How to look inside a proton

How to look inside a protonA proton has a diameter of just 1 femtometer, or 0.000000000000001 meters. This is around 100,000,000,000 times smaller than this dot:


So, looking inside protons requires great ingenuity.

In normal life we can use visible light to study objects. If we shine a light on something, the light bounces off of the object and reaches our eyes, enabling us to see the object. If something is really small, then we can use a microscope to magnify the reflected light, helping us to see the object in more detail. However, there is a physical limit of how small visible light microscopes can see.

Visible light has a wavelength of around 400 to 700 nanometers (0.0000004 to 0.0000007 meters), depending on the color of the light. The proton is around 100,000,000 times smaller than the wavelength of visible light, which means that we cannot use visible light for examining protons. Physically, it just cannot work and standard laboratory microscopes are therefore no use for looking inside protons. To look inside a proton, a much more powerful, considerably larger and quite different type of microscope is needed.

How to look inside a proton

Scientists look inside protons using a technique called Deep Inelastic Scattering, or DIS for short. DIS involves making a proton collide with another particle in order to make the proton break up. Then, by detecting the fragments of proton that are produced in this collision, scientists can learn about the internal structure of the proton, or, put another way, what the proton is made of.

The «Deep» in DIS refers to the fact that the scientists are looking deep inside the proton, not just at its «surface».

«Inelastic» means that the particle that is used to scatter off of the proton does not simply bounce off of the proton. Rather, the proton is actually broken up during the interaction.

«Scattering» means that another particle is used to scatter off of the protons, causing them to break up and reveal their deeper structure.

The scattering particles that are used to break up the protons are usually electrons or positrons (the anti-particle of the electron). However, sometimes muons (a heavier version of the electron) or neutrinos (a massless particle with no electric charge) are also used.

The important point to note is that all of the scattering particles (electrons, positrons, muons or neutrinos) are fundamental particles and therefore cannot be broken up themselves during the scattering process. This


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