Summer 2007 Issue


Light Speed

By Dr. H. Paul Shuch, N6TX

Dr. SETIís spacecraft orbits the sun at
about one 10-thousandth the speed of
light. Its fuel economy is 200-billion
gallons per light year (see text).

Tuning the bands in search of an interstellar CQ, we become aware that the universe is immense. When contemplating its magnitude, we need a whole new yardstick. For astronomers, that yardstick is the light year (LY), the distance light travels in one year.

However, that doesnít tell us very much, does it? I mean, how many of us can close our eyes and visualize the speed of light? I canít. I can board an airliner and know that I am traveling at, say, 78 percent the speed of sound, but even that velocity challenges my comprehension.

Just how fast does light travel, how far does it go in a year, and how can we use that knowledge to assess our place in the universe? I could tell you that a light year is a quarter of the distance to the nearest star. However, from my vantage point under this ocean of air, one star looks pretty much as remote as the next, so that doesnít clarify things at all.

Still, Mach 1, the speed of sound, is a familiar concept to most of us. We know that if we see a lightning flash and then 5 seconds later we hear the thunder roar, the storm must be about a mile away. We know this because sound travels at about one fifth of a mile per second, and light (at least over such limited distances) seems to arrive instantaneously.

Well, in fact, light travels about a million times faster than sound, so we can quantify the speed of light, very approximately, as Mach 1 million. Thus, a light year is about how far weíd go in a million years, traveling at Mach 1, or how far the Concorde (which flew at Mach 2) would have gone in 500,000 yearsóif it didnít have to stop for gas.

Of course, we all have to stop for gas, sooner or later. My little Volkswagen Beetle gets about 30 miles to the gallon on a good day. Just how many gallons of fuel would I need to drive a light year, and how many times would I have to stop to fill up?

The textbooks tell us that one LY is about 6-trillion miles. Letís see if we can sink our teeth into that one. Well, 6 trillion is 6,000 billion, so at 30 miles per gallon, we merely need to divide 6,000 by 30 (that equals 200) and then tack a billion on the end.

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