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![Page 1: Thoughts(on(Quantum(Gravity( - DAMTP · Thoughts(on(Quantum(Gravity(David(Tong(University(of(Cambridge ... Kapustin and Strassler, 9902033 3 Jacobson(Whatwe(don’tunderstand(Informaon(Paradox](https://reader031.fdocuments.us/reader031/viewer/2022022603/5b5c78827f8b9ad2198c7af0/html5/thumbnails/1.jpg)
Thoughts on Quantum Gravity
David Tong University of Cambridge
The Structure of Gravity and Space<me, Oxford, February 2014
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Gravity is Weak
D-BRANES ON ALF SPACES
Gµν =1
M2pl
Tµν − Λgµν
Mpl ≈ 1027 eV
Λ ≈ (10−3 eV )4
Λobserved = Λbare + Λinduced
Λobserved ≈ (10−3 eV )4
Λinduced ≈ (1012 eV )4 (1)
= 1060 Λobserved
Λinduced ≈ (1012 eV )4 (2)
= 10120 Λobserved
Λinduced ≈ (1012 eV )4
Λbare ≈ −(1012 eV )4
�E =e�̂r
4πr2
Λinduced ≈ 1060 Λobserved
Λbare ≈ −1060 Λobserved
Energy =
�d3r �E · �E
=
� ∞
adr
e2
4πr2=
e2
4πa
e2
4πa< mec
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
1
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…Really Weak
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
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How to Overcome This?
• Large extra dimensions? • Planck suppressed operators in infla<on? • Large number of par<cles? • Long periods of space/<me?
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Why do we think it’s even possible to talk about this?
![Page 6: Thoughts(on(Quantum(Gravity( - DAMTP · Thoughts(on(Quantum(Gravity(David(Tong(University(of(Cambridge ... Kapustin and Strassler, 9902033 3 Jacobson(Whatwe(don’tunderstand(Informaon(Paradox](https://reader031.fdocuments.us/reader031/viewer/2022022603/5b5c78827f8b9ad2198c7af0/html5/thumbnails/6.jpg)
A Naïve AUempt at Quantum Gravity
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(in units with )
What goes wrong with the obvious approach?
(This usually goes by the more scary sounding name of “non-‐renormalisability”)
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)−2
c = � = 1
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)−2
c = � = 1
GE2
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
Dimensionless coupling is
Theory is weakly coupled in the IR and strongly coupled in the UV. We can only use it to compute IR quan<<es.
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
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So what happens in the UV?
• New degrees of freedom • String theory • (Also chiral lagrangian for pions, Fermi theory)
• A strongly interac<ng fixed point • Asympto<c safety
• Explicit Cut-‐Off • Causal Dynamical Triangula<on • Loop quantum gravity? • Causal set theory • (Also many condensed maUer contexts)
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A Toy Model: 5d Yang-‐Mills
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
• Theory is weakly coupled in the IR. • It is described by non-‐linear classical field theory
• It is strongly coupled in the UV • And non-‐renormalisable
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5d Yang-‐Mills with a cut-‐off
Lafce spacing: a Gauge coupling: g2
Lafce gauge theory in 5 Euclidean dimensions
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Different Phases
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
Energy Energy
a-‐1
g-‐2
-‐
-‐ -‐
-‐
a-‐1
g-‐2
• Strongly coupled at cut-‐off • In confining phase in IR
• Weakly coupled at cut-‐off • 5d Yang-‐Mills at low energies • Looks nothing like YM at scale g2
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High Energies in Gravity
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High Energies in Gravity
Collide two par<cles at energies much greater than Ep. You create a black hole.
We don’t need quantum gravity to tell us this. Classical gravity is enough. All the quantum stuff is hidden behind the horizon, near the singularity.
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ScaUering in Non-‐Renormalisable Theories
Energy
g-‐2 or lower -‐
known
unknown
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ScaUering in Gravity
Energy
G-‐1/2 -‐
known
unknown
known*
What does this mean?
*Un<l black hole evaporates!
![Page 16: Thoughts(on(Quantum(Gravity( - DAMTP · Thoughts(on(Quantum(Gravity(David(Tong(University(of(Cambridge ... Kapustin and Strassler, 9902033 3 Jacobson(Whatwe(don’tunderstand(Informaon(Paradox](https://reader031.fdocuments.us/reader031/viewer/2022022603/5b5c78827f8b9ad2198c7af0/html5/thumbnails/16.jpg)
A Low Energy Handle on Fundamental Degrees of Freedom
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Black Hole Thermodynamics
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
Bekenstein; Hawking
Likely interpreta<on: eS counts number of black hole microstates. • Fundamental degrees of freedom • Holography • What about de SiUer entropy?
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Thermodynamics and Hydrodynamics
Why is there such a close rela<onship between gravity and thermodynamics?
• Well understood in an< de SiUer space • Thermodynamics of boundary field theory • Navier-‐Stokes = Einstein equa<on
• What is the comparable story with other asympto<cs?
• Gravita<onal metric is an emergent, thermodynamic quan<ty
• But why, then, is it governed by a Hamiltonian system?
Λ = 0
Λ > 0
�f |i�
��
d4x√−gO1(x) δ (O2(x)− 42)�
dQ = TdS ⇒ Rµν − 1
2Rgµν + Λgµν = Tµν
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
3
Jacobson
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What we don’t understand
Informa<on Paradox
• Informa<on preserved • QFT in curved space<me breaking down when curvatures are small and GR says “trust me”
• Informa<on lost • Quantum mechanics breaks down
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What can we measure?
![Page 21: Thoughts(on(Quantum(Gravity( - DAMTP · Thoughts(on(Quantum(Gravity(David(Tong(University(of(Cambridge ... Kapustin and Strassler, 9902033 3 Jacobson(Whatwe(don’tunderstand(Informaon(Paradox](https://reader031.fdocuments.us/reader031/viewer/2022022603/5b5c78827f8b9ad2198c7af0/html5/thumbnails/21.jpg)
Observables
In quantum field theory we compute correla<on func<ons
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
In gravity these are no good. They are not gauge (diffeo) invariant.
What replaces them?
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Boundary Observables
The simplest observables depend on the asympto<cs of space<me.
An<-‐de SiUer Minkowski de SiUer
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
Λ < 0
Λ = 0
Λ > 0
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
Λ < 0
Λ = 0
Λ > 0
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
Λ < 0
Λ = 0
Λ > 0
2
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
2
boundary correla<on func<ons
a =�
mec
e2
4π�c < 1
e2
4π�c ≈ 1
137
lp =
�G�c3
≈ 10−35 m
Eexperiment
EPlanck≈ 10−15
[G] = (Length)2 = (Energy)−2
c = � = 1
GE2
S =1
4g2
�d5x Tr FµνF
µν
[g2] = (Length) = (Energy)−1
T =1
8πGM
S =A
4G
�O(x1) . . .O(xn)�
a � g2
a � g2
Λ < 0
Λ = 0
Λ > 0
�f |i�
2
S-‐matrix
Spacelike boundary at future and past infinity. Observables not observable!
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Rela<onal Observables
Λ = 0
Λ > 0
�f |i�
��
d4x√−gO1(x) δ (O2(x)− 42)�
References
[1] Gibbons and Rychenkova, 9608085
[2] Kapustin and Strassler, 9902033
3
• In AdS, same as boundary observables
• What about in Minkowski and dS?
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Quantum Gravity Hides
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Cosmic Censorship Conjecture
• Weak cosmic censorship: singulari<es buried behind horizons • Why?
• Not true in 5d GR (or 5d Yang Mills)
Penrose
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Tes<ng Quantum Gravity is Harder Than it Should Be!
E
G-‐1/2 -‐
known
unknown
known*
Cosmic Censorship Infla<on ScaUering
Why? What is this telling us?
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Thank you for your aUen<on