After the Second World War, 1940s America was awash with radar equipment and experienced technicians who could make it work. That set the scene for the birth of a new field of science – radio astronomy.
In 1944, the Dutch astronomer Hendrick van de Hulst predicted that interstellar hydrogen ought to emit electromagnetic radiation with a specific wavelength. He noted that a hydrogen atom contains only a proton and an electron that both have a property known as spin. These particles can spin in the same direction or in opposite directions and van de Hulst realized there must be a small energy difference between these two states.
So any hydrogen atom flipping from the more energetic state to the lower energy state must emit a photon, which he calculated would have a wavelength of 21 centimeters and a frequency of 1420 Megahertz. That’s roughly the same microwave wavelength that radar operated in.
