How do we know what we know?

Particles like protons, electrons, and force carriers are too small to see with even our most powerful microscopes. So how do we know that such particles exist and what they're like?

The Particle Family Album

photos: CERN

These images, which reveal tracks of various types of particles, help physicists prove that particles exist. Scientists can learn how particles interact by studying the relationships between different particle tracks in each image.

While we can't see the particles themselves, physicists have designed ingenious experiments that allow them to see the paths, or tracks, of moving particles. Just as skid marks on a road can tell you about a car's behavior just before an accident, particle tracks tell scientists a lot about how the building blocks of matter behave.

In fact, particle tracking has allowed physicists to identify more than a hundred different kinds of particles and learn important information about them -- such as their size and mass, how they interact with other particles, and their role in the universe.

One experiment commonly used to see particle tracks involves a special container filled with alcohol vapor called a cloud chamber. When some particles pass through this chamber, they make alcohol vapor molecules stick together, leaving a track of condensed alcohol.