Physicists Found a New Clue That Could Reveal the Fifth Force

Here’s what you’ll learn when you read this story:

The Standard Model of Particle Physics accounts for four fundamental forces—strong, weak, electromagnetism, and gravity—but for decades, scientists have wondered if an elusive fifth force might be at work.
A new study analyzing the atomic transition of five calcium isotopes constrains the mass of a particle that would carry such a force from somewhere around 10 to 10 million electronvolts.
It’s still possible that these anomalies could be explainable via the standard model.

The Standard Model of Particle Physics is a scientific masterpiece, but even so, it remains unfinished. For example, we still don’t know why there is matter at all (a.k.a. matter-antimatter asymmetry), and then there’s the whole dark matter and dark energy thing.

Another source of some scientific quandary is whether there might be a fifth fundamental force. You might be familiar with the standard four—the strong force, the weak force, gravity, and electromagnetism—but some physicists wonder if a fifth force that couples together neutrons and electrons could also be at work throughout our universe. Now, an international collaboration of scientists from Germany, Switzerland, and Australia have discerned the upper limit of a particle that could carry such a force by looking at transition frequencies of five calcium isotopes. Those masses were penciled out to around 10 to 10 million electronvolts (yes, electron volts are sometimes used as mass measurements—thanks E=mc²). The results of the study were published in the journal Physical Review Letters.

To arrive at this number, the researchers observed the atomic transitions of calcium-40, calcium-42, calcium-44, calcium-46, and calcium-48. An atomic transition occurs when an electron—attracted to the positively charged particles in a nucleus—briefly jumps to a higher energy level. These atomic transitions can vary based on the isotope and are influenced by the number of neutrons present in an atom.

Once the observations were complete, the authors mapped the variations they recorded on what’s called a King plot. According to the Standard Model, this should produce a linear plot. However, that is not what the study found. Due to the high sensitivity of the experiment, the plot ended up being nonlinear, suggesting that the deviations detected by the team could be evidence of a fifth force.

That said, as the authors also note, it could also be attributable to something that is explainable within the Standard Model. However, whatever was causing these deviations, it didn’t detract from the scientists’ ability to set the upper limit of what the mass of the fifth-force boson might be.

The search for this fifth force is a long one, and it’s a scientific endeavor that’s cast quite a wide net. For a while in the 1980s, scientists at MIT thought antigravity could be a fifth force, and another idea known as “quintessence” gained popularity at the turn of the century. Recently, Fermilab in Chicago thought that they might be closing in on a fifth force, though their final results of the “muon g-2” experiment largely confirmed the standard model.

Other efforts have looked at much larger bodies than just atoms for evidence of the fifth force. Los Alamos National Laboratory published a study last year suggesting that by closely analyzing the orbits of asteroids and sussing out any deviations of those orbit, we could learn something about particle forces we don’t understand. That team’s ultimate aim, much like that of the team behind this new paper, was to understand the constraints on where this fifth force might reside.

For now, the search continues, but scientists are taking more and more steps toward a physics-altering answer.

You Might Also Like