You can gaze upward at night to see a veritable ocean of stars, but the visible parts of the universe are just a fraction of the total. The rest, we think, is dark matter and dark energy, which are by their very nature invisible. A new experiment beginning in Germany might finally start unraveling the mysteries of dark matter by shining a light through a solid wall.
As you can imagine, it's hard to study something invisible, but dark matter isn't just invisible—it doesn't interact with ordinary matter except through gravity. We can see the effects of dark matter in the way stars move around in galaxies, but zeroing in on these unknown particles is no simple feat. An international team has just slipped the switch on ALPS II, an experiment at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg that could do just that.
There are several theoretical candidates for dark matter, including weakly interacting massive particles (WIMPs), primordial black holes, and axions. A recent study of Einstein Rings suggested that axions are a closer match for what we see in the universe, and that's the target of ALPS II. The experiment consists of a 250-meter tunnel with an optical cavity that amplifies a laser. As the laser travels down the "Hera" tunnel, it is exposed to an intense magnetic field produced by 12 superconducting magnets. Theoretical work on axions shows that it may be possible to turn a normal photon into an axion—a particle of dark matter—with such a high-power field.
The ALPS II team hopes that at least some of the photons in the laser will morph into axions when they encounter the magnetic field. But how can you detect that if dark matter doesn't interact with anything? ALPS II cleverly gets around that little speedbump by filtering out all the non-transformed light with a sophisticated device known as a wall. So, the laser hits a wall, but any photons that have become axions would pass through it. On the other side, they can transform back into photons to be detected.
The detectors in the tunnel are designed to be highly sensitive because even the most generous estimates acknowledge that photon-axion transformations rarely happen. DESY's Axel Lindner compares it to the odds of "throwing 33 dice and them all coming up the same." Still, if any light leaks through the wall, that would be strong evidence for axions as dark matter, which could profoundly affect how we understand the universe and help to plug some gaping holes in the Standard Model of particle physics.
