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Trouble with physics: Seven experiments to change it all
ROYAL ASTRONOMICAL SOCIETY/SCIENCE PHOTO LIBRARY
played host to an inordinate amount of mathematics that has proved relevant to the real world, far from every equation with which we theorists tinker rises to that level. In the absence of compelling experimental results, deciding what mathematics should be taken seriously is as much art as it is science. Einstein was a master of that art. In the decade after his formulation of special relativity in 1905, he became familiar with vast areas of mathematics that most physicists knew little or nothing about. As he groped towards general relativity’s final equations, Einstein displayed a rare skill in moulding these mathematical constructs with the firm hand of physical intuition. When he received the news that observations of the 1919 solar eclipse confirmed general relativity’s prediction that star light should travel along curved paths, he noted that had the results been different, he “would have been sorry for the dear Lord, since the theory is correct”. I’m sure that convincing data contravening general relativity would have changed Einstein’s tune, but the remark captures well how a set of mathematical equations, through their sleek internal logic, their intrinsic beauty and their potential for wide-ranging applicability, can seemingly radiate reality. Centuries of discovery have made abundantly evident the capacity of
”Deciding which mathematics to take seriously is as much art as it is science”
Seven experiments to change it all
The Higgs boson is (probably) in the bag, but the Large Hadron Collider has plenty more to give. Starting late in 2014, the plan is to double the energy of the proton collisions at CERN’s particle smasher. That should be enough to produce particles predicted by next-generation theories such as supersymmetry. But it is a multibillion-dollar gamble. If it does not pay off, it is back to scrabbling around in cosmic rays or measuring tiny atomic effects to find answers. Richard Webb
Denis Balibouse/Reuters
With theory at an impasse, the next breakthrough in physics is likely to come from an experiment. We introduce seven potential game-changers, starting with the behemoth that’s soon to get bigger…
mathematics to reveal secreted truths about the workings of the world; monumental upheavals in physics have emerged time and again from vigorously following the lead of mathematics. Nevertheless, there was a limit to how far Einstein was willing to follow his own mathematics. He did not take the general theory of relativity “seriously enough” to believe its prediction of black holes, or of an expanding universe. Others embraced Einstein’s equations more fully than he, and their achievements have set the course of cosmological understanding for nearly a century. Einstein instead in the last 20 years or so of his life threw himself into mathematical investigations, passionately striving for the prized achievement of a unified theory of physics. Looking back, one cannot help but conclude that during these years he was too heavily guided – some might say blinded – by the thicket of equations with which he was constantly surrounded. Even Einstein sometimes made the wrong decision regarding which equations to take seriously and which to not. Quantum mechanics provides another case study of this dilemma. For decades after Erwin Schrödinger wrote down his equation for how quantum waves evolve in 1926, it was viewed as relevant only to the domain of small things: molecules, atoms and particles. But in 1957, Hugh Everett echoed Einstein’s charge of a half century earlier: take the mathematics seriously. Everett argued that Schrödinger’s equation should apply to everything because all things material, regardless of size, are made from molecules, atoms and subatomic particles that evolve according to probabilistic rules. Applying this logic revealed that it is not just experiments that evolve in this way, but experimenters, too. This led Everett to his idea of a quantum “multiverse” in which all possible outcomes are realised in a vast array of parallel worlds. More than 50 years later, we still do not know if his approach is right. But by taking the mathematics of quantum theory seriously – fully seriously – he may have had one of the most profound revelations of scientific exploration. The multiverse in various forms has since become a pervasive feature of much mathematics that purports to offer us a deeper understanding of reality. In its furthermost incarnation, the “ultimate multiverse”, every possible > 2 March 2013 | NewScientist | 39
universe allowed by mathematics corresponds to a real universe. Taken to this extreme, mathematics is reality. If some or all of the mathematics that has compelled us to think about parallel worlds proves relevant to reality, Einstein’s famous query – whether the universe has the properties it does simply because no other universe is possible – would have a definitive answer: no. Our universe is not the only one possible. Its properties could have been different, and indeed the properties of other member universes may well be different. If so, seeking a fundamental explanation for why certain things are the way they are would be pointless. Statistical likelihood or plain happenstance would be firmly inserted in our understanding of a cosmos that would be profoundly vast. I don’t know if this is how things will turn out. No one does. But it is only through fearless engagement that we can learn our limits. Only through rational pursuit of theories, even those that whisk us into strange and unfamiliar domains – by taking the mathematics seriously – do we stand a chance of revealing the hidden expanses of reality. n Brian Greene is a theoretical physicist at Columbia University in New York. This article is adapted from his book The Hidden Reality (Allen Lane, 2011)
Big bang cosmology
The dark side Our established picture of the universe is supremely successful – maybe because most of it is made up, says Stephen Battersby
T
2
The Planck probe
esa
Radiation left over from the big bang contains vital clues about the early universe. The most detailed maps of it are coming from the European Space Agency’s Planck satellite, launched in 2009. It can capture the radiation precisely enough to measure cosmological quantities without making many theoretical assumptions, detect the rippling of gravitational waves and test various models of the inflation thought to have occurred during the big bang. It will even let us explore ideas outside of our standard cosmology, such as parallel worlds. Valerie Jamieson
40 | NewScientist | 2 March 2013
O OUR eyes, stars define the universe. To cosmologists they are just a dusting of glitter, an insignificant decoration on the true face of space. Far outweighing ordinary stars and gas are two elusive entities: dark matter and dark energy. We don’t know what they are… except that they appear to be almost everything. These twin apparitions might be enough to give us pause, and make us wonder whether all is right with the model universe we have spent the past century so carefully constructing. And they are not the only thing. Our standard cosmology also says that space was stretched into shape just a split second after the big bang by a third dark and unknown entity called the inflation field. That might imply the existence of a multiverse of countless other universes hidden from our view, most of them unimaginably alien – just to make models of our own universe work.
Cosmology’s dark apparitions raise the question of whether Einstein’s theory is the right one, says Jacob Bekenstein The success of general relativity is embedded in our modern world. True, most solar system and astronomical phenomena are still calculated with Newton’s hoary theory of gravitation, but we would be nowhere without our GPS gadgets, which work only once corrected for the effects of general relativity. General relativity has been tested with great precision within the solar system, and in binary pulsar systems where gravitational fields are very strong, but never on large scales where gravity’s pull is weak. Might the twin embarrassments of dark matter and dark energy mask general relativity’s failure there? Supporters of this idea have had some success. Modified Newtonian dynamics (MOND), proposed by Mordehai Milgrom of the Weizmann Institute in Rehovot, Israel, in the 1980s, relates mass to the gravity it generates in a slightly different way. It describes galaxies better and more parsimoniously than general relativity with dark matter does. Cosmological models constructed from alternative “f(R)” gravitational theories behave as if they contain dark energy, even though they don’t. But no one theory holds all the cards. MOND does not handle motions of individual galaxies within clusters well. Neither does Tensor– vector–scalar (TeVeS) gravity, a relativistic version of a theory I proposed in 2004. The f(R) theories do not adequately describe the anomalous galactic rotations that first led dark matter to be proposed. We might yet strike lucky. If dark energy is the venerable cosmological constant that Einstein shoehorned into his equations of general relativity, its favoured source is vacuum energy. Gravitational fields might conceivably perturb the vacuum enough that concentrations of energy appear in and around galaxies and galaxy clusters, mimicking dark matter. It is impossible to magic up concentrations large enough from current quantum-field theories, but perhaps one day the mystery of the two dark stuffs may be dispelled by the judicious application of known quantum physics. Jacob Bekenstein is a theoretical physicist at the Hebrew University of Jerusalem in Israel
42 | NewScientist | 2 March 2013
Precise recipe This could be Einstein’s cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to
Seeing in the dark Visible matter is only a tiny fraction of what we think the universe contains
Visible matter 4.5% Dark matter 22.5% Dark energy 73 %
explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions. Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what
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Advanced LIGO General relativity predicts that ripples in space-time should constantly be passing through Earth. From 2014 Advanced LIGO, an upgrade of an existing gravitational-wave detector in the US (pictured), will use laser “rulers” several kilometres long to spy spatial disturbances equivalent to Earth moving one-tenth of an atomic diameter closer to the sun. If it sees something, it will be the crowning triumph of Einstein’s relativity. If it doesn’t, it is back to the drawing board with our theories of gravity. Richard Webb ligo
WHAT’s NEXT DISCARD RELATIVITY
Overall, there seems to be about five times as much dark matter as visible gas and stars. Dark matter’s identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see “Flawed genius”, page 45). But it changed cosmology’s standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe’s expansion. The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter’s attractive gravity.
ESO/L. Calçada
galaxies and other structures; dark energy implies that the cosmos will accelerate away into a cold and lonely future; inflation suggests a violent birth. Each member of the shady triumvirate points to new physics. Kirshner sees that as a challenge. “It doesn’t mean there is any flaw in our arguments. It gives a sense not of desperation, but inspiration.” But as long as we have no evidence of dark matter in the lab, or a proven physical basis for dark energy, the possibility remains that we are living under some profound misapprehension – an unknown unknown, something so basic awry in our mathematical model of the universe that as yet we have not been able to imagine the form of our mistake. Might a quantum theory of gravity show us the way forward? Or might some new observation lead us to reformulate our general relativistic cosmology again? We have only the most tenuous of indications where we might look for alternatives. But perhaps if we only discard an unheeded assumption about reality, then a veil will be lifted, all the darknesses banished and the starry night restored. n
WHAT’s NEXT RETHINK INFLATION The best theory of our big bang has logically self-destructed, argues Max Tegmark
”Our lack of answers gives us a sense not of desperation, but of inspiration”
Stephen Battersby is a New Scientist consultant based in London
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LISA Pathfinder
esa
The European Space Agency’s LISA Pathfinder mission will primarily test gravitational-wave detectors, but from next year it could also confirm whether gravity is all general relativity says it is. By flying through the “saddle point” where the Earth and the sun’s gravity cancel out, the craft might probe whether Einstein’s theory still holds when gravitational accelerations are incredibly small. If it does, these gravitational lacunae will be the last resting place of other occasionally fashionable theories, such as Modified Newtonian dynamics (MOND). Stuart Clark
44 | NewScientist | 2 March 2013
Inflation got off to a great start. Beginning with a subatomic speck of a novel, hard-todilute substance, it predicted that this stuff would repeatedly double during a split second to create our big bang and our nearly uniform, flat space. It got even better. Inflation also produced random quantum fluctuations which grew into today’s stars, galaxies and large-scale structures. The theory makes spectacularly accurate predictions. For example, the quantity Omega, which quantifies the flatness of space, should equal 1. It has been measured as 1.003 ± 0.004. Bingo! But like a tenacious ageing professor, inflation refuses to retire. The theory predicts that the process continues forever in distant parts of our cosmos, producing a space that is not just huge but truly infinite, with infinitely many galaxies, stars, planets – and even people like us. Its random fluctuations distribute matter differently in different places, so infinitely many of these people observe an Omega near 1, infinitely many an Omega near 2 – and indeed any other value. So what’s the probability that we’re among those people who observe what we do? The useless, formal answer is infinity divided by infinity. We cosmologists have yet to reach a consensus on how to turn this into something useful. Thanks to inflation, we can predict the probability of virtually nothing any more. I’ve called this the “measure problem”, and view it as one of the deepest crises facing physics today. The way I see it, inflation has logically self-destructed, destroying the predictions that made us take it seriously in the first place. In fairness to inflation, I don’t feel that any competing theory does better. My guess is that once we solve the measure problem, some form of inflation will still remain – but perhaps not the eternal kind. All the problems stem from infinity, specifically the assumption that space can be stretched forever without somehow breaking down. We tend not to question this radical assumption – but we should! Max Tegmark is a cosmologist at the Massachusetts Institute of Technology
Aesthetics tells us what will break the standard-model deadlock: supersymmetry, says Frank Wilczek It is difficult to overstate the power and economy of the standard model. The discovery of the Higgs boson marks its apotheosis. More profoundly, it is the culmination of decades of bread-and-butter work. Just as to find a needle in a haystack one must thoroughly understand both needles and hay, to find the rare traces of the Higgs in the LHC’s explosive “little bangs” we had to know our fundamental physics. The standard model is close to nature’s last word. Yet, in its melange of forces and particles, it does not achieve complete unity and coherence. Maxwell’s equations governing electrodynamics, the oldest part of the standard model, are justly famous for their balance and beauty. The equations of its newest part, those describing the strong nuclear force, are also pleasingly symmetrical, but in place of one electric charge and one force-carrier (the photon), they demand three “colour” charges and eight gluons. The weak nuclear force introduces a further three force carriers. Altogether, the standard model looks a little lopsided. One can dream up bigger and better equations, with even more symmetry, to restore the balance. Supersymmetry is the logical climax of such thoughts. It postulates a basic symmetry that allows forces to turn into materials and materials into forces while the equations as a whole retain the same content. That is done by doubling nature’s library of particles – creating a force-carrying boson for every matter-making fermion, and vice versa. Pursuing this path leads us to an impressive and very concrete numerical success. The new, expanded theory accurately predicts the ratios of the strengths of the strong, weak and electromagnetic forces – parameters that the standard model leaves free. I cannot believe this success is an accident. But in science faith is a means, not an end. Supersymmetry predicts new particles, with characteristic properties, that will come into view as the LHC operates at higher energy and intensity. The theory will soon undergo a trial by fire. It will yield gold – or go up in smoke. Frank Wilczek is a theoretical physicist at the Massachusetts Institute of Technology and a co-recipient of the 2004 Nobel prize in physics 46 | NewScientist | 2 March 2013
”Antimatter came from a theorist’s pen: the positron duly turned up later in cosmic rays”
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wave-particles do not move according to the tidy rules of classical, Newtonian mechanics, but dance to probabilities bounded by bizarre rules in an abstract mathematical space. Quantum mechanics was largely in place by the mid 1920s, and it has yet to fail an experimental test. But when in the late 1920s Paul Dirac and others started to hook up quantum mechanics with Einstein’s special relativity – a vital step in describing particles that shunt around at near-light speeds – things began to take on a life of their own. Dirac’s relativistic equation for the electron had more than one solution, and seemed to predict that a particle existed just like the electron, but with opposite electric charge. The positron duly turned up in cosmic rays five years later. Antimatter had been invented from a theorist’s pen. Quantum field theory, the basis of the standard model, represents the culmination of this logic. The idea of a field transmitting forces goes back to Michael Faraday in the 19th century, but the mathematical structure of quantum fields gives them an odd property: they can create particles from empty space and destroy them again almost at will. Thus, according to the theory of quantum electrodynamics, two electrons repel each other thanks to a photon – the
Dark matter searches Theory points to dark matter being made of so-far-unseen weakly interacting massive particles, known as WIMPs. Over a dozen exquisitely sensitive experiments have been built specifically to catch these slippery customers. Three – DAMA/LIBRA (pictured), CoGeNT and CRESST – have seen things that look suspiciously like them. Others have ruled out the same particles entirely. The trouble is we know too little about what we are looking for. We need more data and additional experiments to understand the experiments. Valerie Jamieson
DAMA/LIBRA/gran sasso
WHAT’s NEXT FOLLOW BEAUTY
can be written on a postcard: six quarks arranged in pairs to make three “generations” identical in all but mass; six leptons, such as electrons and neutrinos, arranged similarly; and a handful of bosons that transmit nature’s fundamental forces between them (see diagram, below right). The essential thing about all these entities is that they are quantum particles. Quantum theory grew from radical discoveries at the beginning of the 20th century, which showed that the wavelengths of radiation emitted and absorbed by atoms could be explained only by assuming that energy is bundled in discrete amounts, or “quanta”. That unleashed an absurd duality at the smallest scales whereby a particle is also a wave and vice versa. These nebulous
term coined by Gell-Mann, finally made quarks respectable by describing their interactions by the exchange of eight gluons that carry a “colour” charge, and showed how, uniquely, this force gets stronger the further you pull two quarks apart. “It could both explain why protons looked as if they were made of quarks and why these quarks could never be pulled out of protons,” says Gross. And that, largely, was it. By 1973, the Beatles had split up and, following a period of mind-boggling theoretical invention, the standard model was in place. There was the unified electroweak theory, to which all particles succumb; and quantum chromodynamics, which affects only quarks and gluons. The model wasn’t just clever, it was beautiful. Its equations had a powerful symmetry that dictated the character of nature’s forces, and told physicists what sort of new particles to look for and where. And, sure enough, the bumps in particle-collider data soon began to appear – together with goosebumps on the skin of the theorists. Evidence for three quarks had already been established in experiments the late 1960s, but by the end of the 1970s physicists in the US had inferred the existence of a fourth and fifth and finally, in 1995, the sixth, “top”, quark. By 2000,
the tau neutrino, the last of the leptons to be discovered, had also been bagged. On the other side of the pond, the gluon was snared at the DESY laboratory outside Hamburg, Germany, in 1979; the W and Z bosons in 1983 at CERN. And finally, last year, the Higgs – the last outstanding particle predicted by the standard model. For Weinberg, the standard model’s triumphant march has been something quite special. “To fool around at your desk with mathematical ideas and then find that, after spending a few billion dollars, experimentalists have confirmed them… there really isn’t anything comparable to it,” he says. So why aren’t he and others like him rejoicing?
WHAT’S NEXT CATCH NEUTRINOS These elusive particles have already given us the key to new physics, says Janet Conrad
Puzzling features For many reasons. Some are aesthetic. Why, for example, do particles come in three generations, with the heaviest quark weighing 75,000 times more than the lightest? The standard model’s equations might be elegant, but to give them their predictive power, they must be fed more than 20 “free” parameters, such as particle masses, by hand. A truly fundamental theory would use the power of quantum theory, or perhaps some deeper idea that nobody has yet thought of, to prune that thicket.
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Neutrino experiments are a hit-and-miss affair. The properties of the ghostly particles are ill-defined, and they interact so rarely that vast floods of them are needed for us to spot anything. The solution could be nuSTORM, a proposed “factory” that will churn out precisely controlled beams of neutrinos or their antimatter counterparts, antineutrinos. That could at last pin down their nature and the number of varieties they come in – and so settle whether any additional types of non-interacting “sterile” neutrinos exist. Valerie Jamieson
48 | NewScientist | 2 March 2013
Roy Kaltschmidt/ Lawrence Berkeley National Lab
Neutrino factories
The standard model is a Goliath among theories: big, powerful, cumbersome and, until recently, seemingly invincible. But a decade ago, it may have already have met its David: neutrinos. According to the standard model, these tiny, mysterious particles come in three different types, or flavours. They have no mass, and interact only through the weak nuclear force, making them very difficult to detect. That at least was the picture until 1998, when the astonishing confirmation came that neutrinos “oscillate”, regularly morphing from one flavour to another and back again. This is a quantum-mechanical effect that can occur only if neutrinos have mass. The particles thus present the first and so far only clear observation of physics beyond the standard model. If neutrinos having mass were the whole story, we might still patch up the standard model. But we are now also seeing evidence for extra oscillations that are very hard to explain in a model with only three neutrinos. This perhaps points to the existence of additional “sterile” neutrino flavours that do not interact through any of the four known fundamental forces, but can morph to and from the active flavours. If this new phenomenon is confirmed, that would land a truly massive blow. Rethinking the masses of known particles is one thing, but making room for a whole new family of particles is something else. It would call for a new theory that explains sterile neutrinos and where they come from. If sterile neutrinos do not interact through any known force, what force do they act through? Might they be a key to the identity of dark matter? We have no clear answers to these questions – but theoretical papers on sterile neutrinos are now coming out at a rate of about two a week. Why study the tiniest particle? Because Goliath lost! Janet Conrad is a neutrino physicist at the Massachusetts Institute of Technology
underlie the standard model suggest space is pixelated into units with sizes of about 10-35 metres, and do not even treat time as a real and observable thing. Asked to choose between the two theories, most physicists’ money is on quantum theory being “right”, because its mathematics is such a successful prism through which to view the world. Others, from Einstein onwards, have taken issue with quantum theory’s seeming “irreality” and spooky, counter-
1900 The birth of quantum theory 1905 Einstein introduces quantum photons of light 1905 Special relativity dictates that light’s speed is always the same
1913 Quantum model of the atom
*1911 Atomic nucleus discovered
ultimate theory. String theory predicts that space has hidden extra dimensions, invoking symmetries embedded in these dimensions to “fold” energy into geometric shapes that look like certain fundamental particles, or mimic the way space curves in the presence of mass. The theory has produced some credible depictions of particles, among them the long-sought graviton, a quantum particle that would carry the force of gravity. It thus takes steps towards a unified picture
intuitive correlations between apparently unrelated objects. If we cannot find a convincing physical reason why these correlations are just so, they argue, perhaps quantum theory is just an approximation to something better. Attempts to go beyond this impasse have drawn on favoured mathematical ideas such as symmetry. One result is supersymmetry, a theory widely regarded as a way station on the road to string theory, a favoured candidate for an
Mid 1920s Quantum mechanics formulated mathematically
1916 General relativity, Einstein’s space-timewarping theory of gravity
1928 Dirac equation describes the electron and predicts *1933 antimatter Antimatter discovered
*1932 Effects of dark matter first observed
*1927 Evidence for an expanding universe: birth of the big bang
Our standard models of particle physics and cosmology have been built on a series of interlinked theoretical and experimental discoveries (*)
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Experiments beaming photons over sometimes hundreds of kilometres have so far only confirmed quantum theory’s outrageous predictions of weird correlations and entanglements between the particles. Soon the ante will be upped with plans to beam quantum transmissions via satellite between continents. It’s a first step to testing quantum theory in space over distances at which relativity’s warping becomes significant – and so seeing what happens when those two great and incompatible theories collide. Richard Webb
50 | NewScientist | 2 March 2013
IQOQI-Vienna
Quantum theory in space
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1940s Quantum theory of electromagnetism 1948 Cosmic microwave background predicted
p
1957 Hugh Everett’s “multiverse” of quantum universes
of all four forces of nature on the basis of quantum theory. But like other proposed frameworks for a theory of everything (see below, right), it has a big flaw. “String theory does predict new things, but they’re almost certainly not testable in the foreseeable future,” says Paul Davies of Arizona State University in Tempe. That failing means the idea of a theory of everything has quietly disappeared, says Renate Loll of Radboud University in Nijmegen in the Netherlands. “For a while you would see it in papers, in the heyday of string theory, but it has gone totally out of fashion.” Chris Isham of Imperial College London goes further. A theory of everything is “psychologically compelling”, he says, but there is no reason to think one exists – or that we can find it. That we have got so far with mathematics is a remarkable fact, but it does not mean we can go all the way. One problem is that mathematics provides infinite ways in which numbers and abstract quantities can be processed – but no indication of what exists beyond it. “Mathematics only reveals truth about abstract objects,” says Deutsch. “Physics
played host to an inordinate amount of mathematics that has proved relevant to the real world, far from every equation with which we theorists tinker rises to that level. In the absence of compelling experimental results, deciding what mathematics should be taken seriously is as much art as it is science. Einstein was a master of that art. In the decade after his formulation of special relativity in 1905, he became familiar with vast areas of mathematics that most physicists knew little or nothing about. As he groped towards general relativity’s final equations, Einstein displayed a rare skill in moulding these mathematical constructs with the firm hand of physical intuition. When he received the news that observations of the 1919 solar eclipse confirmed general relativity’s prediction that star light should travel along curved paths, he noted that had the results been different, he “would have been sorry for the dear Lord, since the theory is correct”. I’m sure that convincing data contravening general relativity would have changed Einstein’s tune, but the remark captures well how a set of mathematical equations, through their sleek internal logic, their intrinsic beauty and their potential for wide-ranging applicability, can seemingly radiate reality. Centuries of discovery have made abundantly evident the capacity of
”Deciding which mathematics to take seriously is as much art as it is science”
Seven experiments to change it all
The Higgs boson is (probably) in the bag, but the Large Hadron Collider has plenty more to give. Starting late in 2014, the plan is to double the energy of the proton collisions at CERN’s particle smasher. That should be enough to produce particles predicted by next-generation theories such as supersymmetry. But it is a multibillion-dollar gamble. If it does not pay off, it is back to scrabbling around in cosmic rays or measuring tiny atomic effects to find answers. Richard Webb
Denis Balibouse/Reuters
With theory at an impasse, the next breakthrough in physics is likely to come from an experiment. We introduce seven potential game-changers, starting with the behemoth that’s soon to get bigger…
mathematics to reveal secreted truths about the workings of the world; monumental upheavals in physics have emerged time and again from vigorously following the lead of mathematics. Nevertheless, there was a limit to how far Einstein was willing to follow his own mathematics. He did not take the general theory of relativity “seriously enough” to believe its prediction of black holes, or of an expanding universe. Others embraced Einstein’s equations more fully than he, and their achievements have set the course of cosmological understanding for nearly a century. Einstein instead in the last 20 years or so of his life threw himself into mathematical investigations, passionately striving for the prized achievement of a unified theory of physics. Looking back, one cannot help but conclude that during these years he was too heavily guided – some might say blinded – by the thicket of equations with which he was constantly surrounded. Even Einstein sometimes made the wrong decision regarding which equations to take seriously and which to not. Quantum mechanics provides another case study of this dilemma. For decades after Erwin Schrödinger wrote down his equation for how quantum waves evolve in 1926, it was viewed as relevant only to the domain of small things: molecules, atoms and particles. But in 1957, Hugh Everett echoed Einstein’s charge of a half century earlier: take the mathematics seriously. Everett argued that Schrödinger’s equation should apply to everything because all things material, regardless of size, are made from molecules, atoms and subatomic particles that evolve according to probabilistic rules. Applying this logic revealed that it is not just experiments that evolve in this way, but experimenters, too. This led Everett to his idea of a quantum “multiverse” in which all possible outcomes are realised in a vast array of parallel worlds. More than 50 years later, we still do not know if his approach is right. But by taking the mathematics of quantum theory seriously – fully seriously – he may have had one of the most profound revelations of scientific exploration. The multiverse in various forms has since become a pervasive feature of much mathematics that purports to offer us a deeper understanding of reality. In its furthermost incarnation, the “ultimate multiverse”, every possible > 2 March 2013 | NewScientist | 39
universe allowed by mathematics corresponds to a real universe. Taken to this extreme, mathematics is reality. If some or all of the mathematics that has compelled us to think about parallel worlds proves relevant to reality, Einstein’s famous query – whether the universe has the properties it does simply because no other universe is possible – would have a definitive answer: no. Our universe is not the only one possible. Its properties could have been different, and indeed the properties of other member universes may well be different. If so, seeking a fundamental explanation for why certain things are the way they are would be pointless. Statistical likelihood or plain happenstance would be firmly inserted in our understanding of a cosmos that would be profoundly vast. I don’t know if this is how things will turn out. No one does. But it is only through fearless engagement that we can learn our limits. Only through rational pursuit of theories, even those that whisk us into strange and unfamiliar domains – by taking the mathematics seriously – do we stand a chance of revealing the hidden expanses of reality. n Brian Greene is a theoretical physicist at Columbia University in New York. This article is adapted from his book The Hidden Reality (Allen Lane, 2011)
Big bang cosmology
The dark side Our established picture of the universe is supremely successful – maybe because most of it is made up, says Stephen Battersby
T
2
The Planck probe
esa
Radiation left over from the big bang contains vital clues about the early universe. The most detailed maps of it are coming from the European Space Agency’s Planck satellite, launched in 2009. It can capture the radiation precisely enough to measure cosmological quantities without making many theoretical assumptions, detect the rippling of gravitational waves and test various models of the inflation thought to have occurred during the big bang. It will even let us explore ideas outside of our standard cosmology, such as parallel worlds. Valerie Jamieson
40 | NewScientist | 2 March 2013
O OUR eyes, stars define the universe. To cosmologists they are just a dusting of glitter, an insignificant decoration on the true face of space. Far outweighing ordinary stars and gas are two elusive entities: dark matter and dark energy. We don’t know what they are… except that they appear to be almost everything. These twin apparitions might be enough to give us pause, and make us wonder whether all is right with the model universe we have spent the past century so carefully constructing. And they are not the only thing. Our standard cosmology also says that space was stretched into shape just a split second after the big bang by a third dark and unknown entity called the inflation field. That might imply the existence of a multiverse of countless other universes hidden from our view, most of them unimaginably alien – just to make models of our own universe work.
Cosmology’s dark apparitions raise the question of whether Einstein’s theory is the right one, says Jacob Bekenstein The success of general relativity is embedded in our modern world. True, most solar system and astronomical phenomena are still calculated with Newton’s hoary theory of gravitation, but we would be nowhere without our GPS gadgets, which work only once corrected for the effects of general relativity. General relativity has been tested with great precision within the solar system, and in binary pulsar systems where gravitational fields are very strong, but never on large scales where gravity’s pull is weak. Might the twin embarrassments of dark matter and dark energy mask general relativity’s failure there? Supporters of this idea have had some success. Modified Newtonian dynamics (MOND), proposed by Mordehai Milgrom of the Weizmann Institute in Rehovot, Israel, in the 1980s, relates mass to the gravity it generates in a slightly different way. It describes galaxies better and more parsimoniously than general relativity with dark matter does. Cosmological models constructed from alternative “f(R)” gravitational theories behave as if they contain dark energy, even though they don’t. But no one theory holds all the cards. MOND does not handle motions of individual galaxies within clusters well. Neither does Tensor– vector–scalar (TeVeS) gravity, a relativistic version of a theory I proposed in 2004. The f(R) theories do not adequately describe the anomalous galactic rotations that first led dark matter to be proposed. We might yet strike lucky. If dark energy is the venerable cosmological constant that Einstein shoehorned into his equations of general relativity, its favoured source is vacuum energy. Gravitational fields might conceivably perturb the vacuum enough that concentrations of energy appear in and around galaxies and galaxy clusters, mimicking dark matter. It is impossible to magic up concentrations large enough from current quantum-field theories, but perhaps one day the mystery of the two dark stuffs may be dispelled by the judicious application of known quantum physics. Jacob Bekenstein is a theoretical physicist at the Hebrew University of Jerusalem in Israel
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Precise recipe This could be Einstein’s cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to
Seeing in the dark Visible matter is only a tiny fraction of what we think the universe contains
Visible matter 4.5% Dark matter 22.5% Dark energy 73 %
explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions. Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what
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Advanced LIGO General relativity predicts that ripples in space-time should constantly be passing through Earth. From 2014 Advanced LIGO, an upgrade of an existing gravitational-wave detector in the US (pictured), will use laser “rulers” several kilometres long to spy spatial disturbances equivalent to Earth moving one-tenth of an atomic diameter closer to the sun. If it sees something, it will be the crowning triumph of Einstein’s relativity. If it doesn’t, it is back to the drawing board with our theories of gravity. Richard Webb ligo
WHAT’s NEXT DISCARD RELATIVITY
Overall, there seems to be about five times as much dark matter as visible gas and stars. Dark matter’s identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see “Flawed genius”, page 45). But it changed cosmology’s standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe’s expansion. The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter’s attractive gravity.
ESO/L. Calçada
galaxies and other structures; dark energy implies that the cosmos will accelerate away into a cold and lonely future; inflation suggests a violent birth. Each member of the shady triumvirate points to new physics. Kirshner sees that as a challenge. “It doesn’t mean there is any flaw in our arguments. It gives a sense not of desperation, but inspiration.” But as long as we have no evidence of dark matter in the lab, or a proven physical basis for dark energy, the possibility remains that we are living under some profound misapprehension – an unknown unknown, something so basic awry in our mathematical model of the universe that as yet we have not been able to imagine the form of our mistake. Might a quantum theory of gravity show us the way forward? Or might some new observation lead us to reformulate our general relativistic cosmology again? We have only the most tenuous of indications where we might look for alternatives. But perhaps if we only discard an unheeded assumption about reality, then a veil will be lifted, all the darknesses banished and the starry night restored. n
WHAT’s NEXT RETHINK INFLATION The best theory of our big bang has logically self-destructed, argues Max Tegmark
”Our lack of answers gives us a sense not of desperation, but of inspiration”
Stephen Battersby is a New Scientist consultant based in London
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LISA Pathfinder
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The European Space Agency’s LISA Pathfinder mission will primarily test gravitational-wave detectors, but from next year it could also confirm whether gravity is all general relativity says it is. By flying through the “saddle point” where the Earth and the sun’s gravity cancel out, the craft might probe whether Einstein’s theory still holds when gravitational accelerations are incredibly small. If it does, these gravitational lacunae will be the last resting place of other occasionally fashionable theories, such as Modified Newtonian dynamics (MOND). Stuart Clark
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Inflation got off to a great start. Beginning with a subatomic speck of a novel, hard-todilute substance, it predicted that this stuff would repeatedly double during a split second to create our big bang and our nearly uniform, flat space. It got even better. Inflation also produced random quantum fluctuations which grew into today’s stars, galaxies and large-scale structures. The theory makes spectacularly accurate predictions. For example, the quantity Omega, which quantifies the flatness of space, should equal 1. It has been measured as 1.003 ± 0.004. Bingo! But like a tenacious ageing professor, inflation refuses to retire. The theory predicts that the process continues forever in distant parts of our cosmos, producing a space that is not just huge but truly infinite, with infinitely many galaxies, stars, planets – and even people like us. Its random fluctuations distribute matter differently in different places, so infinitely many of these people observe an Omega near 1, infinitely many an Omega near 2 – and indeed any other value. So what’s the probability that we’re among those people who observe what we do? The useless, formal answer is infinity divided by infinity. We cosmologists have yet to reach a consensus on how to turn this into something useful. Thanks to inflation, we can predict the probability of virtually nothing any more. I’ve called this the “measure problem”, and view it as one of the deepest crises facing physics today. The way I see it, inflation has logically self-destructed, destroying the predictions that made us take it seriously in the first place. In fairness to inflation, I don’t feel that any competing theory does better. My guess is that once we solve the measure problem, some form of inflation will still remain – but perhaps not the eternal kind. All the problems stem from infinity, specifically the assumption that space can be stretched forever without somehow breaking down. We tend not to question this radical assumption – but we should! Max Tegmark is a cosmologist at the Massachusetts Institute of Technology
Aesthetics tells us what will break the standard-model deadlock: supersymmetry, says Frank Wilczek It is difficult to overstate the power and economy of the standard model. The discovery of the Higgs boson marks its apotheosis. More profoundly, it is the culmination of decades of bread-and-butter work. Just as to find a needle in a haystack one must thoroughly understand both needles and hay, to find the rare traces of the Higgs in the LHC’s explosive “little bangs” we had to know our fundamental physics. The standard model is close to nature’s last word. Yet, in its melange of forces and particles, it does not achieve complete unity and coherence. Maxwell’s equations governing electrodynamics, the oldest part of the standard model, are justly famous for their balance and beauty. The equations of its newest part, those describing the strong nuclear force, are also pleasingly symmetrical, but in place of one electric charge and one force-carrier (the photon), they demand three “colour” charges and eight gluons. The weak nuclear force introduces a further three force carriers. Altogether, the standard model looks a little lopsided. One can dream up bigger and better equations, with even more symmetry, to restore the balance. Supersymmetry is the logical climax of such thoughts. It postulates a basic symmetry that allows forces to turn into materials and materials into forces while the equations as a whole retain the same content. That is done by doubling nature’s library of particles – creating a force-carrying boson for every matter-making fermion, and vice versa. Pursuing this path leads us to an impressive and very concrete numerical success. The new, expanded theory accurately predicts the ratios of the strengths of the strong, weak and electromagnetic forces – parameters that the standard model leaves free. I cannot believe this success is an accident. But in science faith is a means, not an end. Supersymmetry predicts new particles, with characteristic properties, that will come into view as the LHC operates at higher energy and intensity. The theory will soon undergo a trial by fire. It will yield gold – or go up in smoke. Frank Wilczek is a theoretical physicist at the Massachusetts Institute of Technology and a co-recipient of the 2004 Nobel prize in physics 46 | NewScientist | 2 March 2013
”Antimatter came from a theorist’s pen: the positron duly turned up later in cosmic rays”
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wave-particles do not move according to the tidy rules of classical, Newtonian mechanics, but dance to probabilities bounded by bizarre rules in an abstract mathematical space. Quantum mechanics was largely in place by the mid 1920s, and it has yet to fail an experimental test. But when in the late 1920s Paul Dirac and others started to hook up quantum mechanics with Einstein’s special relativity – a vital step in describing particles that shunt around at near-light speeds – things began to take on a life of their own. Dirac’s relativistic equation for the electron had more than one solution, and seemed to predict that a particle existed just like the electron, but with opposite electric charge. The positron duly turned up in cosmic rays five years later. Antimatter had been invented from a theorist’s pen. Quantum field theory, the basis of the standard model, represents the culmination of this logic. The idea of a field transmitting forces goes back to Michael Faraday in the 19th century, but the mathematical structure of quantum fields gives them an odd property: they can create particles from empty space and destroy them again almost at will. Thus, according to the theory of quantum electrodynamics, two electrons repel each other thanks to a photon – the
Dark matter searches Theory points to dark matter being made of so-far-unseen weakly interacting massive particles, known as WIMPs. Over a dozen exquisitely sensitive experiments have been built specifically to catch these slippery customers. Three – DAMA/LIBRA (pictured), CoGeNT and CRESST – have seen things that look suspiciously like them. Others have ruled out the same particles entirely. The trouble is we know too little about what we are looking for. We need more data and additional experiments to understand the experiments. Valerie Jamieson
DAMA/LIBRA/gran sasso
WHAT’s NEXT FOLLOW BEAUTY
can be written on a postcard: six quarks arranged in pairs to make three “generations” identical in all but mass; six leptons, such as electrons and neutrinos, arranged similarly; and a handful of bosons that transmit nature’s fundamental forces between them (see diagram, below right). The essential thing about all these entities is that they are quantum particles. Quantum theory grew from radical discoveries at the beginning of the 20th century, which showed that the wavelengths of radiation emitted and absorbed by atoms could be explained only by assuming that energy is bundled in discrete amounts, or “quanta”. That unleashed an absurd duality at the smallest scales whereby a particle is also a wave and vice versa. These nebulous
term coined by Gell-Mann, finally made quarks respectable by describing their interactions by the exchange of eight gluons that carry a “colour” charge, and showed how, uniquely, this force gets stronger the further you pull two quarks apart. “It could both explain why protons looked as if they were made of quarks and why these quarks could never be pulled out of protons,” says Gross. And that, largely, was it. By 1973, the Beatles had split up and, following a period of mind-boggling theoretical invention, the standard model was in place. There was the unified electroweak theory, to which all particles succumb; and quantum chromodynamics, which affects only quarks and gluons. The model wasn’t just clever, it was beautiful. Its equations had a powerful symmetry that dictated the character of nature’s forces, and told physicists what sort of new particles to look for and where. And, sure enough, the bumps in particle-collider data soon began to appear – together with goosebumps on the skin of the theorists. Evidence for three quarks had already been established in experiments the late 1960s, but by the end of the 1970s physicists in the US had inferred the existence of a fourth and fifth and finally, in 1995, the sixth, “top”, quark. By 2000,
the tau neutrino, the last of the leptons to be discovered, had also been bagged. On the other side of the pond, the gluon was snared at the DESY laboratory outside Hamburg, Germany, in 1979; the W and Z bosons in 1983 at CERN. And finally, last year, the Higgs – the last outstanding particle predicted by the standard model. For Weinberg, the standard model’s triumphant march has been something quite special. “To fool around at your desk with mathematical ideas and then find that, after spending a few billion dollars, experimentalists have confirmed them… there really isn’t anything comparable to it,” he says. So why aren’t he and others like him rejoicing?
WHAT’S NEXT CATCH NEUTRINOS These elusive particles have already given us the key to new physics, says Janet Conrad
Puzzling features For many reasons. Some are aesthetic. Why, for example, do particles come in three generations, with the heaviest quark weighing 75,000 times more than the lightest? The standard model’s equations might be elegant, but to give them their predictive power, they must be fed more than 20 “free” parameters, such as particle masses, by hand. A truly fundamental theory would use the power of quantum theory, or perhaps some deeper idea that nobody has yet thought of, to prune that thicket.
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Neutrino experiments are a hit-and-miss affair. The properties of the ghostly particles are ill-defined, and they interact so rarely that vast floods of them are needed for us to spot anything. The solution could be nuSTORM, a proposed “factory” that will churn out precisely controlled beams of neutrinos or their antimatter counterparts, antineutrinos. That could at last pin down their nature and the number of varieties they come in – and so settle whether any additional types of non-interacting “sterile” neutrinos exist. Valerie Jamieson
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Roy Kaltschmidt/ Lawrence Berkeley National Lab
Neutrino factories
The standard model is a Goliath among theories: big, powerful, cumbersome and, until recently, seemingly invincible. But a decade ago, it may have already have met its David: neutrinos. According to the standard model, these tiny, mysterious particles come in three different types, or flavours. They have no mass, and interact only through the weak nuclear force, making them very difficult to detect. That at least was the picture until 1998, when the astonishing confirmation came that neutrinos “oscillate”, regularly morphing from one flavour to another and back again. This is a quantum-mechanical effect that can occur only if neutrinos have mass. The particles thus present the first and so far only clear observation of physics beyond the standard model. If neutrinos having mass were the whole story, we might still patch up the standard model. But we are now also seeing evidence for extra oscillations that are very hard to explain in a model with only three neutrinos. This perhaps points to the existence of additional “sterile” neutrino flavours that do not interact through any of the four known fundamental forces, but can morph to and from the active flavours. If this new phenomenon is confirmed, that would land a truly massive blow. Rethinking the masses of known particles is one thing, but making room for a whole new family of particles is something else. It would call for a new theory that explains sterile neutrinos and where they come from. If sterile neutrinos do not interact through any known force, what force do they act through? Might they be a key to the identity of dark matter? We have no clear answers to these questions – but theoretical papers on sterile neutrinos are now coming out at a rate of about two a week. Why study the tiniest particle? Because Goliath lost! Janet Conrad is a neutrino physicist at the Massachusetts Institute of Technology
underlie the standard model suggest space is pixelated into units with sizes of about 10-35 metres, and do not even treat time as a real and observable thing. Asked to choose between the two theories, most physicists’ money is on quantum theory being “right”, because its mathematics is such a successful prism through which to view the world. Others, from Einstein onwards, have taken issue with quantum theory’s seeming “irreality” and spooky, counter-
1900 The birth of quantum theory 1905 Einstein introduces quantum photons of light 1905 Special relativity dictates that light’s speed is always the same
1913 Quantum model of the atom
*1911 Atomic nucleus discovered
ultimate theory. String theory predicts that space has hidden extra dimensions, invoking symmetries embedded in these dimensions to “fold” energy into geometric shapes that look like certain fundamental particles, or mimic the way space curves in the presence of mass. The theory has produced some credible depictions of particles, among them the long-sought graviton, a quantum particle that would carry the force of gravity. It thus takes steps towards a unified picture
intuitive correlations between apparently unrelated objects. If we cannot find a convincing physical reason why these correlations are just so, they argue, perhaps quantum theory is just an approximation to something better. Attempts to go beyond this impasse have drawn on favoured mathematical ideas such as symmetry. One result is supersymmetry, a theory widely regarded as a way station on the road to string theory, a favoured candidate for an
Mid 1920s Quantum mechanics formulated mathematically
1916 General relativity, Einstein’s space-timewarping theory of gravity
1928 Dirac equation describes the electron and predicts *1933 antimatter Antimatter discovered
*1932 Effects of dark matter first observed
*1927 Evidence for an expanding universe: birth of the big bang
Our standard models of particle physics and cosmology have been built on a series of interlinked theoretical and experimental discoveries (*)
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Experiments beaming photons over sometimes hundreds of kilometres have so far only confirmed quantum theory’s outrageous predictions of weird correlations and entanglements between the particles. Soon the ante will be upped with plans to beam quantum transmissions via satellite between continents. It’s a first step to testing quantum theory in space over distances at which relativity’s warping becomes significant – and so seeing what happens when those two great and incompatible theories collide. Richard Webb
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Quantum theory in space
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1940s Quantum theory of electromagnetism 1948 Cosmic microwave background predicted
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1957 Hugh Everett’s “multiverse” of quantum universes
of all four forces of nature on the basis of quantum theory. But like other proposed frameworks for a theory of everything (see below, right), it has a big flaw. “String theory does predict new things, but they’re almost certainly not testable in the foreseeable future,” says Paul Davies of Arizona State University in Tempe. That failing means the idea of a theory of everything has quietly disappeared, says Renate Loll of Radboud University in Nijmegen in the Netherlands. “For a while you would see it in papers, in the heyday of string theory, but it has gone totally out of fashion.” Chris Isham of Imperial College London goes further. A theory of everything is “psychologically compelling”, he says, but there is no reason to think one exists – or that we can find it. That we have got so far with mathematics is a remarkable fact, but it does not mean we can go all the way. One problem is that mathematics provides infinite ways in which numbers and abstract quantities can be processed – but no indication of what exists beyond it. “Mathematics only reveals truth about abstract objects,” says Deutsch. “Physics