Thursday, September 30, 2004

Third Floor, Space Junk . . .

One of the less obvious and seemingly minor problems of constructing a space elevator is the conductivity of the material used [“Going Up,” July]. Since nanotubes are quite conductive, I assume that a material made up of 50 percent nanotubes would have some electrical conductivity. So wouldn’t a space elevator mimic on a grand scale the space shuttle’s tether experiment? Also, could the friction from the atmospheric ions or other particles generate a massive static charge, not unlike a natural lightning bolt? If so, this could be viewed as either a real problem or a boon. These space elevators may turn out not to be elevators but power stations! And quite possibly, reality is once again more interesting than fiction.


Ashland, Massachusetts

Carbon nanotubes actually come in two forms—highly conducting and semiconducting. However, when you place carbon nanotubes in a composite, the process destroys much of the electrical conductivity. If in the end we do use a pure carbon-nanotube ribbon, it would conduct like gold. However, we have looked at this option and found that due to the length and the small cross section of the ribbon, the conductivity overall is still not great; there would be little electric current conducted down the ribbon, except possibly in the event of an extreme solar storm, which has not yet been seen. The primary difference between the shuttle experiment and the space elevator is that the shuttle moves at 17,000 miles per hour, dragging the tether through Earth’s magnetic field. The space elevator is stationary; the movement is what generates the large voltages and currents, so the elevator would have little to none. We had hoped at one point we might be able to generate power with the elevator, but no such luck.

Brad Edwards, aerospace engineer

Your fascinating article on using a space elevator to raise payloads to a height of 62,000 miles, where they would have sufficient angular velocity to escape Earth’s gravity, doesn’t mention what would supply the huge increase in angular momentum as the payload, moving at around 1,000 mph, was raised from Earth’s surface to 62,000 miles, where it would be moving at around 17,000 mph. As the payload was raised, it would try to lag behind the tether and pull it out of a straight line, slowing down the upper platform. If the payload was released when it reached the upper platform, common sense says that the platform would need a rocket-powered boost at some point to regain the angular momentum that had been transferred to the payload.


Okemos, Michigan

To an extent, Mr. Swartz is correct: As payloads are moved up and down the elevator, the ribbon is distorted, and it would move the counterweight. Nevertheless, looking at the travel time and the relative masses of the climbers, the ribbon, and the counterweight, we find that the distortion is extremely small and would be quickly corrected because of the forces that are felt by the ribbon and the counterweight. The rotating Earth supplies the needed angular momentum through the anchor and the ribbon. The rotation also provides all the restoring forces required—no rockets are needed to move the counterweight. The best way to look at this may be to think of the space elevator as a pendulum. If you pull the ribbon from its normal position—rising straight up from Earth—the forces will always pull it back.                           

 —Brad Edwards

Sizing Up Intelligence

I was amazed to read in your interview with anthropologist Robert Martin his statement that the evolutionary tree “can tell us that race and gender differences in brain size are unrelated to intelligence” [Discover Dialogue, July]. How can it possibly do that? Race and gender differences are related to brain size. There is ample evidence of such correlations, and I would refer Martin to the work of J. Phillippe Rushton and Richard Lynn, among others, and to publications in the journal Intelligence for documentation. Surely he would not deny such a correlation between different species, yet miraculously he thinks that there is no such correlation in humans.


Grand Island, New York

Comparative studies of mammals reveal clearly that body size is a major factor influencing brain size. The average 10 percent difference in brain size between men and women is predominantly attributable to the average 15 percent difference in body size. However, a comprehensive review of intelligence tests published some years ago in Science showed that there is little overall difference between men and women for the average values in intelligence scores, although men show greater variance. Some differences between the sexes emerged in relation to particular skills. It should also be noted that human brain size declined over the past 30,000 years (until recent times) as body size declined. It would be difficult to argue that this decrease in brain size was paralleled by a decline in intelligence. With respect to testing for differences both between sexes and between races, it should be remembered that all biological systems involve networks of factors, so that simple correlations between two variables are often meaningless. These network effects can be taken into account when applying the statistical technique of partial correlation. If the effects of body size and socioeconomic status are removed with this technique, no significant correlation remains between brain size and intelligence.         

 —Robert Martin

Political Science, Continued

I’ve been reading Discover magazine since my youth and appreciate that it helps make sense of things for someone not involved in the sciences. I have noticed in recent months, however, that there are more and more political statements in your magazine. I promise you that I get my fill of this information elsewhere and find it intrusive in the context of your magazine. I’ll accept your explanation that sometimes the two are inseparable, but the specific references (and always negative) to President George W. Bush make your position apparent and offensive.


Burbank, California

A World of Wonder

Like a robot, I go through the motions of everyday living with a vacant mind. That vacant mind became a mind full of wonder when I happened to pick up a Discover magazine. I have traveled through the universe, touched worlds I never knew existed, marveled at the mysterious work of nature, and trembled at what is still to be discovered. Your magazine opened my stale brain and introduced it to a wealth of questions, answers, puzzles, and beauty, and now I feel as though I am just beginning to live this life. I want to share my discoveries and truly believe that every child should be shown this magazine so that they all can begin this journey of wonder.


Hammond, Indiana



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