Quantum theory and Einstein’s theory of general relativity are two of the greatest successes in modern physics. Each works extremely well in its own domain: Quantum theory explains how atoms and particles behave, while general relativity describes gravity and the structure of spacetime. However, despite many decades of effort, scientists still do not have a satisfying theory that combines both into one clear picture of reality.

Most common approaches assume that gravity must also be described using quantum ideas. As physicist Richard Feynman once said, “We’re in trouble if we believe in quantum mechanics but don’t quantize gravity.” Yet quantum theory itself has deep unresolved problems. It does not clearly explain how measurements lead to definite outcomes, and it relies on strange ideas that clash with everyday experience, such as objects seemingly behaving like both waves and particles, and apparent nonlocal connections between distant systems.

These puzzles become even sharper because of Bell’s theorem. This theorem shows that no theory based on ordinary ideas—such as locality, an objective reality, and freely chosen measurements—can fully match the predictions of quantum theory within our usual four-dimensional view of space and time. These quantum predictions have been repeatedly confirmed in tests of entanglement, first discussed by Einstein, Podolsky, and Rosen (EPR). As a result, simple classical explanations limited to ordinary four-dimensional spacetime cannot fully account for what we observe.

Turn up the voltage, and monogamous quantum relationships fall apart in surprising ways.

Are quantum particles polygamous? New experiments suggest some of them abandon long-standing partnerships when conditions get crowded.

Quantum particles do not behave like isolated dots.

They interact, form bonds, and follow strict social rules. One of the most fundamental divides separates fermions and bosons.

Quantum particles have a social life, of a sort. They interact and form relationships with each other, and one of the most important features of a quantum particle is whether it is an introvert—a fermion—or an extrovert—a boson.

Extroverted bosons are happy to crowd into a shared quantum state, producing dramatic phenomena like superconductivity and superfluidity. In contrast, introverted fermions will not share their quantum state under any condition—enabling all the structures of solid matter to form.

But the social lives of quantum particles go beyond whether they are fermions or bosons. Particles interact in complex ways to produce everything we know, and interactions between quantum particles are key to understanding why materials have their particular properties. For instance, electrons are sometimes tightly locked into a relationship with a specific atom in a material, making it an insulator. Other times, electrons are independent and roam freely—the hallmark of a conductor.