on Wednesday April 10th 2019 you will
probably see the first-ever image of a
black hole that's when the event horizon
telescope will be releasing their
results and I haven't seen them yet but
I think they're going to look something
like this and I can be relatively
confident because well it's gonna look a
bit like a fuzzy coffee mug stain but if
you are disappointed by this image I
think that misses the gravity of the
situation from this image we should be
able to tell whether the general theory
of relativity accurately predicts what
happens in the strong gravity regime
that is what happens around a black hole
what I want to do here is understand
what exactly we are seeing in this image
so here is my mock black hole of science
and this sphere represents the event
horizon that is the location from which
not even light fired radially away from
the black hole could be detected by an
outside observer all of the world lines
end up in the center of the black hole
in the singularity once you're inside
here there is no coming back
not even for light the radius of the
event horizon is known as the
Schwarzschild radius now if we were just
to look at a black hole with nothing
around it we would not be able to make
an image like this because well it would
just absorb all electromagnetic
radiation that falls on it but the black
hole that they're looking at
specifically the one in the center of
our Milky Way galaxy Sagittarius a star
has matter around it in an accretion
disk in this accretion disk there is
dust and gas swirling around here very
chaotically it's incredibly hot we're
talking to millions of degrees and it's
going really fast a significant fraction
of the speed of light and it's this
matter that the black hole feeds off and
gets bigger and bigger over time but
you'll notice that the accretion disk
does not extend all the way in to the
event horizon why is that well that's
because there is an inner most stable
circular orbit and for matter around a
non spinning black hole
that orbit is at three-sport shield
radii now in all likelihood the black
hole at the center of our galaxy will be
spinning but for simplicity I'm just
considering the non spinning case you
can see my video on spinning black holes
if you won't want to find out more about
that so this is the innermost orbit for
matter going around the black hole if it
goes inside this orbit it very quickly
goes into the center of the black hole
and we never hear from it again but
there is something that can orbit closer
to the black hole and that is light
because light has no mass it can
actually orbit at 1.5 Swart shield radii
now here i'm representing it with a ring
but really this could be in any
orientation so it's a sphere of photon
orbits and if you were standing there of
course you could never go there but if
you could you could look forward and
actually see the back of your head
because the photons could go around and
complete that orbit now the photon
sphere is an unstable orbit meaning
eventually either the photons have to
spiral into the singularity or spiral
out and head off to infinity now the
question I want to answer is what does
this black quote-unquote shadow in the
image correspond to in this picture of
what's actually going on around the
black hole is it the event horizon are
we simply looking at this or is it the
photon sphere or the inner most stable
circular orbit well things are
complicated and the reason is this black
hole warps space-time around it which
changes the path of light rays so they
don't just go in straight lines like we
normally imagine that they do I mean
they are going in straight lines but
space-time is curved so yeah they go in
curves so the best way to think of this
is maybe to imagine parallel light rays
coming in from the observer and striking
this geometry here of course if the
parallel light rays cross the event
horizon we'll never see them again so
they're gone that will definitely be a
dark region but if a light ray comes in
just above the event
Rison it too will get bent and end up
crossing the event horizon it ends up in
the black hole even a light ray coming
in the same distance away as the photon
sphere will end up getting warped into
the black hole and curving across the
event horizon so in order for you to get
a parallel ray which does not end up in
the black hole you actually have to go
out 2.6 radii away if a light ray comes
in 2.6 Schwarzschild radii away it will
just graze the photon sphere at its
closest approach and then it will go off
to infinity and so the resulting shadow
that we get looks like this it is 2.6
times bigger than the event horizon you
say what are we really looking at here
what is this shadow well in the center
of it is the event horizon it Maps
pretty cleanly onto onto the center of
this shadow but if you think about it
light rays going above or below also end
up crossing the event horizon just on
the backside so in fact what we get is
the whole back side of the event horizon
mapped onto a ring on this shadow so
looking from our one point in space at
the black hole we actually get to see
the entirety of the black hole's event
horizon I mean maybe it's silly to talk
about seeing it because it's completely
black but that really is where the
points would map to on this shadow it
gets weirder than that
because the light can come in and go
around the back and say get absorbed in
the front you get another image of the
entire horizon next to that and another
annular ring and then another one after
that and another one after that and you
get basically infinite images of the
event horizon as you approach the edge
of this shadow so what is the first
light that we can see it is those light
rays that come in at just such an angle
that they graze the photon sphere and
then end up at our telescopes and they
produce a shadow which is 2.6 times the
size of the event horizon so this is
roughly what we'd see if we happen to be
looking perpendicular to the accretion
disk but more likely we will be looking
at some sort of random angle to the
accretion disk we may be even looking a
John and in that case do we see this
shadow of the black hole you might think
that we wouldn't but the truth is
because of the way the black hole warps
space-time and bends light rays we
actually see the back of the accretion
disk the way it works is light rays
coming off the accretion disk bend over
the top and end up coming to our
telescopes so what we end up seeing is
something that looks like that
similarly light from the bottom of the
accretion disk comes underneath gets
bent underneath the black hole and comes
towards us like that and this is where
we get an image that looks something
like the interstellar black hole it gets
even crazier than this because light
that comes off the top of the accretion
disk here can go around the back of the
black hole graze the photon sphere and
come at the bottom right here producing
a very thin ring underneath the shadow
similarly light from underneath the
accretion disk in the front can go
underneath and around the back and come
out over the top which is why we see
this ring of light here this is what we
could see if we were very close to the
black hole something that looks truly
spectacular one other really important
effect to consider is that the matter in
this accretion disk is going very fast
close to the speed of light and so if
it's coming towards us it's gonna look
much brighter than if it's going away
that's called relativistic beaming or
Doppler beaming and so one side of this
accretion disk is going to look much
brighter than the other and that's why
we're gonna see a bright spot in our
image so hopefully this gives you an
idea of what we're really looking at
when we look at an image of a black hole
if you have any questions about any of
this please leave them in the comments
below and I will likely be making a
video for the launch of the first ever
image of a black hole so I'll try to
answer them
then until then I hope you get as much
enjoyment out of this as I have because
this has truly been my obsession for
like the last week I guess what would be
exciting it is to watch it over time
however it changes I there's a lot of
hope that there are blobs moving around
and you know if you see a blob going
around the front and then it goes on the
back end then you see it in the back
image etc and that's gonna be kind of
cool