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October 18, 2013 1:08 pm
Rolf Heuer, head of Cern, the world’s biggest physics lab, has declared his next research target after triumphantly achieving the first objective: finding the Higgs particle. “We have now completed the Standard Model,” he says. “It is high time for us to go on to the dark universe.”
His words will resonate with an army of scientists currently working on experiments in space, on Earth and in labs deep underground to create what they call “new physics”. Their hunt will go far beyond the Standard Model, which has been built up over the past 50 years to provide an internally consistent but incomplete description of the fabric of reality.
The dark universe is a double problem for new physics. Scientists do not know what makes up the “dark matter” that dominates all the ordinary stuff known to astronomers in galaxies, stars, planets, dust, quasars, black holes and so on. And “dark energy”, a repulsive antigravity force driving the universe apart at an accelerating pace, is even more mysterious.
“It’s a cosmic detective story,” says Kathy Romer of the University of Sussex, who is working on the Dark Energy Camera, an international project to shed light on dark energy by mapping the universe with unprecedented precision. “We’ll collect as much evidence as we can and then have to design the most likely story that explains the facts.” The world’s most powerful digital camera, mounted on a huge telescope in the Andes, will image galaxies as far as eight billion light years away; it will be followed by a space telescope called Euclid, built by the European Space Agency to do a similar job from orbit after 2020.
Besides probing the dark universe, the quest for new physics involves looking for (among other things) a panoply of new subatomic “superparticles”, hidden dimensions of space, the difference between matter and antimatter, a union between gravity and quantum theory, and even signs that our universe is part of an infinite “multiverse”. The search is the bluest of blue-sky research, purely about the intellectual satisfaction of expanding our understanding.
Before exploring the exciting possibilities of “new physics” we should appreciate the stunning scientific achievement represented by the Standard Model. Over the past 50 years many theorists, including the new Nobel laureates Peter Higgs and François Englert, have drawn up a table of fundamental particles and forces which provides a remarkably consistent description of the fabric of reality. Experimentalists, working on increasingly powerful atom smashers, have duly confirmed their predictions.
The last piece fell into place last year with the success of attempts to make the Higgs boson in Cern’s Large Hadron Collider outside Geneva. Scientists needed the particle and its accompanying Higgs energy field to explain how matter acquires its mass in the Standard Model. Although the discovery was expected, Brian Cox, physics professor at Manchester University, believes it deserves all the media fuss – and more. “If anything, Higgs has been underplayed,” he says. “The wonderful thing for me is that an entirely new kind of fundamental particle was predicted on the basis of mathematical principles and then confirmed experimentally almost 50 years later.”
The incomplete nature of the Standard Model leaves no doubt that a rich stratum of new physics awaits discovery. It unifies three fundamental forces, the so-called weak, strong and electromagnetic forces, but it leaves out the fourth force, gravity, and fails to explain the “dark” 95 per cent of the universe. The deepest issues of existence, such as the origins and future of the universe and its possible place in a multiverse of other universes, go far beyond its reach.
The scientists eating and drinking at the world’s greatest physics think-tank and gossip shop, the Cern canteen, are naturally hoping to follow up their Higgs triumph by providing the first evidence of new physics. But the $8bn LHC has discovered nothing beyond the Standard Model during more than two years of smashing together trillions of protons (hydrogen nuclei) at almost the speed of light. It is now shut down for a two-year upgrade as engineers double its running energy to 14 TeV (trillion electron-volts).
Novel particles predicted by some theories of new physics should already have shown up at LHC energies, which reproduce on an infinitesimal scale the newborn universe less than a billionth of a second after the Big Bang, but so far there is no sign of them. “When you’re looking at what might be out there and what our experiments and cosmology have ruled out, then we start to see a picture that in some ways could be quite depressing,” says Tara Shears, a physics professor at Liverpool University. We are sipping morning coffee on the Cern canteen terrace, as the late summer sun illuminates the distant Alpine foothills beneath which the LHC runs. “We know there has to be a deeper understanding out there but it’s quite possible that we will not see [from LHC experiments] any radical departures in our understanding,” she admits. Then, an optimist by nature, Shears adds quickly: “It’s also quite possible that we will,” looking forward to the emergence of new physics when the LHC resumes operations early in 2015.
The most widely anticipated manifestation of new physics is supersymmetry, the theory that all fundamental particles in the Standard Model have corresponding “superparticles” far heavier than their partner. The plethora of new particles provided by supersymmetry could fill several gaps in the Standard Model but so far none has been seen. Many cosmologists believe that superparticles are the most likely candidate to make up dark matter. Astronomers who observe the distribution and movement of galaxies as their telescopes look billions of light years into space (and billions of years back in time) are confident that dark matter is out there – and five times more plentiful than ordinary matter – because of its gravitational effect on the things we can see.
The predicted properties of superparticles would enable them to pass straight through ordinary matter without any interaction. Indeed, millions of superparticles may be whizzing through your body every second, as if you were not there. En masse they would make an impact through gravity but individually they would have no effect on our world.
Physicists hoped that the Higgs discovery itself would give clues to supersymmetry. So far there is no sign of this, says Cern researcher Pippa Wells. “The Higgs is looking very like a simple particle, with no evidence for Higgs-like particles of higher mass predicted by supersymmetry. We see this fascinating, irritating consistency with the Standard Model.”
“We may have been unlucky not to observe new phenomena during the first LHC run,” says Cern theorist Gian Giudice. “The new phenomena may be late to arrive and come when the LHC runs at 14 TeV – which would be a triumph for particle physics. The other option is that we were wrong and this physics does not exist.”
We have moved on from the canteen to Giudice’s office – complete with the traditional blackboard and chalk – in one of Cern’s drab lab buildings. “As theorists we are at a crossroads,” he continues. “At stake is our whole vision of physics. We’ll find out whether, as we go deeper to shorter distances and higher energies, physics becomes simpler and new steps of nature are revealed.”
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Of course, theorists are not waiting idly for new data. Indeed, they are feasting on what is already available. “In spite of the absence of any signal of new physics, the LHC has already provided valuable information for theoretical speculations,” says Giudice, who has constructed a theory with colleagues about the implications of the Higgs particle having a mass of 125 GeV (billion electron-volts). This implies that the universe is in a “near-critical state”, close to a transition to a quite different phase of existence.
The Large Hadron Collider is now shut down for a two-year upgrade to double its running energy to 14 TeV (trillion electron-volts)
To appreciate this, we have to switch from thinking of the Higgs particle, emphasised by the media coverage, and think more about the energy field of which the particle is a manifestation. Paul Newman, head of particle physics at the University of Birmingham, expressed it well in his reaction to the Nobel Prize last week. “At first sight, the Higgs mechanism is a very strange idea indeed,” he says. “It requires the entire universe, even deepest intergalactic space, to be filled with a new field of a fundamentally different kind from anything previously known. The audacity of proposing such a bizarre and all-pervading mechanism based on what was known half-a-century ago [when Peter Higgs and other theorists first put it forward] is simply stunning.” As Brian Cox puts it, “The universe got itself into a position less than a billionth of a second after the Big Bang in which the lowest energy configuration is not empty space but is the Higgs field.”
But calculations by Giudice and colleagues, based on the LHC’s Higgs mass measurement, suggest that this may no longer be the most stable state of the universe. It seems balanced close to a critical point at which it could undergo a “phase change” to a quite different form filled with another energy field. An analogy would be supercooled water poised to freeze or superheated water on the point of boiling. Giudice does not want to alarm people with this “living dangerously” model because there is no evidence that a phase change, which would destroy everything in existence, is imminent. But, even if it happens aeons into the future, it could determine the ultimate fate of the universe.
Intriguingly, through some fiendishly complicated maths, Giudice and colleagues go further and relate the Higgs mass to the idea of multiple universes with differing laws of physics. “It seems that there is selection pressure within the multiverse favouring universes that are ‘living dangerously’ – close to a critical state – and at the same time have conditions likely to lead to the formation of galaxies, stars, planets and possibly to life.”
This “living dangerously” model is just one of many ideas emerging from the current maelstrom of imagination in physics, as experiments around the world and beyond it provide food for theoretical thought. The LHC may be the biggest and most publicised facility but plenty of others are gathering data to answer the big questions about the nature of reality.
Even at Cern, there are other games in town. Its Antiproton Decelerator is the only facility in the world dedicated to making and study in antimatter. “Differences between matter and antimatter are one of the biggest puzzles of all,” says Michael Doser, senior Cern researcher, as he guides me round. “Equal amounts of matter and antimatter must have been formed at the Big Bang – and we have no good explanation for the loss of half the universe.”
According to the Standard Model a particle and its antiparticle, such as proton and antiproton or electron and positron (anti-electron), are identical in every way apart from having an opposite charge. When particle and antiparticle meet, they annihilate each other in an intense burst of energy. In reality there must be tiny differences between them, so that after the mutual destruction of matter and antimatter following the Big Bang, only matter was left.
Cern’s Antiproton Decelerator smashes an accelerator beam into a metal target, generating high-energy antiprotons, which are cooled and tamed through cunning technology for use in the facility’s four experiments. Some antiprotons are even paired up with positrons to make anti-atoms of antihydrogen. “We aim to measure with extreme precision the spectra, mass, lifetime, magnetic moment and other properties of particles and antiparticles, to look for any differences between them,” Doser says. Nothing has turned up yet.
Ordinary matter may make up just 5 per cent of the composition of the universe. The rest is thought to be dark matter and dark energy
Although the scientists have made hundreds of individual antihydrogen atoms and trapped them to study for more than 15 minutes, they have not collected them together. Nor is there any realistic prospect of accumulating a flask antimatter such as the one novelist Dan Brown conjured up for Angels and Demons – which is just as well for everyone’s health and safety.
The antimatter enigma is also being studied at the LHC, Shears says, focusing on rare subatomic particles that disintegrate in a particularly asymmetric way, generating an excess of as much as 25 per cent of matter over antimatter products. Again, scientists have no explanation for the large disparity.
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Beyond Cern, this year’s most important contribution to answering the big questions has come from the European Space Agency’s Planck observatory. It has produced the most detailed map so far of the “cosmic microwave background”, the relic radiation released when the universe was just 380,000 years old and for the first time cool enough for light to pass through it.
From its orbit a 1.5 million km away from Earth, Planck maps minuscule differences in the microwave radiation across the sky, representing temperature variations of less than 0.01 per cent. These are the early seeds of all the large-scale structures in the universe, which were later pulled together by gravity, emerging as the first stars and galaxies after about 200 million years.
“It is a remarkable achievement of cosmology that, if you start with those small fluctuations when the universe was less than half-a-million years old, you can run forward a computer simulation and end up with essentially the structures we see now,” says Martin Rees, Britain’s Astronomer Royal.
But cosmologists are equally interested in using the cosmic microwave map to look back in time to the first moments of the universe. They believe the ripple-like variations come from random quantum fluctuations immediately after the Big Bang 13.8 billion years ago. These were then magnified by a short period of “inflation” when the universe expanded far faster than the speed of light for a fraction of a second, before a phase change inaugurated growth at a more sedate pace.
“The sizes of these tiny ripples hold the key to what happened in that first trillionth of a trillionth of a second,” explains Joanna Dunkley of Oxford university. “Planck has given us striking new evidence that indicates they were created during this incredibly fast expansion, just after the Big Bang.”
There are two of ways of looking at the Planck findings. One emphasises their consistency with the Standard Model. The other focuses on slight anomalies that hint at new physics. These unexpected features include unevenness on the largest scales; one hemisphere is slightly warmer than the other, and a cold spot extends over a huge patch of sky. An extremely speculative interpretation would be that the anomalies represent an imprint of an earlier universe from which ours emerged in the Big Bang. But most cosmologists would be satisfied with a less ambitious explanation. They are looking forward to another release of Planck data next year including, for the first time, an analysis of the polarisation of this early light.
Another achievement by Planck has been to narrow down previous estimates of the composition of the universe. It puts ordinary matter at 5 per cent, dark matter at 27 per cent and dark energy at 68 per cent.
Physicists are reasonably optimistic that dark matter will be identified within the next decade or so. “We know from the astrophysics and computer modelling that dark matter consists of swarms of particles that are electrically neutral and essentially collisionless,” says Rees. “There are various candidates for them, including axions and supersymmetric particles, and several lines of attack aimed at finding them.”
One way is to site instruments deep underground, where they are shielded from extraneous cosmic rays, to detect the extremely rare occasions when a dark matter particle does interact with ordinary matter. Another is to look for signs of secondary particles emitted when – or rather if – dark matter particles annihilate one another or decay in outer space; the Alpha Magnetic Spectrometer, a particle physics detector mounted on the International Space Station, is following this approach.
Dark energy is an even more elusive target. It is a property of empty space – distinct from but possibly related to the Higgs field – which acts like antigravity, pushing everything in the universe apart at an ever-increasing rate. The discovery of dark energy in 1998, made by observing the distribution of distant galaxies through space and time, was cosmologists’ biggest surprise for decades. The Dark Energy Camera, the international instrument built at Fermilab in the US to find out more about it, is about to survey the sky systematically from a 4m telescope on the Cerro Tololo mountaintop observatory in Chile. It cannot directly detect dark energy, whatever it may be, but will infer its properties indirectly with far greater precision than has been possible with previous sky surveys.
At this stage the search for new physics is one of knowledge for knowledge’s sake with no applications in mind. Any proposed use of dark matter, for instance, would be sci-fi speculation – at least until we know its identity. But physicists point out that similar discoveries in the past have had unanticipated applications many decades later. A favourite example is JJ Thomson’s discovery of the electron in 1897, which turned out to be the foundation of modern electronics.
Back at the Cern canteen, listening to the enthusiastic multilingual conversation over the afternoon’s last cups of tea and coffee and the evening’s first glasses of beer and wine, the prevailing mood, especially among the younger researchers, is clearly excitement at the prospect of answering at least a few of science’s biggest questions.
“I’m not too disappointed that we have seen no new physics so far,” says Sudarshan Paramesvaran, a postdoc at Bristol University who is working on the upgrade of one of the LHC’s giant particle detectors. “We know we don’t have the full picture, and we can be creative and think about our own ideas. We’re really pushing the boundaries of knowledge here.”
The birth of our universe about 13.8 billion years ago, in an unimaginably fast expansion of space and energy.
The first growth phase, an infinitesimal fraction of a second after the Big Bang, when an unknown force blew up newborn universe at an exponential rate.
The theoretical framework that unites three of nature’s four fundamental forces (weak, strong and electromagnetic but not gravity).
Fundamental particle named after Peter Higgs of Edinburgh University, associated with a force field that gives matter its mass.
The theory, still unproven, that every particle in the Standard Model is paired with a heavier superparticle.
An unknown property of empty space which contributes 68 per cent of the mass-energy of the universe and counteracts gravity to drive cosmic expansion.
Particles that do not interact with ordinary matter and make up 27 per cent of the universe – identity unknown.
Matter made from the oppositely charged versions of familiar subatomic particles – positrons and antiprotons instead of electrons and protons.
The unproven idea that our universe is just one of a potentially infinite collection of universes – probably with different laws of physics.
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