Almost a hundred years have passed since the introduction of Relativity, an event that dissolved our concept of a simple world with absolutes. Neither space nor time, we learned, could be thought of as weaving the fabric of nature in such a way that all physical laws were to be formulated in terms of observer-independent constructs. Instead, Relativity imposed upon us the need to describe physical events from the perspective of the individual’s frame of reference.

It is now well known that the speed of light in vacuum is independent of the motion of all observers and sources. We do not yet understand why this happens, but an important consequence of this empirical fact is that space and time must be intertwined in such a way that when we measure the velocity of the fastest particles (such as the constituents of light) everyone determines an identical value of distance divided by time for them. Time must therefore`flow’ differently for different observers to make up for the fact that they disagree on the distances they measure.

As radical as this may seem in the context of everyday phenomena, where such effects are so small that we are oblivious to them, a remarkable enlightenment ensues from this theory once the acceleration of particles is incorporated into it. Even Sir Isaac Newton (1642-1727) was aware of the rather odd coincidence that two bodies attract each other gravitationally with a force that depends on their inertial mass. This is the resistance (or inertia) that an object displays when it is being accelerated by a force. It’s the reason, for example, why an astronaut needs to push against something in order to change his motion in space; he does not simply dart haphazardly from point to point without an influence to cause a change in his trajectory. In the early part of the 20th century, Albert Einstein (1879-1955) realized that this was not a chance coincidence at all, but rather an indication that the effects of gravity could be mimicked by an acceleration of one’s frame of reference. In other words, if you were stuck inside an elevator, and you felt a pull towards the floor, you could not determine whether that was due to the force of gravity acting on you downwards, or to a force that was accelerating the elevator upwards. Since the `gravitational’ and inertial masses are equal, the force you experience between your feet and the floor would be identical.

Einstein had what he called “the happiest thought” of his life when it occurred to him that this equivalence constituted far more than a convenient description of gravity using accelerated frames, for it represented a true shift in paradigm for how objects move under the gravitational influence of others. Light, having zero mass, ought not to be affected by gravity in the same way that it influences the motion of massive particles. Yet if Relativity is correct, it is the manifestation of space and time that undergoes modifications from observer to observer, and if in addition the effects of gravity are identical to those of an acceleration between these individuals, then everything in that space-time (not just the massive particles) will respond to the gravitational influence of a massive body. A star, for example, attracts light just as it does any of its planets. Make an object small enough and heavy enough, and its gravitational field will prevent everything from ever escaping it, including light. It’s as if a permanent barrier separates the space-time within the surface of this object from the rest of the Universe. Things can fall into it, but nothing can come out.

An object such as this is called a black hole, and the surface of no return is its event horizon. To be sure, a star needs to be extremely compact before gravity is sufficiently strong to form a black hole. The Sun, for example, would have to be squeezed into an asteroid-sized volume no bigger than six kilometers across. However, since the beginning of the 20th century when Relativity theory was established, evidence has been accumulating slowly, but steadily, that nature does indeed craft environments where these exotic objects may form. By their very nature, black holes are very hard to see---they’re supposed to be `black’, since no light can escape them---so all of the evidence we have thus far is indirect, mostly having to do with how much they weigh. Now, a century after Einstein’s discovery, the astrophysical community is finally on the verge of actually seeing one---at the heart of the Milky Way.

Astronomers have had difficulty looking into the Galactic Center because of the intervening dust lanes and clouds. But this has changed dramatically in recent years due to our ability to peer through these obscuring veils with new telescopes that detect radiation at radio and X-ray wavelengths, where the attenuation is not as severe as it is with optical light. Contrary to expectations, the Galactic Center is a dynamic and bizarre place, with energetic activity dominated by a dark monster with the mass of almost 3 million suns, devouring everything around it.

One of the most exciting recent developments has been the realization that this massive black hole is sufficiently close to us that we should be able to image its event horizon against the backdrop of nearby, radiating gas---a concept that was unimaginable even just a few years ago. Light produced by the infalling gas behind this object is bent by its strong gravitational field and is completely absorbed. The size of the shadow produced by this effect is predicted to be well within the resolving power of radio telescopes being developed during this decade. The image (published in 2001 by the Astrophysical Journal Letters in Vol. 555, p. 83) shows the predicted view that radio astronomers will glimpse when they point their array of telescopes towards the Galactic Center, using very long baseline interferometry at sub-millimeter wavelengths. The radiation coming to us from outside the black hole is due to the hot, moribund gas plummeting towards the event horizon. The shadow is predicted to be about 40 microarcseconds across---a diameter that is only about 3 times smaller than the currently available resolution.

When acquired, this image will not only establish conclusively that the dark matter at the Galactic Center is a massive black hole, it will provide us with the means of directly testing the most captivating prediction of Relativity---the existence of point-like objects shielded forever from us by their event horizon. This event, coming a century after the theory, may be somewhat belated, but nonetheless a fitting tribute to one of the most influential scientific developments yet seen by humanity.