TWO-AND-A-HALF millennia ago, a group of Greek philosophers conjectured that beneath the stunning complexity of the physical world lay an elegant simplicity. The entire universe, they said, was comprised of nothing but particles moving in a void. They called the particles ‘atoms’. What we today call atoms are composite bodies with bits inside them. But the notion that on a small enough scale of size there are fundamental, indecomposable building blocks of the physical world remains a focus for research.
In July 2012, physicists announced a major step forward with the discovery of a particle created in the enormous Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) laboratory in Switzerland. Widely tipped to be the long-sought Higgs boson, the particle seems to be the last piece in the jigsaw of the Standard Model of particle physics, a set of equations that describes much of what is known about the fundamental structure of matter.
But, impressive though this achievement may be, it represents only a partial completion of the project begun in ancient Greece. A glaring omission is the so-called dark matter that makes up the lion’s share of matter in the universe. Astronomers can see the effect of dark matter as it tugs on stars and galaxies, but physicists have no idea what it is. The best guess is that dark matter consists of particles coughed out of the Big Bang that interact with normal matter so feebly that they mostly go right through us unnoticed. Searches for dark matter particles coming from space have so far yielded nothing definite.
MODERN PHYSICS aims not merely to draw up an inventory of particles such as electrons, neutrinos and quarks (the components of protons and neutrons), but also to explain the four forces that act between them: electromagnetism, gravitation and two nuclear forces known as weak and strong. Quantum mechanics permits a description of forces in terms of the exchange of yet more particles – photons in the case of electromagnetism and a variety of less familiar particles for the other forces. The Standard Model involves a partial unification of the forces, by joining electromagnetism and the weak nuclear force in a common scheme. The strong force is successfully described in terms of the exchange of a set of particles known as gluons, but this system is not yet mathematically integrated with the electromagnetic and weak forces. Gravitation, meanwhile, lies outside the scheme altogether.
So, theoretical physicists are yearning for additional unification and simplification. One idea, around since the 1970s, is called supersymmetry. It provides a common mathematical description of particles of matter and the particles that convey the forces. If nature is supersymmetric, there ought to be a whole zoo of additional particles, as each species of known particle should have a supersymmetric partner. The hope is that some of these hypothetical particles might explain the dark matter puzzle. Yet, against expectations, the LHC – by far the best bet for discovering this bizarre particle zoo – has so far shown no sign of any.
Another hope is that the strong force should be fully unified with the electromagnetic and weak forces, but experimental evidence for this is slim. The dream of total unification, in which gravitation is also brought into the scheme, has captivated a generation of theorists, including Einstein, and produced – among other proposals – string theory. This postulates that all particles and forces are manifestations of tiny loops of ‘string’ vibrating in different patterns.
SO HAS THE HIGGS discovery pushed any of these theories further forward? Tantalisingly, it seems the boson lies at the crossroads of supporting the Standard Model, and hinting at exotic new physics that may move us into even stranger realms of theory.
The job of the Higgs boson is to bestow mass on other particles such as electrons and quarks. Left out of the scheme, however, are neutrinos, ghostly particles that have no charge and pass through solid matter almost as if it didn’t exist. Neutrinos make up a very small fraction of dark matter. They have a tiny mass, less than a millionth of that of the electron, the next-lightest known particle. But physicists do not understand how this mass arises.
Even more enigmatic is the so-called dark energy that dominates the universe, making up nearly three quarters of its mass. The best way of describing it is the energy of empty space itself, a weird concept that makes sense only in quantum physics. But theorists are at a loss to explain its observed value, and disagree about whether it will stay constant over time as the universe expands.
All these puzzles point to important physics beyond the Standard Model. Hopes are pinned on more discoveries at the LHC, and perhaps elsewhere, such as dark matter experiments buried far underground. But the completion of the ancient Greeks’ elegant concept will depend more than ever on physicists’ ingenuity. At eight billion euros, the LHC is the costliest particle accelerator ever built. If we want to go beyond the Standard Model of particle physics, we may need to go beyond the standard model of funding too.