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# NSF Ideas Laboratory at NIST: Measuring "Big G" Challenge

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Physics needs you to make one of the most difficult measurements.

Yes, we mean you in front of the monitor. You do not have to have a background in the field.

1. Motivation

The gravitational constant G is a measure of the strength of the gravitational interaction. A first laboratory determination of G was performed in 1798 by Henry Cavendish. Since that time hundreds of measurements were performed worldwide. Despite that long history, our current knowledge of the gravitational constant is comparatively poor. The recent recommended value by the Committee on Data for Science and Technology (CODATA) has a relative uncertainty of 47 x 10-6. Compare this with the relative uncertainties of the Planck constant (0.012x 10-6) or the Boltzmann constant (0.57x 10-6).

The figure below shows the most precise measurements that were conducted in the past 30 years. While the relative uncertainties of some measurements are below 20x10-6 the scatter between the experiments is 6 times as large. In fact, the relative difference between the largest and smallest number exceeds 500 x 10-6.

As shown above, we do not understand either the physics behind gravitation, or the physics used in the instruments used to perform these measurements, or both. New Ideas for new measurements can illuminate both of these aspects, which are very important for science and technology.

Our understanding of gravitation, as described by general relativity may not be complete. Attempts to unify gravitation with quantum mechanics, even though an active area of research, have not yielded a satisfying theory. Could the scatter in the big G data be an indication of a richer field of gravity having hidden variables, that the researchers were not aware of? It would be a shame if new physics is lying before our eyes in the scatter plot above there and we did not fully comprehend it, because we deemed the constant as unimportant. Let's see if we can measure this constant with uncertainties below 10-5.

The experiments measure, depending on their detailed mechanics, small forces, torques, or accelerations. Typically, the gravitational force modulated in these experiments corresponds to the weight of a red blood cell and it has to be measured absolutely with a relative uncertainty of about 10-5. Is the scatter in the data a tell-tale of underestimated biases in small force metrology? Many fields of physics, biology, and chemistry, rely on precise determination of the absolute values of small forces, including the Casimir effect, spring constants of atomic force microscopy (AFM) cantilever, and intermolecular forces in DNA.

2. The Ideas Lab 'Measuring Big G Challenge'

The Ideas Lab is organized by the National Science Foundation, solicitation NSF 16-520. The call for proposal can be found here (http://www.nsf.gov/funding/pgm_summ.jsp?pims_id=505229&org=PHY&from=home).

Timeline and Application Details:

• May 16 2016, preliminary proposal (two-pages)
• July 18 – 22 2016, Ideas Lab at NIST Gaithersburg Campus, invitation only.
• October 26 2016, full proposal

The preliminary proposal is due by May 16th 2016. For the preliminary proposal interested researchers should submit a two-page proposal application. The participation in the Ideas Lab taking place from July 18 – 22 at the NIST Gaithersburg campus is by invitation only from the pool of applicants that submitted a preliminary proposal. After discussions at the Ideas Lab full proposals are due by October 26th.

Who can apply: Any individual interested in participating in the Ideas Lab and who is also eligible for NSF funding. Researchers with no prior experience measuring G are encouraged to apply. A primary goal of the Ideas Lab is to bring new concepts and techniques from outside into an existing field to solve a long standing problem.

3. What is an Ideas Lab?

The Ideas Lab is an intensive, interactive and free-thinking environment, where a diverse group of participants from a range of disciplines and backgrounds gets together for five days - away from their everyday worlds - to immerse themselves in collaborative thinking processes in order to construct innovative approaches. It will involve up to 30 participants that will be expected to engage constructively in dialogue with one another and the Director and mentors to develop collaborative research proposals. Collaboration is an integral aspect of the activity. The Ideas Lab will run over five days starting mid-morning on Day One and finishing mid-afternoon on Day Five. At the outset, the participants will work collaboratively to identify and define the scope of the research challenges relating to measuring G. As the Ideas Lab progresses, participants will dynamically develop and hone novel ideas about how the identified challenges may be addressed, and then use these ideas and approaches to develop research projects, which should contain genuinely innovative and potentially risk-taking investigations. The Ideas Lab will include inputs from a variety of sources and will aim to develop collaborative research projects. Following the Ideas Lab, the teams with the best ideas may be invited to submit a full proposal.