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The particle accelerators used by physicists are not as remote from our everyday experience as one might imagine. The cathode ray tubes of televisions and computer monitors, commonly known as picture tubes, are in fact small, low-energy particle accelerators, creating beams of electrons guided and focused by magnets that hit a phosphorescent screen to produce light. The electrons, having an electric charge, are accelerated by an electric field produced by a voltage difference of about a thousand volts. Accelerating electrons to higher velocities, using voltages in the tens of thousands, allows higher-energy radiation to be released; the x-ray tubes used in diagnostic imaging operate on this principle. Today's high-energy particle accelerators, such as synchrocyclotrons and synchrotrons, accelerate charged particles such as electrons and protons using the same basic principles as ordinary picture tubes, but to much higher velocities. These machines are ring-shaped, often extremely large (some more than ten miles in length), and they accelerate particles to velocities so close to the speed of light that the effects of relativity, such as time dilation and increased particle mass, become important factors. For theoretical physicists, these high speeds are generated to smash the particles against other particles as hard as possible—just like smashing a rock against a wall—just to see what happens. For example, particles once thought to be elementary, like protons, have been shown to consist of yet smaller constituents (quarks, in this case) by observing the scattering patterns that follow certain collisions. A large variety of exotic particles have been created as well in the shower of particles that result from some collisions, and explaining their existence and behavior has deepened theories of fundamental physics. From the explosive aftermath of these artificial high-energy particle collisions, robust theories of the most fundamental constituents of the natural world are being developed.