Fundamental physics today

In the brief article introducing the Standard Model, dated 1967, Steven Weinberg wrote: “Certainly, our model has too many arbitrary features for these predictions to be taken seriously.” In fact, many theoretical ideas developed during those years were incorporated into a realistic model: from the extended gauge theory of weak interactions (and later strong interactions), to spontaneous symmetry breaking and the Higgs mechanism, resulting in a number of completely new predictions, all of which at the time remained to be verified. Over time, each of these predictions has been experimentally confirmed. Today, thanks to the energy and precision of current experiments, such as those conducted at the LHC at CERN, no significant deviations from the Standard Model’s predictions are observed. Have we thus understood everything about fundamental physics? Or are we rather in a situation similar to that at the end of the 19th century, when all physics seemed to be understood before revolutions like relativity and quantum mechanics  radically changed our way of interpreting reality?

It is very difficult to draw historical parallels or predict future developments, but what is certain is that many open questions remain—questions whose answers are not found within the Standard Model. For example: are neutrinos Dirac or Majorana fermions? Why do strong interactions seem not to distinguish between matter and antimatter? How can gravity, which is very weak at the energies we can reach, be integrated with the other fundamental forces? How can we explain the instability of the mass of the Higgs boson? Why do three families of particles exist that are similar but have different masses? How is the mass spectrum of fermions in the Standard Model generated—the elementary particles that make up matter? These are just some of the questions driving scientists around the world to search for new theories and new particles.

Today, we face a fundamental question: do we already have a new theoretical model capable of addressing and solving some of the most important open issues, offering testable predictions for future experiments? The answer is: we do not know yet. It could be that the future Standard Model is already around the corner, with many new predictions to test, or that it already exists but we have not yet given it proper credit. At this stage, experiments are guiding us toward where particle physics will develop next. To make progress, it will be necessary to raise the energy thresholds even further, increase the intensity and sensitivity of our instruments, explore a wide range of possibilities without bias, and look for potential deviations—deviations that must exist even if we do not yet know where they are.

An example of a theory that has generated great interest and hope since the 1980s is Supersymmetry. The Minimal Supersymmetric Standard Model (MSSM) predicts rich phenomenology, with many new particles—supersymmetric partners of the particles in the Standard Model—including a possible candidate for dark matter. Additionally, it could stabilize the mass of the Higgs boson if the supersymmetric particles are not too heavy, enable new sources of matter-antimatter asymmetry, and facilitate the unification of interactions at high energies. However, despite experimental efforts, so far there have been no confirmations of the predictions of this theory. On the contrary, twenty years of research at the LHC have not found any supersymmetric particles, pushing their mass values higher and higher. Although the model cannot be completely ruled out, some of the theoretical motivations supporting it have weakened as the masses of supersymmetric particles increase. Consequently, alternative ideas have emerged to address the still open questions of the Standard Model, which today has no clear successor.

But this uncertainty should not scare us. The history of physics teaches us that the most revolutionary theories often emerge when we least expect them, and that experimental observations can surprise us. Sometimes, theory anticipates experiment, as happened with Einstein’s relativity and, to some extent, with the electroweak theory in the Standard Model; other times, it is the experiment that challenges the theory, which must then evolve to interpret the data. This was the case with quantum mechanics—think of the photoelectric effect—as well as with the theory of strong interactions, quantum chromodynamics, which later became part of the Standard Model and developed under the influence of experiments starting at SLAC  in the late 1960s. However, theoretical physics develops in many directions, many of which are not immediately testable through direct experiments but remain within the realm of ideas and concepts. Among these are studies of phenomena at energies so high that they are unreachable with current tools—such as black hole physics—where it is essential to find a synthesis between gravity and quantum mechanics, through approaches like string theory or quantum gravity. Additionally, researchers explore dualities and relationships between theories applied in different regimes or study theories in extreme limits in an attempt to simplify and understand more complex problems. No matter how far creative thinking may go, the only clearly defined limit is summarized by a famous quote from Richard Feynman: “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”