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Physics meets fashion as bioluminescent dress debuts at Paris Haute Couture Week

Fashion designer Iris van Herpen has unveiled a bioluminescent dress that features 125 million living algae. The garment involved Herpen collaborating with designer Chris Bellamy as well as biophysicists Nico Schramma and Mazi Jalaal from the University of Amsterdam.

bioluminescent dress
Dress to impress: the “living” garment was part of van Herpen’s new fashion collection – Sympoiesis (courtesy: Molly SJ Lowe, for Iris van Herpen)

Bioluminescence is the production of light by a living organism, caused by a chemical reaction such as the molecule luciferin reacting with oxygen to release light.

The bioluminescent dress is composed of a gel material that incorporates millions of single celled bioluminescent algae of the species Pyrocystis lunula, named after their moon-like shape.

In the wild, the bioluminescent algae emit light as a defence mechanism. The flash serves as a warning signal that attracts secondary predators, which hunt the main predator of the cells.

In 2019, Jalaal, Schramma and colleagues began to study how the cells respond to mechanical stresses. By combining microscopy and mechanical tests, they were able to measure the light-emission of the cells and how it depended on deformation, which led to a mathematical model that described the light-production mechanism (Phys. Rev. Lett. 125 028102).

The researchers then worked with Chenghai Li and Shengqiang Cai at the University of California San Diego and bioluminescence researcher Michael Latz from the Scripps Institution of Oceanography in San Diego.

They incorporated the cells in a gel matrix to create a flexible yet resistant substance that emits light upon deformation and movement while at the same time keeping the cells alive.

Bellamy and van Herpen developed and refined the bioluminescent material and incorporated it into a spectacular “living” garment, which on Monday was part of van Herpen’s new fashion collection – Sympoiesis – that was unveiled at Paris Haute Couture Week.

Astronomers observe the biggest booms since the Big Bang

Astronomers at the University of Hawai’i’s Institute for Astronomy (IfA) in the US have detected what they say are the most energetic cosmic explosions known to have occurred since the the universe began. These colossal events, dubbed extreme nuclear transients (ENTs), emit at least 10 times as much energy as the previous record holders, and studying them could open a new window into physical processes that take place at very high energies.

ENTs occur when stars that are at least three times as massive as the Sun pass so close to a supermassive black hole that its colossal gravity shreds them to pieces. The resulting string of matter then spirals into the black hole in a phenomenon known as accretion.

Such events are extremely rare, occurring a few hundred times less frequently than supernovae. However, when they do happen, they release huge amounts of energy, producing long-lasting flares that can then be detected on Earth.

Optical transient surveys have spotted several classes of accretion-powered flares over the past decade or so, explains Jason Hinkle, who led the study as part of his PhD research at the IfA. Examples include tidal disruption events, rapid turn-on active galactic nuclei and ambiguous nuclear transients.

The new ENTs are a different kettle of fish, however. They release between 0.5 × 1053 and 2.5 × 1053 erg (0.5‒2.5 × 1046 J) making them at least twice as energetic as any other known transient. “They are also 10 times as bright (emitting 2 × 1045 to 7 × 1045 erg per second) and remain luminous for years, far surpassing the energy output of even the brightest known supernova explosions,” Hinkle says.

Looking for smooth, high-amplitude and long-lived signals

Hinkle began searching for ENTs at the beginning of his PhD studies by sifting through data from the European Space Agency’s Gaia mission. Gaia is ideal for such a search as it has been observing the full sky since late 2014. As a space-based mission, it also typically has shorter seasonal breaks than ground-based surveys.

Hinkle’s search for smooth, high-amplitude, long-lived signals revealed two possible sources. Designated Gaia16aaw (AT2016dbs) and Gaia18cdj (AT2018fbb), each comes from the centre of a distant galaxy. For Gaia16aaw, that galaxy bears the catchy name WISEA J041157.03-420530.8. Gaia18cdj, for its part, lies within the equally memorable WISEA J020948.15-420437.1

In 2020, astronomers began observing these sources with space-based UV/X-ray missions and ground-based facilities, including the University of Hawai’i’s Asteroid Terrestrial-impact Last Alert System and the W M Keck Observatory. “These gave us the first indication that we were seeing something special,” Hinkle says. “When the Zwicky Transient Facility [a wide-field optical survey] published data on a third similar event, ZTF20abrbeie, also sometimes called ‘Scary Barbie’ (AT2021lwx), in 2023, it gave us additional confidence that we had found a rare, new class of transient phenomena.”

These data show that the brightness of the light emitted from ENTs increases for more than 100 days, peaks, and then slowly declines over a period of more than 150 days. ENTs also produce infrared light, which suggests that circumnuclear dust is being heated up and reemitted at longer wavelengths, Hinkle says.

The fact that Gaia16aaw and Gaia18cdj are located relatively close to the centres of their host galaxies (within 0.68 and 0.25 kpc, respectively) confirms their status as nuclear transients, he adds. Their long timescales and high peak luminosities also suggest that they originate from accretion onto a supermassive black hole. “The way they accrete is very different from normal black hole accretion, however, which typically shows irregular and unpredictable changes in brightness,” Hinkle explains. “Instead, the smooth and long-lived flares of ENTs imply a distinct physical process – the gradual accretion of a tidally disrupted star by a supermassive black hole.”

Several ENTs could be detected per year

According to IfA team member Benjamin Shappee, ENTs provide a valuable new tool for studying massive black holes in distant galaxies. Since they are so bright, they can be seen across vast cosmic distances, equivalent to redshifts between z = 4 and 6. This means they could give astronomers new information about black hole growth when the universe was less than half its present age, during a period when galaxies were forming stars and feeding their supermassive black holes up to 10 times more vigorously than they are today.

Now that astronomers know what to look for, Hinkle says that new survey instruments such as the Vera C Rubin Observatory and NASA’s Roman Space Telescope should turn up several ENTs per year. “From a physics perspective, building a sample of ENTs will give us the best look yet at massive black holes in the early universe, especially the large majority of those that are not otherwise accreting,” he says. “This will serve as an excellent complement to studies of accreting black holes in the early universe with the James Webb Space Telescope, for example.

“We have a great starting point, but as with many things in observational astronomy, we need larger samples to gain a fuller understanding of how these events work and how we can best use them to test fundamental physics.”

The present study is detailed in Science Advances.

Acoustic rainbows emerge from novel sound-scattering structure

Researchers in Denmark have produced the acoustic equivalent of a rainbow, creating a structure that spatially decomposes sound into its component frequencies in free space. Developing such a structure had proven difficult due to the complexity required, but the team at Danmarks Tekniske Universitet (DTU) managed it thanks to an advanced structural design technique. The new architecture could be used to make devices tailored to emit or receive certain frequencies of sound.

Optical rainbows occur when white light is split into its different spectral components, for example by passing through dispersive media such as prisms or droplets. Although acoustic rainbows are less well-known, they follow the same principle, being the spatial decomposition of sound in free space where waves oscillating at different frequencies propagate in different directions. They have previously been created in confined media using arrays of resonant structures that “trap” sound at different positions in space depending on their frequency. Examples include waveguides, solid and/or fluid mixtures and devices known as acoustic circulators.

Acoustic spectral decomposition also occurs in several natural structures, including the outer ear structures, or pinnae, of mammals such as bats, cetaceans and primates. Indeed, the pinnae of primates (including humans) have an intricate geometry that generates complex interference phenomena via scattering of sound waves, thereby enabling the animals to localize external sources of sound.

An acoustic scattering structure

While researchers have previously attempted to imitate such biological designs, these efforts were largely unsuccessful. The new work, which was co-led by Rasmus Ellebæk Christiansen and Efren Fernandez-Grande at the DTU, succeeded in part thanks to a new technique known as computational morphogenesis, or topology optimization.

This technique, which the researchers describe in Science Advances, builds on an earlier morphogenetic design framework for tailoring passive acoustic scattering structures with dimensions on the order of a few wavelengths. Using an iterative process, the team spatially redistributed sound-reflecting material in an air background inside a specified region of space. This enabled them to tailor the sound field emitted from the created structure to match a predefined target emission pattern across a specified frequency band, mimicking naturally-occurring “sound shaping” structures.

“Such a technique is possible today thanks to the rapid growth in computational power in recent years that has allowed us to model and synthesize sound on the large scale,” Ellebæk Christiansen explains. When combined with advanced production techniques like additive manufacturing (also known as 3D printing), he adds that the team benefitted from “nearly unlimited design freedom”, with the new technique enabling the design of metamaterials and nonintuitive structures hitherto deemed unrealizable.

“Our approach to designing the structures is to re-formulate the device design problem carefully and meticulously as a mathematical optimization problem and to use topology optimization to solve this problem,” he explains. “In this way, we do not rely on simplified design rules derived from underlying physics models, on design intuition or on prior design experience to come up with our device geometry. Instead, we use rigorous mathematical modelling and simulation coupled with advanced numerical algorithms.”

Towards new and very different structures/geometries

The geometry and topology of the metamaterial the team created has several features reminiscent of structures present in the pinnae that spatio-spectrally decompose sound, Ellebæk Christiansen says. However, he tells Physics World that the technique may also enable them to develop structures/geometries that offer new possibilities never realized in nature.

One option, Fernandez-Grande suggests, would be to design acoustic materials that reflect different frequencies of sound in different ways – for example, by scattering high frequencies diffusely and redirecting low frequencies towards an absorbing surface. “It might also help in the development of acoustic lenses – that is, sound sources (such as loudspeakers) that control how different frequencies are radiated in space,” Fernandez-Grande adds.

In the future, the researchers would like to transition from their current two-dimensional design to one that is fully three-dimensional. “This would offer significantly more design freedom, and acoustic field complexity, which might allow for even better/more elaborate spatio-spectral sound field control,” Ellebæk Christiansen says.

Inside ATLAS: Sara Alderweireldt explains how the CERN experiment homes in on new physics

This podcast features an interview with Sara Alderweireldt, who is a physicist working on the ATLAS experiment at CERN – the world-famous physics lab that straddles the Swiss-French border and is home to the Large Hadron Collider (LHC).

Based at the UK’s University of Edinburgh, Alderweireldt is in conversation with Physics World’s Margaret Harris and explains how physicists sift through the vast amount of information produced by ATLAS’ myriad detectors in search of new physics.

They also chat about the ongoing high-luminosity upgrade to the LHC and its experiments – which will be finished in 2030 – and the challenges and rewards of working a very long term project.

Construction begins on new £93m European weather-forecasting headquarters

Construction has begun on the new headquarters of the European Centre for Medium-Range Weather Forecasts (ECMWF). Yesterday, senior officials marked the start of construction on the new £93m centre at the University of Reading, which will provide cutting-edge meteorological research and forecasting.

The ECMWF is an independent intergovernmental organization with 35 member and cooperating states. Established in 1975, the centre employs around 500 staff from more than 30 countries at its existing headquarters at Shinfield Park in Reading, UK, and sites in Bologna, Italy, and Bonn, Germany.

As a research institute and 24/7 operational service, the ECMWF produces global numerical weather predictions four times per day and other data for its member/cooperating states and the broader meteorological community.

The new centre at the University of Reading, built by construction firm Mace, is funded by the UK’s Department for Science, Innovation and Technology. When it opens in 2027, it will accommodate up to 300 scientists and staff who will relocate from Shinfield Park.

The centre will carry out work on all aspects of weather prediction, forecast production and research into climate change.

“This state-of-the-art facility places the UK at the heart of international efforts that are helping us to make better sense of our weather and climate,” notes UK science minister Patrick Vallance. “By improving our weather predictions we can optimise our energy consumption estimates, adjust transport schedules effectively and give our farmers time to prepare for extreme weather – helping people and businesses to save money, cut energy use and stay safe.”

Diversity in the UK tech sector must improve, says report

The UK technology industry is struggling with persistent challenges around diversity and inclusion. That is according to a new report by the Department for Science, Innovation and Technology, which concludes that despite some modest recent progress, all minority groups still remain significantly underrepresented in the technology sector.

The tech startup ecosystem is valued at over $1.1 trillion worldwide with the technology sector employing more than 1.8 million people in the UK. Women and people from ethnic-minority groups, however, account for only around a quarter of the technology workforce. People from ethnic-minority groups also only hold 14% of senior roles.

Based on surveys of UK technology industries and a review of existing research on the sector, the report finds that recent diversity gains diminish at mid-career and leadership levels. In the last year, female representation in senior technology positions increased by only 1%, while one in three women are planning to quit their jobs due to a lack of career progression, poor work-life balance and an unsupportive culture.

This persistent “leaky pipeline” is linked to structural and cultural barriers that result in poor retention and promotion of underrepresented people. Cultural attitudes reinforce this gender bias, the report says, with one recent study finding that 20% of men in technology believe that women are “naturally less suited” to technical work. Indeed, a previous national study found that underrepresented minorities were nearly twice as likely to leave a technology job because of unfair treatment than for a better role.

Underrepresentation is particularly stark for Black technologists, who make up only 5% of workers, while just 0.07% of technology employees are Black women. Socio-economic diversity is also poor with only 9% of technology employees coming from poorer backgrounds, compared with 29% in finance and 23% in law. Data also shows that individuals from working-class backgrounds in technology earn, on average, almost £5000 less per year than their peers from more affluent backgrounds.

’Lack of progress’

There is also a lack of diversity when it comes to technology funding, with the report showing that 92% of angel investments in 2022 went to all-white teams, while female and ethnic minority-led startups secured just 2% of venture capital funding. On average female-founded technology businesses receive £1.1m, figures show, while male-owned startups receive £6.2m.

The report also points to one analysis that found that about 14% of technologists identify as disabled, while another put the figure as low as 6%, suggesting a reluctance to disclose disabilities. The later survey also suggests that 53% of technology employees identify as neurodivergent, yet employers claim that just 3% of their staff are neurodivergent.

To improve diversity and inclusion in the technology sector, the report calls for improvements in flexible-working options; diversity, equity and inclusion reporting; improved governance structures; and socio-economic mobility initiatives.

Sarah Bakewell, head of diversity and inclusion at the Institute of Physics, which publishes Physics World, describes the report’s conclusions as “concerning” as “it reveals the lack of diversity in the sector and who funding is allocated to”. Even more worrying, she says, is the lack of progress in boosting the diversity of people in UK tech. “To unleash a new wave of UK innovation, we must attract, develop and retain people from all backgrounds in inclusive work environments where everyone can realise their full potential.”

Black holes could act as cosmic supercolliders

As they approach a black hole’s event horizon, particles of accreting gas can take on opposing orbital trajectories – remarkably similar to the paths produced in manmade particle colliders. Using advanced new models, Andrew Mummery at the University of Oxford, together with Joseph Silk at Sorbonne University, showed how such particles could collide at colossal energies, with detectable collision products that could offer valuable new insights for particle physics.

Within a black hole’s accretion disk, gas particles travel in circular orbits that gradually shrink under its immense gravity. Once an orbit contracts beneath a critical radius, it becomes unstable, and the particles it carries will suddenly plunge toward the black hole.

“Long ago, Roger Penrose showed that these particles could extract energy from the spin of massive black holes in the region where they decay,” Silk explains. “This happens in the ergosphere – the region just outside the event horizon where debris can gain energy from the black hole’s intense gravitational and rotational fields.”

In the theory described by Penrose, a particle approaching a black hole splits into two fragments – possibly through a collision or spontaneous decay. After the split, one fragment falls into the event horizon, while the other gains enough energy from the black hole’s spin to escape its gravity – exiting the ergosphere with more energy than the original particle.

Building on this idea, Silk and two of his previous collaborators – Maximo Bañados and Stephen West – proposed an alternative escape mechanism. Their idea involves gas particles in retrograde orbits (moving opposite to the black hole’s spin) within the accretion disk. Since a retrograde orbit becomes unstable at larger radii than a prograde orbit (movement in the same direction as the black hole’s rotation), these particles fall farther before reaching the ergosphere, allowing them to gain more energy through gravitational acceleration.

Within the ergosphere, Bañados, Silk and West considered how these now highly energetic particles could collide with those originating from prograde orbits, travelling in opposite directions. If this occurred, the relative velocity between the two would be enormous – imparting extreme relativistic energies to their collision products. The trio proposed that some of these products could escape the ergosphere with more energy than either of the original particles.

In their latest study, reported in Physical Review Letters, Silk and Mummery explored this possibility in greater detail. They used models recently developed by Mummery to simulate the flow of particles accreting onto rapidly spinning supermassive black holes.

“We showed that the infalling gas would develop a pattern of turbulent rotating and counter-rotating vortices as it plunged into the black hole’s ergosphere,” Silk explains. The rotation direction of each vortex depends on whether the particles originated from prograde or retrograde orbits within the accretion disk.

When particles travelling in opposite directions collide in the ergosphere, their circular paths resemble the magnetically guided trajectories of protons and heavy ions in manmade particle colliders, such as CERN’s Large Hadron Collider – only on a vastly larger scale. “We found that the collisions occurred at hundreds of times higher energies than those reached in any existing collider, and would approach or even exceed the energies expected for the proposed Future Circular Collider,” Silk notes.

At such colossal energies, Mummery and Silk predict that the collision products could include gamma rays and ultrahigh-energy neutrinos, which might be detectable from nearby supermassive black holes – such as Sagittarius A* at the centre of our own galaxy. As a result, the process could offer an entirely new approach to observations in particle physics.

“Our predicted signatures would complement those of the next generation of giant particle supercolliders planned by CERN and in China, helping to provide evidence of new particle physics beyond the Standard Model,” says Silk. In particular, the duo suggest that these signatures could lead to a highly sensitive probe of dark matter – potentially offering more robust tests for candidates such as weakly interacting massive particles.

Norwegian-US Nobel laureate Ivar Giaever dies aged 96

The Norwegian-born condensed-matter physicist Ivar Giaever, who shared the Nobel Prize for Physics in 1973, died on 20 June at the age of 96. In the late 1950s, Giaever made pioneering progress in the electron tunnelling in superconductors as well as provided a crucial verification of the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity.

Born in Bergen, Norway, on 5 April 1929, Giaever graduated with a degree in mechanical engineering in 1952 from the Norwegian Institute of Technology. Following a year of military service he worked as a patent examiner for the Norwegian government before moving to Canada in 1954 where he began working at General Electric.

Two years later he moved to GE’s research laboratory in New York, where he continued to study the company’s engineering courses. In 1958 he joined the GE’s R&D centre as a researcher.

At the same time, Giaever began to study physics at Rensselaer Polytechnic Institute in New York where he obtained a PhD in 1964 working in tunnelling and superconductivity. That year he also became a naturalized US citizen.

A Nobel life

It was work in the early 1960s that led to his Nobel prize. Following the Japanese physicist Leo Esaki’s discovery of electron tunnelling in semiconductors in 1958, Giaever showed that tunnelling also happened in superconductors, in this case a thin later of oxide surrounded by a metal in a superconducting state.

Using his tunnelling apparatus, Giaever also measured the energy gap near the Fermi level when a metal becomes superconducting, providing crucial verification of the BCS theory of superconductivity.

At the age of 44, Giaever shared half the 1973 Nobel Prize for Physics with Esaki “for their experimental discoveries regarding tunnelling phenomena in semiconductors and superconductors, respectively”. The other half went to Brian Josephson “for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects”.

Josephson told Physics World that Giaever’s experiments were the source of his interest in tunnelling supercurrents. “An interesting point is that none the [physics] laureates that year were professors at the time,” adds Josephson. “[Giaever] and I were too junior, while Esaki was in industry”.

In 1988 Giaever left General Electric and moved to Rensselaer where he continued to work in biophysics. In 1993, he founded the New York-based Applied BioPhysics Inc.

As well as the Nobel prize, Giaever also won the Oliver E Buckley Prize by the American Physical Society (APS) in 1965 as well as the Golden Plate Award by the American Academy of Achievement in 1966.

Gaiever’s career was not withouth controversy. In 2011 he resigned from the APS in protest after the organisation called the evidence of damaging global warming “incontrovertible”.

In 2016 he published his autobiography I am the Smartest Man I Know, in which he details his journey from relatively humble beginnings in Norway to a Nobel prize and beyond.

Space rock quest: meet the people hunting meteorites

Every mystical quest features a journey riddled with challenges, a cast of colourful characters, and a treasure trove that unlocks more intrigue. The Meteorite Hunters: On the Trail of Extraterrestrial Treasures and the Secrets Inside Them is no exception to this canon.

Written by science journalist Joshua Howgego, the book takes the reader on the pursuit of space rocks and how they have unravelled our understanding of the solar system. And, as is so often the way in science as it is with quests, the search and the people you meet along the way are just as interesting as the discoveries themselves.

Towards the end of Meteorite Hunters, Howgego confides that his aim for the book distils down to two questions: “how do you find them, and what do they tell us?”. Indeed, the tale follows this two-act structure pretty neatly. The first half sees the eponymous hunters and their adventures take centre stage, with enough science dotted throughout to set the scene for the second half, which takes us right up to date with the very latest missions to asteroids Itokawa and Ryugu, and the return of the Bennu sample from the OSIRIS-Rex mission. It is a tactic that is kind to the general reader, and there are plenty of interesting anecdotes and characters to keep things from getting too dry, along with some truly astonishing astrophysics.

The journey begins with a look at how people came to understand that rocks can fall from the sky. The truth of course is that civilizations throughout human history have (separately but repeatedly) come to this realization. Howgego highlights how existing knowledge and compelling physical evidence of meteorites from central South American cultures was dismissed as primitive superstitious nonsense by European invaders in the 16th century. It is the perennial story of knowledge being lost during the waves of European colonialism.

Western understanding of meteorites only really gets going in the very late 18th century, and Howgego introduces two key characters who helped cement the topic as a legitimate line of enquiry. Ernst Chladni was a German polymath who wrote the first book on meteorites in 1794 but whose ideas were initially ridiculed. Meanwhile, playwright and journalist Edward Topham had a large meteorite fall on his land in 1795 (witnessed by labourer John Shipley) and went on to the champion the idea of rocks falling from the sky. However, it would take until the mid-1960s, and the anticipation of lunar samples being returned by the Apollo missions, for this area of study to crystallize into the modern field of meteoritics.

Drama and dust

The origin story of many modern meteorite hunters – those who go out searching for these space rocks – often begin in a similar vein to that of Topham, with an inspiring find close to home leading to elaborate expeditions to track down historic falls. The meteorite scientists Howgego interviews are diplomatic when asked about the hunters – after all, they have the resources to investigate reports of fresh falls much more quickly than the hunters can decipher historical reports and local legends. But there is also a real tension between the two camps – there are serious issues with permanent loss of data from the scientific record through mishandling or denial of access to specimens in private collections.

Howgego goes on to discuss efforts to track meteorite falls in real-time, which may be more scientific and systematic but are no less dramatic. Modern programmes involving networks of automated digital cameras can trace their origins back to a resourceful young scientist, Zdeněk Ceplecha, who narrowly escaped the worst of the Stalinist purges in soviet Czechoslovakia. In 1959 he managed to reconstruct the trajectory of an incoming meteorite to within a very respectable margin of modern computations by using long-exposure photographic plates. In a beautiful full-circle moment, the tracking network initiated by Ceplecha followed a 2002 meteorite fall that turned out to have the exact same trajectory as that 1959 space rock – confirming that the two came from the same parent body.

One of the book’s more modern – and most interesting – characters is Swedish jazz guitarist Jon Larsen. His obsession of sifting through tonnes of urban dust for elusive micrometeorites has yielded invaluable (and beautifully photographed) specimens – something dismissed as an urban myth before someone with his patience and ingenuity came along. These pristine remnants of the protoplanetary disc, literal “star dust”, offer unique insights into the earliest days of our solar system.

Alongside his array of characters, Howgego creates a beautiful and accessible rendering of the complex astrophysics underlying the evolution and structure of our solar system as revealed from the study of meteorites. The descriptions of how competing theories have developed and merged also gives a realistic insight into the scientific method in action; consensus building, refinement through accretion of evidence, and an admission that the picture is not yet settled.

The hunt for, and study of, meteorites touches upon an unexpected variety of topics in modern science. But Howgego manages to weave them seamlessly together into a rich fabric, allowing his colourful cast of characters to tell their fascinating stories.

  • 2025 Oneworld Publications 272pp £18.99 hb / £9.99 ebook

New experiment challenges Bohmian quantum mechanics

Schematic diagram of the quantum tunnelling experiment, showing the layout of the two waveguides and how the photons move within and around them
Schematic: The experiment took place in a dye-filled cavity between two mirrors. The bottom mirror was nanostructured to create two waveguides that directed the light. In the primary waveguide, photons were generated by shining a laser at fluorescent dye molecules. This waveguide formed a ramp that gave the photons potential energy. The photons travelled down the ramp until they encountered a step. When they tunnelled into the step, they also tunnelled sideways into the secondary waveguide (an evanescent wave). The rate at which the photons hopped between the two waveguides was used to measure the speed of the particles in the step. (Courtesy: Nature)

A new experiment that measures the quantum tunnelling of photons between two waveguides has produced results that are hard to reconcile with certain deterministic interpretations of quantum mechanics. According to the experimenters, this constitutes a long-sought experimental test of theories that were previously regarded as empirically indistinguishable from conventional quantum mechanics.

In the widely-held Copenhagen interpretation of quantum mechanics developed by physicists such as Werner Heisenberg and Niels Bohr in the 1920s, particles do not have definite properties (such as behaving like a particle or a wave) until they are measured. Instead, a particle’s properties are defined only by its wavefunction, and the square of this wavefunction dictates the probability of the particle being in a particular state when measured.

An alternative interpretation, favoured by physicists such as David Bohm and Louis de Broglie, is that the properties of the particle are everywhere defined by a non-local “guiding equation”.  In the famous quantum double-slit experiment, therefore, the particle does not pass through both slits and interfere with itself. Instead, it passes through one slit or the other, but the probability of it passing through each slit is dictated by the value of the guiding equation. Closing or moving one of the slits alters this equation.

Though most physicists today reject Bohmian mechanics, the differences between it and the Copenhagen interpretation are largely conceptual. “Bohmian mechanics and orthodox quantum mechanics are definitely not physically equivalent – they don’t describe the same things happening in the world,” explains mathematical physicist Sheldon Goldstein of Rutgers University in New Jersey, US. “But they are empirically equivalent – they give the same predictions, the same probabilities, for all possible experiments – which is a kind of striking fact, but it’s true nonetheless.”

A test of Bohmian mechanics?

In the new work, however, Jan Klärs and colleagues at the University of Twente in the Netherlands claim to have devised a test in which the two interpretations predict different results – and Copenhagen wins. To perform this test, the researchers set up two waveguides side by side. When they sent pulses of light down one of the waveguides, light leaked into the other waveguide by quantum tunnelling.  By knowing the strength of the coupling and measuring the quantum tunnelling as a function of distance, they could infer the speed of the photons.

The researchers also introduced a potential step into the first waveguide. As this step was too large for photons to tunnel through, they were largely reflected, but with an exponentially decaying evanescent field inside the step. Bohmian mechanics agrees completely with standard quantum mechanics on the expected density of particles in this field. However, the guiding equation predicts that the velocity of these particles – which can never be measured directly – is zero.

The researchers therefore used the energy of the photons to calculate their expected speeds inside the potential step, and compared this to the tunnelling rate between the two waveguides. They found that particles that were expected to have higher velocity travelled further before tunnelling into the other waveguide. “We interpret this as a speed measurement,” says Klärs. “When you interpret this as a speed measurement, it gives you a speed that is different from the fundamental guiding equation.”

Questions of interpretation

Goldstein, who was not involved in the research, is unconvinced: “There is a theory in Bohmian mechanics where the particles [inside the potential step] are at rest, but for the experiment they give, the Bohmian velocity is not especially relevant to a correct analysis,” he says. “Whatever analysis they’re doing, if they claim that it correctly predicts the analysis based on Schrödinger’s equation, then that would be the conclusion of Bohmian mechanics, and the real thing for them to look at is why was the Bohmian velocity not the thing that corresponds to the result?”

Experimental physicist Aephraim Steinberg of the University of Toronto, Canada is equally sceptical that the work refutes Bohmian mechanics. He points out that the researchers carefully note that the measurements were made in equilibrium, so whether the exponential decay into the step can be interpreted as a speed warrants further discussion by the community.

Nevertheless, he credits their ingenuity. “This particular experiment gave a result that, even after 20 years thinking about tunnelling times, I did not know the answer to,” he says. “There are things in quantum mechanics like ‘how long does a particle spend in a region?’ that sound to our classical ears like they should only have one answer, but that can in fact have multiple answers.”

The research is published in Nature.

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