Frequently Asked Questions
Most of the answers provided here are modified from originals published in the Event Horizon Telescope Collaboration, and are courtesy of the EHT Outreach Committee. We recommend checking the FAQ section on their website for more answers!
Are there more black holes, other than Sgr A*, in our Galaxy?
It is predicted that many smaller black holes, called stellar-mass black holes, exist throughout any galaxy. They are remnants of massive stars that have exploded as supernovae. The EHT studies the 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 image.
The EHT 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?
The EHT Collaboration consider themselves very fortunate that their science is widely viewed as compelling, and that many observatories supported the EHT observations. The following radio telescope observatories were involved in the 2017 observations: Atacama Large Millimeter/Submillimeter Array, Chile, Chajnantur Plateau), (Atacama Pathfinder Experiment, Chile, Chajnantur Plateau), IRAM 30m (Institute de RadioAstonomie Millimtrique, Pico Veleta, Spain), LMT, (Large Millimeter Telescope, Mexico), SMT (Submillimeter Telescope, US, Arizona), JCMT (James Clerk Maxwell Telescope, US, Hawaii), SMA (SubMillimeter Array, US, Hawaii), SPT (South Pole Telescope, South Pole).
Are there any smaller 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*. However, the size of a black hole is proportional to its mass, so these stellar-mass black holes look much smaller than Sgr A*. 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 our Sun (Sun’s mass, or Solar mass, is approximately 333 000 times the mass of the Earth). It orbits a star half the size of the Sun 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 a star of 30 Solar masses with a period of around 6 days.
A black hole, by definition, does not emit radiation. In fact, it captures everything that falls onto it. So how do you observe something that does not radiate?
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 black hole does in fact radiate, so by observing this region, the EHT may observe structures that result from the strong gravity of the black hole.
Can we really photograph a black hole? Are they not entirely dark, since no light can escape them?
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. 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.
What if the black hole shadow is oblate (flattened at the poles) or prolate (elongated between the poles), 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 the EHT 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 still 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.
What is very long baseline interferometry, the technique used by the Event Horizon Telescope?
Very-Long-Baseline Interferometry (VLBI) combines the signals collected by radio telescopes located hundreds or thousands of kilometers apart, stretching the baselines to the maximum lengths possible on Earth to create a “virtual” telescope the size of the whole planet.
This increases the resolution achieved by radio interferometers, such as ALMA, by hundreds of times. This enables astronomers to view the cosmos in sharper detail to such extremes as being able to look at the shadow of the supermassive black holes lurking at the heart of galaxies. This is the ultimate goal of the EHT. The primary targets of the EHT are the two largest supermassive black holes as seen from Earth: Sgr A*, the Milky Way's black hole, and M87*, the black hole in the core of the M87 galaxy. The EHT network looks at the radio waves emitted by the disc of gas around the event horizon of the two black holes.
As the EHT antennas are spread across continents, their signals cannot be combined and analysed on site, like in ALMA. Instead, the signals must be recorded and brought together to be analysed after the observations have been performed.
With the EHT stretching across the whole planet, have we reached the longest possible baselines, hence creating a limit to how sharp we can look at the cosmos? The answer is no, as space-based VLBI is already operating with radio satellites offering even longer baselines than the size of the Earth.
Our planet will not be the ultimate frontier of interferometry but simply the beginning.
Will the EHT take razor-sharp images of the event horizon?
Obtaining sharp images of a 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. Speaking of razor-sharp, here is an interesting calculation. A razor blade edge is typically 400 nanometers wide — less than a millionth of a meter, or roughly one sixty-thousandth of an inch. Held at arm's length, the angular size of a razor blade edge is approximately half an arcsecond. The resolution of the EHT is more than a million times better than that! To get a rough idea of how much better that is, imagine counting individual dimples on a golf ball in Los Angeles... from New York.
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, an EHT five-day observing campaign can produce about a Petabyte (PB) of raw data per observatory or a thousand billion bytes. For example, the total amount of raw data recorded in April 2017 was 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.
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 Einstein’s General Theory of Relativity. It was the direct observation of the immediate environment surrounding a black hole—the event horizon—that had never been achieved, until now. With the release of the first results in April 2019, the EHT has filled a significant gap in our knowledge.
Black holes are laboratories for testing fundamental theories that explain how the Universe works on the largest and the smallest scales. Physicists currently do not understand how to create a single physical theory that works both on extremely large and extremely small scales and, hence, explain the physics of black holes in detail. With the EHT results, scientists are able to directly probe the behaviour of spacetime at the black hole boundary.
In addition to the physical theories, there are many details of plasma physics that are not completely understood. Properties of the hot gas surrounding and being pulled into the black hole beyond the event horizon are not fully known. But, understanding these properties is crucial for interpreting black hole images because it is this glowing plasma that produces radiation captured radio telescope arrays. This glowing hot gas illuminates the shape of spacetime around the black hole and EHT observations will help to better understand the properties and behaviour of these extreme environments.
How realistic are movie depictions of black holes, for example in the "Interstellar"?
The portrayal of the black hole "Gargantua" in the movie Interstellar is somewhat realistic, in that they show the general shape 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. 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 M87* indicate that their accretion discs are much thicker, with a more doughnut-like appearance.