Summer 2008 Issue


Light Speed

By Dr. H. Paul Shuch, N6TX

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, the distance light travels in one year.

However, that doesn’t tell us very much, does it? I mean, how many of us can close our eyes and visualize the speed of light? I can’t. 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.

Therefore, 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, but from my vantage point under this ocean of air, one star looks pretty much as remote as the next, so that doesn’t clarify things at all.

Still, Mach One, the speed of sound, is a familiar concept to many of us. We know that if we see lightning flash and then five seconds later we hear the thunder roar, the storm must be about a mile away. We know this because sound travels at about five miles 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 One Million. Thus, a light year is about how far we would go in a million years, traveling at Mach One, or how far the Concorde (which flew at Mach Two) would have gone in 500,000 years—if it didn’t 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 light year is about 6-trillion miles. Let’s 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.

Okay, 200-billion gallons of fuel. A 20-gallon gas tank means we need to fill up (let’s see, 200 billion divided by 20 equals…) 10-billion times. How long do you suppose that will take us?
Each gas stop consists of pumping the petrol, paying for it, visiting the rest-room, grabbing a cup of coffee, and maybe making a phone call home to keep the spouse apprised of our progress. Let’s say 12 minutes per pit stop. That’s 5 to the hour, which means we spend—let’s see now, 10 billion divided by 5 equals … 2-billion hours lost at interstellar service plazas. I wonder what that is in years.

Now there are 24 hours in a day, and 365 days in a year. Multiplying the two together, we see that a year consists of just under 10-thousand hours. (I told you this would be approximate.) Dividing 2 billion by 10 thousand, we find we’re spending 20,000 years just fueling up. That’s for a hypothetical one-light-year trip, and we haven’t even begun to calculate driving time!

Were this space ship on which we reside traveling at the speed of light, it would of course travel one light year in exactly one Earth year. However, Earth is a slow boat to nowhere. Is it possible that, during its brief history, our planet has traveled light years? In fact, it has, if you count its curved path around our Sun. Let’s calculate the Earth’s annual orbit, in light years:
We start with the known fact that it takes 8 minutes for sunlight to reach us. (We know this because when we flip the switch to turn off the Sun, it takes 8 minutes for the sky to go dark.) Well, that places the Earth 8 minutes from its source of power, so we’ll use 8 light minutes for our orbital radius.

One trip around the Sun takes us just a year. Not counting the Sun’s own motion around the galactic center, the distance we travel in a year is (pi times diameter equals … two pi times radius, equals …) 50 light minutes. Rounding up, we’ll call it a light hour.

Since Earth travels about a light hour per year, and a year is almost 10-thousand hours long (remember?), we can see that we’re orbiting the Sun at one 10-thousandth the speed of light, or Mach 100. Did you realize that you live on a supersonic spacecraft? Furthermore, we see that in about the last 10-thousand years, our planet has traveled about one light year.
Ten thousand years … let’s see … that’s about how long it’s taken us humans to advance from primitive hunter-gatherers to … primitive hunter-DXers.

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