Atom Smashers
The Particle Zoo

Photograph by: Peter Ginter, National Geographic March 2008

The Greeks thought the atom was the smallest thing in the universe; the atom was indivisible. Scientists have since smashed that assumption to pieces—literally. In the past century physicists have discovered hundreds of particles more minute than an atom (subatomic particles). Those that cannot be further broken down are called fundamental, or elementary, particles. For example, matter is composed of molecules that are made up of atoms that are made up of protons, neutrons, and electrons. While protons and neutrons can be broken down into fundamental particles known as quarks and gluons, electrons are themselves fundamental—at least for now. It’s always possible that as physicists deepen their understanding of the universe and wield more powerful technology they will discover an even tinier unit underlying the universe. Fundamental particles make up not only matter but also antimatter (in the form of antiparticles) and the particles that carry forces between other particles (e.g., photons mediate the electromagnetic force; gluons mediate the strong force).


Achenbach. Joel. “At the Heart of All Matter.” National Geographic (March 2008), 90-105.

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Stanford Linear Accelerator Virtual Visitor Center.

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The Subatomic Demolition Derby

Physicists use accelerators to smash subatomic particles together to find the smallest building blocks of the universe. These miles-long devices hurl charged subatomic particles such as protons, electrons, and positrons (antiparticles of electrons) at each other at nearly the speed of light to cause them to collide with immense energies in hopes of producing exotic particles in the resulting debris. Particle collisions can create other particles because energy equals mass (as Albert Einstein so famously noted in 1905 when he introduced the equation E=mc2). Two masses (particles) collide, creating a burst of energy, and then condense into other masses (particles).

What’s the point of spending billions of dollars on accelerators to see these particles? The exotic particles that physicists hunt are rare because the universe has cooled significantly since its earliest moments, when temperatures and energies were trillions of times higher than they are today. Also, many exotic particles are unstable and exist for such a short time (some less than a trillionth of a trillionth of a second) that they disappear before we could even start looking for them in nature. In a laboratory scientists and engineers can manufacture and control high temperatures and energies to create new particles. Massive detectors help them sift through a forest of particles to find their exotic quarry.

Below is a list of laboratories around the world conducting high-energy physics experiments with particle accelerators.

CERN (European Organization for Nuclear Research)
LHC (Large Hadron Collider), proton-proton and lead-ion collider, 17-mile ring, highest-energy accelerator once up and running in 2008

Fermi National Accelerator Laboratory]
Tevatron, proton-antiproton collider, 4-mile ring, currently the highest-energy accelerator in the world

Stanford Linear Accelerator Center
PEP II, electron-positron collider, 2-mile linear accelerator

Brookhaven National Laboratory
RHIC (Relativistic Heavy Ion Collider), gold-ion collider, 2.4-mile ring

Thomas Jefferson National Accelerator Facility
CEBAF (Continuous Electron Beam Accelerator Facility), electron beam focused on target (vs. colliding beams) (targets: hydrogen, carbon, gold, lead), 7/8 mile track using two linear accelerators

Cornell University
CESR (Cornell Electron Storage Ring), electron-positron collider, 0.5-mile ring

KEK B, electron-positron collider, 2-mile ring

The Hunt

Physicists use particle accelerators not only to find the smallest building blocks of the universe but also to shed light on the biggest questions: What is the universe composed of? What laws govern it? How did it come to be? Below is a list of what they’re looking for.

The Higgs boson
One question physicists hope to answer is why or how particles have mass. They postulate that there’s a space-filling field that imbues fundamental particles with mass by interacting via a special particle called the Higgs boson. Finding the Higgs would help them understand mysteries of mass, such as why protons are heavier than electrons and why photons have no mass at all.

Why matter and not antimatter?
Theoretically, the big bang should have produced equal amounts of matter and antimatter that annihilated each other, leaving a virtually empty universe. So why is our universe almost exclusively matter?

Dark matter and dark energy
The stars and galaxies that we can see count for only 4 to 5 percent of the universe. The rest is dark matter (20 to 25 percent) and dark energy (70 to 75 percent). Physicists hope accelerator experiments will shed light on the nature of these dark phenomena.

Supersymmetry theorizes that all particles have heavier counterparts with the same electrical charge but different spin (an intrinsic property of particles). No superparticles have ever been seen, but if they exist, the LHC should be able to produce one, and many physicists suspect that a superparticle could be the basic component of dark matter.

Superstrings and other dimensions
String theory tries to unify physics by explaining all particles and forces as vibrations of one-dimensional strings; it also predicts that space has six or seven more dimensions than we know about. Strings are too small for current particle accelerators to detect, but physicists hope to find indirect evidence of their existence, such as superparticles, particles disappearing into other dimensions, or disturbances in the behavior of ordinary particles.

Quark-gluon plasma
In the first few microseconds after the big bang, the universe was permeated by a state of matter called the quark-gluon plasma. The temperatures and pressures were so high that quarks and gluons were able to exist independently of each other. At the much lower temperatures and pressures that we experience in the universe today, quarks and gluons do not exist freely, rather they are bound together in trios that make up protons and neutrons. Physicists use particle accelerators to create quark-gluon plasma to investigate its properties and learn more about the early universe and the cores of neutron stars.

Why are there so many particles?
Physicists have discovered hundreds of particles, including 57 fundamental particles. Some physicists are suspicious that it takes so many particles to describe the universe, and they theorize that there must be an underlying simplicity to the particle zoo. String theory is one such attempt to find order in the zoo.

In the mid-1800s James Maxwell realized that electricity and magnetism were not separate phenomena but rather aspects of the same force: electromagnetism. In the 1970s the standard model of physics, which still reigns today, showed that the electromagnetic and weak forces combine to form the electroweak force. There are four fundamental forces of nature: electromagnetic, weak, strong, and gravitational. Many physicists wonder if the strong force and gravity can’t also be combined with the electroweak force to effect a unification of forces. This idea is known as the Grand Unification Theory, or GUT, and physicists hope particle accelerators will help them find evidence of unification.


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Other Resources
Center for History of Physics, American Institute of Physics.

Timeline of high-energy physics

Achenbach, Joel. “The Power of Light.” National Geographic (October 2001), 2-31.

Fox, Karen, and Aries Keck. Einstein: A to Z. John Wiley and Sons, 2004.

Krauss, Lawrence. Atom: An Odyssey From the Big Bang to Life on Earth … and Beyond. Little, Brown and Company, 2001.

Lineweaver, Charles, and Tamara Davis. “Misconceptions About the Big Bang.” Scientific American (March 2005), 36-45.

Park, David. The Fire Within the Eye: A Historical Essay on the Nature and Meaning of Light. Princeton University Press, 1997.

Trefil, James. The Nature of Science: An A-Z Guide to the Laws and Principles Governing Our Universe. Houghton Mifflin, 2003.

Tyson, Neil DeGrasse, and Donald Goldsmith. Origins: Fourteen Billion Years of Cosmic Evolution. W. W. Norton and Company, 2004.

Last updated: February 5, 2008

Keywords: particle physics, particle accelerator, Higgs, LHC, CERN, dark matter, dark energy, supersymmetry, string theory, antimatter, quark, subatomic, collider