When the international collaboration behind the Event Horizon Telescope (EHT) unveiled the humanity’s first direct images of the supermassive black holes at the center of galaxy M87 (in 2019) and our own Milky Way galaxy, Sagittarius A* (in 2022), the glowing orange doughnuts of superheated plasma captivated the world. Yet for astrophysicists, those historic first snapshots were only the beginning. By successfully linking an Earth-sized array of radio observatories at a dramatically shorter, higher-frequency radio wavelength of 0.87 millimeters (345 gigahertz), the Event Horizon Telescope has achieved the highest-resolution astronomical observations ever conducted from the surface of planet Earth—sharpening its cosmic eyesight by fifty percent.
Turning the Entire Planet Into a Single Virtual Dish
In optical and radio astronomy, the sharpness (angular resolution) of any telescope is governed by a simple physical law: it equals the wavelength of the light you are observing divided by the physical diameter of your telescope dish. To resolve something as tiny in the sky as the event horizon of the black hole M87*—which sits fifty-five million light-years away, equivalent to reading the date on a coin in Los Angeles from a sidewalk in New York—a single traditional radio dish would need to be as wide as the entire planet Earth.
To build an Earth-sized telescope without covering the continents in metal, the EHT uses a technique called Very-Long-Baseline Interferometry (VLBI):
- Radio observatories scattered across the globe—from the high Atacama Desert in Chile (ALMA) and the summit of Maunakea in Hawaii to Arizona, Spain, Greenland, and the South Pole—swivel in unison to stare at the exact same black hole at the exact same second.
- Synchronized by ultra-precise atomic hydrogen maser clocks that lose less than a tenth of a billionth of a second per year, each observatory records petabytes of raw radio waves onto hard drives that are physically flown on cargo planes to supercomputers in Massachusetts and Germany, where the signals are combined to act as a single virtual telescope dish eight thousand miles wide.
Jumping From 1.3 Millimeters to 0.87 Millimeters (345 GHz)
For the famous 2019 and 2022 black hole images, the Event Horizon Telescope already held the physical limit on half of the resolution equation: you cannot space telescopes on the ground any further apart than the diameter of the Earth itself. Consequently, the only way to get a sharper, higher-resolution image from the ground was to shrink the other half of the equation: observing at a shorter radio wavelength (a higher frequency).
All previous EHT campaigns observed at a wavelength of 1.3 millimeters (230 GHz), which yielded an angular resolution of about twenty microarcseconds. In landmark test observations published in 2024 led by researchers at the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Radio Astronomy, the team pushed the array to observe at 0.87 millimeters (345 GHz)—achieving an unprecedented resolution of 13 to 19 microarcseconds:
- Fighting Atmospheric Water Vapor: Observing space at 0.87 mm from the ground is notoriously difficult because microscopic water vapor in Earth’s atmosphere absorbs high-frequency radio waves and scrambles their arrival times like looking at a coin at the bottom of a rippling swimming pool.
- Dual-Band Phase Transfer: To overcome the atmospheric blur, engineers installed superconducting receivers capable of observing at both 230 GHz and 345 GHz simultaneously—using the stable, clearer lower-frequency signal as a real-time reference guide to track and cancel out atmospheric turbulence on the higher-frequency 345 GHz signal every few seconds.
Why Sharper Black Hole Rings Matter: Photon Rings and Color Vision
Improving the telescope’s resolution by fifty percent—while simultaneously giving astronomers “color vision” by comparing how the plasma glows at two different frequencies at once—unlocks fundamental secrets at the edge of spacetime:
- Separating the Photon Ring From the Plasma: Surrounding a black hole’s dark shadow is the swirling, turbulent gas of the accretion disk, and tucked right inside that gas is a razor-thin, bright ring called the photon ring—composed of light particles trapped in temporary orbits around the black hole by extreme gravity alone. At 1.3 mm, the fuzzy glow of the hot gas blurs together with the photon ring; at 0.87 mm, the hot gas becomes more transparent and the beam is sharp enough to begin isolating the pure relativistic signature of gravity itself.
- Stepping Stone to the Next-Generation EHT and Space Movies: By proving that 345 GHz interferometry works across intercontinental distances using sub-arrays including ALMA in Chile, the Submillimeter Array in Hawaii, and NOEMA in the French Alps, the EHT has laid the foundation for the Next-Generation Event Horizon Telescope (ngEHT)—which will soon transition from snapping still images to filming high-definition, real-time movies of magnetic fields launching relativistic jets out of the hearts of galaxies.
