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A Speedway to the Stars? The research being done by NASA engineer Robert Frisbee to find a feasible method of interstellar travel ["Star Trek," August] is fascinating. I was surprised, however, at the omission of any reference to the double-edged sword of relativity. On one hand, astronauts on a vessel traveling a significant fraction of the speed of light would experience time passing much more slowly than observers back on Earth. This would make the question of preparing for a decades-long voyage unnecessary, as the astronauts might experience it as months, days, or even mere hours if they were traveling fast enough. But as the vessel approached these incredible speeds, it would progressively become heavier, requiring more thrust, or fuel, to accelerate. This added fuel would not only add mass to the vessel but also be subject itself to the relativistic increase. The amount of fuel required to gain even the slightest increase in velocity at that point would grow exponentially.

Stephen D. Laurette Richmond Hill, New York

Robert Frisbee responds: Mr. Laurette is correct in noting the relativistic effects you would encounter as you approached the speed of light. At 50 percent of the speed of light, the relativistic mass increase and time dilation (slowing down) would be about 15 percent above normal. This means that you would seem to be 15 percent heavier and move 15 percent slower than normal when seen by someone back on Earth. However, if you were traveling on the spaceship, you wouldn't feel any heavier or seem any slower. There is an additional effect on the laser sail: The photons that make up the laser beam would be 73 percent "redder" (a longer wavelength), which means they would have less momentum, so the laser would have to increase its power by 73 percent to compensate (plus an additional 15 percent to compensate for the sail's relativistic mass increase). All these factors contributed to our decision to limit cruise velocity to 50 percent of the speed of light; if you went faster, you would have the advantage of a shorter trip, but overcoming the relativistic effects on the vehicles would rapidly become impossible.

After reading "Star Trek" one would have to conclude that travel out of our solar system is impossible. The fusion, fission, and antimatter engines require too much fuel to be practical. The laser sail is doomed by the fact that building a 6,600-mile-wide collecting mirror is simply not feasible, and apart from the financial cost, a 600-mile-wide sail would be torn apart by cosmic debris on a daily basis. And why build a fusion ramjet when there's no fuel in space to run it and its design would not allow it to attain the speed it needs? The fusion or fission engine concepts would be useful in getting around our own solar system, but what's the use in traveling to other planets in our neighborhood? Venus will never be inhabitable, and neither will Mars or any of the Jovian planets or their moons, and changing the environment of an entire planet will never be within our capabilities. It is fun to speculate on ways that humans might accomplish interstellar travel, but in the end it is just more science fiction.

Gil Bell Duluth, Georgia

As I read August's Letter From Discover, a chill swept across me as I realized that we seem to have given up on our planet. Why do we doom ourselves to "foul our own nest irrevocably" within a few hundred years when we actually have the technology to fix it? From NASA's isolation chambers that recycle water for up to 90 days to Bill Gross's revolutionary solar-powered Stirling engine design, isn't it feasible that we could survive on this planet for several thousand years instead of a measly several hundred? I do believe it is important for us to continue reaching out farther into the universe because it can help us understand more about the interconnectedness of the universe and ourselves. But what if we travel all the way to Alpha Centauri only to find out that we should have spent more time caring about what we had right here?

Russell B. Pace III Carnelian Bay, California

Rule keepers Your gallery of slide rules in the August issue ["Slide Rules: The First Nerd Tool"] brought back fond memories and gave me new information to share with my students, for whom a slide rule is strictly a museum piece. In the second photo caption, you said, "The Achilles' heel of the slide rule is that for most calculations it cannot indicate where the decimal point should go." I see that as an advantage compared with modern calculators. As a result of growing up using a slide rule, I learned how to do approximate arithmetic in my head, so when I get a nonsense answer on my calculator, I usually recognize it as wrong. My students usually do not. Also, I was surprised not to see one of the slide rules I used for many years—the E6-B aviation calculator. On one side it has functions designed for flight calculations and on the other a grid for doing relative wind calculations, so that by entering the predicted wind you can calculate your expected drift and ground speed. There are many pilots who would testify that without that little helper, they wouldn't be alive today.

Hugh B. Haskell Cary, North Carolina

The first "Slide Rules" photo caption says, "Chemist Linus Pauling used to astound freshmen at Caltech by multiplying numbers to six decimal places on his pocket slide rule. 'He calculated the last two digits in his head,' says Caltech math professor emeritus Tom Apostol." As one of the astoundees, I recall that Professor Pauling gave a different explanation. He said he worked through the problems in advance using longhand arithmetic, then used the slide rule as a mnemonic device—it gave him the first three digits, and he remembered the rest. He was known to have a photographic memory. As much as I admired Pauling's slide-rule prowess, I decided to replace my slide rule with an abacus. Some of my classmates grumbled that while they were silently sliding their rules during exams, I kept clinking the beads, which unnerved them. In any case, the abacus enabled me to compute six digits accurately without advance preparation, though admittedly a bit more slowly than with a slide rule.

Les Earnest Los Altos Hills, California

To Skip, Perchance to Glide Cameron Walker's article "Walking on Water" [The Physics of . . . Skipping Stones, August] mentions an airplane concept called HyperSoar, which is attributed to Lawrence Livermore National Laboratory and whose movement is suggested as being analogous to that of stones skipping on water. I have no quarrel with the analogy, but the HyperSoar concept is neither new nor novel. The idea of a hypersonic skip-glide airplane goes back at least 60 years to the German engineers Eugen Sänger and Irene Bredt, who were developing hypersonic skip-glide airplanes intended for weapons delivery at intercontinental range or beyond. In the early 1950s, Walter Dornberger at Bell Aircraft Corporation led a project to develop a hypersonic glider called BoMi (bomber missile), which was followed by the development of the X-15. In the late fifties, the Air Force contracted Boeing to develop the X-20, which was conceived as a hypersonic orbital intercontinental glider. The skip feature was deferred owing to aerodynamic heating problems, and the program was eventually canceled.

Robert H. Smith Bainbridge Island, Washington

For Richer, For Poorer As a behavioral neuroscientist, I was impressed by the article on laboratory housing for rodents by Barry Yeoman ["Can We Trust Research Done With Lab Mice?" July], which questioned the face validity of such research, raising important issues regarding environmental enrichment effects on biological functions. What was overlooked, however, was that many people who are injured or mentally ill sequester themselves in relatively impoverished environments (for example, after brain injury or while depressed). When the symptoms of such disorders are investigated using an animal model, the nonenriched lab environment may actually provide a better approximation of the impoverished human condition.

James C. Woodson University of South Florida and VA Medical Center Tampa, Florida

Errata In "Introducing Nonstick Glue" [R&D;, August] we described direct current in terms of volts. Current is measured in amps. In our lead photo caption in August's "Terminator Genes," we said that the Flavr-Savr tomato had a fish gene that extended its shelf life. Flavr-Savr used a tomato gene that was inserted backward, resulting in the inhibition of the regular gene that causes the degradation of ripe fruit. Another biotech company, DNA Plant Technology, did insert genetic material from the arctic flounder into tomato plants in hopes of producing antifreeze proteins that would increase frost tolerance. That effort never yielded a commercial tomato.

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