Published 16 February 2023 by Benjamin Skuse
Fundamental Physics at the Crossroads
The discovery of the Higgs boson in 2012 at the Large Hadron Collider (LHC) was a key milestone in particle physics, slotting into place the last piece of experimental evidence in the Standard Model puzzle. The enormity of the discovery was recognised by the Nobel Committee only a year later, when Peter Higgs and François Englert – who in 1964 independently conceived a mechanism that endows fundamental particles with mass, a manifestation of which is the Higgs boson – were awarded the Nobel Prize in Physics.
But the discovery was also the end of a clear path towards discovery for the community. Since the mid-1970s, particle physicists had a ‘cheat sheet’ to work from in the Standard Model, a framework for understanding the relationships between the various fundamental particles and how the subatomic world functions. With only details left to iron out, where the next big discovery was going to come from was shrouded in mystery.
The Current Situation
Fast-forward to today and the situation remains the same. The LHC recently restarted after a three-year upgrade, now generating more intense particle collisions at slightly higher energies over a planned four-year period. No doubt, the run will provide physicists with a deeper understanding of the known fundamental particles. But what many researchers are really hoping for is a surprise, which can only happen if they spot collisions that don’t fit with the Standard Model at all.
The Standard Model is a supremely successful and well-tested theory that explains how the basic building blocks of matter interact, but it is not the last word in particle physics. It has nothing to say about dark matter, dark energy or gravity, and cannot explain mysteries such as why neutrinos have mass, or why there is so much matter and so little antimatter in the universe.
Even a hint pointing the way to solving just one of these mysteries would be a huge step towards a new era for fundamental particle physics beyond the Standard Model. There’s just one problem: no one knows if there will be any new physics unearthed at higher energies. Without guidance from theory, researchers are shooting in the dark.
Big Bang Theories
A similar situation is seen in other areas of fundamental physics. For instance, inflation is the idea that a sudden, extreme ballooning of the universe in its earliest moments could account for inconsistencies between Big Bang theory and what the universe looks like now. The most promising way to search for evidence of inflation is by scouring the cosmic microwave background (CMB) – first discovered in 1964 by Nobel Prize in Physics recipients Arno Penzias and Robert W. Wilson. The CMB is theorised to contain telltale fingerprints of inflation, known as B-mode polarisation patterns, which could only have been generated by primordial gravitational waves during inflation.
Despite hunting for these signatures for decades, researchers have found nothing at the energies the most likely hypotheses suggest they could hide. Primordial gravitational waves could in principle have generated strong B-mode signatures or incredibly weak ones. Theorists have even devised complex scenarios where inflation produces no B-mode signal at all. As a result, experimentalists have no idea if or when their observations will ever bear fruit.
A Candidate for Dark Matter
Another example is the long-sought hypothetical axion – a leading candidate for dark matter. Though its discovery would solve central issues in the Standard Model, its character is elusive. Besides not knowing if it exists, physicists don’t know its mass or how much it interacts with regular matter. Through diverse and ingenious experiments, much of the most promising ‘space’ in which axions would have the right mass and energy to be dark matter has turned out to be empty. And even if observations prove it cannot be the sole dark matter particle, the axion could still contribute to dark matter in some way at wildly different masses and interaction strengths. The search once again appears endless.
So far, when faced with such interminable tasks, physicists have tended to double down on their efforts. Hunting for inflation’s fingerprints, CMB-S4 will begin operations in 2030 and is expected to be the ultimate CMB experiment you can build on the ground, while LiteBIRD (planned 2028 launch) will search for CMB B-mode signatures in fine detail from space. Meanwhile, a host of new ground-based axion-hunting experiments and the Laser Interferometer Space Antenna (LISA; planned 2037 launch) promise to probe new axion mass and energy regimes. And even more powerful and expensive particle colliders are expected to produce collisions at unprecedented energies in the coming decades, including the International Linear Collider, High-Luminosity Large Hadron Collider and more.
But just as for current experiments, there is no guarantee that new physics hides where these experiments will probe. What happens if they come back empty? Should we label these mysteries as unknowable and move on?
There may come a time when practical challenges in terms of sheer scale and cost limit physics experiments and humanity’s ability to probe the fundamental makeup of reality, but in all the cases mentioned physics is nowhere near those limits yet. And although searching for new physics right now appears to be a never-ending and thankless trudge, every new theoretical idea, null result and scrap of information is a small step towards a truer description of the universe we live in.
The real roadblock to discovery will come if theorists and experimentalists are not given the time, space and funding to prod and poke at tiny deviations from expected results, or abandon ingrained concepts and long-held assumptions in order to explore new ways to try to probe and understand nature – be it through ever more precise experiments or completely novel approaches.
In key areas, fundamental physics is at the crossroads. Taking the right direction will depend on how well we support those who look upon the situation not as a crisis for fundamental physics, but as an opportunity to revolutionise our understanding of the universe.