A large underground tank of liquid xenon waits for a rare event — a nudge from a dark matter particle. Normally, gravity is too weak to notice such a nudge. But if space has hidden dimensions, everything changes: at tiny distances, gravity suddenly strengthens, and the chance of catching an elusive particle grows.
The authors revisited the idea of Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali about large extra dimensions. In such a model, every gravitational "nudge" is accompanied by a multitude of invisible copies — Kaluza-Klein modes (oscillations of gravity in hidden dimensions). Together, they produce an effect comparable to the force acting on an atomic nucleus. Calculations showed that the sensitivity of modern xenon detectors is sufficient to place strict constraints on the mass of dark matter and the fundamental scale of the multidimensional world.
Another approach is the search for gamma rays from the center of the Galaxy. Dark matter particles can collide and annihilate (turn into radiation), resonantly capturing the same Kaluza-Klein modes. This process leaves a bright "fingerprint" in the form of high-energy photons. Comparison with data from gamma-ray telescopes yielded even tighter limits on the possible properties of multidimensional dark matter.
Thus, two completely different experiments, an underground xenon tank and an orbital observatory, converge on one point: if extra dimensions exist, they cannot be arbitrarily large. And dark matter may already be leaving traces that we do not yet know how to read.
In models with extra dimensions, the force of gravity at distances smaller than a millimeter could be billions of times stronger than Newton predicts, as if a magnet held not a paperclip but a car.
In Carl Sagan's novel "Contact," humanity receives blueprints for a machine that uses extra dimensions for instantaneous travel — the physics of ADD models allows such tricks with gravity.