Quantum Tests of the Einstein Equivalence Principle with the STE-QUEST Space Mission (1404.4307v2)

Abstract: We present in detail the scientific objectives in fundamental physics of the Space-Time Explorer and QUantum Equivalence Space Test (STE-QUEST) space mission. STE-QUEST was pre-selected by the European Space Agency together with four other missions for the cosmic vision M3 launch opportunity planned around 2024. It carries out tests of different aspects of the Einstein Equivalence Principle using atomic clocks, matter wave interferometry and long distance time/frequency links, providing fascinating science at the interface between quantum mechanics and gravitation that cannot be achieved, at that level of precision, in ground experiments. We especially emphasize the specific strong interest of performing equivalence principle tests in the quantum regime, i.e. using quantum atomic wave interferometry. Although STE-QUEST was finally not selected in early 2014 because of budgetary and technological reasons, its science case was very highly rated. Our aim is to expose that science to a large audience in order to allow future projects and proposals to take advantage of the STE-QUEST experience.

Quantum Tests of the Einstein Equivalence Principle with the STE-QUEST Space Mission

The paper "Quantum Tests of the Einstein Equivalence Principle with the STE-QUEST Space Mission" details an ambitious proposal for testing the Einstein Equivalence Principle (EEP) using quantum sensors aboard the STE-QUEST satellite mission. This mission was pre-selected by the European Space Agency for its M3 launch opportunity, but ultimately was not chosen due to budgetary and technological constraints. Despite its non-selection, the mission's scientific objectives were highly rated, underscoring the significance of its proposed investigations into fundamental physics.

Scientific Context

The EEP is a cornerstone of Einstein's General Relativity (GR), which alongside Quantum Mechanics (QM), constitutes our understanding of the universe at its most fundamental level. GR has successfully described gravitational phenomena across various scales, from planetary to cosmological. Concurrently, QM has provided a robust framework for explaining atomic and subatomic interactions, culminating in the successful Standard Model of particle physics. However, these two frameworks are conceptually divergent, and contemporary theories seeking their unification frequently predict violations of the EEP.

The STE-QUEST mission aims to investigate these potential violations, particularly in the quantum regime, where distinctions between classical and quantum descriptions of motion could manifest.

Mission Objectives and Methodology

STE-QUEST proposes to test various aspects of the EEP using high-precision atomic clocks, matter wave interferometry, and time/frequency links. The three fundamental facets of EEP are:

• Weak Equivalence Principle (WEP): The universality of free fall, which posits that trajectory is independent of an object's composition.
• Local Lorentz Invariance (LLI): The uniformity of physical laws irrespective of the velocity and orientation of an experiment's apparatus.
• Local Position Invariance (LPI): The constancy of non-gravitational experimental outcomes regardless of location or time.

Weak Equivalence Principle Test

The mission's atom interferometer will compare the free fall of quantum matter waves from two isotopes of Rubidium, targeting an Eötvös ratio measurement with an uncertainty down to $2 \times 10^{-15}$. This precision substantially surpasses existing ground-based quantum tests, which operate at uncertainty levels around $10^{-7}$.

Local Position Invariance Test

The proposed satellite will measure gravitational red-shifts involving clock comparisons at various global stations. It is anticipated that tests in the field of the Sun, Moon, and potentially Earth, will achieve uncertainties as low as $2 \times 10^{-6}$ and $4 \times 10^{-4}$ respectively for the Sun and Moon tests. These tests are crucial for assessing any anomalies in the clock frequencies induced by gravitational potential differences.

Expected Contributions and Implications

By dramatically enhancing precision levels, STE-QUEST could provide conclusive insights into potential quantum interactions with gravity, with prospects of revealing new physics at the intersection of GR and QM. The gravitation's coupling to quantum fields remains one of the most profound avenues of inquiry, with ramifications extending through fundamental physics, cosmology, and potentially yielding clarifying evidence for dark matter and dark energy phenomena.

Given the intricate potential violations of the EEP across its sub-principles, the mission's multi-faceted approach could precisely delineate the nature of these interactions across different quantum states. Such insights will be invaluable in constraining contemporary theories pursuing the unification of particle physics with gravitation.

Legacy and Future Directions

Beyond fundamental physics, STE-QUEST's design and methodologies hold promise for advancements in geodesy, time-frequency metrology, and establishing celestial and terrestrial reference frames. While STE-QUEST was ultimately not selected for the M3 slot, the learned experiences and scientific ambitions could serve as a prototype for future missions exploring quantum aspects of gravity. These future endeavors may well utilize the foundation laid by the STE-QUEST planned investigations, thereby affirming Europe's leadership in this nascent domain of space science.