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Photographing a black hole?

As teased earlier this month, today the Event Horizon Telescope (EHT) project announced and presented the first ever photographs of a black hole — “the last photon orbit.” Another epic story of big science and an international team. The interplay of models and simulations, data capture, and complex processing. And funding.

Much news coverage. Here’s an article by Space.com: “Eureka! Black Hole Photographed for 1st Time.” I’ll add other articles later.

“We have seen what we thought was unseeable,” Sheperd Doeleman, of Harvard University and the Harvard-Smithsonian Center for Astrophysics, said today (April 10) during a press conference at the National Press Club in Washington, D.C.

Doeleman directs the Event Horizon Telescope (EHT) project, which captured the epic imagery. These four photos, which were unveiled today at press events around the world and in a series of published papers, outline the contours of the monster black hole lurking at the heart of the elliptical galaxy M87.

And in case you’re wondering about Sagittarius A*: The EHT team hopes to get imagery of that supermassive black hole soon, Doeleman said today. The researchers looked at M87 first, and it’s a bit easier to resolve than Sagittarius A* because it’s less variable over short timescales, he explained.

In addition, the shape of an event horizon can reveal whether a black hole is spinning, said Fiona Harrison of the California Institute of Technology, the principal investigator of NASA’s black-hole-studying Nuclear Spectroscopic Telescope Array (NuSTAR) mission.

The National Science Foundation (NSF) news has detailed coverage, including papers about the announcement which were published in a special issue of The Astrophysical Journal Letters. See their video “BRIEF, SELF-CONTAINED, NARRATED OVERVIEW of Event Horizon Telescope project and the first black hole.

Space photo
Black hole silhouette. Credit: Event Horizon Telescope collaboration et al.

Here’s the NSF YouTube video of the press event:

Actual press conference starts after ~38 minutes in video.

Other YouTube videos

Perimeter Institute for Theoretical Physics (published on Apr 10, 2019)

The EHT Collaboration consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.

Time – Scientists from The NSF Hold Conference on Results from The Event Horizon Telescope.

Scientists from the National Science Foundation hold a news conference on the groundbreaking results from the Event Horizon Telescope, releasing the first image of a black hole – a dense mass that distorts space and time, with gravity so strong not even light can escape.

Veritasium (published on Apr 10, 2019) – which explains the 40 microarcsecond (μas) image, with links to additional information in the description.

The Event Horizon Telescope Collaboration observed the supermassive black holes at the center of M87 and our Milky Way galaxy (SgrA*) finding the dark central shadow in accordance with General Relativity, further demonstrating the power of this 100 year-old theory.

TEDx Talks (published Dec 7, 2016) via Phys.org Scientist superstar Katie Bouman designed algorithm for black hole image

To take a photo of a black hole, you’d need a telescope the size of a planet. That’s not really feasible, but Katie Bouman and her team came up with an alternative solution involving complex algorithms and global cooperation. Katie Bouman is a Ph.D. candidate in the Computer Science and Artificial Intelligence Laboratory (CSAIL) at the Massachusetts Institute of Technology (MIT), under the supervision of William T. Freeman. She previously received a B.S.E. in Electrical Engineering from the University of Michigan, Ann Arbor, MI in 2011 and an S.M. degree in Electrical Engineering and Computer Science from MIT, Cambridge, MA in 2013. The focus of Katie’s research is on using emerging computational methods to push the boundaries of interdisciplinary imaging. This talk was given at a TEDx event using the TED conference format but independently organized by a local community.

“I’m so excited that we finally get to share what we have been working on for the past year!” the 29 year-old Bouman, a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics

In 2016, Bouman developed an algorithm named CHIRP to sift through a true mountain of data gathered by the Event Horizon Telescope project from telescopes around the world to create an image.

“No one algorithm or person made this image,” wrote Bouman, who in the fall will begin work as an assistant professor at the California Institute of Technology (Cal Tech).

“It required the amazing talent of a team of scientists from around the globe and years of hard work to develop the instrument, data processing, imaging methods, and analysis techniques that were necessary to pull off this seemingly impossible feat,” she said on Facebook.

SciShow Space Channel (published on Apr 19, 2019)
Host: Hank Green
Description: For the first time ever we have visual confirmation that black holes actually exist and we got it with a telescope the size of our planet.

This video is a cogent overview of how the image was processed, particularly regarding resolution, wavelength, image gaps, stitching, number of teams, averaging, and future improvements. Partial transcript below.

In astronomy angular resolution refers to the ability to see two objects that appear close together in space as their own distinct sources. But it really just comes down to how much detail you get into an image, and there are really only two ways to improve it: One is to study light that has a shorter wavelength; the other is to make your telescope bigger — specifically increase its collection area or the size of whatever it uses to collect light.

So to study a black hole like this astronomers are interested in radio wavelengths of about one millimeter, but there’s a catch at those wavelengths — the telescope you need to resolve a black hole would have to be like as big as a planet.

So the EHT collaboration came up with one they turned the entire earth into a telescope …

Here’s a weird thing about telescopes — you could take a bunch of small ones spread them out and get computers to link them all up and pretend to have a telescope that’s as big as the distance between them and this actually works. You get gaps in the images but the same amount of resolution. This technique is called interferometry.

They collected petabytes worth of data which was flown on hard drives to supercomputers in the US and Germany to be processed into a picture, which requires the programs not to just stitch together separate images but eliminate all of the noise coming from stuff that’s not the black hole, and then to fill in all the gaps due to us not having a single telescope dish the size of a planet.

Filling in those gaps is kind of like inferring the melody of a well-known song when you can only hear some of the notes, but it might be difficult to narrow it down to just one song — that’s why the EHT didn’t produce just one image. Initially there were actually four. Four separate teams worked independently from one another to produce the first images to avoid potential bias. They used two different classes of algorithms, but in the end they all came out relatively the same. Most importantly you can see the shadow in the middle of all of them that proves their techniques were working.

After some more refinement the now-famous final image was made by averaging three different processing methods

In the future there are several ways to improve images like these: 1. One is to simply look at the object for longer (the collaboration observed M87 star over four nights between seven and twenty five times each night collecting data for just three to seven minutes apiece). 2. Another is to collect data at other wavelengths of light which will require upgraded technology with faster processing speeds. 3. We could also add in more telescopes as well as add ones that have larger collecting dishes

Other articles

Space.com: “All Your Questions About the New Black Hole Image Answered

Example: Why is the image blurry?

With current technology, that’s the highest resolution achievable. The resolution of the Event Horizon Telescope is about 20 microarcseconds. (One microarcsecond is about the size of a period at the end of a sentence if you were looking at it from Earth and that period was in a leaflet left on the moon, according to the Journal of the Amateur Astronomers Association of New York.)

If you take an ordinary photo that contains millions of pixels, blow it up a few thousand times and smooth it out, you’ll see about the same resolution as seen in the black hole image, according to Geoffrey Crew, the vice chair of the Event Horizon Telescope.

TED.com Video: “Inside the black hole image that made history” — a conversation with Sheperd Doeleman, global team leader for the Event Horizon Telescope project.

Documentaries

Smithsonian Channel via Space.com: ” ‘Black Hole Hunters’ Shows Epic Chase to Capture First Images

The one-hour documentary, called “Black Hole Hunters,” debuts today (April 12) at 9 p.m. EDT and at 9 p.m. PDT, depending on your time zone. It follows Harvard University astronomer Shep Doeleman and his team, which this week released the first images of a black hole, created using a networked set of telescopes called the Event Horizon Telescope (EHT).

Screenshot
EHT Locations
Screenshot
Location Correlation
Screenshot
An Image Processing Algorithm Team

Notes

“We can abstract away all of the astrophysics of the problem and really just think of it as a purely computational imaging problem. We have these sparse, noisy data, and our challenge is to find the image that actually caused it.” — Katie Bouman, Caltech Magazine, Summer 2019

Wiki: Photon sphere

photon sphere is a spherical area or region of space where gravity is strong enough that photons are forced to travel in orbits. The radius of the photon sphere—which is also the lower bound for any stable orbit—is, for a Schwarzschild black hole …

Wiki: Very-long-baseline interferometry

Very-long-baseline interferometry (VLBI) is a type of astronomical interferometryused in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.

Wiki: Multi-messenger astronomy

Multi-messenger astronomy is astronomy based on the coordinated observation and interpretation of disparate “messenger” signals. Interplanetary probes can visit objects within the Solar System, but beyond that, information must rely on “extrasolar messengers”. The four extrasolar messengers are electromagnetic radiationgravitational wavesneutrinos, and cosmic rays. They are created by different astrophysical processes, and thus reveal different information about their sources.

3 thoughts on “Photographing a black hole?

  1. As noted in some of the articles about the photos of the massive black hole at the center of M87, these “photographs” are not like the snapshots we take with our smartphones in several ways. They are not “taken” using visible light. And the processing of radio wave data to generate pixels for the images is exceedingly complex (although smartphone photo processing, especially for low light conditions, has become more and more intensive, even permitting adjusting focus and background blur in post-processing).

    It’s been somewhat of a visceral breakthrough for me using infrared cameras on a daily basis: that all light — all EM radiation, not just the visible portion of the spectrum, behaves according to the same rules, e.g., regarding refraction, reflection, shadowing, etc. [1]

    [1] Well, reflection, for example, up to a point, as explained in this Physics Stack Exchange 2018 thread: “Why does a mirror reflect visible light but not gamma rays?

    Visible frequencies have wavelengths of microns, 10^-6 meters. Gamma rays have a wavelength of 10^-12 meters, picometers.

    In physics, there are two mainframes, the classical frame, which includes Maxwell’s electrodynamics, Newton’s mechanics, and derivative theories, and the quantum mechanical frame which becomes necessary for small distances and high energies, where gammas (photons), electrons, atoms, nucleons, lattices belong.

    The classical electromagnetic wave emerges from zillions of superposed photons. Maxwell’s equations describe very well the behavior of light beams when scattering or reflecting or generally interacting for macroscopic distances and small energies. Reflection, classically, needs a very flat surface so that the phases of the reflected waves are retained. Depending on the material the classical beams may be absorbed, decohered in reflecting from many point sources, or reflected coherently if the scattering is elastic (mirrors elastically and coherently scatter incoming light).

    Gamma rays though force us to go to the micro level, because of the very small wavelength that describes them as a light beam.

    One has to look at the details of the surface, and whether a classical smooth surface for classical reflections can be modelled for gammas, and the answer is, no it cannot.

    The spacing between atoms in most ordered solids is on the order of a few ångströms (a few tenths of a nanometer).

    For micron wavelengths (optical light) the fields built up by atoms with angstrom distances in the lattice appear smooth and can be classically modelled.

    Gamma rays considered as a classical light beam, with their picometer wavelengths see mostly empty space between the atoms of the solid.

    Reflection is caused by electrons reacting to the electromagnetic field by oscillating at the same frequency. When they do this they emit radiation of the same frequency as the incoming light and this is observed as reflection. This works well if the EM frequency is near the eigen frequencies of the electrons. When the frequency is very high the electrons are simply too massive and the forces retaining them not strong enough – think of a mass on a spring – to follow the electric field. So gamma rays can pass through matter.

    Also, regarding gamma ray optics, there are articles like this one: “X-RAY AND GAMMA-RAY OPTICS.”

    Reflection of X-ray and gamma-ray radiation off a medium can be very efficient, albeit at very small grazing angles.

    Or this Quora thread: “Can harmful solar radiation be reflected by mirrors?

    Is there any way to reflect gamma rays?

    Gamma rays and X rays can be reflected by very complex mirrors (cf. X-ray optics – Wikipedia) but at low efficiency and only at shallow angles. A spacecraft usually cannot choose at what angle radiation hits

    And the low efficiency means a lot of the gamma rays will either pass through or interact with the gamma ray mirror causing secondary radiation.

    Other notes:

    Wiki: Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation.

    Physics World: Silicon ‘prism’ bends gamma rays (09 May 2012)

    NASA: Tour of the Electromagnetic Spectrum – Gamma Rays

    Unlike optical light and x-rays, gamma rays cannot be captured and reflected by mirrors. Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector. Gamma-ray detectors typically contain densely packed crystal blocks. As gamma rays pass through, they collide with electrons in the crystal. This process is called Compton scattering, wherein a gamma ray strikes an electron and loses energy, similar to what happens when a cue ball strikes an eight ball. These collisions create charged particles that can be detected by the sensor.

  2. Another example of an article on this event, from Vox.com by Brian Resnick: “How to make sense of the black hole image, according to 2 astrophysicists — Think on this: The light at the center of the black hole picture has been forever removed from the observable universe.”

    Some may not be impressed by the slight blurriness of the image. But there’s so much more to it than what immediately meets the eye. Two astrophysicists — Sheperd Doeleman, the project leader of the Event Horizon Telescope, and Katie Mack of North Carolina State University, who was not involved with the effort — walked me through a few of the coolest aspects of the image that helped me appreciate just wonderfully mind-blowing it is.

    You might think this ring of material, or the innermost edge of it, represents that event horizon. It actually doesn’t. … That boundary is known as the photon orbit, and its diameter is about 2.5 times larger than that of the event horizon.

    The light you do see in this image (which are really representations of radio waves, …) isn’t just coming from the sides of the black hole, it’s coming from behind it, from in front of it, from all directions. Space and time is so warped, that some of the light orbits the black hole in a circle.

    The telescopes used in the Event Horizon effort were radio telescopes. That means they only “see” radio frequencies on the electromagnetic spectrum.

  3. The scientists affiliated with the Event Horizon Telescope project have received much international attention, and now even from the US Congress — the House Science Committee on May 16, as discussed in this Space.com article “The Scientists Behind the First Black Hole Photo Get Nod from Congress — Shep Doeleman, Katie Bouman and other scientists talked black holes with Congress” (May 17, 2019).

    Four scientists affiliated with the Event Horizon Telescope project, which released the first-ever image of a black hole earlier this year, testified at the hearing about how the image was created, the importance of science research and education, and the future of the project itself. Throughout the hearing, the scientists’ enthusiasm for black holes and the new image was clear.

    Talk about capturing people’s (especially young people’s) imagination!

    Throughout her testimony [article includes a video], Bouman focused on emphasizing the importance of collaborating across experience levels and disciplines to succeed at this sort of large, complicated project. “Early-career scientists have been a driving force behind every aspect of the EHT,” Bouman said, referring to the Event Horizon Telescope. “No one algorithm or person made this image; it required the talent of a global team of scientists and years of hard work to develop not only imaging techniques, but also cutting-edge instrumentation, data processing and theoretical simulations.”

    There was a really wonderful, philosophical point by Doeleman:

    “In 1655, there was an image that startled people: It was the first drawing of a flea, by [Robert] Hooke,” he said. “The microscopic world became real for us. All of a sudden something that was invisible to us became real, and it changed the way we thought about our lives.”

    … “Think also about the first X-ray, made by Röntgen, of his wife’s hand — you could see the ring with the bony structure underneath. It made something visible for the first time that was invisible prior to that. And then think of the Earthrise over the moon, the first ‘Blue Marble.’ It really put things in perspective for us, it made us feel connected in a way that we hadn’t before, it made us feel vulnerable.

    “These are iconic images; they’re terrifying, but we can’t look away,” Doeleman said, adding that he believes the new black hole image may join this group of pivotal scientific images.

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