Why “Many” Is Displacing “Small” as the Hottest Frontier in Physics

By Charlie Wood

Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, physics staff writer Charlie Wood outlines how the biggest field in physics is enjoying a golden age.

Is fundamental physics in crisis? Many physicists and journalists — myself included — worry that it is. The Nobel Prize–winning physicist Richard Feynman defined fundamental physics as “the rules of the game,” and we don’t know that much more about nature’s rules today than we did in the 1970s — despite intense efforts in particle physics and astrophysics to reveal new ones.

That plateau in progress is real, but it mainly concerns the rules of the subatomic world, the frontier of the very small. There is another frontier with an entirely different set of rules: the frontier of the very many. Birds form free-wheeling flocks. Snowflakes pile up and create an avalanche. H2O molecules can collectively become vapor, water or myriad forms of ice. When simple objects gather in large numbers, behaviors materialize that are hard, if not impossible, to anticipate.

“The ability to reduce everything to simple fundamental laws does not imply the ability to start from those laws and reconstruct the universe,” wrote Philip Anderson, another Nobel laureate physicist, in his classic essay “More Is Different.” “Instead, at each level of complexity entirely new properties appear, and the understanding of the new behaviors requires research that I think is as fundamental in its nature as any other.”

Anderson was a key player in the subfield most dedicated to exploring this complexity frontier, known today as condensed matter physics. Condensed matter physicists concern themselves with the large-scale properties of matter, especially solids and liquids. Much of the focus is on figuring out how quantum particles — often electrons — behave in vast swarms.

The ability to understand, and therefore tame, electrons’ collective behaviors has had an enormous technological impact. The 1930s-era theory of what electrons are doing inside conductors, semiconductors and insulators led to the transistor and our digital age. In the 1950s, physicists determined why electrons sometimes pair up in a way that allows them to “superconduct,” flowing through a metal with zero resistance, enabling the development of the superconducting magnets in MRI machines and large particle colliders.?

That was just the beginning. Today, condensed matter represents the largest subfield of physics, accounting for at least a third and perhaps nearly half of all working physicists. Many continue to study superconductivity, while others investigate even more exotic phenomena. The discoveries have come at a rapid pace — new instances of superconductivity, exotic forms of magnetism, and situations where electrons conspire to act as if they’ve broken into parts, to name just a few. Experimentalists regularly make unexpected observations, and theorists develop useful models for explaining what’s going on. Particle physics might be stuck, but condensed matter physics is enjoying a golden age of discovery.

What’s New and Noteworthy

One of the biggest developments in condensed matter — if not all of physics — in recent decades has been the discovery of a whole new way for matter to organize itself. In school we learn about phases of matter that you can sketch on a page; a solid forms when molecules snap into a grid, for instance, or a metal becomes magnetized when its atoms all point in one direction. Now there’s topological order, which involves different patterns that can form when quantum particles in a material become entangled with each other. These patterns can lead to strange materials, such as ones that maintain superconducting boundaries and insulating interiors. In recent years, quantum computers have allowed physicists to engineer some of the simplest topological phases, though others remain entirely theoretical.

Another booming area of investigation involves a shift from studying minerals one might dig out of the ground to creating designer 2D materials with precisely tuned characteristics. The revolution began with the 2004 discovery that you could peel a flat honeycomb lattice of carbon atoms off a hunk of graphite with a piece of Scotch tape to form a 2D material dubbed graphene. Then came the blockbuster 2018 discovery that a properly assembled graphene sandwich could superconduct electricity. Now multiple labs have found superconductivity in various stacks of carbon sheets. And they’re stacking other materials to create devices with bespoke electronic properties?that produce all sorts of strange quantum states.

Condensed matter is so vast and active a field that any overview of its various fronts will be woefully incomplete. In the last year alone, physicists created phases of matter where fractional electric charges move freely without relying on powerful magnetic fields; confirmed the existence of a new kind of magnetism; heard the smooth hiss of electrons conspiring to flow in a seemingly continuous current of charge, and completed a 70-year hunt for a motionless wave of electrons known as Pines’ demon.

But even as the field racks up discoveries about collective electron behaviors, it has also experienced several recent controversies. An allegedly eternal form of quantum stability has fallen under suspicion. Splashy papers purporting to have detected an elusive particle useful for quantum computing have been retracted due to sloppy data analysis. And one high-profile claim of room-temperature superconductivity turned out to be a case of outright fraud.

Fraud aside, perhaps such missteps are hard to avoid in a fast-moving field dedicated to uncovering the unforeseeable physical phenomena that lie just over the complexity horizon. More is different, and also very hard.

Around the Web

• The physicist Philip Anderson explains how complicated emergent systems are fundamentally different from their constituent parts in his 1972 essay, “More Is Different.”
• The foundation of much of condensed matter is “band theory,” which explains how the flow of electrons differs in conductors, semiconductors and insulators. The YouTube channel PhysicsHigh explains the basics in this video.
• TED-Ed presents a way to think about what makes “topological” matter topological.
• Last year, one of the most stunning experimental claims in the history of condensed matter became one of the field’s biggest scandals, Dan Garisto wrote for Nature.