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In April 2019, the Event Horizon Telescope
(EHT) collaboration revealed the
first image of the candidate supermassive
black hole (SMBH) at the centre
of the giant elliptical galaxy Messier
87 (M87). This event-horizon-scale
image shows a ring of glowing plasma
with a dark patch at the centre, which is
interpreted as the shadow of the black
hole. This breakthrough result, which
represents a powerful confirmation of
Einstein’s theory of gravity, or general
relativity, was made possible by assembling
a global network of radio telescopes
operating at millimetre wavelengths
that for the first time included
the Atacama Large Millimeter/
submillimeter Array (ALMA). The addition
of ALMA as an anchor station
has enabled a giant leap forward by
increasing the sensitivity limits of the
EHT by an order of magnitude, effectively
turning it into an imaging array.
The published image demonstrates that
it is now possible to directly study
the event horizon shadows of SMBHs
via electromagnetic radiation, thereby
transforming this elusive frontier from
a mathematical concept into an astrophysical
reality. The expansion of
the array over the next few years will
include new stations on different continents
— and eventually satellites in
space. This will provide progressively
sharper and higher-fidelity images of
SMBH candidates, and potentially even
movies of the hot plasma orbiting
around SMBHs. These improvements
will shed light on the processes of black
hole accretion and jet formation on
event-horizon scales, thereby enabling
more precise tests of general relativity
in the truly strong field regime.
