Is 21 cm is a "magic length" with the universe?
- sciart0
- May 1
- 1 min read
KEY TAKEAWAYS
Across the observable Universe, there are some 10^80 atoms, and most of them are simple hydrogen: made of just one proton and one electron each.
Every time a hydrogen atom forms, there’s a 50/50 shot that the proton and electron will have their spins aligned, which is a slightly higher-energy state than if they’re not aligned.
The quantum transition from the aligned state to the anti-aligned state is one of the most extreme transitions of all, and it produces light of precisely 21 cm in wavelength: arguably the most important length in the Universe.