There are four forces that are canon in our universe: gravity, electromagnetism, and the strong and weak nuclear forces. But scientists are searching for evidence of a new, unknown force that could explain some of the wildest mysteries facing humanity.
The effects of the four known forces on matter, from the tiny realm of atoms all the way up to the colossal scale of galaxies, are well documented and mostly understood. But when you consider that about 95 percent of our universe’s mass is made up of shadowy unexplained stuff known as dark matter and dark energy, it’s no wonder that scientists have long suspected that those four forces do not represent the entire blueprint of the cosmos.
To account for perplexing phenomena, scientists have been on the lookout for evidence of a fifth force for decades. Some have hunted for it hundreds of miles underground, inside Earth’s mantle, while others have searched for force-carrying particles that can evade detection around dense objects, like the planet we live on.
Why are scientists looking for a mysterious ‘fifth force’?
If such a fifth force were to be discovered, it would be a historic milestone that would expand our understanding of this odd reality we’ve found ourselves in. It might even explain why dark matter and dark energy are so much more abundant in the universe than the stuff that makes up stars, galaxies, and our own bodies.
But it’s no easy task to detect unknown forces, let alone explain how they might fit into our broader and well-corroborated conception of our universe.
“Whenever scientists come up with one of these theories, they have to think about what other predictions it would make, and does it agree with every experiment that’s been done so far,” said Paul Hamilton, a physicist at UCLA, in a call.
“If you come up with this new force, you better make sure that you don’t mess that up at all,” he noted.
The most recent effort to describe a new force comes from a team led by Attila Krasznahorkay, a physicist at the Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI). The researchers sparked international headlines with a study published on the arXiv preprint server in October, which is not yet peer-reviewed, that presents new evidence for a hypothetical subatomic object known as the X17 particle.
If other teams are able to reproduce the results and come to the same conclusion, it could shed light on dark matter, because the particle’s parameters fit some theories about fifth forces associated with this non-luminous substance.
“All of our knowledge about the visible world can be described by the so-called Standard Model of particle physics,” Krasznahorkay said in an email. “This model, however, does not predict any particles heavier than the electron and lighter than the muon, which is 207 times heavier than the electron.”
“If one finds a new particle in the above mass window, than it would point to some new interaction not included in the Standard Model,” he added.
That’s exactly the type of particle that Krasznahorkay’s team think they may have stumbled across. This object is named X17 for its estimated mass of 17 mega-electron volts (MeV), or about 34 times heavier than an electron—the sweet spot for a potential mediator of a fifth force.
Krasznahorkay’s team first developed the X17 hypothesis years ago, after conducting an experiment that involved shooting protons at isotopes, which are variants of chemical elements.
The researchers were observing how protons caused an isotope known as lithium-7 to become an unstable kind of atom called beryllium-8. As the beryllium-8 atoms decayed, they produced pairs of electrons and positrons that repelled each other, prompting the particles to shoot away at angles.
The team expected to see a correlation between the light energy emitted by the decay process and the angles at which the particles traveled away from each other. Instead, the electrons and positrons moved away at an angle of 140 degrees nearly seven times more frequently than their models predicted, a surprising result.
The anomaly could potentially be caused by the X17 particle briefly emerging as a byproduct of beryllium-8 decay, Krasznahorkay and his colleagues proposed in a 2016 study in Physical Review Letters.
The finding attracted the attention of many scientists, including Iftah Galon, a physicist at Rutgers University. “These guys had this very distinct signal and we thought: ‘wow, if it’s there, then maybe they discovered something we haven’t seen in the past’,” said Galon in a call.
Galon and his colleagues narrowed down the possible properties of an X17 particle and proposed that it might be a new type of boson, in a 2017 study published in Physical Review D. Bosons are particles with a particular angular momentum; many varieties of them have already been detected, such as the Higgs boson.
Meanwhile, Krasznahorkay’s team tried to detect the X17 particle again with a different experimental setup, which is the one described in the new preprint study. This time, the researchers watched tritium decay into helium-4, and once again, they observed the weirdly angled getaways that hint at a particle with a mass around 17 MeV.
“The photon mediates the electromagnetic interaction, the gluon the strong interaction, and the W and Z bosons the weak interaction,” Krasznahorkay explained. “Our X17 particle should mediate a new interaction, which is the fifth interaction.”
The new result lowers the odds that the initial experiment was just a fluke, or that it was caused by an error in the experimental system. “It’s basically a completely different experiment and they still see the same thing,” Galon explained. “So, it would have to have been a very weird systematic effect to appear in two different target setups and two different experimental systems.”
However, it will take a lot more research to back up the team’s findings and interpretations, and the existence of the particle is by no means certain. “To corroborate or dismiss a result, one has to go and do an experiment,” Galon said. “That’s how science works. So, now it is up to the people who are experimentalists.”
To that end, other teams have been on the lookout for the particle. The NA64 experiment at the CERN Super Proton Synchrotron in Geneva, Switzerland, constrained some other parameters of the hypothesis, according to a 2018 study in Physical Review Letters. The Positron Annihilation into Dark Matter Experiment (PADME) collaboration in Frascati, Italy also plans to search for the particle.
Chameleon particles and symmetrons
While the X17 particle is the fifth force du jour, it certainly has some fascinating predecessors. Take the example of chameleons, hypothetical particles that were first proposed in 2004 to account for the strange behavior of dark energy.
Scientists think dark energy is driving the accelerated expansion of the universe, a phenomenon that is pushing galaxies away from each other at an increasing clip for reasons that remain mysterious. One explanation is that a hypothetical new force called quintessence, mediated by unknown particles such as chameleons, could have a repulsive effect on objects at cosmic distances.
Chameleons got their name from their predicted behavior of camouflage near massive objects. “Depending on the environment, the distance that the chameleon field can exert a force changes,” said Hamilton, who has experimented with the chameleon theory.
In outer space, the chameleon field “basically becomes long-range and you can see things like galaxies getting pushed apart,” he continued. “In everyday objects, where the density is really high, it can be incredibly short. If you are more than a few nanometers away, you’re not going to see it.”
“If you have a theory that’s predicting a new force, you have to make sure that it doesn’t violate decades—or even hundreds of years—of results, which is difficult to do"
To try to detect these finicky particles, Hamilton and his colleagues conducted an experiment inside an ultrahigh-vacuum chamber that simulated the low-density environment of space. The team shot laser beams at cesium atoms to see if they could pick up the subtle interference of the chameleon field.
The results, published in a 2015 study in Science, limited the possible physical properties of chameleons, and did not detect a new force. “A lot of my field and what we do is try and make measurements that can help the theorists that are coming up with these theories figure out which are viable and which aren’t,” Hamilton explained.
“Between this experiment and other experiments, it’s getting pretty close to where having the chameleon field be a good explanation for dark energy is ruled out,” he added. “There’s a little small range where it’s still possible, but it’s mostly ruled out.”
Scientists are still exploring other fifth force concepts that might account for dark energy, mediated by proposed particles like symmetrons or dilatons. There are also plenty of proposed fifth forces that are not focused on dark matter and dark energy.
For instance, a team searched deep inside Earth for signs of a hypothetical force similar to magnetism called long-range spin-spin interaction, according to a 2013 study in Science. Like all the other teams who have hunted for fifth forces, the researchers didn’t find one—but that in itself is a valuable result.
“If you have a theory that’s predicting a new force, you have to make sure that it doesn’t violate decades—or even hundreds of years—of results, which is difficult to do,” Hamilton said.
“You would love to discover this new thing and there would be Nobel Prizes and glory and all that,” he concluded. “But it’s pretty hard to find new physics, so typically our mindset is how well we can constrain it.”