Gravitational Physics via Lunar Laser Ranging: Optimizing Data Quality

万有引力是自然界最明显的力，但实际上是基本力中最弱的，因此也是最缺乏检验的。爱因斯坦的广义相对论是目前我们对引力的最佳描述，它从根本上与量子力学不相容，未来可能会被一个更完整的理论所取代。修正后的理论将预测太阳系的微小偏差，这可能对我们对整个宇宙的理解产生深远的影响。月球激光测距（LLR）是一种从望远镜发射的短激光脉冲被美国宇航员和苏联着陆器放置在月球上的反射器反射回来的技术，几十年来，通过高精度地绘制月球轨道的形状，进行了各种领先的重力测试。该小组建议继续利用阿帕奇点天文台的月球激光测距操作（APOLLO）进行前沿观测，努力使重力经受最严格的测试。阿波罗号位于新墨西哥州南部9200英尺的山顶上，引入了毫米级精度的月球轨道测量新制度。然而，不完整的模型至今无法证实其准确性。因此，除了继续进行观测活动外，该小组将设法建立一个校准系统，以确保阿波罗达到其毫米测量目标。拟议的工作将在许多方面使更广泛的社区受益。在智力方面，不断提高我们对引力的认识，为宇宙学家、天体物理学家、粒子物理学家和弦理论家提供了各种各样的信息。这项工作还将对地球和行星科学做出贡献，特别是通过超导重力仪产生的测量结果。阿波罗小组将继续从事教育和外联活动，毫无疑问将继续通过印刷、网络、广播和电视媒体吸引公众的兴趣。在其他有助于阿波罗卓越观测的特性中，在分钟的时间尺度上对所有五个月球反射器的常规范围极大地提高了我们测量月球方向和身体扭曲的能力。这些信息可以更精确地确定月球质心的路径，从而便于对基本重力的测试。同时，更高精度的距离测量，加上来自阿帕奇点天文台超导重力仪和2.5公里外高质量全球定位系统（GPS）站的数据，将极大地提高我们对天文台相对于地球质心（引力测试所需）的瞬时位置的理解，因为它揭示了必须纳入模型的微妙的地球动力学。LLR测量为强等效原理、牛顿引力常数的时间变化率、重力磁学、平方反比定律和首选框架效应提供了最佳的可用测试。除了这些经典的引力测试之外，阿波罗还将允许测试与暗能量、额外维度和违反洛伦兹不变性有关的物理学新思想。这里提出的大部分工作是建立一个基于铯时钟标准、低抖动短脉冲激光器和精确间隔计数器的绝对校准系统。该系统将提供对APOLLO基本测量的独立检查，潜在地识别故障并确认相应的补救措施。

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 and is likely to be replaced by a more complete theory in the future. A modified theory would predict small deviations in the solar system that could have profound consequences for our understanding of the Universe as a whole. Lunar laser ranging (LLR), in which short laser pulses launched from a telescope are bounced off of reflectors placed on the moon by U.S. astronauts and Soviet landers, has for decades produced various leading tests of gravity by mapping the shape of the lunar orbit to high precision. The group proposes to continue conducting leading-edge observations with the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO), in an effort to subject gravity to the most stringent tests yet. APOLLO, situated atop a 9,200 ft summit in Southern New Mexico, introduces a new regime of millimeter-precision in measuring the lunar orbit. However, incomplete models are thus far unable to confirm the accuracy. The group will therefore seek to build a calibration system to ensure that APOLLO meets its millimeter measurement goal, in addition to continuing the observation campaign. The proposed work will benefit the broader community in a number of ways. On the intellectual front, improving our knowledge of gravity informs a diverse range of cosmologists, astrophysicists, particle physicists, and string theorists. The effort would also contribute to Earth and planetary science, especially via measurements produced by the superconducting gravimeter. The APOLLO team will continue a track record of engagement in education and outreach activities, and will no doubt continue to attract public interest through print, web, radio, and television media.Among other attributes that contribute to APOLLO's superior observations, routine ranging to all five lunar reflectors on timescales of minutes dramatically improves our ability to gauge lunar orientation and body distortion. This information allows a more precise determination of the path for the Moon's center of mass, thereby facilitating tests of fundamental gravity. Simultaneously, higher precision range measurements, together with data from a superconducting gravimeter at the Apache Point Observatory and from a high-quality Global Positioning System (GPS) station 2.5 km away, will greatly improve our understanding of the instantaneous location of the Observatory with respect to the Earth's center of mass (needed for the gravitational tests) by exposing subtle Earth dynamics that must be incorporated into the model. LLR measurements provide the best available tests of the strong equivalence principle, the time-rate-of-change of Newton's gravitational constant, gravitomagnetism, the inverse-square law, and preferred frame effects. In addition to these classical gravitational tests, APOLLO will permit testing of new ideas in physics relating to dark energy, extra dimensions, and violations of Lorentz Invariance. A large part of the effort proposed here is the construction of an absolute calibration system based on a cesium clock standard, a low-jitter short-pulse laser, and a precision interval counter. This system will provide an independent check of APOLLO's fundamental measurement, potentially identifying faults and confirming their pursuant remediation.