The Lorentz Invariance Space Test (LIST) on the International Space Station
Probing the constancy of the speed of light in microgravity
LIST is a proposed quantum experiment in space with the primary goal of testing:
The LIST project is part of the Quantum Sensors for Fundamental Physics (QSFP) consortium and is supported by the UK Space Agency (UKSA), which has agreed to nominate the LIST experiment as national payload to the European Space Agency (ESA) for launch to the International Space Station (ISS).
Since the revolutionary Michelson-Morley experiment in the 19th century, the speed of light has been an unchanging speed limit of nature as predicted by Maxwell’s classical electrodynamics which, too, stands the test of time. The resulting Lorentz Invariance underpins Einstein’s Special Relativity and General Relativity as a foundation of modern physics. The well-tested quantum nonlocality, however, demonstrates not all physical processes are limited by the speed of light. Furthermore, attempts beyond the Standard Model to unify gravity with quantum theory, as a most challenging scientific endeavour of the time, suggests the breakdown of Lorentz Invariance at a scale where the discrete nature of space-time is important, leading to a fluctuating speed of light.
The Planck scale provides a natural Quantum Gravity scale. However, precise observations of gamma-ray bursts have shown no evidence of energy dependence of the speed of light even beyond the Planck scale. Furthermore, based on effective field theory, Standard Model Extension suggests that Lorentz symmetry may be spontaneously broken resulting in a variable speed of light at lower than the Planck scale. In a wider context of recent developments in variable fundamental constants, not only the constancy of the speed of light but also the definiteness of the Planck scale might not be certain. Noticeably, Lorentz Invariance tests of non-photon sectors do not detect any variation of the speed of light. This opens up possible anisotropy and inhomogeneity of the speed of light over a wide range of unconstrained energy scales to be tested as an effective approach to the search for new physics (Fig. 1).
The constancy and hence isotropy of the speed of light is the direct consequence of the Lorentz symmetry. More recent Michelson-Morley experiments use ultra-stable optical resonators to search for violations of Lorentz Invariance. Earth-bound experiments have reached a limit of 10-18 in precision for detecting anisotropies in the speed of light. LIST aims to overcome their limitations and perform an experiment with at least a factor of 10 better precision.
This experiment uses quantum technology for realising a precision experiment in microgravity to probe the constancy of the speed of light. The operation of LIST on the ISS will minimise gravitational distortion and environmental noise and to access maximal angular degrees of freedom while avoiding systematic rotation-induced effects. Thus, the advantages of LIST will be to obtain a larger, more accurate and unbiased Lorentz violation parameter space than with any current ground-based experiment, allowing us to test a broader class of theories with higher precision.
2 Scientific Impact
2.1 Quantum sensors for testing and guiding fundamental physics
LIST employs a quantum sensing device to probe fundamental physics. The experiment would be the first of its kind in space and particularly on the ISS. With its ultra-stable, force insensitive cavity used as optical resonator, LIST will allow exploration of the limits of the constancy of light speed for the first time in a microgravity environment in space and improve precision by at least a factor of 10 when compared to the most recent terrestrial results. A detection of a violation of Lorentz Invariance would have major implications on modern physics, pointing to the limits of Einstein’s theory of Special Relativity, which in turn underpins both General Relativity and Quantum Field Theory. Therefore, such an outcome of our proposed project would help guide research towards an ultimate unification of quantum and gravitational theories (Fig. 2), which is widely regarded as the 'holy grail' of modern physics by fundamental and theoretical physicists, applied mathematicians and beyond.
As illustrated in Fig. 1, evidence of Lorentz violation could also point to the existence of new particles as the Standard Model Extension background fields responsible for the spontaneous breaking of Lorentz symmetry. Such particles are of significant interest as potential candidates for Dark Matter, which may induce the 5th force and variations of fundamental constants, currently being sought by particle physicists. Furthermore, they may be closely related to the hidden sectors and scalar fields predicted by the models from String Theory and Loop Quantum Gravity. These areas forge clear mutual phenomenological inputs between LIST and other projects under the QSFP consortium.
2.2 Enabling quantum technology for future experiments in precision metrology
The ultra-stable cavity-stabilised laser technology developed for the proposed LIST project could provide an optical source for future space-based precision interferometer such as LISA and NGGM. Despite being a stand-alone unit, the technology of LIST can also be an integral part of an optical atomic clock. Therefore, space developments of these two are very much interlinked and consequently, the accruing benefits from realising LIST would help establish a space optical atomic clocks across a wide range of space-centric science and technology applications. Examples encompass a network of high-precision optical clocks with a space-based master optical atomic clock and a gravitational wave space telescope.
The very low relative frequency uncertainty at the 10-18 level of these optical clocks allows the establishment of an improved international atomic time scale based on optical clocks and would facilitate a future optical redefinition of the SI second. With the precision of today’s ground-based optical clocks, a gravitational potential at any location can be measured with resolution equivalent to a 1-cm height difference between two high performance clocks. Moreover, just as LIST addresses Lorentz Invariance, an optical atomic clock in space would contribute to better understanding of related fundamental physics laws such as Einstein’s local positional invariance, by comparisons of the gravitational redshift in the Earth’s gravitational field between a space-based clock and ground clocks. Furthermore, a network of ultra-stable oscillators, such as optical atomic clocks, distributed over the Earth’s surface in combination with a space-based clock could serve for detection of Dark Matter. In detail, one could precisely detect the interaction of topological defects related to Dark Matter with these oscillators. For the detection of Dark Matter, it is essential that the resonators or clocks span a large-sized network to be able to match the plausible size of the topological defects and to allow determination of their speed and direction of motion. Hence, a space clock and its associated links to ground clocks would play an essential role in making such a network operational.
In summary, the realisation of LIST will allow the exploration of boundaries of where the laws of modern physics, in particular Lorentz Invariance, are valid. Further, this experiment would benefit the Fundamental Physics in Space community at large and pioneer ultra-stable lasers stabilised to optical cavities for future operation of optical atomic clocks in space.
3 Technological Benefits
The proposed project LIST involves UK-based research at the cutting edge of quantum-technological applications. It combines academic sector science with the capabilities of a UK R&D national institute and UK industry, thereby creating an interdisciplinary and cross-sector environment for atom-optics research dedicated to the development and exploitation of ultra-sensitive, ultra-precise, robust, and compact quantum technology devices for space applications. Hence, it will significantly contribute to the UK’s competitiveness at the international level.
The realisation of LIST will have a major technological impact: on the one hand for future space experiments related to precision metrology; and on the other hand for terrestrial uses of quantum-technological devices due to a raised technology readiness level (TRL) through our developments in LIST. We stress that this project would allow the UK to gain leading advantage in quantum technology and other future science missions and satellite navigation technology in space. Notably, the UK-led LIST experiment on the ISS, benefiting from the world-leading expertise of UK-based researches in technology for optical atomic clocks, would establish UK research alongside the leading ground-based science testing the validity of Lorentz Invariance.
There are programmes for the development of a European space optical atomic clock underway with similar developments worldwide. The space qualification of the LIST experiment would be a crucial contributor to these.
To maximise the international impact of the proposed experiment, we will make our measurement data publicly available in a timely and reliable fashion. This way a very broad community having diverse scientific goals in connection with precision and quantum optical measurement data, including those under microgravity and in space, can benefit from LIST.
4 Research Methods
4.1 Experimental development
LIST uses the output of a diode-based continuous wave laser which is split into two beams with compensation of the beam paths to take out thermal and vibration effects en route to the cavity. The frequency of each beam is stabilised to a cavity resonance of one of the two orthogonal axes of an ultra-stable ultra-low expansion (ULE) dual axis cubic cavity. The anisotropic variation of the speed of light can be probed by searching for a modulation of the beat frequency between the beams with a planar periodicity of half the ISS orbit. The search for more general types of the variation of the speed of light with the precessing ISS orbital plane is further discussed in the theoretical framework section below. The respective LIST payload is designed with a modular architecture, which is very efficient for this type of project. The system consists of a science module, an electronics module, and a computer and control unit.
4.1.1 Science module
To achieve the project objectives, we will use a semiconductor diode laser based on an interference filter design. This design is considered to be very stable and also suitable for space-based experiments. It has already been successfully tested in drop tower and sounding rocket experiments in Germany. We will split the output of this laser into two beams and stabilise each beam against one axis of a high-performance dual-axis cavity capable of reaching a line width of 1 Hz with its approximately 5-cm spacer design as shown in Fig. 3. Ultimately, we will aim for a system, which can achieve stability equal to 1 part in 1015 in 1s or better with crystalline-coated mirrors. For the frequency comparison between the two stabilized laser beams, we will introduce modulation on one of the laser beams.
4.1.2 Electronics and computer control module
As illustrated in Fig. 4, the electronics module will accommodate the locking and modulation boards for the control of the laser frequencies at the science module. Finally, a computer control module controls the science and electronic module and manages the power distribution within the payload. It further provides a communication interface with the ISS infrastructure. To ensure autonomous system operation with a near-continuous up-time, we will implement laser mode auto-recovery and re-lock algorithms within a multi-level state machine approach, with override options for ground control. The data collection system will conform to ESA standards.
For the overall system we consider a maximum weight of 30 kg, a maximum volume of 50 L, and a maximum required power of 30 W. We design the LIST payload to be compatible for accommodation in one of the ISS space station lockers (SSL) in the European drawer rack (EDR) located in the Columbus module (Fig. 5). The SSL offers a total volume of 57 L; power supply, cooling, data exchange, and various other infrastructure required to run the experiment. With the LIST payload installed in an SSL of the EDR, autonomous operation will be activated, but with ground control options for test, adjustment or re-boot if necessary.
4.2 Theoretical framework
Theoretical work will support the development and analysis of the experiments throughout the project. Tailored estimates of sensitivity for parameters describing deviations from the Standard Model and General Relativity will be determined in the ISS environment. The implications of data for confirming and constraining existing and new theories will be provided (Fig. 1). Furthermore, recent theoretical progress on quantum-photon effects and quantum-gravitational interactions will be incorporated in the experiment with the aim to enhance the measurements with the development of new quantum sensors for fundamental physics.
As illustrated in Fig. 6, the LEO of ISS at inclination i = 51° with 90-min period, 6° per day nodal regression and synchronous rotation affords ISS an advantageous platform for LIST as follows. In the ISS Analysis frame (X,Y,Z), we will orientate the crossed cavities to ensure the full angles of space can be surveyed optimally in 2,000 variation periods of orbit-induced beat modulation due to any anisotropy of speed of light every 2 months. Importantly, achieving this without the active rotation of cavity eliminates induced systematic effects experienced by recent tests. In addition, the ISS LEO speed of 30 km/s, significantly faster than the maximum Earth surface spin speed of 460 m/s, allows improved limits of the boost-dependence of the speed of light compared to Kennedy-Thorndike experiments on the ground or involving possible higher orbits.
This setting enables more parameters from the Lorentz group and Lorentz Invariance violating Standard Model Extensions to be probed relative to an arbitrary rest frame, which may be related to the CMB as considered by recent ground-based experiments or a potential Lorentz Invariance violating Dark Matter in independent directions. Furthermore, our ISS-enabled isotropic test for Lorentz Invariance ensures orientation-independent noise to be more effectively integrated out allowing improved angular resolution. Sub-Hz vibration levels on ISS important to LIST are only about 10-6 of the ground level. Non-cyclic differential drifts will be removed in data post-processing. Remaining signals are then correlated to:
5 The Project Team
The LIST Team includes members:
The Cold Atoms Group at UoB led by Kai Bongs brings expertise and a track record in developing transportability of high-precision optical clock setups. Ongoing and previous project involvements comprise, amongst others, research developments performed in a previous EU-FP7 project (
SOC2), the ITN FACT, and the ongoing H2020 RISE project as well as the recently started EC Quantum Flagship project 'iqClock'. Moreover, this partner provides access to some testing facilities for space flight qualification.
The Time and Frequency Group at NPL led by Patrick Gill has already demonstrated ultra-stable lasers with Hz-level linewidths and optical atomic clocks with uncertainties in the in 10-17 fractional frequency range. Notably, the NPL cubic cavity is seen by ESA as leading technology for a number of future ESA missions. All these capabilities have led to a space heritage across a wide range of wavelength and optical frequency standards applications within ESA and EU contracts, totalling some 18 ESA contracts over the last 15 years, of which NPL has primed 9.
The Quantum Gravity Group at UoA headed by Charles Wang contributes its theoretical expertise in gravitational and other force fields coupled to real-world quantum optical and atomic systems. The group has built up pioneering research with major publications on the effects of classical and quantum (gravitational) environments on light and matter in open space and laboratory settings. Its extensive experience in actively interacting with both theorists and experimentalists and leading and collaborating on EPSRC Theoretical Physics projects, Quantum Technology project GG-Top, and STFC and ESA Fundamental Physics in Space projects including HYPER, GAUGE, and STE-QUEST makes the Aberdeen group well suited to provide theoretical support to this project.
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