of each antenna is a set of sensitive receivers. They detect the millimeter waves from the Universe, and turn them into a signal. Together, the ALMA receivers could be called the retina of the observatory. Eventually, each ALMA antenna will have ten different receivers. With 66 antennas, that means ALMA’s ‘retina’ consists of 660 ‘cells’!
Your retina needs to be in the dark to detect the light that enters your eye. Likewise, the receivers of ALMA need to be extremely cold to detect the faint millimeter waves from the sky. Otherwise, the molecules stimulation will generate such a noise that will prevent the antennas to detect such faint waves. Therefore, they are frozen to a temperature of minus 269 degrees Centigrade (minus 452 degrees Fahrenheit)!
The signals from the ALMA receivers are transferred to a big computer. This computer is like the brain of ALMA. It makes sense of all the signals it receives. And it tells astronomers what they are looking at.