Kind: captions
Language: en
this video is sponsored by kiwiko more
about them at the end of the show
this is a picture of the supermassive
black hole at the center of our milky
way galaxy known as sagittarius a-star
the black hole itself doesn't emit light
so what we're seeing is the hot plasma
swirling around it
this is only the second picture of a
black hole ever it was taken by the
event horizon telescope collaboration
the same people who brought you this
image of the supermassive black hole at
the center of galaxy m87
now their original plan was to image
sagittarius a star first since it's in
our own galaxy it is 2 000 times closer
than m87 star
but it's also over a thousand times
smaller so from earth it appears only
slightly larger than m87 star
and there are a number of additional
challenges to observing it
first of all there is a lot of dust and
gas between us and the center of our
galaxy so you can't even see it with
visible light
in this video from the european southern
observatory we zoom in on our galaxy's
core as we get closer and closer at some
point we have to switch over to infrared
light which can better penetrate the
debris
allowing us to see it from earth
over the past three decades we've been
able to peer into the heart of the milky
way
and witness something truly amazing
a collection of stars zipping around on
all kinds of eccentric orbits they go
incredibly fast one of the stars was
clocked going 24 million meters per
second that's 8 percent the speed of
light
all these stars appear to be orbiting
something incredibly massive and compact
but this object isn't glowing brightly
like a star if you watch closely you can
see it flicker now and then
this is what we believe to be a
supermassive black hole
from the motion of the stars around it
we can infer that the black hole's mass
is about 4 million times that of our sun
but all crammed down into a tiny point
the singularity
anything including light that comes
within a short shield radius of this
point can't escape and ends up in the
singularity
so for us to see any radiation from the
black hole it must come from further out
than this
usually from superheated plasma as it
falls in
but for its size sagittarius a-star
doesn't consume much matter it's
unusually quiet and dark the
supermassive black hole at the center of
m87 in contrast is much more active
gobbling up matter from its accretion
disk
plus since it's over a thousand times
bigger it takes a thousand times longer
for objects to orbit it and that means
from earth its appearance over time is
more consistent in contrast to
sagittarius a-star where things can
change on the order of minutes
these visualizations are from luciano
rizzola and colleagues at go to
university frankfurt but the biggest
challenge of all in making an image of
either supermassive black hole is that
these objects are so compact and so far
from earth in the sky they appear very
very tiny
to get a sense of just how tiny take the
whole sky and divide it into 180 degrees
the andromeda galaxy spans about 3
degrees
then divide 1 degree into 60 arc minutes
and 1 arc minute into 60 arc seconds
divide an arc second into 100
into 100 again
and into 100 once more
and this is the size of the black holes
on the sky
it's equivalent to taking a picture of a
donut on the moon
now there is no optical telescope on
earth that could produce such an image
so in this video i want to answer two
questions
how did they do it and what are we
actually looking at
so starting with how did they make these
images of black holes well the first
thing to know is they weren't made with
visible light they were made using radio
waves with a wavelength of 1.3
millimeters so all the observations were
taken by radio telescopes which
essentially look like huge satellite
dishes
when a source emits radio waves they
travel out radially in all directions
but earth is so far away that by the
time they reach our planet the wave
fronts are almost completely flat and
parallel
this is known as a plane wave
a radio telescope works by scanning back
and forth across the sky
when it is pointed directly at a radio
source it produces a bright spot that's
because all the radio waves travel the
same distance bounce off the dish and
are received at the same time
so they are in phase meaning peaks line
up with peaks and troughs with troughs
they constructively interfere
as the telescope moves past the source
some of the radio waves now travel
farther than others and therefore they
meet up out of phase destructively
interfere and the intensity of the
signal drops to zero
to make a sharp image you want this
drop-off to be as steep as possible so
the telescope produces peak intensity
only when aimed directly at the source
and then the intensity drops rapidly
when the dish is moved just a tiny bit
in any direction
there are two ways of achieving this one
is to observe higher frequency radio
waves that way any slight movement of
the telescope represents a greater
fraction of a wavelength this causes
destructive interference to occur sooner
the other way is to increase the
diameter of the telescope this increases
the difference in path length between
radio waves on opposite sides of the
telescope for a given angular adjustment
how narrowly a telescope can identify
the source of radio waves is known as
its angular resolution
you can think of it as the size of the
spot on the sky that the telescope is
sensitive to
it is proportional to wavelength and
inversely proportional to the diameter
of the telescope
the challenge with making a picture of a
black hole is that you're trying to see
the structure in a tiny area of the sky
imagine scanning a radio telescope
across the center of a black hole you
would want to see the bright spot as the
telescope passes over the left edge and
then a dark spot and then another bright
spot as it passes the right edge
the problem is for any individual radio
telescope on earth the angular
resolution is too large so as it passes
over the black hole it would still be
receiving radio waves from the left side
as it begins receiving radio waves from
the right side the resolution isn't high
enough to tell if there's a ring
structure there as we'd expect with a
black hole or if it's just a blob
observing at shorter wavelengths isn't
really an option because that light is
blocked either by our atmosphere or by
the matter around the black hole
so if you want to improve resolution the
only way you can do it is by increasing
the diameter of the telescope
but if you actually do the calculation
you find that the telescope you'd need
would have to be the size of the earth
in order to see the ring of a black hole
which is obviously impossible
but there is a way to do something
that's almost as good
you don't need a complete dish the size
of the earth just pieces of it
individual radio telescopes that are
separated by distances up to the earth's
diameter
as long as you can properly combine the
signals from all these distant
telescopes you get the constructive and
destructive interference required to
achieve the same angular resolution
as an earth-sized dish
this technique is called very long
baseline interferometry
so the event horizon telescope is not
just one telescope but a global network
of radio observatories all these
telescopes observe sagittarius a-star at
the same time
unlike a single telescope you can't
bounce all the radio waves to a central
receiver and add them up in real time so
instead each telescope records the
signal at its location and the exact
time down to the femtosecond
petabytes of data are generated
but now that data needs to be brought
together the fastest way to do it was
actually to carry hard drives as hand
luggage to centralized locations
now think about the data we've got
electrical signals and precise timings
from a number of radio telescopes around
the world
but none of those radio telescopes has
enough angular resolution to see the
ring of the black hole
so how do you combine that data and get
finer detail than any of the inputs
well there is additional information in
the relative distances between these
telescopes and in the time delays
between when a wave front hits one
telescope relative to the others
imagine combining the signals from two
distant telescopes
let's say they both received the same
wave at the same time so those waves
were coming in phase
well then the source must have been
located directly between them the radio
waves would have traveled the same
distance to each telescope to arrive at
the same time
except with just two telescopes that
only narrows it down to a line in the
sky that is equidistant from both
telescopes
the source could have been anywhere on
that line
and it's actually worse than that
it's possible that the source could be
exactly one wavelength closer to one of
the telescopes and that way the radio
waves would still arrive perfectly in
phase
where the difference could be two or
three or four wavelengths but you get
the point
so from one pair of telescopes the
information we get about the source is
actually a series of bright and dark
fringes
telescopes that are close together
produce wide fringes while those that
are far apart produce narrow fringes
so to make an image you need pairs of
telescopes at all different orientations
and different distances apart
each pair makes a different interference
pattern
and then by combining all these patterns
we get an image of the black hole which
created them
but now that we have this picture
what exactly is it showing us
well this is how i explained it when the
first image of a black hole was released
so here is my mock black hole of science
and this sphere represents the event
horizon 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 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 holes 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
hey this video was sponsored by kiwiko
creator of awesome hands-on projects for
kids you know i've used kiwiko with my
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really is no substitute for getting your
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plus it's a ton of fun and to me that's
how learning should be i want my kids to
approach learning as play and i've seen
how this fosters their curiosity and
sparks new ideas kiwico have been long
time supporters of the channel i've
visited their offices which really
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kiwico are offering 30 off your first
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