How do we know any of this?

-To test theories, physicists experiment and use what they already know to find out things that they don't know.

The 1900's Conceptions of the Atom

In the 1900's, it was believed that atoms were mushy, semipermeable balls with little bits of charge strewn around. However, in 1909, a man named Ernest Rutherford set up an experiment to test this theory.

Rutherford shot a stream of Alpha Particles at a sheet of very thin gold foil in front of a screen. The particles would make little flashes of light where they hit the screen. The particles were expected to pass through the gold foil and make their marks in a cluster (atoms were expected to be permeable, neutral balls). However, some of the alpha particles were deflected at large angles. So, Rutherford concluded that there must be something inside an atom for the particles to bounce off of, something small, dense, and positively charged (the nucleus).

Today, most particle physics experiments use the same basic elements that Rutherford used: a beam, a target, a detector.

Light

Light is actually made up of small particles called photons, which also have wave characteristics. We use light to perceive the world around us, but any kind of reflected wave can do the job (bats and echolocation).

The problem with using waves to detect the world around you is that the image quality is limited by the wavelength you use. To see things in higher magnification, you must use waves with smaller wavelengths. When you use smaller wavelengths, you get a sharper image--a higher resolution.

Rough rule of thumb: a particle can only probe down to distances equal to the particle's wavelength. To probe down to smaller scales, the probe's wavelength must be made smaller.

The Accelerator

Since all particles have wave properties, physicists can use particles as their probes (light waves are too long).

A particle's momentum and wavelength are inveresely related (short wavelength = high momentum, long wavelength = low momentum.

Physicists can then use particle accelerators to increase the momentum of a probing particle, thus shortening the wavelength.

Return to Stanford.