Astronomy
Imaging Supermassive Black Hole Shadows with Interferometry
Quick fact
To image the shadow of a supermassive black hole, astronomers used a technique called interferometry, combining radio telescopes across the globe to achieve the resolution of an Earth-sized telescope. This allowed them to see details as small as a grapefruit on the Moon's surface.
Why this is interesting
We all know that nothing, not even light, escapes a black hole. So how did scientists manage to 'see' the black hole at the center of galaxy M87?
Read the full explanation
Understanding Imaging Supermassive Black Hole Shadows with Interferometry
Imagine trying to watch a movie being filmed across the street, but your window is covered by a pinhole. You'd only see a tiny dot of light. To see the movie clearly, you need a large window. Similarly, to see an extremely distant object like a black hole's shadow, you need a telescope as large as the Earth. But building such a giant dish is impossible. Instead, astronomers use interferometry: they place several smaller telescopes far apart and combine their signals. Each pair of telescopes measures a small piece of the image's detail, like seeing a few pixels. By combining these pieces from many pairs, they reconstruct the full image. The Event Horizon Telescope (EHT) does exactly this, linking telescopes from Hawaii to Spain to create a virtual Earth-sized telescope.
A deeper explanation
Interferometry works by using the interference pattern of light waves from a single source detected at two different locations. The pattern encodes the Fourier component of the source brightness at a spatial frequency determined by the separation (baseline) between the telescopes. By combining measurements from many baselines of different lengths and orientations, we sample a range of frequencies. The more baselines and the longer their lengths, the higher the angular resolution and the better the image fidelity. For the black hole shadow, the key is that the shadow's size and shape are set by the black hole's mass and spin, as predicted by general relativity. The EHT used several arrays, including ALMA, to measure the shadow of M87 and later Sgr A. Because the shadow is absorbed and lensed light, the image shows a dark region surrounded by a bright ring of emission. The observed ring diameter matched theoretical predictions, confirming general relativity in a strong gravity regime. The technique also highlights the inevitability of photon orbits near the event horizon, which produce the distinctive shadow.