Taking Newton into space

The test mass retroreflector

Schematic of M R Feldman et al‘s  proposed experiment. The test mass retroreflector, exhibiting harmonic motion within the tunnel of the larger layered sphere, is represented by the filled black circle on the left. Determinations of the round trip light-time from the host spacecraft (on the right) using an onboard ranging system provide measurements of the period of the oscillator.

Newton’s gravitational constant, G, is crucial for fundamental physics: it governs how much spacetime curves for a given mass, is essential for metrology, and might give clues to a deeper understanding of quantum gravity. However, G continues to present unexpected issues in need of resolution. Determinations over the last thirty years have yielded inconsistencies between experiments significantly greater than their reported individual uncertainties, oddly with possible periodic behavior. To push forward, the National Science Foundation (NSF) has recently called for new “high-risk/high-impact” proposals to produce a step-change improvement in measurements (NSF 16-520).

In response, we propose taking advantage of the classic gravity train mechanism by sending a solid sphere with a cylindrical bore hole along its diameter into deep space and measuring the period of oscillation of a test mass moving within the hole using laser ranging techniques from a far removed host spacecraft. As is well known, the oscillator’s period is directly dependent upon G (mass and dimensions of the sphere are known to a certain accuracy before launch), and thus provides a means for determining its value. To ensure stability, the bored sphere must be composed of a dense inner core surrounded by a less dense outer envelope.

Preliminary estimates of the relative uncertainty (63 ppb) yield a near one thousand times improvement versus previous measurements on Earth. As with any new proposal, there are questions that need to be addressed more thoroughly, e.g. precise modeling of stray small force gradients within the hole, charge accumulation due to galactic cosmic ray bombardment, release mechanism from the host, but our goal is to add to the discussion for ways to produce the aforementioned step-change improvement in determining G.

Read the full article for free* in Classical and Quantum Gravity:
Deep space experiment to measure G
M R Feldman, J D Anderson, G Schubert, V Trimble, S M Kopeikin and C Lämmerzahl 2016 Class. Quantum Grav. 33 125013
*until 25 June 2016

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Michael R Feldman, John D Anderson, Gerald Schubert, Virginia Trimble, Sergei M Kopeikin and Claus Lämmerzahl

Michael R Feldman, John D Anderson, Gerald Schubert, Virginia Trimble, Sergei M Kopeikin and Claus Lämmerzahl