麻豆传媒

If you haven鈥檛 got a CERN budget, don鈥檛 despair: find the magic formula and you can recreate the most exotic cosmic objects in surprisingly humble settings

IN MANY an unassuming corner of the globe, you might come across an attic, shed or spare room with a table-top railway. To the uninitiated, these model train sets, with tiny figures standing at platforms and tracks fringed by green sponge bushes, represent a harmless, if peculiar, pastime. To the hobbyist, they are a serious affair 鈥� a labour of love, and a way to run a railway almost as if for real.

Modelling the physics around us (Image: Brett Ryder)

This spirit of tinkering, of exploring and learning about the real world by making smaller-scale models of it, is also alive in the physics lab. Many desirable things lie beyond the practical reach of physicists: recreating the first moments of the universe, playing freely with high-energy particles, wandering the fringes of a black hole.

And so on bench-tops across the world, you鈥檒l see odd apparitions. Whether a black hole fashioned from water waves, or a Higgs boson sculpted from liquid helium, these are 鈥渁nalogues鈥� 鈥� lovingly crafted replicas of physical systems that, primed in the right way, can be made to work just like the original. The hope is they might help overcome some of the practical and financial limitations of larger experiments, and themselves become an engine driving our understanding of the real world.

The idea of doing physics without actually doing it is not new. Purely hypothetical have long been used to investigate the consequences 鈥� or perhaps absurdities 鈥� of physical theories, from ancient Greek times right up to these modern days of relativity and quantum theory. In recent years, powerful computers have given a new way to simulate physical processes, as they roam through lines of code to explore the mathematics that underlies a phenomenon.

Precise replica

But number-crunching has its limitations. 鈥淭here are still many unanswered questions about these systems, and we can only include in our computer codes what we already know about them,鈥� says of Heriot-Watt University in Edinburgh, UK. Analogues work differently. Mathematics is the universal language that underlies physics, and often the same equations pop up in seemingly unrelated areas. Find two physical systems that run according to the same rules, and you can substitute one for the other, crafting a precise replica of the phenomenon you are interested in using materials that exactly emulate the underlying mathematics. 鈥淚t isn鈥檛 the same as using the original system, but it鈥檚 always more interesting than using a computer model,鈥� Faccio says.

鈥淔ind two physical systems that follow the same rules, and you can swap one for the other鈥�

Take the Higgs boson. This particle was the final missing piece in the jigsaw of the standard model, the theory that explains how quantum particles interact through three of nature鈥檚 four fundamental forces. According to an idea first floated in the 1960s, the vacuum is permeated by an invisible field, the Higgs field, that 鈥渟ticks鈥� to fundamental particles to different extents, giving them different masses. Prod this field by injecting a large enough amount of energy into it, and it manifests itself more tangibly: a Higgs boson pops out of the aether.

The discovery of this particle was finally announced to the world in July 2012 by researchers at the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland. One reason it took so long was that, although we knew it would take a lot of energy to make a Higgs, no one knew quite how much. In the end, it could only be found by building a monstrous particle-smashing machine and sifting through the wreckage of many billions of high-energy particle collisions within it.

Perhaps we made things harder than they needed to be. When the idea of the Higgs was mooted by theorists in the mid 1960s, a similar idea was already doing the rounds among physicists studying low-temperature superconductors. In these curious materials, pairs of electrons begin to interact with each other and the surrounding lattice of atoms, leading to the creation of particle-like entities known as Cooper pairs. These pairs move freely, encountering no electrical resistance.

This magic happens only in materials cooled to within a whisker of absolute zero, about -273 掳C 鈥� but the mathematics describing the process is almost identical to that describing the higher-energy Higgs interactions. While it is not easy to stoop to such low temperatures, the kit needed to do it is considerably smaller and cheaper than a particle accelerator. Observing the equivalent processes in the superconductor might provide crucial clues as to how the real Higgs boson is made (see diagram).

It took a while to realise all of this. of Imperial College London was one of six physicists who in 1964 came up with the idea of the mass-giving Higgs mechanism. But at the time, he and his colleagues hadn鈥檛 quite grasped the significance of what the superconductor theory was already turning up. 鈥淲e were certainly aware of this suggestion, but we didn鈥檛 understand it very well 鈥� at least, I didn鈥檛,鈥� he says.

It was a similar story in 1981 when the signature of a Higgs-like creature was actually spotted within a superconductor. At the time, this was a bit of a sideshow. Then, the theory of low-temperature superconductors was well-established, and finding the analogue of the Higgs was just another, slightly incidental confirmation of it. In particle physics, meanwhile, the theorists were still scrabbling for any experimental confirmation at all of their idea for how matter gained mass. 鈥淲e didn鈥檛 realise how big the Higgs would become,鈥� says of Argonne National Laboratory in Illinois, who helped to interpret the superconductor experiments.

Eventually, then, the 鈥渞eal鈥� Higgs was found without the help of any clues from superconductors. But the discovery comes at a time when the analogues are at last poised to really come into their own, says Marie-Aude M茅asson of Paris-Diderot University CNRS in France. That is in part because there has been an explosion of sightings of things like Higgs particles: not just , but also in chilled into the same quantum state and in . Joint conferences and research projects are now springing up to try to share intelligence. 鈥淎s the examples build up, many researchers are now convinced there will be cross-fertilisation between the analogues and particle physics,鈥� says M茅asson.

Simulated symmetries

One hope is that low-energy analogues could reveal new ways of detecting the Higgs in high-energy collisions. But more importantly, they might help in discovering new physics. The Higgs turns out to comply very neatly with all of the standard model鈥檚 predictions 鈥� and this is not necessarily a good thing. There is much that the standard model does not explain, such as the nature of the dark matter that seems to make up most of the mass in the universe, or the dark energy thought to account for the universe鈥檚 apparently accelerating expansion 鈥� and indeed why there is much more matter than antimatter in the universe.

(SUSY) is a framework that makes a better fist of some of these things, making it a favoured next-generation theory. One of its central predictions is that there should be more than one Higgs particle. of Aalto University in Finland and Mikhail Zubkov of the Institute for Theoretical and Experimental Physics in Moscow, Russia, think they might have found some clue as to where those extra particles might be 鈥� in superfluid helium-3. The purity of this rare form of helium makes it highly prized for the study of delicate quantum processes. The discovered Higgs weighs in at around 125 gigaelectronvolts (GeV). Studying the spectrum of excitations in the superfluid helium suggests Higgs particles should also exist at energies of 210 GeV and 325 GeV. These possibilities are not excluded by results collected so far at the LHC, although it is too early to say anything definite, says Zubkov ().

, a theorist at the University of California, Berkeley, thinks he can take things further. Supersymmetry rests on the idea that the standard model鈥檚 messy division of particles into fermions (which make up matter) and bosons (which transmit forces) can be replaced by a more symmetric representation in which every fermion has a boson partner and vice versa. 鈥淲e had hoped to see signatures of SUSY at the LHC, but we haven鈥檛, so that motivated us to look for other ways to realise it,鈥� says Vishwanath.

Those other ways are analogues. Strange things happen within materials around 鈥減hase transitions鈥�, at which their atoms rearrange themselves and the material changes state. For instance, as a piece of molten iron cools from liquid to solid, the quantum-mechanical spins of its electrons all align, making the material magnetic. At closer to absolute zero, quantum fluctuations drive phase transitions that create other, exotic types of symmetry, such as the speed of electrons and the speed of sound-wave packets, known as phonons, travelling through a metal converging until they are identical.

Since electrons are fermions and phonons are bosons, Vishwanath and his colleagues think this emerging symmetry could suggest the mechanisms by which supersymmetry emerges, potentially leading to lab experiments that could explore the finer points of the theory. They are currently discussing what type of analogue might best realise this kind of physics in the lab.

That鈥檚 all well and good, but finding something in an analogue is no guarantee that it exists in the real world, cautions Kibble. In the early 2000s, he was part of a pioneering initiative called that aimed to foster the use of analogues to explore, among other things, his ideas about cosmic strings. These are defects in space-time that many theories predict would have appeared as the early universe expanded and cooled, or perhaps not. 鈥淭hey may well not exist,鈥� says Kibble.

A more current case in point is the magnetic monopole. In nature, anything magnetic always seems to have two poles. But as the physicist Paul Dirac showed in the 1930s, if independently moving magnetic poles were not created in the big bang, we are hard-pressed to explain the explain the existence of single, freely moving electric charges today.

Monopoles are now popping up all over the place in analogues 鈥� first in low-temperature crystals known as spin ice, and earlier this year in a superchilled cloud of rubidium atoms (Nature, vol 505, p 657). But do they exist freely in nature? David Hall of Amherst College in Massachusetts, who did the rubidium experiments, is hedging his bets. 鈥淭hey will exist or not independent of our experiment,鈥� he says. 鈥淏ut it is reassuring that the Dirac monopole structure can exist in nature.鈥�

鈥淟ong-sought magnetic monopoles are popping up in all sorts of analogues 鈥� but do they exist in nature?鈥�

Over the event horizon

Where analogues really come into their own is with objects in the universe that we know exist, but that are impossible to investigate directly. Black holes are a good example. These cosmic monsters are predicted by Einstein鈥檚 theory of gravity, the general theory of relativity. They form when large stars collapse and die, and supermassive versions are thought to skulk at the heart of most large galaxies. They are also portals to the ultimate prize of physics 鈥� a theory that explains what happens when the quantum particles of matter meet the extremes of gravity, the only force not covered by the quantum rules of the standard model.

But given that it emits no light, it is not easy to discern exactly what a black hole is doing. at the University of Nottingham, UK, aims to lift the veil in the lab, using just water and laser light to simulate a black hole鈥檚 emission of Hawking radiation. This process, proposed by the physicist Stephen Hawking in the 1970s, is thought to occur when a fluctuation in the quantum vacuum near a black hole鈥檚 event horizon 鈥� its point of no return 鈥� causes a quantum-entangled pair of matter and antimatter particles to form. If one of the pair falls into the black hole while the other is just far enough away to escape it, the two particles can separate, with one trapped inside the black hole forever and one radiated away.

Weinfurtner鈥檚 analogue actually simulates a 鈥渨hite hole鈥� that, instead of sucking everything in to it, deflects everything away. Reverse the direction of time in the underlying equations, however, and conclusions drawn for white holes are just as valid for black ones.

The analogue consists simply of water flowing along a channel containing a smooth obstacle. The team induced ripples on the surface of the water travelling in the opposite direction, and used a 2D sheet of laser light to analyse the properties of the surface waves as they hit the obstacle region and are reflected off. They found that the amplitude and spread of wave frequencies corresponded to those expected of Hawking radiation at a black hole鈥檚 event horizon (). 鈥淚t was a very clear, conclusive detection of the effect,鈥� says Weinfurtner. 鈥淚t was a big surprise to us how robust these experiments are.鈥�

The work has already triggered theoretical studies into how an entirely classical-physics experiment can even crudely reproduce aspects of Hawking radiation, which is a fundamentally quantum effect. A full-blown lab demonstration 鈥� one that also shows that the particles remain entangled as predicted by the theory 鈥� would require a more sophisticated, quantum analogue. Together with her Nottingham colleague Peter Kruger, Weinfurtner is working out how to detect the effect using supercooled atoms.

Hidden power

In Faccio鈥檚 lab in Edinburgh, the experiments that cram the bench-tops aim to simulate the black hole鈥檚 event horizon using intense laser pulses. By concentrating laser light into a very small spot within a waveguide made of a glass block, he can temporarily change the refractive index of the glass so that it slows down subsequent laser pulses and ultimately repels them. 鈥淲hat makes these analogue experiments so powerful is that from a photon or a water wave鈥檚 perspective, it has no way of distinguishing whether it is crossing the event horizon of a real black hole or is in a waveguide under some weird constraints,鈥� he says.

While their original motivation is to investigate esoteric questions of physics, there could be more worldly pay-offs, too. Using expertise garnered from their experiments, Faccio鈥檚 team is currently working on a way of 鈥渟queezing鈥� pairs of entangled photons out of the vacuum, a recently discovered phenomenon called the dynamical Casimir effect. A cheap and easy source of entangled photons would be a boon for super-secure quantum communication technologies. These already use entangled photons to exchange information, but currently need bulky optical equipment to make them.

Exploring and using such new phenomena are, to Faccio, another big draw of working with analogues. They can do far more than just model an aspect of the world as a model railway does: their true value lies in taking us places where we cannot go ourselves, and telling us which of our ideas about the real world are right, and which effects really do happen out there. 鈥淥nce you鈥檝e demonstrated that they exist,鈥� says Faccio, 鈥測ou can ask, 鈥榗an I actually use them?'鈥� So, a little more than mere idle tinkering.

This article appeared in print under the headline 鈥淪mall world, big physics鈥�