FAQ

It is true that a black hole itself does not emit light. However, the EHT observes the nearby surroundings of a black hole. The gas that surrounds the observational targets of the EHT does in fact radiate, so by observing this region the EHT may observe structures that result from the strong gravity of the black hole.

There are lots of stellar-mass black holes that are much closer to the Solar System than Sgr A* in the Galactic Center. However, the size of a black hole is proportional to its mass, so these stellar-mass black holes look much smaller than the Galactic Center black hole. Even if we can detect some stellar-mass black holes with the EHT, we would not be able to resolve the emission around them on the scale of their event horizon.

The closest currently known black hole is V616 Monocerotis, 3,000 light-years away, with a mass of 11 times larger than our Sun (Sun’s mass, or Solar mass, is approximately 333,000 times the mass of the Earth). It orbits a K-type star (0.5 Solar masses) with a period of about 8 hours, which causes its light to vary periodically. At double the distance, we have Cygnus X-1 (15 solar masses), orbiting an O-type star (30 Solar masses) with a period of around 6 days.

It is predicted that many black holes of roughly stellar mass exist throughout any galaxy. They are remnants of massive stars that have exploaded as supernovae. We study supermassive black holes Sgr A* and M87 because their apparent sizes are much larger than those of stellar-mass black holes when viewed from the Earth, so they are easier to study.

The first image of a black hole is not a classical photograph. It is a radiolight image the result of complex observational and computational interpretation (deconvolution). Further, it is not of the black hole itself, but of the "shadow"—the closest we can come to imaging a completely dark object that consumes all light and matter. The black hole boundary—the event horizon for which the EHT is named—casts this shadow. General Relativity says the superheated material around the black hole will glow and illuminate the strongly warped region of spacetime, making it visible to interferometer observation and measurement.

This animation and this article may help in better understanding how the image of a black hole is formed.

On April 10th 2019, the EHT Collaboration presented its first results -- an image of the supermassive black hole in galaxy M87 -- in multiple simultaneous press conferences around the world. The official EHT press release can be found here.

Scientists of the EHT and their collaborators try to organize observations with a number of different telescopes so that they coincide with observations with EHT observations. The aim of this is to provide multi-wavelength coverage in order to investigate potential correlations in source brightness in various bands across the electromagnetic spectrum. In 2017 and 2018, coordinated observations were performed by various radio telescope arrays operating at wavelengths longer than 1 mm, such as GMVA, VLBA, KVN, HSA, EVN, RadioAstron. A number of optical and infrared telescopes monitored the EHT targets, as did X-ray telescopes Chandra, Swift, NuSTAR, and AstroSat, and high-energy gamma-ray observatories MAGIC, VERITAS, and HESS.

What makes the radio observations at 1 mm wavelength different from observations in any other wavelength band is the spatial resolution. Only observations with the Earth-sized EHT array at wavelength below approximately 2 mm have the theoretical resolving power sufficient to discern the very small size of the event horizons of black holes in SgrA* and M87. Resolving power scales directly with the observing wavelength and inversely with the distance between the furthest telescopes in an interferometric array such as the EHT. Other radio arrays, including global ones like the GMVA, do not operate at short enough wavelengths, while at wavelengths shorter than about a tenth of a millimeter long-distance interferometry becomes technically impossible. As a result, telescopes at other wavelengths either view changes on spatial scales larger than those observed with the EHT, or have a difficulty in discerning where exactly (with respect to the black hole) the changes are happening.

Observational campaigns in both April 2017 and April 2018 were conducted over five observing nights. These data were recorded when the major target sources were optimally observable by night and when the weather was globally best for EHT observations. Each telescope observed EHT targets and various celestial calibrators for approximately 8 hours per night.

However, the EHT collaboration effort has been evolving over more than 20 years and continues to go forward. Such a daring endeavor required developing new hardware and software throughout this period and especially in the last several years, when new telescopes joined the project. Instrument coordination, advanced processing software, hardware retrofitting, and other technological and practical issues were among the issues that needed to be solved in order to align existing telescopes into the globally coordinated observations of the EHT array.

After the first data were recorded in April 2017, they were shipped to central processing facilities—at the Max Planck Institute for Radio Astronomy and the MIT Haystack Observatory—then carefully processed, calibrated, analyzed, and ultimately, interpreted using the cutting-edge computational tools created specifically for this experiment by EHT collaboration members. While "making an image" from processed and well-calibrated data takes a mere few minutes, the development of procedures and tools that enable this required a great deal of time and effort of many dedicated experts.

The Event Horizon Telescope (EHT) Collaboration is a team of two hundred individual researchers situated at a number of research institutions around the world. The EHT is subdivided into different expert teams to address the different challenges of the coordinating a global research effort, such as phasing individual arrays to work as a single telescope of the global network, outfitting the telescopes at the different sites with new hardware and software purpose built for the EHT observations, post-processing the data in the correlator center, calibrating and validating the data, testing algorithms and computing images, interpreting the images obtained, producing numerical simulations to compare with the observations.

More than 200 researchers and engineers from 20 different countries and regions around the world are currently members of the EHT Collaboration. However, even more individuals have contributed to the pioneering work on the first image of a black hole over the different stages listed above, so the total number of co-authors of the main scientific publication, and winners of the 2020 Breakthrough prize in Fundamental Physics, is 347.

The EHT is an evolving network of telescopes. Observations in April 2017 were carried out with eight observatories in six geographical locations around the globe. For the observing campaign in 2018, one new telescope was added to the array, totalling nine observatories at seven sites.

No observations with the EHT array were performed in 2019 and 2020. We plan to add additional observatories to the array for future observational campaigns -- two more are expected to join the array for observations in 2021, totalling eleven observatories at nine different locations around the globe. You can find a virtual tour of all EHT observatories here.

Depending on instrument setup, weather, position within the array, and other factors, a five-day observing campaign can produce about a Petabyte (PB) of raw data per observatory. The total amount of raw data recorded in April 2017 is about 3.5 PB. This data must be recorded to hard disks and manually transported to central processing facilities in Germany and the United States, as transferring it via the Internet would take considerable time. Approximately 5.5 PB data were recorded during the April 2018 observations. Future campaigns are expected to record up to 15 PB per year.

Data storage and processing enthusiasts may want to look up more details in this article.

The portrayal of the black hole "Gargantua" in the movie Interstellar  is somewhat realistic, in that they show the general morphology of these cosmic structures accurately. However, for aesthetic reasons, the movie producers removed one important aspect of the imaging to make the black hole more dramatic: the Doppler effect is switched off in the movie. In reality, the approaching side of the rotating accretion disk would appear brighter and hotter (blueish white), while the receding side would be dimmer and more red. Although the Interstellar production team consulted with physicists (most notably, Nobel-prize winner Kip Thorne) and based Gargantua on mathematical simulations, the movie ultimately showed a more artistic—brighter and more symmetric—rendering of the black hole. Furthermore, "Gargantua" is shown to have a razor-thin accretion disk, whereas astronomical observations of both Sgr A* and the M 87 black hole indicate that their accretion disks are much thicker, with a more donut-like appearance.

If you want to know more, follow this Twitter thread by the EHT member Sara Issaoun (@SaraIssaoun):

Let's unwrap another question we often get about the @ehtelescope 's #EHTblackhole image: Why doesn't the picture look like the black hole simulation in the movie Interstellar? Let's digest the differences one by one.  --- Continue here.

Finding something contrary to our expectations (some of which you may find in our simulations gallery) would certainly be interesting. It would not necessarily mean that the General Theory of Relativity is wrong, but it would imply that we have more physics to understand. Discoveries of gravitational waves from merging black holes by LIGO and collaborators have recently confirmed one of the most fundamental predictions of this theory.