Well, this gets more into the definitions of words, as you can call a
very very low frequency wave a "radio" wave if you want to. Waves with
a frequency of just a few Hertz are called "ULF" waves --
Ultra-Low-Frequency waves, and are used to communicate with submarines,
so they perhaps can be classified as radio waves. The electrical power
in the U.S. gets distributed at 60 Hz (50 Hz in Europe), and so the
power lines emit electromagnetic waves at 60 (50) Hz. Radio cannot use
that frequency for two reasons -- there's too much interference from
power distribution, and the kind of stuff you want to broadcast uses a
wider spectrum than the carrier will allow. Music has frequencies from
about 100 Hz to maybe 5000 Hz and putting that on a 60 Hz carrier won't
do the trick. You can do Morse Code or something like it with the ULF
waves of course. You can detect these very low-frequency waves in the
same way as radio waves -- you just need a very big antenna.
The same goes for the high end of the spectrum. I'm a high-energy
physicist, and we routinely detect single photons with have energies of
50 GeV or so. 50 GeV is the amount of energy an electron would gain if
it went across 50 billion 1V batteries in series. We call these gamma
rays, and do not have another name for them as the energy gets ever
higher. At these energies, the best way to detect them is with what we
call a "calorimeter". You make them typically out of stacks of lead
plates with scintillator material sandwiched in between. An incoming
photon with this energy has so much energy that when it hits a lead
atom, it is able to "pair-produce" an electron and an antielectron,
both with high energies. Then the electron and antielectron fly past
more lead nuclei, decelerate and radiate more high-energy photons,
which can produce more pairs of electrons and antielectrons. At high
energies, thousands of such pairs can be made in a "shower". Some
plastics will emit flashes of light when high-energy particles travel
through them. Then phototubes are used to detect the flashes of light,
making electrical pulses which can be amplified and recorded on
computers.
Tom
(published on 10/22/2007)
