Laser without a cavity I am reading the book 'How the laser happended' by Townes. In it, he mentioned the megamaser discovered in the cosmos. 
It is a maser without a cavity. 
I am a little bit surprised that a maser or laser can function in open space. Can anyone explain why a cavity is not essential for the maser or laser? 
 A: A real-life hydrogen maser does have a RF cavity, but the goodness of the cavity is not essential, as explained below.
For laser/maser, you need to amplify and maintain high phase coherence. The longer the storage time, the narrower the frequency resolution (small frequency uncertainty). If the coherence is stored in light, you need a high-finesse cavity with stable length around your active medium. However, if you can store coherence somewhere else, then the quality of cavity is not important. In the case of hydrogen maser, the coherence is stored in the hydrogen atom. The atoms can collide with walls but usually walls are coated with paraffin or Teflon such that they do not lose their coherence. If the atom is viewed as an oscillator, then its linewidth is much narrower than the RF cavity surrounding it. There is a research effort to adopt this "bad-cavity" approach to optical regime (see "Superradiant laser") 
A: Laser was originally an achronym, standing for Light Amplification by Stimulated Emission of Radiation. A device which amplifies light via stimulated emission of radiation should be called a laser - any other definition would be painfully obtuse. This includes any normal gain medium that's not in a cavity, like your example in space, or for example many optical amplifiers that I use in my own lab. Most lasers are also relatively coherent in space and time since you can only get such light by using a laser, but this isn't a requirement. Coherence is relative, anyways, so you'd be hard-pressed to define a non-arbitrary boundary between light-sources based on coherence.
