The Pros and Cons of Protons
- Layla Abdoulaye
- 6d
- 7 min read
Updated: 3d
What do the Sun and a proton have in common? A surprising amount considering their immense difference in size. They both lie at the center of an incredibly complex system and understanding the inner workings of each have profound insights into how the universe works. While the source of the Sun’s power has long been discovered, physicists still struggle to unearth the mysteries of what goes on inside of the proton.
What’s the Matter With Matter?
Of the three particles that make up atoms: electrons, protons, and neutrons, we know the most about electrons because of their utility. Moving electrons are responsible for the electricity that powers our computers, and a buildup of electrons in clouds can create lightning bolts. But what do we know about protons beyond their positive charge to balance out an electron’s negative charge, and being heavy enough to contribute half of an atom's mass? Professor Axel Schmidt and his team at the Thomas Jefferson National Accelerator Facility are on a mission to find out. Specifically, they are researching how protons move inside a nucleus and potentially outside of one. Though originally believed to be effectively stationary and grouped together with neutrons, new research shows that they can actually be organized into groups (similar to electron shells) or even pair up with a neutron. A better understanding of the constituents of atoms creates a more solid foundation upon which scientists can make new, more applicable discoveries and technology.
A Microscopic Solar System
In the early 1900s, physicists were obsessed with creating a model of atoms that matched the behavior they observed through experiments. One such physicist, Neils Bohr, came up with one that is now most students’ introduction to the atom in a basic chemistry class today (source 1). Although it was later discovered to be completely inaccurate, it was determined that there is actually no way to accurately model an atom. However, the model is helpful to give people an idea of the concept of an atom.
Bohr’s model of an atom can be thought of as similar to the solar system. The nucleus is similar to the sun in that it is much larger and heavier than the electrons that move around it. The electrons can be compared to planets the way they orbit the nucleus and are grouped by the size of their orbit. Like the sun and its composition of mainly hydrogen and helium, the nucleus is made of protons and neutrons. The protons have an equal but opposite charge as the electrons to keep them close and orbiting the nucleus, while neutrons have no charge but are considered the glue holding the nucleus together and preventing the protons from repelling each other. It's important to note the two main forces at work here, because unlike the solar system, subatomic particles don't have enough mass to create strong gravitational pulls. Instead, there are the strong nuclear force and the electrostatic force. The strong nuclear force is responsible for keeping nuclei together and essentially attracts the protons to the neutrons, while the electrostatic force is what's responsible for attracting opposite charges and keeps the electrons orbiting the nucleus.
The Quirks of Quarks
Out of the three particles that make up an atom, only electrons are considered fundamental particles, meaning they cannot be broken down into smaller pieces. Protons and neutrons on the other hand, are made up of quarks. Theorized in 1964 and experimentally discovered in the late 60s, quarks are an integral part of the scientific standard model (the fundamental equations that describe how the universe works) (source 2). But unlike electrons, you can never get a single quark by itself. It must always be accompanied by at least one other quark. This has led to the discovery of an assortment of particles that come from the many different configurations of quarks such as mesons but none are as long lasting or stable as protons and neutrons. One of the most important qualities of quarks is spin. The concept of spin in quantum mechanics can be very abstract so for the purposes of this introduction to quarks we can think of it as similar to the rotation of the Earth. The direction of spin, noted as spin up or spin down, determines properties of the quark. Spin up quarks have a positive two-thirds charge while spin down quarks have a negative one-third charge. Therefore a proton is made up of two spin up quarks and one spin down quark in order to create one positive whole charge. A neutron likewise is composed of two spin down quarks and just one spin up quark to create an overall neutral charge. The existence of quarks was an exciting discovery for particle physicists because it opened the door to a lot of unknowns. For starters, the behavior and characteristics of protons and neutrons are actually those of a group of quarks.
The Subatomic Racetrack
So how do we study the building blocks of the universe? One of the most effective methods of studying subatomic particles is to launch an electron moving at close to 99% the speed of light (almost 670 million miles per hour!) into an atom and observe the collision. But that's simplifying an incredibly complex process.
First, we have to understand how to accelerate electrons to such unbelievable speeds. Since these electrons are so small, we can’t just attach a rocket to the back. Instead, physicists have come up with a clever method using electric fields and magnetic fields (source 3). Electric fields speed up the electron by using electrostatic forces to “push” it while magnetic fields are used to steer the electron because magnetic forces influence the direction of a charged particle.
Since the electrons are moving so fast, they end up travelling really far very fast. So in order to create efficient accelerators, they are circular instead of using a really long straight track to create an effectively endless path. But there is a cost to this racetrack-like design. When travelling in a circle, some of an object’s acceleration has to point inward to keep it from flying out of the circle. This is similar to holding onto the bars of a merry-go-round to counteract the pull that you feel as it spins faster. In the case of the particle accelerator, it creates a limit on how fast the electrons can go because of the energy it loses trying to stay in the circular path. A larger circle requires less effort to stay in, similar to how you would need to slow down to make a sharp turn versus staying at the same speed to go around a gradual bend when driving. So a larger particle accelerator would be able to make electrons go faster. However, that doesn't mean we could make an electron go infinitely fast if we had a big enough accelerator.
One of the most important rules in physics is that nothing can go faster than the speed of light. Often referred to as the universal speed limit, the speed of light is about 186,000 miles per second (source 4). So particle accelerators can make electrons go as close as they want to the speed of light (in some cases up to 99.9999%) but they’ll never quite get there.
Collisions between atoms are a useful type of experiment because we can piece together what was happening before the collision occurred. Going off of some of the fundamental concepts in physics such as conservation of energy and momentum, physicists are able to understand how fast the particles inside the atom were moving before the collision as well as what forces were acting on them and much more. This is how quantities like mass, size, and spin in the case of quarks, of particles are determined.
What's New With the Nucleus?
Out of the three constituents of atoms, the electron is the most familiar. It's relatively easy to isolate them and use them for all kinds of experiments. Protons and neutrons however are much more mysterious. Luckily, the nucleus of a hydrogen atom only contains one proton so although difficult, it is possible. Neutrons however, are too unstable to survive by themselves so they must be studied inside the nucleus. This creates a great challenge because particles in a nucleus are under the influence of various forces, such as the strong nuclear force, which will impact the results of experiments .
Dr. Axel Schmidt, a professor at George Washington University, has joined a long line of physicists before him attempting to uncover the behavior of protons and neutrons and more specifically, the behavior of the quarks inside them. At George Washington University's Thomas Jefferson Lab, there are 2 small particle accelerators. One of which Dr. Schmidt uses to run experiments that can help answer this question: How do the quarks inside protons and neutrons behave under the strong nuclear force?
Dr. Schmidt and his team are approaching this problem using the idea that most protons and neutrons act normally in a nucleus and just a handful of them have unexpected behavior. This would explain why it has been so hard to identify a cause for the discrepancies - because they don’t occur with every particle. Instead, it’s proposed that protons and neutrons can form a short range correlation where the particles get so close together that their constituents start to interfere with each other. The effects of this correlation would largely impact the behavior of the correlated particles in a collision and since the experiment averages the motion of the particles in the collision, this would impact the average velocity and make it appear like all the particles are acting differently than expected.
To test this theory, Dr. Schmidt and his team have developed an experiment that can shed light on the interactions between protons and neutrons. In the experiment, electrons are accelerated to 99.99% the speed of light and then shot into a tube of liquid deuterium. Deuterium is a hydrogen isotope with a nucleus made up of one proton and one neutron, as opposed to a typical hydrogen atom that only consists of one proton at its center. Although this is currently an ongoing experiment, Dr. Schmidt’s team has seen promising results. And you can learn more at the Jefferson Lab homepage here.
While we won’t be seeing drastic improvements in technology in the upcoming years from this experiment, the results could have interesting implications for future experiments and expand our knowledge of quarks, neutrons, and protons. Oftentimes, the applications of research take decades to be integrated into everyday life. Dr. Schmidt’s work at the Jefferson lab is to help bridge a knowledge gap that could help lay the foundation for a multitude of technological advancements.
"What we're trying to understand [is] essentially the way this strong nuclear force works, and better understanding that will help us to build the next generation of technologies."
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