Angular momentum reversed direction while staying conserved
Deep within the ordered architecture of crystals, physicists have witnessed angular momentum—the rotational force binding planets, electrons, and atoms alike—reverse its direction as it passes between vibrations, while the total amount remains perfectly conserved. An international team, working with terahertz laser pulses and a quantum material called bismuth selenide, has provided the first direct experimental proof of a phenomenon long predicted but never seen: that the symmetry of a crystal lattice can flip the direction of rotation without violating any law of physics. Published in Nature Physics in May 2026, the discovery bridges a century-old insight from Einstein and de Haas to the frontier of quantum material control. In learning to read the hidden grammar of atomic motion, science moves closer to writing new languages for information and memory.
- Angular momentum inside a crystal was observed reversing direction mid-transfer—a result so counterintuitive the team coined it a quantum '1 + 1 = −1' effect.
- The tension lies in the gap between long-standing theoretical predictions and the absence of direct proof—a gap this experiment, for the first time, closes.
- Using two precisely timed terahertz laser pulses fired at bismuth selenide, researchers triggered and then measured the reversal, turning an invisible quantum process into observable data.
- The crystal's own rotational symmetry is the mechanism: certain spin states are physically indistinguishable when reversed, allowing direction to flip while total angular momentum holds constant.
- The findings land as both a fundamental confirmation of conservation laws at the quantum scale and a practical signpost toward ultrafast memory devices and information technologies not yet within reach.
Inside a crystal, atoms vibrate in patterns as strictly governed as orbital mechanics. An international team of physicists has now directly observed something that defies intuition yet breaks no rule: when angular momentum transfers between vibrations in a crystal lattice, it can arrive pointing the wrong way. The total amount is conserved—nothing is lost—but the direction reverses. Researchers from the Helmholtz-Zentrum Dresden-Rossendorf, the Fritz Haber Institute of the Max Planck Society, and collaborators across Europe produced this observation by firing intense terahertz laser pulses at bismuth selenide. One pulse set atoms into circular motion; a second measured a connected vibration elsewhere in the crystal. What came back was the reversal.
The phenomenon connects to a principle Einstein and de Haas established over a century ago: magnetism and mechanical rotation are fundamentally linked, and angular momentum—like energy—cannot be created or destroyed, only moved or transformed. How it moves through a solid lattice had remained a theoretical prediction without experimental confirmation. This work, published in Nature Physics in May 2026, provides that confirmation for the first time.
The reversal is permitted by the crystal's own symmetry. Certain rotational states in the lattice are physically identical whether spinning forward or backward, so when angular momentum transfers between them, the direction can flip while the total stays constant—what the team calls an Umklapp-like quantum effect. Doctoral researcher Olga Minakova, who led the experiments, found the result elegant: the laws of physics, she noted, are directly written by the symmetries of nature. Department head Sebastian Maehrlein called it fundamentally new—the kind of result that may one day appear in textbooks.
Beyond its theoretical weight, the discovery carries practical promise. Mastering how angular momentum moves through crystals could give researchers finer control over ultrafast quantum processes, opening pathways to information technologies and memory devices operating at speeds and scales currently out of reach. Even where intuition fails, the universe follows rules—and rules, once seen clearly, can be put to work.
Inside a crystal, atoms vibrate in patterns governed by laws as rigid as gravity. An international team of physicists has now watched something happen inside those vibrations that shouldn't be possible—and yet it obeys every rule of physics. When angular momentum, the rotational force that keeps planets spinning and electrons orbiting, transfers between different vibrations in a crystal lattice, it reverses direction. The atoms keep spinning, the total angular momentum stays constant, but the direction flips. It's like watching a top suddenly spin backward while the room stays perfectly still.
Researchers from institutions across Europe—the Helmholtz-Zentrum Dresden-Rossendorf, the Fritz Haber Institute of the Max Planck Society, and collaborators in Berlin, Jülich, and Eindhoven—made this observation by firing extraordinarily intense terahertz laser pulses at bismuth selenide, a quantum material with particular symmetries. One pulse set atoms vibrating in a circular motion. A second ultrafast pulse measured a connected vibration elsewhere in the crystal. What they found was the reversal: angular momentum moving from one vibration to another, but arriving backward.
The phenomenon traces back to a principle Einstein and de Haas demonstrated more than a century ago: that magnetism and mechanical rotation are fundamentally linked. Since then, physicists have understood that angular momentum—like energy and momentum—cannot be created or destroyed in a closed system. It can only move from one place to another, transform into different forms. But how it moves through the atomic lattice of a solid has remained largely invisible, a theoretical prediction without direct experimental proof. This work, published in Nature Physics in May 2026, provides that proof.
The reversal happens because of the crystal's rotational symmetry. Some rotational states are physically identical even when they spin in opposite directions—a property of how atoms arrange themselves in the lattice. When angular momentum transfers between vibrations, this symmetry allows the direction to flip while the total amount remains conserved. The team describes it as a quantum "1 + 1 = −1" effect, comparing it to an Umklapp process, where the crystal lattice's symmetry effectively reverses the direction of motion. It is the first experimental demonstration of this behavior with lattice angular momentum.
Olga Minakova, the doctoral researcher at the Fritz Haber Institute who led the experiments, found the result elegant: the laws of physics, she observed, are directly dictated by the symmetries of nature. Sebastian Maehrlein, who heads the department at HZDR and led the overall study, called the results exceptionally exciting—something fundamentally new that might eventually appear in physics textbooks. The practical implications are significant. Understanding how angular momentum moves through crystals could enable researchers to control ultrafast quantum processes with greater precision, potentially opening pathways to advanced information technologies and memory devices that operate at speeds and scales currently beyond reach. The work demonstrates that even in the quantum realm, where intuition often fails, the universe follows rules—and those rules, once observed directly, can be harnessed.
Citas Notables
The laws of physics are directly dictated by the symmetries of nature— Olga Minakova, doctoral researcher at Fritz Haber Institute
We have discovered something fundamentally new that will hopefully make its way into the textbooks— Sebastian Maehrlein, head of department at HZDR and study leader
La Conversación del Hearth Otra perspectiva de la historia
Why does the direction flip? If angular momentum is conserved, shouldn't it just move from one place to another without changing?
That's the intuitive question, and it's exactly what makes this surprising. The reversal happens because of how the atoms are arranged in the crystal—its symmetry. Some rotational states are physically identical even when they spin opposite ways. So when angular momentum transfers, the symmetry allows it to arrive backward and still be the same state.
That sounds almost like a trick. How do you know it's really the same angular momentum and not something else?
Because the total amount is conserved. You can measure it before and after. The direction flips, but the magnitude stays constant. It's like the universe found a loophole in its own rules—except it didn't, because the rules still hold.
What made them look for this in the first place?
They were trying to watch angular momentum move through a crystal lattice directly, something that had never been observed before. They used terahertz lasers to manipulate the vibrations and measure what happened. The reversal was unexpected—a discovery, not a prediction they were testing.
Why does this matter beyond the curiosity of it?
Because if you understand how angular momentum moves through materials, you can control it. That means controlling magnetism, controlling quantum processes at ultrafast speeds. It could lead to better memory devices, faster computing. The physics is beautiful, but the applications could be real.
Is this the kind of thing that changes how we think about crystals?
It changes how we think about what's possible inside them. We've known the rules for a century, but seeing them play out this way—watching symmetry actually govern the behavior—that's new. It confirms something theoretical in a way that opens doors.