Kind: captions
Language: en
on wednesday april 10 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 going to 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's 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 secretion disk there is dust and
gas swirling around here very
chaotically it's incredibly hot we're
talking 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 innermost stable
circular orbit and for matter around a
non-spinning black hole
that orbit is at three short 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 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 short 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 innermost 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's 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
horizon 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 short shield
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 back side 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 edge on 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 out 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 secretion disc
is going very fast close to the speed of
light and so if it's coming towards us
it's going to look much brighter than if
it's going away that's called
relativistic beaming or doppler beaming
and so one side of the secretion disk is
going to look much brighter than the
other and that's why we're going to 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 is is to
watch it over time
however it changes right 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 and you see it in the back
image etc then that's going to be kind
of cool