by Isabelle Chiu

 

Black holes have been a mystery to both scientists and the general public alike. But now, with new imaging technology, we have finally confirmed their existence.

The very first image of a black hole (pictured above) was captured by the international Event Horizon Telescope (EHT) collaboration, with eight different ground-based radio telescopes. This glowing, donut-shaped black hole is in the center of the galaxy M87, around 55 million light years away, and it is 6.5 billion times greater than the mass of the Sun.

This discovery was released in the Astrophysical Journal Letters on April 10 and also confirmed in seven different press conferences across the globe. The team observed two black holes in total – M87’s and also Sagittarius A, which is at the Milky Way’s center. Since both galaxies are far from Earth, in order to compile an image with a high-enough resolution, scientists needed an Earth-sized telescope. However, scientists were able to use a technique known as interferometry, where two or more sources of light are merged to create an inference pattern, with eight radio observatories — two within the North American continent, two in Hawaii, two in South America, one in Spain, and one in Antarctica. When combined, the observatories’ resolving pattern was equivalent to a telescope the size of an Earth. A collaboration of this scale had never been done before, and the EHT produced highly anticipated results.

The team recorded every wavelength of data coming their way from Sagittarius A and M87 for five days straight in April 2017. The total data accumulated to multiple petabytes, too much to transmit through cloud sharing software — the raw data had to be collected on hard disks and physically shipped between the different observatories within the EHT. After combining the data from across the globe, the team started data analysis in mid-2018 and decided to first image M87 because it would yield a cleaner picture. This image confirms scientific theories, which are discussed by Ethan Ward here.

Firstly, what is a black hole? A black hole is bound by its event horizon, commonly known as the “point of no return,” and is similar to a one-way membrane. This is defined as the edge of the black hole where gravity is so strong nothing can be seen and nothing can escape. Around it is the accretion disk, a band of spinning matter comprised of stellar debris. In this groundbreaking image, the event horizon appears five times larger than it actually is because the black hole warps space and bends paths of light. Although the accretion disk exists on all sides of the black hole, due to the strong gravity, the accretion disk appears to wrap around the black hole like a shadow, because of the light rays distorted to go around the black hole. This effect is known as gravitational lensing.

However, this isn’t an evenly-distributed halo, as in all images collected, the halo appears significantly brighter on the bottom left. Due to the Doppler-shift effect, the sides rotating toward the observer should appear much brighter than the others. Stars are red/blue shifted, based upon how far it is and whether or not it moves toward or away from the observer. A red-shifted star indicates that it is moving away, as the light shifts toward the red end of the visible light spectrum. A blue shifted star is moving toward the observer, with a higher frequency. Similarly, the brighter, bottom left side of the black hole is due to the radio telescopes picking up on higher frequency signals, meaning it is closer to Earth. Through picking up on the frequency, scientists could see the spin of the black hole, but not its speed.

The image also provided evidence of a photon ring around the black hole. The photon orbit is the gravitational point where light will neither drop in nor escape. This orbit is not stable, and it is circular in 3D, as if it were taking different paths along a sphere. This circular force is unusual, as most forces are elliptical. Outside of the photon orbit is the innermost stable circular orbit (ISCO), which moves mechanically (like a 2D circle). Most of the data collected was from the ISCO, as the photon orbit is more difficult to collect data from. Due to the bending of light around the black hole, the photon orbit creates a shadow effect.

In the astrophysicists’ world, this discovery was revolutionary because it has proven Einstein’s theory of general relativity. Read more about the significance and scientific implications of M87’s image in Ethan Ward’s blog article!

Sources:

Castelvecchi, D. (2017, Mar. 27). “How to Hunt for a Black Hole with a Telescope the Size of Earth.” Retrieved from www.nature.com/news/how-to-hunt-for-a-black-hole-with-a-telescope-the-size-of-earth-1.21693.

Castelvecchi, D. (2019, Apr. 10). “Black Hole Pictured for First Time – in Spectacular Detail.” Nature News, Nature Publishing Group, Retrieved from www.nature.com/articles/d41586-019-01155-0.

“Image – The Surface of Pluto.” HubbleSite, Sept. 1996, hubblesite.org/image/400/news_release/1996-09.

Psaltis, D. “The Black Hole Shadow in the M87 Galaxy.” Berkeley Physics Colloquium. The Black Hole Shadow in the M87 Galaxy, 22 Apr. 2019, Berkeley, UC Berkeley, 1 LeConte Hall.

Mastin, L. (2009, October). Event Horizon and Accretion Disk. Retrieved [April, 22, 2019], from https://www.physicsoftheuniverse.com/topics_blackholes_event.html