Chuong 4 Vu Tru Trong Vo Hat de - Smith.N Studio

8/3/2019 Chuong 4 Vu Tru Trong Vo Hat de - Smith.N Studio

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Trang 101

CHNG 4TIN ON TNG LAI

S bin mt ca thng tin trong cc h en c th lm gim kh nng tin on tng lai cachng ta nh th no?

Smith Nguyen Studio.


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V T R T R O N G M T V H T

Trang 102 Ngi dch: [email protected]; http://datrach.blogspot.com



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T I N O N T N G L A I

Trang 103Ngi dch: [email protected]; http://datrach.blogspot.com

(Hnh 4.1)

Mt ngi quan st trn tri t(mu xanh) ang chuyn ngtrn qu o xung quanh mt tris thy sao Ha (mu ) in trnvng cu cc sao.Chuyn ng biu kin phc tpca cc hnh tinh c th c giithch bng cc nh lut ca New-

ton v khng c nh hng g ns phn ca con ngi.

Nhn loi lun mong mun iu khin tng lai, hoc t nhtl on trc c iu g s xy ra. l l do ti saongnh chim tinh hc li ph bin n th. Chim tinh hccho rng cc s kin xy ra trn tri t u lin quan n chuynng ca cc hnh tinh trn bu tri. y l mt gi thit c thkim chng mt cch khoa hc, khng, n s l mt gi thit cth kim chng mt cch khoa hc nu nh cc nh chim tinh dmmo him ni mt d on chc chn m c th kim tra c. Tuynhin, h cng thng minh ch ni nhng d on m h cth ng vi bt k kt qu no. Nhng pht biu kiu nh Ccmi quan h c nhn c th tr ln mnh lit hn hoc l Bn sc mt c may v ti chnh s khng bao gi b chng minh l saic.

Nhng l do m phn ng cc nh khoa hc khng tin vo chimtinh hc khng phi l nhng bng chng phi khoa hc hoc thiunhng bng chng khoa hc m v n khng ph hp vi nhng lthuyt khc c kim chng bng thc nghim. Khi Copernicus

v Galileo pht hin ra rng cc hnh tinh quay quanh mt tri chkhng phi quay quanh tri t, v Newton tm ra nh lut hp dniu khin chuyn ng ca cc hnh tinh th chim tinh hc trln cc k ng ng. Ti sao v tr ca cc hnh tinh khc trn nntri khi chng c nhn t tri t li c nhng mi tng quanvi nhng i phn t t gi l sinh vt c tr tu sng trn mt tiuhnh tinh (hnh 4.1)? Chim tinh hc cn phi lm cho chng tatin vo s tng quan . Cc l thuyt c trnh by trong cunsch ny cng khng hn g chim tinh hc ch khng c thm

Thng ny sao Ha chim cungNhn m, l thi gian tt bn t hc. Sao Ha yu cu bn sng mt cuc sng theo cchbn cho l ng v thng nhngngi khc cho l sai. V nhng

iu s xy ra.Vo ngy 20, sao Ha s i n phn hc vn ca bn, n linquan n ngh nghip v bn shc nhn ly trch nhim vgii quyt cc quan h kh khn.Tuy vy, n k trng rm bns c c mt s thng sut vtm bao qut tuyt vi v ton bcuc i m bn s nhn c.



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cc bng chng thc nghim cng c cc l thuyt , nhng tavn tin v cc l thuyt ny ph hp vi cc l thuyt c kimchng.

S thnh cng ca cc nh lut ca Newton v cc l thuyt vtl khc dn n tng v quyt nh lun khoa hc (scientificdeterminism). tng ny c mt nh khoa hc ngi Php tn

l Marquis de Laplace a ra ln u tin vo u th k th michn. Laplace cho rng nu chng ta bit c v tr v ta catt c cc ht trong v tr ti mt thi im th cc nh lut vt ls cho php chng ta on c trng thi ca v tr s nh th noti bt k mt thi im no khc trong qu kh v tng lai (hnh4.2).

Ni cch khc, nu quyt nh lun khoa hc m ng th chngta c th on trc c tng lai v khng cn n chim tinhhc. Tt nhin l trn thc t ngay c nhng ci n gin nh nh

lut hp dn ca Newton cng dn n cc phng trnh m chngta khng th gii mt cch chnh xc cho h c nhiu hn hai htc. Hn na, cc phng trnh ny thng c mt tnh cht cbit l hn lon, do , mt thay i nh v v tr v vn tc timt thi im c th dn n mt tnh cht hon ton khc ti ccthi im tip theo. Nhng ngi xem phim Cng vin k Jura(Jurassic Park, hnh 4.3) u bit, mt xo trn nh mt ni nyc th gy ra mt thay i ln mt ni khc. Mt con bm vcnh Tokyo c th gy ra ma cng vin trung tm New York(hnh 4.3). iu phin phc l chui s kin khng c tnh lp

(Hnh 4.2)

Nu bn bit v tr v tc caqu bng m bn nm i, bn cth tnh c n s i n u.

(Hnh 4.3)



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li. Ln sau con bm v cnh, mt lot cc s kin khc s khci v cc s kin ny s nh hng n thi tit. l l do ti saocc d bo thi tit rt khng ng tin cy.

Do vy, mc d v nguyn l th cc nh lut ca in ng lc hclng t s cho php chng ta tnh ton c tt c mi th trongha hc v sinh hc, nhng chng ta vn khng c nhiu thnh cng

trong vic on trc c hnh vi con ngi t cc phng trnhton hc. Tuy nhin, mc d gp phi nhng kh khntrn thc tin nh th, nhng v nguyn tc, phn lncc nh khoa hc vn c an i vi tng chorng tng lai vn c th d bo c.

Thot nhn th quyt nh lun khoa hc c v nhb nguyn l bt nh e da. Nguyn l bt nh nirng chng ta khng th o chnh xc v tr v vn tcca mt ht ti mt thi im. Chng ta o v tr cng chnh xc

bao nhiu th chng ta xc nh vn tc cng km chnh xc bynhiu, v ngc li. Li gii thch v quyt nh lun khoa hc caLaplace cho rng nu chng ta bit v tr v vn tc ca cc ht timt thi im th chng ta c th xc nh c v tr v vn tc cachng ti bt k thi im no trong qu kh v tng lai. Nhnglm th no m chng ta c th lm c iu nu nh ngay tu nguyn l bt nh khng cho chng ta bit c v tr vvn tc ti mt thi im? D my tnh ca chng ta tt th no ichng na, nu chng ta cung cp d liu u vo sai th chng ta

U VO

U RA



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s nhn c cc d on sai lm.

Tuy vy, quyt nh lun c khi phc di mt dng khc trongmt l thuyt mi c gi l c hc lng t, tng thch vinguyn l bt nh. Trong c hc lng t, ni mt cch gn ng,ta c th d on mt cch chnh xc mt na nhng iu m ta

mong mun thc hin trn quan im Laplace c in. Trong chc lng t, mt ht khng c v tr hoc vn tc xc nh nhngtrng thi ca ht c th c biu din bng mt ci gi l hmsng (hnh 4.4).

Ti mi v tr trong khng gian, mt hm sng l mt con s chobit xc xut m ht c th c tm thy ti v tr . Tc thayi ca hm sng t im ny n im khc cho bit kh nng ht c cc vn tc khc nhau. Mt s hm sng c mt nh rt nhn

(Hnh 4.4)

Hm sng xc nh xc sut mht s c cc v tr v vn tc khcnhau theo cch m chng phitun theo nguyn l bt nh.

HM SNG NHN

PHN B XC SUT VNTC CA HT

SNG CC HM SNG

PHN B XC SUT VNTC CA HT

V tr

V tr

Vn t

Vn t



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ti mt im c th trong khng gian. Trong trng hp ny, btnh v v tr ca ht l rt nh. Trn gin ta cng c th thytrong nhng trng hp , hm sng thay i rt nhanh gn nhca sng, hm sng tng nhanh mt sn v gim nhanh phasn kia. iu ny c ngha l phn b xc sut ca vn tc ctri trn mt vng gi tr rt ln. Hay ni mt cch khc, bt nhv vn tc rt ln. Mt khc, chng ta hy xem cc hm sng thoaithoi th bt nh v v tr ln nhng bt nh v vn tc linh. Vy nn, vic m t cc ht bng hm sng khng cho ta v trv vn tc chnh xc. Gi y ta thy rng hm sng l tt c nhngg m ta c th xc nh. Thm ch chng ta cng khng th chorng Cha bit v tr v vn tc ca cc ht nhng giu khng chochng ta bit. Cc l thuyt bin s n (hidden variable) nh thkhng ph hp vi cc quan st thc nghim. Hn th na, Cha bgii hn bi nguyn l bt nh v khng th bit v tr v vn tcca ht; Cha ch c th bit hm sng ca ht m thi.

Tc thay i ca hm sng theo thi gian c cho bi mt

(Hnh 4.5)PHNG TRNH SCHRO-DINGER

S ph thuc ca hm sng theo thi gian c xc nh biton t H lin quan n nnglng ca h ang xt.



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phng trnh gi l phng trnh Schrodinger (hnh 4.5). Nu tabit hm sng ti mt thi im th chng ta c th dng phngtrnh Schrodinger tnh hm sng ti bt k thi im no khctrong qu kh v tng lai. Vy nn, quyt nh lun khoa hc vnng trong l thuyt lng t nhng vi mt mc thp hn.

Thay cho kh nng on trc c c v tr v vn tc, chng tach c th bit c hm sng. Hm sng ch cho bit chnh xc vtr hoc vn tc ch khng th bit c c hai. Do , trong c hclng t, kh nng tin on chnh xc ch bng mt na kh nngtin on trong th gii quan Laplace c in. Vi ngha gii hnny, ta c th ni quyt nh lun khoa hc vn ng.

Tuy nhin, vic dng phng trnh Schrodinger tnh hm sng theothi gian (tc l d on nhng iu s xy ra trong tng lai) hinnhin tha nhn rng thi gian tri i mt cch trn tru mi mi ti

khp cc im trong khng gian. iu ny r rng l ng trong vtl Newton. Thi gian c cho l tuyt i, tc l mi s kin tronglch s ca v tr c nh du bi mt con s c gi l thigian v chui con s tri mt cch trn tru t v hn trong qukh n v hn trong tng lai. c th ni l cm nhn chung vthi gian v l cch nhn m phn ng mi ngi thm ch l phnng cc nh vt l tm nim. Tuy nhin vo nm 1905, nh chngta thy, thuyt tng i hp vt b khi nim thi gian tuyti, trong , thi gian t n khng cn l mt i lng c lp

(Hnh 4.6)

Trong khng thi gian phngca thuyt tng i hp, nhngngi quan st chuyn ng vi

cc tc khc nhau s c cc php o thi gian khc nhau,nhng chng ta c th dng phng trnh Schrodinger trongbt k thi gian no on chm sng trong tng lai.



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ca mt th lin tc bn chiu c gi l khng thi gian. Trongthuyt tng i hp, cc nh quan st khc nhau chuyn ng vicc vn tc khc nhau trong khng thi gian theo cc hng khcnhau. Mi nh quan st c php o thi gian ring ca anh ta hocc ta dc theo hng m anh hoc c ta ang chuyn ng. V ccnh quan st khc nhau s o c cc khong thi gian khc nhaugia cc s kin (hnh 4.6).

Do , trong thuyt tng i hp, khng c thi gian tuyt i chng ta c th nh du cc s kin. Tuy nhin khng thi gian

trong thuyt tng i hp li phng. iu ny c ngha l, trongthuyt tng i hp, thi gian c o bi bt k nh quan stchuyn ng t do s tng mt cch trn tru trong khng thi giant m v cng ca qu kh v cng n dng v cng ca tnglai v cng. Chng ta c th dng bt k php o thi gian no trongphng trnh Schrodinger tnh s ph thuc ca hm sng vothi gian. Vy nn, trong thuyt tng i hp, chng ta vn c mtkiu quyt nh lun lng t.

Trong thuyt tng i rng th tnh hung li khc i v khng

thi gian khng cn phng m b b cong bi vt cht v nnglng trong . Trong h mt tri ca chng ta, t nht l trn ncthang v m, cong ca khng thi gian nh n ni n khngnh hng n quan nim chung ca chng ta v thi gian. Trongtrng hp , chng ta vn c th dng thi gian trong phngtrnh Schrodinger bit s ph thuc ca hm sng vo thi gian.Tuy nhin, mt khi chng ta cho php khng thi gian c th bcong th kh nng, trong khng thi gian c mt cu trc khngcho php thi gian tng mt cch trn tru i vi mi nh quan stnh chng ta trng i, c th xy ra. V d, khng thi gian ging

(Hnh 4.7) THI GIAN DNG

Mt php o thi gian c th nhtthit phi c mt im dng mti quai cm ca chic cc tipxc vi phn tr chnh: ti ccim , thi gian s dng. Ticc im nh th, thi gian khngtng theo bt k hng no. Do, ta c th dng phng trnhSchrodinger tin on hm

sng trong tng lai.

Thigian

Thigian

Khng gian Khng gian

im dng



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nh mt hnh tr thng ng (hnh 4.7).

Chiu cao ca hnh tr c th l php o thi gian, thigian tng i vi mi ngi quan st v chy t m v

cng n dng v cng. Tuy nhin, hy tng tngrng, thay cho hnh tr l mt hnh tr vi mt ci

quai (hoc l mt h giun (wormhole)) tch ri khi

hnh tr sau li nhp li. Do , bt k php o thigian no u c cc im dng ti ni m ci quai nhp vo

hnh tr chnh: cc im m ti thi gian dng li. Ti ccim ny thi gian khng tng i vi bt k ngi quan st

no. Trong mt khng thi gian nh vy chng ta khng th dngphng trnh Schrodinger bit c s ph thuc ca hm sngvo thi gian. Hy cn thn vi cc h giun: bn khng bao gi bitc ci g s chui ra t .

Cc h en l nguyn nhn chng ta ngh thi gian khng tng

i vi cc nh quan st. Tho lun u tin v h en xut hin vonm 1783. Mt cu gio s ca i hc Cambridge, John Michell trnh by lun c sau y: nu ai bn mt ht, nh l mt vinn i bc chng hn, thng ln tri th chuyn ng ln trn s bchm li do lc hp dn v cui cng l ht s dng chuyn ngln trn v s ri tr li (hnh 4.8). Tuy vy nu vn tc bn ban uln hn mt vn tc ti hn c gi l vn tc thot th lc hpdn s khng mnh dng ht li v ht s bay i. Trntri t, vn tc thot vo khong 12 km/giy; trn mt tri th gi



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H EN SCHWARZSCHILD

Nm 1916, mt nh thin vn hc ngi cKarl Schwarzschild tm thy mt nghim tl thuyt tng i ca Einstein i din chomt h en hnh cu. Cng trnh ca Schwar-zschild tip l mt b n bt ng ca thuyttng i. ng chng minh rng nu mtngi sao b c c trong mt vng nh thtrng hp dn ti b mt s tr ln mnh nmc nh sng cng khng th thot c.

Ngy nay ngi sao nh th c gi l mth en, mt vng khng thi gian b bao bimt ci gi l chn tri s kin, n gii hnvng khng gian m tt c mi th, ngay cnh sng cng khng th thot ra n vi

ngi quan st.

Trong mt thi k rt di, cc nh vt l, ngayc Einstein cng nghi ng mt hnh th ccoan nh th ca vt cht c th xut hintrong v tr thc. Tuy nhin, ngy nay chngta hiu rng bt k mt ngi sao khng quay, nng, sau khi t ht nhin liu ht nhns phi suy sp thnh h en hnh cu honho ca Schwarzschild. ng knh (R) cachn tri s kin ca h en ch ph thucvo khi lng ca n, v c cho bi cngthc:R

= 2MG

/c

2

Trong cng thc ny, c l vn tc nh sng,G l hng s Newton, v M l khi lng cah en. Mt h en c khi lng bng khilng mt tri s c bn knh bng hai dm.

tr vo khong 618 km/giy.

C hai vn tc thot u ln hn nhiu vn tc ca cc vin ni bc nhng li nh hn vn tc nh sng (vo khong 300.000km/giy). Vy nn nh sng c th thot khi tri t v mt trimt cch khng my kh khn. Nhng Michell li l lun rng cth c cc ngi sao ln hn nhiu ln mt tri v c vn tc thotln hn vn tc nh sng (hnh 4.9). Chng ta khng th nhn thycc ngi sao v bt k tia sng no c phng i s b lc hpdn ca ngi sao ko tr li.

tng v cc ngi sao ti ca Michell da trn nn vt l caNewton, trong thi gian l tuyt i v thi gian khng m xan nhng s kin xy ra. Vy nn, trong bc tranh vt l c inca Newton, cc ngi sao en khng nh hng n kh nng tinon tng lai ca chng ta. Nhng trong thuyt tng i rng,trong cc vt th khng l lm cong khng thi gian th tnh hungli khc hn.

Nm 1916, ngay sau khi thuyt tng i rng c a ra ln utin, Karl Schwarzschild (ng mt ngay sau khi mc bnh mttrn vi Nga trong i chin th gii ln th nht) tm thy mtnghim ca cc phng trnh trng ca thuyt tng i rng biudin cho mt h en. Trong rt nhiu nm, ngi ta khng hiu hoc



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(Hnh 4.10)

Quasar 3C273, ngun pht sngv tuyn gi sao u tin cpht hin, pht ra mt nng lngrt ln trong mt vng nh. Cch duy nht gii thch hintng sng cao nh th l vtcht ang ri vo h en.

JOHN WHEELER

John Chibald Wheeler sinh nm 1911 ti Jackson-ville, Florida, Hoa K. ng ly bng tin s v tnx ca nh sng ln nguyn t Helium i hcJohn Hopkins nm 1933. Nm 1938 ng lm viccng nh vt l an Mch Niels Bohr pht trin

l thuyt phn hch. Sau mt thi gian, ng lmvic cng vi sinh vin ca ng l Richard Feyn-man, tp trung nghin cu nhit ng hc; nhngngay sau khi Hoa K tham gia i chin th gii 2,c hai u ng gp vo s n Mahattan.Vo nhng nm u thp k 50, ly cm hng tcng trnh v s suy sp hp dn ca nhng ngisao nng ca Robert Oppenheimer vo nm 1039,Wheeler chuyn sang thuyt tng i rng ca

Einstein. Vo thi gian , phn ln cc nh vtl u nghin cu vt l ht nhn v thuyt tngi rng khng thc s c coi l ph hp vi thgii vt l. Nhng gn nh mt mnh Wheeler nthng c m lm thay i lnh vc nghin cuthng qua cc cng trnh ca ng v thng qua vicging dy kha hc u tin v thuyt tng i

i hc Princeton.Rt lu sau, vo nm 1969, ng a ra khi nimh en, m lc by gi ch c mt s t ngitin rng n tn ti, ch trng thi suy sp cavt cht. V t cng trnh ca Werner Israel, ngd on rng h en khng c tc, m ch trngthi suy sp c bt k ngi sao nng khng quayno cng c th c m t bng nghim Schswar-zschild.



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khng nhn ra tm quan trng ca nhng iu m Schwarzschild tm ra. Bn thn Einstein cng khng bao gi tin vo cc h env quan im ca ng cng c phn ln cc nh khoa hc c uytn v thuyt tng i chia s. Ti cn nh chuyn i Paris trnh

by mt bo co v pht hin ca ti cho rng thuyt lng t ng cc h en khng hon ton en. Bo co ca ti kh t nht v volc gn nh khng c ai Paris tin vo cc h en. Ngi Phpcn cm thy rng ci tn trou noir(l en) m h dch ra tingPhp c ngha hi tc tu v nn thay bng ci tn astre occlu tc lngi sao n (hidden star). Tuy vy, d l ngi sao n hoc bt ktn no khc cng khng nhn c s nhn nhn ca cng chngbng ci tn h en. y l tn do Archibald Wheeler a ra. ngl mt nh vt l M, ngi gy nhiu cm hng cho cc cngtrnh trong lnh vc ny.

S pht hin ra cc quasar vo nm 1963 gy ra mt cuc bngpht cc nghin cu l thuyt v h en v cc n lc quan st nhn thy chng (hnh 4.10). y l bc tranh c ghp li. Hyxem xt nhng iu chng ta tin v lch s ca mt ngi sao c khilng ln gp hai mi ln khi lng mt tri. Cc ngi sao nhvy c hnh thnh t cc m my kh nh l cc ngi sao trongtinh vn Thin Lang (Orion, hnh 4.11). Khi cc m my kh coli di lc hp dn ca chnh bn thn chng, cc kh ny s nungnng v thm ch tr nn nng khi ng phn ng nhit hch

bin hydro thnh helium. Nhit to bi qu trnh ny gy nn mtp sut gip ngi sao chng tri li lc hp dn ca n v lm chongi sao khng b co thm na. Ngi sao s trng thi ny trongmt thi gian di, t chy hydro v bc x nh sng vo khnggian.

Trng hp dn ca ngi sao s nh hng n ng truyn cachm sng pht ra t ngi sao . Ta c th v mt gin vi thigian l trc thng ng, khong cch t tm ca ngi sao l trcnm ngang (hnh 4.12). Trong gin ny, b mt ca ngi sao

c biu din bng hai ng thng ng nm hai bn ca tmsao. Ta c th o thi gian bng giy v khong cch bng giynh sng khong cch m nh sng i c trong mt giy. Khita dng cc n v ny th tc ca nh sng l 1; tc l, tc ca nh sng l mt giy nh sng trn giy. Gin ng rng, pha xa ngi sao, xa trng hp dn ca n th ng truyn ca tiasng trn gin l mt ng thng to vi trc thng ng mtgc 45 . Tuy nhin, gn ngi sao th cong ca khng thi giando khi lng ca ngi sao gy ra s lm thay i ng truyn cacc tia sng v lm cho chng to vi phng thng ng mt gc

(Hnh 4.11)

Cc ngi sao c hnh thnh tcc m bi kh ging nh tinhvn Thin lang.



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hp hn.

Cc ngi sao nng s t chy hydro thnh helium nhanh hn mttri rt nhiu. iu ny c ngha l chng s cn kit hydro chtrong mt thi gian ngn khong vi trm triu nm. Sau , ccngi sao ny s i mt vi mt cuc khng hong. Chng c th

t helium thnh cc nguyn t nng hn nh l carbon v oxygen,nhng cc phn ng ht nhn ny khng gii thot nhiu nnglng, do cc ngi sao s mt nhit v mt i p sut nhit gipngi sao chng li lc hp dn. Do cc ngi sao tr nn nh hn.Nu khi lng ca chng ln hn hai ln khi lng mt tri thp sut s khng bao gi ngn chn qu trnh co li. Chng ssuy sp thnh mt im vi mt v hn to nn ci gi l imk d (hnh 4.13). Trong gin thi gian theo khong cch t tmngi sao, khi mt ngi sao co li th ng truyn ca cc tia sng

(Hnh 4.12) Khng thi gian xungquanh mt ngi sao khng b suysp. nh sng c th thot ra t b mt ngi sao (ng thngng mu ). xa ngi sao, nhsng to vi phng thng ngmt gc 45 , nhng gn ngisao, khi lng ca ngi sao lmcong khng thi gian lm cho cctia sng lm mt gc nh hn viphng thng ng.

(Hnh 4.13) Nu mt ngi saosuy sp (ng mu ct nhauti mt im) khng thi gian bb cong n ni nh sng gn bmt b hng vo trong. Mt hen c hnh thnh, mt vngkhng thi gian m nh sng t khng th thot ra c.

Hnh 4.12 Hnh 4.13

Ngi sao Ngi sao

Tia sngTia sng

Chntri

K d

Khng gianKhng gian Thigian

Thigian



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t b mt ngi sao s to vi ng thng ng nhng gc nh hnv nh hn. Khi ngi sao t n mt bn knh ti hn xc nhth cc ng truyn ca tia sng s l ng thng ng trn gin

, tc l nh sng s i trn mt khong cc c nh vi tm ngisao m khng bao gi c th thot i c. ng truyn ti hnca tia sng s lt trn mt b mt c gi l chn tri s kin(event horizon) phn cch vng khng thi gian m nh sng c ththot khi ngi sao v vng khng thi gian m nh sng khng ththot c. Bt k mt tia sng no c ngi sao pht i sau khii qua chn tri s kin s b b cong tr li bi cong ca khngthi gian. Ngi sao s tr thnh mt trong nhng ngi sao en caMichell, hoc nh ngy nay chng ta ni, mt h en.

Chn tri l vng bin gii camt h en c hnh thnh bicc tia sng ch chm khngthot khi h en v gi mt

khong cch khng i vi tmca h en.

Nn nh sng thot ra

Tia sng thot ra trc khichn tri s kin hnh thnh

Nn nh sng b nh hngbi trng hp dn

Nn nh sng b by

Tia sng thot ra ti thi imchn tri s kin hnh thnhK d

Tia sng thot ra sau khichn tri s kin hnh thnh

Khi u chn tri s kin

KHNG GIAN

TH

IGIAN



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Lm th no c th ghi nhn mt h en nu khng mt tia sngno c th thot khi n? Cu tr li l h en vn to ra mt lc ht

hp dn khng i ln cc vt th ln cn khi ngi sao b suy sp.Nu mt tri l mt h en hoc tr thnh mt h en m khngmt i khi lng ca n th th cc hnh tinh vn quay nh chngang quay hin nay.

C mt cch tm kim cc h en l tm kim vt cht quayxung quanh mt vt th khng l, c v khng nhn thy. Ngi ta quan st rt nhiu cc h nh th. C l n tng nht l cc hen khng l xut hin tm ca cc thin h v cc quasar (hnh4.15).

Cc tnh cht ca h en c tho lun cho n nay khng gynn vn ln no cho quyt nh lun. Thi gian s kt thc ivi nh du hnh v tr ri vo h en v chm vo im k d. Tuynhin, trong thuyt tng i, ngi ta hon ton t do khi o thigian vi cc tc khc nhau v ti cc a im khc nhau. Do ,ngi ta c th tng tc ng h ca nh du hnh v tr khi anh tahoc c ta tin n gn im k d, vy nn, ngi ta vn ghi nhnc khong thi gian v tn. Trn gin thi gian khong cch(hnh 4.14), cc mt phng m ti thi gian l hng s (mt ng

thi gian) s rt dy c ti trung tm bn di im m ti k d xut hin. Nhng cc mt phng li ph hp vi php othi gian thng thng ti vng khng thi gian gn phng cch xah en.

Ta c th dng thi gian trong vng khng thi gian gn phng chophng trnh Schrodinger v tnh hm sng ti cc thi im sau nu ta bit thi gian trc . Do , ta vn c quyt nh lun.Tuy vy, cng ng lu rng, ti cc thi im sau, mt phn ca

(Hnh 4.15) MT H EN TM THIN H

Tri: Pht hin thin h NGC4151 t ng knh thin h trngrng.

Gia: ng nm ngang bchnh l nh sng pht ra t mth en tm thin h 4151.

Phi: Hnh ny cho thy tc pht ra oxygen. Tt c cc bng

chng u cho thy rng NGC4151 c cha mt h en c khilng bng mt trm triu lnkhi lng mt tri.

CT: ng ng thi gian

(Hnh 4.14)



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Hnh minh ha mt nh du hnh h cch xungmt ngi sao ang suy sp vo hi 11 gi 59 pht57 giy v chm vo ngi sao ang co li ti bnknh ti hn m ti hp dn mnh n ni khngmt tn hiu no c th thot ra c. Nh du hnhgi tn hiu t ng h ca anh ta v phi thuynang quay cch quanh ngi sao mt khong cch

tng i ln.Mt ai ang nhn ngi sao t pha xa s khngbao gi nhn thy n i qua ng chn tri vo hen. Thay vo , ngi sao c v nh ln l bnngoi kch thc ti hn v ng h trn b mtngi sao s chm dn ri tt hn.

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KHNG GIAN

THIGIAN



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hm sng li bn trong h en ni m khng ai bn ngoi cth quan st thy hm sng . V th, mt ngi quan st ngi nhy cm khng b ri vo h en, khng th chy ngcphng trnh Schrodinger tnh hm sng ti cc thi im trc. lm iu , anh ta hoc c ta cn bit phn ca hm sngnm bn trong h en. Phn hm sng ny c cha cc thng tinv nhng ci ri vo h en. Rt c kh nng l mt lng lncc thng tin v h en vi khi lng v tc quay cho c thc to thnh t rt nhiu tp hp cc ht khc nhau; h en khngph thuc vo bn cht vt th suy sp to nn n. John Wheelergi kt qu ny l h en khng c tc. i vi ngi Php, iuny khng nh cc mi nghi ng ca h.

Kh khn cho quyt nh lun xut hin khi ti thy rng cc hen khng phi l hon ton en. Nh l ta thy trong chng 2,thuyt lng t ng rng cc trng c th khng hon ton bngkhng ngay c trong ci m ta gi l chn khng. Nu chng bngkhng th chng s c c v tr ti im khng v tc thay i haycn gi l vn tc cng bng khng. iu ngy vi phm nguyn lbt nh ni rng ngi ta khng th xc nh chnh xc vn tc vv tr. Thay vo , tt c cc trng cn phi c mt ci gi l thngging chn khng nht nh (tng t nh l con lc trong chng2 phi c thng ging im khng). Thng ging chn khng c thc gii thch theo mt vi cch c v khc nhau nhng thc t lchng tng ng vi nhau v mt ton hc. Trn quan im thcchng, ta khng b b buc khi s dng bt k m hnh no hiu qunht cho bi ton t ra. Trong trng hp ny, s rt c ch khi coithng ging chn khng nh cc cp ht o xut hin cng nhau timt im trong khng thi gian, chuyn ng ra xa nhau ri quaytr li vi nhau v hy nhau ln nhau. o c ngha l cc ht nykhng th c quan st mt cch trc tip, nhng ta c th o ccc hiu ng gin tip, v cc php o ny ph hp vi cc tinon l thuyt vi mt chnh xc ng k (hnh 4.16).

Nu h en hin din th mt thnh phn ca cp ht c th b ri

vo h en li thnh phn kia t do thot vo v tn (hnh 4.17).i vi mt ngi pha xa h en th ht thot ra kia dng nhc pht x t h en. Ta trng i ph ca h en chnh l phca mt vt nng vi nhit t l vi trng hp dn ti chn tris kin bin gii ca h en. Ni cch khc, nhit ca h enph thuc vo kch thc ca n.

Mt h en c khi lng gp vi ln khi lng mt tri s cnhit khong mt phn triu trn khng tuyt i, v mt

NHIT CA H EN

H en pht x ging nh mtvt nng c nhit (T) vch ph thuc vo khi lng

ca n. Ni chnh xc hn lnhit cho bi cng thcsau:

T = hc3/8kGM

Trong cng thc ny (c) lvn tc nh sng, (h) l hngs Plank, (G) l hng s hpdn Newton, (k) l hng sBoltzman.Cui cng l (M) l khilng ca h en, do , hen cng nh th nhit cng cao. Cng thc ny chochng ta thy rng, nhit ca mt h en c khi lngbng vi ln khi lng mttri c nhit ch khongmt phn triu trn khng tuyt i.

H en khng c tc



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(Hnh 4.16, bn tri)Trong khng gian trng rng, cccp ht xut hin trong khonhkhc ri hy ln nhau.

(Hnh 4.17, bn trn)Cc ht o xut hin v hy lnnhau gn chn tri s kin cah en.Mt ht b ri vo h en trongkhi ht kia thot ra ngoi. T bnngoi chn tri s kin dngnh h en ang bc x cc htra khng gian.



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h en ln hn li c nhit thp hn. Do vy, bt k bc x lngt no t cc h en nh th s b chm hon ton trong bc x 2,7 cn st li t v n ln bc x phng v tr m ta tho lun

trong chng 2. Ta c th ghi c bc x t cc h en nh hnv nng hn, nhng dng nh khng c nhiu cc h en nh th.Tht ng tic! Nu ngi ta pht hin ra mt h en th ti s cgii Nobel. Tuy nhin chng ta vn c nhng bng chng kh quangin tip v bc x ny, bng chng ny n t v tr s khai. Nh m t trong chng 3, ngi ta cho rng vo nhng giai onrt sm trong lch s, v tr tri qua mt thi k lm pht, khi v tr gin n vi mt tc cha tng c. Qu trnh gin n trongthi k ny nhanh n ni mt s vt th qu xa chng ta v nh

Cc s kin m ngi quan st khng bao gi nhn thy

(Hnh 4.18) Nghim de Sitter ca cc phng trnh trng ca thuyttng i biu din mt v trgin n theo kiu lm pht. Trngin , thi gian hng ln trnv kch thc ca v tr theophng nm ngang. Khong cchv khng gian tng nhanh nmc nh sng t cc thin h xaxi khng bao gi c th n vichng ta, v do , c mt chntri s kin ging chn tri skin ca h en, mt bin giingn cch nhng vng v tr takhng th quan st c.

Chn tri s kinca ngi quan st

Chn tri s kinca ngi quan st

Mt ng thi gianLch s ngi quan st



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sng ca chng khng bao gi n c vi chng ta. V tr ginn qu nhiu v qu nhanh trong khi nh sng t chng truyn vnhng v chng ta. Do s c mt chn tri trong v tr gingnh chn tri ca h en, phn cch vng m nh sng t c thn vi chng ta v vng m nh sng khng th n vi ta c(hnh 4.18).

Cc lp lun tng t cho tht rng c mt bc x nhit t chn triny ging nh bc x nhit t chn tri ca h en. T bc x nhit,

chng ta mun bit ph c trng ca cc thng ging mt . y, cc thng ging mt ny cng gin n cng v tr. Khi kchthc ca chng tr ln ln hn kch thc ca chn tri s kin thchng s b ng bng, cho nn ngy nay ta c th thy chng nhl nhng thng ging nh trong nhit ca bc x phng v trcn li t thi k s khai ca v tr. Cc thng ging quan st phhp vi cc tin on v thng ging nhit vi chnh xc cao.

Thm ch nu bng chng thc nghim v bc x ca h en khngtrc tip cho lm th tt c nhng ai nghin cu vn ny ung rng bng chng cn phi ph hp vi cc l thuyt khcm c kim chng bng cc quan st. iu ny c ngha rtquan trng i vi quyt nh lun. Bc x t h en s mang nnglng i, iu ny c ngha l h en s mt nng lng v tr nnnh i. V s nh i ny, n lt n, c ngha l nhit ca hen s tng v tc bc x cng s tng. Cui cng th h en sgim n khi lng bng khng. Lc th ta khng bit lm thno c th tnh c vic g s xy ra, nhng kt qu duy nhthp l v ng nhin l h en dng nh bin mt hon ton.Th th ci g s xy ra sau i vi phn hm sng nm trongh en v thng tin v nhng ci ri vo h en m phnhm sng c? D on u tin c th l phn hmsng ny, v cc thng tin m n mang, s thot rakhi h en cui cng th cng bin mt. Tuy nhin,thng tin khng c truyn i mt cch minph, ngi ta nhn ra iu khi nhn cha n in thoi!

Thng tin cn nng lng mang n i v lic rt t nng lng cn st li trng thi



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cui cng ca h en. Cch duy nht c v hp l thng tin bntrong c th i c ra ngoi l n s thot ra lin tc vi bc x chkhng i n trng thi cui cng ny. Tuy nhin, theo bc tranhv mt ht ca cp ht o b ri vo h en v ht kia thot ra ngoi,ta khng trng i ht thot ra ngoi c lin h vi ht b ri vo hen hoc mang i thng tin v ht b ri vo h en. Do vy cu trli duy nht dng nh l thng tin ca phn hm sng bn trongh en b mt (hnh 4.19).

S mt mt thng tin nh th c hm rt quan trng vi quyt nhlun. bt u, ta hy lu rng, thm ch, nu bn bit hm sngsau khi h en bin mt th bn cng khng th chy ngc phngtrnh Schrodinger v tnh xem hm sng nh th no trc khih en c hnh thnh. Hm sng trc ph thuc mt phnvo hm sng b mt trong h en. Chng ta tng ngh l chngta bit qu kh mt cch chnh xc. Tuy nhin, nu thng tin b mttrong h en th suy ngh khng cn ng na. Bt k iu gcng c th xy ra.

(Hnh 4.19)

Bc x nhit t chn tri s kinca h en mang nng lngdng i lm gim khi lngca n. V mt i khi lng,nhit ca h en tng ln vtc bc x cng gia tng lm

cho khi lng ca n gim icng nhanh. Chng ta khng bitiu g xy ra nu khi lng trln cc nh, nhng kt qu hpl nht c th l h en bin mthon ton.



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Mc d vy th ni chung mi ngi nh l cc nh du hnh v trv nhng ngi t vn cho h quan tm n vic tin on tnglai hn l xem xt li qu kh. Thot nhn, c v nh l vic mtmt phn hm sng trong h en khng ngn cn chng ta tin onhm sng bn ngoi h en. Nhng ha ra s mt mt ny li tcng n tin on nh th, nh chng ta s thy khi ta xem xt mtth nghim v t duy do Einstein, Boris Podolsky v Nathan Rosen xut vo nhng nm 1930.

Hy hnh dung mt nguyn t phng x phn r v pht ra hai ht itheo hai hng ngc nhau vi spin cng ngc nhau. Mt ngiquan st m ch nhn vo mt ht th khng th tin on ht squay phi hay quay tri. Nu ngi quan st o c mt ht quayphi th anh ta hoc c ta c th on chc chn l ht kia s quaytri, v ngc li (hnh 4.20). Einstein cho rng iu ny chngminh thuyt lng t tht nc ci: gi s by gi ht kia phabn kia ca thin h, ngi ta vn c th bit ngay tc th hngquay ca n. Tuy vy, phn ln cc nh khoa hc ng rng chnh

(Hnh 4.20)

Trong th nghim t duy Einstein-Podolsky-Rosen, mt ngi quanst o spin ca mt ht s bithng spin ca ht th hai.



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Einstein lm ln ch khng phi thuyt lng t. Th nghim vt duy Einstein-Podolsky-Rosen khng chng minh rng ngi tac th gi thng tin nhanh hn nh sng. M chnh l iu ncci. Ngi ta khng th chn ht quay phi quan st nn ngita khng th on trc c ht pha xa ngi quan st s quay

tri.

Trn thc t, th nghim v t duy ny chnh l nhng iu xy ravi bc x h en. Cp ht o s c mt hm sng tin on rnghai ht s mi c spin ngc nhau (hnh 4.21). iu m chng tamun lm l on spin v hm sng ca ht thot ra. Ta c thlm c iu ny nu ta c th quan st c ht b ri vo h en.Nhng ht trong h en th ta khng th o c spin v hmsng ca n. Vy nn ta khng th tin on spin v hm sng ca

(Hnh 4.21)

Mt cp ht o c mt hm sngtin on c hai ht c spin ngcnhau. Nhng nu mt ht b rivo h en th ta khng th on

c chc chn spin ca ht cnli.



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Cc mng gp nhau

H en

ht thot ra. Ht c th c cc spin khc, cc hm sng khc vicc xc sut khc nhau, nhng n khng c mt spin hoc mt hmsng duy nht. Vy nn, kh nng tin on tng lai ca chng tadng nh b gii hn hn. tng c in ca Laplace v victa c th tin on c v tr v vn tc ca ht cn phi thay i khi

nguyn l bt nh cho thy rng ta khng th o chnh xc c vtr v vn tc. Tuy nhin, ngi ta vn c th o c hm sng vs dng phng trnh Schrodinger bit tng lai s nh th no.iu ny cho php ta chc chn on c t hp ca v tr v vntc l mt na ca nhng iu c th on trc theo tngca Laplace. Chc chn l ta c th tin on cc ht c spin ngcnhau, nhng nu mt ht b ri vo h en th chng ta khng thtin on chc chn v ht cn li. iu ny c ngha l ngi takhng th tin on chc chn bt k php o no bn ngoi h en:

(Hnh 4.22)

Cc h en c th c coi nhl phn giao nhau ca cc mng- p trong cc chiu b sung cakhng thi gian. Thng tin v cctrng thi bn trong h en c th

c lu tr di dng cc sngtrn mng-p.



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kh nng tin on chc chn ca chng ta b gim v khng. Dovy ngnh chim tinh hc c th tin on tng lai khng ti hncc nh lut khoa hc.

Nhiu nh vt l khng thch s suy gim trong quyt nh lun, vdo , h cho rng thng tin v nhng th bn trong c th thotra ngoi bng cch no . Trong nhiu nm, ngi ta ho vngtm thy cch cu ly thng tin. Nhng vo nm 1996, AndrewStrominger v Cumrun Vafa thc hin mt ci tin quan trng.H coi mt h en c to nn bi cc thnh phn c gi l cc

mng-p (p-brane: xem trang 54).

Nhc li l ta c th hnh dung v cc mng-p nh l cc tm chuynng trong khng gian ba chiu v trong by chiu b sung m takhng (hnh 4.22). Trong mt s trng hp nht nh, ta cth chng minh rng s cc sng trn cc mng-p bng lng thngtin m ta trng i h en c. Nu cc ht p vo mng-p, cc ht s to ra thm cc sng trn cc mng . Tng t, nu ccsng ang chuyn ng theo cc hng khc nhau trn cc mng-p

(Hnh 4.23)

Mt ht b ri vo h en c thc coi nh mt vng dy knp vo mng-p (1). N kchthch mt sng trn mng-P (2).Cc sng c th i vi nhaulm cho mt phn ca mng-p bt ra to mt vng dy kn (3).y l mt ht thot ra khi hen.



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m gp nhau ti mt im no th chng c th to nn mt nhsng ln n ni lm cho mt phn ca mng-p b t v thot rangoi nh mt ht. Vy nn cc mng-p c th hp th v pht xcc ht ging nh h en (hnh 4.23).

Ta c th coi cc mng-p nh l mt l thuyt hiu dng; tc l,chng ta khng cn tin rng thc s c cc mng t hon chuynng trong mt khng thi gian phng hay khng, th cc h envn c th x s nh th chng c to thnh t cc mng nh vy.Ging nh nc vy, nc c to thnh t hng t t cc phn t

H2O vi cc tng tc phc tp. Nhng m hnh cht lng lin tcl m hnh hiu dng rt tt. M hnh ton hc v cc h en chorng chng c to thnh t cc mng-p cho cc kt qu tng tvi m hnh cp ht o c m t trc y. Do , trn quan imthc chng, y cng l mt m hnh rt tt, t nht l i vi mts loi h en nht nh. i vi loi h en , m hnh mng-ptin on mt cch chnh xc tc pht x m m hnh cp ht oa ra. Tuy nhin, c mt s khc nhau quan trng: trong m hnhmng-p, thng tin v nhng ci ri vo h en c lu tr bi hm



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sng ca cc sng trn cc mng-p. Cc mng-p c coi nh l cctm trong khng gian phng, v v l do , thi gian s tri i mtcch trn tru, ng truyn ca cc tia sng s khng b b cong, vthng tin trong cc sng s khng b mt. Thay vo , cui cngth thng tin hin ra t h en theo cc bc x t cc mng-p. Vynn, theo m hnh mng-p, chng ta c th s dng phng trnhSchrodinger tnh hm sng ti cc thi im sau . Khng c gb mt c, thi gian s quay u n. Chng ta s c mt quyt nhlun hon ho theo ngha lng t.

Th th bc tranh no trong hai bc tranh trn l ng? Mt phnhm sng s b mt trong h en hay tt c cc thng tin s thotra nh mng-p d on? y l mt trong nhng cu hi ni cmnht trong vt l l thuyt ngy nay. Nhiu ngi tin rng cng trnhgy y cho thy thng tin s khng b mt. Th gii l an ton vc th d on c, s khng c g bt ng xy ra. Nhng iu vn cha r rng. Nu ta chn thuyt tng i rng ca Einsteinmt cch nghim tc th ta phi tha nhn xc xut m khng thigian t tri mnh nh mt ci n v thng tin b mt trong nhngnp gp. Khi phi thuynEnterprise i qua mt h giun, vi iu btng xy ra. Ti bit iu v chnh ti trn phi thuyn chi bi vi Newton, Einstein v Data. Ti rt ngc nhin. Hyxem ci g xy ra u gi ca ti ny!

Mt cnh trong phim Star Trek:The Next Generation, 2001



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