How Was the Black Hole Image Captured? – Very Long Baseline Interferometry Explained

In April 2019, the world was captivated by the first-ever image of a black hole, a cosmic entity so dense that not even light can escape its gravitational pull. This groundbreaking achievement was made possible by a revolutionary technique called Very Long Baseline Interferometry (VLBI). VLBI is a powerful tool that allows astronomers to achieve incredibly high resolution by combining data from multiple telescopes spread across the globe, essentially creating a virtual telescope with a diameter spanning the Earth.

Understanding VLBI

Imagine you want to see a distant object, like a star, with incredible detail. You could use a large telescope with a big mirror, but even the largest telescopes on Earth have limitations. VLBI overcomes these limitations by using multiple telescopes simultaneously and combining the data they collect.

Here’s how it works:

1. Simultaneous Observation: Multiple telescopes, often located thousands of kilometers apart, observe the same celestial object at the same time.
2. Data Recording: Each telescope records the incoming radio waves from the object. These signals are stored as digital data.
3. Correlation: The recorded data from all the telescopes is then brought together and correlated. This process aligns the signals from each telescope, taking into account the time delay caused by the different distances between the telescopes and the object.
4. Image Formation: By combining the correlated data, astronomers create an image of the object with a resolution much higher than any single telescope could achieve on its own.

Capturing the Black Hole Image

The Event Horizon Telescope (EHT), a global network of telescopes, used VLBI to capture the image of Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. The EHT combined data from eight telescopes around the world, including telescopes in Hawaii, Arizona, Chile, Mexico, Spain, and the South Pole.

To capture the image of Sagittarius A*, the EHT had to overcome several challenges:

• The size of the black hole: Sagittarius A* is incredibly small, even though it’s a supermassive black hole. Its event horizon, the boundary beyond which nothing can escape, is only about 25 million kilometers in diameter, which is about 17 times the diameter of our Sun. This means that the EHT needed to achieve a resolution of a few micro-arcseconds, which is equivalent to being able to see a golf ball on the Moon from Earth.
• The distance to the black hole: Sagittarius A* is about 26,000 light-years away from Earth, meaning the light we see from it today left the black hole 26,000 years ago.
• The Earth’s atmosphere: The Earth’s atmosphere can distort radio waves, making it difficult to get clear images.

To overcome these challenges, the EHT used a sophisticated technique called interferometry, which allows astronomers to combine data from multiple telescopes to achieve a resolution much higher than any single telescope could achieve on its own. The EHT also used specialized algorithms to process the data and create the final image.

The Significance of the Black Hole Image

The image of Sagittarius A* was a groundbreaking achievement that provided the first direct evidence of the existence of black holes. This image has helped to confirm Einstein’s theory of general relativity, which predicts the existence of black holes. The image has also opened up new avenues of research into the nature of black holes and their role in the evolution of the universe.

Conclusion

VLBI is a powerful tool that has revolutionized our understanding of the universe. By combining data from multiple telescopes, VLBI allows astronomers to achieve incredibly high resolution, making it possible to study objects that are too small or too distant to be seen with single telescopes. The image of Sagittarius A* is just one example of the many groundbreaking discoveries that have been made using VLBI. As technology continues to advance, VLBI will undoubtedly play an even greater role in our exploration of the cosmos.