At the center of a galaxy, a huge black hole is shrouded in an invisible cloud — a swarm of ultralight particles that it itself spawned. Into this cloud dives another black hole, about the size of a star, and begins a long spiral dance. The approach takes millions of years, but the most dramatic events unfold right at the horizon.
The orbit of the small hole is rarely a perfect circle. Usually it is elongated, like an ellipse, and tilted relative to the big one's equator. Earlier calculations simplified things: the motion was considered circular and confined to a single plane. The new work for the first time honestly accounts for both elongation and tilt. It turns out that an elongated orbit gives rise to a dense picket fence of resonances — moments when the frequency of approach matches the cloud's natural frequency. At such moments, energy exchange is particularly vigorous.
The tilt also brought a surprise. If the orbit is only slightly inclined (less than a critical angle), then at small distances from the big hole, energy begins to flow not from the hole to the cloud, but in the opposite direction — from the cloud to the small hole. For a while, the cloud becomes not a brake but an accelerator. For large elongations, resonances persist at any inclination, but their pattern differs for prograde and retrograde motion relative to the big hole's spin.
The small hole's dance through the cloud leaves a trace in gravitational waves — ripples in spacetime that detectors like LISA will catch. Resonances and the cloud's role reversals will be imprinted in the signal's frequency and loudness. This will help not only to see the invisible cloud but also to probe the strongest gravitational fields in the Universe.
Ultralight particle candidates are hundreds of billions of times lighter than an electron. If an electron weighed as much as a human, such a particle would weigh as much as a speck of dust.
In Robert Wilson's novel "Spin," an invisible membrane alters the flow of time around Earth — here, an invisible cloud alters the fate of a falling black hole.