Testing Gravity Using Millimeter Accuracy Data from APOLLO

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Gravity, the most evident force of nature, is in fact the weakest of the fundamental forces, and consequently the most poorly tested. Einstein's general relativity, which is currently our best description of gravity, is fundamentally incompatible with quantum mechanics. It is therefore imperative that we subject gravity to the most stringent tests available. Lunar laser ranging (LLR), in which short laser pulses launched from a telescope are bounced off reflectors placed on the moon by U.S. astronauts and Soviet landers, has for decades produced some of the leading tests of gravity by mapping the shape of the lunar orbit to high precision. Since 2006, an experiment called APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) has been collecting the best LLR data in the world, in terms of both data volume and precision. In 2016, an upgrade allowed APOLLO to guarantee accuracy at the millimeter level - the first time this capability has existed anywhere. This project will capitalize on this new capability. Besides its fundamental impact to all areas of cosmology, this project has a strong footprint in training students and in communicating the excitement of science to the public.This effort seeks to collect the first-ever set of millimeter-accurate LLR data using APOLLO. Besides continuing to collect the most accurate LLR data ever obtained, this involves developing techniques to calibrate APOLLO range data against the newly-installed absolute calibration system and to characterize APOLLO performance and systematic errors. Additionally, site geodetic measurements will be collected by an on-site superconducting gravimeter and a nearby global positioning system station, incorporating these data into the LLR analysis in order to constrain motion of the ground.