| Fundamental physics has a problem. Some researchers say the field faces a "nightmare scenario." Some say it's "in crisis." To others it's merely "stuck." Whatever word they use, most particle physicists acknowledge that progress has slowed. After a rip-roaring 20th century that saw the discovery of general relativity, quantum theory, and a dozen or so fundamental particles, the first quarter of the 21st century has mainly brought further confirmation of those theories. Physicists know their hard-won understanding of nature's laws is incomplete. They don't know why certain particles have mass, what sort of invisible stuff seems to be holding galaxies together, or what sort of energy is driving the universe's expansion. But their biggest blind spot is one of scale. Physicists have equations that predict how molecules zig and zag, how atoms split, and how the heart of the atom holds together. They can continue zooming into the sub-sub-subatomic world for quite a while, but eventually — for any event playing out on a stage roughly 10⁻³⁵ meters across — they run out of equations. The universe seems to have rules that tell it what to do in those situations (it follows them during black hole formation, for instance), but physicists are ignorant of these instructions. Particle physicists have pushed their frontier of ignorance back to this minuscule realm by repeatedly crashing particles closer and closer together, watching what happens, and developing the mathematics to capture the strange and surprising behaviors they witness. This strategy has culminated in Europe's Large Hadron Collider, a 27-kilometer ring that can summon the energies needed to collide protons and study nature at 10⁻¹⁹ meters. The international physics community aims to build a next-generation collider — perhaps with a 100-kilometer circumference — this century. But the money, time and technology required to go much further boggles the mind. Numerous clever non-collider experiments are searching for subtle deviations from predictions and could lead to a major discovery any day, but researchers are ultimately facing similar problems as their experiments grow more and more intricate. Particle physicists are approaching a technological and financial wall as they attempt to probe ever deeper layers of reality. Fortunately for the incurably curious, physicists are not limited to studying events they can orchestrate themselves. In deep space, stars explode, black holes collide, and the Big Bang's echoes continue to reverberate. These events are so revealingly violent that physicists can only dream of re-creating them in labs. In short, the universe at large is far more extreme than Earth is. So in the twilight of traditional particle physics, theorists and experimentalists are increasingly turning to various types of cosmic fireworks in their search for clues that will help them better understand the fundamental laws of nature. What's New and Noteworthy One of the most promising avenues for new discoveries is the detection of ripples in space-time known as gravitational waves. Researchers have now racked up dozens of chirps originating from cataclysmic crashes between black holes, and they are rapidly developing ways of picking up cosmic clangs indicative of even more dramatic events. Particle physicists are anxiously awaiting the 2030s, when they hope to see a trio of satellites known as the Laser Interferometer Space Antenna (LISA) go into operation. If it flies, LISA will be capable of picking up gravitational waves generated during the first fractions of a second after the Big Bang, an intense era in cosmic history at the frontier of the known laws of physics. In those early moments, some physicists expect, the universe existed in multiple phases at once — like a pot of boiling water. If so, the bubbling would have set off space-time ripples that LISA will be listening for. Another route is to map out — in excruciating detail — the locations of galaxies in the sky. While at first glance they may seem to be randomly scattered about, a careful statistical analysis will reveal that when you find one galaxy, the odds of finding another galaxy a particular distance away are slightly higher than usual. Physicists believe that this mild clustering reflects the spontaneous generation of quantum particles in the immediate aftermath of the Big Bang — and that it therefore offers a way to indirectly study quantum events that took place at the dawn of time. Leading particle theorists are engaged in a years-long effort to determine exactly what sorts of quantum drama would produce exactly what sorts of galaxy clusters in the sky. It's a devilishly difficult theoretical problem that has required them to harness an array of standard theoretical techniques and develop a few new ones. High-energy astrophysics isn't exclusively about reconstructing the birth of the universe. Physicists are also watching out for inexplicable aspects of intense events in the modern universe. They identified just such a mystery in 2022, when a handful of observatories spotted a strikingly bright burst of high-energy light from a dying star. Of particular interest was a single particle of light carrying roughly 30% more energy than is released in an LHC collision. It's almost unthinkable that such a photon would have been able to travel the 2.8 billion light-years to Earth uninterrupted. Some researchers proposed that it morphed into a specific dark matter candidate, made the trip incognito, and then morphed back into a photon shortly before striking Earth. Deciphering unimaginably subtle hints of new physics in the heavens won't be easy. It will likely take many years of theorizing, planning, observing and analyzing. But with a bit of luck, it just might be possible. | 
