 
The Event Horizon Telescope is a global
array of radio telescopes spread across
the earth. All these telescopes are
linked together and that allows us to
achieve absolutely unprecedented levels
of both resolution and sensitivity.
Despite their enormous masses black
holes the ones at the centres of
galaxies that are a billion times as
massive as the Sun are very tiny objects.
We don't see the black hole directly -- we
see the stuff that is surrounding it.
There's material in the vicinity rushing
headlong to fall across the horizon and
that material shines and silhouetted
against that shiny material we see the
tell-tale sign of a horizon -- a black
shadow. In general relativity the shape of
the shadow is almost always nearly
circular. Encoded in that shadow is the
shape of space-time around black holes.
In general relativity
black holes are very simple objects
characterized by only a handful of
numbers and therefore their shadows are
also simple shapes but if general
relativity was not the right theory of
gravity -- if Einstein was wrong -- the
shadows could be complicated shapes and
that is the signal we are going after.
Our prime target is the supermassive
black hole at the center of the Milky
Way we call it Sagittarius A* or
just short SagA*. Sagittarius A*
is the four million solar mass
behemoth at the center of our galaxy and
that makes it the closest supermassive
black hole to Earth.
it also makes it the optimal target for
the Event Horizon Telescope. It has a
diameter of about 50 micro arcseconds so imagine you were to cut the moon in
235 million slices then the thickness of
one of these slices would be about the
diameter of the shadow. This is super
super tiny and yet we can resolve it. In
this research we considered what would
happen if we squish the space-time say
instead of a spherical peach maybe it
looks more like a pear or a pizza.
Changing the shape of the space-time
changes the shape of the shadow cast by
the horizon and using seven years of
observations
taken by the Event Horizon Telescope
already we can say that general relativity
does a pretty good job that it really
can't look like a banana or pizza it
could maybe look like a pear.
however starting in 2017 many more
stations will be part of the Event
Horizon Telescope and the sensitivity
and the resolution will increase
dramatically and instead of telling
apart peaches and pears we're going to
be able to see the hairs on the peach. So
we have demonstrated both with existing
data and with forthcoming data that we
are able to place tight constraints on a
potential deviation from general
relativity we're also able to measure
the spin of Sag A* very precisely
as well as its orientation in the sky.
What this work implies is that the era
of high-precision strong gravity
research has begun, that is with radio
telescopes we're going to not only be
able to watch the exciting dramas of gas
and fields accreting onto black
holes but study the underlying stage set
by the space-time itself.
