sign in sign up
<<
Home , Fifth force

Features Cover story

May the fifth be with you

The increasingly seems to be telling us there is an unexplained presence on the cosmic stage, says Daniel Cossins

ASKED with telling the universe’s drive the cosmic narrative. They tell epic story, cosmologists have put its various actors, from particles to planets, Ton a compelling show. The curtain how to move and behave – things that would rises with a bang before a sweeping, otherwise seem inexplicable (see “What is a unstoppable narrative unfolds. Stars form force?”, page 32). The four fundamental forces and explode, galaxies swirl their way into we know of are , , existence. Black holes munch and merge, the weak and the strong nuclear sending out ripples through the auditorium. force (see “The familiar four”, page 35). Of these, It is a ripping yarn – but the longer we gravity is the outlier, the only one with no watch it, the more it seems not quite to quantum field or particle attached to it and add up. The story is inconsistent. The pace which can’t be described by the “standard changes arbitrarily. Some of the characters model” of particle . Yet gravity, described are ill-drawn, do inexplicable things or are by Albert Einstein’s space-and-time-warping just plain not there on cue. All in all, there general theory of relativity, determines the is enough in this play that goes wrong to universe’s overarching plot lines. make you think someone has lost the plot. The problems with the story of the cosmos Increasingly, we think we know how. begin at the beginning. The big bang theory We had assumed that just four fundamental suggests that temperature and matter density forces keep the cosmic action bowling in the universe should now be a hotchpotch, along. But hints from theory and the result of early random quantum experiment are combining to suggest fluctuations being amplified as the cosmos it might not be just four, but five, six – or expanded. But viewed at the grandest scales, maybe even more. Sketchy though these galaxies and the like seem remarkably evenly indications are, even one new force spread. To square that circle, in the 1980s would be a turn-up for the books. “It would cosmologists invented cosmic inflation, be absolutely momentous,” says Philippe a split-second burst of growth during Brax at the Saclay Institute of Theoretical which the primordial cosmos ballooned

Physics in France. exponentially, flattening out its surface > GENTOWEN

30 | New Scientist | 16 May 2020 16 May 2020 | New Scientist | 31 WHAT IS A FORCE? at the University of California, Irvine, found By holding this magazine, or swiping this hard to believe when he first heard it. down a smartphone screen, you are Similar anomalies crop up from time to time, exerting a force: one that operates and are almost always down to experimental between two objects that are physically error. “But when I went through the paper touching. Drag forces such as friction trail, looking at how they did the experiment, and air resistance are also such “contact I couldn’t see anything wrong,” says Feng. forces”, which influence movement The plot thickened late last year, when the and acceleration, and can be described Hungarian team reported a similar anomaly in by Isaac Newton’s laws of motion. the decay of helium nuclei. Feng reckons both When physicists talk about results are consistent with the existence of a fundamental forces, it is something “protophobic X boson” that interacts over rather different: influences between short distances with the within the things that are apparently not in contact. atomic nucleus in a new way. That would be a This “action at a distance” perplexed startling find. “It would be huge,” he says. Newton when his universal law of “We’re talking about a once every half-century gravitation first suggested it. It was, sort of discovery.” he wrote, “so great an absurdity that I The idea has its critics. Matt Strassler, believe no man who has in philosophical a theorist at Harvard University, points out matters a competent faculty of thinking that making Feng’s proposed new force coexist can ever fall into it”. with the ones we know about “requires some These days, we ascribe such mysteries complicated and not entirely plausible to the action of fields that fill empty trickery”. The properties of the particle, with space. “In the modern understanding, the an intermediate mass and a short range of most basic things in the world are fields,” interaction, are certainly surprising given what says theorist Frank Wilczek at the quantum field theory suggests we should Massachusetts Institute of Technology. expect. “There are two kinds of things you can So what are fields? They are, says add to the that have not been Matt Strassler at Harvard University, wrinkles. A grand plot twist – but one proved maddeningly difficult to edit, passing The tricky part is finding a force that fits observed, but would be consistent with “a fundamental intermediary between “We don’t know that is currently entirely inexplicable. every test we have ever thrown at it, including the bill. For inflation, a one-off event some everything we have observed,” says Wilzcek: two objects”. For three of the four what new actor Problem number two really became the recent detection of gravitational waves 13.8 billion years ago, whatever caused it might very heavy particles, which would carry a fundamental forces we currently know of, apparent around the same time – the 1980s, produced when black holes and other massive have long since left the stage – not that this has short-range force, or very light particles that they are quantum fields that come with to expect, other not the dawn of time – with the observation cosmic objects collide. Meanwhile, ideas stopped people from coming up with inventive would mediate a long-range force. accompanying particles, called bosons, that there isn’t enough visible matter in most that try to alter gravity, such as modified new plot lines, even involving particles and The new particle seems to be neither. Still, that pop in an out of existence to mediate than it must be galaxies to exert the gravitational pull required Newtonian dynamics or MOND – a popular forces we already know (see “The Higgs force Feng says, we need to keep an open mind. “New influences across various ranges: a quantum force” to stop them flying apart as their components way to explain away – don’t awakens”, page 34). physics doesn’t have to come from the place the massless photon, for instance, whirl around. Cosmologists’ second big square with all cosmic observations. When it comes to the other cosmic you expect it.” To really get theorists’ pulses mediating the electromagnetic force. invention was some additional invisible stuff, That adds to the yearning for a new inexplicables, dark matter and , racing, the result needs first to be corroborated The odd one out is the gravitational dark matter, to glue galaxies together – stuff character on the stage, and the belief that a fifth however, we seem to have some hot leads. in an independent experiment. That could field. According to Albert Einstein’s we have failed to find. fundamental force of nature must be waiting Perhaps the hottest, although controversial, come soon, or not: researchers at PADME, general theory of relativity, which The third implausible turn of events came in the wings. “We have several indications,” lead dates from 2015, when a team led by Attila the Positron Annihilation into Dark Matter superseded Newton’s universal law, in the late 1990s, when observations of far-off says Brax. “There’s definitely something there.” Krasznahorkay of the Institute for Nuclear Experiment in Frascati, Italy, for example, have gravity is the product of mass warping exploding stars known as supernovae revealed Research at the Hungarian Academy of been collecting data for over a year now and space-time. The strength of the that the universe’s expansion is accelerating. Sciences spotted anomalies in the decay of expect to have results sometime in 2021. gravitational field at any point is Naively, with only gravity pulling things The dark side short-lived nuclei of the unstable isotope If confirmed, the particle would count essentially the degree to which a massive together, you might expect it to be slowing. But we don’t know what new actor to expect, beryllium-8. These seemed to indicate the among the great surprises that experiments object is curving space-time around it. Our best stab at explaining the “dark energy” other than a quantum force. This tallies with interference of an even shorter-lived, slow- occasionally throw at theorists – a new force In all cases, what separates the we think is responsible for accelerated the idea that even if gravity can’t yet be moving particle. Its mass was about 17 MeV, that interacted so weakly with ordinary fundamental forces from the common- expansion invokes the power of quantum described in quantum terms, most physicists a little more than 30 times the mass of an matter that we just hadn’t spotted it. Brax or-garden ones we tend to notice is that particles popping in and out of empty space. believe it eventually will be, in a long sought electron, and nowhere near that of any known and Burrage, meanwhile, are investigating they can’t be reduced to another force But this comes up with an answer for the size after marrying of relativity and quantum particle. It also happened to look like a boson, the possibility of a type of fifth force that or field, as for example friction or air of the effect roughly 120 orders of magnitude field theory. “Any sensible physicist believes a force-carrying particle like the photon, but adopts a different disguise: it has large effects, resistance can ultimately be reduced too big. “The universe would have expanded gravity’s force-carrying particle exists,” one that interacts very weakly – just the thing but those effects are screened by gravity. to electromagnetic interactions between so rapidly, everything would have been says Frank Wilczek, a particle theorist at the for explaining dark matter’s diffident It is known as a chameleon force, and the different bits of matter. But the question ripped apart,” says Clare Burrage at the Massachusetts Institute of Technology who interaction with the rest of the cosmos. idea is that the particle transmitting it changes of how many of these fundamental University of Nottingham, UK. won a share of a Nobel prize in physics for the The researchers speculated that it might be its mass depending on the local density of intermediaries exist remains unanswered The simplest solution to these problems quantum theory behind the strong nuclear a “”, a new particle that might matter. Chameleon particles would be heavier (see main story). might be just to say that gravity doesn’t work force. Follow that logic and any fifth force has transmit a force between dark matter particles. where the average matter density is high, as for how we think it does. But has to be quantum, too. Like most observers, theorist Jonathan Feng example around , meaning the force >

32 | New Scientist | 16 May 2020 16 May 2020 | New Scientist | 33 THE HIGGS FORCE AWAKENS associated with them would have a smaller by Baojiu Li at Durham University, UK, ran has collaborated with experimentalists to Given the changeable nature of chameleons, As physicists close in on the discovery range in our neighbourhood and so would be simulations that showed that a universe with craft a bowling-ball-sized vacuum chamber it may not even be such an outrageous stretch of a fifth fundamental force of nature practically invisible to us. The mass of these a chameleon force would form galaxies like with a marble-sized metal sphere at its centre to think that a chameleon force might, under (see main story), a pedant might counter particles would be much smaller in the vast those we see. The challenge now is to identify designed to cancel out the effects of the known certain circumstances, change its strength that we have already found it. And they swathes of empty space between galaxies, subtle differences in those galaxies compared forces. It is a trap for chameleons. A chameleon so it assists gravity, rather than counteracting would be right, sort of. where they would have a larger range of with general relativity’s predictions, so that field would be suppressed around the central it – and so bag two birds with one stone by Discovered in 2012 at the Large Hadron influence – just the ticket to explain the next-generation telescopes, such as the sphere and the walls of the chamber but active also addressing dark matter. “There have Collider – CERN’s powerful atom smasher dark-energy effect of distant galaxies racing European Space Agency’s EUCLID satellite, between them – so drop atoms into the been some attempts to see if the chameleon near Geneva, Switzerland – the Higgs away from us ever faster. set for launch in 2022, can look for them. vacuum and any acceleration of them in this can play a role on galaxy and galaxy cluster boson and its underlying field is famous “It is not quite as strange as it sounds,” says region would betray its presence. scales, maybe replacing some of the need for for giving all other known particles their Burrage, pointing out that the massless photon The team reported its first results last year. dark matter,” says Burrage. Indications so far, masses. But the Higgs field isn’t only a undergoes a similar metamorphosis when It’s a trap! It was a bust, there was no sign of the however, seem to suggest that chameleon mass-giver. Under the right conditions, passing through a plasma of charged particles, Burrage thinks we don’t need to wait. Even chameleon. “That is obviously disappointing,” forces can’t explain all the effects we ascribe it can create a push and pull between experiencing a drag and effectively gaining if a chameleon force is a master of disguise, says Burrage. But so far, the researchers have to dark matter, she adds. two particles, too, which would make it mass. Wilczek agrees in principle, while being it can still be exposed here on Earth, she says. ruled out only one particular chameleon Anyhow, rather than a unification of forces, another fundamental force of nature to sceptical of the models themselves. “That sort “You just need to come up with a situation model, and there is plenty of room for an the smart money is on diversification. With the add to the four we already know. of thing is allowed by the rules of quantum where it can’t hide.” upgraded experiment to uncover the real deal: four fundamental forces we already have, we You won’t often hear physicists refer field theory,” he says. That situation is housed in a basement lab a weaker force ever so slightly stronger than have contrived to explain only normal atomic to it as such, however, because the Higgs And it might just work. In 2018, a group led at Imperial College London, where Burrage gravity that might explain dark energy. matter, which appears to make up only 5 per force operates at such a short range that cent of the matter and energy in the universe. it is practically irrelevant – and possibly “It seems unlikely that all the vast majority undetectable. That is why we had to find “A ‘chameleon’ THE FAMILIAR FOUR of the universe would be made of just one or the Higgs particle to confirm the existence two components,” says Brax. “I wouldn’t be of the field. “If we define the Higgs field force would be We currently know of four fundamental surprised if we find more than one new force.” as the interaction, then we’ve already forces governing the basic workings Wilczek agrees, sort of. “I wouldn’t be discovered it,” says theorist Matt Strassler invisible to us of matter in our universe today. scandalised,” he says. “I don’t know what to at Harvard University. “But if we mean where matter expect, but certainly it would be nice to have the pull between two objects it induces, ELECTROMAGNETISM: Explains why more than one.” Indeed, he is pursuing another then we’ve not seen it.” density is high, atoms hold together and how light behaves candidate for an additional fundamental force: All of which makes it a bit surprising GOVERNING THEORY: Quantum one associated with hypothetical, light, long- that theorists have suggested the Higgs like on Earth” electrodynamics (QED) lived particles called axions. These would have field could be the cause of cosmic inflation, MEDIATOR: Photon (predicted many of the properties associated with dark the split-second burst of mega-expansion by Albert Einstein in 1905) matter, as well as helping to explain some at the time of the big bang invoked to MAXIMUM RANGE: Infinite other thorny problems in , account for the perplexing uniformity of such as why events at the subatomic level look the universe at the largest scales. But it is WEAK NUCLEAR FORCE: Accounts the same whether they run backwards or possible to tweak the properties of the for radioactive beta decay and the forwards in time. “This is the fifth force that Higgs field, such that it could have been nuclear fusion that fuels stars I think is most compelling,” says Wilczek. temporarily strong enough to suddenly GOVERNING THEORY: Electroweak theory All these efforts speak to a wider truth, inflate everything in that first moment, (unified theory with QED at high energies) says Brax: that what we have now with our before settling down to the barely MEDIATOR: W and Z bosons (predicted standard cosmological model is akin to a rough detectable strength it has now. in 1968, discovered in 1983) draft of the script for the story of the universe. On further inspection, though, TYPICAL RANGE: 10-18 metres “To embed our model in something larger, it turns out the Higgs works in this way, something we could call a theory, usually that as an “inflaton”, only if you invent at least STRONG NUCLEAR FORCE: Holds involves new particles or fields, and those are one other field to regulate its strength. and neutrons together within the atomic nucleus going to give you new forces,” he says. So although the Higgs force alone probably GOVERNING THEORY: Quantum Or to put it another way, even a smash hit can’t explain inflation, it might plausibly chromodynamics (QCD) like our cosmological model starts to look a serve as a portal to new forces that could. MEDIATOR: Gluons (predicted in 1962, little tired after a while. A sprinkling of new discovered in 1979) players to add to the established figures is TYPICAL RANGE: 10-15 metres starting to look like the best way to reaffirm why everyone found the story so compelling GRAVITY: Keeps galaxies together, in the first place. ❚ the planets moving around the sun and our feet on the ground GOVERNING THEORY: General relativity Daniel Cossins is a feature MEDIATOR: None; gravitons if it were found editor at New Scientist to be quantum RANGE: Infinite

34 | New Scientist | 16 May 2020 16 May 2020 | New Scientist | 35