FAQ

Are there more black holes, other than Sgr A*, in our Galaxy?

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.

You used eight telescopes for a week-long observation of black holes in April 2017. Which telescopes were used, and was it difficult to get so much observing time on all these telescopes?

We consider ourselves very fortunate that our science is widely viewed as compelling, and that many observatories supported the Event Horizon Telescope observations. The following radio telescope observatories were involved in our 2017 observations: ALMA (Atacama Large Millimeter/Submillimeter Array, Chile, Chajnantur Plateau, http://www.almaobservatory.org), APEX (Atacama Pathfinder Experiment, Chile, Chajnantur Plateau, http://www.apex-telescope.org), IRAM 30m (Institute de RadioAstonomie...

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Are there any stellar-mass black holes near the Solar System and can you detect them by using the EHT?

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’...

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What if the black hole shadow is oblate or prolate -- does it mean that there is something wrong with the General Theory of Relativity?

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. Read more about What if the black hole shadow is oblate or prolate -- does it mean that there is something wrong with the General Theory of Relativity?

Will the EHT take razor-sharp images of the event horizon?

Obtaining sharp images of the black hole event horizon is very challenging and the EHT will do its best to produce the sharpest images ever obtained. The quality of the images depends on the arrangement of the telescope array, weather conditions at the telescope sites, as well as blurring of images as the light travels from the black hole toward the Earth. Theoretical simulations, some of which you can find in...

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How much data is recorded during an observation and how it is transferred to the central processing facilities?

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.

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Do you only use radio telescopes or are you also looking at higher-energy emission? What makes radio telescopes superior to, for example, X-ray telescopes?

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...

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Why is it important to study black holes? What is there to be learned from the EHT observations?

As a scientific collaboration, the aim of the EHT is not only to prove the existence of black holes, but also to understand the physics of black holes and their surrounding environments. There is ample indirect evidence from various astronomical studies indicating that black holes exist, including the investigation of nearby objects that are subjected to the gravitational pull of a black hole. These are circumstances well explained by the General Theory of Relativity (GR). It was the direct observation of the immediate environment surrounding a black hole—the event horizon—that had never...

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