Hydrogen Line Radio

Hydrogen is the first and foremost of elements. It has an atomic number of 1 and a single atom is comprised of  one single positively charged proton and a single negatively charged electron. It is estimated to comprise 75% of all the mass in the universe. We think of it usually in the from of hydrogen gas which is actually dihydrogen or H₂, but it has many other forms. What on Earth does this have to do with radio? Hydrogen can emit radio waves.

So let me do a little background here. Today the pedestrian understanding of Atoms isn't much better than Leucippus. It's usually seen as an the smallest unit of matter. It's seen as stable, and foundational. This is of course destroyed by any understanding of subatomic particles... but even these basic atomic building blocks are in flux. Electrons for example can jump between energy levels. It can jump up a level by absorbing light. Then by emitting light if can jump down a level. There are a total of 5 levels each which has a known energy state. More here and here.

Level	Energy
1	-13.6 eV
2	-3.4 eV
3	-1.51 eV
4	-0.85 eV
5	-0.54 eV

When Hydrogen changes energy states it also releases some of that energy as radio waves. This electromagnetic radiation is at a frequency of 1420.40575177 MHz.This spectral line is called "the hydrogen line. The frequency is within the microwave band and is observed in radio astronomy. Actually this "hiss" was first detected in the 1930s.  Dr Hendrik van de Hulst predicted that this would be caused by Hydrogen.  This was then proven in 1951. The frequency works out to a physical  wavelength of 21.1 cm. That's why Carl Sagan and Frank Drake considered it universal enough to use as a unit of measure on the Pioneer plaque to be shot into space.