QUANTA

RECENT SIGHTINGS

Thomas Shannon, The Notes, 1969-1976

Size MattersTO KEEP GETTING SMALLER, FUTURE COMPUTERS

MAY HAVE TO BE BUILT BY ORGANIC CHEMISTS

ciency. The smaller the IC, the shorterthe path an electric signal must travel,making the chip faster. Furthermore, asmaller IC uses less energy to run theelectrical components, thereby keepingthings cool.

Meanwhile, the long and successfulshrinking of the silicon-based chip mayat last be approaching its physical limits.David A. Muller and his colleagues at BellLaboratories in Murray Hill, New Jer­sey, report in a recent issue ofNature thatthe minimum thickness ofelectrical insu­lation needed to properly isolate elec­trodes embedded in a chip is four layersof silicon atoms. And according to onerecent estimate, the present rates ofminiaturization will produce silicon chipswith a thickness of five atoms by 2012.

Rotaxanes are as fast as their silicon­based cousins and consume less energy,but they have one great disadvantage:once oxidized, the oxidationprocess can­not be reversed. Hence, once the flow ofcurrent across a rotaxane layer is turnedoff by oxidation, it cannot be turned backon again. That prevents the molecular lay­er from acting as a simple on-off switch.

To overcome that limitation, theUCLA and Hewlett-Packard team isnowlooking for molecules that can beswitched on again. Heath imagines a mol­ecule that could alternate between theinsulating and conducting state by apply­ing higher or lower voltages than the onesthat would be used to perform the com­putations, so as not to interfere with themain work of the computer. Such aswitch would not only be many timessmaller than a silicon-based transistor, itwould alsoneed only halfthe wires a tran­sistor needs to perform its task.

But the new technology still has a longway to go. Although Heath is optimisticthat the first competitive computersbased on organic molecules could beavailable in a dozen years, other scien­tists in the field do not want to raise somany expectations.

"It's certainly an interesting idea, butit's still far away from [bringing us] mol­ecular-sized computers," says the physi­cist Cees Dekker ofDelft University ofTechnology in the Netherlands. Dekkerbelieves too many technical problemsremain for molecule-based computers toreplace silicon anytime soon. That meanstoy-savvy computer freaks-already

rotaxane molecular complexes, sand­wiched between two electrodes, allowselectrons to travel freely from the neg­ative electrode to the positive one, skip­ping through the rotaxane layer as if itwere a stepping stone in a river. Butwhen the current is reversed, the mole­cule rapidly oxidizes-losing electrons toone ofthe electrodes-and turns into anelectrical insulator, switching off theflow ofcurrent. The team reported theywere able to wire several of the rotax­ane-based switches together to buildsimple devices (AND, OR and NOTgates) that carry out logical operations.

"This was the first step toward build­ing a whole [molecular] computer," saysthe chemist James R. Heath of UCLA,adding that he expects molecular com­puters to operate as many as a billiontimes faster than their silicon cousins.The work of Heath and his team wasreported in the July 16 issue of Science.

UNUKE ITS FORGOTTEN PREDECES­

sor from the 1950s, the molecularcomputer for 1999 seemed particularlywell timed. The driving force behind theminiaturization of logic processors andmemory elements is computing effi-

IN THE EARLY STONE AGE OF COM­

puting, circa 1959, the U.S. Air Forcespent $4.6 million on a project to createwhat it called molecular electronics­"moletronics," in the jargon of the day.It was a spectacular case of bad timing;just a few months earlier, the integratedcircuit (Ie) had been invented, andmoletronics, according to the comput­er historian Paul E. Ceruzzi, was "qui­etly" set aside in favor of the IC, whichthe trade press explained at the time asan "interim step." Forty years later, theintegrated circuit has become the basicworking element of the modem com­puter. But in July "molecular electron­ics" came back into the news, this time,perhaps, in a form practical enough toresuscitate the old prediction that the ICwill one day seem like an "interim step."

The new molecular component is anelectronic switch whose key element isa single V-shaped organic molecule calledrotaxane. A team ofchemists based at theUniversity of California, Los Angeles,working with chemists and computerscientists from Hewlett-Packard Labo­ratories in Palo Alto, California, becameintrigued by a property of rotaxane: itcan act like a switch. A single layer of

6 THE SCIENCES· September/October 1999

accustomed to the "Intel inside" label ontheir pes-must wait for computerswith an "organic molecules inside" tag.

-HOLGER M. BREITHAUPT

HOW COLD IS REALLY COLD? EVERY­

one knows the answer: absolutezero, 273.15 Celsius degrees below thefreezing point of ordinary water. Butcan anything actually get that cold?Theoretical physics says no: the coldestof cold can be approached but neverquite attained. Well, then, does it makeany sense to say matter can be measuredat absolute zero? A group at the Massa­chusetts Institute of Technology saysthe answer is a qualified yes. Matter atabsolute zero can be measured, but notabsolutely.

Working with a so-called Bose-Ein­stein condensate, the MIT investigatorshave quantitatively measured a state thatis characteristic of matter at absolutezero. Moreover, they have made the

measurement without having to extrap­olate from observations ofmatter madeat relatively high temperatures. Thestate of matter is characterized by so­called zero-point motion, and it hasbeen predicted to exist theoreticallysince the early 1920s. It arises, albeitsomewhat magically, out of an invio­lable law ofquantum mechanics knownas the Heisenberg uncertainty principle.

According to classical thermodynam­ics, a system would reach absolute zeroonly when all energy had been removed.That would cause the system to grind toa dead halt. But old laws fall to new, andthe Heisenberg uncertainty principle hassuperceded that classical dogma. It statesthat no particle of matter can have botha precise momentum and a precise posi­tion at the same time. Thus matter at ornear absolute zero can never freeze stock­still, as classical ideas contend; for exam­ple, a truly motionless atom would havea definite position and zero momen­tum-also a precise value-thereby vio­lating the uncertainty principle.

What nature does instead is save theuncertainty principle, but at a cost. Rel­egating both position and momentum tothe shadows of probability, particles ofmatter in a known enclosure retain a

small but nonzero range ofmomentum,even in their lowest-possible energystate, the ground state. Matter in itsground state is not perfectly stationary.Instead, it jiggles, and so no particle everassumes an entirely defmite position­thereby satisfying the dictates of theuncertainty principle. It is the jigglingthat constitutes the zero-point motion.

Zero-point motion has been experi­mentally confirmed for some time, butonly in relatively "warm" statesofmat­ter. The MIT group used a new tech­nique to measure the zero-point effectin a Bose-Einstein condensate. Neverbefore hassuch a body ofconfirming datacome directly from measurements madeon matter whose temperature wasso nearabsolute zero.

Led by the physicist Wolfgang Ket­terle, the MIT group chilled sodiumatoms in a magnetic trap to about 500nanokelvins: 500 billionths ofa Celsiusdegree above absolute zero. The atomscollapsed into a state ofso-called Bose­Einstein condensation, after Saryen­dranath Bose and Einstein, who pre­dicted the effect in 1924. Inside themagnetic trap, which at 500 nano­kelvins is arguably the coldest place inthe universe, the condensed atoms act

host to anothen.Rats pick up the par­asite through contact with cat feces orby eating other animals that have beeninfected. Inside the cells of the rat theparasite reproduces asexually, poten~tiallyinvadingany organ,including the

brain. Although Macdonald and his coworkers propose nopreCisemechanism, they do suggest that the parasite's

pr.esence in the brain somehow proY'Okeskamikaze tendencies in rats, thereby facili­

tating the parasite's transferback to acat­me only host in which it can undergosexual reproduction. Once inside thecat, T. gondii continues its life cycle,

" maturing, reproducing and ultimate­ly ending up back in the litter box,where the cycle startsover again.

Although Macdonald says his""findings are among the first empiri­calevidence that parasitescan alter the

behavior of mammals, T. gondii's trapisnot unique. "Very interesting, but not

unheard of," says the parasitologist LarryS. Roberts of the University of Miami.

Roberts points to several other parasites that per­form similar tricks. A tapeworm known as Tae­nia multiceps, for instance, has been known tocause sheep to stagger, making them mote vul­

nerable to wolves, another of the worm's 1l.osts.-YUVAl ROSENBERG

IN THE AGE-OLD GAME OF CAT AND

mouse, a new player has come tolight. A parasite that commonly infectsrats makes the animalsoddly fearless ofpredators, such as cats.

Biologists at the University ofOx-. . ... . .ford studied several hundred brown rats andfound that 35percent of them were infected with a protozoanknown as Toxoplasma gondii. The infectedrodents showed no signs ofserious illness,hut they behaved quite differently fromuninfectedrats. Theyscamperedaroundmore and lost their natural suspicionof new objeetsandeders. And, in asuicidal tum, some were actuallyattracted to feline scents.

The biological mechanism for therats' personality change remainsunclear, but the Oxford study pointsto the parasite, T. gondii, which has acomplicated life cycle that requires it toinfect cats, at least some of the time."Otherratsinfected with different parasitesdo not change their behavior, so somethingspecial isgoing on," saysthe behavioral ecologistDavid W. Macdonald, who led the research team. Three cots and a rat,

T. gondii-which is notoriously dangerous to English, 13th c.pregnant women,leading doctors to advise themagainstcontact with catsand their litter boxes-e-seemsto haveevolved an ingenious adaptation to help it spread from one

September/Oerober 1999 • THE SCIENCES 7

Q-bitsStarstruck. Nearly thirty years agoastronomers predicted that the rarebut long-livedisotopeiron-60 shouldform in substantialamounts during asupernova, the prodigious explosionthat spectacularly announces thedeath ofa star. Now a multidiscipli­nary team ofinvestigators from threeGerman institutionshasdetected rel­ativelyhigh levelsofthat rare isotopeon the oceanBoor,thusprovidingthefirst direct evidence that a superno­va hasleft its signaturehere on earth,Aftersamplingaplugofsedimenttak­en from the South Pacific near Tahi­ti that represents the past 13 millionyears,theywere ableto conclude thatabout five million years ago a super­nova exploded some ninety light­years away from earth.

Smash hit. How much energy doesit take to reduce objects to smith­ereens?To answer that question, twocomputer scientists from the Uni­versityofStuttgartin Germanydevel­oped a device worthy of the DavidLetterman show-a destruction sim­ulator. Ferenc Kun and flans]. Herr­mann built a computer programdesigned to slamtogether two virtu­aldisks madeofmanysmall, unbreak­able units held together by elasticthreads.When the diskshit eachoth­er at various energies, the computercalculatedwhich threadsbroke. Theresults showedwhat we alreadyknowfrom dailyexperience:acriticalener­gy isrequired to shatterobjects com­pletely. Below that level, the disksmerely broke into chunks.

Head over heels. In what must bethe arachnid version of the heat ofpassion, male redback spiders vaultinto the females' mouthparts. Themove, known as the "copulatorysomersault," often ends in the male'sdemise. Now researchby the behav­ioral ecologists Maydianne C.B. An­drade and Erin M. Banta of CornellUniversity sheds light on how thatself-sacrificial maneuver might beadaptive. Males that successfullyentice females to dine on them fathertwice the number of offspring astheir non-cannibalized competitors,it turns out. Males that fail to be eat­en enjoy less copulation time, andrelinquish any future shot at father­hood becauseofthe damagewroughtby the somersaulton their reproduc­tive organs.

just as they would if the temperaturewere absolute zero.

Ever since the first Bose-Einsteincondensate was made in 1995, the zero­point motion of the condensed atomshad been assumed. The trick was to finda technique to prove it quantitatively,without so jostling the fragile conden­sate that it would disappear. Until theMIT investigation, momentum studieson an undisturbed condensate had beenstymied, leaving physicists with nooption but to tum offthe trap and mea­sure the expanding atom cloud. Butonce the cloud began expanding, themomentum immediately jumped to atleast ten times larger than that of zero­point motion.

Ketterle and his coworkers avoidedthose pitfallsby applying an experimen­tal technique called Bragg spectroscopy.By bouncing laser-generated photons offinely tuned frequencies off the con­densed atoms, the experimenters wereable to deduce the momentum of thechilled atoms. They found that theatoms, instead ofzipping along at about300 meters a second, as they do at roomtemperature, moved like a fly caught inmolasses, some 300,000 times moreslowly. Those results were in line withthe predictions of the uncertainty prin­ciple, thereby proving the chilly state ofthe atoms in the condensate.

-CLIFFORD F. RANSOM

STUDENTS WRESTLING WITH CHEM­istry will cringe at the news: "Peri­

odic Table Updated! More ElementsFound!"

Nuclear physicists blasted two newelements into being this past Aprilat the eighty-eight-inch cyclotron atLawrence Berkeley National Labora­tory in Berkeley, California. Elements116 and 118 were discovered after aserendipitous decision to act on the sug­gestions ofRobert Smolanczuk, a visit­ing Fulbright scholar and theoreticalphysicist from the Soltan Institute forNuclear Studies in Poland. Smolanczuk,who was studying the daunting prob­lem of creating superheavy elements,calculated that collisions between kryp­ton ions and lead targets might form anew element.

"We didn't expect this to work, butwe gave it a try and it paid off spectac-

ularly well," declared Walter D. Love­land, a visiting chemist from OregonState University, who contributed tothe experiment. The payoff was threeatoms, each with 118 protons in itsnucleus. All three atoms quickly emit­ted an alpha particle and decayed intothe equally novel element 116. (Ele­ment 117 remains at large.)

THERE ARE NINETY-TWO NATURAL­ly occurring elements in the peri­

odic table, ranging from hydrogen,whose atoms have just one proton in thenucleus, to uranium, whose atoms haveninety-two. In the past half century,physicists operating particle acceleratorsand nuclear reactors have created twen­ty-three more elements, all of themheavier-in other words, having morethan ninety-two protons. Those so­called transuranium elements have beenput to varied uses. Americium (element95), for instance, is a key component insmoke detectors, whereas plutonium(element 94), the most notorious, is themain explosive component in atombombs. The two newest man-made ele­ments, like many of their counterparts,existed for less than the blink ofan eye:element 118 decayed within a millisec­ond, and element 116 lingered onlyabout five times that long.

To make superheavy elements, physi­cists cause two large nuclei to crash intoeach other, in hopes that the collisionwill form a new kind of atom with anextra-large dose ofprotons in the nucle­us. Such Frankenstein-like creations areusually so unstable that they rarely lastlong enough to be studied. But in Jan­uary investigators at the Joint Institutefor Nuclear Research in Dubna, Russia,announced the creation ofelement 114,which was viable for about thirty sec­onds-a relative eternity.

The latest discoveries have manynuclear chemists and physicists excitedabout the possibility that the so-calledisland of stability, whose existence hasbeen postulated for about thirty years,may soon be reached. The theory pre­dicts that if elements possessingapprox­imately 114 protons and 184 neutronscould be produced, they would have lifespans long enough for investigators toprobe their properties. So these days,investigators are on a roll. The nuclearchemist Ken Gregorich of LawrenceBerkeley, who headed the most recentdiscovery team, thinks that if a bismuthtarget were substituted for the lead one,element 119 might soon eclipse element118. Chemistry students beware!

-LEVIN SANTOS

8 THE SCIENCES • September/October 1999

JO KES ABOUT INCEST IN ROYAL (AND

rural) lineages have come to occupya standard-if not hallowed-place inthe humor hall of fame. But it seemsthe joke may be on all of us, now thatthree physicists have developed an ana­lytical model that quantifies the genet­ic diversity of people living today andin the past.

In deciphering the statistical proper­ties of genealogical trees, the investiga­tors found that not only is it likely thatyou and your neighbor down the blockhave an ancestor in com­mon within the past thirtygenerations, but that manyofus can trace our lineagesback to some pretty impres­sive forebears. "Ifsomeonewere to come to me tomor­row claiming to be adescendant of Charle­magne's, I could now tellhim, with a fair degree ofcertainty, 'Yeah, well, soam I!'" says Damian H.Zanette, a physicist atthe Instituto Balseiro inArgentina and one of theauthors of a recent studypublished in Physical ReviewLetters. In fact, most peopleprobably are related toCharlemagne, or Confu­cius, or Cleopatra-andjustabout anyone else wholived before the thirteenthcentury.

By itself, that conclusionis no surprise. It has longbeen understood that ifyougo far enough back into thepast, the family trees ofeveryone on earth mustmerge together into the small, originalgroup of early humans from which weall descend. But no one knew how toexpressthat funneling ofhumanity in sta­tistical terms until a collaboration arosebetween Zanette, the physicist SusannaC. Manrubia, a postdoctoral fellow at theMax Planck Society in Berlin, and thephysicist Bernard Derrida of the EcoleNormale Superieure in Paris.

Until now, the only mathematicalrule available to genealogists for tracingpeople's lineages back in time assumedthat one has 2/1 ancestors in any given

past generation n. In other words, eachperson currently living must have twoparents, 22 grandparents, 23 great-grand­parents, and so on back through thebranches of the family tree. But apply­ing that rule over the span ofhuman his­tory (about 4,000 generations) leads toa paradox: Each of the nearly six billionpeople on earth has hundreds of timesmore ancestors than the number ofelementary particles in the visible uni­verse-clearly, an impossibility.

Genealogists, who are familiar withthe repeated appearances ofsome indi­viduals in any given family tree, havelong understood the reason for theparadox: populations living long agowere so small that matings were limit­ed, in many cases, to ones between rel­atives. (That would have been particu-

SpencerTunk~Nevada, 1997

larly true for the earliest humans, whoappeared about 100,000 years ago andnumbered no more than a few thou­sand individuals.) In the royal castes ofEurope-convenient real-world exam­ples of the kinds of finite populationsmodeled in the Zanette study-inter­breeding, coupled with diligent gene­alogical record-keeping, producedreadily apparent instances of ancestralrepetition. The family tree of the four­teenth-century English king EdwardIII, for instance, includes nearly 1,000people, some ofwhom appear as many

as six times. Such cases of repetitioncan arise when, for instance, two firstcousins produce offspring. Instead ofeight different great-grandparents, theirchildren will have only six.

The work of Zanette, Manrubia andDerrida shows that beginning far backin history and continuing until aboutthirty generations ago, a pattern ofinheritance involving large numbers ofancestral repetitions applied to thehuman population at large. But after thattime, many population groups were bigenough that fewer intrafamilial pairingstook place. The most notable result aris­ing from the team's model, whichassumes sufficient genetic mixing, wasthat about 20 percent ofthe people wholived thirty generations ago or earlierhave no living descendants, whereas the

remaining 80 percent areancestors of every personalive today.

The new findings-andthe method developed toobtain them-have impor­tant applications for evolu­tionary biologistsand popu­lation geneticistslooking fora simple measure of thedegree ofgenetic diversityina population, human or oth­erwise. As Zanette pointsout, repetitions in genealog­ical trees reduce the geneticdiversity represented byindividuals' ancestors-andtherefore reduce the geneticvariability in the genomes ofthe individuals themselves.Quantifying that reductioncould help evolutionarybiol­ogists evaluate the extent towhich various present-dayanimal populationsaregenet­ically representative of theirancestral populations.

In addition, the re­searchers suggest that theirwork could be applied to anumber of problems unre­

lated to evolutionary biology and popu­lation genetics. The equations used toexpress the relative contributions of dif­ferent ancestors from one generation tothe next are uncannily similar to mathe­matical descriptionsofhow forcesare dis­tributed throughout a randomly arrangedpile of granular particles. The sharedwork ofZanette, Manrubia and Derridamay thus be a key step toward distillingnot only the mathematical patterns ofgenealogical trees, but also the complexworkings ofother disordered systems.

-ANA BERLIN

September/October 1999· THE SCIENCES 9