Lược sử thời gian

8/14/2019 Lc s thi gian

1/226

LC STHI GIAN 1

http://ebooks.vdcmedia.com

MC LC

Li gii thiu ca nh xut bn Bantam Books.................................................................. 3Gii thiu cun sch "Lc s thi gian"........................................................................... 5

Phn 1.................................................................................................................................. 7

Phn 2................................................................................................................................ 12

Phn 3................................................................................................................................ 16

Phn 4................................................................................................................................ 20Phn 5................................................................................................................................ 25

Phn 6................................................................................................................................ 29

Phn 7................................................................................................................................ 33Phn 8................................................................................................................................ 38

Phn 9................................................................................................................................ 42

Phn 10.............................................................................................................................. 46

Phn 11.............................................................................................................................. 51

Phn 12.............................................................................................................................. 55Phn 13.............................................................................................................................. 61

Phn 14.............................................................................................................................. 64

Phn 15.............................................................................................................................. 68

Phn 16.............................................................................................................................. 73Phn 17.............................................................................................................................. 78Phn 18.............................................................................................................................. 83

Phn 19.............................................................................................................................. 89

Phn 20.............................................................................................................................. 93

Phn 21.............................................................................................................................. 99

Phn 22............................................................................................................................ 104Phn 23............................................................................................................................ 109

Phn 24............................................................................................................................ 114

Phn 25............................................................................................................................ 118Phn 26............................................................................................................................ 122

Phn 27............................................................................................................................ 126Phn 28............................................................................................................................ 130

Phn 29............................................................................................................................ 135

Phn 30............................................................................................................................ 139

Phn 31............................................................................................................................ 141


2/226

Stephen Hawking 2


Phn 32............................................................................................................................ 144

Phn 33............................................................................................................................ 148

Phn 34............................................................................................................................ 152Phn 35............................................................................................................................ 156

Phn 36............................................................................................................................ 158

Phn 37............................................................................................................................ 160

Phn 38............................................................................................................................ 162

Phn 39............................................................................................................................ 168

Phn 40............................................................................................................................ 173Phn 41............................................................................................................................ 177

Phn 42............................................................................................................................ 180

Phn 43............................................................................................................................ 183

Phn 44............................................................................................................................ 186

Phn 45............................................................................................................................ 189Phn 46............................................................................................................................ 193

Phn 47............................................................................................................................ 197

Phn 48............................................................................................................................ 201

Phn 49............................................................................................................................ 205

Phn 50............................................................................................................................ 209Phn 51............................................................................................................................ 213

Phn 52............................................................................................................................ 217

Phn cui......................................................................................................................... 224


3/226

LC STHI GIAN 3


Li gii thiu ca nh xut bn Bantam Books

[...] Chng ta ang sng cuc sng hng ngy ca chng ta m hu nhkhng hiu cthgii xung quanh. [...] Ngoi trtr em (v chng cn bit qu tkhng ngn ngit ra nhng cu hi quan trng), cn t ai trong chng ta tn thi gian bn khon, tisao tnhin li nhthny m khng nhthkhc, v tr ra i tu, [...]

Chng ta ang sng cuc sng hng ngy ca chng ta m hu nh khnghiu c th gii xung quanh. Chng ta cng t khi suy ngm v cchto ra nh sng mt tri - mt yu t quan trng gp phn to nn s sng,v hp dn - ci cht keo kt dnh chng ta vo tri t, m nu khc i

chng ta s xoay tt v tri dt vo khng gian v tr, v nhng nguyn tcu to nn tt c chng ta - m chng ta hon ton l thuc vo s bn vngca chng. Ch tr c tr em (v chng cn bit qu t khng ngn ngit ra nhng cu hi quan trng) cn t ai trong chng ta tn thi gian bnkhon ti sao t nhin li nh th ny m khng nh th khc, v tr ra itu, hoc n c mi mi nh th ny khng, liu c mt ngy no thigian s tri git li, hu qu c trc nguyn nhn hay khng; hoc c giihn cui cng cho s hiu bit ca con ngi hay khng? Thm ch cnhng a tr con, m ti c gp mt s, mun bit len l ci g; ci g l

ht vt cht nh b nht, ti sao chng ta ch nh qu kh m khng nhtng lai; v nu lc bt u l hn lon th lm th no c s trt t nh tathy hm nay, v ti sao li c v tr.

Trong x hi ca chng ta, cc bc ph huynh cng nh cc thy gio vncn thi quen tr li nhng cu hi bng cch nhn vai hoc vin n ccgio l mh. Mt s gio l y li hon ton khng thch hp vi nhngvn va nu trn, bi v chng phi by qu r nhng hn ch ca shiu bit ca con ngi.

Nhng rt nhiu mn trit hc v khoa hc li ra i t nhng cu lc vnnh vy. Ngy cng c nhiu ngi ln cng mun t nhng cu hi thucloi v thi thong h nhn c nhng cu tr li kh l lng. Nmtrung gian gia cc nguyn t v cc v sao, chng ta ang m rng chntri khm ph ca chng ta, nhm bao qut c nhng ci rt nh ln nhngci rt ln.


4/226

Stephen Hawking 4


Ma xun nm 1974, khong 2 nm trc khi con tu v tr Viking h cnhxung sao Ha, ti c tham d mt cuc hp t chc Anh, do Hi Honggia London ti tr, bn v vn lm th no tm kim s sng ngoi Trit. Vo gigii lao, ti thy mt cuc hp ln hn nhiu c t chc

phng bn cnh v v t m ti bc vo xem. Th ra ti ang chng kinmt nghi l c knh, l kt np hi vin mi ca Hi Hong gia London, mttrong nhng t chc hc thut lu i nht ca hnh tinh chng ta. hngtrn cng, mt thanh nin ngi trong xe y ang rt chm ri k tn mnhvo cun s m nhng trang u tin ca n cn gi c ch k caIsaac Newton. Khi Stephen Hawking, cui cng k xong tn mnh, nhngtrng hoan h nh sm ni ln, ngay t lc ng l c mt huyn thoi.

Hin nay, Hawking l gio s ton hc ca trng i hc Cambridge, vicng v m trc y Newton, ri sau ny P.A.M Dirac - hai nh nghin

cu ni ting v nhng ci cc ln v nhng ci cc nh - m nhim.Hawking l ngi k tc ht sc xng ng ca h. Cun sch u tin caHawking dnh cho nhng ngi khng phi l chuyn gia ny c th xem lmt phn thng v nhiu mt cho cng chng khng chuyn. Cun schhp dn va bi ni dung phong ph ca n, va bi n cho chng ta mtci nhn khi qut qua nhng cng trnh ca chnh tc gi. Cun sch chang nhng khm ph trn nhng ranh gii ca vt l hc, thin vn hc, vtr hc v ca c lng dng cm na.

y cng l cun sch v Thng ... hay ng hn l v s khng-c-

mt-ca-Thng-. Ch Thng xut hin trn nhiu trang ca cunsch ny. Hawking dn thn i tm cu tr li cho cu hi ni ting caEinstein: Liu Thng c s la chn no trong vic to ra v tr ny haykhng? Hawking nhiu ln tuyn b mt cch cng khai rng ng c nh tm hiu ngha ca Thng . V t n lc , ng rt ra kt lun

bt ngnht, t nht l cho n hin nay, l v tr khng c bin trongkhng gian, khng c bt u v kt thc trong thi gian v chng cvic g cho ng sng th phi lm y c.

Peter Guzzardi


5/226

LC STHI GIAN 5


Gii thiu cun sch "Lc sthi gian"

Cun sch m chng ti gii thiu vi cc bn sau y c tn l "Lc sthi gian" (ABrief History of Time), mt cun sch tuyt diu, c vit bi mt trong nhng nh khoahc vi nht ca thi i chng ta: nh ton hc v vt l l thuyt ngi Anh StephenHawking.

S.W. Hawking sinh nm 1942. Trong cuc sng c nhn, ng gp nhiu bthnh. Nm 1985, ng b sng phi v sau khi phu thut m kh qun,Hawking mt kh nng pht m. Trc , mt cn bnh t lit thn kinh(bnh ALS) gn cht ng vo chic xe y. Hawking ch cn cch lmvic v giao tip vi mi ngi bng mt my vi tnh v mt my tng hpting ni lp lin vi gh. Tuy nhin, tt c nhng bt hnh ny khng qutng c ch ca nh vt l thin ti. Hin nay ng l gio s ti i hcCambridge (Anh), chc v m ngy xa Newton, ri sau l P.A.M.Dirac, m nhim. ng chuyn nghin cu v l thuyt tng i rng.

Nhng kt qu thu c cng vi George Ellis, Roger Penrose,... v nht ls pht hin kh nng bc x ca cc cc len a Hawking ln hngnhng nh vt l ni ting nht th gii.

Cun "Lc s thi gian" c vit xong nm 1987. Ngay t khi ra i, n

tr thnh mt trong nhng cun sch bn chy nht th gii. "Lc sthi gian" ng trong danh mc sch bn chy nht ca New York Timestrong 53 tun, v ti nc Anh, 205 tun lin n c tn trong mc sch bnchy nht ca Sunday Times. Chnh Stephen Hawking cng phi kinh ngc.T trc n nay, cha c mt cun sch khoa hc no c cng chngn nhn nng nhit nh vy (tuy rng nhiu ngi ni, h mua n ch

by t sch ch khng thc sc. Vim ny, cun sch ca Hawkingcng c s phn tng t nh Kinh Thnh hoc cc v kch caShakespeare).

Bng mt li trnh by sng sa, ging vn hi hc, hi nhum mu biquan, Stephen Hawking dn dt ngi c phiu lu sut lch s v tr,t khi n cn l mt im k d vi nng lng v cng ln, cho ti ngynay. Cuc tm kim ca Hawking gip ngi c khm ph ht b mt nyn b mt khc. i khi ng dc gi vo nhng ng nhn tng nh rtc l, ri li bt ngch ra s phi l trong cch ngh, ri ph vmi ngnhn. Cun sch cp n nhng vn nghim trng v hc ba nht ca


6/226

Stephen Hawking 6


vt l l thuyt, nh v n ln, len, khng - thi gian, thuyt tng i,nguyn l bt nh... m khng h lm bn c b ri.

Bn ting Vit m chng ti gii thiu vi cc bn sau y c dch biCao Chi v Phm Vn Thiu, nh xut bn Vn ha Thng tin, H Ni,2000.

Minh Hy


7/226

LC STHI GIAN 7


Phn 1

[...] Bn cht ca thi gian l g? N c im tn cng khng? Nhngt ph mi ytrong vt l hc - mt phn nhnhng cng ngh mi tuyt xo - a ra cu tr licho mt scu hi tn ti dai dng txa xa va nu trn. [...]

Chng 1: Bc tranh ca chng ta v v tr

Mt nh khoa hc ni ting (hnh nh l Bertrand Russell) mt ln c trccng chng mt bi ging v Thin vn hc. ng m t tri t quayquanh mt tri nh th no v n lt mnh, mt tri li quay quanh tmca mt qun th khng l cc v sao - m ngi ta gi l thin h - ra sao.Khi bi ging kt thc, mt b gi nh b ngi cui phng ng dy vni: Anh ni vi chng ti chuyn nhm nh g vy? Th gii thc t ch lmt ci a phng ta trn lng mt con ra khng l m thi. Nh khoahc mm mt n ci h c trc khi tr li: Th con ra y ta ln cig?. Anh thng minh lm, anh bn tr, anh rt thng minh, b gi ni,nhng nhng con ra c xp chng ln nhau mi xung di, ch cn saona.

Nhiu ngi chc thy rng bc tranh v v tr ca chng ta nh mt ci

thang v tn gm nhng con ra chng ln nhau l chuyn kh nc ci,nhng ti sao chng ta li ngh rng chng ta hiu bit hn b gi nh bkia? Chng ta bit g v v tr v bng cch no chng ta bit v n? Vtr ti tu v n si vu? V tr c im bt u khng v nu c thiu g xy ra trc ? Bn cht ca thi gian l g? N c im tn cngkhng? Nhng t ph mi y trong vt l hc - mt phn nhnhng cngngh mi tuyt xo - a ra cu tr li cho mt s cu hi tn ti dai dngt xa xa va nu trn. Mt ngy no , rt c th nhng cu tr li nys trnn hin nhin i vi chng ta nh chuyn tri t quay xung quanhmt tri hoc cng c th trnn nc ci nh chuyn thp nhng con ra.Ch c thi gian (d cho c th no i na) mi c th phn quyt.

T rt xa xa, khong nm 340 trc cng nguyn, nh trit hc HyLp Aristotle, trong cun sch ca ng nhan V Bu tri, ara hai lun chng sng gi chng minh rng tri t c hnh cu chkhng phi l ci a phng. Th nht, ng thy rng hin tng nguytthc l do tri t xen vo gia mt tri v mt trng. M bng ca tri t


8/226

Stephen Hawking 8


ln mt trng lun lun l trn, iu ny chng nu tri t c dng cu.Nu tri t l mt ci a phng th bng ca n phi dt nh hnh elip, nutrong thi gian c nguyt thc mt tri khng lun lun ngay di tm caci a . Th hai, t nhng chuyn du hnh ca mnh, ngi Hy Lp bitrng sao Bc u nhn phng nam dng nh thp hn khi nhn nhng

vng phng bc! (Bi v sao Bc u nm ngay trn cc bc, nn n dngnhngay trn u ngi quan st Bc cc, trong khi i vi ngiquan st xch o, n dng nh nm ngay trn ng chn tri).

T s sai khc v v tr biu kin ca sao Bc u Ai Cp so vi Hy Lp,Aristotle thm ch cn a ra mt nh gi v chiu di con ng vngquanh tri t l 400.000 stadia. Hin nay ta khng bit chnh xc 1 stadiadi bao nhiu, nhng rt c th n bng khong 200 thc Anh (1 thcAnh bng 0,914 mt). Nh vy, c lng ca Aristotle ln gn gp 2 lncon sc chp nhn hin nay. Nhng ngi Hy Lp thm ch cn a ra

mt lun chng th 3 chng t rng tri t trn bi v nu khng th ti saokhi nhn ra bin, ci u tin m ngi ta nhn thy l ct bum v ch sau mi nhn thy thn con tu?

Aristotle nghrng tri t ng yn cn mt tri, mt trng, cc hnh tinhv nhng ngi sao chuyn ng xung quanh n theo nhng quo trn.ng tin vo iu bi v ng cm thy - do nhng nguyn nhn b n no - rng tri t l trung tm ca v tr, rng chuyn ng trn l chuynng hon thin nht. tng ny c Ptolemy pht trin thnh mt mhnh v tr hon chnh vo th k th 2 sau Cng nguyn. Theo m hnh ny

th tri t ng tm v bao quanh n l 8 mt cu tng ng mang mttrng, mt tri, cc ngi sao v 5 hnh tinh bit vo thi gian : saoThy, sao

Kim, sao Ha, sao Mc v sao Th (Hnh1.1). Chnh cc hnh tinh li phi chuynng trn nhng vng trn nh hn gnvi cc mt cu tng ng ca chng

ph hp vi ng i quan st ctng i phc tp ca chng trn bu

tri. Mt cu ngoi cng mang cc thinth c gi l cc ngi sao c nh,chng lun lun nhng v tr cnh ivi nhau, nhng li cng nhau quay ngangqua bu tri. Bn ngoi mt cu cui cng l ci g th m hnh khng bao gini mt cch r rng, nhng chc chn n

Hnh 1.1: M hnh v tr ca Aristotle - Ptolemycoi tri t l trung tm ca v tr.


9/226

LC STHI GIAN 9


cho rng l phn ca v tr m con ngi khng th quan st c.

M hnh ca Ptolemy to ra c mt h thng tng i chnh xc tin on v tr ca cc thin th trn bu tri. Nhng tin onnhng v tr mt cch hon ton chnh xc, Ptolemy phi a ra githuyt rng mt trng chuyn ng theo mt quo i khi a n ti gntri t ti 2 ln nh hn so vi nhng thi im khc. Ptolemy nh phichp nhn im yu , nhng du sao vi th, l c th chp nhn c.M hnh ny c nh thThin cha gio chun y nh mt bc tranh vv tr ph hp vi Kinh Thnh, bi v n c mt u im rt ln l dnhkh nhiu chngoi mt cu cui cng ca cc ngi sao cnh cho thinng v a ngc.

Tuy nhin, mt m hnh n gin hn c mt mc s ngi Ba Lan,

tn l Nicholas Copernicus xut vo nm 1554. (Thot u, c l v snh thquy l d gio, Copernicus cho lu hnh m hnh ca mnh nhmt tc phm khuyt danh). tng ca ng l mt tri ng yn, cn trit v nhng hnh tinh chuyn ng theo nhng quo trn xung quanhmt tri. Phi mt gn mt th k, tng ny mi c ch p nhn mtcch thc s. Hai nh thin vn - mt ngi c tn l Johannes Kepler vmt ngi Italy tn l Galileo Galilei - bt u cng khai ng h hcthuyt Copernicus, mc d nhng quo m n tin on cha n khphon ton vi nhng quo quan st c. V vo nm 1609 mt n chmng ging xung hc thuyt Aristotle - Ptolemy. Vo nm , Galileo

bt u quan st bu tri bng chic knh thin vn ca ng va pht minhra. Khi quan st sao Mc, Galileo thy rng km theo n cn c mt s vtinh hay ni cch khc l nhng mt trng quay xung quanh n. iu nyng rng khng phi mi thin h u nht thit phi trc tip quay xungquanh tri t, nh Aristotle v Ptolemy ngh. (Tt nhin vn c th tinrng tri t ng yn trung tm ca v tr v cc mt trng ca sao Mcchuyn ng theo nhng quo cc k phc t p khin ta c cm tngnh n quay quanh sao Mc. Tuy nhin hc thuyt ca Copernicus n ginhn nhiu). Cng thi gian , Kepler ci tin hc thuyt ca Copernicus

bng cch a ra gi thuyt rng cc hnh tinh khng chuyn ng theong trn m theo ng elip. V nhng tin on by gihon ton nkhp vi quan st.

i vi Kepler, cc quo elip n gin ch l mt gi thuyt tin li vchnh th n cng kh chp nhn bi v cc elip r rng l km hon thinhn cc vng trn. Khi pht hin thy gn nh mt cch ngu nhin rng cc


10/226

Stephen Hawking 10


quo elip rt n khp vi quan st, Kepler khng sao dung ha c nvi tng ca ng cho rng cc hnh tinh quay quanh mt tri l do cclc t. iu ny phi mi ti sau ny, vo nm 1867, mi gii thch c,khi Isaac Newton cng b tc phm Philosophiae Naturalis PrincipiaMathematica (Nhng nguyn l ton hc ca trit hc t nhin) ca ng. Cly l cng trnh vt l hc quan trng bc nht c xut bn t trcn nay. Trong cng trnh ny, Newton khng cha ra mt l thuytm t schuyn ng ca cc vt trong khng gian v thi gian, m ngcn pht trin mt cng c ton hc phc tp dng phn tch ccchuyn ng . Hn th na, Newton cn a ra mt nh lut v hp dnv tr m theo mi mt vt trong v tru c ht bi mt vt khc

bng mt lc cng mnh nu hai vt cng nng v cng gn nhau. Chnhlc ny buc cc vt phi ri xung t.(Cu chuyn k rng, do c quto ri trng u m Newton cm hng pht minh ra nh lut hp dn v

tr chc chn ch l chuyn thu dt. Tt c nhng iu m Newton ni rach l: tng v hp dn n vi ng khi ang ngi trng thi chimnghim v c ny sinh bi s ri ca qu to). Newton ch ra rngtheo nh lut ca ng, lc hp dn s lm cho mt trng chuyn ng theoquo elip xung quanh tri t v cc hnh tinh chuyn ng theo quoelip xung quanh mt tri.

M hnh Copernicus vt b nhng thin cu ca Ptolemy v cng vichng vt b lun tng cho rng v tr c mt bin gii t nhin. Vnhng ngi sao cnh dng nh khng thay i v tr ca chng tr squay xung quanh bu tri do tri t quay xung quanh trc ca n, nn s lhon ton t nhin nu gi thit rng cc ngi sao cnh l nhng thin thging nh mt tri ca chng ta, nhng xa hn rt nhiu. Cn c vo lthuyt hp dn ca mnh, Newton thy rng do cc ngi sao ht nhau nn vcn bn chng khng th l ng yn c. Vy liu chng c cng ri vomt im no khng? Trong bc th vit nm 1691 gi Richard Bentley,cng l mt nh t tng li lc thi , Newton chng t rng iu thc t c th xy ra nu ch c mt s hu hn cc ngi sao c phn btrong mt vng hu hn ca khng gian. Nhng mt khc, ng cng ch ra

rng nu c mt s v hn cc ngi sao c phn b tng i ng utrong khng gian v tn th iu khng th xy ra c, bi v khi skhng c im no l trung tm cho chng ri vo. Lun chng ny lmt v d v nhng ci by m ta c th gp khi ni v s v hn. Trong vtr v hn, mi mt im u c th c xem l mt tm, bi mi mtim u c mt s v hn cc ngi sao mi pha ca n. Cch tip cnng n - m iu ny phi mi sau ny mi c - phi l xem xt mt tnh


11/226

LC STHI GIAN 11


trng hu hn trong tt c cc ngi sao s ri vo nhau v sau t cuhi tnh hnh s thay i nh th no nu ta thm vo mt s ngi sao nac phn b gn nhng u ngoi vng ang xt. Theo nh lut ca

Newton th v trung bnh, nhng ngi sao mi thm vo ny cng hon tonkhng lm c iu g khc vi nhng ngi sao ban u, tc l chng cngri nhanh nh vy. Chng ta c th thm vo bao nhiu ngi sao ty ,nhng chng cng s ri sp vo nhau. By gith chng ta hiu rng khngth c mt m hnh tnh v hn ca v tr trong hp dn lun l lc ht.

y l s phn nh l th v bu khng kh t tng chung ca mt giaion trc th k hai mi, trong khng mt ai ngh rng v tranggin nhoc ang co li. Mi ngi u tha nhn rng hoc v tr tn tivnh cu trong trng thi khng thay i, hoc n c to ra mt thiim hu hn trong qu kh gn ging chng ta quan st thy hin nay.

iu ny c th mt phn l do thin hng ca con ngi mun tin vonhng s tht vnh cu cng nh s tin li m h tm thy trong nghrng v tr l vnh cu v khng thay i, mc d ngay bn thn h cng cth gi i v cht.


12/226

Stephen Hawking 12


Phn 2

[...] Nhng vn : v tr c im btu trong thi gian v c b gii hn trong khnggian hay khng sau ny c nh trit hc Immannuel Kant xem xt mt cch baoqut trong cun Ph phn ssuy l thun tu, mt cng trnh vi (v rt ti ngha) cang, c xut bn nm 1781. [...]

Thm ch ngay c nhng ngi thy rng l thuyt hp dn ca Newtonchng t v tr khng th l tnh, cng khng ngh ti chuyn cho rng nc thang gin n. Thay v th, h li c nh ci bin l thuyt ny bngcch lm cho lc hp dn tr thnh lc y nhng khong cch rt ln.iu ny khng nh hng ng kn nhng tin on ca h v chuynng ca cc hnh tinh, nhng li cho php mt s dn tri v hn ca ccngi sao cn trng thi cn bng: nhng lc ht ca cc ngi sao gnnhau sc cn bng bi lc y t cc ngi sao rt xa. Tuy nhin, ngynay chng ta bit chc chn rng, s cn bng l khng bn: nu nhngngi sao mt vng no ch cn xch li gn nhau mt cht l lc htgia chng s mnh hn v ln t lc y, v th l cc ngi sao s tip tcco li vo nhau. Mt khc, nu nhng ngi sao dch ra xa nhau mt cht llc y s li ln t, v cc ngi sao s chuyn ng ra xa nhau.

Mt phn bc na i vi m hnh v tr tnh v hn thng c xem lca nh trit hc ngi c Heinrich Olbers, ngi vit v l thuyt ny vonm 1823. Thc t th rt nhiu ngi ng thi ca Newton nu ra vn ny, v bi bo ca Olbers thm ch cng khng phi l bi u tin chang nhng l l hp l chng li n. Tuy nhin, y l bi bo u tinc nhiu ngi ch . Kh khn l ch trong mt v tr tnh v hn thgn nh mi mt ng ngm u kt thc trn b mt ca mt ngi sao.

Nh th th ton b bu tri s phi sng chi nh mt tri, thm ch c banm. L l phn bc ca Olbers cho rng nh sng t cc ngi sao xa s bmnht i do s hp th ca vt cht xen gia cc ngi sao. Tuy nhin, dcho iu c xy ra i na th vt cht xen gia cui cng s nng ln, chon khi n cng pht sng nh nhng ngi sao. Con ng duy nht trnhc kt lun cho rng ton b bu tri m cng sng chi nh b mt camt tri l phi gi thit rng, cc ngi sao khng pht sng vnh vin, mch bt sng mt thi im hu hn no trong qu kh. Trong trnghp hp , vt cht hp th cn cha th nng, hay nh sng t cc ngi


13/226

LC STHI GIAN 13


sao xa cha kp ti chng ta. V iu ny li t ra cho chng ta mt cuhi: ci g lm cho cc ngi sao bt sng u tin?

S bt u ca v tr, tt nhin, c ngi ta tho lun t trc rtlu. Theo mt s l thuyt v v tr c t xa xa, v theo truyn thng cangi Do Thi gio/ Thin Cha gio/ Hi gio, th v tr bt u c t mtthi im hu hn nhng cha tht qu xa trong qu kh. Mt l l chng tc s bt u l cm gic cn phi c ci nguyn nhn u tin giithch s tn ti ca v tr. (Trong v tr, bn lun lun gii thch mt skin nh l c gy ra bi mt s kin khc xy ra trc , nhng s tnti ca chnh bn thn v tr ch c thc gii thch bng cch , nu nc s bt u). Mt l l na do St. Augustine a ra trong cun sch cang nhan Thnh phca Cha. ng ch ra rng, nn vn minh cn angtin b, v chng ta nhc ai l ngi thc hin k cng ny hoc ai

pht trin k thut kia. Nh vy, con ngi v c l c v tr na ucha thc tri nghim c qu lu di. V tha nhn ngy ra ica v tr vo khong 5.000 nm trc Cng nguyn, ph hp vi schCha sng to ra th gii (phn Sng th k ca Kinh Cu c). (iu l thl thi im khng qu xa thi im kt thc ca thi k bng h cuicng, khong 10.000 nm trc Cng nguyn, thi im m cc nh kho cni vi chng ta rng nn vn minh mi thc bt u).

Mt khc, Aristotle v cc trit gia Hy Lp khc li khng thch tng vs Sng th v n dnh lu qu nhiu ti s can thip ca thn thnh. Do

h tin rng loi ngi v th gii xung quanh tn ti v s cn tn ti mimi. Nhng ngi ci xem xt l l nu trn v s tin b v h giip nh sau: c nhiu nn hng thu hoc cc tai ha khc xy ra mtcch nh ka loi ngi tt li im bt u ca nn vn minh.

Nhng vn : v tr c im bt u trong thi gian v c b gii hntrong khng gian hay khng sau ny c nh trit hc ImmannuelKant xem xt mt cch bao qut trong cun Ph phn s suy l thuntu, mt cng trnh vi (v rt ti ngha) ca ng, c xut bn nm

1781. ng gi nhng cu hi l s mu thun ca suy l thun tu, bi vng cm thy c nhng l l vi sc thuyt phc nh nhau tin vo lun cho rng v tr c im bt u, cng nh vo phn cho rng v trtn ti mi mi. L l ca ng bnh vc lun l: nu v tr khng c im

bt u th trc bt k mt s kin no cng c mt khong thi gian vhn, iu ny ng cho l v l! L l ca ng bo v phn l: nu v trc im bt u, th s c mt khong thi gian v hn trc n, vy th ti


14/226

Stephen Hawking 14


sao v tr li bt u mt thi im no ? S tht th nhng trng hpng a ra cho c lun v phn u ch l mt l l m thi. C hai uda trn mt gi thit khng ni r ra cho rng thi gian li v tn v phasau bt k v tr c tn ti mi mi hay khng. Nh chng ta s thy sauny, khi nim thi gian mt ngha trc thi im bt u ca v tr. St.Augustine l ngi u tin ch ra iu . Khi c hi: Cha lm gtrc khi Ngi sng to ra th gii? ng khng p: Ngi ang to ra angc cho nhng kt nhng cu hi nh vy. Thay v th, ng ni rngthi gian l mt tnh cht ca v tr m Cha to ra v thi gian khngtn ti trc khi v tr bt u.

Khi m sng tin rng v tr v cn bn l tnh v khng thay i th cuhi n c im bt u hay khng thc t ch l mt cu hi ca siu hnhhc hoc thn hc. Ngi ta c th vin l rng nhng iu quan st c

u ph hp tt nh nhau vi l thuyt cho rng n bt u vn ng mtthi im hu hn no , theo cch sao cho dng nh l n tn ti mimi. Nhng vo nm 1929, Edwin Hubble thc hin mt quan st c tnhcht l mt ct mc cho thy d bn nhn u th nhng thin h xa xicng ang chuyn ng rt nhanh ra xa chng ta. Ni mt cch khc, v trang gin nra. iu ny c ngha l, nhng thi gian trc kia cc vtgn nhau hn. Thc t, dng nh l c mt thi, mi hoc hai mi ngntriu nm v trc, tt c chng u chnh xc cng mt ch v do mt ca v tr khi l v hn. Pht minh ny cui cng a cu hi v s

bt u v tr vo a ht ca khoa hc.

Nhng quan st ca Hubble gi rng c mt thi im, c gi l vn ln, ti v tr v cng nh v v cng c (mt v hn). Dinhng iu kin nh vy, tt c cc nh lut khoa hc v do mi khnng tin on tng lai u khng dng c.

Nu c nhng skin trc im th chng khng thnh hngti nhng ci ang xy ra trong hin ti. Do , stn ti ca chng cth b qua bi v n khng c nhng hu qu quan st c. Ngi ta

c th ni rng thi gian c im bt u v n ln, theo ngha l nhngthi im trc khng th xc nh c. Cng cn nhn mnh rng sbt u ny ca thi gian rt khc vi nhng s bt u c xem xttrc . Trong v tr tnh khng thay i, s bt u ca thi gian l ci g c p t bi mt ng ngoi v tr, ch khng c mt yu t nocho s bt u c. Ngi ta c th tng tng Cha to ra th gii btk mt thi im no trong qu kh. Tri li, nu v tr gin n th c


15/226

LC STHI GIAN 15


nhng nguyn nhn vt l cn phi c s bt u. Ngi ta vn cn cth tng tng Cha to ra th gii thi im v n ln hoc thm chsau theo cch sao cho dng nh c v n ln, nhng s l v ngha nucho rng v trc to ra trc v n ln. Mt v tr gin nkhng loitrng sng to, nhng n t ra nhng hn ch khi Ngi cn thc hincng vic ca mnh!


16/226

Stephen Hawking 16


Phn 3

[...] Ngay t bui bnh minh ca nn vn minh, loi ngi khng bng lng nhnnhng skin nhnhng thri rc v khng gii thch c. H khao kht hiu bitci trt tnm su kn trong thgii. Ngy hm nay chng ta cng vn trn trmunbit ti sao chng ta li y v chng ta tu ti. [...]

ni v bn cht ca v tr v tho lun nhng vn nh: n c im btu hay kt thc hay khng, cc bn cn hiu r mt l thuyt khoa hc lnh th no. y, ti s ly mt quan nim mc mc cho rng l thuyt chl mt m hnh v v tr, hoc v mt phn hn ch no , ca n cng vitp hp nhng quy tc lin h cc i lng ca m hnh vi quan st mchng ta s thc hin. Tt nhin l thuyt ch tn ti trong u ca chng tach khng c mt thc ti no khc (d n c th c ngha g i na). Mtl thuyt c xem l tt nu n tha mn hai yu cu: n phi m tchnh xc mt lp rng ln nhng quan st, trn csca m hnh chcha mt s t nhng phn tty ; v n phi a ra c nhng tinon v cc quan st trong tng lai. V d, l thuyt ca Aristotle chorng mi vt u c cu to nn t bn yu t: t, khng kh, la vnc. N c u im l kh n gin, nhng li khng a ra c mt tinon xc nh no. Trong khi , l thuyt ca Newton v hp dn da trn

mt m hnh cn n gin hn, trong cc vt ht nhau bi mt lc t lvi mt i lng c gi l khi lng ca vt, v t l nghch vi bnhphng khong cch gia chng. Th nhng n li tin on c nhngchuyn ng ca mt tri, mt trng v cc hnh tinh vi mt chnh xccao.

Bt k mt l thuyt vt l no cng ch l tm thi, theo ngha n ch l mtgi thuyt: bn s khng khi no c th chng minh c n. D cho nhngkt qu thc nghim ph hp vi mt l thuyt vt l bao nhiu ln i na,

bn cng khng bao gim bo c chc chn rng kt qu th nghimln ti s khng mu thun vi l thuyt. Trong khi , bc b mt lthuyt bn ch cn tm ra mt quan st khng ph hp vi nhng tin onca l thuyt . Nh nh trit hc ca khoa hc Karl Popper nhn mnh,mt l thuyt tt c c trng bi iu l: n a ra c nhiu tinon m v nguyn tc c th bc b bi quan st. Mi mt ln nhngthc nghim mi cn ph hp vi nhng tin on th l thuyt cn sng st


17/226

LC STHI GIAN 17


v nim tin ca chng ta vo n li c tng thm, nhng nu thm ch chc mt quan st mi t ra l khng ph hp th chng ta cn phi vt bhoc phi sa i l thuyt . t nht l iu c xem l s xy ra,nhng bn cng lun lun c tht vn v thm quyn ca ngi thchin quan st .

Trn thc t, iu thng hay xy ra l mt l thuyt mi thc ra ch l sm rng ca l thuyt trc. V d, nhng quan st rt chnh xc vhnhtinh Thy (m ta quen gi sai lsao Thy) cho thy s sai khc nh giachuyn ng ca n v nhng tin on ca l thuyt hp dn Newton. Stht l nhng tin on ca Einstein hon ton n khp vi quan st, trongkhi nhng tin on ca Newton cha t c iu - l mt trong nhngkhng nh c tnh cht quyt nh i vi l thuyt mi. Tuy nhin, chngta vn cn thng xuyn s dng l thuyt ca Newton cho nhng mc ch

thc tin, bi v s khc bit gia nhng tin on ca n v ca thuyttng i rng l rt nh trong nhng tnh hung m chng ta gp thngngy. (L thuyt ca Newton cng cn mt u im ln na l n d sdng hn l thuyt ca Einstein rt nhiu).

Mc ch ti hu ca khoa hc l to ra c mt l thuyt duy nht ckh nng m tc ton b v tr. Tuy nhin, cch tip cn m phnng cc nh khoa hc thc s theo ui l tch vn ny ra lm hai phn.Th nht l nhng quy lut cho bit v tr s thay i nh th no theo thigian. (Nu chng ta bit mt thi im no v tr l nh th no th cc

nh lut vt l s cho chng ta bit n s ra sao bt k thi im no tipsau). Th hai l vn v trng thi ban u ca v tr. Mt s ngi cmthy rng c l khoa hc ch nn quan tm ti phn th nht; h xem vn v trng thi ban u ca v tr l vn ca siu hnh hc hoc ca tngio. H cho rng Cha, ng ton nng c th cho v tr bt u theo btc cch no m Ngi mun. Cng c th l nh vy, nhng trong trnghp Ngi cng c th lm cho v tr pht trin mt cch hon ton ty. Nhng ha ra Ngi li chn cch lm cho v tr tin trin mt cch rtquy c ph hp vi mt s quy lut. V vy cng s l hp l nu gi thit

rng cng c nhng quy lut chi phi trng thi ban u.Thc ra, rt kh c th xy dng c mt l thuyt m tc ton b vtr trong tng th ca n. Thay v th, chng ta phn bi ton thnh tng

phn v t pht minh ra nhiu l thuyt c tnh cht ring phn. Mi mtl thuyt nh th m t v tin on chc mt lp hn ch nhng quanst, trong khi phi b qua nh hng ca nhng i lng khc hoc biu


18/226

Stephen Hawking 18


din chng bng tp hp n gin cc con s. Cng c th cch tip cn nyl hon ton sai lm. Nu mi vt trong v tr ph thuc vo nhau mt cchcn bn, th s khng th tip cn li gii y bng cch nghin cu cc

phn ca bi ton mt cch ring r, c lp. Tuy nhin, chc chn l cchm chng ta lm ra s tin b trong qu kh. Mt v d kinh in li l lthuyt hp dn ca Newton. L thuyt ny ni vi chng ta rng lc hp dngia hai vt ch ph thuc vo mt con s gn lin vi mi vt - l khilng ca chng, nhng li hon ton c lp vi chuyn vt c lm

bng cht g. Nh vy ngi ta khng cn phi c mt l thuyt v cu trcv thnh phn ca mt tri v cc hnh tinh m vn tnh c quo cachng. Ngy nay, cc nh khoa hc m t v tr da trn hai l thuytc s c tnh cht ring phn, l thuyt tng i rng v c hclng t. Hai l thuyt l nhng thnh tu tr tu vi ca na u thk ny. L thuyt tng i rng m t lc hp dn v cu trc cc vca

v tr, - cu trc t quy m t dm ti triu triu triu triu (1 v hai mibn s 0 tip sau) dm tc l kch thc ca v tr quan st c. Tri li,chc lng t li m t nhng hin tng phm vi cc nh, cmt phntriu triu ca 1 inch. Tuy nhin, khng may, hai l thuyt ny li khngtng thch vi nhau - ngha l c hai khng thu ng thi ng. Mttrong nhng n lc ch yu trong vt l hc ngy nay v cng l tich yu ca cun sch ny, l tm kim mt l thuyt mi c thdung np c hai l thuyt trn - l thuyt lng t ca hp dn. Hinchng ta cn cha c mt l thuyt nh vy v c th cn lu mi c c,nhng chng ta bit c nhiu tnh cht m l thuyt cn phi c. Vnh chng ta s thy trong cc chng sau, chng ta cng bit kh nhiuv nhng tin on m l thuyt lng t ca hp dn cn phi a ra.

By gi, nu bn tin rng v tr khng phi l ty tin m c iukhin bi nhng quy lut xc nh th iu ti hu l cn phi kt hp nhngl thuyt ring phn thnh nhng l thuyt thng nht hon chnh c khnng m t mi iu trong v tr. Nhng trong qu trnh tm kim mt lthuyt thng nht hon chnh nh vy, li vp phi mt nghch l rt cbn.

Nhng nim v cc l thuyt khoa hc c phc ra trn xem rng

chng ta l nhng sinh vt c l tr t do quan st v tr theo chng ta vrt ra nhng suy din logic t nhng ci m chng ta nhn thy. Trong mts nh th, s l hp l nu cho rng chng ta c th ngy cng tin gnti cc quy lut iu khin v tr. Nhng nu qu thc c mt l thuytthng nht hon chnh, th n cng s c th quyt nh nhng hnhng ca chng ta. V nhvy tbn thn l thuyt s quyt nhkt qu vic tm kim l thuyt y ca chng ta! Hn na, ti sao n s


19/226

LC STHI GIAN 19


quyt nh rng chng ta si ti nhng kt lun ng t nhng iu quanst c? Hay l ti sao n khng th quyt nh chng ta rt ra nhngkt lun sai? Hay l khng c mt kt lun no ht?

Cu tr li duy nht m ti c tha ra cho vn vn ny l da trnnguyn l chn lc t nhin ca Darwin. Y tng nh sau: trong bt cqun th no ca cc c th t sinh sn, cng u c nhng bin i trongvt liu di truyn v s gio dng, khin cho c cc c th khc nhau. Skhc nhau c ngha l, mt s c th c kh nng hn nhng c th khctrong vic rt ra nhng kt lun ng v th gii quanh mnh v bit hnhng mt cch ph hp. Nhng c th ny c sc sng v sinh sn mnhhn, v v th, kiu mu hnh vi v suy nghca h s dn chim u th.Trong qu kh, ng l nhng ci m chng ta gi l tr tu v pht minhkhoa hc truyn c ci li th sng st ca con ngi. Nhng cn cha

r rng l liu iu c cn ng trong trng hp khi m nhng phtminh khoa hc ca chng ta c th s tiu dit tt c chng ta v thmch nu khng xy ra iu , th mt l thuyt thng nht hon chnhcng c th khng lm khc i bao nhiu chi sng st ca chng ta.Tuy nhin, vi iu kin v tr tin trin mt cch quy c, chng ta cth hy vng rng nhng kh nng suy lun m s chn lc t nhin chochng ta vn cn c dng trong cuc tm kim mt l thuyt thng nhthon chnh v s khng dn chng ta ti nhng kt lun sai lm.

V nhng l thuyt ring phn m chng ta c a ra nhng tin

on v tt c, tr nhng tnh hung cc oan nht, nn vic tm kim mtl thuyt ti hu v v tr kh c th bin minh trn csnhng ng dngthc tin. (Tuy nhin, cn phi thy rng chnh l l tng t c a ra chng li thuyt tng i v c hc lng t, th m chnh nhng lthuyt ny mang li cho chng ta c nng lng ht nhn ln cuc cchmng vi in t!). Do s pht minh ra l thuyt thng nht hon chnh cth khng gip g cho s sng st ca chng ta. N thm ch cng khng nhhng g n li sng ca chng ta. Nhng ngay t bui bnh minh ca nnvn minh, loi ngi khng bng lng nhn nhng s kin nh nhng th

ri rc v khng gii thch c. H khao kht hiu bit ci trt t nmsu kn trong th gii. Ngy hm nay chng ta cng vn trn trmun bitti sao chng ta li y v chng ta tu ti. Kht vng tri thc, khtvng su xa nht ca loi ngi, bin minh cho s tm kim lin tcca chng ta. V mc ch ca chng ta khng g khc hn l s m ty v tr, ni chng ta ang sng.


20/226


21/226

LC STHI GIAN 21


Nhng php o ca Galileo c Newton s dng lm cscho nhngnh lut v chuyn ng ca ng. Trong nhng thc nghim ca Galileo,khi mt vt ln trn mt phng nghing, n lun lun chu tc dng ca cngmt lc (l trng lc ca n) v kt qu l lm cho vn tc ca n tng mtcch u n. iu chng t rng, hu qu thc s ca mt lc llun lun lm thay i vn tc ca mt vt, chkhng phi l lm chon chuyn ng nhngi ta nghtrc .iu ny cng c ngha l,

bt c khi no vt khng chu tc dng ca mt lc, th n vn tip tcchuyn ng thng vi cng mt vn tc. tng ny c pht biumt cch tng minh ln u tin trong cun Principia Mathematica (Ccnguyn l ton hc), c cng b nm 1867, ca Newton v sau ny c

bit nhnh lut th nht ca Newton. nh lut th hai ca Newton chobit iu g s xy ra i vi mt vt khi c mt lc tc dng ln n. nhlut ny pht biu rng vt s c gia tc, hay ni cch khc l s thay i vn

tc t l vi lc tc dng ln n. (V d, gia tc s tng gp i, nu lc tcdng tng gp i). Gia tc cng s cng nh nu khi lng (lng vtcht) ca vt cng ln.(Cng mt lc tc dng ln vt c khi lng ln gphai ln s to ra mt gia tc nh hn hai ln). Mt v d tng t ly ngay tchic t: ng c cng mnh th gia tc cng ln, nhng vi cng mtng c, xe cng nng th gia tc cng nh.

Ngoi nhng nh lut v chuyn ng, Newton cn pht minh ra nh lutv lc hp dn. nh lut ny pht biu rng mi vt u ht mt vt khcvi mt lc t l vi khi lng ca mi vt. Nh vy lc gia hai vt smnh gp i nu mt trong hai vt (v d vt A) c khi lng tng gp hai.y l iu bn cn phi trng i bi v c th xem vt mi A c lm thai vt c khi lng ban u, v mi vt s ht vt B vi mt lc banu. Nh vy lc tng hp gia A v B s hai ln ln hn lc ban u. Vnu, v d, mt trong hai vt c khi lng hai ln ln hn v vt kia c khilng ba ln ln hn th lc tc dng gia chng s su ln mnh hn. Bygith ta c th hiu ti sao cc vt li ri vi mt gia tc nh nhau: mt vtc trng lng ln gp hai ln s chu mt lc hp dn ko xung mnh gphai ln, nhng n li c khi lng ln gp hai ln. Nh vy theo nh lut 2

ca Newton, th hai kt qu ny b tr chnh xc cho nhau, v vy gia tcca cc vt l nh nhau trong mi trng hp.

nh lut hp dn ca Newton cng cho chng ta bit rng cc vt cng xanhau th lc hp dn cng nh. V d, lc ht hp dn ca mt ngi sao ng

bng mt phn t lc ht ca mt ngi sao tng t, nhng khong cchgim i mt na. nh lut ny tin on quo ca tri t, mt trng v


22/226

Stephen Hawking 22


cc hnh tinh vi chnh xc rt cao. Nu nh lut ny khc i, chng hn,lc ht hp dn ca mt ngi sao gim theo khong cch nhanh hn, th quo ca cc hnh tinh khng cn l hnh elip na, m chng s l nhngng xon c v pha mt tri. Nu lc li gim chm hn, th lc hpdn t cc ngi sao xa s ln t lc hp dn t mt tri.

S khc bit to ln gia nhng t tng ca Aristotle v nhng ttng ca Galileo v Newton l ch Aristotle tin rng trng thi ngyn l trng thi c a thch hn ca mi vt - mi vt s ly trngthi , nu khng c mt lc hoc xung lc no tc dng vo n.c

bit, ng cho rng tri t l ng yn. Nhng t nhng nh lut ca Newton suy ra rng khng c mt tiu chun n nht cho s ng yn.Ngi ta hon ton c quyn nh nhau khi ni rng, vt A l ng yn vvt B chuyn ng vi vn tc khng i i vi vt A hoc vt B l ng

yn v vt A chuyn ng. V d, nu tm gc ra mt bn chuyn ng quayca tri t quanh trc ca n v quo ca n xung quanh mt tri, ngita c th ni rng tri t l ng yn v on tu trn n chuyn ng v

pha bc vi vn tc 90 dm mt gihoc on tu l ng yn cn tri tchuyn ng v pha nam cng vi vn tc . Nu ngi ta tin hnhnhng th nghim ca chng ta vi cc vt chuyn ng trn con tu thtt c cc nh lut ca Newton vn cn ng. V d, khi nh bng bn trncon tu , ngi ta s thy rng qu bng vn tun theo cc nh lut ca

Newton ht nh khi bn bng t cnh ng ray. Nh vy khng c cchno cho php ta ni c l con tu hay tri t ang chuyn ng.

Vic khng c mt tiu chun tuyt i cho sng yn c ngha l ngita khng th xc nh c hai s kin xy ra hai thi im khc nhau ccng mt v tr trong khng gian hay khng. V d, gi s qu bng bntrn con tu ny ln v ri xung chm bn cng mt ch sau khong thigian 1 giy. i vi ngi ng cnh ng ray th hai ln chm bn xyra hai v tr cch nhau 40 m v con tu chy c qung ng trongkhong thi gian gia hai ln qu bng chm bn. S khng tn ti sngyn tuyt i, v vy, c ngha l ngi ta khng th gn cho mt s kin

mt v tr tuyt i trong khng gian, nh Aristotle tm nim. V tr cacc s kin v khong cch gia chng l khc nhau i vi ngi trn tuv ngi ng cnh ng ray v chng c l do g thch v tr ca nginy hn v tr ca ngi kia.

Newton l ngi rt bn khon v skhng c v tr tuyt i, hay nhngi ta vn gi l khng c khng gian tuyt i, v iu khng ph


23/226

LC STHI GIAN 23


hp vi nim ca ng v Thng tuyt i. Thc t, Newton chib, khng chp nhn s khng tn ti ca khng gian tuyt i, mc d thmch iu ngm cha trong nhng nh lut ca ng. ng b nhiungi ph phn nghim khc v nim tin phi l , m ch yu nht l biGim mc Berkeley, mt nh trit hc tin rng mi i tng vt cht v ckhng gian ln thi gian ch l mt o nh. Khi ngi ta k cho tin sJohnson ni ting v quan im ca Berkeley, ng ku ln: Ti s bc bn nh th ny ny! v ng ngn chn ci vo mt hn ln.

C Aristotle ln Newton u tin vo thi gian tuyt i. Ngha l, h tin rngngi ta c tho mt cch ng hong khong thi gian gia hai s kin,rng thi gian hon ton nh nhau d bt k ai tin hnh o n, min lh dng mt chic ng h tt. Thi gian hon ton tch ri v c lp vikhng gian. l iu m nhiu ngi xem l chuyn thng tnh. Tuy

nhin, n lc chng ta phi thay i nhng nim ca chng ta v khnggian v thi gian. Mc d nhng quan nim thng thng ca chng tavn c kt qu tt khi cp ti cc vt nh qu to hoc cc hnh tinh lnhng vt chuyn ng tng i chm, nhng chng s hon ton khngdng c na i vi nhng vt chuyn ng vi vn tc bng hoc gn

bng vn tc nh sng.

Nm 1676, nh thin vn hc an Mch Ole Christensen Roemer lngi u tin pht hin ra rng nh sng truyn vi vn tc hu hn,mc d rt ln. ng quan st thy rng thi gian cc mt trng ca sao

Mc xut hin sau khi i qua pha sau ca hnh tinh khng cch u nhaunh ngi ta chi, nu cc mt trng chuyn ng vng quanh saoMc vi vn tc khng i. Khi tri t v sao Mc quanh xung quanh mttri, khong cch gia chng thay i. Roemer thy rng s che khut ccmt trng ca sao Mc xut hin cng mun khi chng ta cng xa hnhtinh . ng l lun rng iu xy ra l do nh sng t cc mt trng n chng ta mt nhiu thi gian hn khi chng ta xa chng hn. Tuynhin, do nhng php o ca ng v s bin thin khong cch gia tri tv sao Mc khng c chnh xc lm, nn gi tr vn tc nh sng m ng

xc nh c l 140.000 dm/s, trong khi gi tr hin nay o c ca vntc ny l 186.000 dm/s (khong 300.000 km/s). D sao thnh tu caRoemer cng rt ng k, khng ch trong vic chng minh c rng vntc ca nh sng l hu hn, m c trong vic o c vn tc , c bitn li c thc hin 11 nm trc khi Newton cho xut bn cun PrincipiaMathematica.


24/226

Stephen Hawking 24


Mt l thuyt ch thc v s truyn nh sng phi mi ti nm 1865 mi rai, khi nh vt l ngi Anh James Clerk Maxwell thnh cng thngnht hai l thuyt ring phn cho ti thi gian vn c dng m tring bit cc lc in v t. Cc phng trnh ca Maxwell tin on rngc th c nhng nhiu ng ging nh sng trong mt trng in t kth p, rng nhng nhiu ng s c truyn vi mt vn tc c nhging nh nhng gn sng trn h. Nu bc sng ca nhng sng (khong cch ca hai nh sng lin tip) l mt mt hoc ln hn, th chngc gi l sng radio (hay sng v tuyn). Nhng sng c bc sng ngnhn c gi l sng cc ngn (vi bc sng vi centimet) hoc sng hngngoi (vi bc sng ln hn mi phn ngn centimet). nh sng thyc c bc sng nm gia bn mi phn triu n tm mi phn triucentimet. Nhng sng c bc sng cn ngn hn na l tia t ngoi, tia - Xv cc tia gamma.


25/226

LC STHI GIAN 25


Phn 5

[...] L thuyt tngi buc chng ta phi thay i mt cch cn bn nhng nim cachng ta vkhng gian v thi gian. Chng ta buc phi chp nhn rng thi gian khnghon ton tch ri v c lp vi thi gian m kt hp vi n thnh mti tng gi lkhng - thi gian [...]

L thuyt ca Maxwell tin on cc sng v tuyn v sng nh sng truynvi mt vn tc cnh no . Nhng l thuyt ca Newton gt b khinim ng yn tuyt i, v vy nu nh sng c gi thit l truyn vimt vn tc cnh, th cn phi ni vn tc cnh l i vi ci g. Do ngi ta cho rng c mt cht gi l ether c mt khp mi ni, thmch c trong khng gian trng rng. Cc sng nh sng truyn qua ethernh sng m truyn trong khng kh, v do vy, vn tc ca chng l i viether. Nhng ngi quan st khc nhau chuyn ng i vi ether s thynh sng i ti mnh vi nhng vn tc khc nhau, nhng vn tc ca nhsng i vi ether lun lun c mt gi tr c nh. c bit, v tri tchuyn ng qua ether trn quo quay quanh mt tri, nn vn tc canh sng c o theo hng chuyn ng ca tri t qua ether (khi chngta chuyn ng ti gn ngun sng) s phi ln hn vn tc ca nh snghng vung gc vi phng chuyn ng (khi chng ta khng chuyn

ng hng ti ngun sng). Nm 1887, Albert Michelson (sau ny trthnh ngi Mu tin nhn c gii thng Nobel v vt l) v EdwardMorley thc hin mt thc nghim rt tinh xo ti trng Khoa hc ngdng Case Cleveland. H so snh vn tc nh sng theo hng chuynng ca tri t vi vn tc nh sng hng vung gc vi chuyn ngca tri t. V h v cng ngc nhin khi thy rng hai vn tc honton nh nhau!

Gia nm 1887 v nm 1905 c mt s nh, m ch yu l ca vt lngi H Lan Hendrik Lorentz, nhm gii thch kt qu ca th nghimMichelson - Morley bng s co li ca cc vt v s chm li ca ng hkhi chng chuyn ng qua ether. Tuy nhin, trong bi bo cng b vo nm1905, Albert Einstein, mt nhn vin thuc vn phng cp bng sng ch

pht minh Thy S, ngi m trc cn cha ai bit ti, ch ra rngton b tng v ether l khng cn thit nu ngi ta sn lng vt b tng v thi gian tuyt i. Quan nim tng t cng c mt nh ton


26/226

Stephen Hawking 26


hc hng u ca Php l Henri Poincar a ra ch t tun sau. Tuy nhin,nhng l l ca Einstein gn vi vt l hn Poincar, ngi xem vn ny nh mt vn ton hc. Cng lao xy dng nn l thuyt mi nythng c tha nhn l ca Einstein, nhng Poincar vn thng cnhc nh ti v tn tui ca ng gn lin vi mt phn quan trng ca lthuyt .

Tin cbn ca l thuyt mi - m ngi ta thng gi l thuyt tngi - c pht biu nhsau: mi nh lut ca khoa hc l nhnhaui vi tt c nhng ngi quan st chuyn ng tdo bt k vn tcca h l bao nhiu. iu ny ng i vi cc nh lut ca Newton vchuyn ng, nhng by gi l thuyt c m rng ra bao hm c lthuyt ca Maxwell v vn tc nh sng: mi ngi quan st u o cvn tc nh sng c gi tr hon ton nh nhau bt k h chuyn ng

nhanh, chm nh th no. tng n gin c mt s h qu rt ngch . C l ni ting nht l h qu v s tng ng ca khi lng vnng lng c c kt trong phng trnh ni ting ca Einstein: E = mc2v nh lut ni rng khng c vt no c th chuyn ng nhanh hn nhsng. V c s tng ng gia nng lng v khi lng nn nng lngm vt c th nhchuyn ng s lm tng khi lng ca n. Ni mt cchkhc, n s lm cho vic tng vn tc ca vt trnn kh khn hn.

Hiu ng ny ch trc s quan trng i vi cc vt chuyn ng vi vn tcgn vi vn tc nh sng. V d, vn tc ch bng 10 % vn tc nh sng

khi lng ca vt ch tng 0,5 % so vi khi lng bnh thng, trong khivn tc bng 90 % vn tc nh sng khi lng ca n cn tng nhanh hn,v vy s cng mt nhiu nng lng hn tng vn tc ca n ln na.Thc t khng bao gi c tht ti vn tc ca nh sng v khi khilng ca vt s trthnh v hn v do s tng ng gia nng lng vkhi lng, s phi tn mt lng v hn nng lng t c iu .V l do , mt vt bnh thng vnh vin b tnh tng i gii hn chuynng ch chuyn ng vi vn tc nh hn vn tc nh sng. Ch c nhsng hoc cc sng khc khng c khi lng ni ti l c th chuyn ng

vi vn tc nh sng.Mt h qu cng ng ch khng km ca thuyt tng i l n lmcch mng nhng nim ca chng ta v khng gian v thi gian. Trong lthuyt ca Newton, nu mt xung nh sng c gi t ni ny n nikhc th nhng ngi quan st khc nhau u nht tr vi nhau v thi giantruyn xung nh sng (v thi gian l tuyt i). V vn tc nh sng


27/226

LC STHI GIAN 27


chnh bng khong cch m n truyn c chia cho thi gian tn iht qung ng , nn nhng ngi quan st khc nhau so c vntc ca nh sng c gi tr khc nhau. Trong thuyt tng i, tri li, mingi quan st u phi nht tr v gi tr vn tc ca nh sng. Tuy nhin,h vn cn khng nht tr v khong cch m nh sng truyn, v vy hcng phi khng nht tr v thi gian m nh sng tn (thi gian ny bngkhong cch nh sng truyn - iu m cc nh quan st khng nht tr -chia cho vn tc nh sng - iu m cc nh quan st u nht tr). Ni mtcch khc, l thuyt tng i co chung cho tng v thi gian tuyti! Ha ra l mi ngi quan st cn phi c mt bo thi gian ring camnh nh c ghi nhn bi ng h m h mang theo v cc ng hging ht nhau c mang bi nhng ngi quan st khc nhau khng nhtthit phi ch nh nhau.

Mi mt ngi quan st c th dng radar bit mts kin xy ra u v khi no bng cch gi mt xungnh sng hoc sng v tuyn. Mt phn ca xung phnx t s kin trv v ngi quan st o thi gian mh nhn c ting di. Thi gian xy ra s kin khi s bng mt na thi gian tnh t khi xung c gi in khi nhn c ting di trli, cn khong cch tis kin bng na s thi gian cho hai lt i-v nhn vi vn tc nh sng. (Mt s kin, theo nghany, l mt iu g xy ra mt im duy nht trongkhng gian v mt im xc nh trong thi gian).

tng ny c minh ha trn hnh 2.1, n l mt vd v gin khng-thi gian. Dng th tc ny,nhng ngi quan st chuyn ng i vi nhau s gncho cng mt s kin nhng thi gian v v tr khcnhau. Khng c nhng php o ca ngi quan st c

bit no l ng hn nhng ngi khc, nhng tt c cc php o u quanh vi nhau. Bt k mt ngi quan st no cng tnh ra c mt cch

chnh xc thi gian v v tr m mt ngi quan st khc gn cho mt skin, min l ngi bit c vn tc tng i ca ngi kia.

Ngy hm nay o khong cch mt cch chnh xc, chng ta vn cndng phng php ni trn, bi v chng ta c tho thi gian chnh xchn o chiu di. Thc t, mt c nh ngha l khong cch m nh sngi c trong khong thi gian 0,000000003335640952 giy o theo ng

Hnh 2.1: Thi gian c otheo trc ng v khongcch t ngi quan st co theo trc ngang. ng ica ngi quan st trongkhng-thi gian c biudin bng ng thng ngbn tri; ng i ca cc tiasng ti v phn x t skin l cc ng cho.


28/226

Stephen Hawking 28


h nguyn t xesi. (Nguyn nhn dn ti con s l lng ny l n tngng vi nh ngha c tnh cht lch s ca mt: l khong cch gia haivch trn mt ci thc c bit lm bng bch kim c giParis). Nhvy chng ta c th dng mt n v mi thun tin hn, c gi l giy-nh-sng. N n gin l khong cch m nh sng i c trong mt giy.Trong l thuyt tng i, by gita nh ngha khong cch thng qua thigian v vn tc nh sng, nh vy phi tng suy ra rng mi ngi quanst o vn tc ca nh sng s nhn c cng mt gi tr (theo nh nghal 1 mt trong 0,000000003335640952 giy). Khi cn phi a vo khinim ether, v li s c mt ca n khng thc ghi nhn bng cch no,nh th nghim ca Michelson - Morley chng t.

Tuy nhin, l thuyt tng i buc chng ta phi thay i mt cch cn bnnhng nim ca chng ta v khng gian v thi gian. Chng ta buc phi

chp nhn rng thi gian khng hon ton tch ri v c lp vi khng gianm kt hp vi n thnh mt i tng gi l khng - thi gian.


29/226

LC STHI GIAN 29


Phn 6

[...] Chng ta khng thtng tng ni mt khng gian 4 chiu. Ring bn thn ti hnhdung mt khng gian 3 chiu cng vt v lm ri. Tuy nhin vmt s vkhnggian 2 chiu th li kh ddng, chng hn nhvbmt ca tri t. [...]

Theo kinh nghim thng thng, ngi ta c th m t v tr ca mt imtrong khng gian bng ba con s, hay ni cch khc l ba ta . V d,ngi ta c th ni: mt im trong phng cch mt bc tng 7 b, cchmt bc tng khc 3 b, v cao so vi sn 5 b. Hoc ngi ta c th ch rmt im kinh tuyn no, vtuyn bao nhiu v cao no so vi mc

nc bin. Ngi ta c th thoi mi dng ba ta thch hp no m mnhmun, mc d chng ch c phm vi ng dng hn ch. Chng hn, chng tas khng ch v tr ca mt trng bng khong cch theo phng bc v

phng ty so vi rp xic Piccadilly v chiu cao ca n so vi mc ncbin. Thay v th, ngi ta cn phi m t n qua khong cch t mt tri,khong cch t mt phng quo ca cc hnh tinh v gc gia ng nimt trng vi mt tri v ng ni mt tri ti mt ngi sao gn nh saoAlpha ca chm sao Nhn M. Nhng thm ch nhng ta ny cngkhng c dng nhiu m t v tr ca mt tri trong thin h ca chngta hoc ca thin h chng ta trong qun th thin h khu vc. Thc t,

ngi ta c th m t ton b v tr bng mt tp h p cc mng gi lnnhau. Trong mi mt mng, ngi ta c th dng mt tp hp ba ta khcnhau ch v tr ca cc im.

Mt s kin l mt ci g xy ra mt im c bit trong khng gian vmt thi im c bit. Nh vy, ngi ta c th ch n bng 4 con s hayl 4 ta . V ln ny cng th, vic la chn cc ta l ty , ngi tac th dng ba ta khng gian bit v mt o no ca thi gian.Trong thuyt tng i, khng c sphn bit thc sgia cc ta khng gian v thi gian, cng ht nh khng c s khc bit thc sgia hai ta khng gian. Ngi ta c th chn mt tp hp ta mi,trong , chng hn, ta khng gian th nht l t hp ca ta khnggian c th nht v th hai. V d, thay v o v tr ca mt im trn mt t

bng khong cch theo phng bc v ty ca n i vi rp xic Piccadillyngi ta c th dng khong cch theo hng ng bc v ty bc i viPiccadilly. Cng tng t nh vy, trong thuyt tng i, ngi ta c th


30/226

Stephen Hawking 30


dng ta thi gian mi l thi gian c (tnh bng giy) cng vi khongcch (tnh bng giy - nh sng) theo hng bc ca Piccadilly.

Mt cch rt hu ch suy nghv bn ta ca mt s kin l ch v trca n trong mt khng gian 4 chiu, c gi l khng -thi gian. Chng takhng th tng tng ni mt khng gian 4 chiu. Ring bn thn ti hnhdung mt khng gian 3 chiu cng vt v lm ri. Tuy nhin v mt sv khng gian 2 chiu th li kh d dng, chng hn nh v b mt ca trit (B mt ca tri t l hai chiu v v tr ca mt im trn c thc ghi bng hai ta , kinh v v). Ti s thng s dng nhnggin trong thi gian tng theo phng thng ng hng ln trn, cnmt trong nhng chiu khng gian c v theo phng nm ngang. Haichiu khng gian cn li s b qua, hoc i khi mt trong hai chiu c v theo phi cnh. (Nhng gin ny c gi l gin khng-thi

gian, ging nh hnh 2.1). V d, trong hnh 2.2 thi gian

c t hng ln trn vi n v l nm, cn khongcch nm dc theo ng thng ni mt tri vi saoAnpha ca chm sao Nhn m c t nm ngang vin v l dm. Nhng con ng ca mt tri v saoAlpha qua khng - thi gian l nhng con ng thngng bn tri v bn phi ca gin . Tia sng tmt tri i theo ng cho v phi mt 4 nm mi tic sao Alpha.

Nh chng ta thy, cc phng trnh Maxwell tinon rng vn tc ca nh sng s l nh nhau bt k

vn tc ca ngun sng bng bao nhiu, v iu ny c khng nhbng nhiu php o chnh xc.

iu ny suy ra t s kin l nu mt xung nh sngc pht ra mt thi im c bit, ti mt im c

bit trong khng gian, th sau vi thi gian n s lanra nh mt mt cu nh sng vi kch thc v v tr

khng ph thuc vo vn tc ca ngun sng. Sau mtphn triu giy, nh sng s lan truyn, to thnh mtmt cu c bn knh 300 mt, sau hai phn triu giy,

bn knh l 600 mt, v c nh vy mi. iu ny cngging nh nhng gn sng truyn trn mt nc khi chn nm xung h.

Hnh 2.2: Tia sng t mt tri

i theo ng cho v phimt 4 nm mi ti c saoAlpha.

Hnh 2.3: vng trn ln dnca cc gn sng s tothnh mt nn c nh nmng ti ch v ti thi imhn chm vo mt nc.


31/226

LC STHI GIAN 31


Nhng gn sng truyn nh mt vng trn c ln dnmi theo thi gian. Nu ta ngh v mt m hnh bachiu gm b mt hai chiu ca h v mt chiu thigian th vng trn ln dn ca cc gn sng s tothnh mt nn c nh nm ng ti ch v ti thiim hn chm vo mt nc (hnh 2.3). Tng t,nh sng lan truyn t mt s kin s to nn mt mtnn ba chiu trong khng-thi gian 4 chiu. Mt nn c gi l mt nn nh sng tng lai ca s kinang xt. Cng bng cch nh vy ta c th dng mtmt nn khc, gi l mt nn nh sng qu kh - l

tp hp cc s kin m t chng mt xung nh sng c th ti c s kinang xt ( hnh 2.4).

Nhng mt nn nh sng qu kh v tng lai ca mt skin P chia khng gian thnh ba min (hnh 2.5.). Tng laituyt i ca s kin l vng nm trong mt nn nh sngtng lai ca P. y l tp hp ca tt c cc s kin c thchu nh hng ca nhng iu xy ra P.

Nhng tn hiu t P khng th ti c nhng s kin nmngoi nn nh sng ca P bi v khng g c th chuyn ngnhanh hn nh sng. Do vy m cc s kin khng chunh hng nhng g xy ra P. Qu kh tuyt i ca P l

vng nm trong nn nh sng qu kh. y l tp hp cc skin m t nhng tn hiu truyn vi vn tc bng hoc nh hn vn tcca nh sng c th ti c P. Do , tp hp nhng s kin ny c thnhhng ti nhng g xy ra P. Nu bit c mt thi im c bitno nhng g xy ra mi ni trong vng khng gian nm trong nnnh sng qu khca P th ngi ta c th tin on nhng g s xy raP.

Phn cn li l vng khng - thi gian khng nm trong

nn nh sng tng lai hoc qu kh ca P. Cc s kintrong phn cn li ny khng thnh hng hoc chunh hng bi nhng s kin P. V d, nu mt tringng chiu sng chnh thi im ny, th n skhng nh hng ti cc s kin trn tri t ngaythi im bi v chng nm ngoi nn nh sng canh sng khi mt tri tt (hnh 2.6). Chng ta s bit v

Hnh 2.4: Mt nn nh sngqu kh l tp hp cc skin m t chng mt xungnh sng c th ti c skin ang xt.

Hnh 2.5: Nhngmt nn nh sngqu kh v tnglai ca mt s kinP chia khng gianthnh ba min.

Hnh 2.6: Nn nh sng khimt tri tt.


32/226

Stephen Hawking 32


s kin ch sau 8 pht - l thi gian nh sng i t mt tri n trit. V ch khi ny nhng s kin trn tri t mi nm trong nn nh sngtng lai ca s kin mt tri tt. Tng t nh vy, thi im hinnay chng ta khng th bit nhng g ang xy ra nhng ni xa xi trongv tr, bi v nh sng m chng ta thy t nhng thin h xa xi richng t hng triu nm trc. Nh vy, khi chng ta quan st v tr ththc ra l chng ta ang thy n trong qa kh.

Nu ngi ta b qua nhng hiu ng hp dn, nhEinstein v Poincar lm nm 1905, th ta cthuyt tng i c gi l thuyt tng i hp.i vi mi s kin trong khng-thi gian ta u c thdng mt nn nh sng (l tp hp mi con ng khdca nh sng trong khng-thi gian c pht ra s

kin ), v v vn tc nh sng l nh nhau mi skin v theo mi hng, nn tt c cc nn nh sng lnh nhau v cng hng theo mt hng. L thuytny cng ni vi chng ta rng khng g c th chuyn

ng nhanh hn nh sng. iu c ngha l ng i ca mi vt quakhng-thi gian cn phi c biu din bng mt ng nm trong nn nhsng mi mt s kin trn n (hnh 2.7.).

Hnh 2.7: Nn nh sng cacc s kin trn ng ica vt th trong khng gian.


33/226

LC STHI GIAN 33


Phn 7

[...] i vi ngi trn cao mi chuyn pha di xy ra chm chp hn. iu tinon ny c kim chng vo nm 1962 bng cch dng hai ng h rt chnh xc:mttnh v mttchn mt thp nc.ng hchn thp, gn tri t hn,chy chm hn - hon ton ph hp vi thuyt tngi rng. [...]

L thuyt tng i hp rt thnh cng trong vic gii thch s nh nhau cavn tc nh sng i vi mi ngi quan st (nh th nghim Michelson -Morley chng t) v trong s m t nhng iu xy ra khi cc vt chuynng vi vn tc gn vi vn tc nh sng. Tuy nhin, l thuyt ny likhng ha hp vi thuyt hp dn ca Newton ni rng cc vt ht nhau vimt lc ph thuc vo khong cch gia chng. iu ny c ngha l, nulm cho mt vt chuyn ng th lc tc dng ln cc vt khc s thayi ngay lp tc. Hay ni mt cch khc, cc tc dng hp dn truyn vivn tc v hn, thay v n bng hoc nh hn vn tc nh sng nh thuyttng i hp i hi.

Trong khong thi gian t nm 1908 n nm 1914, Einstein nhiu lnth tm mt l thuyt hp dn ha hp c vi thuyt tng i hp, nhng khng thnh cng. Cui cng, vo nm 1915, ng a ra c mt l

thuyt m ngy nay chng ta gi l thuyt tng i rng (hay thuyt tngi tng qut). ng a ra mt gi thit c tnh cht cch mng cho rnghp dn khng phi l mt lc ging nh nhng lc khc m n l kt quca s kin l: khng - thi gian khng phi phng nh trc kia ngi tavn tng, m n cong hay vnh i do s phn b ca khi lng v nnglng trong n. Cc vt nh tri t khng phi c to ra chuyn ngtrn cc quo cong bi lc hp dn, m thay v th, chng chuyn ngtheo ng rt gn vi ng thng trong khng gian cong m ngi ta gil ng trc a. ng trc a l ng ngn nht (hoc di nht) giahai im cnh nhau. V d, b mt tri t l mt khng gian cong hai chiu.


34/226

Stephen Hawking 34


ng trc a trn mt tri t chnh l vng trn lnv n l ng ngn nht gia hai im trn mt t(H.2.8). V ng trc a l ng ngn nht gia hai

sn bay, nn n l ng m nhng ngi dn ng hng khng hng cc phi cng bay theo. Trong l thuyt tng i rng, cc vt lun lunchuyn ng theo cc ng thng trong khng-thi gian 4 chiu,nhng i vi chng ta, chng c v chuyn ng theo nhng ngcong trong khng gian 3 chiu. (iu ny rt ging vi vic quan st chicmy bay trn mt vng i g. Mc d n bay theo ng thng trongkhng gian 3 chiu, nhng ci bng ca n li chuyn ng theo mt ngcong trn mt t hai chiu).

Khi lng ca mt tri lm cong khng-thi gian theo cch sao cho mc dtri t chuyn ng theo ng thng trong khng-thi gian 4 chiu, nhng

n li th hin i vi chng ta l chuyn ng theo qu o trn trongkhng gian ba chiu. V thc t, quo ca cc hnh tinh c tin onbi l thuyt tng i rng cng chnh xc nh c tin on bi lthuyt hp dn ca Newton. Tuy nhin, trong trng hp i vi sao Thy,hnh tinh gn mt tri nht, do cm thy hiu ng hp dn mnh nht vc quo thun di hn, th thuyt tng i rng tin on rng trc dica elip quo quay quanh mt tri vi vn tc 1 trong 10 ngn nm.Mc d hiu ng l rt nh, nhng n c ghi nhn t trc nm 1915v c dng nh mt bng chng u tin khng nh l thuyt caEinstein. Trong nhng nm gn y, nhng lch thm ch cn nh hnna ca quo cc hnh tinh khc so vi nhng tin on ca l thuyt

Newton cng c o bng rada v cho thy chng ph hp vi nhngtin on ca thuyt tng i rng.

Nhng tia sng cng cn phi i theo nhng ng trc a trong khng-thigian. Cng li do khng gian b cong nn nh sng khng cn th hin ltruyn theo ng thng trong khng gian na. Nh vy thuyt tng irng tin on rng anh sng c th b b cong bi cc trng hp dn. Vd, l thuyt ny tin on rng nn nh sng ca nhng im gn mt tri

s hi b un hng vo pha trong do tc dng ca khi lng mt tri.iu ny c ngha l nh sng t mt ngi sao xa khi i qua gn mt tri cth b lch i mt gc nh, khin cho i vi nhng ngi quan st trn mtt, ngi

Hnh 2.8:ng trc a trnmt tri t chnh l vng trnln v n l ng ngn nhtgia hai im trn mt t.


35/226

LC STHI GIAN 35


sao dng nhmt v tr khc (H.2.9). Tt nhin,nu nh sng t ngi sao lun lun i qua gn mttri, th chng ta khng th ni tia sng c b lch haykhng hoc thay v th ngi sao c thc s nm ngch chng ta nhn thy n hay khng. Tuy nhin, v trit quay quanh mt tri nn nhng ngi sao khc nhauc lc dng nhi qua pha sau mt tri v nh sngca chng b lch. V th nhng ngi sao ny thay iv tr biu kin ca chng i vi cc ngi sao khc.

Thng th rt kh quan st hiu ng ny, bi v nh sng ca mt tri lmcho ta khng th quan st c nhng ngi sao c v tr biu kin gn mttri trn bu tri. Tuy nhin, iu ny c th lm c trong thi gian cnht thc, khi m nh sng mt tri b mt trng chn mt. Nhng tin on

ca Einstein khng c kim chng ngay lp tc trong nm 1915 v cucchin tranh th gii ln th nht lc ang lan rng, v phi ti tn nm1919 mt on thm him Anh khi quan st nht thc Ty Phi chng tc rng nh sng thc sb lch do mt tri ng nh lthuyt don. S chng minh l thuyt ca mt ngi c bi ccnh khoa hc Anh c nhit lit hoan nghnh nh mt hnh ng hagii vi gia hai nc sau chin tranh. Do , tht l trtru khi kim trali sau nhng bc nh m on thm him chp, ngi ta pht hin rarng sai s cng ln chiu ng m hnh o. Php o ca h hon tonch l may mn hoc mt trng hp bit trc kt qu m h mun nhnc - mt iu cng thng xy ra trong khoa hc. Tuy nhin, s lch catia sng c khng nh hon ton chnh xc bi nhiu quan st sau ny.

Mt tin on khc ca thuyt tng i rng l thi gian dng nh chychm hn khi gn nhng vt c khi lng ln nh tri t. l bi vmt mi lin h gia nng lng ca nh sng v tn s ca n (tn s lsng nh sng trong mt giy): nng lng cng ln th tn s cng cao. Khinh sng truyn hng ln trong trng hp dn ca tri t, n s mt nnglng v v th tn s ca n gim. (iu ny c ngha l khong thi gian

gia hai nh sng lin tip tng ln). i vi ngi trn cao mi chuyn pha di xy ra chm chp hn. iu tin on ny c kimchng vo nm 1962 bng cch dng hai ng h rt chnh xc: mt t nh v mt t chn mt thp nc. ng hchn thp, gn tri thn, chy chm hn - hon ton ph hp vi thuyt tng i rng. S khc

bit ca tc ng hnhng cao khc nhau trn mt t c mt tmquan trng c bit trong thc tin hin nay khi ngi ta s dng nhng h

Hnh 2.9:iu ny c nghal nh sng t mt ngi saoxa khi i qua gn mt tri cth b lch i mt gc nh.


36/226

Stephen Hawking 36


thng o hng chnh xc da trn nhng tn hiu t v tinh. Nu khi nyngi ta b qua nhng tin on ca thuyt tng i rng, th v tr tnhton c c th sai khc ti vi ba dm!

Nhng nh lut v chuyn ng ca Newton t du chm ht cho nim v v tr tuyt i trong khng gian. Thuyt tng i vt bkhi nim thi gian tuyt i. Ta hy xt hai a tr sinh i. Gi s rngmt a c a ln sng trn nh ni v mt a sng ngang mcnc bin. a th nht s gi nhanh hn a th hai. Nh vy, nu gp linhau mt a s gi hn a kia. Trong trng hp ny s khc nhau vtui tc s rt nh, nhng n s ln hn rt nhiu nu mt a thc hinchuyn du hnh di trong con tu v tr chuyn ng vi vn tc gn vntc nh sng. Khi trv n s tr hn rt nhiu so vi a li tri t. iuny c gi l nghch l hai a tr sinh i, nhng n l nghch l ch nu

nim v thi gian tuyt i vn cn ln qut trong u c chng ta. Trongl thuyt tng i khng c mt thi gian tuyt i duy nht, m thay vth mi c nhn c mt o thi gian ring ca mnh v o phthuc vo ni hang v h chuyn ng nh th no.

Trc nm 1915, khng gian v thi gian c xem l mt sn khu cnhni din ra mi s kin v khng chu nh hng bi nhng iu xy ratrong n. iu ny ng thm ch c vi thuyt tng i h p. Cc vtchuyn ng, cc lc ht v y, nhng khng gian v thi gian vn lin tcv khng bnh hng g. V nghcho rng khng gian v thi gian c

tip tc nh th mi mi cng l chuyn t nhin.

Tuy nhin, tnh hnh hon ton khc trong thuyt tng i rng. By gikhng gian v thi gian l nhng i lng ng lc: khi mt vt chuynng, hoc mt lc tc dng, chng u nh hng ti cong ca khnggian v thi gian v p li, cu trc ca khng - thi gian snh hng ticch thc m cc vt chuyn ng v cc lc tc dng. Khng gian v thigian khng ch c tc ng m cn b tc ng bi mi iu xy ra trongv tr. Chnh v ngi ta khng th ni v cc s kin trong v tr m

khng c khi nim v khng gian v thi gian, nn trong thuyt tng irng s trnn v ngha nu ni v khng gian v thi gian ngoi gii hnca v tr. Trong nhng thp k tip sau, s nhn thc mi ny v khnggian v thi gian lm cch mng quan nim ca chng ta v v tr. tng xa c cho rng mt v tr cn bn khng thay i c th tn ti vc th cn tip tc tn ti vnh vin c thay th bng khi nim mt vtrng, ang gin n, mt v tr dng nh bt u mt thi im


37/226

LC STHI GIAN 37


hu hn trong qu kh v c th chm dt mt thi im hu hn trongtng lai. Cuc cch mng ny l ti ca chng tip sau. V nhng nmsau n cng l im xut pht cho hot ng ca ti trong lnh vc vtl l thuyt. Roger Penrose v ti chng tc rng chnh thuyt tngi rng ng v tr cn phi c im bt u v c th cim kt thcna.


38/226

Stephen Hawking 38


Phn 8

[...] Bc tranh hin i vv tr khi u chmi vo nm 1924, khi nh thin vn ngiMEdwin Hubble chng tc rng thin h ca chng ta khng phi l thin h duynht. Thc t cn c nhiu thin h khc v gia chng l nhng khong khng giantrng rng rng ln. [...]

Chng 3: V tr gin n

Nu ta nhn ln bu tri vo nhng m quang ng, khng trng, nhng vtsng nht m chng ta nhn thy c l l cc hnh tinh: sao Kim, sao Ha,sao Mc v sao Th. Cng c rt nhiu cc ngi sao tng t nh mt trica chng ta nhng rt xa. Mt s nhng ngi sao cnh , thc t, lidng nh thay i - d l rt t - v tr tng i ca chng vi nhau khi trit quay xung quanh mt tri: chng hon ton khng phi l cnh! Sdc iu ny l do chng tng i gn chng ta. Khi tri t quanh xungquanh mt tri, t nhng v tr khc nhau chng ta thy chng trn nn canhng ngi sao xa hn. l mt iu may mn, v n cho php chng tao c mt cch trc tip khong cch t nhng ngi sao n chng ta:chng cng gn th cng c v di chuyn nhiu hn.

Ngi sao gn chng ta nht l sao Proxima ca chm sao Nhn M c tmthy cch chng ta khong 4 nm nh sng (ngha l nh sng t n phi mt4 nm mi ti c tri t), hay khong hai mi ba triu triu dm. a scc ngi sao khc thy c bng mt thng nm cch chng ta trongkhong vi trm nm nh sng. so snh, bn cn bit rng mt tri chcch chng ta c 8 pht nh sng! Nhng ngi sao thy c dng nhnm ri rc trn ton b bu tri m, nhng chng c bit tp trung trongmt di m ngi ta gi l di Ngn h (Milky Way). Rt lu v trc, vokhong nm 1750, a s cc nh thin vn cho rng s xut hin ca di

Ngn h c th gii thch c nu phn ln cc sao nhn thy nm trong

mt cu hnh a duy nht - mt v d v ci m hin nay chng ta gi lthin h xon c. Phi my chc nm sau, nh thin vn William Herschelmi khng nh c tng ca mnh bng cch cn mn lp mt bsu tp v v tr v khong cch ca mt s rt ln cc ngi sao. Thm chnh th, nhng tng ny chc chp nhn hon ton vo u th kny.


39/226

LC STHI GIAN 39


Bc tranh hin i v v tr khi u ch mi vo nm 1924, khi nhthin vn ngi M Edwin Hubble chng t c rng thin h cachng ta khng phi l thin h duy nht. Thc t cn c nhiu thin hkhc v gia chng l nhng khong khng gian trng rng rng ln. chng minh iu ny, ng phi xc nh khong cch n cc thin hkhc . Nhng thin h ny qu xa chng ta, nn khng ging nhngngi sao gn, chng dng nh thc s cnh. Do Hubble buc phi sdng cc phng php gin tip o khong cch. Ngi ta bit rng chi biu kin ca cc ngi sao ph thuc vo hai yu t: nh sng n phtra bao nhiu (tc trng ca n) v n xa chng ta ti mc no. i vinhng ngi sao gn, chng ta c th o c c chi biu kin lnkhong cch ca chng v nh vy chng ta c th tnh c c trngca chng. Ngc li nu chng ta bit c trng ca cc ngi sao ccthin h khc chng ta c th tnh c khong cch bng cch o chi

bin kin ca chng. Hubble thy rng c mt s loi sao lun lun c cng trng khi chng gn ta c tho c, do ng rt ra kt lunrng nu ta tm thy nhng ngi sao loi cc thin h khc th chng tac th xem rng chng cng c cng trng - v nh vy c th tnh ckhong cch n thin h . Nu chng ta c th lm iu cho nhiungi sao trong cng mt thin h m kt qu tnh ton u cho mt khongcch nh nhau th hon ton c th tin c vo nh gi ca chng ta.

Theo cch Edwin Hubble xc nh c khong cchn 9 thin h khc nhau. By gith chng ta bit rng thinh ca chng ta ch l mt trong s vi trm ngn triu thinh c th nhn thy c bng cc knh thin vn hin i, mimt thin h li gm khong vi trm ngn triu ngi sao.Hnh 3.1. l nh ca mt thin h xon c m chng ta nghrng thin h ca chng ta sc nhn ging nh th dicon mt ca ngi sng mt thin h khc. Chng ta sng

trong mt thin h c b ngang rng chng mt trm ngn nm nh sng vquay chm; cc ngi sao nm trong cc nhnh xon ca thin h quay xungquanh tm ca n vi vn tc gc mt vng trong hai trm triu nm. Mt

tri ca chng ta cng ch l mt ngi sao bnh thng mu vng, c kchthc trung bnh v nm mp trong ca mt nhnh xon c. K t thiAristotle v Ptolemy, thi m chng ta nghrng tri t l trung tm ca vtr, cho ti ngy nay, - qu tht chng ta i c mt chng ng rtdi.

Hnh 3.1: nh camt thin h xonc.


40/226

Stephen Hawking 40


Nhng ngi sao xa chng ta n ni, i vi chng ta, chng ch l nhngchm sng nht nht. Chng ta khng th thy c kch thc cng nhhnh dng ca chng. Vy th bng cch no ta c th ni v cc loi saoring bit khc nhau? i vi i a s cc ngi sao, ch c mt nt ctrng m chng ta quan st c - l mu nh sng ca chng. Newton

pht hin ra rng nu nh sng mt tri i qua mt lng knh n s tchthnh cc mu thnh phn (cn gi l quang ph ca n) nh mu ca cuvng. Bng cch hng knh thin vn vo mt ngi sao ring l hay mtthin h ngi ta c th quan st mt cch tng t quang ph ca nh sngt ngi sao hay thin h . Nhng ngi sao khc nhau c quang ph khcnhau, nhng chi tng i ca cc mu khc nhau lun lun chnh xcht nh ngi ta mong i tm thy trong nh sng ca nhng vt pht sngnng . (Thc t, nh sng c pht ra bi mt vt khng trong sut nng c phc trng ch ph thuc vo nhit ca n - quang ph nhit.

iu ny c ngha l chng ta c th bit nhit ca ngi sao t quang phnh sng ca n). Hn na, chng ta cn tm thy rng mt s mu rt xcnh khng c mt trong quang ph ca ngi sao, v nhng mu vng mt khc nhau i vi nhng ngi sao khc nhau. V chng ta bit rng minguyn t ha hc hp th mt tp hp c trng nhng mu rt xc nh,nn bng cch i chiu nhng mu ny vi nhng mu vng mt trongquang ph ca mt ngi sao, chng ta c th xc nh c chnh xc nhngnguyn t no c mt trong kh quyn ca ngi sao .

Trong nhng nm 1920, khi cc nh thin vn bt u quan st quang phca cc ngi sao thuc nhng thin h khc, h tm thy mt iu rt c

bit: c nhng tp hp c trng cc mu vng mt ging ht nhi vinhng ngi sao trong thin h chng ta, nhng chng b dch i cng mtlng tng i v pha ca quang ph. hiu c ngha ca iuny, chng ta trc ht cn phi tm hiu v hiu ng Doppler. Nh chng ta thy, nh sng thy c gm nhng thng ging, hay nhng sng, trongtrng in t. Tn s (hay s sng trong mt giy) ca nh sng l rt cao,tri di t bn n by trm triu triu sng trong mt giy. Cc tn s khcnhau ca nh sng c mt ngi nhn thy nh nhng mu khc nhau.

Nhng nh sng c tn s thp nht nm pha ca quang ph v nhngnh sng c tn s cao nht nm pha tm ca n. By gichng ta hyhnh dung mt ngun sng cch chng ta mt khong khng i, t nhmt ngi sao, v pht sng nh sng c tn s khng i. R rng l tn sca cc sng m chng ta nhn c cng chnh l tn s m chng cngun pht ra. (Trng hp dn ca thin h cha mnh gy ra hiung ng k). By gigi th rng ngun sng bt u chuyn ng hng


41/226

LC STHI GIAN 41


v pha chng ta. Khi ngun pht mt nh sng tip theo th n gn chngta hn, v vy thi gian nh sng ti c chng ta s t hn so vikhi ngun sng ng yn. iu ny c ngha l thi gian gia hai nh sngti chng ta l nh hn v do s sng m chng ta nhn c trong mtgiy (tc l tn s) s ln hn so vi khi ngun sng ng im. Tng ng,nu ngun sng i ra xa chng ta th tn s m chng ta nhn c s thphn. Do , trong trng hp nh sng iu ny c ngha l nhng ngi saochuyn ng ra xa chng ta s c quang ph dch v pha ca quang ph(hin tng dch v pha ) v nhng ngi sao chuyn ng v pha chngta s c quang ph dch v pha tm. Mi quan h ny gia tn s v vn tc- c gi l hiu ng Doppler - l mt kinh nghim hng ngy. Hy lngnghe mt chic xe t chy trn ng: khi chic xe tin li gn, ting ngcca n nghe bng hn (tc l tn s sng m cao hn), cn khi n i ra xam ca n nghe trm hn. i vi cc sng v tuyn cng tng t nh vy.

Thc t cnh st dng hiu ng Doppler xc nh vn tc ca cc xe t bng cch o tn s ca cc xung sng v tuyn phn x t cc xe .


42/226

Stephen Hawking 42


Phn 9

[...] Thm ch ngay c Einstein, khi xy dng thuyt tngi rng vo nm 1915, cnginh ninh rng v tr cn phi l tnh. V thng phi sa i l thuyt ca mnh iu c thxy ra, bng cch a vo nhng phng trnh ca mnh ci c gi l"hng sv tr". [...]

Sau khi chng minh c s tn ti ca cc thin h khc, trong nhng nmtip sau, Hubble dnh nhiu thi gian lp mt kho d liu v khongcch gia cc thin h v quan st quang ph ca cc thin h . Vo thigian y, nhiu ngi nh rng cc thin h chuyn ng hon ton ngunhin, cho nn h chi tm thy nhng quang ph dch v pha tm cngnhiu nh nhng quang ph dch v pha . Do , ngi ta ht sc ngcnhin khi pht hin ra rng a s cc thin h u c quang ph dch v pha: ngha l gn nh tt c chng ang chuyn ng ra xa chng ta! iucn ngc nhin hn na l pht hin m Hubble cng b nm 1929: thm ch dch v pha ca thin h cng khng phi l ngu nhin, m n t lthun vi khong cch gia thin h v chng ta. Hoc ni mt cchkhc, thin h cng xa th n chuyn ng ra xa cng nhanh! C ngha lv tr khng phi l tnh nh trc kia ngi ta vn tng, m n thc tang gin n, khong cch gia cc thin h ngy cng tng ln theo thi

gian.

Pht minh v trang gin n l mt trong nhng cuc cch mng trtu vi ca th k 20. Vi nhn thc mun mng, th vic ch ngcnhin m t hi ti sao trc kia khng ai ngh ti iu l chuynqu d dng. Newton v nhng ngi khc l ra phi thy rng v tr tnhsm hay mun ri cng s co li di nh hng ca hp dn. Nhng bygi, ta hy c gi th rng v trang gin n. Nu n gin n chm, thlc hp dn s lm cho n cui cng s ngng gin nv sau s bt uco li. Tuy nhin, nu v tr gin nvi vn tc nhanh hn mt vn tc giihn no , th lc hp dn s khng bao gi mnh lm dng n li vv tr s tip tc gin nmi mi. iu ny cng hi ging nh khi ngi ta

phng mt tn la ln khng trung t mt t. Nu n c vn tc nh th lchp dn cui cng s lm n dng li v bt u ri xung. Ngc li, nutn la c vn tc ln hn mt vn tc ti hn no (khong by dm


43/226

LC STHI GIAN 43


trong mt giy), th lc hp dn s khng cn mnh ko n li na, vn s tip tc ri xa tri t mi mi.

Tnh cht ca v tr l ra c th hon ton c tin on t l thuythp dn ca Newton bt k thi im no ca th k 19, 18, thm ch cui th k 16. Nhng v nim tin vo v tr tnh qu mnh ti mc n vncn dai dng cho ti u th k 20. Thm ch ngay c Einstein, khi xydng thuyt tng i rng vo nm 1915, cng inh ninh rng v trcn phi l tnh. V th ng phi sa i l thuyt ca mnh iu c th xy ra, bng cch a vo nhng phng trnh ca mnh cic gi l "hng s v tr". Einstein a vo mt lc phn hp dnmi, m khng ging nh nhng lc khc, n khng c xut x t mtngun c bit no, m c to dng ngay trong cu trc ca khng-thigian. ng t ra yu cu l khng-thi gian c xu hng ni ti l nra, v

iu l cn bng chnh xc vi lc ht ca ton b vt cht trong vtr, sao cho kt qu thu c l mt v tr tnh. Dng nh ch c mtngi mun chp nhn thuyt tng i rng dng ban u ca n, lnh vt l v ton hc ngi Nga Alexander Friedmann. V trong khiEinstein v cc nh vt l khc tm mi cch lng trnh s tin on vmt v tr khng tnh, th Friedmann chp nhn v bt tay vo gii thchn.

Friedmann a ra hai gi thit rt n gin v v tr: l v trng nht theo mi hng m chng ta quan st, v iu ny cng ng

vi bt k v tr quan st no. Ch t hai tng , Friedmann chngtc rng chng ta khng th chi v tr ch l tnh. Thc t, vo nm1922, t nm trc pht minh ca Hubble, Friedmann tin on chnh xciu m Hubble tm ra!

Gi thit cho rng v tr nhn y ht nhau theo mi hng r rng l khngng vi thc t. V d, nh chng ta thy, nhng ngi sao khc trongthin h chng ta to nn mt di sng ni bt trn nn tri m, tc l di

Ngn h. Nhng nu chng ta quan st nhng thin h xa th s lng ca

chng tng i ging nhau. Nh vy, vi th th v tr c th xem lnh nhau theo mi hng, vi iu kin l ta phi nhn n qui m ln sovi kch thc gia cc thin h, v b qua nhng sai khc qui m nh.Trong mt thi gian di, iu ny bin minh cho gi thit caFriedmann nh mt php gn ng th i vi v tr thc. Nhng gn yhn, mt s tnh cmay mn ch ra rng gi thit ca Friedmann thc tl s m t kh chnh xc v tr ca chng ta.


44/226

Stephen Hawking 44


Nm 1965, hai nh vt l M lm vic phng th nghim ca hng BellTelephone New Jersey l Arno Penzias v Robert Wilson ang tin hnhtrc nghim mt my d sng cc ngn rt nhy. (Sng cc ngn cng gingnh nh sng nhng vi tn s ch c 10 ngn triu sng trong 1 giy).Penzias v Wilson rt bn khon khi h pht hin ra rng my d ca hghi c qu nhiu ting n hn mc cn thit. Ting n ny dng nhkhng n theo mt phng c bit no. u tin h pht hin c phnchim trong my, sau h kim tra mi kh nng c th hng hc, nhngtt cu b loi tr. H cng bit rng mi loi ting n bn trong bu khquyn s mnh hn khi my d khng hng theo phng thng ng, biv cc tia sng truyn trong kh quyn s thu c gn ng chn trinhiu hn l trn nh u. Nhng ting n thi qu y li nh nhau theomi phng m h hng u d ti v nh vy n phi ti t bn ngoi khquyn. Ting n ny cng nh nhau c ngy ln m trong sut c nm bt

k tri t vn quay quanh trc ca n v quay quanh mt tri. iu nychng t bc x phi ti tbn ngoi h mt tri, thm ch tngoi cthin h chng ta, v nu khng n s thay i khi chuyn ng ca trit lm cho my d hng theo nhng hng khc nhau. Thc t, chngta bit rng bc x ti c chng ta phi i qua phn ln vng v trquan st c v v n nh nhau theo cc phng khc nhau nn v tr cngcn phi nh nhau theo mi phng, nu ch xt trn qui m ln. By githchng ta bit rng bt k nhn theo phng no, th ting n cng ch

bin thin khng bao gi vt qu mt phn vn. Nh vy, Penzias vWilson hon ton tnh c pht hin c mt bng chng kh chnh xckhng nh gi thit th nht ca Friedmann.

Gn khong thi gian , hai nh vt l Mgn i hc Princeton l BodDicke v Jim Peebles cng ang quan tm ti cc sng cc ngn. Hanglm vic theo mt xut ca George Gamow (ngi mt thi l sinhvin ca Alexander Friedmann) cho rng v trthi ku phi rt nngv c, ng thi pht sng nng, trng. Dicke v Peebles l lun rngchng ta hin nay vn cn c th thy c nh sng chi li ca v trthi ku, bi v nh sng t nhng phn rt xa ca v tr ch by gimi

n c ch chng ta. Tuy nhin, s gin n ca v tr c ngha l nhsng phi dch rt mnh v pha khin cho by gichng ta thy ndi dng bc x viba (sng cc ngn). Dicke v Peebles ang chun b tmkim bc x th Penzias v Wilson nghe ni v cng trnh ca h v haing hiu ngay rng mnh pht hin c chnh bc x . V th mPenzias v Wilson c trao gii thng Nobel v vt l nm 1978 (mtiu hi chua cht i vi Dicke v Peebles, y l cha ni ti Gamow!).


45/226

LC STHI GIAN 45


Giy thot nhn th ton b bng chng - bng chng xc nhn rng vtr nhn nh nhau theo bt k hng no m chng ta quan st - c th dnn nghcho rng c mt ci g c bit v v tr ca chng ta trong vtr. c bit, c th nghrng nu chng ta quan st thy tt c cc thin hkhc ang chuyn ng ra xa chng ta, th chng ta cn phi trung tm cav tr. Tuy nhin, cng c mt cch gii thch khc: v tr cng phi nhnhau theo mi hng khi n c quan st t bt k mt thin h no khc.

Nhng, nh chng ta thy, chnh l gi thit th hai ca Friedmann.Hin chng ta cha c bng chng khoa hc khng nh hay bc b githit . Chng ta tin n ch trn csca s khim tn: s l qu ni btnu v tr l nh nhau theo mi phng xung quanh chng ta, nhng likhng nh th xung quanh cc im khc trong v tr. Trong m hnh caFriedmann tt c cc thin h u chuyn ng ra xa nhau. Tnh hung nykh ging mt qu bng bay, trn mt c v nhiu chm mu, ang

c thi cng ln tt. Khi qu bng cng ln, khong cch gia ccchm mu tng ln, nhng khng th ni chm mu no l trung tmca sgin n. Hn na cc chm cng xa nhau th chng chuyn ngra xa nhau cng nhanh. Tng t nh vy, trong m hnh ca Friedmann vntc m hai thin h chuyn ng ra xa nhau t l vi nhng khong cchgia chng. Nh vy, m hnh ny tin on rng mi s dch v pha ca mt thin h t l thun vi khong cch t n n chng ta, ng nhHubble pht hin. Mc d thnh cng ca m hnh v tin on ca n vnhng quan st ca Hubble, nhng cng trnh ca Friedmann t c bitti, cho ti khi nhng m hnh tng tc pht minh bi nh vt l MHoward Robertson v nh ton hc Anh Arthur Walker, gii thch phthin ca Hubble v s gin nu ca v tr.


46/226

Stephen Hawking 46


Phn 10

[...] Nhiu ngi khng thch tng xem rng thi gian c im btu, c lbi v ndy da vi s can thip ca thn thnh. (Nh thThin Cha gio, tri li, nmngay ly m hnh v n ln v vo nm 1951, v chnh thc tuyn bn hon ton phhp vi Kinh thnh). [...]

Mc d Friedmann ch tm ra mt, nhng thc t c ti ba loi m hnh khcnhau cng tun theo hai gi thit cbn ca Friedmann. Trong loi u tin(loi m Friedmann pht hin) v tr gin n chm lc ht hp dngia cc thin h khc nhau, lm cho s gin nchm li, v cui cng thdng hn. Sau cc thin h bt

u chuyn ng li gn nhau, v v tr co li. Hnh3.2 cho thy, khong cch gia hai thin h ln cnthay i t 0, tng ti cc i ri li gim dn xung 0.Trong loi th hai, v tr gin nnhanh ti mc lc hthp dn khng bao gic th lmqu trnh

dng li, mc d n c th chm li i cht. Hnh 3.3

cho thy c s tch xa ca hai thin h cnh nhau trongm hnh ny. N bt u t 0, v n mt lc no ,cc thin h chuyn ng ra xa nhau vi tc u.Cui cng l kh nng th ba: v tr gin n va nhanh trnh c qu trnh co v trng thi ban

u. Trong trng hp , s tch bit - nhc thytrn hnh 3.4 - cng bt u t 0 ri tng ln mi mi.Tuy nhin, vn tc m cc thin h chuyn ng ra xanhau ngy cng nh, mc d khng bao git ti 0.

Mt c im ni bt ca m hnh u tin caFriedmann l v tr khng phi l v hn trongkhng gian, nhng khng gian li khng c bin.

Lc hp dn mnh ti mc khng gian b cun trn li v lm cho n nan nh b mt ca tri t. Nu ngi ta chi theo mt hng trn b mtca tri t, ngi ra s khng bao gigp mt ro chn khng th qua c

Hnh 3.2: Khong cch giahai thin h ln cn thay it 0, tng ti cc i ri ligim dn xung 0.

Hnh 3.3: Hai thin h ngycng tch xa nhau.

Hnh 3.4: S tch bit giahai thin h cng bt u t0 v tng ln mi mi.


47/226

LC STHI GIAN 47


hoc ri xung qua mt mp vc, m cui cng s quay v ch xut pht.Trong m hnh u tin ca Friedmann khng gian ng nh th, ch ckhc l n c ba chiu ch khng phi hai chiu nh b mt tri t. Chiuth t - thi gian - cng hu hn, nhng n ging nhng thng c haiu, hay ni cch khc l hai bin: bt u v kt thc. Sau ny chng ta sthy rng khi ngi ta kt hp thuyt tng i rng vi nguyn l bt nhca chc lng t th c khng gian ln thi gian u c th hu hn mkhng c mp hoc bin.

tng cho rng ngi ta c th chu du vng quanh v tr v kt thc chnh ch xut pht s lm nn mt cu chuyn vin tng khoa hc hay,nhng khng c nhiu ngha thc tin, v ngi ta c th chng tcrng v tr s co li tr v kch thc zero trc khi ngi ta i ht mtvng quanh n. Bn cn phi i nhanh hn nh sng mi kp v ch xut

pht trc khi v tr co chung, nhng iu li khng c php.

Trong loi m hnh u tin ca Friedmann, v tr n ra ri co li, khnggian b un trn li ging nh b mt tri t. Do , n l hu hn. Trongloi m hnh th hai - v tr n ra mi mi - khng gian b un theo cchkhc, n ging nh hnh mt chic yn nga. Nh vy, trong trng hp nykhng gian l v hn. Cui cng trong m hnh th ba, vi tc gin ntihn, khng gian l phng (v do cng v hn).

Nhng m hnh no ca Friedmann m tng v tr ca chng ta? Liu v

tr cui cng c ngng gin nv bt u co li hay s gin nmi mi? tr li cu hi cn phi bit tc gin nhin nay ca v tr v mt trung bnh hin nay ca n. Nu mt nh hn mt gi tr ti hn no c xc nh bi tc gin n, th lc ht hp dn s qu yu lmdng qu trnh gin n. Nu mt ln hn gi tr ti hn, lc hp dn slm dng qu trnh gin nmt thi im no trong tng lai v s lmcho v tr co li.

Chng ta c th xc nh c tc gin nhin thi bng cch dnghiu ng Doppler o vn tc m cc thin h chuyn ng ra xa chngta. iu ny c th thc hin c mt cch rt chnh xc. Tuy nhin,khong cch gia cc thin h li c bit khng chnh xc lm, do chngta o chng ch bng phng php gin tip thi. V vy, hin nay chng tach bit c l v trang gin n5% n 10% trong mi khong thi gianl ngn triu nm. Tuy nhin, s bt nh v mt trung bnh hin nay cav tr thm ch cn ln hn na. Nu chng ta cng khi lng ca tt c


48/226

Stephen Hawking 48


cc ngi sao nhn thy trong thin h ca chng ta v cc thin h khc li,th tng s cn nh hn mt phn trm khi lng cn thit hm s ginnca v tr li, thm ch c trong trng hp nh gi thp nht tc gin n. Tuy nhin, thin h ca chng ta v cc thin h khc cn cn phicha mt lng ln vt cht ti m chng ta khng th thy c trctip, nhng chng ta bit n phi c v nh hng sc ht hp dn ca n lnquo ca cc ngi sao trong cc thin h. Hn na, a s cc thin hc tm thy nm thnh tng cm, v tng t nh trn, chng ta c thsuy ra s c mt ca mt lng vt cht ti cn ln hn na trong khonggia cc thin h thuc mt cm bi tc ng ca n ln chuyn ng cacc thin h. Khi cng tt c lng vt cht ti li, chng ta cng ch minhn c khong mt phn mi khi lng cn thit hm s gin nli. Tuy nhin, chng ta khng th loi tr kh nng c th c mt dng vtcht khc c phn bng u trong ton v tr m chng ta cn cha

pht hin c v n c th s nng mt trung bnh ca v tr ln ti gitr ti hn hm s gin nli. Do vy, bng chng m hin nay chng tac gi rng v tr c l s gin nvnh vin, nhng iu m chng ta thcs tin chc, l nu thm ch cui cng v tr c co li i na, th iu cng cha xy ra t nht trong vng mi ngn triu nm na, bi v n vncn gin n t nht cng tng y thi gian. iu ny chc khng khinchng ta phi lo lng: ti thi gian nu chng ta khng nh cc ngoi h mt tri, th loi ngi cng dit vong t lu cng vi mt trica chng ta!

Tt c cc m hnh ca Friedmann u c mt c im l mt thiim no trong qu kh(khong gia mi v hai mi ngn triunm trc) khong cch gia hai thin h ln cn phi bng 0. thiim - thi im m chng ta gi l v n ln (Big Bang) - mt ca vtr v cong ca khng-thi gian l ln v hn. V cc nh ton hc khngth thc s thao tc vi cc s v hn, nn iu ny c ngha l thuyt tngi rng (m cc m hnh ca Friedmann da trn ) tin on rng c mtim trong v tr m ti chnh bn thn l thuyt cng khng dng cna. Mt im nh vy l mt v d v ci m cc nh ton hc gi l im

k d. Thc t mi l thuyt khoa hc ca chng ta u c pht biu trngi thuyt cho rng khng - thi gian l trn v gn nh phng, v vy chngs khng cn dng c im k d v n ln v cong ca khng-thi gian l v hn. iu ny c ngha l thm ch nu c nhng s kin xyra trc v n ln, th ngi ta cng khng th dng chng xc nhnhng g xy ra sau , bi v tnh tin on c cng s khng cn na tiv n ln. Tng ng, nu ch bit nhng g t sau v n ln - chnh l


49/226

LC STHI GIAN 49


trng hp ca chng ta - th chng ta cng khng th xc nh c nhngg xy ra trc . i vi chng ta, th nhng s kin xy ra trc v nln c th khng c hu qu g, v vy chng khng to thnh mt b phntrong m hnh khoa hc v v tr. Do , chng ta s ct b chng khi mhnh v ni rng thi gian bt u t v n ln.

Nhiu ngi khng thch tng xem rng thi gian c im bt u,c l bi v n dy da vi scan thip ca thn thnh. (Nh thThinCha gio, tri li, nm ngay l

Documents

Lược sử thời gian