Reconstructing Spacetime from Entanglement

Transcription

1 Reconstructing Spacetime from Entanglement Hal Haggard in collaboration with Eugenio Bianchi October 29th, 2013 High Energy Physics Seminar Radboud University, Nijmegen

2 Finite region quantum gravity What is the physics of a finite region of a quantum spacetime? Finite regions can incorporate diffeos of GR into a quantum context avoid notorious difficulties, e.g. [Arkani-Hamed et al]. 1

3 Entanglement thermality Entanglement leads to even finite regions of spacetime being hot. We illustrate this claim with a spherical entangling surface. 2

4 Entanglement insights Casini, Huerta & Myers have extensively studied the entanglement entropy of spherical causal domains. [Casini, Huerta & Myers 11] Bianchi has been shedding interesting light on black holes through entanglement. [Bianchi 1-12, 2-12] Disappearance of distinction between statistical and quantum fluctuations. [Bianchi, HMH, Rovelli 13] Focus on QFT while aiming for spin networks and loop gravity. 3

5 Outline 1 Introduction to loop gravity and the discreteness of space 2 What kind of entanglement? 3 Where (and when) is the region? 4 Why the entanglement spectrum? 4

6 Loop Quantum Gravity Briefly construct Hilbert space of loop gravity: H. Similarities to Fock space of QED and to lattice gauge theory (e.g. QCD). Built on graphs: L links l N nodes n source and target: s : l s(l) and t : l t(l) Graph Γ 5

7 Fock Space Massive scalar field: One particle: H 1 = L 2 (M ), M the Lorentz hyperboloid. n particles, H n = L 2 (M n )/ with permutations. Factorization symmetrizes states. All states up to N particles N H N = H n. n=0 Fock space H Fock = lim N H N. 6

8 Lattice Gauge Theory Lattice Γ with L links l, N nodes n and gauge group G H Γ = L 2 (G L ). States ψ(h l ) H Γ acted on by gauge transformations ψ(h l ) ψ(g s(l) h l g 1 t(l) ), g n G. Gauge invariant Hilbert space is H Γ = L 2 (G L /G N ). 7

9 Loop Quantum Gravity General graph Γ, called a spin network, an SU (2) lattice gauge theory. If Γ Γ then H Γ H Γ. H Γ = L 2 (SU (2) L /SU (2) N ), H Γ = H Γ / H = lim Γ H Γ 8

10 Loop Quantum Gravity General graph Γ, called a spin network, an SU (2) lattice gauge theory. If Γ Γ then H Γ H Γ. H Γ = L 2 (SU (2) L /SU (2) N ), H Γ = H Γ / H = lim Γ H Γ 9

11 Physical Picture Quanta of gravity are grains or chunks of space 10

12 Volume Polyhedral Volume: [Bianchi, Doná and Speziale] ˆV Pol = The volume of a quantum polyhedron 11

13 Minkowski s theorem: polyhedra The area vectors of a convex polyhedron determine its shape: A A n = 0. 12

14 Minkowski s theorem: polyhedra The area vectors of a convex polyhedron determine its shape: A A n = 0. Only an existence and uniqueness theorem. 13

15 Minkowski s theorem: a tetrahdedron Interpret the area vectors of tetrahedron as angular momenta: A 1 + A 2 + A 3 + A 4 = 0 For fixed areas A 1,..., A 4 each area vector lives in S 2. Symplectic reduction of (S 2 ) 4 gives rise to the Poisson brackets: {f, g} = 4 A l l=1 ( f A g ) l A l 14

16 Minkowski s theorem: a tetrahdedron For fixed areas A 1,..., A 4 p = A 1 + A 2 q = Angle of rotation generated by p: {q, p} = 1 15

17 Bohr-Sommerfeld Quantization of Harmonic Osc. Require: J = pdq = (n + 1 γ 2 )2π. 16

18 Dynamics Take as Hamiltonian the Volume: H = V 2 = 2 9 A 1 ( A 2 A 3 ) Area of orbits given in terms of complete elliptic integrals, ( 4 ) 4 J(E) = a i K(m) + b i Π(αi 2, m) E i=1 i=1 17

19 V Tet = A 1 ( A 2 A 3 ) A 1 = j + 1/2 A 2 = j + 1/2 A 3 = j + 1/2 A 4 = j + 3/2 4 2 = Numerical = Bohr-Som [PRL 107, ]

20 Table j 1 j 2 j 3 j 4 Loop gravity Bohr- Accuracy Sommerfeld % % % % % % % % % % % %

21 Outstanding challenge How to identify the ground state of a general relativistic theory? Want to coordinate individual grains of space to recover Minkowski space from this quantum theory. Use entanglement 20

22 Outline 1 Introduction to loop gravity and the discreteness of space 2 What kind of entanglement? 3 Where (and when) is the region? 4 Why the entanglement spectrum? 21

23 Boundary conditions for the remainder For simplicity I restrict to: Scalar field ϕ(x), with m = 0 on flat D = spacetime unless otherwise stated. Metric signature (, +, +, +). Results apply to any conformal field theory. 22

24 Entanglement Pure state Ψ H A H B. Schmidt decomposition: Ψ = i λ i i A i B with i A and i B orthonormal bases in H A and H B respectively. Leads to the reduced density matrix ρ B = Tr A Ψ Ψ = λ 2 i i B i B i 23

25 Entanglement entropy S E Tr ρ B log ρ B = i λ 2 i log λ 2 i For example, Ψ = D i 1 D i A i B = S E = log D Interested in bipartite entanglement of pure states. But, S E is a single number encodes limited information... 24

26 Entanglement spectrum Can always write ρ B = e H E, i.e. H E log ρ B, the entanglement Hamiltonian". Already diagonalized H E : ρ B = i e ɛ i i B i B with ɛ i (i = 1, 2,... ) [λ 2 i = e ɛ i ] the entanglement spectrum. Provides thorough understanding of entanglement. Study for spacetime fields. [Li & Haldane 08] 25

27 Outline 1 Introduction to loop gravity and the discreteness of space 2 What kind of entanglement? 3 Where (and when) is the region? 4 Why the entanglement spectrum? 26

28 Spherical causal domain Cauchy development of 3-ball with boundary 2-sphere of radius R: Spatial 3-ball B Cauchy development D(B). Entangling surface the boundary 2-sphere B = S 2. Choose adapted coordinates that preserve S 2 : similar to how polar coords fix (0, 0)... 27

29 Diamond coordinates Use hyperbolas Diamond coords (λ, σ, θ, φ): sh λ t = R ch λ + ch σ sh σ r = R ch λ + ch σ with λ (, ), σ [0, ). The Minkowski metric becomes ds 2 R 2 = (ch λ + ch σ) 2 [ dλ2 + dσ 2 + sh 2 σdω 2 ], a conformal rescaling of static κ = 1 FRW. 28

30 Conformal completion Diamond coordinates can be extended to all of Minkowski space E.g. region II: sh t = R λ ch σ ch λ, sh σ r = R ch σ ch λ λ σ. 29

31 Rindler Limit Large σ limit: t = 2Re σ sh λ = l sh λ R r = 2Re σ ch λ = l ch λ coord transformation to (left) Rindler wedge. The proper distance from right corner is l = 2Re σ. 30

32 Current Congruence ξ µ = ( λ) µ with current J µ = T µν ξ ν, µ J µ = 0 ( µ T µν )ξ ν + T µν µ ξ ν = T µν (µ ξ ν). ξ µ is a conformal Killing = µ J µ = 1 2 θt µ µ (θ = ρ ξ ρ ). For dilatation invariant field theory T µ µ = 0 (on shell) so and J µ is a Nöther current. µ J µ = 0, 31

33 Entanglement (or foliation) Hamiltonian The conserved charge is C = R Tµν ξ µ dσν with Tµν the stress-tensor. It generates the spatial foliation discussed above: Explicitly this charge is, Cin = 1 2R Z B r 2 dr d Ω 1 2 ~ ϕ) ~ (R2 r 2 )(ϕ 2 + ϕ + ϕ2 32

34 Outline 1 Introduction to loop gravity and the discreteness of space 2 What kind of entanglement? 3 Where (and when) is the region? 4 Why the entanglement spectrum? 33

35 Density matrix from Euclidean path integral Minkowski vacuum: Spherical density matrix: Rindler density matrix: sin λe cos λe + ch σ Bipolar sh σ coords r =R cos λe + ch σ te = R Z ρb = Dϕe SE = e 2πCin 34

36 Temperatures ρ B = e 2πC in /Z: thermal density matrix satisfying KMS condition. 2) Martinetti-Rovelli temperature T MR = 1 dλ 2π dτ = 1 (ch λ+ch σ) 2π R ) Geometric temperature T G = 1 2π ds 2 = R 2 (ch λ+ch σ) 2 [ dλ 2 + dσ 2 + sh 2 σdω 2 ] 3) Thermometer (Unruh) temp. T U = a 2π = 1 sh σ 2π R [M & R 02, Wong et al 13] Thermometer measures T µν ξ µ u ν The region is not clearly in equilibrium. [Chirco, HMH & Rovelli 13] 35

37 Conformal symmetry Curved spacetime Lagrangian density (D=4 spacetime dimensions) L = 1 2 g[g µν µ ϕ ν ϕ + (m R(x))ϕ2 ]. For ḡ µν = Ω 2 (x)g µν and ϕ = Ω(x) 1 ϕ the m = 0 action is invariant. With ḡ µν = η µν the EOM transform as ϕ = Ω 3 [ 1 6 R]ϕ, with = g µν µ ν. Using this, sphere modes are ū I k(x) = (2k) 1 2 R (ch λ + ch σ)π kj (σ)y M J (θ, φ)e ikλ. 36

38 Sphere vacuum and spherons Minkowski case: ϕ(x) = d D 1 k[a k u k (x) + a k u k (x)] The vacuum satisfies a k 0 M = 0. Cut out all in-out entanglement to get sphere vacuum Sphere case: ϕ(x) = k [si k ui k + si k ui k The sphere vacuum satisfies sk I 0 S = s II + sii k uii k + sii k k 0 S = 0. s I k creates spherons, excitations localized within the spherical entangling surface. uii k ] 37

39 Two-point functions Leveraging conformal symmetry we can achieve a more substantial characterization of the vacuum, finding its two-point functions: D + (x, x ) = Ω D 2 2 (x) D+ (x, x )Ω D 2 2 (x ) 1+1 spacetime: D + (x, x ) = 1 4π log λ2 + σ spacetime: D + (x, x ) = (ch λ + ch σ) σ(ch λ + ch σ ) 4π 2 R 2 sh ( σ) [ λ 2 + σ 2 ] 38

40 Sorkin-Johnston vacuum Recently researchers working on causal sets have been investigating a remarkable state, the Sorkin-Johnston vacuum: [Johnston 09, Sorkin 11] proposed vacuum for a causal set within a causal diamond. It is defined only through referencing the diamond s interior has no entanglement with outside and so does not satisfy the Reeh-Schlieder theorem. Afshordi et al have just investigated the Sorkin-Johnston vacuum in 2D analytically and numerically... [Afshordi et al 12] 39

41 Comparison... and amongst many other things they found a surprise: near the L and R diamond corners the Sorkin-Johnston two-point function is that of a static mirror in Minkowski spacetime. How does the sphere vacuum compare? In the limit λ = λ = 0, σ, σ : D + 2D 1 4π log (log l l ) D + 4D 1 2π(l 2 l 2 ) log l l The Rindler two-point functions. [Troost & Van Dam 79] (Recall l = 2Re σ is the proper distance from the right corner.) = the sphere and Sorkin-Johnston vacuums differ in 2D. Is this still true in 4D? 40

42 Entanglement spectrum Let us return to considering the Minkowski vacuum 0 M. Explicitly constructed the entanglement Hamiltonian diagonalize it to recover the entanglement spectrum λ i = e ɛ i/2 and 0 M = i λ i i in S i out S. This shows that despite the absence of in-out entanglement you can recover the Minkowski vacuum through a well-tuned superposition of sphere states. 41

43 Spin network entanglement This suggests an intriguing perspective on spin networks. Individual nodes are like the interior of the sphere with no entanglement between a given node and its neighbors. Can we choreograph entanglement to yield the Minkowski vacuum? A wealth of condensed matter research on entanglement to draw from (MPS, PEPs, etc). 42

44 Conclusions Interesting connections to causal sets. What is the nature of the Sorkin-Johnston vacuum in 4D? Exhibited an example outside the hypotheses of the Reeh-Schlieder theorem. Looking to engineer the Minkowski vacuum and its entanglement from spin network superposition. Numerous possibilities More general entangling surfaces Deeper insights from condensed matter Anomalies 43

45 Credits Burning orb image: Spherical spin network: Z. Merali, The origins of space and time, Nature News, Aug. 28, 2013 Special thanks to the Cracow School of Theoretical Physics, LIII Course, 2013 for a lovely school and setting while this work was begun. And to the Perimeter Institute for their gracious hosting of visitors and support during the continuation of this work. 44