Lecture 33 - Celestial Masers
A maser, like a laser, is a source of bright electromagnetic radiation but in the microwave regime. When microwave radiation is incident upon molecules present in space, sometimes they molecules respond by amplifying it, by producing a 'maser'.
Naturally-occurring masers are found near stars and supermassive black holes, as well as in the atmospheres of Jupiter and other Solar System objects. In the most dramatic cases, a water vapor maser can radiate more energy at a single wavelength than does the Sun over its entire visible spectrum.
In a historic paper published in 1917, Einstein introduced the concept of “stimulated emission of radiation” by atoms and molecules. The first MASER (Microwave Amplification by Stimulated Emission of Radiation) based on this idea was made in 1953, and the first LASER was made in 1960. After the advent of millimetre wave astronomy, astronomers have discovered a wide variety of MASERS in celestial bodies. In this lecture, we systematically develop the concept of two-level systems in quantum mechanics, and stimulated emission from them. Then we discuss the Ammonia MASER and Hydrogen MASER, in terms of two-level quantum systems.