I KH&CN nude ngoai
< » < » <)> <)>
TINH THE T H 0 I GIAN
GS Cao Chl Tinh the thdi gian (time crystals) la trang thai m6i cua vat chat chufa nhOng cau hinh hoat dpng tUdng tUnhif trong cac dong ho.
Tinh the thdi gian co the giup chung ta hieu them vu tru va lo den til mot l<hia canh khac? Giup chung ta hieu vat chat mot each l(hac hoac md rong tam nhin ve vat chat? Neu cd che tinh the thdi gian la mot cd che phd quat trong vu tru thi tinh the thdi gian co the la mot quan diem nhan thu'c mdi ve vu tru?
Unit the ihili gian la giP
Nam 2012, Frank Wilozek (Giai Nobei Vat ly nam 2004 ve tUdng tac manti) da daa khiai niem mdi - tinti the thai gian vao vat iy. Ban than ong da rat ngac nhien v i cao trang thai mffl cua vat chat co nhOng doi xilng trong thai gian, tuong tu nhu cac tinh the thong thuang co doi xUng trong khong gian.
NhU ta biet, tinh the vdi nhOng nguyen tCf hay phan ta CO cau true lap lai lam cho cac chat ran on dinh va cUng. Tinh the co cau true lap lai, trong ngon noO vat ly ngudi ta goi la tinh the b! vi pham do! xUng khonq gian tu phat {•) ndi dang ly phai cd su dich chuyin toan phin thi tinh t h i chi od sU djch chuyen tCfng doan mdt).
Tinh t h i thdi gian la mot khai nidm hoan toan mdi Dd la nhOng trang thai vat chft ma hinh the lap Z theo nhOng khoing thdi gian (thay vi thed_^nhOng khoang khdng gian). Do la cac he trcng dd do, xung thdi gian bjvi pham tu phat.
Khai niem tinh the thdi gian xuat hien nam 2012 nhUng den nam 2017 cac nha vat ly mdi lan dau tien kham pha nhUng chat lieu mdi ve tinh the thdi gian.
Cac kham pha nay cd y nghTa rat quan trgng doi vdi may tinh lugng tCf trong tUdng lai, giiip chung hoat ddng d mUc do phUc tap va on dmh cao hon nhd khai thac cac quy luat cua ddng luc hgc lUdng tif.
Ngoai ra, cdng nghe nay cung dUdc dung de che tao nhieu loai dong hd lUdng tCf mdi cd do chfnh xac cao.
Djch chuyen thdi gian sau day dUdc ghi la x va dUdo goi la tau (chO Hy Lap). Cac tinh the dUdc dac trung bdi khoang each d, vay cac tga dd se la nd.
Khi chung ta dich chuyen tinh the khoang each dthl tinh the trung lai vdi chfnh nd. Nhu vay tinh the co mot doi xUng han che ve dich chuyen (khdng djch chuyen mdt khoang bat ky dUdc - hinh 1),
Ta ndi rang ddi xUng djch ehuyen tdng quat bi pha vd dSn den mot djch chuyen han che hdn dac trung bdi d. Tinh the cd 2, 3 chieu. Chung cd the cd cac ddi xi/ng rieng quay hoac tjnh tien (hinh 2).
62 Viptm
S6'4 nam 2020KH&CN nUdc ngoai I
r l l n h 1 . Do) x u t i g cua t i n h t h e .
AC Josephson Effect Hieu Ling AC Josephson Hai chi^u (d^n Alhmabra) Ba c h i l u {tinh t h ^ kim cifong)
H i n h 2 . D o i x i h i g t i n h t h i t r o n g 2 v a 3 c h i l u .
GS Wilczek nhan djnh rang, cd the ndi rdng sd Dhieu Ien 4 chieu khdng - thdi gian. Hai cau hdi Xd dd 20 the phat sinh: cac tinh the trong khdng thdi gian 30 the mieu ta nhiing he nao trong the gidi thuc tai?
i/a nhiing hinh loai (pattern) dd cd cho phep chiing ta xac djnh cac trang thai cua vat chat hay khdng?
Nhiing hien tUdng gi ed the quy ve hien tUdng imh the thdi gian? Dd la: Trai dat lap lai hudng trong
<hdng gian qua tC/ng chu ky thdi gian khoang 1 ngay, he Trai dat - Mat trdi lap iai cau hinh tUng chu
<y thdi gian khoang 1 nam. Cac ddng ho ciing lap iai
;au hinh lidn hoi. Cac dong hd nguyen tCf da od do :hfnh xao cao song chung ta van cd the tang them 3d chi'nh xac nhd sif dung tinh the thdi gian.
Pha u0 doi xiitig tir phat
Khi mot chat Idng hoac khf bien thanh tinh the hi ddi XLfng se bj thu hep lai. Hien tUdng giam ddi
<Ung nay sinh ra vi sU giam nhiet do va khdng can not tac dpng ngoai lai nao khac. Ta cd the ndi rang, (hi chat long bien thanh tmh the, vat chat da pha vd
36i xifng dich chuyen mdt each "tU phat" (dd la mdt Hinh 3. Hieu iJng josephson.
chuyen pha manh), Su chuyen pha nay kem theo bUc xa nang iLidng (dudi dang nhiet nang) va chung ta ed the biet trudc.
Tathay phat sinh 3 dac trung: doixUng bi thu hep, mdt chuyen pha manh va mot dac trung khac la dp ciing ran. Can cU vao 3 dac trUng dd, cd the thay ban chat sau xa ciia hien tUdng la do cac nguyen tCf mudn rdi vao mdt cau hinh vdi nang lugng thuan ldi hdn.
Wiiczek cho rang x cung cd the bj pha vd tU phat.
Dd la hien tUdng xay ra trong eac tinh the thdi gian.
Nam 2012 Wilczek (cung vdi Shapere) da cdng bo cdng trinh, dUa vao vat ly ly thuyet khai niem tinh the thdi gian: tinh the thdi gian la he trong do x b| pha vd doi xUng tU phat. 6 day, chung ta phai sif dung dmh ly I Noether: x cd ban gan vdi bao toan nang IUdng.
Khi mdt he pha vd ddi xCfng x thi nang lUdng khdng bao toan. Pha vd ddi xUng tU phat xay ra la can thiet ve mat nang lUdng. Hay tUdng tUdng, vien kim cUdng tudng la vmh eCfu song dUdi nhiet dd rat cao cung bien thanh tro, tham ehf cd the thanh nhUng id den vi md dudi tac ddng eiJa thang giang lUdng tif.
Hnh the thtii gian the he triTdc va sau
Hieu Ung AC Josephson (lay tSn nha vat ly Brian Josephson) eho ta hinh anh mot gia dinh cac tinh the thdi gian. Hieu Ung xay ra khi ap dat mdt hieu the V Ien doan noi 2 vat lieu sieu dan (hinh 3). Trong thf nghiem nay, ta quan sat dUdc mot ddng dien xoay
Mot dong fflen xoay chieu xuat hien trong doan noi
w
Measured as akemating current acrtBS the junction
So 4 n a m 2 0 2 0 I B N O N G H I V i S N a i H
63
I KH&CN nude ngoai
chieu tan sd 2eV/h chay qua doan ndi. Ddi xUng i bj thu hep ve mot ddi xUng x nhd hdn vdi tan sd h/2eV.
Hieu ifng AC Josephson la mot pham tru cho khai niem tinh the thdi gian. Chu y, de nudi V phai cd ac quy, nhu vay khi ac quy chay thi nhiet nang tda ra.
Neu thay sieu dan bang sieu chay ta eung cd the thu dUde ket qua tUdng tU. Nhiing tinh the thdi gian kieu AC Josephson dupc ggi la tinh the thdi gian the he trudc. NhOng tinh the thdi gian tCf 2017 dUpe ggi la tinh the thdi gian mdi.
Thang 3/2017, Christopher Monroe (Dai hoc Maryland, My) da tao nen tinh the thdi gian gdm mot day ions ytterbium (Yb) bang each sif dung tia laser, nam cham, gUdng, thiet bi quang hgc, tac ddng Ien cae nguyen tif, khien chiing dao dpng ciing nhau, sau dd tach rieng nhOng nguyen tCf rieng le, bay chung bang dien tU, ehuyen sang budng chan khdng va chidu chung bang tia laser.
Thang 8/2017, Mikhail Lukin va cdng sU (Dai hpc Harvard, My) da tao nen tinh the thdi gian bang each tac ddng Ien mang kim cUdng nhu'ng xung laser ed ehu ky. GiiJa cac xung thi spin' tie'p tuc tUdng tac vdi nhau. Toan he thu dUpc mdt cau hinh chu ky, song chu ky nay khac vdi ehu ky ciia cac xung laser. Ndi each khae, he da vi pham ddi xUng tU phat (hinh 4).
Time >•
Thai gian
Xung w song Tirang \6c ^m vi sSng Microwave Intwactions Microwave p u l s e — I i p u l s e — i
c&u m\ spin cOa cdc lam vang N , trong Rim cirong Cau hinh spin rfi6i
Spinpallemof Mteraative nitrogen vacancy spin pattern centeis in diamonds
Hinh 4. Thi nghiem Lukin.
Cac thf nghiem neu tren da md ra hudng tao vit lieu mdi trong vat ly, song nhieu van de da dUdc dat ra: can ndi rdng pham tru nay den nhieu hien tUdng trong thien nhien; can xet phdi hdp sU vi pham ttf phat eija khdng gian va thdi gian; cung can xet den cac quasicrystals (tinh the chua dUde xep hang theo cau hinh lap lai) nhU chat Idng, kfnh thdi gian (time glass), chgt lieu cd dp cifng (rigidity) lan cac l$ch chuan nhd.
Ve T irong uu tru ua Id den
SiJ on dinh nhat djnh nao dd dan den bao toan i trong vu tru hgc. Trong cac md hinh, eae nha thi§n van cho rang vu tru bao toan doi xUng thdi gian.
Song chiing ta da biet vu try dang dan nd, md hinh trang thai vUng (steady-state) cho rang vat chat lu6n tao ra lam cho mat dp trung binh ciia vu trii dudng nhu khdng thay doi. Song md hinh viing dd cd th^
khdng sdng sot vdi thdi gian. Vu try hien nay khac vdi vu tru 13,7 ty nam trUdc. Trong y nghTa dd TCO the tu phat bj pha vd ve toan eye, Nhieu nha vu tru hgc cho rang, vu tru chiing ta la mdt vu try ehu kj (cyclic universe). NhOng y tudng dd gan vdi y tudngj ve tinh the thdi gian. Cd the chang ban than khong -•
thdi gian dUdi nhOng dieu kien ap suat, nhiet dd cao]
ciing mat di x- nghTa la mat dl djch chuyen thdi gian?l Nhu the tinh the thdi gian cd t h i giup chung ta hieu them vu tru va lo den tU mdt khia canh khac?
Giup Chung ta hieu vat chat mdt each khac hoac md rgng tam nhin cua chiing ta ve vat chat? Neu cd che tinh the thdi gian la mdt cd che pho quat trong vQ tru thi tinh the thdi gian co the la mot quan diem nhan thUe mdi ve vu try?
TAI LI|U THAM KHAO
1. Frank Wilczek (2019), "The exquisite precision of time crystals", Scientific American, 11, pp.31.
2, Krzysztof Sacha and Jakub Zakrzewski (2018), "Time crystals: A review". Reports on Progress in Physics. 81(1), article No.016401
'Spin !a lUcing chuyen dong quay ma mot vat co, xet ca khoi luong va hinh dang cua no. Day con dUdc goi la moment tJong iuong cua mot vat.
64 KHOAHOC I L k l
eiNG NGHE Viet IBI S6 4 nam 2020
