A gravity gradient is measured interferometrically from two light beams which each reflect from both of two freefalling test masses. The light beams project in beam arms which remain equal in length as the two test masses freefall except for different effects of gravity on each test mass and any initial relative velocity difference imparted to the test masses. The optical path length of the beam arms also change equally and oppositely during freefall, to amplify the interferometric effect by four times. A high level of common mode rejection eliminates many spurious influences.