Picture yourself listening to a radio in the dead of night, hoping to catch the whisper of some distant station. Instead, all you get is the hiss of the ether. That is roughly how physicists listened for a signal from dark photons: 36 hours of silence, but that silence turned out to be worth more than any amount of noise.
Dark photons are hypothetical particles thought to be possible constituents of dark matter. Unlike ordinary photons, they have a tiny mass, and they interact with matter extremely reluctantly. In theory, though, they can mix very weakly with ordinary light. To catch that mixing, researchers build special detectors called haloscopes. The DALI prototype at the Institute of Astrophysics of the Canary Islands is not your usual sealed metal box but an open resonator: a stack of 20 zirconium oxide plates, rather like a shelving unit, set inside a copper mirror. This design keeps a large volume even at high frequencies.
The setup scanned a frequency range around 6.9 GHz, which corresponds to a dark photon mass of roughly 28.5 microelectronvolts (billions of times lighter than an electron). Over 36 hours of effective observation, not a single narrow line appeared that would betray a dark photon turning into an ordinary one. The sensitivity reached χ ≲ 6.8×10⁻¹⁴. Put into everyday terms, that means for every trillion ordinary photons there would be at most one dark photon: like hunting for one special grain of sand on a beach hundreds of kilometers long.
Previously there were no sufficiently sensitive laboratory measurements in this mass range, and nobody knew how strongly dark photons might mix with ordinary light. Now the tightest limit has been set. This does not mean dark photons do not exist: they could be fainter than the current prototype can detect. But for the full-scale DALI, which will have a hundred times the area, this is great news: the concept works, and there is room to grow.
No practical applications beyond fundamental physics are in sight for this result just yet. But as with listening through the night, the very fact that we have learned to tell silence apart brings us closer to the moment when the whisper will finally be heard.
A sensitivity of χ ≲ 6.8×10⁻¹⁴ means that if a dark photon and an ordinary photon mixed with that strength, there would be roughly one dark photon for every trillion ordinary photons. It is like hunting for one special grain of sand on a beach hundreds of kilometers long.
The idea of hidden photons that permeate our Universe and only rarely reveal themselves echoes the "dark sector" concept from science fiction, for instance in Liu Cixin's novel The Dark Forest, where invisible particles play a key role in the plot.