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Gravitational waves will allow scientists to test Einstein?s theory of General Relativity in the previously unexplored strong-field regime. Einstein?s theory of general relativity, as the most accepted theory of gravity, has been greatly constrained in the quasi-linear, quasi-stationary regime, where gravity is weak and velocities are small. Gravitational waves may carry information about highly dynamical and strong-field gravity that is required to generate measurable waves. Coalescing compact binaries are the most promising sources of gravitational waves accessible to ground-based interferometers, such as Advanced LIGO. Made of neutron stars and/or black holes that orbit each other hundreds of times a second just before they collide, the resulting waves are imprinted with information about the individual objects and the dynamical coalescence process. After reviewing the basic properties of gravitational waves, I will present an overview of the detector design and provide an update on the current status of Advanced LIGO and its ability to probe the strong gravity regime.

Some 40 years ago Hawking showed that if the black hole has a smooth horizon, then information will be lost when the black hole radiates. In string theory black holes appear to have a complete set of `hair'; these black hole states are called fuzzballs, and they radiate like normal bodies with no information loss. It was recently argued that structure at the horizon will necessarily feel like a `firewall' to an infalling observer. We will show that this need not be the case, since one can have `fuzzball complementarity' where an approximately smooth horizon appears as a `dual' description.

It has now become clear that the radio jet in the giant elliptical galaxy M87 must turn on very close to the black hole. This implies the efficient acceleration of leptons within the jet at scales much smaller than feasible by the typical dissipative events usually invoked to explain jet synchrotron emission. Here we show that the stagnation surface, the separatrix between material that falls back into the black hole and material that is accelerated outward forming the jet, is a natural site of pair formation and particle acceleration. This occurs via an inverse-Compton pair catastrophe driven by unscreened electric fields within the charge-starved region about the stagnation surface and substantially amplified by a post-gap cascade. For typical estimates of the jet properties in M87, we find excellent quantitive agreement between the predicted relativistic lepton densities and those required by recent high-frequency radio observations of M87.

The radio galaxy M87 is an excellent laboratory for investigating the formation process of the relativistic jet and the production mechanism of high energy particles and photons in the vicinity of super-massive black holes. VLBI observations at 1.3 mm can address at least two issues concerning the fundamental nature of M87. The first is the event-horizon-scale structure of the jet launching region, and another one is the production mechanism of very high energy (VHE; > 100 GeV) photons at there. In this talk, I report on new 1.3 mm VLBI observations of M87 with the EHT during the VHE enhancement in 2012. These observations provide the first measurements of closure phase, imposing new constraints on accretion-disk/jet models for M87, and also the first constraints on the innermost structure of the relativistic jet on scales of a few Rs during VHE variability. I discuss results and their implications for jet/disk models and also VHE models for M87, as well as future prospects of EHT observations of M87.

Because of the existence of a prominent jet, M87 provides an ideal source with which the EHT can critically test current jet launching paradigms. The EHT has already placed weak limits on the black hole spin based on generic arguments regarding limits on the image size. However, careful modeling of the jet structure can produce much more stringent constraints on both the black hole properties and the mechanisms responsible for the generation of the observed radio emission. I will describe how this modeling is performed, what the remaining chief uncertainties are and why we believe they can be controlled, and how existing EHT observations of M87 support models of electromagnetically launched jets powered by black hole spin.

VLBI observations at the highest possible frequency penetrate the opacity barrier in the nuclear regions of radio-galaxies and blazars, which are synchrotron self-absorbed at longer wavelength. This facilitates a direct and sharper than ever view into the 'heart' of Active Galactic Nuclei (AGN), into region in which BH physics and general relativity effects become important and where radio jets are launched. Here we report on new results from global 3mm and 1.3mm VLBI observations adding the APEX and IRAM to the Event Horizon Telescope. New images and core size estimates for a number of AGN jets and for Sgr A* are presented and discussed.