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String Theory Preprint typeset in JHEP style - PAPER VERSION STRING THEORY Paul K. Townsend Department of Applied Mathematics and Theoretical Physics Centre for Mathematical Sciences, University of Cambridge Wilberforce Road, Cambridge, CB3 0WA, UK. Email: [email protected] Abstract: Part III Course Notes. 24 Lectures. Contents 1. Why study String Theory? 2 2. The relativistic point particle 3 2.1 Gauge invariance 3 2.2 Hamiltonian formulation 4 2.2.1 Gauge invariance and first-class constraints 8 2.2.2 Gauge fixing 9 2.2.3 Continuous symmetries and Noether's theorem 10 2.2.4 Quantization: canonical and Dirac's method 12 3. The Nambu-Goto string 13 3.1 Hamiltonian formulation 15 3.1.1 Alternative form of phase-space action 16 3.1.2 Gauge invariances 16 3.1.3 Symmetries of NG action 17 3.2 Monge gauge 18 3.3 Polyakov action 19 3.3.1 Relation to phase-space action 20 3.4 Conformal gauge 21 3.4.1 \Conformal gauge" for the particle 23 3.4.2 Residual invariance of the string in conformal gauge 24 3.4.3 Conformal invariance 26 3.5 Solving the NG equations in conformal gauge 28 3.6 Open string boundary conditions 30 3.7 Fourier expansion: closed string 31 3.8 Open string 34 3.8.1 Fourier expansion: Free-ends 34 3.8.2 Mixed free-end/fixed-end boundary conditions 36 3.9 The NG string in light-cone gauge 38 3.9.1 Light-cone gauge for mixed Neumann/Dirichlet b.c.s 40 3.9.2 Closed string in light-cone gauge 40 4. Interlude: Light-cone gauge in field theory 41 4.0.3 Maxwell in light-cone gauge 41 4.0.4 Linearized Einstein in light-cone gauge 42 { 1 { 5. Quantum NG string 43 5.1 Light-cone gauge quantization: open string 43 5.1.1 Critical dimension 45 5.1.2 Quantum string with mixed N/D b.c.s 47 5.1.3 Quantum closed string 48 5.2 \Old covariant" quantization 49 5.2.1 The Virasoro constraints 53 6. Interlude: Path integrals and the point particle 56 6.1 Faddeev-Popov determinant 57 6.2 Fadeev-Popov ghosts 59 6.3 BRST invariance 62 6.4 BRST Quantization 63 7. BRST for the NG string 66 7.1 Quantum BRST 69 7.1.1 Critical dimension again 71 8. Interactions 73 8.1 Ghost zero modes and Sl(2; C) 74 8.2 Virasoro-Shapiro amplitude from the path integral 76 8.2.1 The Virasoro amplitude and its properties 80 8.3 Other amplitudes and vertex operators 82 8.3.1 The dilaton and the string-loop expansion 84 8.4 String theory at 1-loop: taming UV divergences 88 8.5 Beyond String Theory 90 1. Why study String Theory? • Because it's a consistent theory of perturbative quantum gravity, i.e. of gravi- tons. But the perturbation theory has zero radius of convergence, and so doesn't actually define a \theory". You need M-theory for that, but that's not yet a \theory" either. • As a possible UV completion of the standard model. But the \landscape prob- lem" makes it unclear whether String Theory will have any predictive power. • Because it has applications to everything else (GR, QCD, fluid dynamics, con- densed matter, . ) via AdS/CFT correspondence. But maybe you don't really need String Theory. { 2 { • Because it lies on the borderline between the trivial and the insoluble. It's the \harmonic oscillator of the 21st century". • None of the above. 2. The relativistic point particle What is an elementary particle? • (Maths) \A unitary irrep of the Poincar´egroup". These are classified by mass and spin. • (Physics) \A particle without structure". The classical action for such a particle should depend only on the geometry of its worldline (plus possible variables describing its spin). Let's pursue the physicist's answer, in the context of a D-dimensional Minkowski space-time. For zero spin the simplest geometrical action for a particle of mass m is Z B Z Bp Z tB p dx I = −mc2 dτ = −mc −ds2 = −mc −x_ 2 dt ; x_ = ; (2.1) A A tA dt where t is an arbitrary worldline parameter. In words, the action is the elapsed proper time between an initial point A and a final point B on the particle's worldline. We could include terms involving the extrinsic curvature K of the worldline, which is essentially the D-acceleration, or yet higher derivative terms, i.e. " # Z p `K 2 I = −mc dt −x_ 2 1 + + ::: ; (2.2) c2 where ` is a new length scale, which must be characteristic of some internal structure. In the long-wavelength approximation c2K−1 ` this structure is invisible and we can neglect any extrinsic curvature corrections. Or perhaps the particle is truly elementary, and ` = 0. In either case, quantization should yield a Hilbert space carrying a unitary irrep of the Poincar´egroup. For zero spin this means that the 2 particle's wavefunction Ψ should satisfy the Klein-Gordan equation ( − m ) Ψ = 0. There are many ways to see that this is true. 2.1 Gauge invariance Think of the particle action Z p I[x] = −mc dt −x_ 2 (2.3) { 3 { as a \1-dim. field theory" for D \scalar fields” xm(t)(m = 0; 1; ··· D − 1). For a different parameterization, with parameter t0, we will have \scalar fields” x0(t0), s.t. x0(t0) = x(t). If t0 = t − ξ(t) for infinitesimal function ξ, then 0 x(t) = x (t − ξ) = x(t − ξ) + δξx(t) = x(t) − ξx_(t) + δξx(t) ; (2.4) and hence δξx(t) = ξ(t)_x(t) : (2.5) This is a gauge transformation with parameter ξ(t). Check: p 1 d(δ x) 1 2 ξ _ 2 δξ −x_ = −p x_ · = −p ξx_ + ξx_ · x¨ −x_ 2 dt −x_ 2 p p d −x_ 2 d p = ξ_ −x_ 2 + ξ = ξ −x_ 2 ; (2.6) dt dt so the action is invariant for any ξ(t) subject to the b.c.s ξ(tA) = ξ(tB) = 0. The algebra of these gauge transformations is that of Diff1, i.e. 1-dim. diffeomorphisms (maths) or 1-dim. general coordinate transformations (phys). Gauge invariance is not a symmetry. Instead it implies a redundancy in the description. We can remove the redundancy by imposing a gauge-fixing condition. For example, in Minkowski space coords. (x0; ~x) we may choose the \temporal gauge" x0(t) = c t : (2.7) 0 0 Since δξx = c ξ when x = c t, insisting on this gauge choice implies ξ = 0; i.e. no gauge transformation is compatible with the gauge choice, so the gauge is fixed. In this gauge Z Z 1 I = −mc2 dtp1 − v2=c2 = dt −mc2 + mv2 1 + O v2=c2 ; (2.8) 2 where v = j~x_ j. The potential energy is therefore the rest mass energy mc2, which we can subtract because it is constant. We can then take the c ! 1 limit to get the non-relativistic particle action 1 Z I = m dt jd~x=dtj2 : (2.9) NR 2 From now on we set c = 1. 2.2 Hamiltonian formulation p If we start from the gauge-invariant action with L = −m −x_ 2, then @L mx_ p = = p ) p2 + m2 ≡ 0 : (2.10) @x_ −x_ 2 { 4 { So not all components of p are independent, which means that we cannot solve for x_ in terms of p. Another problem is that mx_ 2 p H =x _ · p − L = p + m −x_ 2 ≡ 0 ; (2.11) −x_ 2 so the canonical Hamiltonian is zero. What do we do? Around 1950 Dirac developed methods to deal with such cases. We call the mass-shell condition p2 + m2 = 0 a \primary" constraint because it is a direct consequence of the definition of conjugate momenta. Sometimes there are \secondary" constraints but we will never encounter them. According to Dirac we should take the Hamiltonian to be the mass-shell constraint times a Lagrange multiplier, so that Z 1 I = dt x_ · p − e p2 + m2 ; (2.12) 2 where e(t) is the Lagrange multiplier. We do not need to develop the ideas that lead to this conclusion because we can easily check the result by eliminating the variables p and e: • Use the p equation of motion p = e−1x_ to get the new action 1 Z I[x; e] = dt e−1x_ 2 − em2 : (2.13) 2 At this point it looks as though we have `1-dim. scalar fields” coupled to 1- dim. \gravity", with \cosmological constant" m2; in this interpretation e is the square root of the 1-dim. metric, i.e. the \einbein". p • Now eliminate e from (2.13) using the e equation of motion me = −x_ 2, to p get the standard point particle action I = −m R dt −x_ 2. Elimination lemma. When is it legitimate to solve an equation of motion and sub- stitute the result back into the action to get a new action? Let the action I[ ; φ] depend on two sets of variables and φ, such that the equation δI/δφ = 0 can be solved algebraically for the variables φ as functions of the variables , i.e. φ = φ( ). In this case δI ≡ 0 : (2.14) δφ φ=φ( ) The remaining equations of motion for are then equivalent to those obtained by variation of the new action I^[ ] = I[ ; φ( )], i.e.
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