At three o'clock in the morning, the deep-space monitoring array beyond Pluto's orbit suddenly lit up with red alarms. Duty officer Lin Long heard an extremely faint yet exceptionally clear signal in her headphones—as if someone had gently plucked a string somewhere in the distant void. She immediately pressed the red button, routing the data stream into the "String Light Cloud Brain's" quantum decoding channel. Three minutes later, the "Prometheus Broadcast" relay center on Phobos was awakened; five minutes after that, one hundred and twenty-eight linguists, mathematicians, musicians, and quantum communication engineers were urgently pulled into an encrypted conference room. Mozi, leaning on his ebony cane, stepped into the synchronization hall for the first time in the seventh winter since Yue'er's passing. He looked up at the holographic screen, where only one flashing message appeared:
"Quantum coherent signal received from M31, confidence level 99.997%."
The Andromeda Galaxy, 2.54 million light-years away—humanity's initial call had received an answer at this very moment.
The Prometheus Broadcast began in 2042, jointly initiated by Mozi, Yue'er, and Xiuxiu. It employed Yue'er's newly formed "Information Geometry Field Theory" compression algorithm, encoding humanity's hundred thousand years of art, science, and emotional history into a quantum string capable of maintaining coherence on a cosmic scale, launched via the "String Light Cloud Brain" on the world's most powerful Fermi Gamma-ray Array. Theoretically, this signal would maintain a decodable entanglement structure even after traversing millions of light-years—like an unbreakable spider's thread, stitching Earth together with unknown civilizations. Seventy years had passed; people had long regarded this as a romantic commemorative ceremony, no one truly expecting to hear an echo. Yet the echo came, and in a nearly poetic manner: it was a quantum-state "Ode to Joy," the fourth movement of Beethoven's Ninth Symphony, but rewoven into a "music-mathematics dual-state entity" combining classical harmony with quantum superposition.
The decoding process lasted forty-six Earth days. First, the signal was injected into a cryogenic rubidium atom pool, with Raman sideband cooling to eliminate thermal noise; then through quantum tomography, the signal's high-dimensional density matrix was reduced to a Hilbert subspace processable by classical computers; the final step, and the most difficult—translating quantum amplitudes into audible melodies. Musicologists had initially worried they would hear unrecognizable electronic noise, yet what came through the headphones was the familiar D-major chord that brought tears to their eyes, only with each beat simultaneously existing in both "sounded" and "unsounded" states, as if the entire orchestra were playing inside Schrödinger's box. Even more incredible was that if Yue'er's field theory symmetry operators were applied to this quantum musical phrase, its energy spectrum perfectly matched the theoretically predicted "emotional topological invariants"—as if the sender not only understood human music but also comprehended Yue'er's yet-to-be-published cosmic grammar.
Mozi first listened to the complete fragment alone in his old residence by Mirror Lake in Jiuquan. The night was quiet, quantum headphones completely isolating the external world; he heard string instruments gently undulating in the vacuum, like someone breathing in the darkness. When the chorus section arrived with a thunderous presence, the sound was no longer mere acoustic waves, but clusters of entangled photons simultaneously "existing—disappearing" on his eardrums, each superposition stirring minute currents traveling along the vagus nerve straight to his heart. He suddenly remembered Yue'er's last smile on her deathbed—that smile carried the same duality: farewell and eternity occurring simultaneously. He removed the headphones, discovering tears had already flowed along his wrinkles to the corner of his mouth, salty yet sweet. He whispered to the invisible singer: "The universe is singing."
In the technical report, people described this miracle with calm terminology: the sender employed "intergalactic quantum entanglement relay," using the Andromeda Galaxy's central black hole as a natural amplifier, embedding entangled photon pairs into the B-mode component of cosmic microwave background polarization; the musical structure followed the E8×E8 extension of the "information geometry symmetry group," precisely corresponding to the "empathy particle" propagation mode in Yue'er's field theory; the signal's end contained a self-extracting "mathematical key" written in the arrangement of non-trivial zeros of the Riemann zeta function, proving the sender mastered the complete underlying grammar of human mathematics. In more secretive appendices, linguists pointed out that the quantum music's "harmonic-discrete dual-state encoding" was highly isomorphic to the "dual-pulse laser phase modulation" algorithm Xiuxiu had used years ago for lithography machine error compensation—as if the sender were paying homage to Xiuxiu's personally designed precision optical path.
Xiuxiu did not wait for the official announcement. She was in the "light moss" experimental module beyond Pluto's orbit, personally connecting the signal to the biological quantum interface composed of "light moss." The moment the music began, the entire chamber filled with a soft blue-green glow; billions of silicon-based microorganisms, as if guided by an invisible conductor, synchronized their flashing into breathing rhythms. She remembered thirty years earlier, when she first placed DNA origami circuits into a vacuum chamber, seeing similar fluorescent flashes—that was the first time she felt the boundary between "machine" and "life" dissolve. Now, the song from Andromeda made boundaries disappear once more: music was no longer air vibration but photons, electrons, excitons within "light moss" cells; it was the "String Light Code" she, Yue'er, and Mozi had collectively written echoing through interstellar space. She reached out to touch the chamber wall; her fingertips registered faint warmth, like holding a hand extending from the vacuum. She wrote in her log: "What they sing to us is not just the Ode to Joy, but the universe comforting loneliness."
When the signal cycled for the seventh time, she suddenly discovered an extremely subtle phase drift hidden within the chorus section—if the three notes of the D-major tonic chord were mapped to RGB primary colors, and the phase differences plotted as spatial curves, it formed a fractal pattern strikingly similar to ginkgo leaf veins. Ginkgo, the first tree they three had planted before the research institute's entrance decades ago. She instantly understood: the sender was saying, "We see you, and we see your story." In that moment, tears streamed down her face, yet she smiled like a child recognized by distant relatives.
Before the official announcement, Mozi insisted on adding a "non-technical acknowledgment." He wrote: "We thank the sender for responding with music rather than weapons; we thank them for leaving gentle uncertainty within quantum superposition; we thank them for allowing Yue'er's departed field theory to blossom again in the starry sky; we thank them for reminding us—no matter how many light-years apart, communication and understanding remain civilization's first instinct." At the statement's end, he appended a personal dedication: "If you've ever trembled from loneliness in the deep night, please remember: on this side of the galaxy, three people once felt warmth through tears because of your song."
After the signal went public, Earth entered a peculiar festive atmosphere. City squares at midnight synchronized playback of quantum "Ode to Joy"; people donned simple quantum headphones, embracing, crying, kissing within choruses simultaneously "existing—not existing." Astronomers launched the #CosmicIsSinging hashtag on social media, reaching over six billion impressions in three days; musicians transformed quantum phases into new genre "entangled electronica," topping global charts; programmers compiled the signal's Riemann key into open-source libraries, enabling any terminal to generate small segments of "Andromeda harmony." Regions previously tense from resource or belief conflicts temporarily ceased fire during shared music minutes—soldiers removing headphones discovered enemy faces equally streaked with tears.
On the technical level, humanity gained more than art and emotion. Quantum communication engineers designed "black hole relay station" prototypes based on the signal's relay patterns: utilizing stellar-mass black holes' gravitational lensing effects to project entangled photon pairs millions of light-years away, theoretically reducing real-time communication delays to "cosmic Planck time" levels. Xiuxiu's team transplanted the music's phase modulation technology into next-generation "lithography-light painting" hybrid systems, achieving atomic-scale "musical etching"—chips no longer mere computing tools but solidified musical movements. And Yue'er's unfinished "emotional topology" paper was completed through musical signal verification: her predicted "empathy particle" was confirmed as a topological excitation capable of transmitting emotional information at quantum levels, its propagation speed unconstrained by light speed yet requiring sender and receiver to share isomorphic "topological genus" in "emotional space"—in other words, only civilizations truly understanding each other's pain can hear one another's songs.
On Mozi's eighty-sixth birthday, he drifted alone on Mirror Lake in a light sail boat. He turned the quantum headphone volume to maximum, letting "Ode to Joy's" superposition states flood his eardrums. The lake water reflected countless stars; each "existence—disappearance" in the headphones resembled a star blinking. He suddenly realized the signal's 2.54 million light-year journey to Earth coincided precisely with the seventh year after Yue'er's passing, the third year after Xiuxiu's retirement, and his own first winter deciding to "return the world to the world"—this was no coincidence; the universe was telling him in the language of light: separation is not an endpoint but echo's beginning. He looked toward Andromeda's direction, softly humming the old song: "Freude, schöner Götterfunken..." Voice hoarse, yet carrying childlike excitement. Lake ripples formed, like countless tiny hands keeping rhythm. In that moment, he knew the sender could hear, could see—on both sides of the galaxy, loneliness facing loneliness, then singing together.
The signal continues updating daily. Humanity gradually learns to listen with humbler posture: no longer rushing to decode hidden technologies, no longer attempting to respond with trade or war, but quietly awaiting each chorus section's subtle phase drift—it resembles the sender's heartbeat, reminding receivers: behind music lies emotion, behind emotion lies consciousness, behind consciousness lies a civilization seeking sparks in cosmic darkness just like themselves. Mozi wrote this realization into his final elementary natural science lesson plan, titled simply—"The Universe Is Singing, We Learn to Cry."
When class ended that day, he assigned homework: close your eyes, imagine an invisible string extending from your heart toward the starry sky, try plucking it with your breath. Children earnestly complied; the classroom filled with faint yet synchronized breathing sounds, like countless tiny wings beating simultaneously. Mozi stood beside the podium; quantum headphones happened to deliver quantum "Ode to Joy's" final beat—that beat simultaneously "is" and "isn't," like a gentle sigh. He gently removed the headphones, placed them at the podium's center, letting music continue superimposing in the air. Children didn't open their eyes, yet many small faces revealed faint smiles, as if they truly heard harmonies from distant galaxies, within which loneliness translated to understanding, night translated to dawn, separation translated to eternity.
When the dismissal bell rang, Mozi walked out alone. On Mirror Lake, sunset dyed the water surface golden, like a giant string spanning heaven and earth. He paused, took a deep breath, and softly spoke toward the invisible distance: "Thank you for hearing us." The wind carried faint echoes, like an unfinished lyric, or an unlanded sigh. He knew that echo would persist, like the never-decaying "empathy particle" in Yue'er's field theory, like the never-defocusing extreme ultraviolet laser in Xiuxiu's lithography machines, like himself—an old man finally learning to listen quietly within the cosmic chorus—the never-extinguishing string light in his heart.