arXiv subjects

Anna M. Nobili

Publications and source records attributed to Anna M. Nobili.

7 recordsLinked to original sources

arXiv2026

Ground measurements of the gravitational redshift questioned:re-establishing the physical bases

Motivated by alleged inconsistencies in the scientific and educational literature, Asenbaum, Overstreet and Kasevich (2024; https://doi.org/10.1088/1402-4896/ad340c) aim to clarify some fundamental concepts in the physics of gravitation. To this end they reexamine the first experimental measurement of the gravitational redshift by Pound and Rebka in 1960, claiming that it did not in fact measure the gravitational redshift predicted by Einstein almost half a century earlier, but rather a Doppler shift originating from non-gravitational reaction forces. We show that their conclusion arises from a misunderstanding of the reference systems involved, along with an unphysical interpretation of non gravitational forces. Thus, our work restores the Pound and Rebka experiment to its rightful place in Physics. Beyond the specific paper addressed, this analysis re-establishes the physical bases of simple, yet fundamental issues of gravitational physics.

gr-qc↗

arXiv2025

On the first test of the Weak Equivalence Principle in low Earth orbit

The Weak Equivalence Principle is the founding pillar of General Relativity and as such it should be verified as precisely as possible. The Microscope experiment tested it in low Earth orbit, finding that Pt and Ti test masses fall toward Earth with the same acceleration to about 1e-15, an improvement of about two orders of magnitude over ground tests. Space missions, even if small, are expensive and hard to replicate; yet, the essence of physics is repeatability. This work is an assessment of the Microscope results based on the laws of physics and knowledge from previous experiments, focusing on the limiting thermal noise and the treatment of acceleration outliers. Thermal noise reveals anomalies that we explain by stray sub-microVolt potentials caused by patch charges, giving rise to an unstable zero. The measurements were affected by numerous acceleration spikes occurring at the synodic frequencies relative to the Earth (the signal frequency) and the Sun, which we interpret as evidence of a thermal origin. In Microscope authors' analysis, the spikes were removed and the resulting gaps replaced with artificial data (up to 35, 40 per cent of the sessions data), which retain memory of the gaps and may simulate or cancel an effect (signal or systematic). An alternative approach based exclusively on real measured data would avoid any ambiguity. The lessons of Microscope are crucial to any futures improved mission.

gr-qc↗

arXiv2020

Systematic errors in high-precision gravity measurements by light-pulse atom interferometry on the ground and in space

We focus on the fact that light-pulse atom interferometers measure the atoms' acceleration with only three data points per drop. As a result, the measured effect of the gravity gradient is systematically larger than the true one, an error linear with the gradient and quadratic in time almost unnoticed so far. We show how this error affects the absolute measurement of the gravitational acceleration $g$ as well as ground and space experiments with gradiometers based on atom interferometry such as those designed for space geodesy, the measurement of the universal constant of gravity and the detection of gravitational waves. When atom interferometers test the universality of free fall and the weak equivalence principle by dropping different isotopes of the same atom one laser interrogates both isotopes and the error reported here cancels out. With atom clouds of different species and two lasers of different frequencies the phase shifts measured by the interferometer differ by a large amount even in absence of violation. Systematic errors, including common mode accelerations coupled to the gravity gradient with the reported error, lead to hard concurrent requirements --on the ground and in space-- on several dimensionless parameters all of which must be smaller than the sought-for violation signal.

physics.atom-ph↗

arXiv2018

Testing the Equivalence Principle in space after the MICROSCOPE mission

Tests of the Weak Equivalence Principle can reveal a new, composition dependent, force of nature or disprove many models of new physics. For the first time such a test is successfully carried out in space by the MICROSCOPE satellite. Early results show no violation sourced by the Earth for Pt and Ti test masses with random errors (after 8.26d of integration time) of about 1 part in 1e14, and similar systematic errors.It improves by 10 times over the best ground tests with rotating torsion balances despite 70 times less sensitivity to differential accelerations, thanks to the much stronger driving signal in orbit. The test is limited by thermal noise from internal damping in the gold wires used for electrical grounding. This noise was shown to decrease when the s/c was set to rotate faster than planned. The result will improve by the end of the mission, as thermal noise decreases with more data. Not so systematic errors. We investigate major non-gravitational effects and find that the Pt-Pt sensor does not allow their separation from the signal. The early test reports an upper limit of systematic errors in the Pt-Ti sensor which are not detected in the Pt-Pt one, hence would not be distinguished from a violation. Once all the integration time is used to reduce random noise there will be no time left to check systematics. MICROSCOPE demonstrates the huge potential of space for WEP tests of very high precision and indicates how to reach it. To realize the potential, a new experiment needs the spacecraft to be in rapid, stable rotation around the symmetry axis, needs high quality state-of-the-art mechanical suspensions, and must allow systematic checks. The design of the "Galileo Galilei" (GG) experiment, aiming to test the WEP to 1 part in 1e17 unites all the needed features, indicating that a quantum leap in space is possible provided the new experiment heeds the lessons of MICROSCOPE.

gr-qc↗

arXiv2017

Relevance of the weak equivalence principle and experiments to test it: lessons from the past and improvements expected in space

Tests of the Weak Equivalence Principle (WEP) probe the foundations of physics. Ever since Galileo in the early 1600s, WEP tests have attracted some of the best experimentalists of any time. Progress has come in bursts, each stimulated by the introduction of a new technique: the torsion balance, signal modulation by Earth rotation, the rotating torsion balance. Tests for various materials in the field of the Earth and the Sun have found no violation to the level of about 1 part in 1e13. A different technique, Lunar Laser Ranging (LLR), has reached comparable precision. Today, both laboratory tests and LLR have reached a point when improving by a factor of 10 is extremely hard. The promise of another quantum leap in precision rests on experiments performed in low Earth orbit. The Microscope satellite, launched in April 2016 and currently taking data, aims to test WEP in the field of Earth to 1e-15, a 100-fold improvement possible thanks to a driving signal in orbit almost 500 times stronger than for torsion balances on ground. The `Galileo Galilei' (GG) experiment, by combining the advantages of space with those of the rotating torsion balance, aims at a WEP test 100 times more precise than Microscope, to 1e-17. A quantitative comparison of the key issues in the two experiments is presented, along with recent experimental measurements relevant for GG. Early results from Microscope, reported at a conference in March 2017, show measurement performance close to the expectations and confirm the key role of rotation with the advantage (unique to space) of rotating the whole spacecraft. Any non-null result from Microscope would be a major discovery and call for urgent confirmation; with 100 times better precision GG could settle the matter and provide a deeper probe of the foundations of physics.

gr-qc↗

arXiv2016

Do general relativistic effects limit experiments to test the universality of free fall and the weak equivalence principle?

The Universality of Free Fall and the Weak Equivalence Principle, which are at the basis of General Relativity, have been confirmed to 1 part in 10^13. Space experiments with macroscopic test masses of different composition orbiting the Earth inside a low altitude satellite aim at improving this precision by two orders of magnitude (with the Microscope satellite, launched on 25 April 2016) and up to four orders of magnitude (with the 'Galileo Galilei' - GG satellite). At such a high precision many tiny effects must be taken into account in order to be ruled out as the source of a spurious violation signal. In this work we investigate the general relativistic effects, including those which involve the rotation of both the source body and the test masses, and show that they are by far too small to be considered even in the most challenging experiment. (Paper to appear on Physical Review D)

gr-qc↗

arXiv2016

Fundamental limitations to high-precision tests of the universality of free fall by dropping atoms

Tests of the universality of free fall and the weak equivalence principle probe the foundations of General Relativity. Evidence of a violation may lead to the discovery of a new force. The best torsion balance experiments have ruled it out to 10^-13. Cold-atom drop tests have reached 10^-7 and promise to do 7 to 10 orders of magnitude better, on the ground or in space. They are limited by the random shot noise, which depends on the number N of atoms in the clouds. As mass-dropping experiments in the non-uniform gravitational field of Earth, they are sensitive to the initial conditions. Random accelerations due to initial condition errors of the clouds are designed to be at the same level as shot noise, so that they can be reduced with the number of drops along with it. This sets the requirements for the initial position and velocity spreads of the clouds with given N. In the STE-QUEST space mission proposal aiming at 2x10^-15 they must be about a factor 8 above Heisenberg's principle limit, and the integration time required to reduce both errors is 3 years, with a mission duration of 5 years. Instead, offset errors at release between different atom clouds are systematic and give rise to a systematic effect which mimics a violation. Such offsets must be demonstrated to be as small as required in all drops, must be small by design and must be measured. For STE-QUEST to meet its goal they must be several orders of magnitude smaller than the size of each individual cloud, which in its turn must be at most 8 times larger than the uncertainty principle limit. Even if all technical problems are solved and the clouds are released with negligible systematic errors, still they must be measured. Then, Heisenberg's principle dictates that the measurement lasts as long as the experiment and the systematic nature of the effect requires many measurements for it to be ruled out as a source of violation.

physics.ins-det↗