As a student at Caltech, Townes was told to give up physics by his
doctor because all the reading was too much for his eyes. As a compromise, he concentrated
on experimental physics, thinking working with instruments and equipment would be
less of a strain. This turned out to be a fortuitous decision, as the development
of the maser (the microwave version of the optical laser) required a unique blend
of experimental technique and theoretical knowledge.
In the 1940s, Townes worked on radar as part of the war effort. MIT's
radiation lab had successfully produced working radar at the 10 cm wavelength, and
even 3 cm, but the military told them to try and produce 1.25 cm radar. The point,
of course, is that the smaller the wavelength, the more precise the resolution of
what the radar can resolve. Townes told Washington that 1.25 cm radar could not
work, because that wavelength of radiation coincided with a frequency at which water
vapor absorbs radiation...but the military replied that the decision had already
been made and they could not stop. Naturally, when it was built, it did not work.
The radar beam went out a couple of miles, and then would fade out, being absorbed
by water vapor which was present in the atmosphere.
Building on the knowledge gained by the failed microwave radar, Townes
continued on in microwave research after the war. Ammonia also has a 1.25 cm absorption
line, and Townes made the first maser using ammonia as the masing material.
Today, the laser is ubiquitous, and sometimes appears as if it can
be made from almost anything...Arthur
Schawlow, Townes brother-in-law, to illustrate this, tried to make an edible laser, from Jell-O...although
that did not work, by using just regular gelatin and fluorescent dye, the required
edible laser was made.
Feynman once remarked to Townes privately, as they were discussing
the laser, that the way to tell a great idea is that, when people hear it, they
say "Gee, I could have thought of that." Perhaps. But the history suggests otherwise...
Columbia theorist L.H. Thomas told Townes the maser could
not work. After Townes demonstrated it, Thomas stopped talking to him! The great
Bohr told Townes it could not work (due to line broadening) -- even after it had
been demonstrated. Von Neumann's initial reaction was also that it could not work,
but on being told it had been done, went away and after 15 minutes of thought decided
that yes, it could.
The reason Bohr, Von Neumann, Thomas and others thought it could
not work of course is due to the time-energy uncertainty principle. (To remind you,
the time-energy uncertainty principle roughly says that if a time evolving state
has been in existence for a time t, then the uncertainty E in its energy is such that
Et >= hbar/2) Now, the time a photon spends in a maser/laser cavity is on the
order of .0001 seconds, and so the uncertainty in its energy should have been greater
than what Townes was claiming (Laser light has a very high spectral purity which
translates into a very low uncertainty in the energy of its photons)...However,
as there are many molecules
in the masing or lasing medium, we do not know which molecule has underwent
a transition, and so we do not learn the energy or frequency of any particular molecule
from the laser light. Hence, the uncertainty principle is not being violated. A
slightly similar situation occurs in the Mossbauer
effect.
Here is a video of Townes
discussing the history of the laser.