Astronomy
Using Radio Interferometry to Image Black Hole Event Horizons
Quick fact
To resolve the event horizon of M87's black hole, we needed a telescope the size of Earth—achieved by linking radio dishes across the globe, creating a virtual instrument with 10,000 times the resolution of the Hubble Space Telescope.
Why this is interesting
Black holes swallow all light, so they are literally invisible—yet in 2019, we saw one for the first time. How can humanity capture an image of something that emits no light?
Read the full explanation
Understanding Using Radio Interferometry to Image Black Hole Event Horizons
Imagine you want to read a newspaper from a mile away. Your eyes are too small to see the letters—they blur together. A telescope with a larger lens collects more light and sharpens the image. But even the best optical telescope cannot see a black hole's event horizon from Earth—it's far too small. However, there's a trick: if you take two telescopes far apart and combine their signals perfectly, they act like a single telescope with a mirror the size of the distance between them. This is called interferometry. For black holes, radio waves are the key because they can travel through dust and gas, and because we can record the exact arrival times of these waves at each telescope. By carefully synchronizing many radio telescopes across Earth with atomic clocks, we create a virtual telescope almost as large as the planet. This technique, called very-long-baseline interferometry (VLBI), gives us the sharpest vision in astronomy.
A deeper explanation
The technique works by combining the signals from multiple telescopes to simulate a single enormous dish. When a radio wave arrives at two different telescopes, there is a tiny time difference. By recording each signal with a precise timestamp and then mathematically combining them—a process called correlation—we can measure the interference pattern caused by the source. This pattern encodes the fine details of the object. The maximum achievable angular resolution is approximately the wavelength of the observed waves divided by the maximum distance between telescopes (the baseline). For the Event Horizon Telescope, using wavelengths around 1.3 mm and baselines spanning thousands of kilometers, the resolution is about 20 microarcseconds—enough to see a grapefruit on the Moon. This allowed astronomers to image the shadow of the supermassive black hole in M87, a dark region ringed by glowing gas, confirming the predictions of general relativity about the event horizon's size. Interferometry thus turns Earth into a lens, letting us gaze at the edge of a black hole and test fundamental physics.