The hydrogen line is an important instrument in modern astronomy — from studying distant stars to looking for extraterrestrial intelligence (if it exists).
Each hydrogen atom is made of one proton and one electron.
Both these particles have a property called spin.
Colloquially, we can say the spin can point up or down at any given time.
When the spins of both particles are pointing up (or down), they are said to be aligned.
When they are pointing in opposite directions, they are anti-aligned.
When the spins are anti-aligned, the atom will have more energy than if they are aligned.
The atom will shed this ‘excess’ energy when the electron flips its spin, emitting electromagnetic radiation of wavelength 21 cm.
In some detectors, this emission shows as a characteristic line, thus the name ‘hydrogen line’.
Scientists discovered this emission in 1951 and it transformed radio astronomy.
By looking for this emission, they could detect clouds of cold, neutral hydrogen atomic gas in interstellar space.
Soon after, they were able to create a map of the Milky Way galaxy for the first time, and discover its spiral arms.
Today, astronomers are also using the 21 cm as a probe to spot the first light emitted by the universe’s earliest galaxies.
Because the emission doesn’t require complicated technologies to spot, astronomers also think it can be used to send information across space that, some day, an alien civilisation might detect
COMMENTS