Protons are tiny. A proton has a diameter of just 1 femtometer, or 0.000000000000001 meters. This is around 100,000,000,000 times smaller than this dot:
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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.
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 means that only the proton is broken up and makes analyzing the fragments produced decidedly simpler.
So, how do scientists make electrons collide protons?
The answer is that they use a special machine called a particle accelerator. This is a large underground circular machine with a circumference of several kilometers. The protons are accelerated around the machine in one direction and the electrons in the opposite direction. The protons and electrons are accelerated using really powerful magnets up to speeds close to the speed of light.
Then, at a predetermined position, the protons and electrons are made to collide with each other. The protons and electrons have to be traveling so fast in order for them to have sufficient energy for the electrons to actually break up the protons, rather than just bounce off of them.
When the protons are broken up in this way, the fragments produced spray out in all directions. Scientists then use special detectors located around the point of collision to allow them to see what comes out of the broken up proton and thereby learn about the proton’s structure.
Each time a collision happens the proton breaks up slightly differently and different fragments are produced. This means that in order for scientists to gain an accurate and detailed picture of the internal structure of a proton, they need to repeat the collisions and detect what happens many billions of times.
Luckily, particle accelerators can be filled with millions of electrons and protons, producing collisions many many times a second. Even so, these experiments need to run for several years in order for scientists to collect sufficient information to fully understand what the inside of a proton looks like.
Today, several such experiments have been running for many years, in locations all over the world. Together, these experiments have enabled scientists to build up an incredibly detailed and accurate picture of the rich and complicated internal structure of protons.
