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Although gravitational waves have never been directly detected, the existence of gravitational waves is in little doubt as their effects have been measured precisely, if indirectly. Any theory of gravity consistent with special relativity will exhibit gravitational waves, and the predictions of general relativity should be quantitatively reliable for LISA because the long-standing best evidence for gravitational waves is the orbital decay of the Hulse-Taylor binary pulsar, which radiates at frequencies only marginally below LISA's operating band. Therefore LISA will be able to detect the gravitational waves predicted by any reasonable theory of gravity.

In the same way that electromagnetic radiation accompanies acceleration of electric charges, gravitational radiation accompanies quadrupolar acceleration of any kind of mass or energy, perturbing spacetime with a dimensionless metric-strain amplitude. LISA senses this by monitoring the changes in the distances between inertial test masses. LISA uses precision laser interferometry across 5×10⁶ km of space to compare separations between test masses that are protected by the spacecraft from non-gravitational disturbances. LISA coherently measures spacetime strain variations, including frequency, phase, and polarisation, all of which reflect large-scale properties of the systems that produce them and are therefore direct traces of the motions of distant matter.

LISA is an astronomical observatory of unprecedented versatility and range. Its all-sky field of view ensures that it can observe every source of gravitational waves, without having to compromise between observations. Its coherent mode of observing allows it to resolve and distinguish overlapping signals and locate them on the sky. Its unparallelled sensitivity allows it to study sources within the Galaxy and out to the edge of the Universe. Finally, LISA’s wide frequency band (more than three decades in frequency) allows it to study similar sources of widely different masses and cosmological redshifts. Because gravitational waves penetrate all regions of time and space with almost no attenuation, LISA can sense waves from the densest regions of matter, the earliest stages of the Big Bang, and the most extreme warpings of spacetime near black holes.

The key components of the LISA mission concept are the interferometric measurement of the changes of a large baseline (5×10⁶ km), free-falling test masses that define the endpoints of the baseline, suitable orbits of the spacecraft to avoid orbit maintenance (and hence disturbances on the test masses) and a mission lifetime of five years. With this mission concept and an instrument sensitivity model that captures the anticipated sensitivity of the measurement the science performance of LISA can be demonstrated: enough sources with sufficient signal-to-noise ratio are detected in the mission lifetime to fulfil the science objectives.

The classical distinction between spacecraft and payload doesn’t fit LISA very well, as the spacecraft is not just providing the infrastructure for the instruments, but must be designed and built with the gravitational requirements of the free-falling test masses in mind. The usual structural and thermal analysis of the spacecraft has therefore been extended to include gravitational effects as well to ensure that the requirements on gravity gradient at the position of the test masses is fully met. In addition, the payload controls the position of the spacecraft during science operations, rendering the spacecraft effectively a part of the instrument. The importance of the co-design (and the co-operation) of spacecraft and payload is captured in the term “sciencecraft”. The core features of the payload have been stable since more than a decade &ndaash; the interferometric measurement system, the telescope, the gravitational reference sensor, and the micropropulsion system. Their design has evolved and over the time has now reached considerable maturity – many of the design features of, e.g., the optical bench have been shown in laboratory prototypes, during testing of the LISA Pathfinder, and will finally be demonstrated on orbit during LPF operations. The disturbance reduction system for LISA is identical to the one that is being built as the flight model for LISA Pathfinder, the micropropulsion system enjoys full heritage from LISA Pahfinder as well. Other critical components of LISA that are not needed for LISA Pathfinder have been demonstrated experimentally to fully meet the requirements of LISA, such as the phasemeter post-processing techniques to remove the residual laser phase noise and the mechanism to compensate for the angle between send and receive beam.