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 electrons 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.
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.