Cheese! - The first-ever image of a black hole ( and its movie)
They cannot be seen, yet they are not hiding. Nothing can escape to them, yet they are not
chasing anything: they are just so...attractive. At first, Einstein didn't believe they could
exist in physical reality, but today we know he was wrong.
Because they exist. We know it for sure: in 2020, professor Roger Penrose shared the Nobel
Prize for his famous discovery that their formation is a robust prediction of the general
theory of relativity. I didn't even mention them, but you all know
what I am talking about. Some scientists spent their lives studying
these objects: Stephen Hawking was just one of the many.
Ladies and gentlemen, fasten your seatbelts and follow me on this journey because today
I am going to tell you the story of how we took a picture of a Black Hole(M87).
In 2019 the world saw, for the first time ever, the image of a black hole.
It was made a huge effort to bring this result home, and plenty of scientists from all over
the world were involved in the project. I will tell you the story that lies behind this
event, I will introduce some scientists to you that made this possible, and I am going
to explain to you how we did it, how much important it was for science, and why.
But...let's start with some information about black holes:
despite their name, black holes are anything but empty space.
Rather, they are some great, giant amount of matter packed into a very small area - think
of a star ten times more massive than the Sun squeezed into a sphere approximately the
diameter of New York City.
The idea of an object in space so massive and dense that not even light itself could
escape, it has been around for centuries. Most famously, black holes were predicted
by Einstein's theory of general relativity, which showed that when a massive star dies,
it leaves behind a small, dense remnant core. If the core's mass is more than about three
times the mass of the Sun, the equations showed, the force of gravity overwhelms all other
forces and produces a black hole. We can say that one Star's End is a Black Hole's Beginning.
But...black holes...are black! Do you have any idea how should we make to observe them?
Scientists can't directly observe black holes with telescopes that detect x-rays, light,
or other forms of electromagnetic radiation, because they trap electromagnetic radiation.
The answer is: we use indirect methods. We can infer the presence of black holes and
study them by detecting their effect on other matter nearby. For example, if a star passes
close to a black hole, the black hole can tear the star apart as it pulls it toward
itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate
into space. Recent discoveries offer some tantalizing
evidence that black holes have a dramatic influence on the neighborhoods around them
- emitting powerful gamma-ray bursts, extraordinarily energetic radiation. This type of radiation
occurs, for example, when mass is converted into energy during fission reactions that
run nuclear reactors on Earth. But in the case of black holes, gamma radiation is even
more energetic than that produced in nuclear reactors and is the product of very different
processes. This powerful emission devours nearby stars
and spurs the growth of new stars in some areas while stalling it in others.
Although the basic formation process is understood, one mystery in the science of black holes
was that they appeared to exist on two radically different size scales. On the one end, there
are the black holes that are the remnants of massive stars (we will refer to them as
“Babies”). On the other end of the size spectrum are
the giants known as "supermassive" black holes, which are millions, if not billions, of times
as massive as the Sun (we will call them “Giants”) Astronomers found that supermassive black
holes lie at the center of virtually all large galaxies, even our own Milky Way. Scientists
Andrea Ghenz and Reinhard Genzel demonstrated such a link in our Galaxy, and for that they
shared the 2020 Nobel Prize in Physics with Roger Penrose.
Historically, astronomers have long believed that only Babies and Giants black holes could
exist and that no mid-sized black holes exist. However, using the twin LIGO detectors located
in Livingston, Louisiana, and Hanford, Washington, and the Virgo detector located near Pisa,
Italy, they have detected gravitational waves from the most massive binary black hole merger
ever discovered. The two spinning black holes merged when the Universe was only about 7
billion years old, which is roughly half its present age, and formed a larger black hole
weighing a whopping 142 times the mass of the Sun — a so-called intermediate-mass
black hole. Dubbed GW190521, the gravitational-wave signal was detected on May 21, 2019, at 03:02:29
UTC.
Now we are ready to dive into the most exciting part of the video: how did we take a picture
of a black hole?
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The answer is very simple: we let him say “cheese”!
Ok, joking.
As we said before, by its very nature, a black hole cannot be seen, but the hot disk of material
that encircles it shines bright. Against a bright backdrop, such as this disk, a black
hole appears to cast a shadow. We took a picture of that shadow. So we are not facing a black
hole: we are more likely to say that we are admiring his shadow.
The stunning image shows the shadow of the supermassive black hole (it's a giant one!)
in the center of Messier 87 (M87), an elliptical galaxy some 55 million light-years from Earth.
This black hole is 6.5 billion times the mass of the Sun.
On jpl website we can read about the technique that was used to take the picture:
“The ability to image an object so distant still eluded scientists.
A team formed to take on the challenge, creating a network of telescopes known as the Event
Horizon Telescope, or the EHT. They set out to capture an image of a black hole by improving
upon a technique that allows for the imaging of far-away objects, known as Very Long Baseline
Interferometry, or VLBI. Telescopes of all types are used to see distant
objects. The larger the diameter, or aperture, of the telescope, the greater its ability
to gather more light and the higher its resolution (or ability to image fine details). To see
details in objects that are far away and appear small and dim from Earth, we need to gather
as much light as possible with very high resolution, so we need to use a telescope with a large
aperture. That’s why the VLBI technique was essential
to capturing the black hole image. VLBI works by creating an array of smaller telescopes
that can be synchronized to focus on the same object at the same time and act as a giant
virtual telescope. In some cases, the smaller telescopes are also an array of multiple telescopes.
This technique has been used to track spacecraft and to image distant cosmic radio sources,
such as quasars. Each telescope in the array focuses on the
target, in this case, the black hole, and collects data from its location on Earth,
providing a portion of the EHT’s full view. The more telescopes in the array that are
widely spaced, the better the image resolution.”
Just to give you some terms for comparison, their feat has been compared to resolving
the shape of a doughnut on the surface of the Moon from Earth.
“This is an amazing accomplishment by the EHT team,” said Paul Hertz, director of
the astrophysics division at NASA Headquarters in Washington. “Years ago, we thought we
would have to build a very large space telescope to image a black hole. By getting radio telescopes
around the world to work in concert like one instrument, the EHT team achieved this, decades
ahead of time.”
This marked, once again, the importance of radio astronomy and international cooperation:
we are able to improve our knowledge only if we work together as humanity like there
are no divisions.
EHT observations could lead to images of strong gravity effects that are expected near a black
hole, and to the direct detection of dynamics near the black hole as matter orbits at near-light
speeds. This capability would open a new window on the study of general relativity
in the strong-field regime, accretion and outflow processes at the edge of a black hole,
the existence of event horizons, and fundamental black hole physics.
The EHT leverages considerable global investment to create a fundamentally new instrument with
an angular resolving power that is the highest possible from the surface of the Earth.
Over the coming years, the international EHT team will mount observing campaigns of increasing
resolving power and sensitivity, aiming to bring black holes into focus.
Now, ladies and gentlemen, I want to introduce to you the famous 29-year old computer scientist
that has earned plaudits worldwide for helping develop the algorithm that created the first-ever
image of a black hole. Katie Bouman led the development of a computer
program that made the breakthrough image possible. In an interview, she said:
“No telescope actually takes a picture. What happens is, the light from the black
hole travels 55 million light-years and then every dish collects a single stream of the
light that it sees at the same time. That’s recorded onto these hard drives.
The hard drives are sent on airplanes to a central location, where they’re computationally
processed. But it’s incomplete. The process of imaging
is taking the incomplete information that we get from a couple of places on our virtual
telescope, and trying to fill in all the missing information to get the picture an actual Earth-sized
telescope would have produced. That is a hard problem: there’s an infinite
number of possible images that could have been created from the sparse measurements
that we took. The goal of imaging is to find the image that not only reconstructs and matches
the data that we measured, but also is the one that is most likely.
We have to impose some information about what the image should look like in order to recover
that image. Some stuff that we impose is natural and easy — we know that light is positive.
You can’t have negative light. Other things we might impose would be how
smooth the image is. You wouldn’t expect an image of a black hole to look like the
white noise you get when you pull a cable out of your television.
You really don’t want to accidentally tell our imaging algorithms that, for example,
“Oh, what is likely is this ring shape,” because then we just recover that ring back,
and we’ve learned nothing. We worked separately for a month, not talking
to each other about anything. Then after one month, we all gathered together
in Cambridge, Massachusetts, and we put all the images up on a screen at one time. I think
that was the most amazing moment because even though each of the other images had different
underlying assumptions and looked different, this ring appeared in all of the images.
The ring was always the same size, and it was brighter in the south. That was huge.”
Excitedly bracing herself for the groundbreaking moment, Dr. Bouman was pictured loading the
image on her laptop." Watching in disbelief as the first image I ever made of a black
hole was in the process of being reconstructed," she wrote in the caption to the Facebook post.
Katie Bouman's picture became a symbol of this discovery.
All's well that ends well, but this story is a long-lasting one because the historic
first image of a black hole unveiled last year has now been turned into a movie. There
is a short sequence of frames that shows how the appearance of the black hole’s surroundings
changes over the years as its gravity stirs the material around it into a constant maelstrom.
To create them, the Event Horizon Telescope took old data on the black hole and combined
these with a mathematical model based on the image released in 2019, to show how the surroundings
have evolved over eight years. The work, which appeared on 23 September in The Astrophysical
Journal, offers a taste of what the team might be able to do in the near future, as its techniques
improve. “In a few years, it could really start to look like a movie,” says lead author
Maciek Wielgus, a radio astronomer at Harvard University in Cambridge, Massachusetts.