-Black holes don't exactly ham it up for the camera:
since light cannot escape from these massive objects,
they're difficult to spot, let alone photograph.
But despite the immense challenges
of a black hole photo shoot,
the Event Horizon Telescope Collaboration
managed to snap the first picture
of a black hole in history
using a virtual Earth-sized telescope.
The international team aims to unravel
the mysteries of supermassive black holes,
including the one lurking at the center of our own galaxy.
♪♪
It's a real pleasure to welcome Ziri Younsi,
an astrophysicist, and UKRI Stephen Hawking fellow
at University College London's
Mullard Space Science Laboratory,
and he's also a member
of the Event Horizon Telescope Collaboration.
Thank you so much for talking to us today.
-My pleasure, Becky. Thanks for having me on.
-Well, this telescope is so fascinating,
both in terms of the really mind boggling targets
that it is taking pictures of -- black holes --
but also just in terms of its sheer scale.
So I just wanted to start by asking you,
what is the telescope and how does it work?
-It's actually a very interesting question
because it isn't just one telescope.
It's actually a network of telescopes
spread all across the world.
And for this famous image
that we first saw in April 2019,
that was actually the combined efforts
of eight radio telescopes spread all around the world,
including even in the South Pole.
And so it's eight telescopes that are connected together
through a very special technique,
because obviously you're recording that information
at each telescope,
but that -- the radiation that comes from the black hole,
which I'm sure will talk about a little bit later
in more detail, arrives at slightly different times.
So we have to use a very special technique called interferometry
to connect and synchronize these telescopes.
And so we record the data
at all of these different telescopes simultaneously,
and it's all stored on hard disks.
And then it's all brought together at the very end
to -- to put the data together synthetically.
So it's not one telescope.
It's actually a big network.
-And it's an Earth-scale network.
Is does that make it the biggest interferometer
that exists right now?
-Yes, that's absolutely correct.
It's the biggest interferometer that exists right now
and it works at radio wavelengths.
And it is precisely because of this scale,
because it's Earth-size,
that we can achieve the resolutions
that we need to resolve a black hole,
because even though these supermassive black holes,
like the one in M87,
in the galaxy Messier 87, rather,
that we saw, it's an enormous black hole,
but it's actually still very, very, very far away.
So it's angular size on the sky is absolutely miniscule.
So it's an unprecedented technological feat, really,
to be able to achieve the kind of resolutions
that we achieved as a collaboration
to resolve it in the first place.
-And why was Messier 87 the target?
You know, there's black holes at the center
of all these galaxies; what made this one
the photogenic target for this?
-Well, that's a great question.
And actually -- it's actually
a perfectly legitimate question too --
why do we focus on that black hole?
Well, actually, the one we really want
is in the galactic center.
That's our own supermassive black hole,
very special place in astronomers' hearts,
and we've been studying it for a very long time.
But that's quite a difficult one to image.
Messier 87 is one of the biggest black holes in the Universe.
It's absolutely enormous.
It weighs in at over 6.5 billion times
the mass of our sun.
So it's up there,
it's in the top few black supermassive black holes
that we know, and because of its enormous size
and relatively it's quite close to us,
what that means is that if you look at the --
at the actual size of this thing compared to its distance --
its angular size is just within the limits of us being able
to resolve it with the EHT, the event Horizon Telescope.
-Well, it's so -- it's funny to hear, like,
the easiest black hole to image,
but it's actually like --
-Still pretty tough. -Yeah, right? Yeah!
It's -- they're notoriously very difficult to see
because, of course, they don't emit light.
Light can't escape them.
So could you talk a little bit about the challenges of that
and the kind of resolution that this telescope needs
to see something that's not only 55 million light years away,
but also is a black hole!
-As you said, like, it's very difficult to see
because you can't directly see a black hole.
It has an event horizon.
And so anything which crosses that event horizon,
even light itself, will be forever trapped.
And light which strays very close to the black hole
can actually orbit the black hole multiple times
in spherical orbits.
So it's kind of like if you were near a black hole
very close to a black hole event horizon,
you would see multiple images of the back of your own head
because light which bounces off your head,
which would orbit multiple times before it reaches you
and each one would be one orbit, two orbits, and so on.
So you see very weird things.
Light doesn't behave as it does here on Earth
and travel in straight lines.
So even your perception of what you see would be totally warped.
So there's a few ways
that we use to explain this when we give, like, public talks,
and I like -- my two favorites, actually,
are, the first one is, it's like trying to resolve an orange
on the surface of the moon with your naked eye;
or another would be holding your finger out in front of you
and being able to see individual atoms in your own finger.
So that's the -- that's the scale of
the resolving power of the EHT.
So -- and even now, we can just about resolve
the size of the event horizon itself.
So we're not yet on scale smaller than that.
And in time we will be, but this is just the first step.
-That is just a phenomenal achievement.
And yeah, thinking about looking at the atoms in my hand
is just -- really puts it into perspective.
So, you know, when this image did come out in 2019,
we being space nerds here at the show, of course loved it,
but I was actually really happy to see the public.
I mean, people who had never really
thought about black holes I saw a posting on Twitter
and social media.
It really was this big public splash
and people were really engaged with it.
And I think it was a great way to -- to show that, you know,
show something visual about a black hole
that brought in a whole new audience of space lovers.
So what was that like for you as a member of the collaboration
to see how excited people were about this image?
-Oh, it was -- it was amazing.
And I have to say, it was not expected.
It was completely unanticipated, actually, at least on my part.
We didn't expect that that image would resonate with people
so -- so strongly.
And because -- okay, so from my own background
is I've been working on these kind of images for years,
ever since I was a graduate student.
And so to me, it wasn't a surprise to see the image
in the sense that we just saw, "Oh, it kind of looks a lot
like what the theoretical calculations predict,"
and, "Oh, we have a cool picture now."
And obviously this is the picture
that would be used and shared around the world.
But we didn't expect it would be covered by so much media;
there would be the positive, you know,
the coverage was phenomenal.
And then secondly, you didn't expect it
to sort of touch people in quite --
quite a human and almost sort of existential ways.
Right? So, like, it -- it's just quite an evocative image
for a lot of people, ourselves included.
And the response was amazing.
So some people would see, like, you know,
bagels or cosmic donuts and stuff like this.
And then other people would see the Eye of Sauron,
you know, so -- and to some --
and, you know, and then for other people still,
they would -- they would see it as this, like,
beautiful, mysterious object.
Others would see it as like, you know,
the edge of the gates of hell and this kind of stuff.
-It was wonderful to see, and, you know,
because black holes are often kind of more in the theory,
in the mathematical calculation realm and stuff,
to -- to have that, that's -- that's something
that's much more relatable.
We should explain that it's not the Eye of Sauron
or anything malevolent.
You know, what are we actually looking at, what's that ring?
And why is it brighter on the one side than the other?
That ring we believe to actually be extremely hot gas or plasma
that's spin-- it's circulating.
It's orbiting around the black hole
and it's gradually falling inwards.
And that plasma is extremely hot
and it's emitting a lot of radiation.
And the reason why it's brighter in one part of the image
more towards the bottom half,
a dimmer on the top half
is due to do with the fact that the black hole
is actually spinning quite rapidly, we believe.
And because it's spinning rapidly,
part of it is approaching us.
And then, of course, conversely,
another part of it receding away from us.
So the part which is approaching us is actually brighter
because it's Doppler boosted into our line of sight.
A little bit like when you hear an ambulance come by
and it's approaching, you hear it getting louder,
right, but when it's moving away,
it's -- [distorted]
the sound is stretched,
and it appears to sound a little bit quieter than it would be
if it was stationary. -Along those lines,
like, what can we learn from this line of study?
And particularly like, you know, black holes, obviously,
they're really interesting on their own merits,
but what can they tell us broadly about the Universe?
And how can the Event Horizon telescope
shed light on this kind of stuff?
-Black holes have the strongest gravitational fields
of the Universe, so if you can start to understand
more about what's going on about gravity in that regime,
then you understand more about what space and time really mean,
and you understand more about what it is
to have cosmic expansion and where we come from.
And so, you know, it really touches at the heart
of basic human understanding of what governs the Universe,
because gravity really is the weakest force
by many, many orders of magnitude,
compared to the three other fundamental forces.
And yet it's -- it seems to be the force which dominates.
-Absolutely, and it's so it's interesting to me that,
you know, black holes being,
you know, these non-luminous, very mysterious objects,
but actually illuminating -- -Yeah, absolutely.
-...so many different parts of science.
Yeah, they are -- they are truly amazing laboratories.
You know, speaking of giant observational projects like EHT,
I read that you were also part of the LISA collaboration.
This one is just mind boggling to me.
And I'd love if you could just talk
a little bit about that project and what it will --
-Yeah, sure -- yes, I joined LISA -- so this is
the Laser Interferometer Space Antenna mission.
A few -- about two years ago, actually,
when I moved back to the UK.
But so what is LISA?
It's a space-based gravitational waves observatory.
But it's very special -- so first of all,
it isn't going to launch in at least the mid 2030s.
So it's a way away still.
What LISA will be able to do
is extend the sensitivity
such that you'd be able to look at supermassive black holes.
And they're a lot more interesting,
scientifically speaking, for various reasons.
But understanding what's going on
with binary supermassive black holes
will tell us a lot more about the early Universe,
it will tell us a lot more about black hole physics itself.
And it allows us to probe Einstein's theory
in this really dynamical regime.
And also, there's going to be a lot of stuff
that we don't even expect to detect,
that we can't anticipate --
sort of serendipitous science,
which is really true science.
We will find things, I'm sure, which we just don't anticipate.
And that's really the exciting thing.
And actually, what LISA will enable us to do
is look at objects like neutron stars
in our galactic center orbiting Sagittarius A tar
and use the precise timing properties
on the waveforms that we detect to actually measure
and probe spacetime structure, and to actually map spacetime.
And so instead of saying, right,
we want to measure the mass or spin of a black hole,
we can actually use this this object,
which is like a test mass,
and it's very precise, we know the mass,
because neutron stars don't vary much in mass --
and you can actually, like, almost perform cartography
of spacetime in principle.
So you can really measure
the geometrical structure of the spacetime,
and unambiguously answer once and for all
the questions about the mass, the spin,
which theory of gravity, is Einstein correct?
So I think in the 2030s and beyond,
we're getting into the realm of, like,
really understanding gravity a lot more intimately.
♪♪