General Relativity (GR) will soon celebrate its 110th birthday, holding up against all experimental enquiry. Nonetheless, unification theories attempting to quantize gravity, such as string theory, ...are gaining footing. These hypothesize additional scalar, vector, and tensor long-range fields that couple to matter (Will, 2014), introducing violations to GR. Although such violations have never been detected, it is likely that GR will not be the ultimate theory of gravity. What is certain is that gravity tests are alive and well, pushing the validity of GR to new scales and accuracies, or -potentially- suggesting alternative routes for new physics.
Building upon the legacy of Voyager and Pioneer missions, which demonstrated the capability to survive in the outer reaches of the solar system, the Interstellar Probe mission concept (McNutt et al., 2022) aims to characterise our heliosphere through state-of-the-art instrumentation, opening new frontiers also for GR testing. In this work, we investigate the possibility of constraining the Nordtvedt parameter η and the mass of the graviton via the Compton wavelength λC, by simulating the processing of 10 years of radiometric data from the Interstellar Probe. Station calibration and clock synchronisation, as well as limiting spacecraft precession manoeuvres are highlighted as key strategies for obtaining high-quality estimates. In the most favourable scenario, η can be constrained to less than 1.5·10-5, reducing the uncertainty obtained via Lunar Laser Ranging (Hofmann and Müller, 2018), and a lower bound of 1.4·1014 km is set for λC, improving the estimates obtained from planetary ephemerides (Bernus et al., 2020) and gravitational wave detection (Abbott et al., Jun 2021). Extending ranging measurement acquisition to 20 years improves the results tenfold. This experiment interrogates fundamental physics from a unique dynamical setting, investigating possible violations of the Equivalence Principle (EP) underlying GR.
The NASA Discovery-class mission VERITAS, selected in June 2021, will be launched towards Venus after 2027. In addition to the science instrumentation that will build global foundational geophysical ...datasets, VERITAS proposed to conduct a technology demonstration for the Deep Space Atomic Clock (DSAC-2). A first DSAC successfully operated in low-Earth orbit for more than two years, demonstrated the trapped ion atomic clock technology, and established a new level of performance for clocks in space. DSAC-2 would have further improvements in size, power, and performance. It would host a 1× 10-13 grade USO to produce a frequency output with short-term stability of less than 2× 10-13/√τ (where τ is the averaging time).However, due to funding shortfalls, DSAC-2, had to be canceled. The initially foreseen presence of an atomic clock on board the probe, however, raised the question whether this kind of instrumentation could be useful not only for navigation and time transfer but also for fundamental physics tests. In this work, we consider the DSAC-2 atomic clock and VERITAS mission as a specific example to measure possible discrepancies in the redshift predicted by General Relativity by using an atomic clock onboard an interplanetary spacecraft. In particular we investigate the possibility of measuring possible violations of the Local Lorentz Invariance and Local Position Invariance principles. We perform accurate simulations of the experiment during the VERITAS cruise phase. We consider different parametrizations of the possible violations of the General Relativity, different operational conditions, and several different assumptions on the expected measurement performance. We show that DSAC-2 onboard VERITAS would provide new and improved constraints with respect to the current knowledge. Our analysis shows the scientific value of atomic clocks like DSAC-2 hosted onboard interplanetary spacecraft.
Torsion pendulum revisited Bassan, Massimo; De Marchi, Fabrizio; Marconi, Lorenzo ...
Physics letters. A,
10/2013, Letnik:
377, Številka:
25-27
Journal Article
Recenzirano
Odprti dostop
We present an analysis of the motion of a simple torsion pendulum and we describe how, with straightforward extensions to the usual basic dynamical model, we succeed in explaining some unexpected ...features we found in our data, like the modulation of the torsion mode at a higher frequency and the frequency splitting of the swinging motion. Comparison with observed values yields estimates for the misalignment angles and other parameters of the model.
•We describe experimental results showing an unexpected libration of the oscillations.•Lagrangian model of an ideal torsion pendulum for small oscillations.•Lagrangian model for a torsion pendulum affected by small misalignments.•Comparison of this last model with the data achieved.
The Geodesy and Geophysics of Jupiter and the Galilean Moons (3GM) experiment aboard the JUpiter ICy moons Explorer (JUICE) will measure the gravity fields of Europa, Callisto and Ganymede. For the ...first two moons the data will be acquired during flybys, while for Ganymede a 9-months orbital phase is planned. This latter phase is divided in a 5-month elliptical orbit phase (GEO) and a 4-month circular phase (GCO-500). The GCO-500 will provide the first gravity data from an orbiter for Ganymede, enabling the measurement of the Ganymede's gravity field up to the degree 35–45 of spherical harmonics expansion. The large amount of data collected by 3GM will provide enough details to potentially identify regional (hundreds of km) surface structures. Remarkably, Ganymede's outer ice shell is characterized by the presence of dark and bright terrains: evidence of older, dirty ice and younger, cleaner ice, respectively.
In this work we investigate the possibility to detect gravity anomalies related to the surface distribution of bright and dark terrains using the 3GM data. By assuming a range of i) surface density contrasts for the dark and bright terrains based on an estimation of the impactors flux, ii) average surface topographies, iii) internal structure configurations, and comparing several models for the external gravity field of Ganymede, we simulate the expected gravity field as it would be reconstructed by JUICE. Our results show that 3GM data might allow to discriminate and separate the gravitational contributions from the deep interior and the surface distribution of dark and bright terrains.
•JUICE mission•Ganymede simulated gravity anomalies•dark/bright terrains density contrast estimation•Ganymede's core simulated gravity field
While surface‐confined Ullmann‐type coupling has been widely investigated for its potential to produce π‐conjugated polymers with unique properties, the pathway of this reaction in the presence of ...adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X‐ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″‐dibromo‐p‐terphenyl or 1,4‐dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host–guest chemistry.
On‐surface Ullmann‐type coupling with linear precursors typically forms a 1D organometallic intermediate en route to a 1D polymer. Introducing oxygen leads to the unexpected transformation of this intermediate to highly ordered 2D organometallic networks. The network size and shape can be rationally controlled by changing the precursor length and substrate symmetry, making them promising templates for host–guest systems.
General relativity is supported by great experimental evidence. Yet there is a lot of interest in precisely setting its limits with on going and future experiments. A question to answer is about the ...validity of the strong equivalence principle. Ground experiments and lunar laser ranging have provided the best upper limit on the Nordtvedt parameter sigmaeta=4.4x10 super(-4). With the future planetary mission BepiColombo, this parameter will be further improved by at least an order of magnitude. In this paper we envisage yet another possible testing environment with spacecraft ranging towards the nearby Sun-Earth collinear Lagrangian points. Neglecting errors in planetary masses and ephemerides, we forecast sigmaeta=6.4(2.0)x10 super(-4)(5 yr integration time) via ranging towards L sub(1) in a realistic (optimistic) scenario depending on current (future) range capabilities and knowledge of the Earth's ephemerides. A combined measurement, L sub(1)+L sub(2), gives instead 4.8(1.7)x10 super(-4). In the optimistic scenario a single measurement of one year would be enough to reach approximate3x10 super(-4). All figures are comparable with lunar laser ranging, but worse than BepiColombo. Performances could be much improved if data were integrated over time and over the number of satellites flying around either of the two Lagrangian points. We point out that some systematics (gravitational perturbations of other planets or figure effects) are much more in control compared to other experiments. We do not advocate a specific mission to constrain the strong equivalence principle, but we do suggest analyzing ranging data of present and future spacecrafts flying around L sub(1)/L sub(2)(one key mission is, for instance, LISA Pathfinder). This spacecraft ranging would be a new and complementary probe to constrain the strong equivalence principle in space.
Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in ...opto‐ and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real‐time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550°C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll‐like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).
Real‐time low energy electron microscopy observations reveal the growth dynamics and stability of Ge quantum dots formed on Si(111) at temperatures between 450°C and 600°C. A mix of metastable, flat islands and tall, faceted islands are produced at 550°C and below, whereas growth at 600°C yields uniform large, triangular islands.
BepiColombo is a joint ESA/JAXA mission to Mercury with challenging objectives regarding geophysics, geodesy, and fundamental physics. The Mercury Orbiter Radioscience Experiment (MORE) is one of the ...on-board experiments, including three different but linked experiments: gravimetry, rotation, and relativity. The aim of the relativity experiment is the measurement of the post-Newtonian parameters. Thanks to accurate tracking between Earth and spacecraft, the results are expected to be very precise. However, the outcomes of the experiment strictly depend on our "knowledge" about solar system: ephemerides; number of bodies (planets, satellites, and asteroids); and their masses. In this paper we describe a semianalytic model used to perform a covariance analysis to quantify the effects on the relativity experiment, due to the uncertainties of Solar System bodies' parameters. In particular, our attention is focused on the Nordtvedt parameter eta used to parametrize the strong equivalence principle violation. After our analysis we estimated sigmaeta<, ~4.5x10 super(-5), which is about 1 order of magnitude larger than the "ideal" case where masses of planets and asteroids have no errors. The current value, obtained from ground-based experiments and lunar laser ranging measurements, is sigmaetaapproximate4.4x10 super(-4). Therefore, we conclude that, even in the presence of uncertainties on Solar System parameters, the measurement of eta by MORE can improve the current precision of about 1 order of magnitude.
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