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One quantum transition makes light at 21 cm
Photons come in every wavelength you can imagine. But one particular quantum transition makes light at precisely 21 cm, and it's magical.
Unlike the planets in our Solar System, which could stably orbit the Sun at any distance if they possessed the right speed, the protons, neutrons, and electrons that make up all the conventional matter we know of can only bind together in a specific set of configurations. Credit: Tiltec/Wikimedia Commons If we had the capability of sensitively mapping this 21 centimeter emission in all directions and at all redshifts (i.e., distances) in space, we could literally uncover the star-formation history of the entire Universe, as well as the de-excitation of the hydrogen atoms first formed in the aftermath of the hot Big Bang. Credit: Saptarshi Bandyopadhyay Of course, there’s another possibility that takes us far beyond astronomy when it comes to making use of this important length: creating and measuring enough spin-aligned hydrogen atoms in the lab to detect this spin-flip transition directly, in a controlled fashion.
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