SUMMER 2005 T H E B E N T O F T A U B E T A P I 17

by Trudy E. Bell

Engineering Gutsy Science on a Shoestring—

Testing Einstein

RESSED IN Awhite clean-roombunny suit, MITphysicist RainerWeiss crawled on

his hands and kneesinside a frigid half-mile-longsection of what appeared to bea 4-foot-diameter stainless-steel culvert erected aboveground on a concrete slab inremote southeastern Washing-ton. When he shone his flash-light at the base of the firstlight baffle—an interior ringwith teeth as vicious as a woodsaw’s—and saw a “puddle ofsoot-like dust,” he recounted, “Iwas horrified.”

Why Weiss’s panic? 1) Thestainless-steel culvert was ac-tually one section of a beam tube—intended to be theworld’s largest vacuum chamber, enclosing the light pathof the world’s largest Michelson interferometer, for detect-ing gravitational waves predicted by Einstein. 2) Any par-ticulates inside the beam tube would destroy theexperiment, so the beam tube was being assembled underClass 100 clean-room conditions. 3) Therefore, no more than100 particles larger than 0.5 micrometers across were sup-posed to be in each cubic foot of air inside the beam tube,much less growing piles of sooty, sharp-edged ceramic dustthat could be picked up on the end of a finger. 4) Accordingto the terms of the fixed-price contract between the Cali-fornia Institute of Technology and the beam tube’s contrac-tor Chicago Bridge and Iron (CB&I), the university hadsigned on the dotted line to assume the entire financial riskof the project and to develop solutions if something wenthorribly wrong.

Therefore, since Weiss was science liaison for the beamtube, this ceramic dust unexpectedly splintering off the

shark-tooth light baffles—andits implications for success orfailure of the third-of-a-billion-dollar experiment—wassquarely, sickeningly, Weiss’sproblem.

Several such major panicmoments dogged the decade-long construction of the twin“telescopes” of the Laser In-terferometer Gravitational-wave Observatory, known forshort as LIGO (pronouncedLYE-go). Some, including theceramic dust, were temporaryshow-stoppers while franticscientists and engineershuddled to figure out whatnext (“Fear is a great motiva-tor,” wryly observed LIGO di-rector, Caltech physicist Barry

C. Barish). What’s striking about the construction of LIGO,though, is not that things went wrong (on monumental en-gineering projects, they always do), but the “enginuity”involved in righting matters again—and indeed, in theproject’s sheer scale and daring conception.

The payoff? This very summer (2005), after more thana year of testing and tweaking to maximize sensitivity,both LIGO telescopes (now completely dust-free) are be-ginning regular operation 24/7. No, neither one looks likeany telescope you’ve ever seen; from a distance, there isno shiny metallic dome or giant parabolic dish of mesh toherald a major optical or radio astronomy observatory. Allthat is visible at each site is a long, low, featureless,rounded concrete enclosure marching off for 4 kilometers(2.5 miles) in two perpendicular directions into nothing-ness: a sagebrush desert 13 miles north-northwest ofRichland, WA, for LIGO-Hanford [shown above], and for-ested wetlands 25 miles east of Baton Rouge, LA, forLIGO-Livingston.

What do you get when you cross an exotic predictionby Einstein with daring astrophysicists and a

determined funding agency having only limited means?Twin new Laser Interferometer Gravitational-wave Observatories.

d

LIGO inHanford, WA.

18 SUMMER 2005 T H E B E N T O F TA U B E TA P I

Yet, these two odd, L-shaped telescopes 3,000 km apartare working together to search for direct evidence of oneof Einstein’s weirdest and most important predictions forboth physics and astronomy: gravity’s bending of the ge-ometry of space and time.

ENGINEERING SPECSIf you can’t readily wrap your mind around Einstein’s as-trophysics, never mind. Understanding cosmologicalesoterica is actually immaterial for this construction story.

Focus instead on the staggering engineering specifica-tion necessary for each LIGO telescope to have a prayerof testing Einstein’s prediction. Basically, each telescopehas one single heroic task: continuously comparing thephysical lengths of its two 4-km long perpendicular arms—specifically, by comparing the time infrared laser lighttakes to travel the distance between each arm’s main pairof mirrors—to detect a relative displacement of 10-16cm,smaller than a thousandth (0.001) of the diameter of a sub-atomic neutron. The pattern and magnitude of any rela-tive displacement would signal the passage of agravitational wave, as well as yield information about thewave’s source.

Even more stringent (if that’s even conceivable), be-cause of the high cost of such a mammoth experiment, thescientists and engineers wanted to build both LIGO tele-scopes so they would have a minimum lifetime of 30 years.Thus, they wanted to ensure that its lasers, mirrors, anddetectors could be upgraded regularly—even to accom-modate technologies not yet invented. Also, once the beam

tube was evacuated, they wanted to be able to do all nec-essary upgrades and maintenance without breaking beam-tube vacuum for the next three decades. In short, “buildingLIGO was like building a giant spacecraft,” Weiss said.“Once it’s launched, you can’t get to it again. It has to bedead reliable, because you don’t want to lose the mission.”

The ambitious LIGO project resembled a spacecraft inthree other ways. It took about as long to plan and build asa major unmanned interplanetary mission (the basic planwas conceived in the 1970s and ’80s, and ground was bro-ken in 1994 at Hanford and 1995 at Livingston, with con-struction finishing at both in 2004). It called for physicistsand engineers to work hand-in-glove, with one of each head-ing up every major system (physicist Weiss co-led the de-velopment of the beam tube with engineer Larry Jones,and physicist Michael E. Zucker co-led the development ofthe vacuum system with engineer John Worden). And itsconstruction cost was almost as dear, totting up to a bot-tom line of $292 million for the scientific facilities for thetwo sites.

Figure 1 Aerialphotograph showsthe entire L-shapedtelescope of the LaserInterferometerGravitational-waveObservatory nearLivingston, LA. Eacharm of the facility is 4km (2.5 miles) long.Each telescope is agigantic Michelsoninterferometer, whosesole purpose is tomeasure continuouslythe relative lengths ofthe two perpendicu-lar arms; according toEinstein’s generaltheory of relativity,should a gravitationalwave from a violentcelestial event passthrough the Earth,one arm of LIGOshould contract andanother shouldlengthen in a specificpattern.

“Building LIGO was like building a gia“Once it’s launched, you can’t get to it again

because you don’t want to los

AN OVERVIEW OF LIGO

BENTSU05_p17-24_FE_Bell 6/21/05, 1:24 PM18

SUMMER 2005 T H E B E N T O F T A U B E T A P I 19

That steep price tag was itself an albatross: “Our prob-lem was having to build LIGO on a budget the NSF al-ready considered dreadfully expensive, but actually wasnot enough money,” Weiss explained. Thus, “building some-thing we could afford drove most of the technology and mostof the anxiety. Our fundamental problem was the money—

always the money.”Because the project’s contractors were apprehensive

at the high risks of something going wrong with preciouslittle cash for contingencies, Caltech and the LIGO projectassumed many of the financial and physical risks that usu-ally devolve to contractors. They had to do all the R ofR&D ahead of time so companies could bid on essentiallyfixed-price contracts specifying that there would be vir-tually no change orders during construction—and if some-thing did go horribly wrong, it was on LIGO’s head tofigure a way out.

Ironically, those high-risk terms did not have the ef-fect of driving the pairs of LIGO scientists and engineersto proven, standard engineering handbooks to make surethey didn’t mess up. Instead, they threw out the hand-books for just about every system in LIGO, to press thelimits of known physics to invent superior means of achiev-ing both affordability and reliability.

Of all the LIGO systems, the two most central to theguts of the experiment were each telescope’s beam tubeand vacuum equipment. Because the beam tube, vacuumsystem, sheltering concrete enclosures, and various testsaltogether accounted for $116 million—a full 40 percent ofthe project’s cost—the saga of the beam tube and vacuumsystem dramatically demonstrates the lengths the scien-tists and engineers went to in order to innovate foraffordability and reliability.

The resulting tale of adventure engineering for high-stakes pure science is fully comparable to the mid-20th-century’s now-legendary building of the world’s largest200-inch Hale telescope on Palomar Mountain.

MUCH ADO ABOUT ‘NOTHING’Each LIGO telescope is an interferometer using a high-power (nominal 10 watts) infrared laser whose beam issplit and sent down and back both arms 100 times to com-pare the phase—that is, precise light travel time—of thereturning light wave fronts, and thus the precise lengthsof the arms.

Right from the start “it was very easy to realize thatLIGO needed a high-vacuum beam tube,” explainedBarish, “because the scattering of light off moleculesthrough 4 km of air is considerable”—actually, 400 km with

the multiple reflections. Such atmospheric scattering is,of course, what causes sunbeams to be visible in dusty orhazy air.

The problem was not that scattering reduces efficiency.The truly serious problem was that scattered light couldreflect off the beam tube’s inner walls and into the ulti-mate detector, adding noise. That noise was particularlydreaded because the beam tube is not isolated from groundmotions. Ground vibrations transmitted to any reflectedlight waves would shift their phase, or the timing of theirwave fronts. In fact, ground vibrations would make noisefrom reflected scattered light exactly mimic a signal ex-pected from an incoming gravitational wave! Hence, tominimize reflections the beam tube was made wide—fully1.2 meters (4 feet) across—and the serrated light baffles[Fig. 2] were installed.

But wait, there’s more. “We not only wanted a vacuum,but also we wanted the vacuum to be there for future gen-erations,” Barish continued. “Thus, we didn’t want the sci-ence to be limited by how good the vacuum is.” The LIGOscientists wanted a vacuum of about 10-9 or 10-10 torr—about equivalent to the atmospheric pressure at an alti-tude of 100 miles above the Earth (twice the height of thenominal edge of space). So far, so good: 10-9 or 10-10 torr isroutine for laboratory-sized high-vacuum chambers of afew liters as well as in commercial cathode-ray tubes forcomputer monitors. Also, the LIGO scientists did not wantto allow outside air to leak into the tube faster than 10-9

torr liters per second—again, standard for small high-vacuum systems.

a giant spacecraft,” Weiss said,again. It has to be dead reliable,to lose the mission.”

Figure 2 Teeth asvicious as a shark’sserrate the inner edge ofthe light baffles placedalong the length ofLIGO’s beam tubes.Caltech physicist Kip S.Thorne calculated theexact pattern of varyingwidths and heights forthe teeth so as todestroy any scatteredlight reflected from thebeam tube’s walls—essential, because anysuch reflected light wouldadd noise that exactlymimicks the appearanceof a gravitational wave.

20 SUMMER 2005 T H E B E N T O F TA U B E TA P I

The challenge was that “we were asking for the same kind of leak perfor-mance [standard for table-top systems] on this enormous volume,” empha-sized Weiss. At 4 km long and 1.2 m wide, each beam tube enclosed a volume ofsome 4.5 million liters, far larger than any previous high-vacuum system everbuilt for space simulation chambers, telescope-mirror–coating chambers, oreven classified nuclear tests.

Lastly, unlike particle accelerators, which have vacuum pumps installedabout every 10 meters along the beam line, “we wanted to get away with pump-ing only from the ends, like sucking through a 2.5-mile-long straw,” recountedMIT physicist Michael E. Zucker, now head of LIGO-Livingston, who co-de-signed LIGO’s vacuum system. Why that radical spec? “Pumping is expen-sive—you have to buy pumps, supply power, and instrumentation,” Zuckerreminded. “It’s all money.”

Minimal pumping, however, meant that “the beam tube itself had to do twothings exceptionally well: not outgas [from the steel itself] and hold air out—that is, not leak from welds,” Zucker continued.

Both of those specs meant radical new manufacturing techniques.

OUTRAGING THE EXPERTSEven with very pure commercial steel, molecules of normal atmosphericgases—water vapor, carbon dioxide, carbon monoxide—stick to the surface.In a beam tube the size needed for one of LIGO’s arms, “a cup of water mightbe bonded to the surface,” estimated Weiss. The bonds are weak, so the mol-ecules are always coming loose and sticking somewhere else, with the resultthat the vacuum is bad. So vacuum engineers usually heat the steel chamberto drive off the water, and then pump out the vapor molecules. Even then, leta chamber sit for a few weeks at room temperature and it will slowly fill withlow-pressure hydrogen gas, as mobile hydrogen atoms deeply embedded amongthe iron atoms migrate to the surface. Thus, vacuum chambers are usuallypumped continuously from as many places as possible.

As early as 1990 though, Caltech scientists wondered whether they couldbuild beam tubes out of steel specially pre-treated to remove atoms and mol-ecules that usually outgas into a vacuum chamber. Through basic metallurgi-cal research plus the building of a 2-foot-diameter prototype beam tube at theCaltech synchrotron laboratory, they found that baking coils of sheet stainlessat 450°C for 36 hours and annealing them in very dry air would reduce theamount of hydrogen in the steel by a factor of 1,000. There was a limit, however:reduce the hydrogen too much and the steel would become brittle and perme-able, a risk that affected subsequent phases of construction. Nonetheless, theythought, using low-hydrogen steel might allow pumping only from the ends.

For the second task of keeping air out, the LIGO scientists and engineersrelied on CB&I’s welding expertise, assisted by MIT Professor Thomas W.Eagar, P.E., [Massachusetts Beta ’72] who had helped to solve welding prob-lems that the U.S. Navy had encountered in atomic submarines. Traditionalvacuum tubing is made by rolling up a sheet of steel and giving it one longitu-dinal weld, often by hand. Although that gives the shortest weld, it’s labor-intensive and not efficient for constructing 16 km of large-diameter tubing.For speed and efficiency, the prize goes to the spiral mills used for makinglarge water lines and drainage culverts: a coil of rolled steel is loaded into themachine, which pulls the end off at an angle to form a helix whose edges aremated and welded inside and out to form a spiral seam (rather like the spiralseam seen on the cardboard tube that holds a roll of paper towels). But spiral-welding technology was not applied for high-vacuum use, the main problembeing the sheer length of the weld (about 15 km for each 4-km beam tube).CB&I and Dr. Eagar developed techniques for cleaning the welding wire andusing dry gases so as not to reintroduce hydrogen into the seams. “So we mar-ried the inexpensive technique of the spiral mill with the technology of mak-ing high-quality welds that don’t leak,” stated Caltech physicist Stanley E.Whitcomb, LIGO’s overall deputy director.

“We had a helluva time!”Weiss recalled, “We weretold over and over by

people who make highvacuum systems that

pumping just from theends was too big of

a gamble.”

Figure 3 Apparatus invented by CB&I to testthe circumference of each girth weld for leaks.

SUMMER 2005 T H E B E N T O F T A U B E T A P I 21

Last, to save both materials and transportation costs,just how little steel could they get away with using? “Forthe beam tube to collapse, it has to go out of round,” ex-plained Frederick J. Raab, Zucker’s northern counterpartas head of LIGO-Hanford. “If the beam tube can’t go out ofround, it can’t collapse.” Using techniques well-known tovacuum engineers, the LIGO scientists and engineers cal-culated that they could get away with walls as thin as 3mm (1/8 inch) if the beam tube were stiffened by exteriorperpendicular rings welded every 30 inches along its length.Getting the numbers right to shave off so much weightposed some risk, especially with the relatively fragile low-hydrogen steel. Not only was there some risk that the weldsmight reintroduce hydrogen into the steel, but also in a tubethat long “there’s one gigajoule in the vacuum—that’s theenergy needed to remove the air,” said Raab. “That’sequivalent to the energy needed to launch a mass of onekilogram into a 10,000-km orbit. If we buckled the beamtube, we’d be filling out lotsa paperwork.”

When the LIGO team leaders proudly presented theirreduced-pumping proposal to the National Science Foun-dation in the early 1990s, many outside reviewers “wereoutraged by our whole concept,” Weiss recalled. “We had ahelluva time! Some believed we had underestimated thecost and everything else by a full factor of 10. We were toldover and over by people who make high-vacuum systemsthat we were under-pumping that big thing; they recom-mended a hundred times the ratio of pumps to area thanwe have, that pumping just from the ends was too much ofa gamble. Advisors from the DOE [Department of Energy]wanted to kill the project, saying we were trying to do it onthe cheap.”

Although back then the NSF didn’t have experience withsuch a huge project, Weiss declared, “we have to give theNSF a lot of credit. They stuck with it.”

BUILDING LIGOApproval in hand, CB&I began laying in materials—get-ting lucky, as “the Russians decided to screw the West bydumping cheap steel on the market just when we decidedto buy,” Weiss recounted. “We were the beneficiaries, buy-ing all the stainless we needed for about half our budgetedprice”—a precious savings on the strapped project.

In 1996, CB&I designed and installed a $1.5-million cus-tom spiral mill in a building rented in an industrial parkwest of Pasco, WA, for LIGO-Hanford and then later toreit down and took the equipment to a similar rental in Mag-nolia, LA, for LIGO-Livingston [Fig. 4]. The spiral seamswere fusion-welded inside and out in a dry nitrogen atmo-sphere to prevent the absorption of hydrogen. Then thetubes were cut off in 20-meter tube segments (the longestpractical to truck on U.S. roads). At that point, Raab re-counted, the 3-mm stainless “could just barely hold its ownweight up,” each tube segment visibly flexing as it cameoff the mill.

Immediately, each tube segment was fitted every 30inches with external 1-1/2-inch wide stiffening rings madeout of quarter-inch (6-mm) steel, with a solid weld on oneside and a scatter weld on the other, again using a dry

Figure 4 A spiral mill at Chicago Bridge and Iron (CB&I) isshown manufacturing segments of LIGO’s beam tubes.

Figure 6 Spray rig for cleaning residues of hydrocarbonsfrom inside each beam tube segment was designed by CB&I.Lowered on a hose into each tube segment, it sprayed thetube’s interior with detergent, rinsed it with water, followedby a worker’s pouring of a strip of spectroscopic-gradeisopropyl alcohol, whose run-off was inspected at a lab forminute levels of hydrocarbons.

Figure 5 This clam-shell coffin was used on each individual20-m beam-tube segment to test the spiral welds for leaks.

22 SUMMER 2005 T H E B E N T O F TA U B E TA P I

nitrogen atmosphere. On the segments intended for thevery ends of the beam tube, heavy-duty support rings werealso welded. After each segment was inspected for round-ness, both ends were stretched to a standard diameter andsquared off. Then, “two guys with jewelers’ loupes in-spected every welded seam from inside and out andmarked if it needed rework [by hand],” Raab said.

But that visual check was only the first of CB&I’s rig-orous leak checks. The next step was to seal off both ends

of the segment, lay the capped segment into what lookedfor all the world like a blue coffin [Fig. 5], and close theclamshell doors. The sealed segment was evacuated andconnected to a helium leak detector. Then the annular cy-lindrical space outside the segment in the rest of the “cof-fin” was crudely evacuated and backfilled with helium gas.If any helium gas were detected inside the segment, itmeant the beam-tube wall or some spiral weld still leaked,and the segment was marked for rework. Last, each tubesegment was cleaned [Fig. 6], both ends were capped andthe segment wrapped in white plastic for trucking to thesite.

At Hanford on the beam tube’s foundation, the seg-ments were uncorked in a mobile clean-room shed, andone squared end girth-welded to extend the length of thegrowing beam tube. After visual inspection with a jewel-ers’ loupe, the girth weld was itself leak-checked by aningenious portable device using helium gas [Fig. 3]. Then,the serrated light baffles were installed inside the seg-ment.

It was at this stage around January 1997, when the firstbeam tube at Hanford was a quarter built, that Weiss washorrified to find the black dust near each light baffle. The

baffles were being installed in winter, and the periodicfreezes and thaws were causing the antireflection glazeto stress-fracture around the sharp points of their serratedteeth. Immediately, he called for a stop to the glazing, andhad every spring-loaded glazed baffle pulled out of the firstinstalled kilometer and replaced with untreated baffles—requiring the workers to crawl as much as 5/8 mile intothe tube, a task “both scary and tiring!” exclaimed Jones.“To their credit, CB&I didn’t flinch, and didn’t add ridicu-lous charges for this task.”

“It was one of our real panic moments,” added Weiss.“We knew we were not going to get another shot at build-ing another beam tube in that 30-year minimum lifetime.”

It was at this point that Barish began to have night-mares. Each beam tube was actually two 2-km halves,separated by a gate valve at a mid-station. As Hanford’sfirst arm reached the half-way mark, it also came to itsfirst true acid test—the 2-km leak test.

Repeatedly, Barish woke in cold sweats, wondering “notif we would find a leak, but multiple leaks.” One leak ev-eryone felt could be localized fairly readily. But if therewere more than one leak, finding them would involve fancyschemes, all of which would be time-consuming headaches.Moreover, the 2-km leak test was the one test that the

LIGO scientists and engineers had not been able to proto-type at full scale. Jones summarized everyone’s dread:“Locating multiple leaks was the monster no one wantedto face.”

For the basic test, the half beam tube was evacuated,and Weiss and his colleagues anxiously monitored the airsignatures from a residual-gas analyzer for trace amountsof nitrogen and argon. First one day passed. Then another.Nothing showed on the analyzer’s output. Gradually,

Figure 7 Master CB&I welder crimp-welds and inspectsthe girth joint between two beam tube segments on site atLIGO-Hanford.

Figure 8 When each beam tube, one the world’s largest high-vacuumchambers—was half-completed, the 2-km section was covered withinsulation in preparation for a month-long bake-out at 150° to 160°Cto drive off gases.

SUMMER 2005 T H E B E N T O F T A U B E T A P I 23

Weiss, Barish, and the others began to have the courageto believe their eyes. In a vacuum chamber 1.2 meters wideand 2 kilometers long, “it turns out CB&I was so carefuland built it so well that we had no leaks”—not in any ofthe eight 2-km sections at both sites, exclaimed Barish,still marveling. Yes, in 44 linear miles of welds, there wasno leak at the level of 1 x 10-10 torr-liters per second. Zero.Exclaimed Raab, “One thing CB&I knew was welding.They were good. Nothing that ever got past ‘em everleaked.”

THE GREAT LIGO BAKE-OUTRemember the water vapor andother atmospheric molecules thatlike to stick to the surface of thesteel? There was no escaping: to at-tain the high vacuum the LIGO sci-entists wanted, they would have tobake the beam tube to drive out theunwanted molecules, just as is donein every other high-vacuum cham-ber. “But how do you heat somethingthat’s 1-1/4 mile long to a reasonabletemperature?” asked Weiss. “Howlong do you bake it? And how hot?”

Back to basic principles and twosmaller prototypes. The LIGO scien-tists and engineers didn’t want torisk overheating the 3-mm steel,which would make the low-hydrogenmetal permeable. To get rid of wa-ter vapor and any residual hydrocar-bons, they calculated that 150° or160°C would do—that’s about 300° or325°F, a temperature that cooks calla “moderately slow oven,” aboutright for baking meatloaf or bananabread.

Most critically, the entire 2-kmtube had to be heated absolutely uni-formly. “A cold spot is death in abake-out, because vapors condenseon the cold spot and stay in the sys-tem,” explained Zucker. So not onlydid the beam tube have to bewrapped in aluminized cloth with 6 inches of fiber glassinsulated batting; “we also had to insulate all holes, nozzles,and places where legs stick out and touch the ground,”Zucker said [Fig. 8]. Assuring a uniform temperature alsomeant baking the beam tube for a long time—a month foreach 2-km section, the LIGO scientists determined.

LIGO scientists and engineers ran the bake-out withtechnical assistance from CB&I. The idea: run currentthrough the beam tube itself so it would heat up like theresistance wire inside a toaster. They borrowed 480-voltAC three-phase magnet power supplies from the Univer-sity of Chicago’s Fermilab particle accelerator; these theyput on flatbed trucks and hooked to a 13.8-kilovolt line,

and transformed the AC to DC at 60 volts that drew some2,000 amperes. They wheeled up the power supplies to thebeam tube, snaked off heavy 0.8-inch copper cables, andbolted them with spade lugs to 12 points on the heavy-duty support rings at each end of the 2-km tube section.“We also instrumented the tube with thermocouples andslowly brought up temperature a few degrees at a time,”Raab recalled. “We ran it for 30 days and 30 nights.”

They did the 2-km bake-out a total of eight times, oncefor each 2-km section, four times at each site. “It was veryeffective,” said Raab. It was also phenomenally expensive.The project budgeted $3 million for the eight 2-km bake-outs, fully half of which was just for electrical power—a

megawatt per month for a total of eightmonths.

OUT TO THE VIRGO CLUSTER“You know the old expression that goessomething like ‘engineering…is the artof doing that well with one dollar, whichany bungler can do with two after a fash-ion’,” said Zucker, patting the side of theLIGO-Livingston beam tube with pride.“LIGO is the biggest hole in the atmo-sphere ever built.” (Some readers mayquibble with Zucker’s definition of “big-gest” because NASA’s space power fa-cility at Glenn Research Center’s PlumBrook Station near Sandusky, OH, is ac-tually about a third larger—27,000 cu-bic meters compared to the combined20,000 cubic meters of the four LIGObeam tubes—but the SPF’s vacuum is“only” 10-6 torr instead of LIGO’s 10-9 or10-10 torr.)

“If we had designed the beam tubewith its aperture and length like high-energy physicists design accelerators,we could have never paid for it,” saidWeiss. “We think we’ve done somethingimportant by proving we could [take anunorthodox approach]. We thought wecould take what CB&I did and give in-dustry some new technology for vacuumsystems. But nobody wanted it. It’s one

of the sad stories of LIGO. Many of the CB&I engineersretired or lost their jobs. There was no market for the tech-nology.”

The risks aren’t over even yet. This summer, bothLIGO-Hanford and LIGO-Livingston have reached theirspecified design sensitivity. Like two ears or two eyes thatverify what each other detects and gain some idea of di-rection, they are seeking evidence of gravitational wavesemitted by colliding black holes and other massive celes-tial violence well beyond our Milky Way galaxy. Will theirL-shaped arms change length in a way indicative thatEinstein was right and that a gravitational wave has justpassed through?

“You gotta handit to the NSF,”

added Raab, “Wenever promisedthem anythingother than a

high-risk, high-payoff program.

This kind ofexperiment isdone only by

optimists. Pessi-mists don’t do

this kindathing.”

24 SUMMER 2005 T H E B E N T O F TA U B E TA P I

ReferencesFor descriptive background on the basics of gravitational waves, Einstein’sprediction, and several classes of detectors (of which LIGO is just one),see Marcia Bartusiak’s Einstein’s Unfinished Symphony: Listening to the Soundsof Space-Time (Joseph Henry Press, 2000). For a 40-year history of thepeople and politics as well as the physics, see Harry Collins, Gravity’sShadow: The Search for Gravitational Waves (University of Chicago Press,2004). For wider background on Einstein’s general theory of relativity,consult the classic text Gravitation by Charles W. Misner, Kip S. Thorne[California Beta ’62], and John Archibald Wheeler (W. H. Freeman, 1973),which despite its heft (topping 5 pounds at 1,279 pages) and age (thinkvintage wine) is utterly charming for including so much history and evenpoetry among the mathematics.

For a shorter explanation, see the remarkably clear overview “Gravi-tational Waves and the Effort to Detect them,” by Peter S. Shawhan,American Scientist 92: 350–357 (July–August 2004). A detailed account ofall the beam tube leak tests appears in “LIGO Beam Tube Componentand Module Leak Testing,” by W. A. Carpenter, P. B. Shaw, L. Jones, and R.Weiss, given at the 46th International Symposium of the American VacuumSociety (Paper No. 1708, Program No. VT-ThM7, October 28, 1999). Otheroverview articles include “LIGO: An Antenna Tuned to the Songs of Grav-ity,” by Gary H. Sanders and David Beckett, Sky & Telescope (October2000): 41–48 and “LIGO and the Detection of Gravitational Waves,” byBarry C. Barish and Rainer Weiss, Physics Today (October 1999). Anotherarticle, “When Stars Collide,” now in press by Trudy E. Bell is scheduledfor publication in the August 2005 issue of Smithsonian’s Air & Spacemagazine.

LIGO maintains an extensive website with many photographs andbackground papers at www.ligo.caltech.edu/. Tinkerers can build a cute1:10,000 scale micro-LIGO Michelson interferometer for under $150according to Fred Raab’s instructions at www.ligo-wa.caltech.edu/teachers_corner/lessons/IFO_9t12.html (suitable for a high-school scienceproject and for fully understanding the measurement technique).

If you personally want to help search for gravitational waves, lendthe idle CPU time of your computer to LIGO through theEinstein@Home grid computing project launched this year. Sign up ateinstein .phys.uwm.edu/intro.php; the E@H home page is ateinstein.phys.uwm.edu/; more about BOINC (the Berkeley Open Infra-structure Network Computing architecture that makes distributed com-puting possible for five unrelated projects) is at boinc.berkeley.edu/. I’vebeen running E@H since January; it’s worth it alone for the beautifulthree-dimensional, rotating celestial-sphere screen-saver that keeps trackof your credits.

AcknowledgmentThe author expresses deep gratitude not only to the sources namedin this article for their candor and time in interviews, but also tomany others who granted complete access to both LIGO sites andthe Caltech 40-meter and who supplied extraordinarily usefulbackground materials. Special thanks go to David Beckett, Den-nis Coyne, Dale Ingram, and Richard J. McCarthy for scores oforiginal photographs.

Image SourcesAll figure photographs provided by LIGO.

Trudy E. Bell,(shown at left, atopLIGO-Livingston) isthe managing edi-tor for the Journalof the Antique Tele-scope Society andhas an M.A. in thehistory of scienceand American intel-lectual history fromNew York Univer-sity (1978). A

former editor for ScientificAmerican (1971-78) and IEEE Spectrum (1983-97),she has written nine books and more than 300articles on the physical sciences, technology, bicy-cling, and history of exploration. Her books includethree titles on the solar system for middle-schoolstudents (Byron Preiss Visual Publications/SmartApple Media, 2003) and the Institute of Electricaland Electronics Engineer’s millennium book Engi-neering Tomorrow: Today’s Technology Experts Envisionthe Next Century (with Dave Dooling and JanieFouke, IEEE Press, 2000). She is now writing a bookfor Joseph Henry Press with the working title ofHeaven’s Geographers: How Expeditionary ScientistsMade Astronomy Celestial.

Her previous articles for THE BENT were “TheVictorian Global Positioning System” (Spring 2002),“The Victorian Space Program” (Spring 2003), “Tak-ing Engineering by Storm” (Winter 2004), “Questfor the Astronomical Unit” (Summer 2004), and“Struck by Lightning” (Winter 2005).

“I don’t know,” Barish stated frankly. “We have to belucky. [LIGO’s sensitivity] is where it’s plausible to seegravitational waves. But our proposal couldn’t guaranteewe would succeed. We always knew we’d need upgrades.”

Hand-in-glove, LIGO scientists and engineers are al-ready well along in designing those upgrades. Meanwhile,the LIGO Scientific Collaboration already encompasses 500scientists from 40 institutions in seven countries. SaidBarish, “That’s pretty amazing for a fledgling new fieldwhere we haven’t seen a damn thing yet!”

“You gotta hand it to the NSF,” added Raab. “We neverpromised them anything other than a high-risk, high-pay-off program. This kind of experiment is done only by opti-mists. Pessimists don’t do this kinda thing.”