ENRICHED STABLE

ISOTOPES

Produced to Your Specifications!

A gas centrtfuge cascade

From KILOGRAMS To TONNES

At Cost Effective Prtces!

Contact us for production availability of

Isotopes of the following elements:

Sr Cd CI Cr Fe Ga Ge Hg In I r Kr Mo Ni Os Pb Pt Re S Sb Se Si Sn Ta Te TI V W Xe Zn

AIChem nc. Pine Ridge Office Park, Suite 202·B

702 Illinois Ave., Oak Ridge, TN 37830 Phone: Dr. Bruce Clark, Dlr. of Mktg.

(615) 482-0028

he asks. "Or do you do the best you can? I respect the Embers for what they're doing." -].H.

PHYSICAL SCIENCES

Ice House Did a growth spurt of mountain ranges initiate the ice ages?

B eginning about 2.5 million years ago vast sheets of ice pushed south from the Arctic and blan­

keted much of North America and Eu­rope. Since then the ice has advanced and retreated every 100,000 years or so. The periodic recurrence of these ice ages has been linked to wobbles in the orbit of the earth known as the Milankovitch cycles, but these cycles predate the ice ages by millions if not billions of years. Why did the earth become susceptible to glaciation only recently?

Workers from the Lamont-Doherty Geological Observatory propose that a growth spurt of major mountain rang­es over the past five million years may have triggered the glaciation. They suggest that the higher topography of the Rocky Mountains and the Himala­yas could have disturbed jet streams in the Northern Hemisphere, diverting frigid arctic air south. Computer simu­lations lend credence to this hypoth­esis. The rapid uplifting might also have led, rather circuitously, to a de­crease in the amount of carbon diox­ide in the atmosphere and hence to a global cooling-a kind of negative greenhouse effect.

Elaborating on this latter hypothesis in Geology, Maureen E. Raymo, William F. Ruddiman and Philip N. Froelich note that higher elevations result in higher erosion rates. Lofty terrain at­tracts more precipitation, and water is more erosive spilling down a slope than on a flatland; moreover, uplifting continually exposes fresh rock to the weather. The weathering of highland rocks produces positively charged ions-including sodium, potassium, magnesium and calcium-that are car­ried by streams and rivers to the ocean. An increased influx of positive ions makes the oceans more alkaline and decreases the amount of carbon dioxide dissolved in the seawater, in part by locking it up in other carbon compounds. The net effect, since lev­els of carbon dioxide in the atmos­phere and in the ocean are at equilibri­um, is a decrease in the amount of atmospheric carbon dioxide.

22 SCIENTIFIC AMERICAN November 1988

Over very long time scales the deple­tion of atmospheric carbon dioxide caused by weathering is balanced by other mechanisms, such as the release of the gas from volcanic eruptions and metamorphic activity. But Raymo, Ruddiman and Froelich suggest that over the past five million years these restorative processes have not kept pace with the depletion caused by increased weathering. They cite nu­merous studies showing that during this period most of the earth's major mountain ranges have been uplifted at a faster rate than in the preceding five million years. The rate of uplift of the Himalayas and the Andes, in partic­ular, has more than doubled. Core samples extracted from the ocean floor also indicate that by-products of weathering have been deposited in ocean sediments more rapidly over the past five million years.

Apparently millions of years passed before the uplifting cooled the earth's climate enough to bring on glaciation. Even though the same processes con­tinue today, Raymo points out that this slow and subtle negative green­house effect is not likely to offset the positive greenhouse effect that is now in the news: the global warming ex­pected to result from the buildup of atmospheric carbon dioxide initiated by fossil-fuel burning and other hu­man activities. -].H.

Plus <::a Change ... Once again: The gravitational constant is constant

The idea that Newton's gravita­tional constant G may actually change with time has fascinat­

ed physicists for a century. The Aus­trian physicist Ernst Mach stated that the forces acting on water in a spin­ning bucket, for example, are due to accelerations relative to the distant galaxies. This idea, known as Mach's principle, led naturally to several the­ories in which G varies as the universe expands.

A different type of "variable G" the­ory was proposed by P. A M. Dirac in 1938. He noted that the ratio of the size of the universe to the size of an atom very nearly equals the ratio of the electrical force between an elec­tron and proton to the gravitational force between the same two parti­cles-and that both ratios are the huge number 1039•

Everything in the two ratios except the size of the universe is ordinarily considered constant; it is far from ob-

© 1988 SCIENTIFIC AMERICAN, INC

© 1988 SCIENTIFIC AMERICAN, INC

SCIENTIFIC AMERICAN In Other Languages

LE SCIENZE L. 3,500/copy L. 35,000/year L. 45,000/iabroad) Editorial, subscription correspondence:

Le Scienze S.p.A., Via G. De A lessandri, 11 20144 Milano, Italy

Advertising correspondence: Publietas, S.p.A., Via Cino de Ouca, 5, 20122 Milano, Italy

ofj-�I� .. l� Y880/copy Y9600/year Y13,OOO/iabroad) Editorial, subscription, advertising correspondence:

Nikkei Science, Inc. No. 9-5, I-Chome, Otemachi Chiyoda-ku, Tokyo, Japan

INVESTIGACION Y CIENCIA 450 Ptas/copy 4950Ptas/year $35/labroad) Editorial, subscription, advertising correspondence:

Prensa Cientifica S.A., Calabria, 235-239 08029 Barcelona, Spain

SCIENCE 27FF/copy 265FF/year 3l5FF/year labroad) Editorial, subscription, advertising correspondence:

Pour la Science S.A.R.L., 8, rue F�rou, 75006 Paris, France

SP�J}lJU 9.80 OM/copy 99 OM/year 112.200M/labroad) Editorial, subscription correspondence:

Spektrum der Wissenschaft GmbH & Co. Moenchhofstrasse, IS

0-6900 Heidelberg, Federal Republic of Germany

Advertising correspondence: Gesellschaft Fur Wirtschaftspublizistik Kasernenstrasse 67

0-4000 Duesseldorf, Federal Republic of Germany

4.t4 1.40R MB/copy 16RMB/year $24/1 abroad) Editorial, subscription correspondence:

IST IC-Chongqing Branch, PO. Box 2104, Chongqing, People's Republic of China

BMHPEHAn\H 2R/copy 24R/year $70/labroad) Editorial correspondence:

MIR Publishers 2, Pervy Rizhsky Pereulok 129820 Moscow U.S.S.R.

Subscription correspondence: Victor Kamkin, Inc. 12224 Parklawn Drive, Rockville, MO 20852, USA

TUDOMANY 98Ft/copy 1,176Ft/year 2,l00Ft/labroad) Editorial cor.respondence:

TUOOMANY H-1536 Budapest, Pf 338 Hungary

Subscription correspondence: "KULTURA" H-3891 Budapest, PI. 149 Hungary

f�\ I KO/cOfY 10KO/year $40/labroad) Editoria I subscription, advertising correspondence:

MAJALLAT AL-OLOOM PO. BOX 20856 Safat, 13069 - Kuwait

Advertising correspondence all editions: SCIENTIFIC AMERICAN, Inc. 415 Madison Avenue New York, NY 10017 Telephone: 1212) 754·0550 Telex: 236115

vious why the universe should be just the right size to make the ratios equal. Their apparent equality may be a cos­mic coincidence but if, for unknown fundamental reasons, the two ratios are in fact always equal, then it follows almost immediately that G should de­crease as the universe expands. More recently, superstring theories have also predicted that the constants of nature, including G, may change as the universe ages.

In spite of the many predictions of a nonconstant constant, no experiment to date has found any evidence of variation. Astronomical observations of the orbital period of the moon (which would change if G changed) and Viking lander data on the orbital period of Mars show that any change in G must be at a rate of less than about 3 x 10-11 part per year.

A larger G in the past would also have made the abundance of helium formed during the big bang larger than the 24 percent predicted by the usual value of G. Yet astronomical observations show that helium makes up no more than 25 percent of the universe's mass. This restricts G's var­iability to a limit of about 2 x 10-11 part per year, or to considerably less in certain cosmological models; G could have changed at most by 20 percent since the big bang.

Now Thibault Damour, Gary W. Gib­bons and Joseph H. Taylor report in Physical Review Letters that observa­tions of the binary pulsar designat­ed PSR 19 13 + 16 give similar results : G can change by no more than ( 1 ± 2.3) x 10-11 part per year (2.3 rep­resents two standard deviations).

The binary pulsar, which consists of a neutron star orbiting around anoth­er compact object, has been studied intensively since its discovery in 1974. Its orbital period and the rate of change of the period are known to extremely high accuracy-the latter to about 13 decimal places. Einstein's general theory of relativity actually predicts that the period of the binary pulsar will change as the two obj ects emit gravitational radiation and grad­ually spiral into each other. The ob­served rate of change of the period is in excellent accord with relativity, which assumes G is constant.

If G varied slowly, however, the equations describing the pulsar's or­bit would be altered and would pre­dict a different rate of change in the orbital period. The small differen­ces between the observations and rela­tivity's predictions constitute the max­imum allowed effect of a variable G. This leads to the limit quoted above;

24 SCIENTIFIC AMERICAN November 1988

as the authors say, this limit is con­sistent with zero. Some things nev­er change. -Tony Rothman

Wobbly Evidence Doubts remain on "sightings" of planets near other stars

In the hunt for planetary systems other than our own, tantalizing re­ports have consistently led to dis­

appointment. In the late 19 50's Peter van de Kamp of Swarthmore College attributed wobbles he had observed in the position of Barnard's Star, the sun's second-nearest neighbor, to the gravitational tug of an unseen giant planet. Unfortunately the wobbles did not turn up in later, more precise ob­servations. In 1985 workers from the University of Arizona and the National Optical Astronomy Observatories said they had actually spotted-by means of a complex imaging technique known as speckle interferometry-a planet­like object near another of the sun's neighbors. Again follow-up observa­tions suggested that the original find­ings were erroneous.

Undaunted, groups headed by Bruce Campbell of the University of Victoria in British Columbia and by David W. Latham of the Harvard-Smithsonian Center for Astrophysics recently nom­inated two new candidates for planet­hood. These obj ects seem unlikely to vanish outright; the question is wheth­er they are actually planets or more massive, starlike objects.

Like van de Kamp, Campbell and Latham search for wobbles in the posi­tions of stars. But van de Kamp em­ployed astrometry, in which the ap­parent distance between a nearby star and others so distant that their posi­tions seem fixed is measured. Camp­bell and Latham instead examine the spectra of stars for Doppler shifts in­dicative of motion toward and away from the earth. This technique can monitor stars far beyond the range of astrometry, as long as they are bright.

Spectral analysis provides informa­tion about motion in only one di­mension, however, and so it cannot indicate how the companion's orbital plane is oriented. Consequently the technique can establish only a lower limit for the mass of a companion. The lower limit follows if one assumes the orbital plane is parallel to the line of sight (edge on to viewers on the earth) so that the companion's full gravita­tional effect can be seen.

After seven years of monitoring a group of stars, Campbell has dis-

© 1988 SCIENTIFIC AMERICAN, INC