By now you have probably seen it. The image that became iconic on the day of its public release. A century after the mathematics showed it was possible, but widely dismissed as an object that really did not exist in the universe. Decades after it was given a catchy name and the first indirect evidence of its reality was reported, several years after it was depicted in artists’ rendering and popular movies, the object from which light could not escape was “seen”. More properly, the seeing was the shadow of a black hole surrounded by a donut of light (see photograph below). It was the product of the remarkable technical achievement of combining signals from multiple observatories to affect an earth sized radio telescope capable of resolving an object the size of our solar system from a distance of 55 million light years (about 300,000,000,000,000,000,000 miles). That is roughly equivalent to viewing an orange on the surface of the moon.


What is a black hole? We know that the Earth’s gravity will recapture a ball thrown upwards, but not a rocket launched at greater than 25,000 miles per hour, the “escape velocity” from the surface of the Earth . Even before Einstein, some visionary scientists proposed that a source of gravity could be so intense, that even light travelling at 670 million mph could not escape its pull. In 1915 Einstein published the theory of general relativity, the marvelously successful theory of gravity that supplanted Newton’s concept of “mysterious action at a distance” with a new treatment of the geometry of space-time. Rather than treating objects as attracted to another mass by gravity’s force, general relativity describes the way in which mass (and energy) deform space, and objects, including light, merely follow the contours of curved space. The common analogy is to imagine a trampoline or mattress with a bowling ball causing a depression in the surrounding surface, while a marble moving nearby follows the path of least resistance and spirals inward. To paraphrase the physicist James Wheeler: “curved space tells matter how to move, while matter tells space how to curve.” The concept is simple and elegant, but the mathematics to solve specific problems are daunting. One year after its publication, Einstein was surprised to receive a letter from the young mathematician Karl Schwarzshild, then stationed on the Russian front of World War I, which presented an exact solution of the general relativity equations for a spherical mass of sufficient heft that would cause space-time to curve so much that all matter and light would be trapped within. The boundary from which nothing could escape became known as the “event horizon”. Einstein famously congratulated Schwarzschild for his mathematical achievement but contended that such objects did not really exist. The universe need not contain every phenomenon that fit the equations of a theory. Few physicists took up this question, but in 1939, Robert Oppenheimer and Hartland Snyder calculated how a massive star, devoid of its nuclear fuel, would implode indefinitely to a point of “singularity.” Nothing but its gravitational field would persist for outside observers. 25 years later, the term “black hole” was applied to such an object.

The mind-bending properties of a black hole continue to be a subject for the scrutiny of great minds of theoretical physics. General relativity describes matter and space on the large scale, while quantum mechanics describes the properties of the very small with outstanding predictive power. But the two theories have fundamental differences in their mathematical underpinnings, including the very nature of space, which render them incompatible anywhere they are both needed to describe reality. That being where intense mass is confined in tiny spaces. The two places where this collision of theories occurs are at the beginning of the big bang universe, and in black holes. General relativity predicts that nothing will halt the collapse to a singularity of a star greater than about ten times the mass of the sun when it has exhausted the outward pressure of its nuclear fusion. Nor would anything halt the in-fall of an unwary space traveler as she falls into a black hole. But can the universe really have mass contract to an infinitely tiny point? Many scientists hope that an eventual theory of quantum-gravity will show that such a singularity is prevented. The quest for this theory remains one of the greatest challenges of current physics.

The first “detection” of a black hole came not from seeing one directly, but from analyzing its interactions with neighboring stars. For over a decade beginning in the 1960s, improvements in orbiting X-ray observatories provided details about a powerful source of X-rays named Cygnus X-1. Evidence mounted that an optically visible star was orbiting an optically dark companion that was the source of the X-ray emission. The mass and motion of the visible star told that the unseen companion had a mass of about 16 times that of the sun, well into the theoretical range for inevitable collapse into a black hole. The X-ray emission was posited to arise from the violent movement and collisions of particles as the black hole gulped down matter drawn off the companion star. During the years that the observations improved, physicists Kip Thorne and Stephen Hawking had a famous bet as to whether Cygnus X-1 was indeed a black hole. Perhaps Hawking’s concession while visiting Kip Thorne’s office at Caltech in 1990 could be considered the advent of general acceptance that black holes really do exist in our universe. Since then, many other black holes in the size range of stellar masses have been found by measuring their effects on orbiting stars. And in the last three years we have seen the spectacular detection by the LIGO observatories of gravity waves produced by pairs of black holes of 20-30 solar masses in the final moments when they spiraled together to coalesce into a single black hole.

But we now know that black holes far larger than stars abound in the universe.
In 1963, Maarten Schmidt was puzzling over recently detected star-like objects that had incomprehensible spectra. He finally realized that the spectral lines that puzzled astronomers were actually familiar ones that had been enormously red-shifted. Hence, they must originate from exceedingly bright sources at a great distance from our galaxy. Seen as a speck of light beyond our Milky Way, such quasars can outshine all of the billions of stars in their home galaxy. At first, it seemed incomprehensible that such unworldly energy could be produced in a small space. But astronomers realized that gravity is a highly efficient source of available energy, far more so than chemical or even nuclear reactions. Matter falling into a black hole of millions or billions of the mass of our sun is heated by friction as it spirals in from an “accretion disc” of matter. Obviously, by the time such matter falls beneath the event horizon to can no longer give off light of any wavelength, but on the way in, much of the kinetic energy of motion is converted into the emission of radio, visible, ultraviolet and x-rays that are seen in quasars and other “active galactic nuclei”. Once considered an exotic class of objects, astronomers have now found that virtually all large galaxies harbor super-massive black holes at their core. Some weigh billions of solar masses, while our own Milky Way Galaxy has its own core black hole weighing in at 4 million times the mass of the sun.

This brings us to the audacious proposal that black holes could actually be seen. Painters and computer graphics experts created images, and the Nobel-winning gravitational physicist Kip Thorne advised on the visuals of black holes for the movie Interstellar. Single telescopes fall far short of the power to see one. But astronomers have been linking two or more radio telescopes and combining their signals by interferometry to effectively work together as one larger dish. Ever-widening the scope of linked, distant telescopes has greatly increased the resolving power of observations. Shepard Doeleman of Harvard audaciously proposed that linking radio telescopes a world apart could achieve the resolving power to image a black hole. To take on this challenge, the Event Horizon Telescope team grew to over 200 scientists and 8 radio observatories located in four continents. To combine the observations into a virtual one via interferometry requires the melding of radio signals with exquisite timing so they are virtually simultaneous. The world’s most accurate atomic clocks were used to time-stamp all the recorded data from the radio telescopes. Internet connections were inadequate to transmit the enormous load of data, so it was recorded and physically shipped to computer centers in the U.S. and Germany for analysis. Instruments developed by Berkeley SETI scientists contributed to the remarkable electronic and analytical scope of the operation.


The first target was the super-massive black hole in the galaxy M87. Astronomers had already seen that massive jets of charged particles extend thousands of light years from a central source, but the engine powering the emissions remained invisible (see photo above of the emission jet taken with the Hubble Telescope). With the weather cooperating at the many locations, simultaneous observations were made for much of a ten day period in April 2017. Nearly two years were needed to interpret the data and reconstruct an image from the signals seen from all the telescopes. These were compared to hundreds of computer simulations which had applied the mathematics of general relativity to simulated parameters including black hole mass, rotation, orientation of the axis of rotation of the black hole and the surrounding accretion disc and more. On April 10, 2019 the Event Horizon Telescope team lit up the world of science by displaying the image and reviewing the project at simultaneous press conferences. The iconic image shows a dark “hole in space” surrounded by a ring of light rendered somewhat fuzzy by the limit of resolution. (The term “light” is used in a general sense; the radiation detected here has a wavelength in millimeters that is not visible to the eye, and is rendered in arbitrary colors.) That dark edge denotes the inner limit of the stable orbit of photons around the black hole. It is about twice the size of the actual event horizon. Relativity effects greatly distort the path of light emitted from the surrounding accretion disk and from background sources. One can think of the black hole acting as such a powerful lens that it not only bends light rays towards us, but causes some to whip around in orbits like a satellite circling the earth. Photons deviating inward from the ‘last stable photon orbit” become lost forever within the event horizon, while others can travel towards us. The best fit of the image with the computer simulations, as well as the known direction of the radio jets, indicates that we are viewing the black hole from nearly above its axis of rotation and it is spinning clockwise from our perspective. Its spherical shape is consistent with the predictions of general relativity. The increased brightness of the lower quadrant is due to relativistic enhancement of light waves moving towards us. The calculated mass of the black hole is 6.5 billion suns, packed within an event horizon roughly the size of our solar system.


The Event Horizon Telescope team expects to release further analysis that includes polarization measurements to map the intense magnetic fields that whip around the black hole and concentrate and empower the energetic beams of charged particles that spew out in polar directions from M87 and many other quasars and active galactic centers. Future observations at shorter wavelengths and addition of more telescopes are planned to improve the resolution of the image. The next great leap in imaging resolution will require placing radio telescopes in orbit or on the moon.

In the near term, we await the image of the second target of the Event Horizon Telescope, the black hole in the center of our own Milky Way Galaxy, called Sagittarius A*. Its mass, estimated from the motion of stars and gas orbiting very close to the center of the galaxy, is a mere 4 million solar masses. It’s a pipsqueak compared to the M87 black hole, but only 25,000 light years away. It is so much closer that the angular size should be approximately the same as the black hole in M87. Although the observations have already been made, the data reduction has its own challenges. Peering through the disc of our galaxy encounters much obscuring material, and this smaller black hole may be gobbling matter at an irregular rate, causing more rapid changes in the brightness and form of a detected image. While a challenge in image acquisition, such a variation could help astronomers understand the particulars of black hole growth. Perhaps the next black hole image to be released from the Event Horizon Telescope team will be a movie rather that a still picture. Scientists and the interested public await these next images as they continue to transform the elusive mathematical concept of a black hole to a physical entity that can be seen and understood.

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Richard M. Lawn

Superbly written article by Prof. Lawn ... and with a tantalizing ending that will keep us on the edge of our seats.

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I enjoyed reading the article and can't wait for the sequel (movie).

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Thank you for a well written, concise, and understandable explanation.

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I'm a little curious about the, quite frequently heard, issue, that when a member of a binary system collapses to a black hole, it strips material off the companion star. I would expect the black hole to have pretty much the same mass as the original star, perhaps slightly less, which was not stripping material from its neighbour when it was a regular star. Sure, the mass is now concentrated into a much smaller object, and the gravitational field close to the hole is stronger than it was, but it is much smaller., and hence, further away from the companion. How is this issue justified?

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Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.

I'm a little curious about the, quite frequently heard, issue, that when a member of a binary system collapses to a black hole, it strips material off the companion star. I would expect the black hole to have pretty much the same mass as the original star, perhaps slightly less, which was not stripping material from its neighbour when it was a regular star. Sure, the mass is now concentrated into a much smaller object, and the gravitational field close to the hole is stronger than it was, but it is much smaller., and hence, further away from the companion. How is this issue justified?

It doesn't happen immediately after collapse. It starts after some time as their distance decreases accordingly. For examples of evolutions look at this poster.

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I can not wait for the movie on this. Thank you for the very well written information.

I'm assuming that the slightly pentagonal shape of the ring of light around this black hole is simply an artefact of the data processing?

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Saludos !
Interesante como siempre el articulo de agujeros negros ...esperaremos " ver" el agujero de nuestra galaxia.

Thank you for this very informative post and image of the Black Hole.... it is amazing.
I am not a scientist, but SETI is one of my interests. I like to think my small participation
is of some help.

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Süper makale, harika fotoğraf. Tebrikler emek verenlere.

Thank you for a well written and image！！！

Thank you for a wonderful article and images

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>>> It doesn't happen immediately after collapse. It starts after some time as their distance decreases accordingly. For examples of evolutions look at this poster.

The stars existed for many years before the event which created the black hole. They were not approaching each other then. Now, the mass of one of the stars is reduced. Why should they start to approach each other? I would have thought quite the reverse.

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Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.

It's an incredible and combined effort of refined instruments and technics, going farther in the knowledge of our
universe (and what's "outside") requires more and more refined equipment and techniques, nevertheless all this
effort can be easily become useless if the scientific community and goverments allow a few rich an bored egotistical
people to stink the heavens with satellites with the only purpose of become even more rich, all the observational
techniques are being threatened if this "fashion" of satellite constellations is allowed to become a reality, all the
visible, near visible and radio observations will suffer the interference of artificial "almost randomic" objects going
through the visible field of view many times during the observation period not only adding light and radio signals
but also blocking out the light and signals coming from far far away and very weak most of the time, proyects like
SETI can become useless quickly if the scientific community working alongside with the goverments don't stop this
insane practice, there are other hazards that hundred of thousand of small satellites can cause when reach end of
life or get off of their intended orbits, I could see the IAU and ESO warning about this, I could not find any official
message from SETI nor from the scientific community as one, is a paradox that these few people/companies use the
knowledge achieved through the scientific research method but they finally use it against science, technological
predators work fast to achieve their goals, once the problem is there, nobody can do anything to remove them, moving
fast to stop this should be prioritary for science researchers.

Dark&Silent skies

Regards
Carlos

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I was reading about a plan for a huge new constellation earlier today, Amazon want to put up 3,236 satelites for their project...

https://www.theverge.com/2019/4/4/18295310/amazon-project-kuiper-satellite-internet-low-earth-orbit-facebook-spacex-starlink

... the article lists a few other schemes.

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Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.

Excellent write up! I am an amateur, at best, and this was easy to follow and understand. I continue to be fascinated by our tiny existence in the grand scheme of things!

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I was reading about a plan for a huge new constellation earlier today, Amazon want to put up 3,236 satelites for their project...

https://www.theverge.com/2019/4/4/18295310/amazon-project-kuiper-satellite-internet-low-earth-orbit-facebook-spacex-starlink

... the article lists a few other schemes.

there are at least 4 companies trying to do the same, combined the number of rubbish floating over our heads and interrupting the light and
signals from the remote space will be so high that will perform as a solid mesh, try looking through a high density mesh in movement and you
will have a notion, I even wonder whether the DSN will be affected when sending and receiving data/instructions from and to the very remote
spaceprobes as Voyager 1&2, New Horizons, etc, this may become the already difficult communication with these remote probes even harder
to the point of almost impossible, if precise reprogramming is not sent at the exact moment, any delay may cause the lost of the probe, remember
the two way light time exceed one earth's day, you must wait so long to know whether the packets arrived complete, if not, repeat it all again.

In the end money and power rules the world, scientist only give them another powerful tool to make money and gain power, but they will end
killing the golden eggs chicken...

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Absolutely. What is needed is a small constellation of relays satelites orbiting the moon, and a real scientific observatory, deep space comms station and laboratory on the "dark side" . Of course, there is nobody that would pay for such an endeavour.

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Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.

I'm watching these shows all the time on the Science Channel: fascinating to "know" end. So far away, so huge, so thoroughly beyond comprehension. Everything in the Universe serves a purpose, can't wait to find out what they are for a Black Hole. My thought has always been that they have something to do with recycling as a form of re-creation, it's just that right now we can't know what's on the other side of one of these things. Smart money says there are those out there wondering the same thing about the very same hole only in direct contrast to this side of it, and where it leads to. If they could only imagine us.

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