Light is one of the most studied phenomena in physics — and yet it still surprises. At the Princeton Plasma Physics Laboratory, researchers running mathematical calculations on individual photons have uncovered properties that challenge assumptions physicists have held for decades. Their findings don’t just refine textbook knowledge: they may have direct implications for one of science’s most ambitious goals — generating clean energy from nuclear fusion.
A photon property that refuses to change
The core discovery centers on polarization — the direction, left or right, that a photon’s electric fields rotate as they travel. Eric Palmerduca, a graduate student in the Princeton Program in Plasma Physics, and Hong Qin, a principal research physicist at PPPL, found that this property is topological, meaning it holds constant regardless of what materials or environments a photon passes through. The findings were published in Physical Review D.
Because polarization constrains which direction a photon can travel, a beam made up of photons sharing a single polarization can’t fill every region of a given space. That limitation carries real consequences for how physicists think about light sources.
The result also challenges a long-held implication drawn from the Hairy Ball Theorem — a mathematical principle stating that you can’t comb a sphere of hair flat without creating at least one cowlick. Physicists had interpreted this to mean no light source could send photons in all directions at once. Palmerduca and Qin found that interpretation was incomplete: the theorem doesn’t account for the fact that photon electric fields can rotate, and once you include that rotation, the constraint dissolves entirely.
Spin and orbit: a debate settled
For familiar massive objects, rotational motion comes in two separable forms. Earth spins on its own axis while simultaneously orbiting the sun — two independent motions that don’t interfere with each other. This separability holds for everything with mass.
Photons are different. Palmerduca and Qin showed that for massless particles, angular momentum can’t be cleanly divided into spin and orbital components. “Most experimentalists assume that the angular momentum of light can be split into spin and orbital angular momentum,” Palmerduca said. “Our work helps settle this debate, showing that the angular momentum of photons cannot be split.”
That inseparability follows directly from the photon’s topological properties, linking both discoveries into a coherent picture. The findings also amend the influential particle classification system developed by Eugene Wigner, a former Princeton professor widely regarded as one of the most important theoretical physicists of the 20th century. Wigner’s framework accurately describes particles with mass but produces inaccurate results for massless particles like photons. By applying topology, Palmerduca and Qin modified Wigner’s classification to work in all directions simultaneously.
From single photons to fusion plasma
Understanding individual photons was never the end goal. The researchers were working toward a harder problem: using intense light beams to excite topological waves inside tokamaks — the ring-shaped devices designed to contain fusion plasma.
Topological waves are sustained disturbances that tend to form at the boundary between two different regions, such as the plasma inside a tokamak and the surrounding vacuum. They’re not exotic phenomena. Similar waves occur naturally in Earth’s atmosphere, where they contribute to El Niño — the periodic warming of Pacific Ocean water that shapes weather patterns across the Americas.
Deliberately generating these waves in plasma could potentially sustain the extreme temperatures that fusion requires. Qin describes the technique in tactile terms: “Just as using a hammer to hit a bell causes the metal to move in such a way that it creates sound, the scientists want to strike plasma with light so it wiggles in a certain way to create sustained heat.”
Beneficial waves and harmful ones
The challenge ahead isn’t simply creating topological waves — it’s creating the right kind. Some could pull heat away from the plasma rather than sustaining it, undermining the very goal the technique is meant to serve.
Qin frames the problem through an unexpected analogy. “Topological waves are like new breeds of insects,” he said. “Some are beneficial for the garden, and some of them are pests.” The researchers plan to investigate how to selectively excite the helpful varieties while identifying and suppressing the deleterious ones.
That work will require moving beyond theory. “We have a clearer theoretical understanding of the photons that could help excite topological waves,” Qin said. “Now it’s time to build something so we can use them in the quest for fusion energy.” The next phase focuses on developing experimental tools capable of translating these mathematical insights into practical plasma heating techniques — a step that could bring fusion energy incrementally closer to reality.