In 2019, the Event Horizon Telescope (EHT) collaboration completed analysis of 27 petabytes of data from the supermassive black holes at the Milky Way's center and in M87. This groundbreaking effort delivered humanity's first images of a black hole's event horizon. But how does the EHT accomplish this remarkable feat?
Black holes are detectable through distinct signatures, despite emitting no light themselves.
Their immense gravity warps spacetime in a compact region. By observing these gravitational effects from nearby massive objects, astronomers infer black hole presence and estimate their mass.
Black holes profoundly influence their surroundings. Nearby material faces extreme tidal forces, accelerates, and heats up, emitting radiation in the accretion disk.
Spectral analysis of this radiation reveals the black hole at the source. Additionally, merging black holes produce detectable gravitational waves.
The EHT surpasses these indirect methods by directly imaging the event horizon.
This relies on two key astronomical principles: resolution and light-gathering capability, now technologically feasible.
Related: Event Horizon Telescope Delivers First Details of Sagittarius A Black Hole Structure
Black holes demand ultra-high resolution due to their compactness. Imaging their shadow—the absence of light—requires collecting vast amounts of light with pinpoint accuracy.
Typically, larger telescopes provide both better resolution (via wavelength sensitivity) and light collection (via collecting area). However, black hole sizes scale with mass but inversely with distance. Imaging Sagittarius A* demands Earth-sized resolution.
Building a single such telescope is impossible, so the solution is a global network. This sums light-gathering from multiple dishes while resolution matches the longest baseline between them, via very long baseline interferometry (VLBI).
In essence, light gathering depends on total area, but resolution leverages maximum separation.
This video illustrates how the Event Horizon Telescope operates:
This yields 15 microarcsecond (μas) resolution, akin to spotting a fly on the Moon. Only Sagittarius A* and M87's black hole exceed this angular size.
M87's black hole, 50-60 million light-years away, boasts over 6 billion solar masses—1,000 times larger than our galaxy's.
The EHT observes these targets simultaneously with its array, reconstructing high-resolution images from sufficient data.
Success hinges on capturing enough light to delineate the event horizon shadow against surrounding emission.
Black holes accelerate charged particles, generating magnetic fields and synchrotron radiation. Radio wavelengths, being lower-energy, are ideal—all such black holes emit detectable radio waves, as confirmed for these targets. At EHT resolution, the 'dark void' of the event horizon emerges.
