Who wouldn't dream of seeing a black hole up close (but not too close)? In this second article we will take a look at the advances in technology that allow us to view the black hole at the center of the Milky Way.
First, we should understand the obstacles. As we are situated in a disk galaxy, there are large numbers of stars situated exactly in between us and the Galaxy's center. This introduces a kind of 'light pollution' in the form of starlight which interferes with our ability to see faint emission emanating from the accretion disk surrounding the black hole.
This is analogous to watching a friend holding a piercingly-bright flash light on a dark path. You see the flashlight but not the physical features of your friend who you know must be holding the light.
By choosing the color The Milky Way's central supermassive black hole carefully, we are able to view the black hole at a color in which the stars weakly shine. This color is in the radio. Even with this color advantage, the emission from the material falling into the black hole is still very faint, requiring high sensitivity. Sensitivity for radio receivers scales with bandwidth. By recent advances in digital electronics we have been able to develop data collection at high bandwiths which will make this a 21st century experiment.
Secondly, as we discussed last time, black holes are physically small. We approach the spatial resolution that we need by combining or 'phasing up' the light incident on many telescopes spread out across the whole world. By phasing up the signal we achieve a resolution equivalent to that of the Earth. In other words, we use telescopes worldwide in unison as an interferometer to operate a kind of 'telescope' with an aperture the size of Earth.
Finally, these enormous interferometers have higher spatial resolution at higher frequencies. Technology continues to develop in the direction of enabling observations at a wavelength of 1 mm and even in the sub-millimeter. At the end of the day (or perhaps of this decade), we will be able to take pictures of the enigmatic monster at the Galactic center. Theorists are working hard to predict what we will see in this grand experiment called the Event Horizon Telescope.