Our basic understanding of matter and energy is impressive, but falls well short of a complete theory of reality, says Valerie Jamieson
IS ANYTHING real? The question seems to invite only one answer: of course it is. If in doubt, try kicking a rock. Leaving aside the question of whether your senses can be trusted, what are you actually kicking? When it boils down to it, not a lot. Science needs remarkably few ingredients to account for a rock: a handful of different particles, the forces that govern their interactions, plus some rules laid down by quantum mechanics. This seems like a solid take on reality, but it quickly starts to feel insubstantial. If you take a rock apart, you’ll find that its basic constituent is atoms – perhaps 1000 trillion trillion of them, depending on the rock’s size. Atoms, of course, are composed of smaller subatomic particles, namely protons and neutrons – themselves built of quarks – and electrons. Otherwise, though, atoms (and hence rocks) are mostly empty space. If an atom were scaled up so that its nucleus was the size of the Earth, the distance to its closest electrons would be 2.5 times the distance between the Earth and the sun. In between is nothing at all. If so much of reality is built on emptiness, then what gives rocks and other objects their form and bulk? Physics has no problem answering this question: electrons. Quantum rules dictate that no two electrons can occupy the same quantum state. The upshot of this is that, no matter how hard you try, you cannot cram two atoms together into the same space. “Electrons do all the work when it comes to the structure of matter we see all around us,” says physicist Sean Carroll at the California Institute of Technology in Pasadena. That’s not to say the nucleus is redundant. Most of the mass of an atom comes from protons and neutrons and the force binding them together, which is carried by particles called gluons. And that, essentially, is that. Electrons, quarks (mostly of the up and down variety) and gluons account for most of the ordinary stuff around us. But not all. Other basic constituents of reality exist too – 17 in total, which together comprise the standard model of particle physics (see illustration, below). The model also accounts for the mirror world of antimatter with a complementary set of antiparticles. Some pieces of the standard model are commonplace, such as photons of light and the various neutrinos streaming through us from the sun and other sources. Others, though, do not seem to be part of everyday reality, including the top and bottom quarks and the heavy, electron-like tau particle. “On the face of it, they don’t play a role,” says Paul Davies of Arizona State University in Tempe. “Deep down, though, they may all link up.” That’s because the standard model is more than a roll call of particles. Its foundations lie in symmetry and group theory, one example of the mysterious connections between reality and mathematics (see page 10). The standard model is arguably even stranger for what it doesn’t include. It has nothing to say about the invisible dark matter than seems to make up most of the matter in the universe. Nor does it account for dark energy. These are serious omissions when you consider that dark matter and dark energy together comprise about 96 per cent of the universe. It is also totally unclear how the standard model relates to phenomena that seem to be real, such as time and gravity. So the standard model is at best a fuzzy approximation, encompassing some, but not all, of what seems to comprise physical reality, plus bits and pieces that do not. Most physicists would agree that the standard model is in serious need of an overhaul. It may be the best model we have of reality, but it is far from the whole story. ■
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