String Theory:String Theory:an unforeseeablean unforeseeable

historyhistory

G. G. VenezianoVeneziano(CERN et Coll(CERN et Collèège de France)ge de France)

What would seem a foreseeableoutline,

starting from the beginning,

is actually not such an easy one …

Weizmann Institute, 1967 ?

HD, HR, SF, MV, GV, ??, JD

CERN,1968?CERN,1968?

??

π

π

π

ω

?

1968 < t1968 < t0 0 < 1974?< 1974?

νν Counting of states,Counting of states, T THagedornHagedorn

νν Harmonic oscillators, stringHarmonic oscillators, string’’s preludes preludeνν Ghost-killing, vertex operators, Ghost-killing, vertex operators, VirasoroVirasoro

algebraalgebraνν NambuNambu--Goto Goto action, l.c. quantizationaction, l.c. quantizationνν NS & R ext.s, SUSY, GSO projection &NS & R ext.s, SUSY, GSO projection &

superstringsuperstringνν SherkSherk-Schwarz reinterpretation-Schwarz reinterpretation

A different approach:A different approach:what has been/iswhat has been/is

String TheoryString Theory’’ss

place in our understanding of the place in our understanding of thefundamental interactions?fundamental interactions?

A sketch of our understanding as aA sketch of our understanding as afunction of timefunction of time

ElectricElectric

WeakWeak

GravityGravity

MagneticMagnetic }}ClassicalClassicalElectro-Electro-MagnetismMagnetism(Maxwell)(Maxwell)

QEDQED

EWEW

NuclearNuclear QCDQCD

Classical Classical GeneralGeneralRelativityRelativity(Einstein)(Einstein)

SMSM

StringString 1968 1968

StringString 1984 1984

FermiFermi19341934

QGDQGD

tt

We shall followthe greengreen arrows

Neutron Neutron ββ--decaydecay

nnpp

dduu

WW--

eeνν

uudd

ππ−−

νν

µµ−−

WW--

eeνν

nn ppdddduu

uu

ππ−−

µµ−−

νν

EW theory EW theory FermiFermi

Charged-Charged-pion pion decaydecay

pointlikepointlikeinteractioninteraction

is the killeris the killer

No surprise, No surprise, a posterioria posteriori, that Fermi was, that Fermi wasled to his led to his effectiveeffective theory of theory of weakweak

interactionsinteractions

……and that such a theory cannot be validand that such a theory cannot be validat very short distancesat very short distances

What about the neutron, the proton, theWhat about the neutron, the proton, thepionpion??

I will claim that something similarI will claim that something similarhappened there for happened there for strong interactionsstrong interactions

From QCD to the old stringFrom QCD to the old string

E = M cE = M c22 = T r = T r (T= dE/dr = Tension)(T= dE/dr = Tension)

Quarks are held together by theQuarks are held together by the confining potential confining potential of a of acolourcolour-electric flux tube-electric flux tube

J = M v d < cJ = M v d < c-1-1 T r T r22 J < J < αα’’ M M22 ((αα’’ = c = c3 3 /2/2ππT) T)

quarkquark antiquarkantiquark antiquarkantiquarkquarkquark

rr

NaNaïïve ve classicalclassical picture: picture:

JJ

MM22

forbiddenforbidden

allowedallowed

J = J = αα’’ M M22

Smells of a dSmells of a dééjjàà vu vu……

The The linearly risinglinearly rising Regge Regge trajectories oftrajectories ofearly sixties!early sixties!

J/hJ/h

MM22

fermionsfermions

bosonsbosons

1

11/2

dJ/dMdJ/dM22 ~ 0.9 ~ 0.9 GeVGeV-2-2

ρ

Δπ

N

So far we So far we neglected Quantum Mechanicsneglected Quantum Mechanics……

νν Classical physicsClassical physics is valid for is valid for largelarge quantum quantumnumbers, highly excited levelsnumbers, highly excited levels

νν When the QCD When the QCD flux tubeflux tube is long compared to is long compared toits transverse size, it can be considered asits transverse size, it can be considered asone-dimensional, a one-dimensional, a string-likestring-like object object

νν Thus at large distances strings provide anThus at large distances strings provide aneffectiveeffective description of QCD description of QCD

No surprise that a theory of strings wasNo surprise that a theory of strings wasdiscovered starting from some strikingdiscovered starting from some striking

properties of the properties of the hadronic hadronic worldworld

……the surprise came elsewhere..the surprise came elsewhere..

WHAT WENT WRONG?WHAT WENT WRONG?

LargeLarge quantum effects occur at quantum effects occur at smallsmall quantum quantumnumbers, i.e. for the lightest statesnumbers, i.e. for the lightest states

They depend on exactly what kind of stringThey depend on exactly what kind of stringone is talking about, its thickness, whatone is talking about, its thickness, what

makes it, or sits at its endsmakes it, or sits at its ends

ThatThat’’s where thes where the QCD QCD and the oldand the old hadronic hadronicstring differ string differ substantially!substantially!

““Casimir Casimir EnergyEnergy”” of fundamental string of fundamental string

νν The zero-point energies of each harmonic oscillatorThe zero-point energies of each harmonic oscillatorin the string in the string addadd up, up, ……to something to something negativenegative

νν Massless Massless statesstates become possible, actually become possible, actuallyinevitableinevitable, even if they have , even if they have up to 2hup to 2h worth of worth of JJ..

νν States with States with half-integer spinhalf-integer spin are also necessary are also necessaryfor consistency => superstrings and discovery offor consistency => superstrings and discovery ofsupersymmetry supersymmetry in early seventiesin early seventies……

JJ

MM22

2h2h

Classical boundaryClassical boundary

fermionsfermions

TheThe quantum quantum spectrumspectrum(@small coupling)(@small coupling)

The badThe badquantum news: no such states inquantum news: no such states in hadronic hadronic physics !physics !

JJ

MM22

2h2h

3/2h3/2hhh

1/2h1/2h

ClassicallyClassically forbidden forbidden

Classically Classically allowedallowed

J/hJ/h

MM22

fermionsfermions

bosonsbosons

1

11/2

dJ/dMdJ/dM22 ~ 0.9 ~ 0.9 GeVGeV-2-2

ρ

Δπ

N

FurthermoreFurthermore……The string was The string was ““ITIT”” there was there was nothingnothing more basic more basic

insideinside, no , no internalinternal structure, no constituents structure, no constituents

QCD has QCD has quarks and gluonsquarks and gluons, its string is a , its string is a derivedderivedobject, endowed with a finite thickness, made ofobject, endowed with a finite thickness, made of

pointlikepointlike constituentsconstituents

The final verdict, as always, belonged to The final verdict, as always, belonged to experimentsexperiments

It came clear from the study of hard-processesIt came clear from the study of hard-processes

ITIT’’S QCD!S QCD!

Quarks and gluons are there and, till theseQuarks and gluons are there and, till thesedays, look like elementary, point-like objectsdays, look like elementary, point-like objectsThe F-string is just an The F-string is just an effectiveeffective description descriptionof the complex phenomenon of confinementof the complex phenomenon of confinement

Finding the Finding the true QCD string true QCD string is still an area ofis still an area ofintensive researchintensive research

More d.o.f.More d.o.f. than those of the naive D=4 string than those of the naive D=4 stringare needed (see coming talks..)are needed (see coming talks..)

The unforeseeable comebackThe unforeseeable comeback

∂∂ Casimir Casimir Energy of the F-StringEnergy of the F-StringThe The massless massless states of the superstring nicelystates of the superstring nicelyfit with what is required for the carriers offit with what is required for the carriers of

gauge and gravitational forcesgauge and gravitational forcesA A disasterdisaster for the old string becomes a for the old string becomes a firstfirsttremendous tremendous bonusbonus for a theory of all for a theory of allinteractionsinteractions

EMEM

WEAKWEAK

STRONGSTRONG

GRAV.GRAV.

FORCEFORCE

PLAYERSPLAYERSCARRIERSCARRIERS BRICKSBRICKS

W,ZW,ZGluonsGluons GraviGravi--tonton

γγ ννllChCh..qq ννRR ? ???

???? ??

HH

= = carriescarries = = feelsfeels

•• Lack of internal structureLack of internal structureννAbsence of point-like constituents in the Absence of point-like constituents in the string strongly inhibits hard processesstring strongly inhibits hard processesννIts characteristic Its characteristic quantumquantum size, size, lls s = = √h/Th/Texponentially damps exponentially damps pptt > h/l > h/lss

Quantum mechanics, rather than clashing withQuantum mechanics, rather than clashing withGeneral Relativity, frees gravity from itsGeneral Relativity, frees gravity from its(in)famous UV divergences:(in)famous UV divergences:

a a secondsecond extraordinary extraordinary bonusbonus!!

Can we have the cake and eat it?Can we have the cake and eat it?((avoiravoir le le beurre beurre etet l l’’argent du beurreargent du beurre))

We want the constituents when they areWe want the constituents when they areneeded and no internal structure where itneeded and no internal structure where it

would harm!would harm!

Recipe for the cake/string:Recipe for the cake/string:

νν scale downscale down the characteristic size the characteristic size llss by some by some 1919ordersorders of magnitude, from 10 of magnitude, from 10-13 -13 to to 1010-32-32 cm cm

νν Quarks (and Quarks (and all all other other playersplayers) are ) are ““slimslim”” F-stringsF-strings..Protons, neutron, Protons, neutron, pions pions are are ““fatfat”” QCD strings! QCD strings!

νν We have almost point-like quarks, but there is We have almost point-like quarks, but there is newnewphysicsphysics just above the just above the PlanckPlanck length length scalescale

llPP = = √GGNNhh/c/c3 3 ~ 10~ 10-33-33 cm cm

LetLet’’s have a quick look at some of them!s have a quick look at some of them!

String/Black-Hole phase diagram

M

RRS S = l= ls s (M = M(M = MP P gs-1 , S ~ gs

-2)

gs ~ gauge couplingStrings Strings ≠≠ BH BH

Black Holes (= Strings? )Black Holes (= Strings? )RRS S > l> lss

RRS S < l< lss

Fate of black hole evaporationFate of black hole evaporationthrough Hawking radiationthrough Hawking radiation

M

RS = ls

gs

StringsStrings

Black HolesBlack Holes

trajectory of evaporating BH

Would be singularity: avoided thanks to ls≠0?

Other phase transitions can also be studiedOther phase transitions can also be studied

M

RS = ls

gs

StringsStrings

Black HolesBlack Holes

Fate of Big Bang singularityFate of Big Bang singularity

νν Big bang Big bang singularitysingularity is is avoidedavoided too if the curvature too if the curvatureradius (basically Hradius (basically H-1-1) cannot shrink below l) cannot shrink below lss

νν Reopens question of the beginning of time andReopens question of the beginning of time andallows one to consider allows one to consider new cosmologiesnew cosmologies whereby a whereby along era long era preceded/preparedpreceded/prepared a big bang-like event a big bang-like event……

(see (see May 2004May 2004 issue of issue of Scientific American, LeScientific American, Le Scienze Scienze, , ororJune 2004June 2004 issue of issue of Pour la SciencePour la Science))

QuiQui

Present horizontime

Towards the Big Bang, t=0Towards the Big Bang, t=0

End of inflation

now

Present distance from remote cluster

space

STANDARD INFLATION

Η−1

HereHere

Our horizon todaytime

ls

end of pre-bang phase

now

Present distance from far cluster

space

PRE BIG BANG

Η−1

Towards empty spaceTowards empty space@ past infinity@ past infinity

Will all this bring our understandingWill all this bring our understandingof gravity, astrophysics andof gravity, astrophysics and

cosmology up to particle-physicscosmology up to particle-physicsstandards?standards?

Remains to be seenRemains to be seen…………but perhaps the coming talks willbut perhaps the coming talks will

already provide some hintsalready provide some hints