December 2004 - Materials Science Institute - University of Oregon

ARTIST’S CONCEPTION depicts how a tether system might operate on an exploratory mission
to Jupiter and its moons.
from the rotating Earth’s surface would
continuously increase its angular inertia;
therefore, its angular velocity would
steadily decrease as it rose, to keep its angular
momentum constant. This would
compel the rising elevator and mass to increasingly
drift west, which would cause
the tether to bend, pulling the attached
space station closer to the elevator and
to Earth’s surface. If the cable did not
snap under the stress, the station would
steadily increase its eastward angular
orbital velocity (again to keep the angular
momentum constant) as it was being
forced to descend diagonally along the
stressed, increasingly bending tether. The
descending station could possibly collide
with the elevator simultaneously just as
both strike the surface while attached
to an again taut tether, stretching from
its attachment point, 36,000 kilometers
westward along the equator’s surface.
Julian Kane
Hofstra University
LORENZINI AND SANMARTÍN REPLY: Ambrose
worries about confusing students with
the expression “centrifugal force,” a concern
shared by professors teaching fi rst-year undergraduate
physics. This concern has led
authors of textbooks (Physics, by Kane and
Sternheim, 1983; Physics, by Cutnell and
Johnson, 1989) to systematically avoid the
term. Yet many professional physicists do refer
to “centrifugal forces” (for instance, Physics:
Classical and Modern, by Gettys, Keller
and Skove, 1989; the Nobel Prize–winning
Richard Feynman in his Feynman Lectures
on Physics, Vol. I). It is a valid concept within
a rotating frame of reference. We phrased the
article to imply such a frame but regret not
mentioning it explicitly.
The space elevator tether Kane refers to
would actually have to be 144,000 kilometers
long (four times longer than the altitude of
the geostationary orbit). The system’s mass
and the cable tension would be so great an
elevator with almost negligible mass with respect
to the system, moving along the cable
at a reasonable speed toward geostationary
orbit, would bend the tether to a small extent
but not tip the massive system over. The
conservation of angular momentum is maintained
by a slight slowing of Earth’s rotation.
PROBLEMS WITH BUNKER BUSTERS
Michael Levi’s article “Nuclear Bunker
Buster Bombs” doesn’t mention the
most important factor in the success
of these weapons: reliable intelligence.
How would we know the bunker’s location
(within tens of meters) as well as its
depth, geology and degree of hardening
and whether it is a command center or
contains biological or chemical munitions?
Even troops on the ground may
require months to locate targets. Before
the Iraq invasion, offi cials spoke with
certainty about the location and nature
of the country’s weapons of mass
destruction, yet none have been found.
Given the diffi culty in obtaining reliable
intelligence, the viability of these weapons
is questionable.
John Howard
Ventura, Calif.
Letters
EMPIRIC- OR FAITH-BASED PHYSICS?
Regarding Laurence M. Krauss’s comment
about the challenge of teaching
evolution in schools [“Questions That
Plague Physics,” an interview by Claudia
Dreifus]: I am always shocked to
hear about the educational situation in
the U.S. It is incredible that fundamentalists
who are science illiterates have a
say in what should be taught. I wonder
how long the U.S. populace can afford
this poor grounding in science. Surely
the government’s bad record in curtailing
energy consumption and pollution
is related to the widespread lack of a
proper science education.
Gerhard Buzas
Innsbruck, Austria
The Krauss interview reveals a fuzzy
distinction between science and science
fi ction. Krauss says that scientists
empirically prove things and “toss out
things that have been disproved.” It
seems that testing and tossing may be
needed for notions such as string theory
and extra dimensions. Until these
theories can be confi rmed with observations,
they remain mere mathematical
musings, good only as a challenge
for students. The danger is that esoteric
theories could become accepted as fact
solely on the basis of the author’s status,
which brings us perilously close to
the power of the priesthood and faithbased
physics.
Colin Gordon
Santa Barbara, Calif.
ERRATA In “Nuclear Bunker Buster Bombs,”
by Michael Levi, a 10-kiloton nuclear weapon
detonated one meter underground in soil
would destroy targets within 50 meters, not
fi v e m e t e r s .
In “Electrodynamic Tethers in Space,” by
Enrico Lorenzini and Juan Sanmartín, Kevlar
was described as a carbon fi ber; it is an aramid
(short for “aromatic polyamide”) fi ber.
“SETI at Home” [Data Points, News Scan]
should have reported the number of countries
participating in SETI as 226 countries,
possessions and other territories.
14 SCIENTIFIC AMERICAN DECEMBER 2004
COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
ALFRED T. KAMAJIAN