There are moments in physics when reality itself seems to wobble—when theories whisper truths that defy common sense yet remain mathematically undeniable. The Unruh effect is one of these mysterious predictions. Conceived nearly half a century ago, it stands at the shimmering boundary between Einstein’s theory of relativity and the strange fabric of quantum mechanics.
According to this effect, what we call “empty space” is not truly empty. Even a vacuum seethes with restless energy, where particles and antiparticles flicker in and out of existence like sparks in a cosmic storm. But here comes the paradox: whether we see these ghostly particles depends on how we move. To a stationary observer, the vacuum appears silent. To an accelerating observer, however, that silence transforms into a glow of heat—a warm sea of particles that shouldn’t exist.
This is the Fulling-Davies-Unruh effect, often shortened simply to the Unruh effect. It suggests that acceleration itself can conjure a kind of “phantom warmth” out of the void. For decades, this prediction has remained tantalizing yet unreachable, locked behind technological barriers. But now, a team of researchers at Hiroshima University believes they have found a key to unlock this long-elusive door.
Their groundbreaking proposal, published in Physical Review Letters on July 23, 2025, offers a way not only to glimpse the Unruh effect directly, but also to push the frontiers of fundamental physics and future technologies.
The Vacuum That Isn’t Empty
To understand why this matters, imagine sitting in deep space, far from stars or galaxies, in what seems to be absolute emptiness. Quantum theory insists that this void is not truly void. Instead, it is alive with fluctuations—tiny, ephemeral pulses of energy, where pairs of particles appear, then annihilate, vanishing back into nothingness.
Ordinarily, these ghost particles stay hidden. But the Unruh effect predicts that if you were to accelerate—say, in a rocket—you would suddenly perceive the vacuum as a hot bath of radiation. It is not that the particles suddenly sprang into being, but rather that your motion changed your relationship to the vacuum. Reality itself would feel warmer, simply because you moved differently through it.
This strange consequence highlights one of the most profound insights of modern physics: what you see in the quantum world is inseparable from how you move and measure it. In other words, reality is not absolute; it is relational.
The Experimental Challenge
If the Unruh effect is so fundamental, why has it never been observed? The problem lies in the staggering scales required. To feel even the faintest hint of this quantum warmth, an observer would need to accelerate at an almost inconceivable rate—around 10²⁰ meters per second squared. That’s a hundred quintillion meters every second, every second. For comparison, a fighter jet pulling its pilot through extreme maneuvers subjects them to accelerations of only a few hundred meters per second squared.
Such immense accelerations are beyond the reach of traditional experimental setups. For decades, the Unruh effect seemed destined to remain a beautiful but untouchable prediction, filed alongside black holes and cosmic inflation as phenomena we could describe but not touch.
A Bold New Approach
The Hiroshima University team, led by Professor Emeritus Noriyuki Hatakenaka and Assistant Professor Haruna Katayama, decided to tackle the problem from a fresh angle. Instead of trying to push linear acceleration to impossible extremes, they turned to circular motion and the strange behaviors of superconducting circuits.
Specifically, they proposed using metastable fluxon-antifluxon pairs—tiny, paired disturbances that form in superconducting rings called annular Josephson junctions. By exploiting advances in microfabrication, they can create circuits with extraordinarily small radii. In such confined spaces, particles moving in circular motion can experience effective accelerations vast enough to generate detectable Unruh radiation.
In this setup, the Unruh effect wouldn’t appear as a faint heat haze but as a dramatic, measurable shift: the sudden splitting of fluxon-antifluxon pairs. This splitting event creates a clear voltage jump across the superconducting circuit—an unmistakable, macroscopic signature of the effect.
“It’s surprising and almost poetic,” said Hatakenaka. “The tiniest fluctuations of the quantum vacuum can trigger a voltage spike you can measure with ordinary instruments. The invisible whispers of empty space reveal themselves in a shout of electricity.”
From Quantum Fluctuations to Voltage Jumps
Here lies the genius of their approach. Detecting the Unruh effect doesn’t require directly capturing elusive phantom particles. Instead, it leverages how their influence destabilizes fragile quantum systems. The “quantum warmth” induced by acceleration nudges the fluxon-antifluxon pairs until they break apart. When that happens, the circuit responds dramatically: a sudden voltage jump.
By carefully analyzing the statistical distribution of these jumps, the researchers can map the Unruh temperature—the effective “heat” produced by acceleration—with extraordinary precision. And because these jumps shift solely with acceleration while all other parameters remain fixed, the signature becomes unambiguous.
What once seemed beyond reach is suddenly within experimental grasp.
A Window Into Spacetime
The implications of observing the Unruh effect extend far beyond ticking a box in theoretical physics. It would provide one of the most direct demonstrations yet of the deep link between relativity and quantum mechanics—the two great pillars of modern physics that have long resisted reconciliation.
Relativity tells us how spacetime bends and flows under gravity, while quantum mechanics describes the jittery behavior of particles and fields at the smallest scales. The Unruh effect sits exactly at their intersection, a phenomenon where space, time, motion, and quantum energy converge. Verifying it experimentally could sharpen our understanding of spacetime itself, and possibly hint at pathways toward a long-sought unified theory of physics.
Looking Ahead
The Hiroshima University team is far from finished. Their next steps involve analyzing the complex decay processes of fluxon-antifluxon pairs, especially the role of macroscopic quantum tunneling—a phenomenon where particles slip through barriers that should be impenetrable. Understanding these processes will refine their experimental framework and bolster the robustness of their method.
But their ambitions stretch further still. By coupling their system to other quantum fields, they hope to probe even deeper connections, perhaps uncovering clues to mysteries like quantum gravity or the nature of dark energy.
Beyond pure science, their methods also carry potential for technological applications. Ultra-sensitive detectors of quantum fluctuations could revolutionize fields like quantum sensing, information processing, and precision measurement. The very techniques used to catch the “phantom heat” of acceleration might one day power the technologies of tomorrow.
“We aspire for this work to open new avenues in fundamental physics,” said Katayama. “But equally, we hope it inspires others to keep reaching into the unknown—to keep asking what spacetime and reality are truly made of.”
The Human Story in the Physics
In the end, the story of the Unruh effect is not just about equations or circuits. It is about human beings daring to chase after whispers of the impossible. For decades, the effect has existed as a tantalizing rumor in the language of mathematics. Now, for the first time, scientists may be close to hearing its voice in the laboratory.
Physics often asks us to look beyond the ordinary, to stretch our imaginations until they tremble. The Hiroshima University team has done just that, transforming a once unreachable prediction into a testable reality. In doing so, they remind us that science is not only about solving puzzles—it is about deepening our connection to the universe, uncovering the hidden warmth in the vacuum of existence itself.
More information: Haruna Katayama et al, Circular-Motion Fulling-Davies-Unruh Effect in Coupled Annular Josephson Junctions, Physical Review Letters (2025). DOI: 10.1103/mn34-7bj5
