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St r i n g T h eo r y an d t h e Mu l t i v e r s e *

er i o u s A b o u t T h i s St u f f ?!

*O r : A r e T h ey R eal l y S

8. 225 / STS. 042, Ph ys i cs i n t he 20t h C ent ur y

Pr of es s or D a vi d K ai s e r , 9 N o v em ber 2020

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1. Q ua nt um

Gr a vi t y? 2. A M e s s y Lan d s cap e

3. A P a c k a g e Deal

ht tps://ocw.mit.edu/help/faq-fair-use/ 2

Th r e e o f t h e f o u r k n o w n f o r c e s o f n a t u r e a r e i n h e r e n t l y quant um - me c h a n i c a l ; g r a v i t y h a s b e e n t h e s t u b b o r n h o l d - out .

Hu n d r e d - Ye a r C h a l l e n g e

Ph ys i ci s t s ha v e l ong r ecogni zed th a t g r a v ity coul d be des cr i bed i n te r m s o f p a r tic le e x c h a n g e , w ith a ma s s l e s s , s p i n - 2 par t i cl e: t he “ gr a v it o n .”

M. B r o n s t e i n

Left: Image is in the public domain.

LI G O det ect ed cl as s i cal gr a vi t at i onal r adi at i on… st i l l n o e v i d e n c e f o r i n d i v i d u a l gr a vi t ons .

3

In fi n i ti e s R e tu rn

On e c a n mo d e l g r a v i t o n exc h an ge l i ke o t h er q u an t u m fie ld th e o r ie s :

g   = g   0 + h   ,

h  

Th a t l e a d s t o in f in it ie s . B u t unl i k e i n t he ot he r c a se s, no fi n i t e s e t of “ c ount e r t e r m s” c a n

rem o v e al l o f t h e i n f i n i t i es .

from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

4

Th e so u r c e fo r (q u a n tu m ) g r a v ita tio n a l e ffe c ts is ma s s (r a th e r th a n e le c tr ic c h a r g e ), s o v ir tu a l p r o c e s s e s d i v e r g e ev en m or e st ro n g l y wi t h m o m e n t u m k .

In fi n i ti e s R e tu rn

e ef f = e 0 + e vi r t u al

m ef f = m 0 + m vi r t u al

On e c a n mo d e l g r a v i t o n exc h an ge l i ke o t h er q u an t u m fie ld th e o r ie s :

g   = g   0 + h   ,

h  

Th a t l e a d s t o in f in it ie s . B u t unl i k e i n t he ot he r c a se s, no fi n i t e s e t of “ c ount e r t e r m s” c a n rem o v e al l o f t h e i n f i n i t i es .

 ef f =  0 +  vi r t u al + …

QE D Qu a n t u m G r a v i t y

“Ol d Sc h o o l ” Qu an t u m G r a vi t y i s not re norm a l i z a b l e .

5

St r i n gy I d e a s

flu x tu b e s .

Du r i n g t h e l a t e 1 9 6 0 s , w h i l e w o r k i n g on G eof f r ey C hew ’ s S - ma t r i x a p p r o a c h to th e s tr o n g n u c le a r fo r c e , s e v e r a l th e o r is ts b e g a n p la y in g w ith s tr in g - lik e

Left © Capucettorosso on Wikimedia Commons. Above

Th a t w o r k w a s q u i c k l y o v e r s h a d o w e d b y th e s u c c e s s o f th e q u a r k m o d e l a n d Q C D , by t h e m i d - 1970s : ‘ r egul ar ’ quant um f i el d th e o r y w a s b a c k , a n d fe w s a w th e n e e d fo r a s t r ange s t r i ng - lik e tr e a tm e n t o f th e nucl ear f or ce.

6

Wo r l d s h e e t s t o t h e R e s c u e

10 - 15 m

10 - 35 m

Ye t t h e s t r i n g t h e o r i s t s h a d

not i ced s om et hi ng f unn y: th e ir e q u a tio n s s e e m e d to cont ai n a m as s l es s , s pi n - 2 par t i cl e w i t hi n t he l o w - ener gy s pect r um .

Ma y b e t h e y h a d b e e n l o o k i n g a t t h e wr o n g s c a l e : s tr in g th e o r y w a s n ’ t a b o u t n u c le a r fo r c e s ; it m ig h t b e a b o u t gr a vi t y .

Mo r e o v e r , s t r i n g t h e o r y l o o k e d fi n i t e .

Po i n t p a r t i c l e s m e e t a t p o i n t - lik e v e r tic e s ;

1 - d s t r i ngs s w eep out wo r l d s h e e t s , a n d hence v er t i ces ar e sp re a d o u t in sp a c e t i m e . N o i n f i n i t e in te g r a ls !

7

Wo r l d s h e e t s t o t h e R e s c u e

10 - 15 m

10 - 35 m

Ye t t h e s t r i n g t h e o r i s t s h a d

not i ced s om et hi ng f unn y: th e ir e q u a tio n s s e e m e d to cont ai n a m as s l es s , s pi n - 2 par t i cl e w i t hi n t he l o w - ener gy s pect r um .

Ma y b e t h e y h a d b e e n l o o k i n g a t t h e wr o n g s c a l e : s tr in g th e o r y w a s n ’ t a b o u t n u c le a r fo r c e s ; it m ig h t b e a b o u t gr a vi t y .

Bu t t h e r e w a s a c a t c h : th e s im p le s t m o d e l re q u i re d 26 sp a c e t i m e d i m e n si o n s to r e m a in se l f - co n s i s t en t . M o d el s t h at i n co r p o r at ed “s u p e r s y m m e t r y ” ( f o r e v e r y p a r t i c l e w i t h h a l f - in te g e r s p in th e r e e x is ts a n in te g e r - sp i n

pa r t ne r ) re q u i re d “ o n l y ” 10 s pa ce t i m e

di m e ns i ons .

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St r i n g Re v o l u t i o n s

Wo r l d w i d e P h y s i c s P u b l i c a t i o n s , 1 9 7 0 - 2005

Al l Ph y s i c s a r t i c l e s / 1 0 0 St r i n g t h e o r y a r t i c l e s

1s t R e v . 2nd R e v .

9

Qu e s t i o n s ?

10

Leak y G r a vit y?

r

Us u a l a s s u m p t i o n : u n l e s s s o m e t h i n g s t r a n g e h a p p e n s , gr a vi t y w oul d beha v e ve r y di f f er ent l y i n a uni v er s e w i t h ext r a di m ens i ons .

N lin e s o f fo r c e tr a v e r s e

a ( d - 1) - su rf a c e :

N ~

A

1

r d -1

In 9 s p a tia l d im e n s io n s , th e fo r c e o f g r a v ity w o u ld fa ll a s 1 / r 8 !

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Cu r l ’ Em U p

Th e e x t r a d i m e n s i o n s c o u l d b e h i d d e n f r o m v i e w i f t h e y we r e co m p acti fi ed — cu rl ed u p t o v ery s m al l s cal es .

Th i s i s s t r a i g h t f o r w a r d t o do w i t h one ext r a di m e nsi on.

Bu t wi t h si x ext r a d i m en s i o n s , t h ere are ~ 10 5 di st i nc t possi bi l i t i e s. ( 1986)

12

…

No t a P r e t t y L a n d s c a p e

Th e p r o b l e m g e t s e v e n w o r s e ( 2 0 0 3 ) : flu x e s a n d m e m b r a n e s c a n w r a p a r o u n d th e s h r u n k e n d im e n s io n s , y ie ld in g a t le a s t 10 500 di s t i nct v acuum s t at es .

Ot h e r c o s mi c n u mb e r s :

Da v i d K a i s e r ( $)

age o f u n i v er s e = 10 17 s

ma s s o f g a l a x y

Je f f B e z o s ( $ ) = 1 0 5

ma s s o f e l e c t r o n

= 1 0 71

13

Ev er y obs er v abl e quant i t y of our uni v er s e ( m i , g i , H ,  , … ) w o u ld d e p e n d on pr eci s el y w hi c h of t hes e v acua our uni v er s e “ l anded” i n. So w hat pi c ked out our s ?

more information, see ht tps://ocw.mit.edu/help/faq-fair-use/

A. G u t h

Mu l t i v e r s e

Et e r n a l I n f la t io n :  < H . E v e n a s in fla tio n ends w i t hi n one r egi on of s pacet i m e, t he vo l u m e o f o t h e r r e g i o n s c o u l d c o n t i n u e t o gr o w exponent i al l y .

On c e i n f l a t i o n b e g i n s so m e p l a c e , i t c o u l d c o n tin u e fo r e v e r !

A mu l t i v e r s e co u l d exi s t , popul a t e d b y a n in f in it e se t of se pa r a t e bubbl e uni v e r se s.

a gi v en bubbl e uni v er s e.

Al l 10 500 di s t i nct s t r i ng v acuum st a t e s m i g h t b e re a l i z e d an i nf i ni t e num b er of t i m es , o n e e a c h w ith in

Guth © source unknown. Multiverse (above) © Andrei Linde / Stanford. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

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Ev i d e n c e o f D e s i gn ?

If an y o f th e p ar am eter s ( m i , g i , H ,  , … ) w er e ev en sligh tly d iffer en t, life as w e kn o w it p r o b ab ly co u ld n o t exist.

A v e ry o l d a r g u m e n t!

Image is in the public domain.

Be r n a r d d e F o n t e n e l l e , Co n v e r s a t i o n s o n t h e Pl ur a l i t y of W or l d s (1 6 8 6 )

Images are in the public domain.

Is a a c N e w to n , le tte r s to

th e o lo g ia n R ic h a r d B e n tle y , 1690s .

15

An t h r o p i c Re s p o n s e

An t h r o p i c p r i n c i p l e : T h e n a tu r a l c o n s ta n ts in o u r obs er v abl e uni v er s e had t o f al l w i t hi n nar r o w r anges ; ot her w i s e no cos m ol ogi s t s w oul d be her e t o as k about t hem .

Cr i t i c s : “ d a n g e r o u s ” ; “d i s ap p o i n t i n g”; “a vi r u s ”; “an ab d i cat i o n ”…

Wi t h 10 500 di s t i nct ph ys i cal st a t e s o u t t h e re , e a c h re a l i z e d a n in fin ite n u m b e r o f tim e s , pur e ra n d o m c h a n c e coul d be enough to “ e x p la in ” w h y w e h a p p e n e d to ev ol v e w her e w e di d.

Su s s ki n d ’ s ep i gr ap h :

“ Y o u r H i g h n e s s , I h a v e n o n e e d of t hi s h y pot he s i s . ”

16

Qu e s t i o n s ?

17

A P a c k a ge D e a l

Str in g th eo r y h as so me gr eat featu r es. Bu t it co mes wi t h s o m e s t r a n g e f e a t u r e s a s we l l :

• I t r e q u i r e s a n a s - ye t undet ect ed s ym m et r y am ong al l t he kno w n par t i cl es ( “ s uper s ym m et r y” ) .

• I t c a n o n l y b e f o r m u l a t e d i n N > 4 s p a c e t i m e d i m e n s i o n s . Th e m o s t p o p u l a r v a r i a n t s h a v e N = 1 0 o r 1 1 .

• An d , d e s p i t e t h e fo u n d e r s ’ h o p e s , it le a d s to a hug e num ber of pos s i bl e mo d e l s , w i t h ( a s y e t ) n o wa y t o c h o o s e b e t w e e n th e m .

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St r i n gs a n d t h e Ac a d e m y

US P h y s i c s P h . D. s , 1 9 7 5 - 2005

1s t R e v .

2nd R e v .

19

A H o l e i n T e x a s : T h e SSC

Co n g r e s s a p p r o v e d fu n d in g in 1 9 8 5

Pr oj e c t e d c os t : 1 Bi g D i g (~ $ 1 5 B)

20

- 50% i n

1 year !

SSC f u n d i n g

be gi ns

SSC

can cel l ed

Th e R i s e a n d F a l l o f t h e S S C

Co n g r e s s v o t e d to k ill th e S S C in Oc t 1 9 9 3 ; $ 3 B

had al r ead y been s pent .

To t a l U S Ph y s i c s Fundi ng , 1993 t o 1999:

DO E : - 25%

DO D: - 58%

NS F : - 5%

21

De a t h o f S S C a n d L i f e o f S t r i n g T h e o r y

22

St r i n gs Ar o u n d t h e W o r l d

Pe r c e n t a g e c h a n g e b e t w e e n d e c a d e a v e r a g e s , 1 9 8 5 - 94 v s . 1995 - 04

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Mi l l e n n i a l P h y s i c s

Ear l y i n t he 21s t cent ur y , ph ys i cs s eem s j us t as w r apped up i n th e flo w o f c u ltu r e a n d p o litic s a s it d id th r o u g h o u t th e 2 0 th cent ur y .

Images are in the public domain.

Ph ys i ci s t s em er ged f r om W or l d W ar I I w i t h s i gni f i cant pol i t i cal cl out . T hat kept t he m oney f l o w i ng and pr o vi ded co v er agai ns t va r i o u s c r i t i q u e s . A f t e r t h e C o l d W a r , t h e p o l i t i c a l a n d c u l t u r a l la n d s c a p e h a s c h a n g e d .

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Ke e p o n T r u c k i n ’ …

Po s t e d o n c r e a t i o n i s t w e b s i t e o n e w e e k af t er o u r art i cl e w as p u b l i s h ed :

“W e h a d t o s h o w y o u i n t h e i r o w n wo r d s w h a t t h e s e M I T e g g h e a d s a r e sa y i n g . … Gu t h a n d K a i s e r n e e d t o t a k e up t r uc k dr i v i ng . Th a t w o u l d g e t t h e m out of t he i r i v or y t o w e r s a t M I T a nd i nt o th e r e a l w o r ld , w h e r e th e y w o u ld b e fo r c e d to lo o k a t tr e e s , m o u n ta in s ,

we a t h e r , e c o l o g y a n d a l l t h e o t h e r obs e r v a bl e th in g s o n o u r p r i v ile g e d p la n e t th a t a r e in e x p lic a b le b y c h a n c e : r e a litie s th a t p r o c la im de s i g n , pu r po s e , i n t e n t i o n .”

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STS.042J / 8.225J Einstein, Oppenheimer, Feynman: Physics in the 20th Century

Fall 202 0

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