Scientists may have discovered a fifth fundamental force of nature. But it's an extraordinary claim requiring extraordinary evidence. What does it all mean?
In physics, we know of four fundamental forces that explain our interactions with the world around us. Research may be getting us closer to naming one more. But what does all this mean?
The four fundamental forces
These are the four known forces of nature.
- Gravity keeps our feet on the ground and our planet in orbit around our star. But while gravity operates over infinite distances—, and so governs the motions of black holes and entire galaxies—it is also by far the weakest of the four forces.
- The strong force holds atomic nuclei together. Just as its name suggests, it's ~1038 times stronger than gravity—that’s 1 with 38 zeroes. However, it only operates, over a range of a femtometer, or about the size of one of those nuclei.
- The weak force, which is still 1 million times stronger than gravity, is responsible for the radioactive decay of atoms.
- The electromagnetic force governs the ability of charges to attract and repel each other.
Each force is transmitted by a messenger particle, a type of elementary particle called a boson, that makes its presence known. For the electromagnetic force, this so-called force carrier particle is the light particle called the photon. Gluons are the force carrier particles that “glue” nuclei together via the strong force. The weak force has three known carrier particles called W and Z bosons.
The Standard Model, or the theory that pulls all of these pieces together to explain how matter and the four fundamental forces interact, works well to explain much of our universe as we observe it. The theory has been rigorously tested and is supported by a multitude of observations, but there are still a few missing pieces needed to make this puzzle complete. For starters, the graviton, the carrier particle for gravity, is purely theoretical. So far, it hasn't been found. The Standard Model also doesn’t explain dark matter, the as-yet-undetected substance that makes up 27 percent of the universe.
Theorists have proposed additions to the Standard Model, like the existence of what they call “dark photons,”exotic particles that could act as carrier particles for a new force that affects only dark matter. These dark photons are what Dr. Attila Krasznahorkay at the Institute for Nuclear Research at the Hungarian Academy of Science and his collaborators were after when they instead found what some are calling evidence for a possible fifth force.
Fifth force déjà vu
Now, this isn’t the first time Dr. Krasznahorkay has been in the news. Back in 2016, he and collaborators published the results of another dark photon search.
Here’s how it works. The number of protons in an atomic nucleus determines what element you have, and each element tends to have a standard number of neutrons. But an atom can sometimes lose a few neutrons or pack in a few extra. Those neutron-light or neutron-heavy atoms, called isotopes, can be unstable, meaning they decay easily back into the more stable form of the element with the standard number of neutrons. In that decay the process, they tend to emit other particles.
So in the Hungarian lab, the scientists created unstable nuclei so that those nuclei would then decay, potentially forming new particles in the process. Back in 2016, they created beryllium-8 nuclei—those are beryllium nuclei that are short a neutron —with a proton accelerator by aiming high-speed protons at a thin layer of lithium-7. The unstable beryllium-8 then quickly decayed into electron-positron pairs.
This new boson didn’t seem to be the dark photon they were looking for—it doesn't have the right properties—but it was a potential new particle nonetheless.
But they found more of these pairs than predicted, which suggests that another mystery particle is being made first and then that particle is decaying into these extra electron-positron pairs. This new boson didn’t seem to be the dark photon they were looking for—it doesn't have the right properties—but it was a potential new particle nonetheless.
Assuming the result was real and not due to some kind of experimental error, the particle would have a mass only 34 times the mass of an electron or an energy of 17 MeV (mega electron volts), which is fairly light as particles go. So it's been called the X17 boson.
Their reports of a new particle were mostly overlooked until a U.S.-based group did their own analysis. The group, led by Dr. Jonathan Feng of the University of California, Irvine, suggested the particle had all the characteristics of a force carrier particle and thus could, in fact, be evidence of a fifth force. The particle may have escaped detection thus far because it is “protophobic”—it interacts with neutrons and ignores protons, the opposite of how a normal photon acts.
More fifth force evidence?
In an attempt to find the particle again, the Hungarian scientists in their most recent experiment fired protons at tritium, a rare isotope of hydrogen, to form helium-4 nuclei that are excited into an unstable state. When those unstable nuclei decayed, they once again saw more electron-positron pairs than predicted by the Standard Model that suggested the presence of a boson with an energy of 17 MeV (mega electron volts).
So is this irrefutable evidence of a fifth force? Well, some scientists are still skeptical. For starters, the signal is a small one, only seven times the standard deviation, so experimental error is still possible. Some suggest caution with the results before they are repeated by another group, pointing out that the Hungarian group has made claims of other bosons in the past and later abandoned those claims without a thorough explanation. But other groups have looked for potential issues with their experimental setup and haven’t found any. And this second potential detection of the same X17 boson certainly makes the claim more robust.
Recalling words from the astronomer Carl Sagan, extraordinary claims tend to require extraordinary evidence. Rearranging the Standard Model to include an additional force would be extraordinary. Why has it been in hiding for so long? What kinds of interactions does it oversee?
Our detector technology today is good enough that, at least as far as the X17 boson is concerned, it puts this extraordinary evidence at our fingertips. So let’s get searching.