On August 25, the live-streamed segment "Charles Zhang's Physics Lesson," part of the "Basic Science Conference Series Dialogue" on Sohu Video Focus, concluded with its final episode. Charles Zhang, founder, chairman, and CEO of Sohu.com and a physics PhD, engaged in a three-hour deep conversation with Xiao-Gang Wen, the Cecil and Ida Green Professor of Physics at MIT. Their discussion, themed "The Next Revolution in Physics: Extremely Small, Extremely Large, or Extremely Complex?", explored the physical picture emerging from quantum entanglement to macroscopic world phenomena.
Beginning with their respective跨界 experiences, they traced the development of topological states of matter, gradually unveiling a new emergent worldview behind the fractional quantum Hall effect and the "ocean of qubits." They also engaged in philosophical debates on research methodologies, the boundaries of AI capabilities, and the value of basic science communication, providing a concluding chapter to this series of scientific dialogues spanning microscopic, mesoscopic, and cosmic scales.
This conversation was the finale of this year's "Basic Science Conference Series Dialogue" under "Charles Zhang's Physics Lesson." On August 12, Zhang had discussed the emergent patterns of classic disordered systems like sandpiles and foams with Professor Ruwei Liu, director of the Center for Soft Matter and Living Matter at the University of Pennsylvania. On August 18, he explored the controversies and boundaries of the standard cosmological model with Professor Subir Sarkar, professor emeritus at the University of Oxford.
From High-Energy to Condensed Matter: New Scientific Discoveries Often Emerge at Interdisciplinary Boundaries
Wen initially trained in high-energy physics under string theory master Edward Witten before transitioning to condensed matter physics. He recalled that although both fields study physics, they seem to use two different languages: high-energy physics, influenced by relativity, often treats time and space together, while condensed matter physics typically treats time separately, focusing more on how matter organizes in space. Zhang likened this to someone originally skilled in wielding a "broadsword" having to switch to a "spear" in another field. Without learning the new field's language, one risks being unrecognized by both sides.
Although the two physics languages cannot be mixed, it is precisely this dual perspective that allows Wen to find unique entry points when examining condensed matter problems. Many of his early important works benefited from this difference. "New growth points in science often lie not at the center of one field, but at the boundaries between different fields, where new things are most easily generated." Zhang compared this to "hybrid vigor" in biology, where stacking multiple genes can produce stronger vitality, and often interdisciplinary crossover is a crucial source of innovation.
Topological Order and Category Theory: New Foundations of Condensed Matter Physics
Why do materials have different forms? Zhang posed this question using the everyday example of water freezing into ice. Wen explained that liquid water remains invariant under continuous translations, while after freezing, water molecules form a lattice, remaining invariant only under specific translation distances—hence, water and ice have different symmetries. This symmetry-based analysis is the traditional paradigm for describing different states of matter.
Just as traditional theories prevailed, Wen proposed the concept of "topological order" in 1989. This is a new state of matter that cannot be characterized by symmetry: their symmetries are the same, but their lowest energy states differ, and they are not easily disrupted by local impurities. If local impurities are viewed as deformations, this insensitivity to deformation is precisely the mathematical meaning of "topology." Related research has provided a new framework for understanding novel phenomena in condensed matter systems and earned Wen the Dirac Medal, the highest honor in theoretical physics.
Because of the novel properties of topological order, the mathematical tools used to describe matter's symmetry are no longer sufficient. Category theory, a mathematical theory that emphasizes the relationships and organizational structures between things, can be used to classify topological order. Wen admitted that category theory is indeed "daunting," with even mathematicians rarely studying it, but he has still benefited greatly from it and laments being held back by old ways of thinking. He encouraged young students to start directly with category theory, viewing the world from the most cutting-edge perspective of "relations."
Zhang added that category theory deserves wider popularization. He believes new physics revolutions often come with new mathematical tools, and category theory, as an important new mathematical framework for describing many-body entanglement, deserves more attention and understanding from physics researchers.
Where Does Light Come From: The Answer May Lie in an "Ocean of Qubits"
After years of deep research in topological order and long-range entanglement, Wen proposed a rather颠覆性 conjecture: the light we encounter daily might also be a macroscopic manifestation of quantum entanglement. This conjecture is not groundless. While studying a new type of quantum matter called spin liquids, he found that the propagation patterns of spin waves perfectly match Maxwell's equations describing electromagnetic waves. Later, with collaborators, he confirmed that this peculiar phenomenon stems from a special quantum entanglement structure, which they named string-net condensation.
String-net condensation can be understood intuitively: imagine the vacuum as an "ocean" composed of qubits, where qubits can connect to form closed strings. These strings constantly move and fluctuate, creating a dynamic "string liquid." The density distribution of strings corresponds to the familiar electric field: along the string direction, string density remains constant, with density fluctuations only perpendicular to the string—matching the properties of electric fields perfectly. The endpoints of strings correspond to electric charges—once a string breaks, a charge appears at the break point. Thus, the electromagnetic field transmitting interactions and the charges constituting matter both emerge naturally from the same quantum entanglement structure.
"Traditional grand unified theories can only incorporate three fundamental forces into one framework but cannot include the fermions that constitute matter. The string-net condensation and qubit ocean picture unifies both force and matter," Wen said. However, this theory cannot yet explain gravity. Wen has attempted to extend "strings" to "membranes," using a "membrane liquid" model to derive Einstein's gravitational equations, but the mathematical structures do not match, and this direction has not yet achieved a breakthrough.
How to Unravel the World's Mysteries: Seeking Essence in Quantum Entanglement
Starting from string-net condensation, Wen proposed a new worldview, summarized in three keywords: extremely small, extremely large, and extremely complex. The extremely small is simple to construct—just qubits, with only 0s and 1s, no other internal structure. The extremely large world is immensely complex, with Maxwell's equations, Dirac equations, and various states of matter. Connecting the two is the "extremely complex" middle ground, physically known as quantum entanglement. Entanglement is an organizational structure that links the simplest qubits to the rich world we see.
"What matters more is not what the fundamental building blocks of the world are, but its organizational structure," Wen said. This is an "emergent worldview." Knowing the fundamental building blocks are qubits is insufficient; what truly determines the world's appearance is the entanglement structure between qubits. In this new cosmological view, what the basic units of the world are matters less; what truly determines the face of all things is the entanglement structure between qubits.
Zhang believes that from Aristotle to Socrates, humanity has always thought that finding the smallest constituent unit would unlock the world's mysteries, but in reality, the relationships and organizational structures between units are the more essential thing. This is also the research approach of condensed matter physics, which differs most from traditional reductionism.
"Conjectures," "Wandering," and AI: The Methodology Debate in Theoretical Physics
Throughout the conversation, Wen repeatedly emphasized a unique physics research method: bold conjecture. "I often have many conjectures, guessing this and that, but I must judge which guess is right and which is wrong. Once I feel a conjecture is correct, I immediately accept it." Wen stated that mathematicians pursue rigor, but physicists, as long as they believe a conjecture is correct, proceed forward based on it.
Wen compared this method to "building a mansion": first guess the overall structure, leaving the strict details of screws and nuts for later generations. This contrasts interestingly with the mathematician's path of step-by-step rigorous proof. Zhang also advocates this "free-spirited" learning style, saying, "I just understand some basic principles and start deriving on my own." He believes this is the best and most efficient way to learn.
Regarding AI's role in theoretical physics research, Wen believes AI's help is mainly in computation and verification. Many integral calculations previously impossible can now be delegated to AI, greatly accelerating the speed of conjecture verification, allowing faster guessing. However, AI cannot yet generate new definitions or concepts. He stated that once structure and definitions are clear, the remaining logical reasoning might be left to AI. But before definitions emerge, that vague, uncertain creative stage can only be handled by the human brain.
Zhang summarized with a metaphor: AI is like a horse, and humans are the riders—once the direction is pointed out, the horse can run fast. This judgment aligns with his earlier views in conversation with Sarkar: AI can be a powerful tool, but true conceptual breakthroughs still require humans.
Understanding Science Is Prerequisite for Effective Popularization: Frontline Researchers Should Undertake Outreach Responsibilities
During the interactive segment, young scholars and students from Tsinghua University, Nanjing University, the Chinese Academy of Sciences, and other institutions enthusiastically posed questions on topics including the prospects of topological quantum computing, the field theory correspondence between high-energy and condensed matter physics, experimental searches for new states of matter, and pathways to quantum gravity.
Also present were Sohu Video hosts @Yu Ying-Professor of Condensed Matter Physics, @Zhang Chunfeng-Physics Professor, @phyzc (Professor at China University of Petroleum), and @Gao Qingyi (AI scholar), who discussed the significance of scientific research and communication. In their view, the Sohu Video Focus stream is like "gathering faint glimmers," bringing together experts and scholars from physics, mathematics, AI, and other fields, building a "prairie fire of science communication" where different disciplines collide across boundaries.
From personal research and teaching to public science communication, Wen and Zhang shared their perspectives. Wen admitted that science communication is harder than doing research. Popularization must sacrifice rigor, but after sacrificing rigor, it easily becomes fabrication. Only frontline researchers can strike that balance—conveying the spirit and direction correctly without mathematical details. He called on frontline scientists to undertake outreach duties, as distilling, abstracting, and presenting one's research concisely is itself an exercise for researchers, potentially generating new ideas during the reorganization process.
Furthermore, Wen believes doing science communication also benefits researchers in return, suggesting scientists try "drawing a cartoon without mathematical formulas" to abstract concepts, because drawing cartoons forces abstraction and capturing the essence of concepts. Zhang added that letting knowledgeable frontline researchers handle popularization is key. "If popularization relies on various analogies, it becomes misleading." The significance of this dialogue also confirms Zhang's statement: "Use mass media to spread the most profound science, and use popularization methods to disseminate the most rigorous knowledge."