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May the Fifth Force Be with You Features Cover story May the fifth force be with you The universe increasingly seems to be telling us there is an unexplained presence on the cosmic stage, says Daniel Cossins ASKED with telling the universe’s Forces 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 gravity, electromagnetism, existence. Black holes munch and merge, the weak nuclear force 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 physics. 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 neutrons 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 standard model 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 dark matter – 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 dark energy, 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.
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