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2-9-8076/Gravity waves/~pril WS/Head I ' i The tantalizing quest for
2 gravity waves
3
4' When scientists finally detect
s a form of energy they have never ~J-"I " seen but know exists, ~ll " e,,, bJ' \.-=:''2..-> '~~9~ a new era in astronomy
By Arthur Fisher
, ,
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• ~ I
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1 ElITE UJ 1111 III II III III II UliJ+lill : IillJ~-,--;L-i-Wj~_'_LUjljLLI-LWjJJLj_LU1LUllj-L--'-;:;l-L-- 10 2 10 310,4 10' 5 1 0 "610 70 28 I 2-9-8076/Gravity Waves/April PS ! Overline for box:
21 ~~Jrn](~XOOOfXJ(~n~~~l!OOd:roiti::nn)q{ I 31 Two indirect proofs for gravity waves I ! 4,,
5 Overline I
u Battling the limits of modern technology
8
Quote for p. 88
;',1 ~iDC~KXl>lli~
"In about fifteen years, we
, ? '"I will want big, space-based
13 1 laser systems, using, say a
J,4 frame. I! lO-kilometer R~H~R 1"1
16
1 17' I 181 J
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I 298076/gravity waves/af/apr/caps-l I 11 L!iWto credi t--uper photo with man=:>
1f)2 Alan J. Knapp
3 G cre::.t--lower photo :0::::>
4 Cliff Olson
1.
1 ~
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20 -- 2--+ .-~"+--'~--' ~- 3 '.' . /;I j 21ii9S076/ cravi ty waves/af/anr/cans-12J.._. "" ,\.../
t~),
3. Cryogenic bar detector (above) I <
" gets check by Prof. David Douglass;
S at Uni versi ty of Rochester.
c Alurnim.111 cylinder, inside vYin-
7 dowed shield, has been rolled
8 out of red cryostat, which cools
9 system to a fe-VI degrees above
10 absolute zero. If a gravity
1" wave passes through the bar,
,. its length will change by a
1C minute 8'l10unt, a change that
1~ will be detected by supercon-
15, duoting devices at each end of
16' the bar.
17, Early testbed, left, for ~
18' a lUN-noise laser interferometer
19! detector was built by Robert L.
201 Forward ~ Gaylord E. Moss and .l _ 0 0'0 I 'U o _
298076/gravity waves/af/apr/caps~J
Larry Miller at Hughes Research
2 Labs. First such antenna, built
3 in 1972, had an effective length V 1 4 of eight meters. There are noV!
5 plans to build laser detectors
6 more than a kilometer long.
7
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10
11
12
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15 i
16 i
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20 1 -oj ~~-81.~; "U 3 '0 410 5 0 610 70
293076/gravity waves/af/apr/cap$-4 (!)
2
3 First bar-type gravity wave detector I ~---'"'-..-"'--~-.--~-- -- I 4 ! (above), developed by Joseph we~er at V"e 1 I 5 University of Jifl8.ryland, appears lat EinEitein
6 I I exhi bi t at Smithsonian Insti tut~on. Robert I I 7 For!lard's proposed free-space l~ser aniJenna -1 ..,. ~, 1 I , I s to detect gravity waveS from th~ Crab Nebula I ~'. . h . rt' t' d' t' ht " t " lS s.:.. own In a lS s ren l lOD, I·,iY'lg.:..~ • !VeD ral. i :[ 10 satellite of three has laser an:; beam , s~litter. I
11 Two outer' spacecraft have reflec~ors. Gravity
Ii I I
."'" '.'.'a-\7e \vould change distance betwern satelli tes.
I I 1 13 " Massi ve cIJ7 0genic alUJ11i.nlul1 detec~or nOYi ! bar -----...... - ~---'~~'-"".... -.---.,.. ""-t --- ...... ",. -""+-- I I[ 1< operating at Stanford University: (right~ above) , I
15 was built by team headed by Will~am Fairbafr:. i 16 Photo shows large cryostat wi th ~nd rempved,
17 gi ving view of five-ton bar wi th}n, wi tf. , , ! 18 instr'umentation ~~d suspension. lDetail~ and I ' J dimensi ons are given by drawings 1 opposite I ! ~\ pl.ge.) Use of liquid nitrogen anI! liqui • • , , F I I I , , , , i , , I'" " ElITE UJ i I II iii II II I IIJ I IllJJ--llJiL;~J~LJ~ "--L~~+-'-'LJl~~-4JlJ-,; ~Ll~-1~' _' J c~-Ll-i,_-'-',,_-' 10 210 3!0! 4'0 5'0 6!0 7'0' E:, , 298076/gravity waves/af/caps!apr-5 I I helium cools detector to about ltwo deijrees I '
2 above absolute zero. I
"~ ,,--n_::;V Bin Bw~ I I
4 ~ber)David K. LYD1h I 5 ~Stanf6td/~ Univ~rsity
6 cv
7 State-of-the-art bar detector at Stanford Univers~ty is isolated
8 from all external noise by special mech~~cal filters. 9 cv 11 @ Dipole electromagnetic waves and quadrupole gravity waves
I When electromagnetic wave (light, etc.) strikes charged J ~
131 particle, it oscillates in a simple, back-and-forth dipole
141 mode, perpendicular to the direction of the wave.
15
16 v
17 When gravity wave strikes mass, one dimension will ~
I 18 contract, other will expand, im pl~~e Iperpendicular to direction i 19 of wave. These quadrupole moti9ns revJ1rse on next half-cycle of wave. I 20 I --riV Lie PICA TTTH TTl I 111Trtr~T~;~' ,'- "
EliTE tLJ-1~J"""'1"",1'" _L1_J~-,-L1---+lJ--"l~ __ L -,~ i 1" l' 0 20
2980761gravi ty waveslaf I capslapr-6 tV hre.e.. 2 II kinds ~, of gravity wave detectors 3 ------Doppler ranging uses radio signal between 4 spacecraft and Earth. When gravity wave
passes, distance between two changes,
introducing detecta~ Doppler frequency shift.
f CD
'-_~_____Bar detector changes length V.0;-11'ftJ gravity
, wave passes. Ultrasensitive transducers alor€
or at ends of bar translate minute motion
to voltages tha t must be amplified.
i d 'G)
E), I t.a~er i!!!-erfe;:..o~ bOlLT1ces light bea'!', many 1 I 161 i times between mirrors mmmted Tn masses. !
171 Gravi ty wave changes dista.'1ce ~etween ,mirrors, 1 18' I 1 producing interference patternlon scrden.
19 i
20 • < ...... ~Drawina ~~O~~~ ,,- -~ O'U I 'U blU 2-9-8076/Gravity waves/Apri:':~/~";o~ The first evidence 0 any rind forx the existence I
21 of gravity waves comes npt fro sensing them directly I I 3 but from XRIK observing their effect on the behavior of
/~. \ 4 / a bizarre astronomical object called a binary pulsar. I 5 I ~7\)f A pulsar, believed to bel a raplidlYR spinning neutron ~~! /' 6 I <:0' / star, emits strong radiol signa;ls in periodic beeps. But ,//- 7 , I i pulsar PSR 1913+16, discovereq by a team of University S
9 of Massachusetts astronolers in 1974 with the world's I 10 i largest radio telescope ~t Arecibo, P.R., is unique. I sequence, 11 I Its beeps decelerate an~ acce~erate in a regular ~axxKXR
I 12 lasting about eight hou~s. From this, the astronomers,
I, 'I 13 led by Joseph Taylor, deduced ,that the pulsar was rapidly
\f.fr"~,; 14 I y I orbiting around another Iver rndssive object--another neu ~ I Ii 15 tron star £i)~;a;-,"".lI"'''''''ll:nUillt.....OaWb~l''61ii
20 duce a considerable quantty of gravity waves, and that I I I ELITE 4~HU+ [ i [ [ I [ [ [ [ IIlJ I IllJlJlJ II L: I i I II I I UUjlillIUlljilltlm-Ll~u I i I~EJ r, 1 1 0 2 0 -310. -4lo 5 0 610 7 '0 ----sJe 2-9-8076/Gravity waves/tpril SiR? ". - the energy radiated should ~e SIOrlY extracted from the
2 orbit of the system, gradua~ly de~reasing its period as
3 the superdense stars spirallcloser to one another. Ein-
4 [. I I stein's equations predict aldecretse of one-ten-thousandth I I ~ ':)00: 10,1: 7 .;.-" 5[ \)" Q \ \ '-' l ...) \')'''1 i -' , ( " --~- l I of a second per year for~g s,steffl.-1\nd after four years 6
of observations, Taylor's t~am announced, in late 1978, 7 . I s that ultraprec~,e measuremeIjlts of the radio signals gave
'I .I....-! t·",-4' . i OV',~?'J"""''t, ' [ 9 a value l!!:f almost exac. y-w&..t had been calculaEsa. The ~ [ ,
I I I 10 closeness of the match not pn1y provides strong--even i 11 i though indirect--proof of the existence 'of gravity waves,
12 I , but also further bolsters E~nstein's theories of gravity I I I I 13 [ I against some compe ting theoties. l"ntiS could be tIre death 14 kttell for maRY sf the ot:~-t:heeti~-or--gravitati-on,JL~ said Stanford' 5 GHffo:E'a Hi,4.i: ;-m1--~uthoritT-on- -the-subject. As Taylor said of what te ca1~ed "an accidental dis- I';:
I [ I ' covery originally," the astkonomeks had an ideal situa- 16 I i e i ! 17 ion/or tlis ting relativity ~heory~-a moving clock (the I I I I 18 l pulsar) with a very prescis~ rate of ticking and a high -. Sc'" 300 [\",[o",.i.,.; ~c~ $<'<-1" 10 velocity" "It's a1smost as ~f we had designed the sy~tim I 2'>J ourselves and putR it out there sbecifica11y to do this measurement." I ' l' 0 2i O ~---31o- ----4T6 T-'----516 ----,- ~f;lo--~--~-710~- ~ ~81G 2-9-8076/Gravity waveS/Fis~er/APjil PS/4-t r;", Another indirect ind]catio that gravity waves do I J indeed i exist came more rdcentl~, and more dramatically. J i It stemmed from an event hat S~i~l has astromomers x 4 !mlnutes, reeling, At exactly 15 houns, 52,mNRi 5 seconds ~x 5 I i Greenwich time on March 5, 11979, la gamma ray burst of 6
, unheard of intensity flashed through our solar system 7 I I
8 from somewhere in space, tJigger~ng monstrous blips on I I ' 9 detectors aboard a motley (ollection of nine different
le satellites that form an international network of gamma ! '
~. p , ..... t ~-, '1 ( C 1 ~ \' fay burst detector? (~aiIlll1 '!'I'fiYS-r4ft.t:enSBly-errergetic-,
1\ I , 'p
---¥eJ;".-~1Jrt -wave'length-f ooQ--f'"e lec-tI: omagne tic- -rad1ation~ ! I W 13
-( ~IOi Ler d,an x ffij * For ~t1Ra~~ life (lR Ear.tb... 14 ! ! . ---.they----£anno-t -.pene tra ooserved lS te--4;he-~er.a,--but-a'l:'e-;---,\-r-: , unding 16 -from nigb::al.'tt't:Ude",""a-Hcm~lts:m~TOCy-,-ets>-ID1d-m=e
-.1 I 1I, ""ntiy ..., "re~-f_ ;."anl- c,
1.," : I '[ho oboo",,,,,•• , .""~'I' ""~,""""by doe UnHed I I 19 I I ! Sates, France, West Germany, andjthe Soviet Union. ~ I , Zv: I I 'blaBo of ga!l1H1@ rays hj t 11.._ fjkSI;-E~U's~nera-, : -- I 0'" 2-9-8076/Gravity waves/Fis!jler/Ap il PS/~ 1'., 'f - Beer I I fvr=the--tlete'ction 2 :alii"",," Venus Ot\7'tte'!'-;-t:~-e1.a-l~ HliUKiRlI:J!:XK!I¥XSRK I 3 -sf 'riuciedtribla&ts-)) l~ -am:t<-trol!:{iO?:-~-·' , 4 "This March 5 gannna rty event,was extraordinary," ~:fuRxKk:i::Rx I 5 says Thomas Cline of NASA $oddar~ Space Flight Center, 6 who with his colleague Reuten Ralaty and other U.S., R ERRxafxKXgx~pxafxasKxa I I 7 French, and Russian astropmysisicists have been analyzing gliiiR3:xk~xS!!mRxKkR:>CNhappllb;: ! I 8 it ever since. "It was notilike the gannna ray bursts x 9 that have been seen a hundJed ti~es in the last decade. : I 10 , I It's a first and only, like something that's seen once 11 c. I in a scientifi", lifetime." i Call'i'd by some "the whopper," 12 0 00 /, ' 'Xm( tmme s , i it was some ximHs moreilintenJe than any seen before, 13 and reached its peak faste thaniany x other. I 14 KRR Because the surge lof gatma rays was detecg~d~ I I 15 by so many satellites separated in space, astronomers I ' ~ere abt tn ) I i -.i ts./ and iden- 161 im~iirffla:~i~~iate the posiiion 01l1k€ source--KkRxfxxxt 17 tify it with a visible object--the first time such a feat~ XimRxsH~xKXfRRKxRli3:xkRRRX~R!!mpixsRR3:~ ,i I - 18 had been accomplished. The lobjec~ was a supernova reITM 'I "7' 19 dubbed N49 I I nant/in the Large Magellan1c Clound (LMC) , a neighboring 20 , I galaxy roughly 150,000 lightR yedrs away. I I . 110 1 4 10 '--5 ~~--~' 2 0 3 10 I 10'---610 7'0 8 10 2-9-8076!Gravity wave!!F~'s er!APtil PS!~ i~> IV' ' AxxXk~xR~xxxx~fx±kRx ~XR~X,XXRmXHRXXKruRXRX~xm~KX Ramaty, Cline, and col eague~ posit that the geneais 2 asx); I .-/:, of the g, ",oma I star, (the ray bura' wa, a qu:t:;,ng neur'on ",.... 31 r b ~ltra-dene~ ]dl!r?--=c~my_a_cj:>_ 0 l ~ ~lllltt 5 a supernova eXPlasion0'w beli~ve,,, ~s :l::e; ':t~at I I, 6 a neutron star can undergo I a transformation analogous 1 7 1 ' to an avalanche. Snow fall~ on aimountain until there's I 8 1 I a slide. Similarly, dust and other material collect on 9 ~ • lie • I , a neutron star unt~~ ~t ca~ t stknd being as heavy as , ! 10 i~ ! , I it is. Then there's a star:1fUake:, either in the crust 11 i i I , or in the core, and the stkr shakes itself at arrequency 12 I I I , 13 of about 3,000 Hz, a note you could hear if you were 1~ listening to it in H an a~osPhele. The surface of the i --only five to ten miles ,n diaineter-- 15 star/is heaving up and do xkmisax several feet, thous- 1 t) 16 ands of times a second. rtF ma~et~sphere is shaken, and 17 I I that's ~t produces, indi~ectl), the gamma rays. But I ! l ' , i " I I that's secondary, inK our ~odel,1 to the gravitational 19 , , ~ ! I i wa~s caused by the oscillation !of the neutron star. 2,~ , t J 1 I I --,----;-J-~.-"-___;;r;; I 1~~~~~~l;~~~-+~~-j~~.y~-+-"-"-'-q-LLLLc!-LLLLi~L 10 20 30 410 5' 0 610 7'0 8'J 2-9-8076/Gravity waves/Fifher/A ril PS/;"..4- f' .' -~. "Could we detect thes~? The, answer is no, After all, I 21 ~ I e this is only a kind of af ~r-gufgle, thousands of y~ars 3 after the star's original cOllatse--the supernova. It's 4 I . i like a tremor after a maJrr aarkhquake, maybe only one ! 5 i ! I i percent as big. It's not tOing fO be easily detected by 6 S ! present a antennaw that ate maybe capable of seeing a 7 1 I . 8 supernova. Added to WhiCh~ this'is in the next galaxy ./-- ~! I ~ ( about five timeaJ I I ,~" 9 ~~f~-rther away tfuan the same kin~ of event in i 1 10 i the center of our own gal~xy." ·i' Nevertheless, Cline called all the U,S. gravity wave 12 experimenters who could h~ve be~n "on-line" during the 13 ! learm' whether gamma ray burst, to XRRXNkRXX they had seen anything. I 14 , , I Ofll: them all, only Joseph:Weber'had an antenna working 15 16 that March day, and he had observed nothing remarkable. 17 The gamma ray detectors aboard the satellites were 18 not capable of sensing th~ 3,000 Hz frequency predicted I 19 If the~ had, says Cline, it would by the s tar quake mode I, lffiid!:xWIDiiaxka:lI:R I 20 have been "a very direct ~ink" to the existence of grav- , . . . , • , ' • , , 1 " """ I I I ' I , ; I ' ; I I I Iii I I I i I I : i I I I iii ii, : iii I i I j , ~ ELITE 1IIIIIIIIIIIIIIIIIJH~JUli'H~ I II II! I! IIIII!! 1111111111 i II11 JlIIII~" lJ 1 0 2 0 3 0 . 4 0 -slo 6 0 7 0 -----atJ 2-9-0876/Gravity waves/Fi~her/Ar' ril PS/~ " .' '7 itational radiation. I 2 But the star quake mod~l mak~s another prediction: 3 the ~ity waves generat~d shopld carry off an eneumous L ~ more than _t!:~~Il_th.:.L~~_n:na:::.aY!'t?~ , 'b t' 4 -4" r ~I s ar s Vl ra lon amount of energy, and thu~ snuff out the ~KgiitaKi8RX I I 5 very quickly. "The nice tting, "I says Goddard's Reuven 6 II I l.i ,I ~f'c .. .. c, Ramaty, "is that the ~~~_eee,i;i;c:(;;i~ d~mping time :'ol8ed~ ~ 7 gravity waves in this evert corresp#nds to what 8 exa~tly I 9 we observed: The main par~ of t~e burst lasted just 15 I 10 hundredths of a second, and that's what we calculate f ,~ . .,---,. for ithe second time 11 i\... :--- .. -:~ ---- 1 -- from our model. So we now have ~KxKK~indiEect evi- ! I , 12 ! . i I. dence for the eXlstence of gravlty, waves. Of course, they 13 i both have some prob lems, ~s do lall indirec t checks. They 14 won't replace direct evi~nce." 15 16 , 17 18 19 20 ~ - I 1~~+~~~-~-4~-r' I" I I 1i.LLLJ-.LLLLl!i-LLjlJ Ililllljllll!.LLU 1 0 2 0 3 10, 410 5 a 610 -. ~O 8 0 2-9-8076/Gravity waves/Aptil PSVI In the vast reaches of the cosmos, cataclysms are 2 a commonplace: something omentlus is always haD~ening. 3 sun Perhaps the blazing deathlof an! exhausted sxax, or the 4 I melding of two black holer' or t warble deep inside a 5 S'U(L, ~~ I ~ neutron star. \Jha1! £l!U- IJII even~ i;ot; spews out a torrent 6 of radiation bearing huge!amoun~s of energy. The energy 7 I !e-I 8 rushes through space with i the sf,4'led of light, it blankets I I I 9 our solar system, it swee~s thrqlUgh the Earth ••• and no 10 one notices. I 11 , But there is a small Jand, of, experimenters, perhaps , : I , - I I "",. ovl~-w\ ".....\~ scattered~!' • I 1 ~enty group~XR:li romlCalifd,rnia to Canton, determined I 13 ) I that some day they will n1tice. Pushed to the edge of con- 14 c ..porary cechnology and 1eYOndi baccling th, apparenc 15 I limits of natural law itsJlf, they are developing what 16 I 6:'" trM '1, • 17 will be the most sensitiv j\Rati1li@i r;.Jever built. I I 18 And eventually, they are jure, theY will detect these 19 maddeningly intangible Ph~noment--gravitY waves. 20 I ELITE fJ-llWW II U.lll-111 I 1 II II fJ IIIUJJ.l.llllll j IIJ+llLllllllllillllill-f.ill-.w.illlj i 11II UJ! I To 20 --:ilo 4 0 50 6lJ 7'0 8 10 2-9-8076/Gravity waves/AJril P$-2 o Even though gravity ~aves tm~re formally called grav- 2 itational radiation) hav~ nevet been directly detected, 3 virtually the entire sci~ntifi¢ community is convinced 4 they exist. It has to be~ "We ]j:.now that there are grav- 5 I icy wa v",," ,.Y' Riohard IDavi TPcogram Leadar for 6 Space Physics and Astrop~ysicslat the Jet Propulsion Lab- 7 oratory in Pasadena. "Otherwise, we would have to re-en- 8 1 I 9 gineer all of modern phy~ics .•• it would be unthinkable." I 10 This assurance stemsi in part, from the bedrock on 11 ~ ! which gravity wave notions are founded: Albert Einstein's I . 12 , theory of general relatiyity, hich though still being I t 13 I, I tested, remains untopple~ (PS flec '79). Says Cqltech 14 i I astrophysicist Kip Thorn~, "I bon't know of any respect- 15 I able expert in gravitati¢n theory who has any doubt that 16 I . , 17 gravity waves exist. The~only way we could be mistaken 18 would be if Einstein's general, relativity theory were · I 19 wrong, and if all the competing theories were also wrong, · I 20 i • I· because they also pred~c t: grav~ty waves. " I ELITE WJljJJJLIJJJ I 1IIIIIIJLLfJJJJ IruJJJ IIIJlllt 1111 Illil.IJllJ IJm+WJJ1Jll+LLUi 'LU 10 20 310. ~01 ~O 610 7'0 8"c 2-9-8076/Gravity waves/A*ril P~--3 In 1916, Einstein prclcicte& that when matter acceler- 2 ated in a suitable way, he mOj'ing mass would launch 3 ripples in the invisiblelmesh f space-time, tugging mo- I 4 n theiuniversal sea mentarily at each point KxXkKfXPRKKKd as they 5 I I passed by. The ripples--gravitr waves--would carry energy 6 . I ~ I and travel at the speed if ~glt. 7 I I In many ways, this prediction was analogous to one 8 I . i , 9, made by James Clerk Maxw~ll, the brilliant British phy- , I 1C sicist who died in the y~ar of Einstein's birth--1879. , I , b 11 Maxwell stated that the ~cc~leration of an electric charge .,I ,' 12 ! I would produce electroma~etic tadiation--a whole gamut I 13 i v I, of waves, inc~ding light, that would alIi travel at R 14 I. ,i the same constant veloCi~y, Hi~ ideas were r;diculed by 15 I ! e many of his contemporarifSW BUll: a ~re decade after his 16 I 17 death, he was vindicated1when feinrich Hertz both gen- I I .c.he 18 erated and detected radiI:> wave~ in~aboratory. 19 Why, then, more thaJ 60 yelrs after Einstein's bold ~J forecast, has no one seel, mudh less generated, a gravity I I I ! I I Iii ; i j i ( 1-;- ; , I' ,"" I I' I' I II II I II I iii I I I I II til II I I I II I i I J I Iii 11-1_I : , u 2'0 3 0 , 4 0 50 60 7'0 8 10 2-9-8076!Gravity waves/~pril ~S/4 wave? Why despite incre~ible dbstacles are physicists hunt 2 xiRgxKKm±NaKa~ still s~eking!them, making the ~MRKK 3 for gravity waves a king of mqdern quest for the holy 4 grail, one of the most 1xCiti~g ~~::::~Ftite " <,..: I I whole history of scienc~? i :r , I I I To find out, I visi~ed beJh mit!! experimenters who 7' I I I I are building gravity waJe det~ctors and KR!! theoreticians I, I 9 whose esoteric calculat~ons gJmde them, In the process, : land how the attempts to I I rl X±ma::Ke.XX 10 I learned about the pro~lems, !a::Raxa::1uru:xXKRe.xpa::lC3:XXKX I ' : i to solve them are alreaqy producing useful spin-off~~i 11 -! I ~ ~a::~~ff¥xxfxKM~~RKKfMi~xmxRkiRaXNiiix~3:KKRKKXKxp3:NRxfMi, i ,.. I ! 12 iu II E iield f 1!!83,f11re m svl;. ~nd I learned about the ul- n_ I I I i I i 13 timate payoff ~if the quest is successful: a new and ..) i I 1~ ~or the first time, what potent tool for penetra J,ing,K~!!xxe.~xRKx~3:xkiRgx3:fxKKRxx ,I I' 15 on: RXj'l.e. physicist has cjalled i"the most dramatic and URXl1:£.XKR:;::X I Iv 16 overwhelming events in bhe un~erse." 17 18 19 ~I tU It I" II I I I I I III'' I 4-LU-';i~~"1 0 2 1'-LLl-l-'-'-Ll410 3 10 '~:r-LllJJJJ4 0 510 6'1~JJ..L4-DJ--4-l~~- 0 7 '0 8 ~ 2-9-8076/Cravity waves/April psi The fundamental probel~ iB g avity wave detection 2 is that gravity as a forcelis fe~ble in the extreme, some I I 3 40 orders of magnitude weater thrln the electromagnetic 4 40 1 force. (That IS 10 ,or a ~ followed by 40 zeros.) If the 5 I -eleeL10tr=-and j3rgtop that make -tl-pl-a-~~ 6 fiele I I Imumi togeilier "by grallj ty h.tl:: 8 aXKXRgxIlX 14 I tational radiation is quadrupolel (see drawings, p. ) . I I ! i 15 i 1 W~..Je. i ~I If a gravitYl'generate~ ~ If or example, mea super- 16. 1':- " nova in =r galaxy, pam.! chroigh tha paga you ara now 17 reading, the quadrupole eJfect would make first the length 18 i I \ (01 I]l(efPi~~) I - ) 18 expand and the width contrkct~ d then the reverse. But I I I I I , 20 the amount of energy deposited in the page would be so I ! I""f(../A j I I ! I J j I I ! I I 1 I j I I I I j ! iii j iiI iii I I I fi Iii iii iii : iii I : iii iii 1 i :; ITT~T7! ELITE j II IIIII III II I I II I I I I I I I I I I [ I I I II I I I [[ -4U-l1 IJlil.jJ j I I IJJJJ+W-Wlil.jJJ.J-!:-jJL' , 10 20 30.410 51"0 -~Q 7'0' 8'c, 2-9-8076/Gravity waves/Ap~il PSJ6 ~x ch~nge i~ dim~ion infinitesimal that the R~RKiffRliX~HaRgR would be less 2 than tha diama'ar of a pr4,on. trYing '0 da,ac' a gravi'y 3 wave, then, is like stand~ng in,the surf at Big Sur and 4 I I i 5 listening for a kiss blowq acro~s the Pacific. I A Ie I 6 As for genera tAng dete~tabl1 waves on Earth, a la j 7 Hertz, theoreticians longlago dismissed the possibility. 8 "Sure, you mak~ gravity wives every time you wave your 9 :L , fist," says Rajner Weiss, la professor of physics at MIT. I I 10 I ., [ , "But anything you will ever be able to detect must be 11 ~e~ i ,I made by~massive bodies mo.Jing very fast. That means 12 ! I 13 even ts in space." I I , I I 14 Astrophysicists have 10rkedjUP whole catalogs of such (iravi ty wa,,~s ~.? 15 events, each associated w~t~a ~ifferent energy,~MxpNX* I. 16 ! I x different characteristic freq~encies, and different i 17 I ~ .... c.!vd.-e... probabilities of occurrenC;e. Th~y "l$::;&m£ll'lg!,'9.",qj"'!!H~ th e sup- ,I • 18 I posed continuous backgrou1d grayitational radiation of 19 i I ~bQuo 2 I " AA iati:" emitted by pul,ar'land :r::"' ,y,tem' co""i't\Og 3 of s.perdense objects fle~ -st~ And then there r' cO r±k4 4 are the singular events: the bitths of black holes in 5 globular clusters,HRaxiR galacttc nuclei, and quasars; 6 Rxix neutron star quakes; land sfpernovas. 7 I I I I II j I -_ 8 Kip Thorne has even pUj: togej:her what he calls "a pos~ = I I I I I ! 9 sceJilarimj:"in which every galaxy I 10 ssive halo of perhaps 100 billion"dead" I 111 J " " I i binary systems c~~sisting of etther two black holes, 12 a black hole and a ne starp or two neutron stars 13 revolving around each oth~r. emit gravity waves I I I I I and as a result spiral cl?ser at ! I I 16 the last hundredth oil a stcondJ Thorn~says, "they could 17 convert 10 percent of thelr masl into ener~a fanta~ic I final splash of gravity w~ves."i There would be ~rhaJ3s I 19 I ten of these events per y~ar ini our galaxy, Thorne ass\s, 201 I M / 1 ~d they c~~e de~ect~t:.l~ some of the adva~ce~__ ~ ___ ' ELITE U-li4-U-U I! !! ! I, ! I I ! I ! I II I" I 'I' I II I I U~JJj j ifJ-I I I ll~lLjl1JLjil1lPllL\ I II i \ i iJl.j.J 10 20 30, 410T~ ---sI0 6,) 7'0 SiC I 2-9-8076/Cravity waves/Arlril P$/8 .de tes:;.tQrs D.QRS..Qllling on. :j.il'le. -~ 2 Probably the prime c~ndidafe for detection~ ~ is 3 William Fairbank, p ofesslr of ~t ~x~fRs&Nxx~fx~¥±s~ physics at Stanford 4 University, calls "the ~ost dfamatic event in the his- 5 I I I ' tory of the universe"--a lsupel'ova. As a star such as our 6 v I I j sun ages, it co~rts pari of its mass into nuclear energy, 7 ! perhaps one percent in 5 Ibilli¢n years. RKt:H "'fue only 8: I ' I 9 reason a large star like Is the !sun doesn't collapse," n I 10 ex;plains Fairbank, "is becaus~ the very high temperature 11 . . I h ' . h ULd ~n ~ts core generates enoug pressure to w~t S~~n grav- 12 ~ ~tational forces. But as Iii t codls from burning its ru::x 13 I I I I v fuel, the gravitational forces ',begin to o,ercome the II: 14 i electrical forces that k~ep its particles apart. It 15 collapses faster and fas~er, and if it's a supernova, 16 17 the star's outer shell bLasts off. :if In the last thous , J 18 , ! - an~th of a second, it co1JlapseEl to a neutron star.... ? !'l I I 19 ~ if the or"'ginal star was Ilarger than three soJa r masses, r !! 20', I XI!! maybe' to a black hole. " Dur:ing that collapse, ixxrix I ELITE WllllllllllllllllllllllllllllH1HH~+!~+U+UIIIlJH+J IIIIII U~ llil~J-W~jJJJ-W~"; 1 0 2 0 3 0 4 ,0 I, ~O 60 7'0 8·8 2-9-8076/Gravity waves/Atril PS/9 it will radiate anywhere from ~ne percent to 10 2 percent of its mass as e~ergy ~n the form of gravity 3 waves. I 4 I One way of character}zing ~he energy of a gravity we' ,) '-- 5 I l 1 is the strain it In uCjf In arI y matter it impinges on. 6 'de.t . If the mass has a dimens on of!a given length, then the 7 I ! i strain equals the 1 th~t length--produced by 8 change in I J I 9 the gravity wave--di~ided by the length. Gravity waves i I ~ su~p~e~I~t~i~O~-- 10 have very, very, tiny s d:a ins. ! 'F&! a ldri;x -thi 0 spec- s 11 (~ Vf'''''" 1" ) eacalar !Esllar expl.Q.&-iDt~~~oc-curring in our galaxy . ~ ~ i ~f'" ~'f' I",,:,. v J~ S)..I"f' '."1' /( ,- ," (:J I i'f 12 1_~ 0 would produce a strain of Eart~~ 50me\lftere be8leel'l' ~,,' ." ) './.0 I. 13 t; / 1oI!,...' - ( \-")-, - b I-'- ~ e r" ~ ,. '{ 'f" ~~ :' y 0 '\ (. CI k~ ~ _ j-, '-, ~ . _ . _19 -17 1 X -1(') and 10 -.. .a !MSS l~G Cll!~~, I ~ • t . I~, .. , '« J ' if··j , ( .... j.,.~ T- I' 'cJ - .,; •. 14 d,~....."t,,.. 0 7 v-r,..__ ~I...... _, I".. ~ I =:d-th~~'a'o'o4,.t~:wa,l,Uil-b.ru;l";'·..t:b..e-..-J:~timi&t:i:c)~·s Era in -va 1ue , ~ , - 17 ''\ 15' (;.-..,d, ~I.,'/~\C'ltJ ~Jov'c.. IA.\,~I. {re. "t'r,' , >- .. ~ ..J " ~ I -]~ of 10 ,then the rna 88 ¥o1:l1e flungate a trtfttng- 10 . 16 i I ! 17 j;;m ... Thtat io eRe one l:I'l];l~redtb! the diame:tet:-G-i an .a tom; (1 : I i ---,-~- r I (' 18 I tJ11clAPS0a the credit of tireRess experijlffilenters, there I ' lei ~ ~ i I ~re detectors capable of'sensing that iota of a minim , I 20 l I of a scruple. ~ti tfl8l1IKUI, ~~ I ELITE f.llli I 1 III II! I! I II " I' ' II II " , I '-LLLLLLLLtr' ~ I " ,-LLL'+'-'-.LJ+LLLLj" " u_ . .~~ 1 0 2 0 3 0 4 0 5 0 610 7'0;' 810 2-9-8076/Gravity waves/Apri PSII the dense center oC 2 ---~---o--'-.:..r--'-~ - --~ galaxies, a supernova can b~ expected to occur in any 3 I given galaxy roughly about nee ip 30 years. NIC's e!:I:1@,1f 4 ~ " seen BIle -in 5 I onT galany f817 {/\'Olr 2Q(l years, hit tlta-t'-S-baMcally-be- 6, I I from-- 0 til'" '1 i €loZ -•. -~se 99 De i cent"trf--uu~x".--i~~ured-kyxbiKIiXS:Ke:ll;ax 7 I I hy il'lter 11 witlt 1 mpUiIl:'Ey." . , ;~- 12 I ; ~ once every 30 yeatrs.,-f'rONe ;'QliL, is a depressing \7 I . 13 I long interval. Over l& and o~er a~ain, j; the scientists 14 I 'i 1 I I I spoke to despaired of dOirg meaningful work if it had 15 , , i I to depend on such a rara av:is. Professor David Douglas, 16 I 17 of the University of Roches'ter tdld me: "To build an ex- 18 periment to detect an event once every 30 years--rnaybe-- I 19 .s S i i~ not a very sati~fying oc~upation. It's hardly a very I 20 I I good PhD project for a graduate ~ssistant, and i tr S not i , U b _ 2-9-8076/Gravity waves/Apr~l PS/ll even a good lifet;ime car~~r project--you might be 2 unlucky." And Irwin Shapir1, proressor of geophysics and 3 physics at MIT echoes the laintl "What scientist wants ~ 4 I to gamble his entire produitive tife on this?" 5 i I What if weE. don't conflne ourselves to events in our 6 I I 1 I Jr'nstead of the own galaxy, but look farthrr afitld? ~ax~iKgMx«XMRxaK 7 '~opelessly rare" (in the words ?f one researcher) super- 8. I . I nova~in our galaxy, what we ~ooked for them in a x 9 tf I really large arena--the Virgo clhster, which has some 10 I I I ~ 111 2500 galaxies, HHE. where sypernota~ ought to be popping I fr00 to snce a month! or so~) 2 121 ~ every few days Tha t 'f: Catch (22) • The Virgo c Ius ter . I 13 I is about 1,000 times farthrr awar than the center of our 14 I . own galaxy. So a RXmXiaK shperno~a event from the xkax ~ . I I 15 ~hose effect?:On Earth wbuld ~ cluster would despatch grafity waveysome million times 16 (1,000i§ times 1,000, aCCbrdingl to weaker Athe inverse square'law gbverning all radiative 17 energy). And that means bU'ldinJ a detector a million 18 l, I 19 Urn" =" "no ie,v,. """t, i, to fio ld of ,d,n",," I I 20 ,ay, Ronald n"v" of cali"h ard 'he Un'b"",y 0, , I J I I I I I I i I : tLiTt L+-LUJ,V JJJ II I II1II II 1IIII1IIII II \ I I III I ill I I III IIIIIIJJ 1IIII i I I I I 1}JJJ.+lL.LLj'_U I r' 10 20 30,410 50 60 7 10 8'0 2-9-8076/Gravity waves/Aril Pi/12 ,\ 1 Glasgow, Scotland, "wher such ,en,llormous increases in 2 sensitivity are neededa are here, in gravity 3 wave detection." Trying o det1ct a supernova in a dis- 4, I tant galaxy means having Ito me1sure, at the least, a dis- 5 I s I placement one-millionth ..~ze If' 0 an atomlC nuc 1eus. 6 I , " ! Paradoxically, it is Ith;'S \1ery quality C!l:f that mx 7 I i gives gravity waves the abil~ty to be, as Kip Thorne 8 JI I I 9 says, "a very powerful tdol fox: astronomy. True, they i, iI I e I 10 go through a gravi ty wav~ d~tec tor lli:XX wi th impunity. i 11 the graviqy waves But that means xtuqrxaixC!l: ! generated during the I 12 birth of a black hole caJ also [get away through all the I I 13 ! i neither surrounding matter wi th llrnpuni1Y'" AndZl RSld>ii:RgXRXKlq:X 14 ~ I r",dio waves, RC!l:X li~ht,nor gamma rays'For x~KiC!l: can. During 15 I I a supernova, astronomers Ican the explosion with showers 16 s~e I I 17 of electromagnetic radia9ion, iut only hours or days I i 18 after the initial maSSiV]' imPllsion--the gravitational 19 collapse. Then, while a ,eutro~ star or black hole is ! I 2:"; I , being formed, nothing bu1 gravtty waves (and, theoreti- c'-' >c [.L'-U-i ' , , , I ' " I" I , I-'~~LW-l' , , , , , , I I ~JiLLlI I I I I I_LLUJiLLjill,J~JiLLj_LLLlL,_ 10 20 310 40 510 610 70sb 1 2-9-8076/Gravity waveslApri~ PS/1G 1 I cally, neutrinos) can escap~, rep~esenting 90 to 95 per- ) I cen t of the total energy ra~La ted' away. I 3 "We've opened, at least "rCi"~lY' "" che e1eccromagnec'c 4' , I " windows on to the universe, "I says rome. "With gravity 5! I wave astronomy, we will oper a un1ique new window, onto 1 G , ' \ ! I ! v ! fascinating, exp~ive events tha't cannot really be s~d- 7' I I s ied any other way--births ard col'lisions of black holes, 9 star quakes, collapses to n~utron; stars. This is the real 1 I i i ~ 10 bread and butter of modern ~igh-e~er~y astrophysics. It 11 I, , is the area, relatively une~p10reld, where relativity is i i 12 I absoute1y dominant. It's ani exciting frontier, and a i 13 i long fran tier. " 14 I I I But first, as the cookbooks say, you must catch your 15 I gravity wave. Until the 1950's, one presumed that the 16 I +I I , 17 task was even feasible. Then Joseph Weber, a physicist I I I 18 at the University of MarY1~d, b~gan to ponder the prob- ! ~~'J Co ~'Y" 19 1em of building a gravity ~ave detector, and ~ 20 so. It is no exaggeration do say 'that he fathered the 1 iU 1 ;t'U J IU o'U biU 8 ":; 4 'u I 7'0 2-8-8076/Gravity waves/AprVl pS/T 11 entire field. By 1967, he d hil assistants had built 2 the first operating gravit wave Idetector--a massive I 3 I aluminum bar, isolated as a~ possible from external I 4 vibra tions, and girdled by IPieZo~lectric crys tal sensors, I 5 I , which translated changes i~ the bar's dimensions into 6 I I electrical signals. Weber ~port~d a number of events 7 ! I reoorded on and a dHe9,or a' Argonne that he 8 do', ""1 I~ I \ 'rt"n--:' I \ ~ -- ~:-,-- .- / 91 concluded were gravity wavels (PS M c' / '72) J'~ stim- I I 10 ulated a host of other expe~imenJers to build their own I 11 \ J.A. detectors. RN Designed by s~ch investigators as/Tyson I 12/ i I at Bell Labs and David Douglass at Rochester, they fol- I I , I ! 13 I pionE1ering lowed the same principles ~s Web~r's/bar detector, but 14 1 I , h t . .. Id be. WLt grea er sensLtLvLty. ~~f,ey an su sequ~t experLmenters 15 ' I I , 1 were unable to confirm webel,s f~~dingS; in fact, at the 16 I 17 leve 1 Weber's bar was capab!le of,1 theoretic ians believe 1 18 " wa, n,'erly 'mpo,,'ble 1: haV11 detected gravity waves. 19 "Either Joe Weber was wrong,," one of them told me, "or the 2'1' who le universe is cockeyed .1" 1 1 io.L.l , '- ~~~ '110' , 2ID~" I'" 3Io-L.LLLLLJ~IoI-Lt-Li~-" I'" ~1:l'lLl-l-'-'-L~it-.LLLf-.L,Lj1iO~ 2-8-8076!Gravity waves!Aprtl PS! Today, three basic kin~s of wave detector I 2 are being developed. One i basi&ally a Weber resonant I 3 antenna, ~ bari much ref~ned; the sec nd isl the laser interferometer; 1 I 4 space-based and the third is a s~stem called Doppler ran- I 5 '~I : ~ VI '"' ging. Each has its ~ advfntage~, and its own devilish ~ 6 I I engineering problems. 7 I Farthest along is the resonant bar, mostly because 81 , I I 9 it has been in the works l~ngesti. The more massive such ! 10 I .!. 11 d a bar is, the better (b ecause, ~t w~ respon to a grav- ! r j S i ,.- 0 '( T '" j --- '--t ity wave better). And it tij2pends on the quality of res- A 1 I " I ! :~I ".--, I onat~ng, or "ringing," fbr a time after it has been 13 I , struck by the wave. The lorger ~t rings, the better an 14 I I experimenter is able to pit:k oui the effect of the wave. 15 ! ). I That is measured by' the value called "Q,"--the 16 qua1ty 17 higher the Q the better. For a Jhile David Douglass and I I (~U!- b 18 others, including Soviet sbientists'~ Lseeking to make I i I ' 19 1 ! detectors out of such ver~ higj:l1Q materials as sapphire 20 I crystal balls. But Douglas:s, fot one, has returned to I ! _ U - v U'U , 'U to 2-9-8076!Gravity WaVeS!Aail PS116 The reasons: n wall ys aluminum, ~KKKi~xkR«aHKR KNX of aluminum have been 2 found with very high Q's; l~axKi~xkR~KHKR sapphire can't I 3 be fabricated H~ in massiie chu*ks (one of his detectors 4 ,-- has a six-ton aluminum bat)~ an ~axKi~xkR~KHKRX~f---- expense: i 5 i i "A 60-pound pure sapphirelcrysttl," he told me,"would 6 I I I ' cost about $50,000." I 7 1 ~irtuallY everyo e -; i 8 Likeitx:!'&MMHtXKB: 1"--;;1 se developing bar antennas, I i Douglass has abandoned rOQm-temperature detectors and 9/ I I i I 10 I i turned to cryogenic detecfors, ~ooled down ~N as close I 11 o I to absji!1ute zero as possiple. That includes groups at , 12 Louisiana I , i Perth, Australia, MNRx«k i Stafe University, aMR Rome; 13 I I , Weber himself at the University: of Maryland; and William 14 Fairbank I I RamiiK~X and colleagues at Stan[ord University. 15 I I ! I I 16 Fairbank told me why he lo~-temperature route was I , 17 essential: "At room temperaturei, the random thermal motion (;L t Ii """ J I 'h '" I l 18 of the bar is 300 times ab big ~s the displacement we're " I I 19 , I trying to detect. The onl~ way Ito approach the sensitiv- I I 20 I I ",,--""'" .;.. /l ~ ities we're after is to get, ridI of that thermal~- ~IL'A I , I ! . , I : I : I l i I I I I I I i I I I I I I I Iii I I I : I Iii I I I I iii I Iii iii iii iii: i I. i I I I ELITE 1~111~Jll I IjJJ I II I II IIII I II I I I liJJ I I I I I I II I I II 111+11L JllJ I LllJ~ 1111 UJJl.,jjjjlJ_ij I , 11 0 2 0 3 0 . 4 0 5 0 4 618 710 8 i: Gravity waves/2-9-8076/Adril P$/17 by cooling the bar. " detector's 2 When I visited the Sttanforl campus, the gie:amiRg 3 5-ton aluminum bar was s~aled inside its cryostat, a kind 4 0' ~over-sized Thermos b1ttle. I The whole assembly looked 5 could use if you like something you ~xkL*W~xxi&XctKR wanted to freeze 6 1 I , I Frankenstein's monster for a few centuriesAnd the envir- i I ...s; 7 I I oil i I onment was suitable tO vast! drafty concrete building 5, l t 9' that could have been an ~bandoned zeppelin hangar. I '~ 10 I 1 This Rax~ antenna, and others like ~ it, are de- I I I l '. . 1 • signed to respond to graVlty waves with a frequency of I I I 121 I about 1,000 Hz, characteristic of supernova radiation. 13 I Obviously, the antenna m*st be isolated as far as possible 14 I I from any external vibration at, or around that frequency,1 I I -Y 15 I ~ /, ; J. ~ Stanford I I v me IS the Raixuxk group does wiXk by s~spending the 16 I, I 17 cylinder with special sptings, i consisting of alternating I I I ! 18 iron and rubber bars in that i p called an isolation stack 19 , I (see drawings, p. I). "Otherwise, with our sensi- I I i ' I I 20 I I \ \, i tivity," Fairbank says, this detector would make a dandy 2-9-8076/April PS/18 :!.::' C't I, .f u v ,,,,, \!~ It" seismograph--just what wb don't wantJ~The Stantord 2 suspension system attenu~tes outside noise by a factor drop a safe 3 30 of 10 ,enough so that ou copld KxxikRXKRRX in its ! 4 V1./f1.n1.ty,C" W1.t'h out d'l.stu b,1 l.pg t hide, etector. 5 William Hamilton, aJ LSD, lhO is building an antenna ,I I 6 I i very similar XRX¥ to Stahford'~ (eventually it will be- I I 7 i I , come part of a Rome-Pertp-Baton Rouge-Stanford axis 8 1 , coihcidences) takes another 9 looking for gra~it~ H ware , ,seismiV- I , route toward~isolation. Ifhe very low tempera ture 10 I ! I 11 i I bar ~ of the device allows him to levi ta te the :a"'E'Kl'tf'l mag I i I 121 it is coated with h thin netically; HRHxxakK~iNxRkxRXN film of niobium- I 13! I ! tin alloy, a material thrt becbmes superconducting near 141 absolute zero. If electr~magnelts are placed under the , , . I +! I ::I bar, the persisteI~/currerts running through its coating ~ i I 17 will XRH~ interact with ~he maretic field so that the ,inJ, 18 bar "cera"y floac, ).(t'r. I 19 I Superconductivity is also Ithe key to one of the most 1 20 I I perplexing of all engine~ring Iproblems: designing a trans- 1 "'1~lj.lllllillilllllllllllllllllllll; , I I , , I : I " I ! ! I I I I I' I I ; ;, "I I I , ElITE WJ I II I II II I IJ+UJ I II ! I I II I I I I JlJ I I I I IJ+ 11111 i IlJ 11111 11I11 1lllllllJ.ll4i 11II UJ I 10 20 30. 40 5 0 6 0 710 8 0 2-9-8076/April PS/19 . sens~n ducer capable of mRaKHxtRg th1 tiny displacements of x 2 these antennas and cOnVEirting!them to a usef~l voltage i 3 that can be amplified arj.d mea~ured. "You can't buy such I things," says David Doutlass, I "you have to make them, :1 "t', I )' J and go beyond 'h' ''"I' jf tl" I ar'. Bo" Doug1m and I I ...,! Fairbank use superconducting, devices. whose elegant design , I I : orders of magni tude I makes them exquisitely 8' $ensitive--faxxm~xRxKkaRxKkRx ! I i ;• iI 91 more than the piezoelectric ctystals originally used,- although their approachls differ in details. 10 E~KkxXRi~x~xKkRxaRXiRR: ! I 11 Superconducting dev:iJces may also one day--a day far I"· vi in the future--allow gr~vity ~ave astronomers to perform 12 I i i a feat of legerdemain ca,lled '!quantum non-demolition." 13 I I To oversimplify, this evading a fundamental limit m~ana ! 14 • for all resonant detectdrs, one that is imposed by the 15 I . laws of quantum mec h anJ.qsI as Jhe displacements become 16 i I, i I . 17 ever smaller. That probl!em, will have to be faced if bar antennas 181 EHXHRKNXX are ever to bel sens~tive enough to detect grav- 19 -G- i ty waves from xkE! superrovas lin the Virgo cluste~ ~ ~ I I I I \ "\ I \ I :."".~ ~t H'" l, r ....;-. : \ ,....,have iii ,¥iliedis4Eed (It-_4~~~. <:mlly-"'O \~ '- . , f:UI!: Iii I II I' I I I 1'..LLLUUJ-f-1 I I III I I 1 IL ! 1,1 ,I ~ I I i I ! 1~~,j+Lill i I II 1 i~.4i 1 0 210 3 0 4 '0 ' 510 610 1_ - II I ,I 2-9-8076/Gravity waves/fa ~ "One of the reasons 1"e're !turn~ng to laser detectors," /J' c... 2 says Ronald Drever,"~ to avoid the qrtum limit problem. 3 Because we can make measurements over a much larger 4' I, region of space, we x effectively see a much larger sig- I 5' j nal. We don't have to look for such minute changes as 6 in a bar antenna." 7' I aroon ion 81 Laser interferometers bounce~ laser beam back and I 9 forth many times between two mirrors. (A generalized ap- ! 10 proach to the scheme appears in the drawing on page . ) Ii. As a gravity wave ripples through each mirror in succes- sian, the length of the light path ~ changes, resulting 13 n in a change in the interference patter~ns that appear ... 1 I 4' '.... .- ,f I - • I' 1.- ~ r' e: r ~ t ~-.u ~ ~ r~141~ ~(Ht 3 jt vots ,IPJ't''''r -; fb!tl f L \ ;,*,,",t'+ E !l! 5. ) '. !~ "---- ' on a screen. Numbers of such detectors are in the plan 151 ~ I 16 ning and building stages, including ones at MIT, designed " 17 by Rainer Weiss, a pioneer in the field; at the Max P~ck I {1 of jLi >(" ~), r J IIKn G ermany; 18 i Institute in MHH±~R at the pniversity of Glasgow; and I, 19 i at Caltech. Both the Glasgow nd Caltech groups are headed I t 20 by the peripatetic Drevir. I L V ;J'U O'U ;U C '_I ov I "U I 2-9-8076/Gravity waves/fpril PS/2l "The one in Glasgo~ has 110-meter arms;" he told me I ! ~ I , I 2 \ and is working now. The one we're working on at caltech 3 also has 10-meter arms"but will be stretched to 40 me- 41 ters as soon as a building for it is ready. This will 1 5 serve as a prototype for a much larger version--a kilom to ..s NU~J!.. ete~lometers long." ; Of course, laser interferometers have engineering 8, probEilns that 9 problems too, HKpHXXHXXY become exacerbated as they 1[\ grow larger. The laser beams must travel through vacuum ~ i a pipes, and isolating pipes/kilometerx long will not be I 12: I simple. But Drever is convinced it can be done. "Maybe 13 we'll put it in a mine, or in the desert," he says. This 14: e. I device may be JadY by 1986, and has, Drever thinks, a 15 1 J.. , ' I ev ~'\.Jc,.: ''/ chance of detecting supernovas in the Virgo cluster. 16 " 17 One additional advantage of such laser detectors is 18 that they are not restricted to a narrow frequency range, , 1 , I 19 I ' as are the resona nt ant$nnas, j but would be sensitive to I I 20 % a broad band of frequenlies ftom a few Her~ to a few I I ELITE filL+lLiJi1~j-Lj H I i I IllJ~~ I I I II I : . I I . 1-,-+~jLi LLLlJJlJj~-LLlJ~iLUj~ I I • I . I ' _,_LLl.-,-; ,I 1'0 2TO 3 0 I 410:i -r 510 610 7'0 810 2-9-8076/Gravity waves/fpril rS/22 , , 1 thousand Hertz, and coull:l ther'efore theoretically detect 2: black hole events, which have lower frequencies than , gravity waves from supernovas. To detect gravity waves with muchR lower frequencies, such as those from binary systems, you need very long baselines. "In about fifteen years, "says Rainer Weiss, we will want big, spacehased laser systems, using, say, a lO-kilometer frame in space. That way we could avoid all seismic noise."(one early .. ' proposal for a K~~~H laser system in space is illustrated on page . ) 1L , I The th~rd kind of gravity wave detector already ex- after a fashion. ists in space, HXXH0H~H It is called Doppler x L.': Here's how space ranging, and is very simple--in theory. X0HXK±mpX¥x~0mx 1 S scientist Richard Davies of RH Jet propulsion Laboratory MKN~KtHxE¥kxxHa±0XhHXwHHRXE~KXHxKRNXHXK~~~H~K~xx¥xKRHH 16 puts it: "You send a radio signal from Earth to a space 17 HX~KaX±x¥xwaxHx~aKKHKxxHK0H~HxXHHxK0tKxxK¥KxHM¥X craft, and a transponder aboard the craft sends the sig- 18 , , I 19 nal back to you. If a gravity; wave passes through the I 20 solar system, it alters I the distance between the two, ,I tUIt -' -,-~-+"- L\i O-l-~-+""-~~O--'-"-'-'+"' 3 10 ~-4rQi--',-~-~-sro-- El1-o-~ I : , ' 2-9-8076/Gravity waves/fpr il iPS/23 , ' " and when you compare the frequency of the signal you sent + ~ you see that OBey are out to the one you get back, ¥~MXBHxH~xxax~~~xHXXKH±XX. different--the Doppler Shift.~ "We want to detect gravity waves with very low frequen- cies, maybe a thousandth of a Hertz, using interplanetary spacecraft and the Deep Space Net that is used to track them. Such waves could be emitted from a collapsing system with a mass of a million to ten million suns, or from o clA-uble stars that orbit each other in hours." Some preliminary tries at Doppler ranging have been carried out on various space missions, including Viking ! and Voyager. But those spaeecraft were equipped with S x~xMaxKxaRBxYN¥a~Hxxx~XXRHX~HxHxx~xaRHxK band radio equipment, at about 2700 megacycles, which is prone to interference effects from the solar wind. "If we could shift the frequency up three times, to the 17 X-band," says Davies, we could gain an order of magnitude If in sensitivity." Ax~xax±x¥XMRXHxHX~Hx±mHRXXW 1 ~) I A gravity wave expe~iment using X-band radio had been , 20 OU :.J'U O'U I U b;U I "U , 2-9-8076/Gravity waves/fiseer( April PS/24 planned for the International' Solar Polar Mission. But, Gmmitteb on Gravitational Ph}'s_i_c-.: of th0 2 according to MIT's IrWln Shapiro, who chalred thelNatlonal , I 3' I Academy of Science's €~mmX Space Science Board, the plan 4' I has been dropped. "Maybe it will get on the Galileo l!:l±x 5i Jupiter probe later in the 80's, but nothing's been decided yet." 7 i Which of these methods will yield the first direct 8i 9 evidence of gravity waves? And when will that first con- I 1Q tact come? No one really knows, and the gravity wave , l' seekers themselves are extremely diffident about making claims and predictions. But some time within the decade 13 seems at least plausible. '" I i In the meantime, gravity wave research is paying i SI unexpected dividends. "It has opened up," says Kip 16' 17 Thorne, "a modest new chapter in quantum electronics. 18i Bec{!fc\se it is pushing so hard, against the bounds of 19 I modern technology, it is open~ngI up new techniques. that , 1 20 I I will have fall-out else~here,l For example, some of the 1'U 21U :.; IU 4 ,\.: blU b'U IU 8'S 2-9-B076/Gravity waves/Apri,l PS/25 , ! same kinds of problems have' to be faced in cornmuni- cating with submarines via extremely low freqgency waves." In the long run, however, the ~&zJ'9~aba search for ~ (j' Y""" ~ , I ~ d. 1 '/ t /..4 I;, ~ J ' ( d Y I V ~ gravi ty waves if. Ae'i~~ ls-y ~b tU:\el'~Wild\d151c I:t.ilil"'~ little farther, all scientists, and all mankind: to see a X±¥XHXX~KNXHX + to understand a little more, than we have ever done b~ore. E 15 17 18 19 20