Vũ trụ trong vỏ hạt dẻ (Chương 5)

8/14/2019 V tr trong v ht d (Chng 5)

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CHNG 5BO V QU KH

Liu c th du hnh thi gian c khng?Mt nn vn minh tin tin c th quay li v thay i qu kh c khng?


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

Trang 132 Ngi dch: [email protected]; http://datrach.blogspot.com


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B O V Q U K H

Trang 133Ngi dch: [email protected]; http://datrach.blogspot.com

Ti nh cuc nhiu ln vi mt ngi bn v l mtngi ng nghip ca ti Kip Thorne, anh khng phi lmt ngi theo xu hng c chp nhn trong vt l ch vnhng ngi khc chp nhn. iu ny lm cho anh c can m trthnh nh khoa hc nghim tc u tin nghin cu v du hnh thigian nh l mt kh nng hin thc.

Nghin cu cng khai v du hnh thi gian i hi phi rt kho lo.

Ta c nguy c vp phi, hoc l, s phn i kch lit v vic lngph tin cng vo nhng th v b, hoc l, yu cu vic nghincu phi c xp vo mc tiu qun s. Sau ht l lm th no mchng ta c th t bo v khi nhng ngi c chic my thi gian?H c th thay i lch s v iu khin th gii. Ch c vi ngitrong chng ta in r nghin cu v mt vn qu khngng n v mt chnh tr nh th trong th gii vt l. Chng ta chey thc t ny bng cc thut ng khoa hc m ha cho vic duhnh thi gian. Kip Thorne

(1)Stephen Hawkingi vo h giun ngy6/1/1997 (2)

Trong tng lai,ngi ta chng minhc rng qu trnhtin ha ng t cciu kin chung banu c th khng baogi to ra k d trntri

(3)Stephen Hawkingk vo bn nhcc ngy 5/1/1997


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Mt phi thuyn ctcnh vo lc 12:00

Phi thuyn tr li vo lc11:45, mi lm phttrc khi n c gi i

Phi thuyn du hnhqua cc chu trnh trong

khng thi gian cong


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B O V Q U K H


C s cho tt c tho lun v du hnh thi gian l thuyt tngi rng ca Einstein. Nh chng ta thy trong cc chng trc,cc phng trnh Einstein lm cho khng gian v thi gian trnn ng bng vic m t cch m chng b cong v bin dng bivt cht v nng lng trong v tr. Trong thuyt tng i rng,thi gian c nhn ca mt ngi no c o bi ng h eotay ca h v thi gian lun tng ging nh vic thi gian tngtrong l thuyt Newton hay khng thi gian phng trong thuyttng i hp. Nhng by gi c kh nng khng thi gian b bcong n ni bn khng th thot ra khi phi thuyn v tr li trckhi bn c gi i (hnh 5.1).

C mt cch iu ny c th xy ra nu c cc h giun (wormhole) cc ng ng ca khng thi gian c nhc n trongchng 4 ni cc vng khc nhau ca khng gian v thi gian. tng l bn li phi thuyn ca bn vo mt ming ca h giun v ira ming kia n mt ni khc v ti thi im khc (hnh 5.2).

Nu cc h giun tn ti th chng s l li gii cho vn gii hntc trong khng gian: phi mt mi ngn nm i ngang qua


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thin h bng mt phi thuyn chuyn ng chm hn nh sng nhyu cu ca thuyt tng i. Nhng bn c th i qua h giun n l bn kia ca thin h v tr li ng vo ba ti. Tuy nhin,nu ta chng minh rng h giun tn ti th bn cng c th quayli trc khi bn c gi i. Vy nn, bn c th ngh rng bnc kh nng lm iu g nh l cho n tn la trn b phng trnh khng phi ln ng v tr u tin. y l mt bin i canghch l ngi ng (grandfather paradox): iu g s xy ra nubn quay tr li v git ng bn trc khi b bn ra i (hnh 5.3)?

Tt nhin, y ch l nghch l nu bn tin rng bn c ch t do lm iu bn mun khi bn quay tr li qu kh. Cun sch ny

(Hnh 5.2) PHIN BN TH HAI CA NGHCH L ANH EMSINH I

(1)Nu mt h giun nng, bn c th i vo h giun v ra cngmt thi im.

H GIUN NNG

i vo lc 12:00 i ra lc 12:00


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B O V Q U K H


(2) Ta c th hnh dung ly mt u ca h giunsu t phi thuyn v u kia tri t.

(3) V hiu ng nghch l anh em sinh i (*thigian trn phi thuyn ang chuyn ng vivn tc ln s chm hn thi gian trn tri t

ND*), khi chic phi thuyn tr li, thi giantri i u phi thuyn chm hn thi gian u tri t. iu c ngha l nu bn bcvo u tri t v bc ra u phi thuyn tithi im sm hn.

Ming h giun bntrong phi thuyn

Ming h giuntrn mt t

T tri t i voh giun lc 12:00

Ra khi h giun vophi thuyn lc 10:00

Phi thuyn tr litri t t h giun


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DY V TR

Cc dy v tr l cc vtth di, nng v c thitdin rt nh. Cc dy ny cl c to ra ti cc thi ks khai ca v tr. Mt khi ccdy c hnh thnh, chng bko di ra do v tr gin n,v by gi, mt dy v tr c

th ko di theo ton b vngv tr c th quan st.

S xut hin ca cc dy vtr n t cc l thuyt miv ht c bn. Chng tinon rng, ti cc giai onban u nng bng, vt cht trong mt pha i xng, rtging nc trng thi lng rt i xng: ging nhau ti

mi im v theo mi hng ch khng ging trong tinhth nc ng bng c cccu trc ri rc.

Khi v tr lnh i, tnh ixng ca pha ban u nyc th b ph v theo cccch khc nhau cc vng xanhau. Kt qu l vt cht vtr cc vng c cc trng

thi c bn khc nhau. Do ,s hnh thnh ca chng lmt h qu khng th trnhkhi ca vic cc vng khcnhau c cc trng thi c bnkhc nhau.

(Hnh 5.3)

Mt vin n bn vo qu kh qua mt h giun c nh hng n x thhay khng?

s khng i vo tho lun trit hc v ch t do. M thay vo ,cun sch s tp trung vo vic cc nh lut vt l c cho phpkhng thi gian b b cong n mc m cc vt th v m nh lmt chic phi thuyn c th quay tr li qu kh ca chnh n chay khng. Theo l thuyt Einstein, mt chic phi thuyn cn phichuyn ng chm hn vn tc nh sng a phng v i theo mtci gi l l trnh thi gian (timelike path) trong khng thi gian.Do , ta c th pht biu cu hi trn bng cc thut ng chuynmn: khng thi gian c chp nhn cc ng cong thi gian ng(closed timelike curve) hay khng tc l cc l trnh tr li imxut pht ca n nhiu ln? Ti s gi nhng ng nh th l chutrnh thi gian (time loop).

Chng ta c th c gng tr li cu hi ny theo ba mc . Mc th nht l l thuyt tng i rng ca Einstein, l thuyt chorng v tr c mt lch s tt nh m khng c bt k bt nhno. i vi l thuyt c in ny, chng ta c mt bc tranh khhon thin. Tuy nhin, nh chng ta thy, l thuyt ny khngth ng hon ton v chng ta thy vt cht tun theo nguyn l btnh v cc thng ging lng t.

Do , chng ta c th t cu hi v du hnh thi gian trn mt mc th hai, mc ca l thuyt bn c in. Trong l thuyt ny,chng ta xem xt vt cht hnh x theo thuyt lng t, vi tnh btnh v thng ging lng t, nhng khng thi gian hon ton ttnh v c in. y, bc tranh km hon thin hn, nhng t nhtchng ta c mt vi tng v vic s tip tc nh th no.


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NH L KHNG CAGODEL

Vo nm 1931, nh tonhc Kurt Godel chngminh nh l khng niting ca ng v bn cht caton hc. nh l pht biurng trong bt k mt h tin hnh thc no, ging nh

ton hc ngy nay, cc cuhi v vic chng minh hoc bc b cc tin xc nhcc h, lun lun tn ti. Nicch khc l Godel chngminh rng lun tn ti nhng bi ton khng th gii bng bt k tp hp cc quy tchoc qu trnh.

nh l Godel t gii hn

cho ton hc. iu ny gysng st cho gii khoa hc vn p nim tin by lucho rng ton hc l mt hthng cht ch v hon thinda trn cc chuyn tc l-gic. nh l Godel, nguynl bt nh Heisenberg vvic khng th tin on stin ha ngay c trong cch quyt nh lun (tr thnh

hn lon) to nn mt tphp cc gii hn cho tri thckhoa hc, iu ch c thc nhn thy vo th k haimi.

(Hnh 5.4)Khng thi gian c c php ng,

tc l tr li im xut pht ban uca chng nhiu ln hay khng?

Cui cng, c mt l thuyt lng t hp dn y , d n c thl g i chng na. Trong l thuyt ny, khng ch c vt cht m ckhng gian v thi gian u bt nh v thng ging, ngay c viclm th no t ra cu hi l du hnh thi gian c kh thi haykhng vn cn cha r rng. C l iu tt nht chng ta c th lml hi xem nhng ngi trong cc vng khng thi gian gn c in

v khng c tnh bt nh gii thch cc php o ca h nh th no.H c ngh rng du hnh thi gian c th xy ra trong nhng vnghp dn mnh v thng ging lng t ln hay khng?

bt u vi l thuyt c in: khng thi gian phng ca thuyttng i hp (tng i khng c hp dn) khng cho php duhnh thi gian v cng khng cho php khng thi gian b b congm ta bit trc y. Chnh v th m Einstein rt sng st khiKurt Godel pht hin ra mt khng thi gian mt v tr trn yvt cht ang quay vi cc chu trnh thi gian xuyn qua ti mi

im (hnh 5.4) l nh l Godel.

Li gii Godel cn mt hng s v tr, hng s c th hockhng th tn ti trong t nhin, nhng ri cc li gii khc ctm thy m khng cn mt hng s v tr. Mt trng hp c bitl th l trng hp khi hai dy v tr (cosmic string) chuyn ngvi tc cao i qua nhau.

Khng nn nhm ln cc dy v tr vi cc dy ca l thuyt dy


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mc d chng khng phi l hon ton khng c lin h g vi nhau.Chng l cc vt th c di vi thit din rt nh. S c mt ca

chng c tin on trong mt s l thuyt ht s cp. Khng thigian bn ngoi mt dy v tr n l phng. Nhng l mt khngthi gian phng vi mt gc hnh ci nm b ct i, u nhn caci nm l dy v tr. N ging nh mt ci nn: ly mt hnh trnln bng giy v ct i mt cung trn ging nh ct i mt gc bnhngt, ta c mt gc bnh hnh ci nm vi cnh nhn ca mingbnh chnh l tm ca hnh trn. Sau vt phn cung trn b cti v dn hai ng ct ca phn giy cn li vi nhau v bn s cmt ci nn. Ci nn ny biu din khng thi gian m trong dyv tr tn ti (hnh 5.5).

Lu rng v b mt ca nn chnh l tm giy phng ban u (tri phn giy hnh ci nm), bn vn c th gi n l phng ngoitr ci nh. Bn c th nhn ra rng c cong nh nn nh vomt hnh trn xung quanh nh nn c ng knh nh hn hnhtrn c v trn tm giy trn ban u vi cng khong cch ttm hnh trn. Ni cch khc, mt hnh trn xung quanh nh s cchu vi ngn hn hnh trn c cng bn knh c v trn khng gianphng v mt cung trn ca n b mt i (hnh 5.6).

Tng t nh vy, trong trng hp dy v tr, phn khng thigian hnh ci nm b ct khi khng thi gian phng lm ngn icc hnh trn xung quanh dy v tr nhng khng nh hng nthi gian v khong cch dc theo cc dy . iu ny c nghal khng thi gian xung quanh mt dy v tr n l s khng cbt k chu trnh thi gian no, th nn ta khng th du hnh v qukh. Tuy nhin, nu c mt dy v tr th hai chuyn ng tngi vi chic dy th nht th hng thi gian ca dy th hai sl t hp ca cc hng thi gian v khng gian ca dy th nht.Tc l, phn khng gian hnh ci nm c ct t ci dy th hai slm ngn i khong thi gian v khng gian i vi ai chuynng cng ci dy th nht (hnh 5.7). Nu cc dy v tr chuynng vi vn tc gn bng vn tc nh sng tng i vi nhau thkhong thi gian tit kim c xung quanh c hai dy c th lnn mc ta c th tr v trc khi ta c gi i. Ni cch khc, sc cc chu trnh thi gian m ta c th theo n m du hnh v qukh.

Khng thi gian ca cc giy v tr c cha vt cht vi mt nng lng dng v ph hp vi nn vt l m ta bit. Tuy vy,

(Hnh 5.5)


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B O V Q U K H


Dy v tr

t

x

y

Cc gc hnh nm b ctkhi khng thi gian ccc cnh khng song song

(Hnh 5.6)

Mt gc ca mt dy v tr lm ngn ikhong cch ca phn cn li nhng thigian khng b nh hng.

Mt gc b ct t mt dy v tr chuyn ng khccng s lm ngn i khong cch trong khng gianv thi gian ca dy v tr th nht

(Hnh 5.7)


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s cong ca khng thi gian m to nn cc chu trnh thi gian tridi cho n v tn trong khng gian v tr li n qu kh v tntrong thi gian. Do vy, ta c th du hnh thi gian trong cc khngthi gian t khi chng c to ra. Ta khng c l do g tinrng v tr ca ring chng ta c to ra theo mt cch b b congnh vy, v chng ta cng khng c cc bng chng ng tin cy vcc v khch ca tng lai (Ti khng tnh n gi thuyt im lngcho rng cc a bay UFO n t tng lai v cc chnh ph bitnhng che du i. H s che giu chuyn khng th kn nh thc). Do , ti s gi thit rng khng c cc chu trnh thi giantrong qu kh xa xi, hay l mt cch chnh xc hn, khng c ccchu trnh thi gian trong qu kh ca mt s mt phng trong khngthi gian m ti gi l S. Lc cu hi s l: liu cc nn vn minhtin tin c th xy dng mt chic my thi gian hay khng? Tcl liu n c th thay i khng thi gian ca tng lai ca S (phatrn mt S trn gin ), lm cho cc chu trnh thi gian c xuthin trong mt vng hu hn hay khng? Ti ni mt vng hu hnbi v d nn vn minh c tr thnh tin tin n bao nhiu ichng na th nn vn minh c l ch c th iu khin mt phnhu hn ca v tr m thi.

Trong khoa hc, vic trnh by mt cch chnh xc mt vn thng l cha kha gii quyt vn , v y l mt v d tt. nh ngha my thi gian hu hn c ngha g, ti quay tr li vimt s cng trnh trc y ca ti. C th du hnh thi gian trongmt vng khng thi gian c cc chu trnh thi gian cc l trnhchuyn ng chm hn tc nh sng, nhng tuy vy, chng li cth tr li thi gian v a im m chng khi u nh vo cong

(Hnh 5.8)

Nn vn minh tin tin nht cth b cong khng thi gian trongmt vng hu hn. Chn tri duhnh thi gian l bin gii ngncch vng khng thi gian, nc hnh thnh bi cc tia sng pht ra t cc vng hu hn.

Trong vng , ta c th du hnhvo qu kh.

S

CHN TRI DU HNH THI GIAN PHT X HU HN

Tt c cc tia sng trongchn tri du hnh thigian n t vng mt cao ny

chn tri duhnh thi gian


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ca khng thi gian. Vti gi thit rng khng

c cc chu trnh thigian trong khng gianxa xi, nn cn phi cmt ci gi l chntri du hnh thi gian(time travel horizon) bin gii phn cchvng c v khng cchu trnh thi gian(hnh 5.8).

Cc chn tri du hnhthi gian kh ging ccchn tri h en. Trongkhi chn tri h en chnh thnh bi cc tia sng sutb ri vo h en th chn tri du hnh thi gian c hnh thnhbi cc tia sng sut gp nhau. Vy nn ti chn tiu chun ca tiv my thi gian ci m ti gi l chn tri pht sinh hu hn(finitly generated horizon) l mt chn tri c hnh thnh bi

cc tia sng xut hin t mt vng khng thi gian b buc. Nicch khc, cc tia sng khng n t v tn hoc t im k d mt mt vng hu hn c cha cc chu trnh thi gian vng khngthi gian m ta gi thit nn vn minh tin tin ca chng ta csinh ra.

Khi chn nh ngha ny lm c s ca my thi gian, chng ta cli th c th s dng chic my m Roger Penrose v ti pht trin nghin cu cc k d v cc h en. Thm ch khngs dng cc phng trnh Einstein, ti c th chng minh rng, nichung, mt chn tri pht sinh hu hn s c cha mt tia sng mthc ra t gp chnh n tc l mt tia sng c chuyn ng vngquanh mt im. Mi ln tia sng quay li n s b dch chuyn vpha nh sng xanh, v dn dn n s tr ln xanh hn. Cc nhsng ca xung nh sng s dn dn gn nhau hn v nh sng squay vng trong mt khong thi gian ngn hn. Trn thc t, mtht nh sng ch c mt lch s hu hn v c xc nh bi phpo thi gian ring ca n, mc d n chuyn ng vng quanh trongmt vng khng thi gian hu hn v khng va phi mt im kd.

Cu hi l: mt s nn vnminh tin tin c th xy

c my thi gian haykhng?


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Ngi ta c th khng quan tm n chuyn mt ht nh sng cchm dt lch s ca n hay khng. Nhng ti vn c th chng

minh rng c nhng l trnh chuyn ng chm hn vn tc nhsng v c thi gian sng hu hn. Cc l trnh ny c th l nhnglch s ca ngi quan st, ngi quan st li b by trong mtvng khng thi gian hu hn ngay trc chn tri v s bay vngquanh, nhanh dn nhanh dn cho n khi t n tc nh sngtrong mt thi gian hu hn. Do , nu c mt ngi ngoi hnhtinh xinh p trong mt a bay mi bn vo chic my thi gianca c ta th bn phi cn thn khi bc chn vo y. Bn c th bri vo mt trong nhng lch s tun hon b by c thi gian huhn (hnh 5.9).

Cc kt qu ny khng ph thuc vo cc phng trnh Einstein mch ph thuc vo cch m khng thi gian b cong to ra cc chutrnh thi gian trong mt vng hu hn. Tuy nhin, by gi chng tac th hi liu nn vn minh tin tin phi s dng loi vt cht g b cong khng thi gian c th xy dng c mt chic mythi gian c kch thc hu hn? Vt cht c th c mt nnglng dng khp ni ging nh trong khng thi gian dy v trm ti m t trc y hay khng? Khng thi gian dy v tr khng tha mn yu cu ca ti l cc chu trnh thi gian xut hin

trong mt vng hu hn. Tuy nhin, ngi ta c th ngh rng yucu ch l do cc dy v tr di v hn. Ngi ta c th tngtng rng ngi ta c th xy mt ci my thi gian hu hn sdng cc vng dy v tr hu hn v c mt nng lng dng khp mi ni. Tht ng tic khi phi lm tht vng nhng ngimun quay li qu kh ging nh Kip, nhng iu khng ththc hin c khi m mt nng lng l dng khp mi ni.Ti c th chng minh rng, xy mt my thi gian hu hn, bncn nng lng m.

Trong l thuyt c in th mt nng lng lun lun dng, do, loi tr my thi gian vi kch thc hu hn ra khi mc ny. Tuy nhin, tnh hung li khc trong l thuyt bn c in, khim ta coi vt cht tun theo thuyt lng t nhng khng thi gianli tt nh v c in. Nh chng ta thy, nguyn l bt nh cac hc lng t ni rng cc trng lun thng ging ln xungngay c trong chn khng v c mt nng lng v hn. Do ,ta phi loi tr i lng v hn nhn c mt nng lnghu hn m ta quan st c trong v tr. Vic loi tr ny c thlm nng lng m, t nht l m cc b. Ngay c trong khng gian

(Hnh 5.9, pha trn)

S nguy him khi du hnh thigian.

(Hnh 5.10, trang k)

Vic tin on h en pht xv mt i khi lng ng rngthuyt lng t to ra cc nng

lng m i vo h en qua chntri s kin. i vi h en co lin khng, mt nng lngtrn chn tri s kin phi l m, l du hiu cn c xy dngmy thi gian.


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B O V Q U K H


phng, ta c th tm thy cc trng thi lng t c nng lng m

cc b mc d nng lng ton phn l dng. Ta c th t hi lcc gi tr m ny c thc s lm khng thi gian b b cong mtcch thch hp xy mt my thi gian hu hn hay khng, dngnh l c. Nh chng ta thy trong chng 4, thng ging lngt c ngha l ngay trong chn khng trng rng cng y p cccp ht o, cng xut hin, ri xa nhau ri quay tr li hy lnnhau (hnh 5.10). Mt thnh vin ca cp ht o c nng lngdng v thnh vin kia c nng lng m. Khi c mt h en thnhvin c nng lng m c th b ri vo h en v thnh vin cnng lng dng c th thot vo v tn, , thnh vin c nng

lng dng c quan st nh l bc x mang nng lng dngra khi h en. Cc ht c nng lng m ri vo h en s lm choh en mt khi lng v t t bay hi, kch thc ca chn tri skin s b suy gim (hnh 5.11).

Vt cht thng vi mt nng lng dng gy ra hiu ng hpdn m v b cong khng thi gian lm cho c tia sng i li gnnhau ging nh hn bi trn tm cao su trong chng hai b congng i ca cc hn bi nh hn li gn nhau ch khng bao gilm cho chng xa nhau. (Hnh 5.10)

Sinh

Hy

HtPhn ht

(Hnh 5.11)


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iu ny ng rng din tch ca chn tri h en lun tng theothi gian ch khng bao gi b gim i. i vi chn tri ca hen ang gim kch thc th mt nng lng trn chn tri phil m v b cong khng thi gian lm cho cc tia sng i ra xa

nhau. l iu ti nhn ra u tin khi ti i v bung ng ngaysau khi con gi ti ra i. Ti s khng ni khi cch y bao lu,ch bit rng nay ti c chu ngoi ri.

Vic cc h en bay hi cho thy rng trn mc lng t, i khimt nng lng c th l m v lm b cong khng thi gian theohng cn thit xy dng mt my thi gian. Do chng ta cth tng tng rng, mt s nn vn minh rt cao c th b tr mt nng lng m to nn mt my thi gian c th s dngcho cc vt th v m nh l cc phi thuyn. Tuy nhin, c mt skhc bit quan trng gia chn tri h en c hnh thnh bicc tia sng bay vng quanh h en, v chn tri ca my thi gian c cha cc tia sng chuyn ng vng. Do , ta c th trng imt nng lng v hn trn chn tri bin ca my thi gian vng khng thi gian m ta c th i v qu kh. iu ny cxc nh bi cc tnh ton rt tng minh trn mt s nn tng n gin cho cc tnh ton chnh xc. iu c ngha l mt ngihay mt my d khng gian khi th vt qua chn tri i vo trongmy thi gian s b ph hy bi mt vnh ai bc x (hnh 5.12).Nn tng lai c v nh l mt mu en vi vic du hnh thi gian hay l mt mu trng cht chc nh?

Mt nng lng ca vt cht ph thuc vo trng thi ca n nnc th mt nn vn minh tin tin c kh nng to ra mt vt chthu hn trn bin ca my thi gian bng vic vt b hoc loitr cc ht o chuyn ng vng quanh trong mt chu trnh kn. Tuyvy, vn cha r mt chic my thi gian nh th c th n nh haykhng: mt nhiu lon nh nht nh l vic ai i ngang qua chntri vo trong my thi gian s lm tng cc ht o chuyn ngtun hon v khi ng vnh ai nh sng. y l mt cu hi m

Chu ngoi ti, William Macken-zie Smith.


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B O V Q U K H


cc nh vt l nn t do tho lun, khng nn ci khinh thng.Thm ch ngay c khi du hnh thi gian l bt kh th th vic tmhiu ti sao n bt kh th l iu rt quan trng.

tr li dt khot cu hi ny chng ta cn xem xt cc thngging lng t khng ch ca cc trng vt cht m c ca bnthn thi gian na. Ta c th trng i cc thng ging gy ra mt m nht nh trong cc l trnh thi gian v trong ton b khinim thi gian trt t. Thc vy, ta c th coi bc x t cc h enging nh mt s r g ra ngoi nh cc thng ging lng t cakhng thi gian, c ngha l chn tri khng hon ton xc nhmt cch chnh xc. V chng ta vn cha c mt l thuyt hp dnlng t hon thin nn tht kh c th ni cc hiu ng ca thngging khng thi gian l g. Tuy nhin, chng ta c th hy vng cc mt s gi t tng theo cc lch s ca Feynman m cm t trong chng 3.

(Hnh 5.12)

Ta c th b mt vnh ai bc xxa s khi bc chn vo chntri du hnh thi gian.


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Mi lch s s c mt khng thi gian cong vi cc trng vt chttrong . V chng ta gi thit ly tng theo tt c cc lch s khd, khng ch cc lch s tha mn mt s phng trnh m tng cnbao gm cc khng thi gian cong du hnh v qu kh (hnh5.13). Do , cu hi l ti sao du hnh thi gian khng xy ra khp ni? Cu tr li l thc ra du hnh thi gian xy ra cp vi m, nhng chng ta khng ch n n. Nu ngi ta p dng tng ly tng theo cc lch s ca Feynman i vi mt ht, ngita phi ly c cc lch s trong cc ht chuyn ng nhanh hnnh sng thm ch cn i ngc thi gian. c bit l s c cc lch

s m trong cc ht chuyn ng vng quanh theo nhng chutrnh kn trong thi gian v khng gian. iu ny ging nh trongphim Groundbog Day, trong mt phng vin phi sng trongmt ngy c lp i lp li nhiu ln (hnh 5.14).

Ngi ta khng th quan st mt cch trc tip cc ht vi cc lchs c chu trnh kn nh th bng cc my o ht. Tuy vy, cc hiung gin tip ca chng c thy trong rt nhiu th nghim.Mt trong nhng th nghim l c mt dch chuyn nh trong

(Hnh 5.13)

Ly tng theo cc lch s caFeynman tnh n cc lch strong cc ht quay ngc lithi gian v ngay c cc lch s lcc vng ng trong thi gian vkhng gian.

Chu trnh kn

L trnh caht i ngcthi gian theomt chu trnh

Cc lch s ca ht

TNG THEO CC LCH S

Ht

KHNG GIANTHIGIAN


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B O V Q U K H


nh sng pht ra t cc nguyn t hydro, gy ra bi cc in tchuyn ng trong cc chu trnh kn. Mt th nghim khc na l c mt lc nh xut hin gia cc tm kim loi song song gy rado c t hn cc lch s chu trnh kn c th khp vo gia cc tmkim loi so vi cc vng bn ngoi l mt li gii thch tngng khc cho hiu ng Casimir. Vy nn s tn ti ca cc lch schu trnh kn c khng nh bng thc nghim (hnh 5.15).

Ngi ta c th nghi ng vic cc lch s chu trnh kn liu c linquan g n vic b cong khng thi gian hay khng v chng xuthin ngay c trong khung nn c nh nh l khng gian phng.Nhng nhng nm gn y chng ta tm ra rng cc hin tngvt l thng c th c m t theo hai cch tng ng nhau.Ngi ta hon ton c th ni rng mt ht chuyn ng trn mtchu trnh kn trong mt khung nn c nh hoc ngi ta cng cth ni rng ht nm trong trng thi c nh v khng gian v thigian thng ging xung quanh n. Cu hi ch l bn ly tng theocc l trnh ca ht trc ri sau ly tng theo khng thi giancong hoc ngc li.

Cc chu trnh kn

(Hnh 5.15)

(Hnh 5.14)

Ht i ngc thigian

Ht i xui thigian

KHNG GIANTHIGIAN


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Vy nn, dng nh thuyt lng t cho php du hnh thi gian cp vi m. Nhng iu ny khng c nhiu tc dng lm vi

mc ch vin tng khoa hc nh l vic quay li qu kh v gitngi ng chng hn. Vy cu hi l: xc xut trong vic ly tngtheo cc lch s xung quanh khng thi gian vi cc chu trnh thigian v m c cao hay khng?

Ngi ta c th xem xt cu hi ny khi nghin cu tng theo cclch s ca cc trng vt cht trong mt chui cc khng thi giannn (background spacetime) ang xch li gn nhau c cc chutrnh thi gian. Ngi ta trng i ci g kch tnh xy ra khi ccchu trnh thi gian xut hin u tin, v iu ny c xc nhn

trong mt th nghim n gin m ti kho cu vi mt sinh vinca ti l Micheal Cassidy.

Cc khng thi gian nn trong chui m ti nghin cu c lin hrt mt thit i vi ci c gi l v tr Einstein, khng thi gianm Einstein a ra khi ng tin rng v tr l tnh ti v khng itheo thi gian, khng gin n m cng khng co li (xem chng1). Trong v tr ca Einstein, thi gian chy t qu kh v cng ntng lai v cng. Tuy nhin, cc hng khng gian li hu hn vt ng, ging nh b mt tri t nhng c nhiu hn mt chiu.

Ta c th mng tng khng thi gian ny ging nh mt hnh trvi trc thng ng l hng thi gian v mt ct l ba chiu cakhng gian (hnh 5.16).

V tr ca Einstein khng th hin v tr m chng ta ang sng vn khng gin n. Tuy vy, n l mt khung nn rt thun tin trongvic s dng khi nghin cu du hnh thi gian v v tr ngin khi ta ly tng theo cc lch s. By gi ta b qua chuyn duhnh thi gian, hy xem xt vt cht trong v tr ca Einstein quayxung quanh mt s trc no . Nu bn ng trn mt ci trcquay, bn c th vn ti mt im trong khng gian, ging nhbn ng trung tm ca tr tri nga quay ca tr con. Nhng nubn khng ng tm th bn s chuyn ng trong khng gian khibn quay quanh trc. Bn cng xa trc quay th bn chuyn ngcng nhanh (hnh 5.17). Do , nu v tr l v hn trong khnggian, th cc im xa trc quay s quay nhanh hn vn tc nhsng. Tuy nhin, v v tr ca Einstein l hu hn theo cc hngtrong khng gian, nn s c mt tc quay ti hn m di nkhng c phn no ca v tr quay nhanh hn vn tc nh sng.

(Hnh 5.16)

V tr ca Einstein ging nhmt hnh tr, n hu hn theokhng gian v khng i theothi gian, mi ni trong v tr cth quay vi tc nh hn tc nh sng.

KHNG GIANTHIGIAN


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B O V Q U K H


By gi ta hy nghin cu vic ly tng theo cc lch s ht trongmt v tr Einstein quay. Khi quay chm, mt ht c th theo rtnhiu l trnh khi dng mt lng nng lng cho. Do , tngtheo tt c cc lch s ca ht trong khung nn ny s c bin ln. iu ny mun ni rng xc xut ca khung nn ny s cao khily tng theo tt c lch s khng thi gian b un cong tc l, nl mt trong nhng lch s c xc xut cao hn. Tuy vy, khi tc quay ca v tr Einstein t n gi tr ti hn, th ra ngoica v tr chuyn ng vi vn tc tin n vn tc nh sng. Chc mt l trnh c php trong khun kh c in trn ra v tr,l l trnh cho ht chuyn ng vi vn tc nh sng. iu ny cngha l tng theo tt c cc lch s ht s nh nn xc xut ca cckhung nn ny s thp hn xc xut khi ly tng theo tt c cc lchs khng gian cong. Tc l, chng c xc xut thp nht.

(Hnh 5.17)

Trong khng gian phng, phn xa trc quay ca mt a cng cth quay nhanh hn vn tc nh

sng.

Quanh nhanh hnvn tc nh sng

Quanh chm hnvn tc nh sng Trc quay

QUAY TRONG KHNG GIAN PHNG


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B O V Q U K H


th kt lun rng xc xut ca cc khung nn ny tin n khng khichng tin n cong cn thit cho cc chu trnh thi gian. Nicch khc, xc xut c cong cho mt my thi gian l bngkhng. iu ny cng c ci m ti gi l Gi nh bo ton lch s(Chronology Protection Conjecture): l cc nh lut vt l cgng trnh du hnh thi gian i vi cc vt th v m.

Mc d cc chu trnh thi gian c th xy ra nh vic ly tng theocc lch s, nhng cc xc xut li cc k nh. Da trn lun c

lng tnh ti ni trc y, ti c tnh xc xut m Kip Thornec th quay li v git cht ngi ng ca anh ta l mt phn mit t t t t t t t t t.

y l mt con s rt nh, nhng nu bn nhn gn vo bc tranhca Kip th bn c th nhn thy nhng ng m ra bc tranh. N tng ng vi xc sut khi mtk v li no t tng lai quay li qu khv git cht ngi ng ca anh ta anh takhng th c mt y c na.

Nh nhng tay c bc, Kip v ti nh bcv iu . Nhng vn l chngti khng th c cc vi nhauv by gi chng ti cngmt pha. Mt khc, ti skhng nh bc vi btk ai na. Anh ta c thn t tng lai v bitrng my thi gian hot ng.

Bn c th t hichng sch ny cphi l mt phn binphp che y my thigian ca chnh ph haykhng. C th bnng y.

Xc sut m Kip c th quayli qu kh v git nging ca anh ta l mt phnmi m mi m su mi(1/101060).

Ni cch khc xc sut nh hn mt phn ngnt ngn t ngn t ngn tngn t.


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