The nonlocality of gravity doesn't mean that Lorentz invariance is broken, Lorentz invariance and locality are separate concepts. It just means that to define the state of the universe at a certain point, you need to know what is going on everywhere, the state space isn't decomposing into a basis of local operators.

"Locality" is a bit of an overloaded term, and for this discussion I will assume that it means there are bosonic operators at every point which commute at spacelike separation (Bosonic fields and bilinears in Fermi fields). This means that that the orthogonal basis states at one time are all possible values of the bosonic field states on a spacelike hypersurface, and over Fermi Grassman variables if you want to have fermions.

If you extend this idea to curved spacetime and to arbitrarily short distances, you get a completely ridiculous divergence in the number of black hole states. This was the major discovery of 'tHooft, which is the basis of the holographic principle.

To see this, consider the exterior Schwarzschild solution, The local t temperature is the periodicity of the imaginary time solution, and it diverges as 1/a where a is the distance to the horizon (this distance is measured by the metric, which is diverging in r coordinates, so it is not $r-2m$ for r near the horizon, but proportional to $\sqrt{r-2m}$. With this change of variables, the horizon is locally Rindler).

Assuming that the fields are local near the horizon, the thermal fluctuations of the fields consists of a sum over the entropy of independent thermal field fluctuations at the local temperature. You can estimate the entropy (per unit horizon area) in these fluctuations by integrating the entropy at any r with respect to r. The entropy density of a free field (say EM) at temperature T goes as $T^{3}$, so you get

$$ \int_{2m}^A {1\over (r-2m)^{1.5}} dr $$

The convergence at large A is spurious, the redshift factor asymptotes to a constant in the real solution, so you get a diverging entropy. This is sensible, it is just the bulk entropy of the gas of radiation in equilibrium with the black hole. But this integral is divergent near the horizon, so that the black hole Hawking vacuum in a local quantum field theory in curved spacetime is carrying an infinite entropy skin.

This divergent entropy is inconsistent with the picture of a black hole forming and evaporating in a unitary way, it is inconsistent with physical intuition to have such an enormous entropy in an arbitrary small black hole, it is just ridiculous. So any quantum theory of gravity with the proper number of degrees of freedom must be nonlocal near a black hole horizon, and by natural extension, everywhere.

The divergence is intuitive--- it is saying you can fit an infinite amount of information right near the horizon, because nothing actually falls in from the extrerior point of view. If the fields are really local, you can throw in a Gutenberg bible and extract all the text by careful local field measurements a hundred years later. This is nonsense-- the information should merge with the black hole and be reemitted in the Hawking radiation, but that's not what the semiclassiclal QFT in curved space says.

'tHooft first fixed this divergence with a brick wall, a cutoff on the integrals to make the entropy come out right. This cutoff was a heuristic for where locality breaks down. In order to fix information loss, around 1986, he considered what happens when a particle flies into a black hole, and how it could influence emissions. He realized that the only way the particle could influence the emissions was through the gravitational deformation the particle leaves on the horizon.

This deformation is nonlocal, in that the horizon shape is determined by which light rays make it to infinity. The backtracing showed that an infalling particle leaves a gravitational imprint on the horizon, like a tent-pole bump where it is going to enter. He could get a handle on the S-matrix by imagining that the bumps are doing all the physics, the horizon motion itself, and this bump-on-the-horizon description was clearly similar to the vertex operator formalism in string theory, but with crazy imaginary coupling, and all sorts of wrong behavior. This is now known to be because he was considering a thermal Schwartzschild black hole, rather than an extremal one. In extremal black holes, the natural analog to 'tHoofts construction is AdS/CFT.

### String theory

In string theory, you have a nonlocality which was puzzling from the beginning--- the string scattering is only defined on-shell, and the only extension to an off-shell formalism requires you to take light-cone coordinates. This was considered an embarassment in string theory in the 1980s, because to define a space-time point, you need to know off-shell operators which you can Fourier transform to find point-to-point correlation functions.

In the 1990s, this S-matrix nonlocality was reevaluated. Susskind argued heuristically that a highly excited string state should be indistinguishable from a large thermal black hole. One of the arguments was that the strings at weak coupling at large excitation numbers are long and tangled, and should have the right energy-radius relationship.

Another of Susskind's arguments is that a string falling into a black hole should get highly thermally excited, and get longer, and it becomes as wide as the black hole at 'tHooft's brick wall, so that the brick wall is not an imaginary surface to cut off an integral, but the point where the strings in the string theory are no longer small compared to the black hole, and the description is no longer local.

Susskind argued that at large occupation numbers, it is thermodynamically preferrable to have one long string rather than two strings with half the excitation. This is essentially due to the exponential growth of states in string theory, to the Hagedorn behavior. But it means that the picture of a string falling into a black hole is better considered a string merging with the big string which is the black hole already.

The d-branes were also identified with black holes by Polchinsky, and the dualities between D-branes and F-strings made it clear that everything in string theory was really a black hole. This resolved the mystery of why the strings were described by a 2d theory which was so strangely reproducing higher dimensional physics--- it was just an example of 'tHooft's holographic descriptions.

All this stuff made a tremendous pressure to find a real mathematically precise realization of the holographic principle. This was first done by Banks Fischler Shenker and Susskind, but the best example is Maldacena's.

### AdS/CFT

In AdS/CFT, you look near a stack of type IIB 3-branes to get the near-horizon geometry (which is now curved AdS, not flat Rindler, because the black holes are extremal), and you identify the dynamics of string theory near the horizon with the low-energy theory on the branes themselves, which consists of open strings stuck to the branes, or N=4 SUSY SU(N) gauge theory (the SU(N) gauge group comes from the Chan-Paton factors, the N=4 SUSY is the SUSY of the brane background, and the superconformal invariance is identified with the geometeric symmetry of AdS).

The correspondence maps the AdS translation group to involve a *dilatation* operator on the field theory, so that if you make an N=4 field state which is sort-of localized at some point in AdS, and you move in one of the AdS directions, it corresponds to making the blob bigger without changing its center. This means that there is absolutely no locality on the AdS side, only on the CFT side. Two widely separated points are represented by a CFT blobs of different scale, not by a CFT state of different position, so they cannot possibly be commuting, except in some approximation of low energy. The CFT is local, but this is boundary locality, analogous to light-cone locality, not a bulk locality. There is no bulk locality.

This nonlocality is so obvious that I don't know how to justify it any more than what I said. There aren't four dimensions of commuting bosonic operators in the N=4 theory, just 3 dimensions. There aren't five spacetime dimensions, just four. The remaining dimension is emergent by different scales in the CFT. So this example is airtight--- string theory is definitely nonlocal, and nonlocal in the right way suggested by 'tHooft and Susskind's arguments.

This post imported from StackExchange Physics at 2015-04-11 10:31 (UTC), posted by SE-user Ron Maimon