This week, the Event Horizon Telescope collaboration is getting ready to make a major announcement about the Milky Way that has space nerds everywhere on the edge of their seats. EHT had a similarly coordinated set of press conferences around the world to present the first-ever image of a black hole.
The incredible effort sparked by over 100 scientists and engineers from all over the world who devised a solution to a seemingly impossible challenge: to use radio telescopes to enlarge the magnetic field lines around M87* (pronounced M87-star). There''s also a wealth of data that they are looking for in depth about the enormous and mysterious structure.
What sort of effort were required to capture an image of something that gives no light? How did that initial effort stifle the foundation for what will happen this week?
What is the purpose of the Event Horizon Telescope collaboration?
The Event Horizon Telescope collaboration involves an effort of more than 100 astronomers, engineers, and scientists from around the world to combine their own knowledge, resources, and expertise to see the outermost visible edge of a black hole known as the event horizon.
This is not just the work of astronomers and telescope researchers, but also data and computer scientists who must combine more than a dozen streams of overlying data that together form the image.
Why is photographing a black hole so arduous?
It might seem obvious that a black hole is difficult to see because it does not give away any light, instead sucking it into itself, and that isnt wrong. However, a black hole isnt always invisible, and there are several ways we can see them.
Weve had the ability to see the gravitational effect that a black hole has on its surrounding space for many years now. Sometimes, this is in the form of other stars in orbit around it, where the stars'' orbits can''t be explained by what we can see. A lot of people in galaxies are concerned about the possibility of a star coming to the surface of a black hole.
Another is to obtain an accretion disk around the black hole. If a black hole is actively consuming material, the material forms into a flattened disk around it from its angular momentum around the black hole. As the material closer to the black holes event horizon the exact distance from the black holes is at its highest level, where the escape velocity from the black holes gravity exceeds the speed of light that the material orbits the black hole at larger fractions of the speed of light.
Whatever this material was before, by the time it is embedded in the accretion disk, it has been transformed into a hot ionized plasma that accelerates in the disk. Because light cannot come out from the event horizon itself, you can see a total void or shadow in the center of this extremely radio-bright radiation, with the light from the accretion disk behind it being bent by the intense gravity around the black hole into a kind of halo.
The first is that the radiation being removed from the accretion disk is among the most effective radiation in the world. We are effectively staring into the Sun with a naked eye and attempting to see the sunspots.
The actual dimensions of the black holes are tiny. Even the largest supermassive black holes, which can accommodate ten or 20 billion solar masses, would only be four miles wide. Even the largest supermassive black holes, which can easily accommodate ten or 20 billion solar masses, have diameters that are quite small inside our solar system. These are millions of light years away from us.
So, coming back to the analogy of our Sun, finding a black hole is like looking at the sun with the naked eye and looking for a dark sunspot in the city. All of this together is what makes seeing a black hole so incredibly difficult, and why EHTs'' feat was so impressive. So how did they do it?
How a black hole image is taken
The amazing thing about the universe is that light does not disappear outside of a black hole, and that light cannot spontaneously appear where it was before, and if that light strikes our retinas or instruments, we may see it. By using lenses, we can focus the light from the most distant stars and galaxies in the universe and transform it into something we can see.
Since radio waves and X-rays are equally as light as the frequencies of the visible spectrum, our sensors and telescopes have everything they need to see to see the shadow of the event horizon of a black hole. The challenge is to develop a lens that is large enough to capture the light they receive into a visible image.
The lense is the antennas dish that reflects radio light in a way that focuses the image, but when it comes to seeing the shadow of the event horizon of Sagittarius A* (Sgr. A*), the Milky Ways supermassive black hole, the black hole itself isn''t all that large. It has a diameter of 26 million miles, which is about the distance between the Sun and the mean orbit of Mercury.
It''s also about 25,000 light-years away from us, thus its incredible distance makes it appear even smaller. In order to capture an image of something so small from far away, you would need an enormous telescope to focus that tiny amount of light on something we could see; additionally, you would need a radio antenna as wide as the Earths diameter itself.
Evidently, no such radio antenna may be built, so the story would be ended, but thats where the EHT comes in. We might not be able to build an Earth-sized radio telescope, but we have radio telescopes throughout the world, and if we were to turn them all to the same radio source and record data at the same time, then you would get more than two dozen streams of data.
Because the correlation in data streams is perhaps more important than the data itself. We can then map the distance between all of these radio telescopes and mathematically investigate how the distance between two points on Earth''s surface should affect the resulting data streams. That difference can then be algorithmically altered to transform a network of radio telescopes into a single Earth-sized virtual telescope with the capability to zoom in on the event horizon of a black hole.
So, in August 2018, 38 radio telescopes turned their sensors towards Sgr A* and M87*, which despite being at vastly different distances and sizes from us looked almost the same apparent size when seen from Earth. The amount of data collected was so enormous that it had to be processed and inserted into a central lab.
It would be months before all of the data could be shipped where it required to go, primarily from one station in Antarctica, which took nearly a year to ship back to the processing facility in the United States.
Sgr A* has proven itself to be much more elusive, with one of its magnetic poles pointing almost dead-on to Earth. This is why it would take a while to describe a firefighter while they are actively shooting you in the face with a firehose.
This is absolutely raising the stakes for whatever EHT researchers have discovered, and is a part of this week''s announcement so exciting. Basically, the arrangement for the M87* publication, which is being teased as a Milky Way announcement, is similar to that used to be the heart of the galaxy, and we may also know if it''s just as weird and exotic as it appears.