The Confrontation between General Relativity and Experiment (1403.7377v1)

Abstract: The status of experimental tests of general relativity and of theoretical frameworks for analyzing them are reviewed and updated. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational-wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

• The paper presents a comprehensive analysis of general relativity's experimental tests across different historical and modern regimes.
• It details the methods used to verify Einstein's equivalence principle and applies the PPN formalism to assess deviations from classical predictions.
• The review highlights advancements in gravitational wave observations and outlines their implications for studying strong-field gravitational phenomena.

An Overview of Experimental Tests of General Relativity

Clifford M. Will's review paper titled "The Confrontation between General Relativity and Experiment" provides a comprehensive analysis of the ongoing experimental tests and theoretical frameworks associated with general relativity (GR). The paper explores tests of Einstein's equivalence principle (EEP), scrutinizes post-Newtonian parameters, evaluates strong-field regimes, and assesses gravitational wave phenomena, aiming to confront GR with vast experimental data and theoretical challenges.

The experimental tests of GR can be broadly classified into four main periods: Genesis (1887-1919), Hibernation (1920-1960), a Golden Era (1960-1980), and the Quest for Strong Gravity (1980-present). During the Golden Era, a range of technological advancements enabled precise tests of GR. This set the stage for present-day experiments focusing on strongly gravitating bodies and gravitational waves, characterized by the search for potential deviations from GR in such regimes.

Tests of the Foundations of GR

Key to assessing GR is the verification of EEP, composed of three parts: the weak equivalence principle (WEP), local Lorentz invariance (LLI), and local position invariance (LPI). WEP, often probed by evaluating whether bodies with different compositions fall with the same acceleration in a gravitational field, has been tested through sophisticated torsion balance experiments and atom interferometry. LLI, which posits that experimental outcomes are independent of uniform relative motion, is examined via Michelson-Morley like experiments and Hughes-Drever tests, scrutinizing potential velocity-dependent energy shifts. Lastly, LPI, asserting that experimental results should not depend on location or time, is evaluated through gravitational redshift experiments, initially explored by Pound and Rebka, and now tested with interstellar data and atomic clocks aboard spacecraft.

PPN Formalism and Metric Theories

The Parametrized Post-Newtonian (PPN) formalism provides a framework for comparing GR with alternative metric theories by examining deviations from Newtonian predictions through a set of parameters ($\gamma$, $\beta$, etc.). GR precisely predicts these parameters concerning classical light-bending, time-delay, and potential-energy depression effects. The PPN formalism has been instrumental in gauging predictions from scalar-tensor theories, vector-tensor theories, and others, which are often employed to examine deviations in gravity’s behavior where GR's predictions are precise.

Strong-Field Regimes and Gravitational Waves

Testing GR in strong-field regimes involves phenomena like the periastron shift of Mercury and gravitational wave emissions from compact binary systems. The latter has become a focus, especially with the advent of modern gravitational wave observatories like LIGO and VIRGO. These observatories aim to detect waveforms from gravity-driven inspirals, mergers of compact astrophysical objects, and other cataclysmic events, offering a vivid playground for examining GR's predictions. Theoretical advancements in modeling these waveforms, up to 3.5 post-Newtonian order, are imperative, given the high accuracy required for data analysis.

Future Developments

Successful experimentation continues to bolster GR's predictions. However, the potential for discovering new physics not explained by GR promises further developments in both theoretical and experimental domains. Scalar-tensor theories, $f(R)$ models, and quantum gravity considerations hint at possible modifications at various scales. Moreover, the constraints derived from astrophysical observations, such as black hole/no-hair theorem verification and neutron star behaviors, remain fertile grounds for future exploration. The ongoing investigations into LIGO/VIRGO’s extensive data tapes are expected to either further substantiate GR’s formidable resilience or reveal novel insights beyond the current paradigms.

This ongoing scientific endeavor underscores both the robust foundations laid by Einstein’s theory and the evolutionary trajectory of contemporary physics seeking to unveil the deeper fabric of the universe.