New high-precision laser spectroscopy measurements on thorium-229 nuclei could shed more light on the fine structure constant, which determines the strength of the electromagnetic interaction, say physicists at TU Wien in Austria.

The electromagnetic interaction is one of the four known fundamental forces in nature, with the others being gravity and the strong and weak nuclear forces. Each of these fundamental forces has an interaction constant that describes its strength in comparison with the others. The fine structure constant, α, has a value of approximately 1/137. If it had any other value, charged particles would behave differently, chemical bonding would manifest in another way and light-matter interactions as we know them would not be the same.

“As the name ‘constant’ implies, we assume that these forces are universal and have the same values at all times and everywhere in the universe,” explains study leader Thorsten Schumm from the Institute of Atomic and Subatomic Physics at TU Wien. “However, many modern theories, especially those concerning the nature of dark matter, predict small and slow fluctuations in these constants. Demonstrating a non-constant fine-structure constant would shatter our current understanding of nature, but to do this, we need to be able to measure changes in this constant with extreme precision.”

With thorium spectroscopy, he says, we now have a very sensitive tool to search for such variations.

Nucleus becomes slightly more elliptic

The new work builds on a project that led, last year, to the worlds’s first nuclear clock, and is based on precisely determining how the thorium-229 (²²⁹Th) nucleus changes shape when one of its neutrons transitions from a ground state to a higher-energy state. “When excited, the ²²⁹Th nucleus becomes slightly more elliptic,” Schumm explains. “Although this shape change is small (at the 2% level), it dramatically shifts the contributions of the Coulomb interactions (the repulsion between protons in the nucleus) to the nuclear quantum states.”

The result is a change in the geometry of the ²²⁹Th nucleus’ electric field, to a degree that depends very sensitively on the value of the fine structure constant. By precisely observing this thorium transition, it is therefore possible to measure whether the fine-structure constant is actually a constant or whether it varies slightly.

After making crystals of ²²⁹Th doped in a CaF₂ matrix at TU Wien, the researchers performed the next phase of the experiment in a JILA laboratory at the University of Colorado, Boulder, US, firing ultrashort laser pulses at the crystals. While they did not measure any changes in the fine structure constant, they did succeed in determining how such changes, if they exist, would translate into modifications to the energy of the first nuclear excited state of ²²⁹Th.

“It turns out that this change is huge, a factor 6000 larger than in any atomic or molecular system, thanks to the high energy governing the processes inside nuclei,” Schumm says. “This means that we are by a factor of 6000 more sensitive to fine structure variations than previous measurements.”

Increasing the spectroscopic accuracy of the ²²⁹Th transition

Researchers in the field have debated the likelihood of such an “enhancement factor” for decades, and theoretical predictions of its value have varied between zero and 10 000. “Having confirmed such a high enhancement factor will now allow us to trigger a ‘hunt’ for the observation of fine structure variations using our approach,” Schumm says.

Andrea Caputo of CERN’s theoretical physics department, who was not involved in this work, calls the experimental result “truly remarkable”, as it probes nuclear structure with a precision that has never been achieved before. However, he adds that the theoretical framework is still lacking. “In a recent work published shortly before this work, my collaborators and I showed that the nuclear-clock enhancement factor K is still subject to substantial theoretical uncertainties,” Caputo says. “Much progress is therefore still required on the theory side to model the nuclear structure reliably.”

Schumm and colleagues are now working on increasing the spectroscopic accuracy of their ²²⁹Th transition measurement by another one to two orders of magnitude. “We will then start hunting for fluctuations in the transition energy,” he reveals, “tracing it over time and – through the Earth’s movement around the Sun – space.

The present work is detailed in Nature Communications.

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A new heat engine driven by the temperature difference between Earth’s surface and outer space has been developed by Tristan Deppe and Jeremy Munday at the University of California Davis. In an outdoor trial, the duo showed how their engine could offer a reliable source of renewable energy at night.

While solar cells do a great job of converting the Sun’s energy into electricity, they have one major drawback, as Munday explains: “Lack of power generation at night means that we either need storage, which is expensive, or other forms of energy, which often come from fossil fuel sources.”

One solution is to exploit the fact that the Earth’s surface absorbs heat from the Sun during the day and then radiates some of that energy into space at night. While space has a temperature of around −270° C, the average temperature of Earth’s surface is a balmy 15° C. Together, these two heat reservoirs provide the essential ingredients of a heat engine, which is a device that extracts mechanical work as thermal energy flows from a heat source to a heat sink.

Coupling to space

“At first glance, these two entities appear too far apart to be connected through an engine. However, by radiatively coupling one side of the engine to space, we can achieve the needed temperature difference to drive the engine,” Munday explains.

For the concept to work, the engine must radiate the energy it extracts from the Earth within the atmospheric transparency window. This is a narrow band of infrared wavelengths that pass directly into outer space without being absorbed by the atmosphere.

To demonstrate this concept, Deppe and Munday created a Stirling engine, which operates through the cyclical expansion and contraction of an enclosed gas as it moves between hot and cold ends. In their setup, the ends were aligned vertically, with a pair of plates connecting each end to the corresponding heat reservoir.

For the hot end, an aluminium mount was pressed into soil, transferring the Earth’s ambient heat to the engine’s bottom plate. At the cold end, the researchers attached a black-coated plate that emitted an upward stream of infrared radiation within the transparency window.

Outdoor experiments

In a series of outdoor experiments performed throughout the year, this setup maintained a temperature difference greater than 10° C between the two plates during most months. This was enough to extract more than 400 mW per square metre of mechanical power throughout the night.

“We were able to generate enough power to run a mechanical fan, which could be used for air circulation in greenhouses or residential buildings,” Munday describes. “We also configured the device to produce both mechanical and electrical power simultaneously, which adds to the flexibility of its operation.”

With this promising early demonstration, the researchers now predict that future improvements could enable the system to extract as much as 6 W per square metre under the same conditions. If rolled out commercially, the heat engine could help reduce the reliance of solar power on night-time energy storage – potentially opening a new route to cutting carbon emissions.

The research has described in Science Advances.

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A new microscale version of the flumes that are commonly used to reproduce wave behaviour in the laboratory will make it far easier to study nonlinear hydrodynamics. The device consists of a layer of superfluid helium just a few atoms thick on a silicon chip, and its developers at the University of Queensland, Australia, say it could help us better understand phenomena ranging from oceans and hurricanes to weather and climate.

“The physics of nonlinear hydrodynamics is extremely hard to model because of instabilities that ultimately grow into turbulence,” explains study leader Warwick Bowen of Queensland’s Quantum Optics Laboratory. “It is also very hard to study in experiments since these often require hundreds-of-metre-long wave flumes.”

While such flumes are good for studying shallow-water dynamics like tsunamis and rogue waves, Bowen notes that they struggle to access many of the complex wave behaviours, such as turbulence, found in nature.

Amplifying the nonlinearities in complex behaviours

The team say that the geometrical structure of the new wave-on-a-chip device can be designed at will using lithographic techniques and built in a matter of days. Superfluid helium placed on its surface can then be controlled optomechanically. Thanks to these innovations, the researchers were able to experimentally measure nonlinear hydrodynamics millions of times faster than would be possible using traditional flumes. They could also “amplify” the nonlinearities of complex behaviours, making them orders of magnitude stronger than is possible in even the largest wave flumes.

“This promises to change the way we do nonlinear hydrodynamics, with the potential to discover new equations that better explain the complex physics behind it,” Bowen says. “Such a technique could be used widely to improve our ability to predict both natural and engineered hydrodynamic behaviours.”

So far, the team has measured several effects, including wave steepening, shock fronts and solitary wave fission thanks to the chip. While these nonlinear behaviours had been predicted in superfluids, they had never been directly observed there until now.

Waves can be generated in a very shallow depth

The Quantum Optics Laboratory researchers have been studying superfluid helium for over a decade. A key feature of this quantum liquid is that it flows without resistance, similar to the way electrons move without resistance in a superconductor. “We realized that this behaviour could be exploited in experimental studies of nonlinear hydrodynamics because it allows waves to be generated in a very shallow depth – even down to just a few atoms deep,” Bowen explains.

In conventional fluids, Bowen continues, resistance to motion becomes hugely important at small scales, and ultimately limits the nonlinear strengths accessible in traditional flume-based testing rigs. “Moving from the tens-of-centimetre depths of these flumes to tens-of-nanometres, we realized that superfluid helium could allow us to achieve many orders of magnitude stronger nonlinearities – comparable to the largest flows in the ocean – while also greatly increasing measurement speeds. It was this potential that attracted us to the project.”

The experiments were far from simple, however. To do them, the researchers needed to cryogenically cool the system to near absolute zero temperatures. They also needed to fabricate exceptionally thin superfluid helium films that interact very weakly with light, as well as optical devices with structures smaller than a micron. Combining all these components required what Bowen describes as “something of a hero experiment”, with important contributions coming from the team’s co-leader, Christopher Baker, and Walter Wasserman, who was then a PhD student in the group. The wave dynamics themselves, Bowen adds, were “exceptionally complex” and were analysed by Matthew Reeves, the first author of a Science paper describing the device.

As well as the applications areas mentioned earlier, the team say the new work, which is supported by the US Defense Advanced Research Project Agency’s APAQuS Program, could also advance our understanding of strongly-interacting quantum structures that are difficult to model theoretically. “Superfluid helium is a classic example of such a system,” explains Bowen, “and our measurements represent the most precise measurements of wave physics in these. Other applications may be found in quantum technologies, where the flow of superfluid helium could – somewhat speculatively – replace superconducting electron flow in future quantum computing architectures.”

The researchers now plan to use the device and machine learning techniques to search for new hydrodynamics equations.

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