[Paper Review] Optical Design for the Laser Astrometric Test of Relativity

Slava G. Turyshev, Michael Shao+1 more|ArXiv.org|Feb 25, 2005

History and Developments in Astronomy3 citations

TL;DR

This paper proposes the Laser Astrometric Test of Relativity (LATOR), a space-based optical interferometry mission using laser ranging between two microspacecraft and the International Space Station to test general relativity with unprecedented precision. By measuring gravitational light deflection near the Sun and Shapiro time delay at interplanetary scales, LATOR aims to determine the Eddington parameter γ to 1 part in 10^9, probing for scalar-tensor gravity effects with 30,000× greater sensitivity than Cassini.

ABSTRACT

This paper discusses the Laser Astrometric Test Of Relativity (LATOR) mission. By using a combination of independent time-series of highly accurate gravitational deflection of light in the immediate proximity to the Sun along with measurements of the Shapiro time delay on the interplanetary scales (to a precision respectively better than $10^{-13}$ radians and 1 cm), LATOR will significantly improve our knowledge of relativistic gravity. The primary mission objective is to i) measure the key post-Newtonian Eddington parameter $γ$ with accuracy of a part in 10$^9$. $(1-γ)$ is a direct measure for presence of a new interaction in gravitational theory, and, in its search, LATOR goes a factor 30,000 beyond the present best result, Cassini's 2003 test. Other mission objectives include: ii) first measurement of gravity's non-linear effects on light to $\sim$0.01% accuracy; including both the traditional Eddington $β$ parameter and also the spatial metric's 2nd order potential contribution (never been measured before); iii) direct measurement of the solar quadrupole moment $J_2$ (currently unavailable) to accuracy of a part in 200 of its expected size; iv) direct measurement of the ``frame-dragging'' effect on light by the Sun's rotational gravitomagnetic field to one percent accuracy. LATOR's primary measurement pushes to unprecedented accuracy the search for cosmologically relevant scalar-tensor theories of gravity by looking for a remnant scalar field in today's solar system. The key element of LATOR is a geometric redundancy provided by the laser ranging and long-baseline optical interferometry. We discuss the mission and optical designs of this proposed experiment.

Motivation & Objective

• To test the pure tensor metric nature of gravity by measuring the Eddington parameter γ with 1 part in 10^9 accuracy, surpassing Cassini’s 2003 result by a factor of 30,000.
• To measure gravity’s non-linear effects on light propagation with ~0.01% accuracy, including the previously unmeasured second-order spatial metric potential contribution.
• To directly measure the solar quadrupole moment J₂ to 1 part in 200 of its expected value, resolving a current observational gap.
• To detect the Sun’s gravitomagnetic 'frame-dragging' effect on light with 1% accuracy, probing rotational gravity effects.

Proposed method

• Utilizes long-baseline optical interferometry and laser ranging between two microspacecraft and a beacon station on the ISS to achieve geometric redundancy and precise baseline determination.
• Employs narrowband filters, coronagraph optics, and heterodyne detection to suppress solar background light and enable signal acquisition near the solar limb.
• Leverages the orbital motion of the ISS to provide variable baseline projections, eliminating the fringe ambiguity problem common in static interferometric systems.
• Uses μg accelerometers to mitigate vibrations from the ISS structure, ensuring stable interferometric measurements.
• Relies on existing flight-proven optical technologies developed for the Space Interferometry Mission (SIM), enabling rapid mission development and high reliability.
• Operates above Earth’s atmosphere to avoid atmospheric astrometric noise, ensuring high-precision measurements unattainable from ground-based systems.

Experimental results

Research questions

• RQ1 Can the Eddington parameter γ be measured with a precision of 1 part in 10^9, providing the most stringent test of general relativity’s tensor metric structure to date?
• RQ2 Can the second-order non-linear contribution to light deflection in the solar gravitational field be measured with ~0.01% accuracy, revealing effects beyond standard post-Newtonian theory?
• RQ3 Is the solar quadrupole moment J₂ measurable directly with 0.5% accuracy, resolving its current uncertainty in solar system models?
• RQ4 Can the gravitomagnetic frame-dragging effect on light propagation be detected with 1% accuracy, confirming rotational gravity effects predicted by general relativity?
• RQ5 Does the experiment detect a remnant scalar field from cosmological evolution, indicating a deviation from pure tensor gravity and supporting scalar-tensor theories?

Key findings

• The LATOR mission is designed to measure the Eddington parameter γ with a precision of 1 part in 10^9, representing a 30,000-fold improvement over the Cassini 2003 measurement.
• The experiment will measure the second-order non-linear light deflection term in the solar gravitational field with ~0.01% accuracy, enabling the first direct observation of this effect.
• The solar quadrupole moment J₂ will be measured directly to an accuracy of 1 part in 200 of its expected value, resolving a current observational uncertainty.
• The frame-dragging effect on light due to the Sun’s rotation will be measured with 1% accuracy, providing a novel test of gravitomagnetism.
• The mission’s optical design, based on geometric redundancy and laser interferometry, eliminates fringe ambiguity and enables high-precision astrometry in the solar vicinity.
• The use of the ISS as a stable platform, combined with advanced optical technologies, makes LATOR feasible as a Medium Explorer (MIDEX) class mission with demonstrated technology readiness.