ergetic than we are aware of in daily life, the deep connection between the two realms inspires researchers in elementary-particle physics and lends added significance to their investigations. In fact, the properties and interactions of the elementary particles have much to say about the properties of the world around us.

A century ago, the first elementary particle-the electron-was identified. A revolutionary view of the way matter in the universe is put together was provided by experimental evidence that electrons were basic constituents of all atoms and that they carried electricity. The theory of quantum mechanics explained the paradoxical motion of electrons in the atom and the formation of molecules. Eventually, a vast range of phenomena-the stiffness of steel, the way gasoline burns, the colors in the surface of a bubble, the ability of x rays to reveal tumors inside the body-could be accounted for by the quantum mechanical behavior of the electron. This new perspective, a view of the world on a more fundamental scale than the everyday world of our experience, led to a century of spectacular science and technological innovation. Understanding the behavior of the electron and the photon (the quantum of light) has been critically important to the fields of chemistry, materials science, and biology, as well as to the development of modern computing and communication.

Particle physicists further zoomed in on the subatomic realm with increasingly powerful instruments. Forces were revealed on the subatomic level that no one had predicted, the best example being the strong nuclear force that holds the atomic nucleus together. Experiments revealed the existence of hundreds of different-and unexpected-particles. Eventually patterns emerged and theories were put together and tested; today, elementary-particle physics provides the basis for understanding an astonishing variety of phenomena-including those in our daily lives-in terms of just a few truly elementary particles and the forces between them.

The remarkable state of our understanding of elementary particles, embodied in the present theory called the ''Standard Model," has taken shape over the last 30 years. The Standard Model provides an organizing framework for the known elementary particles. These consist of "matter particles," which are grouped into "families," and "force particles." The first family includes the electron, two kinds of quarks (called "up" and "down"), and a neutrino, a particle released when atomic nuclei undergo radioactive decay. There are two more families consisting of progressively heavier pairs of quarks and a corresponding lepton and neutrino. All normal, tangible matter is made up only of particles from the first family, since the others live for very short times. Why are there three families? This question is one of the great unsolved mysteries of elementary-particle physics.

The matter particles exert forces on one another that are understood as resulting from the exchange of the force-carrying particles. Electric and magnetic forces arise when particles exchange photons (the familiar repulsion or attraction



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