Today, let us ride the waves. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
I was caught off guard the
other day when a friend expressed surprise that
radio waves and sound waves moved at different
speeds. Well, why wouldn't someone think that --
doesn't the radio deliver sound? At first, I
remarked that radio waves are inaudible, they
surround us all the time, and the purpose of a
radio is to convert those fast-moving waves into
slow-moving sound waves.
Then, the very next day, a NASA article announced
that sound waves had been observed
emanating from a black hole in the Perseus cluster
of galaxies, hundreds of millions of light years
away.
That "sound" was a pulse with the astonishingly low
frequency of one cycle every nine million years. We
can see those pulses traveling through highly
rarified gases in the space between galaxies. It's
disquieting to hear them called sound,
even if they are propagated in much the same way as
audible sound.
Here on Earth, we are intimate with two kinds of
waves. In one, inertia carries a disturbance through
some material. That's the way a sound wave, an ocean
wave, or a violin string works. As inertial waves
move back and forth in a violin string, the string
drives an inertial wave that we can hear in the air
around the violin.
The other class of waves is electromagnetic. Such
waves move far faster than inertial waves, and they
take a dizzying variety of forms: X-rays, light,
thermal radiation, radio waves -- but they all move
at the speed of light. It takes about a seventh of
a second for such waves to circle the earth. Sound
waves would take more than a day to make that trip.
Electronic information, traveling on the Internet,
rides in fiber-optic cables, on radio waves, or
through copper wire. One reason that copper
conducts electricity so well is that it's rich in
free electrons. Those electrons bounce about within
the copper, much as air molecules move around you.
Electrons move a lot faster than air molecules, but
nowhere near the speed of light. When you impose a
voltage on a wire you send a wave through
the electrons. That wave carries energy at near the
speed of light. Just as you might bob about, in an
almost stationary ocean, while a powerful wave
moves past you, the elec-trons hardly move as
electric waves pass through them.
And so it's not true that electricity is a flow of
electrons through a wire. Rather it's
energy flowing through the electrons.
Electricity is slowed slightly, as it passes
through a wire, and it moves only about two-thirds
the speed of light. Even then, at sixty cycles per
second, the wavelength of alternating current is a
full two thousand miles.
All those numbers are so extreme they might well
leave us disoriented. So try this concluding
thought. As you listen to this program, the sound
of my voice is taking ten times longer to get from
your radio speaker to your ear, than it took to get
from the transmission tower to your radio.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
For the NASA description of the Perseus waves,
see:
http://science.nasa.gov/headlines/y2003/09sep_blackholesounds.htm
For more on the concept of an electronic gas, see,
e.g., Tien, C-l. and Lienhard, J.H.,
Statistical Thermodynamics. New York:
Hemisphere Pub. Corp., 1979 (Chapter 6)
My thanks to several people: Carol Lienhard
originally suggested the episode. Jack Wolfe,
Stuart Long, and N. Shamsundar (all in the UH
College of Engineering) provided technical counsel
Both images are from The Boy Scientist,
1925.
A representation of the Michelson-Morley
measurement of the speed of light.
The Engines of Our Ingenuity is
Copyright © 1988-2003 by John H.
Lienhard.
