# Recent Changes

### Sunday, July 5

1. Social Activities edited ... Other activities: specify and put your name here to find others with this interest. Workshop …
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Other activities: specify and put your name here to find others with this interest.
Workshop dinners
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Thursday 6/10
Group dinner at Freebirds in Isla Vista. Meeting in front of KITP at 6pm. Sign up below!
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3. IMG_6401.JPG (deleted) uploaded Deleted File
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### Monday, June 29

1. Talks & Discussions edited top The purpose of these tables is to maintain a schedule of scientific talks and discussions rel…

top The purpose of these tables is to maintain a schedule of scientific talks and discussions relevant to your program.
Before you depart, PLEASE fill out an activity report and report your publications to KITP. The KITP needs feedback from the community: this not only serves help to improve and develop our services; it also provides an evaluation tool that is crucial to our continued funding.
It is also very important to acknowledge KITP for papers to which your visit contributed, and where feasible list KITP as an institutional affiliation. All KITP activities are supported by NSF PHY11-25915.

Go to current week.⬇
Week 1
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9:49 am

### Tuesday, June 23

1. Discussion - Online edited ... Online discussion This page provides a space for additional discussion of workshop topics H…
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Online discussion
Holography
The holography challenge -- Steve Giddings 6/23/15
Early in the workshop there was significant discussion on the problem of "decoding the hologram." There are some different views about how/whether a sharp "decoding map" can be defined, and what is necessary to be able to sharply resolve and describe all the relevant bulk physics, particularly on scales small as compared to the AdS radius.
At the final workshop discussion, I stated the following "holography challenge:"
Suppose we assume that H_bulk = H_bdy. Provide a precise formlation, purely in terms of CFT data, of bulk observables that reduce to observables of local quantum field theory on scales <<R_ADS in a controlled approximation for small but nonzero g_s.
Some of those present indicated that they thought that the HKLL construction meets the challenge, and none objected to the statement that it does, at least on an AdS background. Of course, as has been discussed, there are additional issues about other backgrounds, such as black holes.
Please share your thoughts whether you think HKLL does indeed meet the challenge -- or whether you see limitations to or problems with the statement that it does. (In particular, there are questions about a systematic expansion in g_s, which I raised in my talk during the "hologram week;" resolution of these seem to require certain properties of smearing functions. There is also the set of questions which resulted in the discussion about code subspaces, or the role of gauge invariance.)

Inflationary cosmology
IR safe cosmo observables -- Martin Sloth 4/28/15
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### Sunday, June 14

1. Talks & Discussions edited ... Aud S. Giddings NVNL: Nonviolent unitarization: overview, questions, Wed, June 17 3:30 …
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Aud
S. Giddings
NVNL:Nonviolent unitarization: overview, questions,
Wed, June 17
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### Saturday, June 13

1. Discussion - Online edited ... In the above refs, the cancellation was checked including the quadratic terms of the slow-roll…
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In the above refs, the cancellation was checked including the quadratic terms of the slow-roll parameters, assuming that all the slow-roll parameters are small. The interpretation of the result is different, but at the algebraic level, the same result was derived by Steve and Martin in
http://arxiv.org/abs/1005.1056
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et al.
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invariant quantity.
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configuration space.
Let me understand your point more clearly. If loop corrections have a different slow-roll parameter dependence from the contributions due to the distance change, the cancellation of the divergence between them never happens. Are you claiming that even for the (local) gauge invariants, the IR divergences and the secular growths are not canceled away? And actually I’m not sure what do you mean by “all true invariants”?
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Question for Tom about black hole entropy in HST - Ted Jacobson, 6/1/15
I'm puzzled about the role of the background Minkowski metric. As I understood it, HST is constructed so the dimension of Hilbert space attributable to a causal diamond goes like the area of the bounding sphere, where the area is measured with respect to the fixed, Minkowski metric. Now suppose we have a black hole. We know the entropy should go like the area of the horizon measured with respect to the emergent, "hydrodynamic" metric. But at the underlying HST level, a different area would presumably be assigned to this surface by the Minkowski metric. This suggests that the dimension of the underlying Hilbert space has no particular relation to the black hole entropy. But if that's the case, then what is the motivation for the HST axioms in the first place? Probably I'm misunderstanding something basic about HST...
Reply to Ted's question - Tom Banks, 6/12/15
Let me start from the assumption that black holes evaporate in a unitary manner in models of quantum gravity in asymptotically flat spacetime. It follows that in the remote past and future there is a complete set of Minkowski geodesics. The HST formalism works with a set of time-like trajectories, which asymptote to these geodesics. We assign a quantum system to each such trajectory with a time parameter which is asymptotically the proper time along those geodesics.
Now consider a process in which a black hole is created in the collision of two jets of gravitons. I argued in my penultimate lecture that our matrix model could model that situation because of the definition of particle jets in terms of constrained states in the matrix model Hilbert space. When the number of constraints defining the incoming state at time - T in the subfactor of the Hilbert space describing a particular interval of proper time 2T , becomes of order T^2 then it is highly improbable that the final state will be anything other than an generic unconstrained state in the Hilbert space. This description was carried out using a trajectory which was at rest in the c.m. of the collision and was placed right at the center of the black hole. My first controversial contention is that one can extend the description of the physics along this trajectory to infinite proper time after the black hole is formed and that it becomes a Minkowski geodesic in the remote future, after the black hole has evaporated. The classical black hole interior metric is geodesically incomplete, so denies this possibility. I would claim that this is not a signal that the quantum mechanics stops, but simply that a finite proper time after horizon crossing, the quantum system is in an equilibrium state associated geometrically with the stretched horizon . The stretched horizon of a black hole formed in collision and evaporating into particles is a timelike cylinder of finite extent. The entropy associated with this cylinder (number of fermionic quantum degrees of freedom necessary to describe it) changes slowly with time, eventually going to zero. In the matrix model, this is modeled by saying that a certain subfactor of the entire Hilbert space is in a generic time dependent state of T^2 degrees of freedom, typically not satisfying of order ET constraints. The probability of finding this subsystem in such a constrained state is of order
e^{- ET} (note T is a time, not a temperature) and this is interpreted as Hawking radiation of a jet of energy E.
Now we come to the crucial question: what should we consider to be the degrees of freedom causally connected to this "trajectory inside the black hole" at proper times larger than a Schwarzschild time after horizon crossing. Classical GR does not give an answer to this question, because it doesn't allow us to think about proper times that long. For a while, up through our first paper on firewalls, Willy and I considered modeling this by a singular QM evolution which stops at a finite time (this just means that as one approaches that time the system goes through every unitary in the unitary group of its Hilbert space an infinite number of times), but that makes no sense because the black hole evaporates. The proper answer is to think about the QM of H_{in} as describing the evolution of the equilibrated stretched horizon and its interaction with the external world. Since at each proper time after local equilibrium sets in, there is SOME probability that the black hole will emit massless Hawking quanta, we have to take the causal diamond of the trajectory inside the black hole to grow as it would in Minkowski space. That is to say, the QM MUST contain states, which are included in the factor of the Hilbert space acted on by $H_{in} (T + t)$. This Hamiltonian is a function of (T + t)^2 fermions and we want to think about t as being much larger than T. The state of "a black hole with a jet of Hawking quanta of energy E" satisfies of order T (T + t) constraints such that the matrix elements between two blocks one of size E and the other of size (T - E) vanish, as well as all of those matrix elements connecting those two blocks to a larger block of size t . The latter constraints are "inherited from the initial state at time - (T + t) while those between the two small blocks are created by a fluctuation in the equilibrium dynamics of the subsystem of T^2 fermions. Geometrically one can think of this causal structure as describing the causal connections between some time like trajectory on the stretched horizon, but if I pick any given trajectory I break the spherical symmetry. So we should think of this as the causal structure of a trajectory in the dead center of the black hole, but the quantum system associated with that trajectory doesn't have any interior particles to talk about and just describes the dynamics of DOF on the stretched horizon. A geometrical slogan that captures what's going on is that the quantum dynamics of H_{in} (\tau ) along trajectories inside the black hole takes place in a Hilbert space describing the domain of dependence of the stretched horizon. This Hilbert space grows with increasing proper time just like the corresponding Minkowski surface (all of this is in the limit of large black hole mass) . At proper times $\tau$ large compared to T, the black hole itself is described by a small subset of the DOF inside the diamond.
I won't repeat what I said in my last lecture about how this framework allows for the description of a system falling into the black hole long before evaporation but long after formation, and how the consistency condition between the description along trajectories which enter the black hole along with the in-falling system, and those which are a few Schwarzschild radii away allows one to describe "a system in the black hole interior" for a period of time of order the Schwarzschild radius.
In Ben's lecture you complained that the causal diamond picture didn't make sense "because we were doing field theory". The point Willy and I have been trying to make is that we are not doing field theory. All we know from string theory and AdS/CFT is that the computation of a certain subset of all boundary correlators (S matrix in the Minkowski case) in a restricted kinematic regime are approximately reproduced by field theory. Many people have taken this as license to use the full field theory Hilbert space apparatus, with some UV cutoff, to describe what's going on in local regions. I believe we should have known for a long time that this is wrong. The restrictions imposed on the Hilbert space by black hole physics cannot be accounted for by a UV cutoff. Field theory IMHO is almost as conceptually remote from the correct theory as QM is from CM. Willy and I have also discovered something somewhat more surprising. In our formalism, most of the DOF associated with a causal diamond, cannot be localized in the bulk of that diamond. The best I can come to a field theoretic description of what's going on is to say that the only field theory states in a causal diamond, which are really there in the theory of quantum gravity, are modes that are pure gauge everywhere except on the boundary.
I hope this helps.

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