In Berlin’s Altes Museum, an ancient Greek statue depicts a boy with arms outstretched skyward, its form almost perfectly symmetrical save for a slightly arched right heel. This balance, however compelling, may not have been solely an artistic choice. German mathematician Hermann Weyl proposed that our attraction to symmetry stems from its fundamental role in the universe’s inherent order.
Weyl’s early 20th-century work illuminated symmetry, and by extension, beauty, as a cornerstone of modern physics. Here, symmetry transcends mere visual balance; it signifies that natural phenomena remain consistent across different locations, times, and various transformations. This principle underpins the conservation of energy and even offers an explanation for existence itself. Weyl noted symmetry’s profound, almost metaphysical importance, describing it as a timeless human endeavor to grasp and manifest order, beauty, and perfection.
Contemporary physicists continue to pursue ever-greater symmetries in theories like supersymmetry and string theory. Yet, recent findings suggest a more profound underlying law: a propensity for extreme levels of quantum entanglement. If validated, this would necessitate a fundamental reevaluation of reality, shifting our perspective from one governed by geometric precision to one shaped by a peculiar interconnectedness between things. Ian Low, a theorist at Northwestern University and Argonne National Laboratory, remarked that such discoveries provide a new avenue for understanding, stating, “Prior to this, we had no idea where symmetry comes from.”
The Genesis of Symmetry in Physics
Historically, symmetry in physics began to surface with Galileo Galilei in the 17th century. Galileo’s groundbreaking notion was that motion is relative, lacking an absolute fixed point. An object’s movement is only discernible against the backdrop of other, differently moving objects. Three centuries later, Albert Einstein extended this principle to gravity, proposing that its presence is perceived only when resisted, such as by the ground beneath one’s feet. In freefall, like in a plummeting elevator, the sensation of gravity would disappear.
Both examples illustrate symmetry: nature behaves consistently under different circumstances. For Galileo, a cannonball’s roll is the same on the harbor or a ship’s deck. For Einstein, a person in freefall experiences weightlessness akin to floating in space. However, it was mathematician Emmy Noether who definitively articulated the profound implications of symmetry. In 1918, Noether proved that any fundamental symmetry in the laws of nature corresponds to a conserved quantity. For Galileo’s and Einstein’s symmetries, while not immediately obvious, familiar examples exist. If an experiment yields the same outcome across different spatial locations, this spatial symmetry directly implies momentum conservation. Similarly, reproducibility of an experiment on subsequent days signifies temporal symmetry, directly implying energy conservation. Noether’s theorem equipped physicists with a powerful tool, revealing a unified, underlying order within seemingly disparate physical laws.
Expanding the Scope of Symmetry
In the ensuing decades, physicists like Weyl explored subtler forms of symmetry. Hidden within the characteristics of fundamental particles like electrons and photons, these “gauge” symmetries not only indicated conserved quantities but also predicted the existence of new fundamental particles. Indeed, one by one, these particles—gluons, quarks, W and Z bosons, and the Higgs boson—were discovered. Collectively, they form the bedrock of the standard model of particle physics, a theory of unparalleled success.
Even within the standard model, not all observed phenomena exhibit perfect symmetry: the universe is not uniform, particles possess varied masses, and matter vastly outweighs antimatter. However, rather than posing contradictions, these instances of “broken symmetry” have reinforced the belief that symmetry remains the fundamental framework against which the universe’s complexity is measured. The late theorist Philip Anderson famously remarked, “It is only slightly overstating the case to say that physics is the study of symmetry.” Werner Heisenberg, a pioneer of quantum mechanics, considered symmetry the “original archetype of creation.”
Challenges to Symmetry’s Dominance
By the late 20th century, this conviction had solidified into scientific orthodoxy. Theorists pursued deeper understanding through concepts like supersymmetry, postulating heavier “superpartners” for known particles, and string theory, which posits fundamental vibrating strings instead of particles, incorporating even more intricate symmetries. Yet, doubts began to surface. Supersymmetry’s predicted partners remain undiscovered, and string theory has yet to yield testable predictions. In 2023, string theory proponent Leonard Susskind acknowledged that its prevailing formulation was stalled, stating with “100% confidence that we don’t live in that world.”
Quantum Entanglement Enters the Discourse
Despite these challenges, symmetry remains central to many theoretical physicists’ work. Martin Savage at the University of Washington employs quantum computing principles for symmetry-based calculations of proton-neutron collisions, crucial for understanding phenomena like supernovae. Quantum computers leverage particle entanglement—where properties are interdependent rather than individual—to achieve a form of parallel processing unachievable with classical computing. This capability promises to simplify complex analyses like proton-neutron scattering.
In 2018, researchers studying scattering problems through a quantum computing lens observed a peculiar phenomenon: maximum internal symmetry occurred when the particles’ overall entanglement was at its minimum. This suggested that nature, while elegant, might favor symmetry alongside the least quantum interaction possible. “It was an absolutely startling pattern,” noted Savage.
Presenting these findings, David Kaplan, a colleague of Savage’s, questioned whether this minimal entanglement was a mere coincidence or a significant hint. Ian Low, who was present at the conference, found the observation particularly intriguing. Returning to his work, Low, along with Thomas Mehen and Marcela Carena, demonstrated that in general two-particle collisions, the least entangled outcome corresponded to the most symmetric one within low-energy physics. Further research revealed that suppressing entanglement in high-energy Higgs boson scattering led to maximum symmetry, aligning with experimental observations. This consistent correlation between symmetry and low entanglement began to garner significant attention from the particle physics community, as Low reported.
The Elusive Nature of Entanglement
The question arose: should theorists prioritize entanglement over symmetry? Entanglement itself is a complex concept, mathematically sound but conceptually elusive. In the standard model, a particle is a quantum of energy at a specific point. Entanglement allows for two quanta of energy to be shared between locations, meaning the particles’ states are mutually dependent—they haven’t “decided” their individual positions. This phenomenon, while predicted by equations, has never been directly observed; a particle always appears localized when measured. This enigma suggests a reality where, in the absence of observation, space and time may lose their objective meaning, making entanglement a potentially fundamental, yet ghostly, aspect of physical law.
The “Flavour” Mystery and Entanglement’s Role
While earlier findings were insufficient to displace symmetry, a recent discovery in March offered a potential paradigm shift. Particle physics grapples with the “flavour” mystery: why fundamental particles like quarks and leptons exist in three near-identical copies, differing only in mass. “The flavour puzzle of the standard model is the white whale,” stated Jesse Thaler, a theorist at MIT. “Of all the mysteries of the universe, I expect to understand dark matter before understanding flavour.”
Intrigued by previous entanglement research, Thaler and Sokratis Trifinopoulos investigated how minimizing entanglement affected quark flavour mixing. They anticipated that maximum symmetry would result from minimal entanglement, implying no mixing. Instead, they found that minimized entanglement produced precisely the subtle flavour mixing observed in experiments—a result that astonished the scientific community. “It’s come out of nowhere,” remarked Low, expressing bewilderment at the seemingly too-good-to-be-true outcome.
What makes this result particularly perplexing is that physicists have long attempted, without success, to explain flavour mixing using symmetry arguments. Entanglement suppression, however, appears to provide a direct explanation. Thaler struggled to comprehend the connection, asking, “What should entanglement have to do with anything? Who cares what value it is?”
One emerging explanation links entanglement levels to the “quantumness” of nature. While quantum rules govern everything, entanglement varies, influencing the degree of interdependence. Excessive entanglement could lead to a state where distinct atomic structures and even recognizable matter cease to exist, creating a “quantum soup.”
The existence of structured entities like atoms, molecules, and stars depends on a narrow window of stability, suggesting the universe might be fine-tuned to maintain low entanglement, enabling complexity and life. Savage muses, “In some ways, it’s not an explanation, but it minimises how disturbed I am by all of this.” This idea aligns with recent work by Low and colleagues who calculated how particle scattering generates varying degrees of “quantum magic”—a metric for deviation from classical behavior. Their findings indicated a universe that, while fundamentally quantum, favors classical-like outcomes, particularly concerning the relative strengths of electromagnetism and the weak force.
Entanglement’s mathematical framework is increasingly guiding theoretical physics. While traditional string theory faces decline, many its proponents now explore holographic models where strings are not physical entities but projections from a flatter realm. In this holographic view, the smooth, symmetric space of higher dimensions is not fundamental but arises from entanglement patterns in a lower-dimensional quantum world. Conversely, researchers like Ivette Fuentes at the University of Southampton have demonstrated how three-dimensional geometry influences entanglement, suggesting that cosmic expansion since the Big Bang has likely increased it.
Despite these interconnected concepts, their link to entanglement suppression within the standard model remains unclear. Entanglement’s elusive nature, transcending space and time, continues to challenge concrete understanding. “It’s what keeps me up at night,” confessed Low.
The Potential End of Symmetry?
Significant doubts linger regarding entanglement as a definitive answer. If minimal entanglement correlates with high symmetry, it doesn’t necessarily prove causation. Some researchers propose the reverse to be equally plausible. Furthermore, symmetry principles are integral to defining particles within the standard model; without symmetry, the concept of entanglement itself becomes problematic.
A growing segment of quantum theorists dismiss such criticisms as remnants of an outdated worldview. They contend that objects are secondary, and reality is defined by relationships encoded in fundamental entanglement patterns. Nevertheless, many hold onto established principles. Fuentes fondly recalled, “When I came across Noether’s work, I thought it was the most beautiful theorem. I don’t think I’m ready to let go of symmetry.”
The enduring appeal of symmetry lies in its intrinsic connection to beauty. For ancient Greek sculptors, beauty was synonymous with balance and proportion, reflecting a divine cosmic order. This philosophy has resonated through scientific history. The seal of the Institute for Advanced Study, a hub for minds like Einstein and Weyl, features the words “truth and beauty.”
However, beauty is subjective and evolves over time. Just as idealized Greek forms differ from modern artistic abstractions like Henry Moore’s, scientific paradigms shift. Ptolemy’s perfect planetary orbits, once lauded for their beauty, yielded to Johannes Kepler’s ellipses. Ultimately, the ascendancy of minimal entanglement as a guiding principle will hinge on its predictive power, particularly in explaining phenomena like the existence of three quark flavours. As Thaler pragmatically stated regarding its aesthetic appeal, “By definition, the theory that matches experiment is beautiful.”