We humans like to measure and then categorize things by size or some other defining characteristic. To do this in a way that promotes understanding and, more importantly, commerce, some sort of standardization is required. In the ancient world, and in the United States today, we measure things using "English Units," though the modern-day English now use the metric system—but more on that later. Not really useful for our discussion of interstellar travel, but relevant because of its history (and arbitrary origins), is the "inch." The inch, as a unit of measure, is based on the width of the average human thumb. The "foot" is based on the length of the average human foot and the "yard" is the distance from the tip of the nose to the tip of the middle finger. A much more useful measure for our purposes is the "mile," though its origin is no less arbitrary: a mile was originally defined to be the distance covered by a Roman legion after taking 1,000 paces.

Modern standards of measure now prevail, and the "mile" is now more precisely defined as being 5,280 feet, the "foot" is 12 inches and the "inch," well, the inch is now defined as being roughly 2.54 centimeters (0.01 of a meter). The centimeter is part of the "metric system," which is actually much easier to use and is based on the distance traveled by light in a vacuum. The 17th General Conference on Weights and Measures in 1983 defined the meter as the distance that makes the speed of light in a vacuum equal to exactly 299,792,458 meters per second.1 The speed of light in a vacuum is one of the fundamental constants of nature. Since the speed of light defines the meter, experiments made to measure the speed of light are also interpreted as measurements of the meter. The meter is equal to approximately 3.2 feet or 39.3 inches.

In twenty-first century America, most people define relatively nearby distances in units ofboth distance and time. For example, how many times have you heard people say that "the beach is about a 2-hour drive from

1 Kleppner, D., "On the Matter of the Meter," Physics Today, March 2001.

here," or "my aunt lives about 3 hours from here." They are, of course, referring to the time it takes a car, traveling on nearby roads under average traffic conditions and usually obeying the speed limit, to reach a stated destination. For illustrative purposes—one that we will call upon again later—let's say that the distance from City A to City B is 100 miles (62 kilometers). Traveling on an interstate highway with a 70-miles-per-hour speed limit and not taking into account any of the normal driving distractions, this journey will take 1.4 hours. Driving from New York to Los Angeles (2,800 miles), across the same continent that used to take early American pioneers the better part of a year to traverse, the average American can now achieve the distance in 40 hours (assuming no rest, bathroom, food, or gasoline stops!).

On a global scale, the human species seems to have conquered distance. The Earth's circumference is 24,902 miles. If we assume that the Earth has an interstate highway around its equator, then traveling around the world at a speed to which the average person is accustomed (in their car), would take 355 hours. Of course, modern air travel makes such a notion quaint. If one were to take such ajourney traveling with an air speed of 500 miles per hour, this around-the-world jaunt would take merely 50 hours. Taking our latest technological leap and assuming you can readily get on board the International Space Station, which is circling the Earth at approximately 17,000 miles per hour, the journey would take merely 1.5 hours. Those poor Roman soldiers, from whom we derived the definition of a "mile" to begin with—assuming they can march at a rate of 15 minutes per mile—would take 259 DAYS (non-stop) to cover this distance.

Does this mean that we humans have conquered "distance" and that extending this capability across the solar system and to the stars is just a simple extrapolation of what we've achieved here on Earth? Sadly, the answer is "no."

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