"Atom smasher" is a colloquial term for a particle accelerator, a device that accelerates elementary particles (protons, electrons, and into each other at significant fractions of the speed of light to probe their inner makeup. Physicists have been building particle accelerators since 1931, when a 9-inch cyclotron atom smasher was built at the Berkeley Radiation Laboratory in Berkeley, California. Particle accelerators are key tools used by physicists to probe the structure of space, time, and matter. Instead of exploring outer space like telescopes, particle accelerators explore the "inner space" of the atom.
The latest and greatest atom smasher is the $5 billion US Dollars Large Hadron Collider (LHC) beneath the Franco-Swiss border near Geneva, Switzerland. This device, which is one of the most expensive and largest technological constructs ever built, lies in a tunnel 27 km (17 mi) in circumference, up to 175 m (570 ft) underground. It accelerates particles up to 99.9999991% of the speed of light, smashing them together with an energy many billions of times greater than those experienced by typical particles in the air or earth colliding with one another due to thermal excitation. The energy of collisions in a particle accelerator is given in electron volts, which says how many electron-equivalents the energy of the collisions is.
Atom smashers have historically built to answer questions like, "What is the behavior of antimatter?" "Which smaller particles make up the atom?" "What are the precise characteristics of the forces between and within these atoms?" and "What fundamental particles exist?" Using an atom smasher, physicists have learned huge amounts about the fundamental structure of matter, which can found applications in numerous technological fields, including our understanding of nuclear power and how to make maximally efficient solar cells.
The most modern atom smasher, the LHC, was built to answer questions like, "How do particles get their mass?", "Are electromagnetism and the strong and weak nuclear forces different manifestations of the same underlying force?", "Why is gravity so weak in comparison to the other fundamental forces of nature?", "Why are there apparent violations of the symmetry between matter and antimatter?", and "What is dark matter and dark energy?" These questions perplex many physicists and are at the forefront of the field. Only with these huge particle accelerators can physicists recreate conditions as the universe existed a fraction of a second after the Big Bang, shedding light on these difficult questions.