Collaborative Research: Multi-Mode Apparatus to Resolve the Discrepancy Concerning Big G

在自然界的所有基本常数中，万有引力常数 G 的已知精度最低。当前围绕 G 知识不确定性的情况令基础物理学和精密测量界感到困惑。世界上最好的实验产生的值彼此不相容，并且与最精确的实验的不确定性相差约 40 倍。此外，了解 G 的真实值在各个领域都很重要，因为它对于在量子引力理论中统一广义相对论和量子力学是必要的。此次合作促成的项目将进行仔细控制的计量实验，测量精度将达到百万分之一。由于过去G测定之间的差异部分可以追溯到所使用的方法，因此该小组将在同一装置内结合不同的方法来确定G，希望从每种方法中获得高精度的G值，但期望使用不同方法获得的值将模仿社区当前的情况，即不同的方法，无论多么精确，都会产生不同的结果。通过在同一装置中进行实验，我们的努力将有助于理解现有实验结果之间的差异。该项目将建立一个扭摆设施，专门用于在同一装置内使用三种不同的实验技术以前所未有的灵敏度测量牛顿引力常数 G。由于不同实验组最近的测量结果分散广泛，或者精度较低，因此商定的 G 值仍然难以确定。精密测量界认为测量值的分散性和测量精度相对较低是需要解决的问题。该项目将基于之前扭摆实验中引入的想法构建一个系统，但将通过设备的多模式性质扩大测量的范围和广度。在主模式中，G 将通过测量在存在精心设计的吸引子质量（也在单独的转盘上旋转）的情况下在转盘上旋转时保持扭摆纤维不扭曲所需的角加速度来确定。这种角加速度反馈模式已经产生了迄今为止最精确的 G 测量，但只执行了一次。与之前的工作相比，所提出的系统将通过使用先进的测量和表征技术来实现更小的计量误差。使用相同的设备，G将通过测量存在吸引子质量的扭摆的谐振频率的变化来确定，并且通过测量摆的热感应振荡来去除吸引子质量。在第三种方法中，当吸引子质量处于两个不同位置时，G 将通过大幅度确定摆的谐振频率变化来确定。每种技术的测量预计相对误差约为 2 ppm。该项目中进行的测量将引起物理和计量学不同领域的科学家的广泛兴趣，并且该方法可能有助于解释为什么之前的实验导致 G 测量结果存在差异。除了广泛的科学兴趣之外，本科生和研究生也将是该项目成功不可或缺的一部分。他们将接受实验物理学和精密测量技术的培训。该项目将从一个联邦政府认可的乡村拉美裔服务机构招募人才，为来自不同背景的第一代大学生和本科生提供培训和教育，该机构在校园内的研究机会有限。来自三所不同大学的学生将保持联系，增强他们对不同学术文化的接触并提供交流机会。作为拟议活动的一部分，将在社区聚会活动、招募活动和课堂环境中开发和使用与力量原理相关的演示。

Of all the fundamental constants of nature, G, the universal gravitational constant, is known with the least precision. The current situation surrounding the uncertainty in the knowledge of G is puzzling the fundamental physics and precision measurement communities. The world's best experiments yield values which are incompatible with one another and differ by about 40 times the uncertainty of the most precise experiment. Furthermore, knowing the true value of G is important in various fields, as it is necessary in efforts to unify general relativity with quantum mechanics in a quantum theory of gravity. The project enabled by this collaboration will be to carry out carefully controlled metrological experiments where the precision of the measurements will be in the part-per-million. Since part of the past discrepancies between determinations of G can be traced back to the methodology used, the group will combine different approaches to determine G within the same apparatus, hoping to obtain highly precise values of G from each approach, but with the expectation that the values obtained using different methodologies will mimic the current situation in the community, namely, that different methodologies, no matter how precise, yield different results. With the experiments carried out in the same apparatus our effort would then help understand the current discrepancies among existing experimental results.The project will establish a torsion pendulum facility dedicated to measuring the Newtonian gravitational constant G with unprecedented sensitivity using three different experimental techniques within the same apparatus. An agreed upon value for G remains elusive as recent measurements by different experimental groups have scattered widely, or have had low precision. The spread in measured values and the relatively low precision of the measurements is recognized by the precision measurement community as something that needs to be addressed. This project will build a system based upon the ideas introduced in previous torsion pendulum experiments, but will expand the scope and breadth of the measurements by the multi-mode nature of the apparatus. In the primary mode G will be determined by measuring the angular acceleration needed to keep a torsion pendulum's fiber from twisting while it rotates on a turntable in the presence of carefully designed attractor masses (that also rotate on a separate turntable). This angular acceleration feedback mode has yielded the most precise measurement of G to date, yet it has only been performed once. Compared to previous efforts, the proposed system will achieve smaller metrology errors by using advanced measurement and characterization techniques. Using the same apparatus, G will be determined by measuring the change in the resonant frequency of the torsion pendulum with the attractor masses present and removed by measuring the thermally induced oscillation of the pendulum. In the third approach, G will be determined by large amplitude determination of the change in the resonant frequency of the pendulum when the attractor masses are at two different positions. Each technique is expected to provide a measurement with a relative error of approximately 2 ppm. The measurements performed within this project will be of broad interest to scientists in diverse fields of physics and metrology, and the approach may shed light on why previous experiments have resulted in discrepant measurements of G. In addition to broad scientific interest, undergraduate and graduate students will be integral to the success of the project. They will be trained in experimental physics and precision measurement techniques. The project will provide training and education for first-generation college students and undergraduates from diverse backgrounds by recruiting from a rural, federally-recognized Hispanic Serving Institution that has limited research opportunities on campus. Students from three different universities will be in contact, enhancing their exposure to different academic cultures and providing networking opportunities. As part of the proposed activities, demonstrations associated with the principles of forces will be developed and used at community gathering events, recruiting events and in classroom environments.