Very Long Baseline Interferometry

Very Long Baseline Interferometry (VLBI) is a technique in radio astronomy that synthesizes the resolving power of a single telescope from an array of geographically separated radio antennas. By recording the phase-coherent voltage signals at each station — synchronized by atomic clocks — and later correlating the data, VLBI achieves angular resolutions that would require a single dish tens of thousands of kilometers across. The technique transforms the Earth's surface itself into an aperture, making VLBI not merely an engineering solution but a reconceptualization of what an astronomical instrument can be: a network, not an object.

The method was pioneered in the late 1960s and has since become essential for imaging compact radio sources, measuring precise positions of celestial objects for astrometry and geodesy, and studying the fine structure of quasar jets and black hole environments. The Event Horizon Telescope represents the current frontier of VLBI, operating at millimeter wavelengths where atmospheric opacity and coherence time impose stringent limits.

The fundamental constraint on VLBI is not hardware but physics: the coherence time of the signal — the window over which phase remains stable — limits the usable bandwidth and baseline length. At shorter wavelengths, atmospheric turbulence degrades coherence faster, which is why the EHT's 1.3-millimeter observations required simultaneous calibration at all sites using a separate atmospheric model. This calibration problem is itself an inverse problem, inseparable from the imaging problem it serves.