Why Symmetry Matters Mario Livio Celebrates the Guiding Light for Modern Physics

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Symmetries feature in the stained-glass ceiling of the Palace of Catalan Music in Barcelona, Spain. Why symmetry matters Mario Livio celebrates the guiding light for modern physics.

ymmetries lie at the heart of the laws If the new boson is confirmed as the acceleration and gravity, and the warping of of nature. Early scientific giants such long-sought Higgs, our faith in symmetry as space-time by masses. as Galileo Galilei, René Descartes a core concept in physics will be cemented. Imposing symmetry on all frames Sand Isaac Newton did not speak in those Further tests of symmetry’s power as a guide of reference was Einstein’s motivation for terms, but symmetries were implicit in their to physical laws will follow, in the quantum developing relativity theory. He wanted to ideas of a comprehensive framework of the realm and through concepts such as super- find a way of describing the laws of nature Universe. And symmetries lie explicitly at symmetry. But what is symmetry and how that would look the same to all observers, the basis of modern physics, from general is it broken? whether at rest, moving at a constant velocity relativity to quantum field theory. or accelerating. To a physicist, symmetry is a broader THE LAWS OF NATURE In looking for symmetries, Einstein was concept than the reflective form of butterfly Universal laws are symmetric under following in the footsteps of James Clerk wings, or the rotational similarity of a translation in space and time — they don’t Maxwell, a physicist who died the same triangular roundabout sign. In physics, to be change from place to place or from today year Einstein was born, in 1879. Purely for symmetrical is to be immune to possible to tomorrow. As astronomers know, hydro- mathematical balance, Maxwell added to changes. Symmetry represents those stub- gen atoms billions of light years away are equations describing electromagnetism an born cores that remain unaltered even under subject to the same physics as on Earth. extra term that related electrical currents to transformations that could change them. Because physics has no preferred direction, a resulting magnetic field. In doing so, he Sometimes symmetries break down. The natural laws are also symmetric under rota- unified all electric and magnetic phenom- announcement on 4 July that two teams at tion. Whether we measure orientation with ena then known, as well as the laws of optics. the Large Hadron Collider (LHC) at CERN, respect to the farthest quasar or the nearest Europe’s particle-physics laboratory near coffee shop, the rules are the same. INTERNAL SYMMETRIES Geneva, Switzerland, had discovered a Such symmetries are true of Newton’s laws The symmetries that give rise to the Higgs Higgs-like boson marked the end of a heroic of gravity and motion, which he applied in boson are different. They originate in the quest to confirm a ‘symmetry-breaking’ the seventeenth century to falling apples, probabilistic world of quantum physics — mechanism. That such a process endows ocean tides, the Moon and planetary orbits. ‘internal symmetries’ reflect the robust- particles with mass was suggested in 1964 They are also true of those laws’ twentieth- ness of universal laws to changes in the by theoretical physicist Peter Higgs and two century successors: Albert Einstein’s special identity of some elementary particles. Just other groups (Robert Brout and François and general theories of relativity, with their as a particle’s position may be uncertain Englert; and Gerald Guralnik, Carl Hagen revolutionary ideas about the constancy according to quantum rules, so may its and Tom Kibble). of the speed of light, the equivalence of identity. Some types of particles are thus

472 | NATURE | VOL 490 | 25 OCTOBER 2012 © 2012 Macmillan Publishers Limited. All rights reserved COMMENT interchangeable within the equations. that treats the W+, W– and Z0 particles as The mechanism by which it did so (the Electrons and neutrinos can be swapped equivalent, for instance, applies to the Higgs mechanism) involves a quantum field without altering the laws of nature. This electron and neutrino. The symmetry (the Higgs field), which has a non-zero value symmetry is central to the ‘electroweak requirement dictates which particles and associated with every point in space. The theory’, developed in the 1960s by physicists interactions are necessary for a given theory. Higgs particle is a ripple, a parcel of energy, Steven Weinberg, Sheldon Glashow and Yet the cosmos does not always manifest in the Higgs field. Abdus Salam. The theory unifies electro- perfect symmetry. The equations describing The Higgs field tugs on W and Z particles, magnetism with the weak nuclear force, the electroweak interaction, for example, are restricting their communication of the weak which is responsible for some radioactive symmetrical. They do not change when a force to an extremely short range (less than decays (such as a neutron to a proton) and photon is swapped with a W or Z particle. about one-ten-thousand-trillionth of a underlies nuclear reactions in stars. The But the solutions to these equations — the centimetre). In other words, it gives the W Higgs boson has a role in breaking the particles themselves — are not identical. The and Z particles inertia, or mass. In similar symmetry of weak interactions. photon is massless, and the W and Z particles fashion, the molasses-like Higgs field gives The strong nuclear force, which acts are about 100 times heavier than a proton. mass to other fundamental particles, such as among quarks to make up protons and The symmetry of the governing equations is electrons and quarks. neutrons, and binds protons and neutrons somehow lost, or broken, in physical reality. Because the vacuum does not carry to form atomic nuclei, is also subject to electrical charge, the photon travels symmetries. The laws of physics are blind, BROKEN SYMMETRIES unhindered. So the photon remains mass- for instance, to exchanges of up quarks and The concept of spontaneous symmetry- less and can render the electromagnetic force down quarks (protons are made of two up breaking allows physicists to preserve over long distances. quarks and one down quark). a symmetric theory while confronting So far, the particle discovered in July at The standard model of particle physics puzzling observations. the LHC looks a lot like the Higgs boson. combines the electroweak theory with In the 1930s, the Jewish–Russian physicist More tests are needed to prove it. First, the quantum chromodynamics, the theory of Lev Landau realized that phase transitions experimentalists must determine the strong interactions within the nucleus. The are accompanied by a loss of symmetry. quantum spin of the new boson (the Higgs model’s great achievement is that it unifies Take magnetic materials. When hot iron is is predicted to have no spin). Second, they three of the four known forces through cooled below its Curie temperature (around need to measure the rates at which it decays which particles interact (gravity stubbornly 770 °C), it acquires bulk magnetism. Its into other particles and compare those to continues to resist unification). internal magnetic fields, pointing in random theoretical expectations. Even if the boson Internal symmetries are local, or directions when hot, collectively settle on passes these tests, symmetry and its breaking independent of the locations of particles. one orientation. The symmetry is broken. do not leave centre stage. Quarks (or electrons and neutrinos) at dif- The equations describing the magnetic field One of the major steps beyond the ferent points in space and time may be within the iron are symmetric — they have standard model involves supersymmetry exchanged without consequence. Fields, with no preferred orientation. It is the physical — the idea that each particle we know has a their associated gauge bosons, mediate these state of the iron that changes. not-yet-discovered superpartner, with a spin remote interactions. The electromagnetic Higgs and his colleagues realized in 1964 removed by half a quantum-mechanical unit. interaction is carried by the photon. In elec- that symmetry-breaking could be applied to Supersymmetry is manifestly broken; other- troweak theory, the photon is joined by the particle physics. They proposed that a frac- wise the superpartners would have had the three carriers of the weak nuclear force: W+, tion of a second after the Big Bang, as the same masses and charges as the known par- W– and Z0 particles. Gluons shepherd the Universe expanded and cooled, it went ticles and would have been detected already. strong force between quarks. through a dramatic phase transition (see A broken supersymmetry opens the door to a The symmetries associated with a given ‘Fundamental forces’). The internal symmetry host of other potential bizarre processes, such fundamental force act on all associated of the weak interactions, which held true at as an electron transforming into a muon. elementary particles, not just the force very high energies, broke when the Universe’s There are no signs as yet from the LHC mediators. The same electroweak symmetry energy dropped below some threshold. of supersymmetric particles, but this could change. Although the simplest versions of supersymmetry seem to have been ruled out, FUNDAMENTAL FORCES no one knows what to expect when the LHC After the Big Bang, the four forces divided as the cooling Universe underwent phase transitions. increases its energy in two years. The Higgs boson broke the symmetry between the electromagnetic and weak nuclear forces. Of course, the ultimate goal remains an all-embracing theory that will unify gravity Temperature of 1032 1027 1015 3 Universe (K) with the other interactions. We still do not know if the underlying principle of such a theory is symmetry, but a confirmation of STRONG NUCLEAR FORCE the newfound boson as the Higgs will show, once again, that symmetry is a guiding light ELECTROMAGNETIC FORCE through nature’s labyrinth. ■

Electroweak symmetry WEAK NUCLEAR FORCE Mario Livio is an astrophysicist at the broken Space Telescope Science Institute, Baltimore, GRAVITATIONAL FORCE Maryland 21218, USA. He is author of The Equation That Couldn’t Be Solved: In ation How Mathematical Genius Discovered Time after Big Bang 10–43 10–35 10–12 5 × 1017 the Language of Symmetry (Simon & (seconds) (present day) Schuster, 2005). e-mail: [email protected]