Black Hole

The Mission to Image a Black Hole


Image of Black Hole
Image Source: Event Horizon Telescope Collaboration

On April 10th, 2019, the first image of a black hole was released to the press, and soon the news travelled across the world.

The black hole imaging process took years to implement, the first proposition from 1999. 20 years later, through use of dramatically improving technology and worldwide collaboration, we have the ability to accurately capture images of supermassive black holes, providing observational evidence to prove theories in astrophysics and answer questions about the strange nature of a black hole.

A Telescope the Size of Earth


The Event Horizon Telescope is an Earth-sized telescope made up of an array of telescopes throughout the world. Creating the images depended on a large amount of digital data from telescopes in Hawaii, Mexico, Chile, Arizona, Spain, and even Antarctica. There are plans to expand the array to include telescopes in France, Greenland, and another in Arizona.

The EHT project also has global partnerships with organizations such as the European Southern Observatory and the East Asian Observatory, truly working to be as global as possible.

The Utmost Precision Required to Image a Dot


Though supermassive black holes are, well, supermassive, they are still incredibly small to observe and require state-of-the-art equipment and resources to image.





VLBI

The EHT project assembles a Very Long Baseline Interferometry network (VLBI), consisting of dishes around the world. Since these dishes are not directly connected, the information must be kept precise and stable, so the VLBI uses atomic clocks, or hydrogen masers, that time-stamp recorded data. For better synchronization, the VLBI also uses GPS clocks, with precision up to a millionth of a second. The VLBI also uses short wavelengths, as small as 1.3mm, for accurate data.


Image Source: ALMA

Current Technology

The ever-improving technology the world has experienced has also greatly contributed to the success of imaging the black hole. A single telescope produces about 350 terabytes of data, and petabytes (billions of gigabytes) of data were used with highly specialized supercomputers just to hold and render data. The recording rate has greatly increased for the EHT, recording up to 64 giga-bits a second.


Image Source: Twitter/MIT Haystack Observatory

Simulations

Simulations are also essential in providing reconstructions and scenarios to predict how the black hole may appear. Not only may they provide sample images, but researchers have also made use of virtual reality and other interactive tools to get an in-depth look on black holes.


Image Source: J.Davelaar 2018

Filling in the Blanks


Despite the EHT's long range, there is still a lot of missing data to fill in, and imaging algorithms were developed to fill in the gaps. The more telescopes, the more accurate the image may be, but there are still an infinite number of possibilities for a "true" image as there are still ambiguous qualities to a sparse amount of data.

This resolution test acts as an example of how the algorithm may be able to fill in the gaps. At the beginning stage, the image could be any number of possibilities, but to make it less ambiguous, more data needs to be contributed so the information can be more accurate. For instance, without enough detail, it can be confused whether the object is a head of lettuce, a flower, or maybe even crumpled up paper. Nearing the end, it can be seen that the object is a rose, but it still isn't as clear as the true, complete image. Perhaps in the near future we can have the technology to capture cosmic images as detailed as the true rose image.

Why look at Black Holes?


Image of Black Hole
Image Source: Daily Mail

Testing General Relativity

Part of the reason why black holes need to be imaged is to have a better understanding of how Einstein's theory of general relativity (GR) relates to something with a gravitational pull as intense as a supermassive black hole. GR connects spactime curvature with the distribution and motion of energy, and also that suggests that due to the attractive gravitational force of massive objects, planets, stars, and black holes can deform spacetime in their vicinity. This theory was debated by scientists because black holes have extremely strong spacetime curvatures. The recent M87 image proved that Einstein's theory was correct as the image produced a circle, rather than showing signs of squashing or stretching.





Astrophysics

To answer questions about black holes and their relationships with matter, actual images are needed, and not just simulations, so that genuine evidence can be used to test hypotheses. For instance, visible evidence may be able to prove theories on how exactly friction may be able to heat up gas to a point of brightness, even though gas atoms are dilute. There may be other factors at play, including hypotheses about turbulence through rotating magnetic fields, and understanding what the accretion disk around the black hole looks like will determine the accuracy of the hypothesis.

Image of Black Hole
Image Credit: MIPT




Image of Black Hole
Image Credit: The Hubble Heritage Team (STScI/AURA) and NASA/ESA

M87 and Jets

Another major phenomenon that scientists wish to understand is the strange case of the relativistic jet. M87 creates a jet of matter which completely extends about 255 thousand light years from its black hole center. High resolution images of M87 may explain how these jets occur in the first place, as not all black holes have this jet phenomena, such as Sgr A*





Sgr A* and Brightness

Another black hole that EHT is focused on is Sgr A* (Sagittarius A Star), as it is the closest supermassive black hole from Earth. However, there are some complex differences from Sgr A* compared to other supermassive black holes, such as M87. For instance, although Sgr A*'s accretion disk is bright, it is not hardly as bright as other supermassive black holes. As of May 2019, an image is not yet produced if Sgr A*, but once one is available it should hopefully answer questions about why it is substantially dimmer than other supermassive black holes.

Image of Black Hole
Image Credit: NASA/UMass/STScI/D.Wang et al.

What is a Black Hole, Really?


Image of Black Hole
Image Source: medium.com

Black holes are compact, yet massive, places in space where the gravitational pull is so strong that even light cannot escape. We often picture a black hole as some sort of space vacuum cleaner. While it is true that black holes "eat" matter, causing it to grow, the matter has to be close enough to the black hole to get sucked in. Some matter, however, just isn't quite close enough to be immediately sucked in, nor is it so far away that it is unaffected. This matter is able to orbit around the black hole due to the black hole's gravity, and may later get sucked in.

For the time that the matter orbits the black hole, it creates an accretion disk. An accretion disk forms a flat disk around the event horizon. The matter is gaseous and hot, so hot that it produces a bright light, and some accretion disks around supermassive black holes can be brighter than all of the stars of its host galaxy combined. That's pretty bright!

Though Sgr A* is fairly dim compared to other supermassive black holes, it's still a few hundred times the brightness of our sun.





What makes the Invisible Visible?

Taking pictures of black holes depend more on the surroundings than it does a black hole itself. Black holes are invisible in space, and if it weren't for the bright accretion disks on the event horizons of black holes, we might not be able to discern whether it's a black hole or not. Supermassive black holes are better subjects to take pictures of because their accretion disks are bigger and brighter than smaller black holes. Though there are at least 6 black holes closer to Earth, they are more difficult to observe than Sgr A* and M87 because they aren't supermassive and don't have highly visible accretion disks.

Image of Black Hole
Image Credit: Hotaka Shiokawa

M87 and Sgr A*




The black hole that was imaged is in the center of Messier 87, part of the Virgo constellation. The supermassive black hole closest to us, Sgr A*, lives at the center of the Milky Way galaxy in the constellation Sagittarius. Click the buttons below the map to see where the black holes are in relation to what else we may see in the night sky.





Distance

Even though black holes have intense gravitational power, we don't need to worry about getting sucked in anytime soon. Even our closest supermassive black hole, Sgr A*, is a good 26,000 lightyears away. Hover the places on the line to see where the black holes are in relation to other cosmic entities.




Earth

Starting Point: 0

Neptune

4 light hours

Apep

8,000 light years

Sgr A*

26,000 light years

Andromeda Galaxy

2.537 million light years

NGC 5195

25.11 million light years

M87

55 million light years










Size

Sgr A* is pretty small for a supermassive black hole, but it's still a good 30 times larger than the size of our sun. The relationship with distance and size of Sgr A* and M87 make it possible to image these black holes. Sgr A* is small but close, and M87 is huge but far away. The diameters are compared below.



Earth to Sun

7,917.5 mi | 864,340 mi

Earth

Sun to SgrA*

864,340 mi | 27.3 million mi

Sun

SgrA* to M87

27.3 million mi | 23.6 billion mi

Sgr A*










Resolution

The factors of size and distance affect the angular resolution of the images to be produced by telescopes. Arcseconds describe the resolution of objects, using wavelength and diameter of the lens. Although the resolution for M87 is much less than that of Sgr A*, the EHT telescope was able to resolve the issue using state-of-the-art technology. Below is a comparison of resolution.





M87 Resolution

22 micro-arc-sec

Sgr A* Resolution

53 micro-arc-sec





Sgr A*'s Upcoming Image


We've imaged M87, the furthest and biggest of the two supermassive black holes, yet that is only one photo. Studies have shown that M87 and Sgr A* have several differences, such as producing jets or being strangely dimmer than other black holes. M87's has already begun to answer questions about black hole physics, including proof of Einstein's Theory of General Relativity, but comparing M87 with Sgr A*'s unique nature should continue to improve scientific understanding about black holes. Constantly improving technology also has a hand in producing high-resolution cosmic images, and the future of space imagery will greatly improve due to the implementation of such technologies.