By Don Lincoln, Fermilab
Depending on some choices of which advances are important, the 1920s and the 1930s were the era where we worked out the details of atoms. However new technologies were becoming available that allowed scientists to push even deeper into our understanding of the microcosm.
There was the other way scientists investigated subatomic matter. In 1912, Austrian physicist Victor Hess discovered that radiation was hitting the Earth from space called the cosmic rays.
By the 1930s and 1940s, scientists had dragged detectors to the tops of mountains to better study this constant radioactive rain from the heavens. They certainly saw things like protons, neutrons, and electrons, but they discovered things that don’t generally exist on Earth, new kinds of particles that aren’t found in atoms.
Fundamental Forces of Nature
By the early 1930s, scientists knew of four different forces: electromagnetism and gravity, but also the strong and weak nuclear forces. Gravity is too weak to matter in the subatomic realm, so there’s three that matter, and the different particles each interacted with a different combination of these forces.
The proton and electron both have electrical charge, while the neutron is electrically neutral. That means that the proton and electron feel the electromagnetic force, while the neutron doesn’t. The proton and neutron are tied together tightly in the center of atoms, which means that both particles experience the strong nuclear force.
On the other hand, the electron doesn’t experience the strong force. That’s why the electron can zoom around the nucleus of the atom at large distances. It turns out that all three particles feel the weak nuclear force, but there are particles that don’t. For instance, the photon, which is the particle that transmits ordinary light, is impervious to the weak nuclear force.
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Over the course of the years, many new particles were discovered that were broadly like protons and neutrons. Protons and neutrons have about the same mass and they were clumped together into a class of particles that we call baryons, which comes from the Greek word barus, which means heavy.
But the protons and neutrons weren’t alone. There were other baryons, with exotic names, generally using Greek letters, like the Delta and Xi, the Sigma and the Lambda, but also the N, and the interestingly named Cascade.
There were patterns in the particles properties that suggested that they were governed by a common set of rules and that the rules allowed certain types of baryons to exist and furthermore that those rules forbid other baryons to exist.
In 1960, scientists had discovered many different baryons, but had no concrete idea about what was going on. Researchers knew of many baryons, basically as if they discovered lots of Rubik’s Cubes, but didn’t know that the cubes could twist. And then, two guys realized that the cubes could twist.
In 1964, Murray Gell-Mann and George Zweig independently looked at all of the baryons that had been discovered and realized that they could explain everything—both the baryons that they knew about and why other baryons hadn’t been discovered—if the proton and neutron were made up of even smaller particles that we know call quarks.
If their idea was right, then the proton and neutron were each made up of two different kinds of quarks, with the confusing names up and down. Both proton and neutron each contained three quarks, with the proton containing two up-type quarks and one down-type quark, while the neutron contained one up-type and two down-types. Other baryons had different configurations of quarks, like one had three up-type quarks, while one had three down.
In order to explain the patterns in all of the observed baryons, Gell-Mann and Zweig actually had to propose yet a third type of quark, with the name strange (of all things). But strange quarks aren’t found in protons and neutrons. In fact, since 1964, researchers have found another three, bringing the total number of quarks up to six.
Testing the Quark Hypothesis
First, you need a bunch of protons—a container of hydrogen. Each hydrogen atom contains one proton and one electron and the two particles have hugely different masses, so it should be easy to distinguish the two. The second thing is you need a beam of particles that you can accelerate to high energies and easily manipulate.
So, in 1968, researchers at Stanford Linear Accelerator Center took a beam of electrons and bashed them into protons to see what they could see. Now, if protons have anything inside them, the first thing is to find out that they have a size.
Findings from the Test
Well, remember that the proton and electron both have an electric charge, which means that they feel an attractive force. Shoot an energetic electron in the vicinity of a stationary proton and the electron will be deflected as it passes the proton. The behavior of electric forces is well understood in this situation; if an electron passes very near a proton, it feels a very strong force and is deflected a lot. If it passes far from a proton, it is deflected only a little.
Furthermore, there is a lot of space that is far away from a proton and only a little that is close. What the Stanford scientists found is exactly what the theory of electromagnetism predicted until they started looking at very strong deflections, ones that would only occur if an electron passed within one quadrillionth of a meter of a proton (that’s ten to the minus-15th meters, or a femtometer).
For less violent deflections caused by long-distance interactions, the data and prediction agreed perfectly. For deflections caused by close interactions, the data and prediction disagreed, and the only sensible explanation was that the proton is a little sphere with a radius of a femtometer. If the electron passes farther away than a femtometer, its passage is predicted by basic electric force theory. If it comes closer than that, the electron literally bounces off or passes through the proton.
Common Questions about the Discovery of Baryons and Quarks
By the early 1930s, scientists knew of four different forces: electromagnetism and gravity, but also the strong and weak nuclear forces.
In 1964, Murray Gell-Mann and George Zweig independently looked at all of the baryons that had been discovered and came up with the concept of quarks.
Protons and neutrons have about the same mass and they were clumped together into a class of particles that we call baryons.