The way matter has to relate, to interact, is through what we commonly call forces. The simple act of drinking a glass of water is an example of this: we can hold it because an electrostatic repulsion appears between the electrons in our hand and those in the glass, and by lifting it we are counteracting the action of another force that acts on it, the gravity.
As in the previous case, all the processes that we observe in our daily lives are reduced to the action of two fundamental forces: gravity and electromagnetics . However, they are not the only ones. There are two others of which we have no experience because their range is restricted to distances from the atomic nucleus: the strong nuclear force , responsible for holding the protons and neutrons together inside the atomic nucleus, and the weak nuclear force , which causes a certain type of disintegration. radioactive, beta decay.
Although most physicists think that there are only these four, a small percentage of them believe that we have missed one. The search for that hypothetical and elusive fifth force has been carried out on two totally different scales: the subatomic and the cosmological.
New particles, new forces
In 2011, physicists at the Tevatron, the Chicago Fermilab particle accelerator, claimed to have found evidence of the existence of a totally unknown particle outside the standard model: the W’ and Z’ bosons, whose existence would open the door to a new force . This is so because according to quantum physics, when two particles interact, what they are doing is exchanging the carrier particle of the force in play: in the case of the electromagnetic force, it is a photon.
However, neither CERN nor Tevatron could subsequently confirm its existence, so the fifth force reverted back to its ghostly existence. But not for long: in 2015 the ATLAS experiment at CERN’s Large Hadron Collider (LHC) claimed to have found evidence of the existence of these bosons, but it didn’t go any further.
And in 2021 the LHCb experiment (also from CERN) pointed to the possible detection of a hypothetical particle called a leptoquark capable of interacting with both quarks (the components of protons and neutrons) and leptons (such as electrons). If the existence of any of these particles had been confirmed, the door to the existence of a new force of nature would be completely open.
the mysterious spin
But the search also goes the other way. In 2013 the fifth force returned to the fore thanks to one of the characteristics that all subatomic particles have: spin. Physicists originally imagined that if the electron were a small charged sphere, then it could rotate on itself like planets do: that – put very crudely – is spin. But here the analogy ends; while a planet’s spin can take any value, the electron’s spin can only take two, which are arbitrarily assigned as +½ and -½.
Gradually, however, it became clear that, whatever the spin was, it in no way corresponded to a rotation of the electron. This was a purely quantum-relativistic result, without any parallel to classical physics. The interpretation of what spin is remains obscure even today, but we do know that in the presence of a magnetic field it can be aligned in two directions: up and down. This strange property of the electron is what allows, for example, magnetic resonance imaging to be done in hospitals.
This mysterious property has led some physicists to think that the fifth force might be some kind of long-range spin-spin interaction between subatomic particles. This is what the physicist Larry Hunter proposed in 2013 when he analyzed how the interaction between the spin of electrons, protons and neutrons in a laboratory could be with that of the electrons inside our planet. The experimental problem to beat is that, as this new force is of a very weak intensity (a million times smaller than gravity), the interaction of the spin with the Earth’s magnetic field masks any other effect. So far this path continues without bearing any fruit.
A fifth cosmological force?
On the other hand, the existence of extra dimensions in the universe also allows a fifth force to enter between the folds of the theory, in such a way that it would modify gravity at small distances . To discover it, different experiments have been carried out whose objective is to determine if there is any deviation from the law of the inverse square of the distance that determines the extent of gravity. Some experiments deep in a mine in Australia and deep in the Greenland ice have found small discrepancies (of the order of 2%) between measured and predicted values, but they are not conclusive.
Other physicists search for a new force operating on cosmological scales. Astrophysicist Bhuvnesh Jain of the University of Pennsylvania thinks that as we look deeper into the universe, the laws of gravitation point to the existence of a force operating on the scale of galaxy clusters. For this astrophysicist, having discovered the accelerated expansion of the universe implies two possibilities: either Einstein’s general relativity is correct on large scales but there is a strange form of energy, dark energy , which causes this expansion, or that energy does not exist but that general relativity is not correct on large scales, which means that there is a fifth force operating at cosmological distances.
In 2012 Jain, along with colleagues, re-examined the luminosity of a class of stars called Cepheids , which vary in brightness with tremendous regularity, in 25 galaxies. The existence of a hypothetical fifth force would imply the existence of differences in the period of the Cepheids without possible explanation. However, what they found was a perfect agreement between the observations and the predictions of general relativity.
Other scientists, such as Jo Bovy and Glennys Farrar, search for it in dark matter . Their model predicts the existence of a long-range, non-gravitational, attractive force that does not affect ordinary matter. So how could its existence be proven? Because being attractive, this force would force dark matter to aggregate, which would end up having a gravitational effect on ordinary matter, accelerating the formation of structures in the universe.
Despite the efforts to demonstrate the existence of a fifth fundamental force in the universe -which began in the late 1980s- the truth is that it has shown a certain ghostly air: there are those who say they have seen it but in the end everything remains in borage water.
References
Jo Bovy and Glennys R. Farrar, Connection between a Possible Fifth Force and the Direct Detection of Dark Matter, Phys. Rev. Lett. 102, 101301
Hunter, J. Gordon, S. Peck, D. Ang, J.-F. Lin. Using the Earth as a Polarized Electron Source to Search for Long-Range Spin-Spin Interactions. Science, 2013; 339 (6122): 928
Diaz, B.; Schmaltz, M.; Zhong, Y.-M. (2017) “The leptoquark hunter’s guide: pair production”. Journal of High Energy Physics. 97 (10): 97
CDF Collaboration, T. Aaltonen et al (2011) “Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV”, Phys.Rev.Lett. 106:171801
