Science has progressed exponentially over the past few centuries, but still, nobody’s quite completely certain what happens inside of an atom. The good news is that two rival teams of scientists believe that they have figured it out, and they are both working hard to demonstrate that their own version of the concept is the correct one.
In order to form a base for this article, we are going to introduce you to electrons. Electrons are particles that can be found in atoms, whizzing around in so-called “orbitals” (which are just trajectories around the nucleus where it’s most likely to find an electron). There is a gap, a whole lot of empty space separating the orbitals from the nucleus, which is a tightly packed knot of neutrons and protons that make up most of the mass of an atom. Protons and neutrons cluster together thanks to a force called “the strong force”. The number of neutrons, protons, and electrons is important because they determine the chemical properties of an atom: Carbon has a certain number of protons and neutrons while oxygen has a different number. Also, knowing the number of protons/neutrons inside a nucleus helps determine whether the atom is radioactive and stable.
However, facts stop here mainly because nobody knows how protons and neutrons (referred to nucleons) behave inside an atom. Outside of an atom, protons have various sizes and shapes. Each and every one of them consists of three smaller particles which are named quarks, and the interaction between quarks is so strong that no external force should be capable to deform them, not even forces as powerful as the ones between particles in a nucleus. However, scientists have kept speculating that this theory is wrong to some extent for decades now. Some experiments proved that, inside a nucleus, protons and neutrons seem much bigger than they should normally be. Physicists have formulated two competing theories that try to explain the weird discordance, and the proponents of each theory are positive that the other is incorrect. However, both camps agree that, regardless of the correct answer, it must all originate from a field that is beyond theirs.
For over 80 years physicists have known that nucleons travel in small orbitals within the nucleus, according to Gerald Miller, a nuclear physicist at the University of Washington. The nucleons, which are deeply confined in their movement have a very little amount of energy and they don’t bounce around too much because of the strong force restraining them.
CERN physicists discovered in 1983 that some electron beams bounced off iron in a different way than they did from free protons, Miller said. This was unusual because protons inside hydrogen were similarly sized with protons inside iron, meaning that electrons were supposed to bounce off in the same way.
At first, this puzzled researchers. However, time passed and they acknowledged the issue: They discovered the EMC effect that makes neutrons and protons inside of heavy nuclei act like they are considerably larger than when they are outside of the nuclei.
One possible explanation is the fact that quarks are pretty limited in the matter of movement because of the strong force acting upon them.
However, it was proven that, at any given time, about 20% of the nucleons in a nucleus are outside their orbitals, pairing with other nucleons and interacting in “short-range correlations”, making the interactions between the nucleons much higher-energy than usual, ultimately breaking down the walls separating quarks inside individual protons or neutrons.
The quarks that make up a proton and those making up another proton begin occupying the very same space, causing a stretch and a blur of protons or neutrons, accordingly, physicist Or Hen says. Quarks grow a lot, albeit for a small duration, skewing the average size of the complete cohort inside the nucleus, thus resulting in the EMC effect.
The EMC effect
Most physicists acknowledged and accepted this interpretation of the EMC effect, according to Hen.
However, not everybody believes that Hen’s team figured out the whole scheme. Ian Cloët, a nuclear physicist at Argonne National Laboratory in Illinois, thinks that the conclusions provided by Hen’s work aren’t fully backed up by experimental data.
Cloët believes that the EMC effect is still unresolved because the standard model of nuclear physics already takes into account for a lot of the short-range pairing described by Hen’s work.
Cloët suggests that “if you use that model to try and look at the EMC effect, you will not describe the EMC effect. There is no successful explanation of the EMC effect using that framework. So in my opinion, there’s still a mystery.”
It ultimately might all be explained by QCD (quantum chromodynamics), the set of rules that dictate quarks’ behavior. Unfortunately, QCD is way more complicated than nuclear physics: At the moment, the complete QCD equations that describe all the quarks inside a nucleus are way too difficult to solve, as Hen and Cloët agreed. Not even a quantum computer could help solve the equations in their current state.