What is the primary purpose of the Event Horizon Telescope?

• A. To visualize distant galaxies in visible light.
• B. To image the shadow of a supermassive black hole using millimeter-wavelength VLBI.
• C. To detect gravitational waves.
• D. To measure exoplanet transits.

Answer: (B)

The Event Horizon Telescope is designed to see features at the scale of a black hole’s event horizon by linking many radio telescopes around the world into one Earth-sized instrument. This very long baseline interferometry at millimeter wavelengths lets us achieve the incredibly fine angular resolution needed to image the shadow that a black hole casts against the glowing material in its immediate surroundings. Observing at these wavelengths also helps minimize scattering and lets the hot, compact region near the horizon shine through, so we can directly view the silhouette predicted by general relativity. The result is a direct look at the region governed by extreme gravity, not a survey of visible-light galaxies, a search for gravitational waves, or a study of exoplanet transits.