By now, everyone must have seen and marveled at the first ever direct image of a black hole, which was released on Wed April 10 by the Event Horizon Telescope (EHT) team. There is a slew of good articles on the subject that contain the images, descriptions of how the world-wide team of researchers used a set of 8 telescopes spread across the globe to produce the images and how the massive 5 petabytes of data was transported using disk drives rather than the Internet.
Here we provide some supplementary information to help understand the EHT effort and the images themselves. Since I am not an astrophysicist, I am hoping that others will points out if I made any major mistakes in the analysis or the descriptions. The diary is rather large, but you can read selected sections while skipping others, if needed.
A Summary of the News
The EHT team started on this quest over two years ago, and used 8 radio telescopes to capture radio waves from two supermassive black holes (SMBH) in April 2017 and April 2018.
EHT researchers focused on two black holes — Sagittarius A*, the SMBH at the center of our own galaxy, with a mass 4 million times that of our sun and the monstrous SMBH with 6.5 billion solar masses, located in the supergiant elliptical galaxy M87, 53.5 million light years away.
The released image is from the April 2017 run and is that of the M87 black hole. Data on Sag A* has not been released yet.
Six papers contain the details of the images and the science behind the collaborative work. There is lot more info in the data than the iconic image.
The Name Pōwehi
The black hole has an unofficial nickname — Pōwehi, meaning embellished dark source of unending creation. It is a name sourced from the Kumulipo, the primordial chant describing the creation of the Hawaiian universe. Pō, profound dark source of unending creation, is a concept emphasized and repeated in the Kumulipo, while wehi, or wehiwehi, honored with embellishments, is one of many descriptions of pō in the chant.
How to Image a Black Hole
Supermassive black holes (SMBHs) are big in terms of mass, but their size and volume, as defined by the black hole event horizon, is relatively small. These objects and their immediate surrounding would occupy but a fraction of a pixel when imaged using the best telescopes available.
Here is an image taken by the Hubble Space Telescope of the galactic core of M87 showing a blue plasma jet that extends 4,900 light-years (ly) across space. The black hole at the center of M87 is only 0.0041 ly across, 1.2 million squared times smaller than the area covered by this image.
Here is an X-ray image of M87 taken by Chandra and overlaid with radio images taken by the VLA; it covers a field 85,000 light years across.
Here is an interesting animation that takes a viewer on a journey into the center of M87.
So, imaging the black holes required a telescope with a very wide aperture, almost as wide as the Earth. Hence, astronomers used a network of smaller radio telescopes spread around the globe as a wide aperture virtual telescope. Data was collected simultaneously by these telescopes, with precisely synchronized clocks and then processed using software and algorithms to tease out the details of the images. The black holes were imaged at radio waves
at a wavelength of 1.3 mm (230 GHz), not visible light.
Transporting the Data
The observations generated 5 petabytes of data, which had to be transported to the data processing centers in the U.S. and Germany. 5 petabytes is equivalent to 5,000 one terabyte disk drives. At a whopping and expensive 1 Gbps Internet link, it would have taken 1.5 years to transfer all the data. Hence, data was transferred the old-fashioned way — by physically transporting high-performance helium-filled hard drives to the Max Planck Institute for Radio Astronomy in Germany and to the MIT Haystack Observatory.
Data from the South Pole telescope had to wait until December 2017, when summer and the first planes of the season arrived at the South Pole.
A Few Videos
Here are a few videos that provide a good overview of the EHT, the telescopes, black holes, M87, the researchers and the technology used in this multi-year effort.
The Smithsonian Channel will be broadcasting a new documentary on the EHT project this Friday (9:00 p.m. and 12:00 a.m.).
The Faces and Voices of the Collaborators
The EHT collaboration involves more than 200 researchers from Africa, Asia, Europe, North and South America.
The EHT consortium consists of 13 stakeholder institutes -
- Academia Sinica Institute of Astronomy and Astrophysics (Taiwan)
- University of Arizona
- University of Chicago
- East Asian Observatory (Hawaii, run by Japan, China, S. Korea and Taiwan)
- Goethe-Universitaet Frankfurt
- Institut de Radioastronomie Millimétrique (French-German-Spanish)
- Large Millimeter Telescope (located in Mexico)
- Max Planck Institute for Radio Astronomy (Germany)
- MIT Haystack Observatory
- National Astronomical Observatory of Japan
- Perimeter Institute for Theoretical Physics (Canada)
- Radboud University (Netherlands)
- Smithsonian Astrophysical Observatory
How Big are these Black Holes?
A black hole itself occupies zero volume in space, but its size is generally defined by the Event Horizon, the boundary of the region surrounding it from which no light can escape.
The Event Horizon radius can be calculated with the simple equation R = 2GM/c2, where G is the gravitational constant, M is the mass of the black hole and c is the speed of light. Alternatively, the mass of the black hole can be calculated given an estimate of the event horizon radius.
Here are some calculations that illustrate the equation -
|
M87 |
Sag A* |
Notes |
Distance from Earth (million ly) |
53.5 |
0.0256 |
|
M (Suns) |
6.50E+09 |
4.30E+06 |
Mass in solar mass equivalents |
M (kg) |
1.29E+40 |
8.55E+36 |
1 Solar mass = 1.989E+30 kg |
Event Horizon Radius
R (m)
|
1.92E+13 |
1.27E+10 |
R = 2GM/c2
G = 6.674E-11 m3kg−1s−2
c = 3E+8 m/s
|
R (km) |
1.92E+10 |
1.27E+07 |
|
R (billion km) |
19.174 |
0.013 |
|
R (AU)
|
128.17 |
0.08 |
1 AU = 149.6 million km = average Sun-Earth distance |
Photon ring radius (AU) |
333.24 |
0.22 |
The photon ring radius is about 2.6 times the event horizon radius |
Ring angle as seen from Earth (μas) |
40.6 |
56 |
μas = micro arc seconds = 1/3,600,000,000 of a degree
An arc second is 1/3600 of a degree
Ring angle (radians) = ring diameter / distance from Earth
|
Measured ring angle (μas) |
42 ± 3 |
|
As measured by EHT |
For comparison, the average Sun-Pluto distance is 39.5 AU and the current distance of Voyager 1 from the Sun is 141 AU.
Note that black hole image consists of a dark circle surrounded by a bright circular area. The ring refers to a region that extends beyond the dark area about half-way into the thickness of the surrounding bright ring. The event horizon (EH) itself cannot be seen. The ring radius is about 2.6 times that of the EH (more on why later). The ring radius we see in the image has a radius of 333 AU, while the black hole itself has a radius of 128 AU. The image itself is blurred around the ring line due to the limited resolution of the EHT.
The following xkcd comic shows the M87 black hole image overlaid with the size of the solar system.
The following is a slightly more accurate and complete picture (made by me) of the scale of the M87 black hole relative to the Solar system. Yes, M87* is big, the event horizon is as wide as the solar system.
The Asymmetric Ring
Another notable feature of the image is that the ring surrounding the black centre is brighter on one side than on the other. That’s because the brighter portions arise from regions that are moving toward Earth at nearly the speed of light, while the dimmer parts come from regions that are moving away.
The Orange/Yellow Color
Is the light emanating from the region around the black hole orange and yellow in color, as depicted in the images?
No. The images were made using radio waves, which cannot be seen by the human eye, let alone be seen as a specific color. The scientists used a yellow/orange color scheme to highlight the brightness differences in various parts of the image. They could have some other color scheme, it would not make any difference.
The Physics behind the Ring
The following video does a marvelous job of explaining the event horizon, the accretion disk swirling around the black hole and how light bends around the black hole to create the shadow and the ring, which is 2.5x - 2.6x the event horizon size. The ring we are seeing is of light from behind the black hole and around it bent around by gravity and directed towards us. Light entering the shadow region tends to fall into the black hole.
Here is a video from NSF explaining the concept using animation.
The following tweet shows images predicted by simulations of black holes depending on their orientation with respect to Earth.
The spin axis of the M87 black hole is estimated to be tilted by 17 degrees in the direction away from Earth.
Would have been so cool if the image looked like the 3rd one in the first row, but that requires an orientation where our line of sight is in the plane of the accretion disk.
Einstein and General Relativity
Most people associate black holes with Einstein, even though he did not believe they could exist. However, his theory of General Relativity predicted the existence of black holes. It took decades and many other researchers to develop the theory of black holes. The latest measurements provide another confirmation of General Relativity and its predictions of physics near a black hole.
The black hole in the galaxy M87 is known as a Kerr black hole, because it is a rotating black hole.
A Short Primer on Black Holes
What is a black hole you ask? Here is a short primer.
A black hole is a region of space where matter has collapsed in on itself, due to the force of gravity. This collapse results in a huge amount of mass being concentrated in an infinitely small volume. The gravitational pull of this region is so great that nothing can escape – not even light.
As a star ages, its nuclear fusion reactions stop because the fuel for these reactions gets depleted. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the outer layers of material and radiation blasts out into space. What remains is the highly compressed and extremely massive core.
After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.
The observable universe is estimated to contain 1019 black holes. hubblesite.org/...
Black holes come in various sizes and need not be super massive -
The smallest black hole that can be formed by natural processes at the current stage of the universe has over twice the mass of the Sun.
Check out the video below and the one at www.pbs.org/… called Black Hole Apocalypse.
Follow the twitter chain below for another concise description of Black Holes.
Black Hole Event Horizon
The boundary of the region around a black hole from which no escape is possible is called the event horizon. Particles, including photons, that pass through the event horizon are swallowed by the black hole. Inside the event horizon, all "events" (points in space-time) stop, and nothing (not even light) can escape. Our current theories of physics do not apply inside a black hole.
The radius of the event horizon is called the Schwarzschild radius, named after astronomer Karl Schwarzschild, whose work led to the theory of black holes. The Schwarzschild radius = 2GM/c2, where G is the gravitational constant, M is the object mass and c is the speed of light.
Types of Black Holes
There are two types of black holes:
- Schwarzschild - Non-rotating black hole
- Kerr - Rotating black hole
Black Hole Observation
Blacks holes cannot be directly observed since they do not generate or reflect electromagnetic radiation. The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, from which the jets – ultra-powerful beams of energy – are shot out into space, perpendicular to the plane of the disk, creating some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location.
A black hole, due to its massive gravity, creates the gravitational lens effect, which bends electromagnetic waves, including light, from other celestial objects that pass near the black hole. The following gif shows an animated simulation of gravitational lensing caused by a Schwarzschild black hole going past a galaxy in the background. A secondary image of the galaxy can be seen within the black hole Einstein ring on the opposite direction of that of the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole.
Does Nothing Escape a Black Hole?
Hawking radiation is blackbody radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974.
An explanation of the process is that vacuum fluctuations cause a particle–antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole while the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). This causes the black hole to lose mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunnelling effect, whereby particle–antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.
Rotating black holes can lose energy as described by the Penrose process (named after the English physicist Roger Penrose); the black hole loses some of its angular momentum in the process. The energy loss is made possible because the rotational energy of the black hole is located not inside the event horizon, but on the outside of it in the ergosphere, in which a particle is propelled with the rotating spacetime. See en.wikipedia.org/… for details.
Epilogue
This effort and discovery is a tour de force of human ingenuity and endeavor. Even though much is known about black holes at a theoretical level, direct measurements help solidify the science and help weed out alternate theories. The effort itself is a technological tour de force, in terms of the math, science and computational advances required to collect data from around the world with extremely high precision and process peta bytes of information, guided by theory and simulations. There is lot more data to come and lot more discoveries to be made.
Why is this important for us at DK? Politics is not just about the news of the day and about trump’s latest antics. It is also about science, about understanding the Universe and our place in it, about peeking into and planning for the future.
What do you think?
References
- eventhorizontelescope.org
- The papers — iopscience.iop.org/…
- Astronomers Capture First Image of a Black Hole — www.eso.org/...
- 10 Deep Lessons From Our First Image Of A Black Hole's Event Horizon — www.forbes.com/…
- First-ever picture of a black hole unveiled — www.nationalgeographic.com/
- What the Sight of a Black Hole Means to a Black Hole Physicist — www.quantamagazine.org/...
- M87 — en.wikipedia.org/…
- NASA Black Hole page — www.nasa.gov/…
- Black hole wiki — en.wikipedia.org/...
- It's Black Hole Friday — (some artist renderings and tweets about black holes) — www.dailykos.com/…
- A Star is Spaghettified — www.dailykos.com/…