Last month the first-ever image of a black hole was revealed to the world. Naturally in the internet age, the fuzzy orange ring quickly found itself photoshopped into all sorts of memes. Many people focused on the image’s underwhelming quality, yet the significance of it is far from underwhelming.
Black holes are points in space that are so dense they create deep gravitational wells – the pulling force of gravity means nothing, not even light, can escape from it. By definition, therefore, they are invisible, which is why the centre, the “shadow”, is black in the image. However, the hole is bounded by a well-defined edge called the event horizon – the region where an object approaching a black hole reaches a “point of no return”, unable to escape its gravitational pull. Gas, dust and other stellar debris close to the event horizon but not quite fallen into the black hole, form a band of spinning matter called an accretion disk. The extreme gravitational pull of black hole superheats the matter in the accretion disk causing it to emit radiation, represented by the orange ring in the image. But the accretion disk isn’t really orange – rather to the naked eye these emissions would probably appear white, but the scientists involved in the project chose to colour the radio signals orange to create a gradient for emission strength.
The first-ever image captured the Supermassive Black Hole (SMBH) which is 55 million light years away at the centre of Messier 87, a supergiant elliptical galaxy in the constellation Virgo. It was created using data collected by the Event Horizon Telescope (EHT), a network of eight ground-based linked radio telescopes across the world spanning locations from the South Pole to Mexico to Spain, in an effort involving more than 200 scientists. The success of the project therefore hinged on clear skies simultaneously in each location and excellent coordination amongst all the teams involved. Radio waves were measured because they can pass through the accretion disk and interstellar dust to reach the telescopes. This was done in a process called very-long-baseline interferometry, whereby the distance between the telescopes is calculated using the time difference between the arrival of the radio waves at each telescope, thus creating a “virtual” telescope the size of the Earth.
The sheer volume of data the EHT generated was so great that it was quicker to physically ship half a tonne of hard drives to a central location, the MIT Haystack observatory, than to send it over the internet. To give you a sense of scale, 5 petabytes of data were produced which is equal to about 5,000 years of MP3 audio. However, this did mean waiting for half a year for the South Pole data, which could only be shipped out at the end of the Antarctic winter. Once the data were collated the next crucial stage began – piecing the data together. This entailed developing new algorithms that could not only combine the data, but also filter out noise caused by factors like atmospheric humidity which warps radio waves.
The resultant image appears fuzzy because of the black hole’s distance from us. Despite being 100 billion km wide, the black hole is so far away that, from Earth, the angle it makes in the sky is only 40 microarcseconds (one microarcsecond is about the size of a full-stop at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth). The EHT had also been observing a closer black hole at the centre of our Milky Way galaxy. However, it was easier to produce an image of the black hole in Messier 87 because of its far larger size and more intense emissions due to stronger gravitational activity.
Up until now we’ve had to use illustrations and digital simulations to visualise black holes. The new image is the first time we’ve seen one, since their prediction in 18th Century, and is consistent with predictions of Einstein’s General Relativity. Prior to the image, scientists have relied on indirect evidence – signals coming from nearby objects – to try to understand black holes. Being able to produce an actual image of one is a major step towards a deeper understanding of our Universe.
Featured image credit: EHT
