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If we cannot see electrons and protons -- or smaller particles such as quarks -- how can we be sure they exist?

Stephen Reucroft and John Swain, professors of physics at Northeastern University in Boston, Mass, teamed up to provide this answer:

The central concept here is what we mean by "see." Normally when we say that we "see" an object, what we mean is that we detect with our eyes particles of light called photons, which come from some source like a lightbulb or the sun and bounce off the object.

The idea of being able to "see" things by observing particles that scatter from them is common to particle physics experiments that study tiny objects like electrons, protons, and quarks (out of which protons are made).

For a classical physics picture of how this works, you might imagine being in a large dark room with an object whose shape you don't know. If you have a bucket of tennis balls, you can start to build up a picture of what the object looks like by tossing the balls at it. Instead of tennis balls, particle physicists use small particles, such as electrons at very high energies. Recalling Albert Einstein's famous formula, E=mc2, lots of energy can be traded for a little bit of matter.

So when we use very high energies to see deeply into matter a new phenomenon can occur: not only do we see what's there, but we create matter in the form of new particles. This gives us yet another way to study the structure of the world at its most profound level.

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