01 APRIL 2018

The Event Horizon Telescope will combine data from a worldwide network of radio telescopes to image the shadow that a black hole casts on the surrounding plasma.

Within months of the publication of Albert Einstein’s general relativistic field equations in 1915, Karl Schwarzschild had derived the equations’ first nontrivial solution—the black hole spacetime. Ever since then, the physics and astronomy communities have had a love–hate relationship with black holes. It took almost half a century before they were considered anything more than a mathematical curiosity. Today the existence of black holes is widely accepted, but they remain perplexing nevertheless. In most attempts to unify quantum field theory and general relativity, black holes present paradoxes that are hard to resolve.Formally speaking, a black hole is a vacuum spacetime with all the mass concentrated in an infinitesimally small region at the center. At large distances from the concentration, the gravitational field behaves like that of any other object. However, a black hole is surrounded by a virtual surface, called the event horizon, from which nothing can escape, not even light. For a nonspinning black hole, the radius of the event horizon, called the Schwarzschild radius RSRS, is equal to 2GM/c22GM/c2, where *G* is the gravitational constant, *M* is the mass of the black hole, and *c* is the speed of light. An ongoing project called the Event Horizon Telescope (EHT) is now attempting to image black holes with horizon-scale resolution.

#### SEEING IN THE DARK

In the observation of gravitational waves recognized by the 2017 Nobel Prize in Physics, detectors at the Laser Interferometer Gravitational-Wave Observatory listened to spacetime ringing as two black holes coalesced. Imaging black holes will give EHT scientists a different way to investigate physics just outside the horizons of these enigmatic objects. Specifically, an image can provide spatially resolved information about strong-field gravitational effects in stationary spacetimes and about the interaction of the horizon with the surrounding matter. However, by their very definition, horizons do not emit light. It is therefore difficult to see how they lend themselves to imaging.To see black holes, the EHT looks for the silhouettes they cast on background emission. Photons that are directed radially outward from a black hole can escape its gravitational field only if they are outside the event horizon. Photons that are not radially directed can be trapped at even greater distances. In fact, any photon with an inward radial momentum component is destined to cross the horizon once it passes the so-called photon orbit radius. As long as there is a source of photons outside the black hole, such as hot material falling into the black hole, there will be radiation on which the black hole will cast a shadow, a silhouette that can be imaged. Figure 1 shows a simulation of what the EHT might see.

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