very-long-baseline interferometry

If linking telescopes a few kilometres apart sharpens the view, why stop there? Very-long-baseline interferometry, or VLBI, takes the idea to its limit: it links radio telescopes on different continents — even on opposite sides of the planet — to synthesise an instrument as wide as the Earth, achieving the sharpest images astronomy can produce.

In ordinary interferometry the dishes are cabled together so their signals combine instantly. Across thousands of kilometres that is impossible, so VLBI takes a different route. Each telescope records its raw radio data along with ultra-precise timestamps from atomic clocks. Afterwards the recordings from every site are shipped to a central processor and combined, with the clocks used to line the waves up to the nanosecond. The effective baseline is the distance between the most widely separated dishes — up to the diameter of the Earth, about 12,700 km — giving a resolution thousands of times finer than the Hubble telescope.

VLBI delivers angular resolution measured in millionths of an arcsecond, fine enough to watch jets erupt from the cores of distant galaxies and to track continental drift by the wobble of the dishes themselves. Its crowning achievement is the Event Horizon Telescope, a global VLBI network that combined dishes worldwide to resolve the shadow of the supermassive black hole at the centre of galaxy M87, and later of our own Milky Way — imaging structures only the size of a few times a black hole's event horizon, billions of light-years closer to impossible than anything before.