Why is ALMA so high?

ALMA is the highest observatory in the world. The Chajnantor Plateau, where the 66 ALMA antennas are located, is at five kilometers above sea level. It would take you about an hour to walk that distance – straight up! Only one building in the world is slightly higher – a train station in the Himalaya Mountains in Tibet.

Many big telescopes and observatories are at high mountaintops. The reason is that the atmosphere of the Earth hampers astronomical observations. In the mountains, the air is thinner and clearer, so you have a better view of the stars.

But in the case of ALMA, it is even more important to be at a very high site. That’s because ALMA studies millimeter waves from the Universe – a special form of ‘invisible light’. Millimeter waves from outer space can only be seen from very high mountaintops. At sea level, ALMA would be completely blind!

You can do an experiment to understand why that is so. All you need is the microwave oven in the kitchen at your home. Ask your parents to assist you! A microwave oven is used to heat stuff. As the name indicates, it uses microwaves to do that. Microwaves are in fact millimeter waves – the same type of radiation that ALMA studies.

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First put a cup of cold water in the microwave oven. Turn on the microwave at full power. After about two minutes or so, the water will be quite hot. The water is heated by the radiation’s energy, because water absorbs millimeter waves.

Now put a piece of hard plastic in the microwave oven, like a solid block of LEGO bricks. Again, turn on the microwave at full power for about two minutes. You will notice that the plastic hasn’t become hot at all. That’s because plastic doesn’t absorb millimeter waves, so it isn’t heated by the energy of the microwaves.

With this experiment, you have learned that water molecules absorb millimeter waves. The atmosphere of the Earth also contains water molecules. They absorb the millimeter waves from the Universe. Therefore, these waves can’t be studied at sea level. They are all absorbed before they reach the surface of the Earth.

But if you’re at a very high altitude, there’s not so much atmosphere above your head anymore. Not all the millimeter waves from the Universe are absorbed. That’s why ALMA has to be at a very high site. And that’s why the air above the ALMA antennas has to be very dry, so it contains little water. Chajnantor is the perfect spot!