There have been dramatic articles in the news media suggesting that a Nobel Prize has essentially already been awarded for the amazing discovery of a “fifth force.” I thought I’d better throw some cold water on that fire; it’s fine for it to smoulder, but we shouldn’t let it overheat.
There could certainly be as-yet unknown forces waiting to be discovered — dozens of them, perhaps. So far, there are four well-studied forces: gravity, electricity/magnetism, the strong nuclear force, and the weak nuclear force. Moreover, scientists are already fully confident there is a fifth force, predicted but not yet measured, that is generated by the Higgs field. So the current story would really be about a sixth force.
Roughly speaking, any new force comes with at least one new particle. That’s because
- every force arises from a type of field (for instance, the electric force comes from the electromagnetic field, and the predicted Higgs force comes from the Higgs field)
- and ripples in that type of field are a type of particle (for instance, a minimal ripple in the electromagnetic field is a photon — a particle of light — and a minimal ripple in the Higgs field is the particle known as the Higgs boson.)
The current excitement, such as it is, arises because someone claims to have evidence for a new particle, whose properties would imply a previously unknown force exists in nature. The force itself has not been looked for, much less discovered.
The new particle, if it really exists, would have a rest mass about 34 times larger than that of an electron — about 1/50th of a proton’s rest mass. In technical terms that means its E=mc² energy is about 17 million electron volts (MeV), and that’s why physicists are referring to it as the X17. But the question is whether the two experiments that find evidence for it are correct.
In the first experiment, whose results appeared in 2015, an experimental team mainly based in Debrecen, Hungary studied large numbers of nuclei of beryllium-8 atoms, which had been raised to an “excited state” (that is, with more energy than usual). An excited nucleus inevitably disintegrates, and the experimenters studied the debris. On rare occasions they observed electrons and positrons [a.k.a. anti-electrons], and these behaved in a surprising way, as though they were produced in the decay of a previously unknown particle.
In the newly reported experiment, whose results just appeared, the same team observed the disintegration of excited nuclei of helium. They again found evidence for what they hope is the X17, and therefore claim confirmation of their original experiments on beryllium.
When two qualitatively different experiments claim the same thing, they are less likely to be wrong, because it’s not likely that any mistakes in the two experiments would create fake evidence of the same type. On the face of it, it does seem unlikely that both measurements, carried out on two different nuclei, could fake an X17 particle.
However, we should remain cautious, because both experiments were carried out by the same scientists. They, of course, are hoping for their Nobel Prize (which, if their experiments are correct, they will surely win) and it’s possible they could suffer from unconscious bias. It’s very common for individual scientists to see what they want to see; scientists are human, and hidden biases can lead even the best scientists astray. Only collectively, through the process of checking, reproducing, and using each other’s work, do scientists create trustworthy knowledge.
So it is prudent to await efforts by other groups of experimenters to search for this proposed X17 particle. If the X17 is observed by other experiments, then we’ll become confident that it’s real. But we probably won’t know until then. I don’t currently know whether the wait will be months or a few years.
Why I am so skeptical? There are two distinct reasons.
First, there’s a conceptual, mathematical issue. It’s not easy to construct reasonable equations that allow the X17 to co-exist with all of the known types of elementary particles. That it has a smaller mass than a proton is not a problem per se. But the X17 needs to have some unique and odd properties in order to (1) be seen in these experiments, yet (2) not be seen in certain other previous experiments, some of which were explicitly looking for something similar. To make equations that are consistent with these properties requires some complicated and not entirely plausible trickery. Is it impossible? No. But a number of the methods that scientists suggested were flawed, and the ones that remain are, to my eye, a bit contrived.
Of course, physics is an experimental science, and what theorists like me think doesn’t, in the end, matter. If the experiments are confirmed, theorists will accept the facts and try to understand why something that seems so strange might be true. But we’ve learned an enormous amount from mathematical thinking about nature in the last century — for instance, it was math that told us that the Higgs particle couldn’t be heavier than 1000 protons, and it was on the basis of that `advice’ that the Large Hadron Collider was built to look for it (and it found it, in 2012.) Similar math led to the discoveries of the W and Z particles roughly where they were expected. So when the math tells you the X17 story doesn’t look good, it’s not reason enough for giving up, but it is reason for some pessimism.
Second, there are many cautionary tales in experimental physics. For instance, back in 2003 there were claims of evidence of a particle called a pentaquark with a rest mass about 1.6 times a proton’s mass — an exotic particle, made from quarks and gluons, that’s both like and unlike a proton. Its existence was confirmed by multiple experimental groups! Others, however, didn’t see it. It took several years for the community to come to the conclusion that this pentaquark, which looked quite promising initially, did not in fact exist.
The point is that mistakes do get made in particle hunts, sometimes in multiple experiments, and it can take some time to track them down. It’s far too early to talk about Nobel Prizes.
[Note that the Higgs boson’s discovery was accepted more quickly than most. It was discovered simultaneously by two distinct experiments using two methods each, and confirmed by additional methods and in larger data sets soon thereafter. Furthermore, there were already straightforward equations that happily accommodated it, so it was much more plausible than the X17.]
And just for fun, here’s a third reason I’m skeptical. It has to do with the number 17. I mean, come on, guys, seriously — 17 million electron volts? This just isn’t auspicious. Back when I was a student, in the late 1980s and early 90s, there was a set of experiments, by a well-regarded experimentalist, which showed considerable evidence for an additional neutrino with a E=mc² energy of 17 thousand electron volts. Other experiments tried to find it, but couldn’t. Yet no one could find a mistake in the experimenter’s apparatus or technique, and he had good arguments that the competing experiments had their own problems. Well, after several years, the original experimenter discovered that there was a piece of his equipment which unexpectedly could absorb about 17 keV of energy, faking a neutrino signal. It was a very subtle problem, and most people didn’t fault him since no one else had thought of it either. But that was the end of the 17 keV neutrino, and with it went hundreds of research papers by both experimental and theoretical physicists, along with one scientist’s dreams of a place in history.
In short, history is cruel to most scientists who claim important discoveries, and teaches us to be skeptical and patient. If there is a
fifth sixth force, we’ll know within a few years. Don’t expect to be sure anytime soon. The knowledge cycle in science runs much, much slower than the twittery news cycle, and that’s no accident; if you want to avoid serious errors that could confuse you for a long time to come, don’t rush to judgment.