# Possible Tests of Fundamental Physics with GINGER

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## Abstract

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## 1. Introduction

- The RLG is a specific kind of interferometer built as a closed path optical cavity, usually defined by four mirrors located at the vertices of a square: two counter-propagating laser beams are excited inside the cavity. Interference of the beams transmitted by each mirror gives information on the non-reciprocal effects experienced by the two counter-propagating beams caused by the geometry or the laser dynamics. Since the interferometer has two equal paths, the differences due to such non-reciprocity effects are extremely small. However, there are other non-reciprocal effects related to the spacetime structure or to fundamental asymmetries, which make RLGs suitable for fundamental physics investigations.

## 2. Ring Laser

^{−1/2}range (at frequency $<0.1$ Hz), more than a factor of 10 below the expected one [22]. This experimental result is clearly not compatible with the conventional shot noise evaluation, which assumes the two counter-propagating beams are independent and does not take into account couplings between them.In a forthcoming study, we intend to develop a model that, tracing back from the detector scheme, accounts for all the complex interdependent dynamics of the counter-propagating beams with the laser medium and the mirrors. Nonetheless, the reported experimental noise level limit suggests that a realistic final sensitivity target of GINGER should be around 1 part in ${10}^{11}$ of the Earth rotation rate [22].

## 3. GINGER: Fundamental Physics Issues

**Figure 3.**Modified gravity roadmap summarizing the possible extensions of general relativity [32].

- The detection of effects due to spacetime curvature around the Earth (de Sitter effect) and Earth mass rotation (Lense–Thirring effect). This measurement requires comparing the IERS Earth rotation vector with the corresponding GINGER rotation vector. Testing extensions/modifications of general relativity by using PPN formalism [14,33]. Some expected measurements could be seen as upper limits; thus, any enhancement in sensitivity and accuracy may pave the way for further theoretical insights beyond general relativity in gravitational theories. The interplay between gravitomagnetism and fundamental physics tests has a large impact; recent reviews about gravitomagnetism and related theories and tests are now available [26,34].
- In principle, tests could also be performed on metric-affine theories, e.g.; teleparallel gravity [35] theories, which assume that the connection on the spacetime manifold constitutes a fundamental field variable and that is independent from the metric.
- Testing Lorentz violations described by the standard model extension (SME) [15,36]. It has been highlighted that SME terms with dimensions d = 4 and d = 5 can disrupt symmetry for counter-propagating beams in a RLG, and GINGER could significantly contribute to the quest for Lorentz violation. In this case, the signal could also be inferred by comparing GINGER with IERS data. Notably, this test is based on observations at fixed frequency rather than a DC level, so high accuracy is not imperative.
- Investigating whether fluctuations stemming from spacetime granularity could potentially exhibit observable signatures in high-frequency RLG spectra [37,38]. Intuitively, the natural length and time scales linked with spacetime quantum nature are the Planck length, and its fluctuations generate white noise, which is investigable using a frequency comb with harmonics at integer multiples of the RLG free spectral range. This point is linked more to the development of RLG, interferometers very different from the ones based on the Michelson scheme.
- Gravitational waves might excite Earth’s normal modes. Detecting such signals seems feasible theoretically, provided the sensitivity exceeds ${10}^{-16}$ rad/s. Recently, in a proposal, Marletto and Vedral highlighted the possibility of exploring, via a quantum version of the Sagnac interferometer, the quantum nature of gravity, assuming the validity of the equivalence principle in its quantum version [39].
- The unification of GR and quantum mechanics remains an unresolved issue in contemporary physics. Experimental techniques in quantum optics have recently achieved the precision necessary to investigate quantum systems under the influence of non-inertial motion, such as being stationary in gravitational fields or experiencing uniform accelerations. In this context, exploring entanglement phenomena or quantum mechanics tests in non-inertial reference frames would be intriguing [37,42].
- Mechanical rotation modifies the manifestation of photon entanglement [42].
- The impact of light scalars coupled conformally and disformally to matter on the geodetic and frame-dragging has been recently evaluated [43]. This has shown that GINGER could provide measurements of $\Lambda >3.1$∼${10}^{-17}$ eV.

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Notes

1 | UGGS is part of the research center of Camerino, project STRIC, approved in 2022. https://sisma2016.gov.it/wp-content/uploads/2022/06/Ordinanza-n.-33-del-30.06.2022-PNC-Sisma_signed.pdf (accessed on 21 February 2024). |

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**Figure 1.**Schematic layout of our RLG prototypes. Four vacuum chambers are located at the corner of a square to host the four super-mirrors, aligned in order to define a square optical cavity. The chambers are connected by vacuum-tight pipes and the whole system is filled with a mixture of Helium Neon gases. In the middle of one of the sides, a pyrex capillary tube is placed, along with external electrodes, used to power the laser by radio frequency excitation. The laser emits at 633 nm; red lines indicate the light beams. The mirrors are equipped with piezoelectric actuators, two of them are shown in the figure (PZT1 and PZT2). They are used to control the geometry, although the RLG can be operated uncontrolled. On the bottom left mirrors, the transmitted light beams interfere at a beam-splitter cube (IBS), the corresponding beat-note is recorded by the photodiodes and stored to be analyzed. On the top left corner, the two output beams (called monobeams PH1 and PH2) are directly recorded by photodiodes. The Sagnac frequency is reconstructed using the beat note signal; monobeams are used to correct the typical systematics of the laser: backscattering and null-shift.

**Figure 2.**Pictorial view of GINGER, the two RLGs are visible. Compared to the previous project, sections and orientation have been changed.

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**MDPI and ACS Style**

Di Somma, G.; Altucci, C.; Bajardi, F.; Basti, A.; Beverini, N.; Capozziello, S.; Carelli, G.; Castellano, S.; Ciampini, D.; De Luca, G.;
et al. Possible Tests of Fundamental Physics with GINGER. *Astronomy* **2024**, *3*, 21-28.
https://doi.org/10.3390/astronomy3010003

**AMA Style**

Di Somma G, Altucci C, Bajardi F, Basti A, Beverini N, Capozziello S, Carelli G, Castellano S, Ciampini D, De Luca G,
et al. Possible Tests of Fundamental Physics with GINGER. *Astronomy*. 2024; 3(1):21-28.
https://doi.org/10.3390/astronomy3010003

**Chicago/Turabian Style**

Di Somma, Giuseppe, Carlo Altucci, Francesco Bajardi, Andrea Basti, Nicolò Beverini, Salvatore Capozziello, Giorgio Carelli, Simone Castellano, Donatella Ciampini, Gaetano De Luca,
and et al. 2024. "Possible Tests of Fundamental Physics with GINGER" *Astronomy* 3, no. 1: 21-28.
https://doi.org/10.3390/astronomy3010003