A way forward for fundamental physics in space

www.nature.com/npjmgrav REVIEW ARTICLE OPEN A way forward for fundamental physics in space A. Bassi1,2, L. Cacciapuoti 3 ✉, S. Capozziello4,5, S. Dell’Agnello6, E. Diamanti7, D. Giulini 8, L. Iess A. Landragin 12, C. Le Poncin-Lafitte 12, E. Rasel13, A. Roura 14, C. Salomon15 and H. Ulbricht16 9 , P. Jetzer10, S. K. Joshi 11 , Space-based research can provide a major leap forward in the study of key open questions in the fundamental physics domain. They include the validity of Einstein’s Equivalence principle, the origin and the nature of dark matter and dark energy, decoherence and collapse models in quantum mechanics, and the physics of quantum many-body systems. Cold-atom sensors and quantum technologies have drastically changed the approach to precision measurements. Atomic clocks and atom interferometers as well as classical and quantum links can be used to measure tiny variations of the space-time metric, elusive accelerations, and faint forces to test our knowledge of the physical laws ruling the Universe. In space, such instruments can benefit from unique conditions that allow improving both their precision and the signal to be measured. In this paper, we discuss the scientific priorities of a spacebased research program in fundamental physics. 1234567890():,; npj Microgravity (2022)8:49 ; https://doi.org/10.1038/s41526-022-00229-0 INTRODUCTION The Standard Model (SM) of particle physics and General Relativity (GR) are the two pillars of our current understanding of Nature. Both theories have been probed individually with ever-increasing precision and are consistent with nearly all experimental observations. However, they fail to explain dark matter (DM), dark energy (DE), or the imbalance between matter and anti-matter in the universe. Yet DM and DE represent 95% of the energy content of our universe while known matter (atoms, molecules) amounts to only 5%. Today, DM and DE have an unknown origin and there is a great deal of experimental and theoretical activity to solve this puzzle. In summary, the clustering of large-scale structures and the accelerated behavior of cosmic fluid could be addressed whether finding out new (unknown) forms of matter or assuming that gravity behaves in a different way at infrared scales. Furthermore, the lack of a self-consistent theory of Quantum Gravity prevents the unification of SM and GR at ultraviolet scales. The open questions in fundamental physics investigated in this paper are: ● ● ● ● Validity of Einstein’s Equivalence Principle; Origin and nature of dark matter and dark energy; Decoherence and collapse models in quantum mechanics; Quantum many-body physics. They will be addressed from different research corners and with different experimental methods: ● ● ● ● Ultracold atoms; High-stability and -accuracy atomic clocks; Matter-wave interferometry; Classical and quantum links. In view of these issues, the cosmos is a particularly attractive laboratory as it provides particles (cosmic rays) or objects (black holes, neutron stars) which are not produced in man-made laboratories (Even if very relevant to fundamental physics, gravitational waves are not discussed here as already addressed in a parallel paper on astrophysics research in space.). OPEN PROBLEMS IN FUNDAMENTAL PHYSICS Einstein’s Equivalence Principle The Equivalence Principle (EP) is at the foundation of Einstein’s GR. It states the universal coupling of matter to the gravitational field, which in turn implies that the impact of gravity onto matter can be understood in terms of a common geometric structure of space-time. The relevance of EP is twofold: First, it clearly goes beyond GR and will serve as a decisive tool for discriminating competing theories of gravity1. Second, understanding its role and impact for couplings to the genuine quantum matter will be a first and decisive step in probing the interface between Quantum Theory and GR in a way guided by experiments, with possible farreaching implications as regards possible reconciliations of the incompatible foundations of these theories. The first formulation of EP, also known as the Weak Equivalence Principle (WEP), states the universality of free fall (UFF), which is meant to say that the center-of-mass motion of a sufficiently unstructured test body only depends on the initial conditions and not on the details of body’s further constitution. In a Newtonian setting, this is sometimes stated as the strict equality of body’s inertial mass mi with its gravitational mass mg, though these two concepts of masses do not easily generalize to other frameworks outside Newtonian physics. The consequences of UFF include the 1 Department of Physics, University of Trieste, Strada Costiera 11, 34151 Trieste, Italy. 2Istituto Nazionale di Fisica Nucleare, Trieste Section, Via Valerio 2, 34127 Trieste, Italy. European Space Agency, Keplerlaan 1 - P.O. Box 299, 2200 AG Noordwijk ZH, The Netherlands. 4Dipartimento di Fisica ‘E. Pancini’, Università di Napoli ‘Federico II’, INFN, Sezione di Napoli, via Cinthia 9, I-80126 Napoli, Italy. 5Scuola Superiore Meridionale, Largo S. Marcellino 10, I-80138 Napoli, Italy. 6Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati (INFN-LNF), via E. Fermi 54, 00044 Frascati (Rome), Italy. 7LIP6, CNRS, Sorbonne Université, Paris, France. 8Institute for Theoretical Physics, Leibniz University Hannover, Appelstrasse 2, 30167 Hannover, Germany. 9Sapienza Università di Roma, 00184 Rome, Italy. 10Department of Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland. 11Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UK. 12SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, LNE, Paris, France. 13Leibniz Universität Hannover, Institut für Quantenoptik, Welfengarten 1, 30167 Hannover, Germany. 14Institute of Quantum Technologies, German Aerospace Center (DLR), Wilhelm-Runge-Straße 10, 89081 Ulm, Germany. 15Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, Paris, France. 16School of Physics and Astronomy, University of Southampton, SO17 1BJ, Southampton, United Kingdom. ✉email: 3 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA A. Bassi et al. 2 impossibility of locally distinguish effects due to a gravitational field from those arising in a uniformly accelerated reference frame2. For strictly uniform gravitational fields this pertains to ordinary Quantum Mechanics (QM)3. Generally, this entails that it is always possible to locally describe the first-order neighborhood of any space-time point with the language of Special Relativity. This is a crucial aspect; indeed, in 1920 Einstein himself addressed the EP as "the happiest thought of my life”4, (p. 265). Today the general formulation of the EP, known as the Einstei (...truncated)