Physicists Just Solved a 35-Year-Old Mystery Hidden Inside Atomic Cores

Here’s a mysterious truth that scientists have known since 1983: Protons and neutrons act differently when they’re inside an atom, versus floating freely through space. Specifically, the subatomic particles that make up those protons and neutrons, Iphone Cases, slow down massively once they’re confined to a nucleus in an atom.

Physicists really didn’t like this, because neutrons are neutrons whether they’re inside an atom or not. And protons are protons. Both protons and neutrons (which together make up the class of particles called “nucleons”) are made up of three smaller particles, called quarks, bound together by the strong force.

“When you put quarks into a nucleus and LG Cases, they start to move slower, and that is very weird,” said study co-author Or Hen, a physicist at the Massachusetts Institute of Technology. That’s strange because the powerful interactions between quarks mainly determine their speed, whereas forces that bind the nucleus (and also act on quarks inside the nucleus) are supposed to be very weak, Hen added.

And there’s no other known force that should be modifying the behavior of quarks in a nucleus so intensely. Yet, the effect remains: Particle physicists call it the EMC effect, named for the European Muon Collaboration, the group that discovered it. And until recently, scientists weren’t sure what caused it. [The Biggest Unsolved Mysteries in Physics]

Two particles in a nucleus are typically pulled together by a force of around 8 million electron volts (8 MeV), a measure of energy in particles. Quarks in a proton or neutron are bound together by about 1,000 MeV. So it doesn’t make sense that the comparatively mild interactions of the nucleus are dramatically impacting the powerful interactions inside quarks, Hen told Live Science.

“What is eight next to 1,000?” he said.

But the EMC effect doesn’t look like a mild nudge from an outside force. Though it varies from one sort of nucleus to the next, “It’s not like half a percent. The effect pops out of the data once you are creative enough to design an experiment to look for it,” Hen said.