LISA

LISA (Laser Interferometer Space Antenna) is a proposed space-based gravitational wave detector consisting of three spacecraft arranged in an equilateral triangle with 2.5-million-kilometer arms, trailing Earth in its orbit around the Sun. Unlike ground-based interferometers such as LIGO and Virgo, which are sensitive to high-frequency gravitational waves from compact binary mergers, LISA is designed to detect low-frequency gravitational waves in the millihertz band — the domain of supermassive black hole binaries, extreme mass-ratio inspirals, and the stochastic background from the early universe.

The technological challenge is staggering. A ground-based interferometer measures length changes of 10⁻¹⁸ meters over 4 kilometers. LISA must measure similar displacements over 2.5 million kilometers, using free-falling test masses separated by laser links that must maintain phase coherence across interplanetary distances. The spacecraft are not rigidly connected; they drift, and the arm lengths change continuously due to orbital dynamics. The measurement is therefore not of a static Michelson interferometer but of a time-varying, drag-free constellation in which the test masses are shielded from solar radiation pressure and the lasers must track each other across millions of kilometers.

LISA's sensitivity band complements that of ground-based detectors. Where LIGO detects the final seconds of a stellar-mass black hole merger, LISA will detect the years-long inspiral of supermassive black hole binaries — systems with millions of solar masses — years or decades before they merge. This provides not only earlier warning but also a different kind of information: the waveform encodes the masses, spins, and orbital eccentricities of the binary, and the long observation baseline allows precise tests of general relativity in the strong-field regime.

The mission, a collaboration between ESA and NASA, is scheduled for launch in the mid-2030s. It represents the next stage in the evolution of gravitational wave astronomy: from ground-based, high-frequency, transient detection to space-based, low-frequency, continuous monitoring. The combination of LISA and ground-based detectors would create a multi-band observatory, tracking the same objects from the low-frequency inspiral phase through the high-frequency merger and ringdown. This is not merely more sensitivity. It is a new observational modality.