A black hole is defined as a region of spacetime where gravity is so strong that nothing, not even light, can escape once it crosses the event horizon. The question of whether these objects are “stationary” is confusing because the term has two different meanings in black hole physics. While in everyday language “stationary” means not moving, in general relativity, the term refers to the unchanging nature of the spacetime geometry around the black hole. All known black holes are in motion and constantly interacting with their cosmic environment.
The Theoretical Meaning of Stationary
The scientific definition of a stationary black hole relates to the stability of the gravitational field around it, not its location in space. In general relativity, a black hole is considered “stationary” if the geometry of the surrounding spacetime remains constant over time. This means the gravitational field measured by an outside observer does not change.
The simplest theoretical model, known as a Schwarzschild black hole, is perfectly “static,” meaning it is non-rotating and uncharged. Its spherical spacetime geometry is time-independent, making it a case of a perfectly stationary black hole.
A more realistic, and still stationary, model is the Kerr black hole, which accounts for rotation. Even though the Kerr black hole is spinning, its external spacetime structure remains stable and unchanging over time, thus classifying it as stationary.
How Black Holes Move Through Space
When considering the physical placement of a black hole, the answer is clear: they move, just like every other object in the universe. Black holes possess mass, and as such, they are subject to the same laws of motion and gravity that govern stars and planets. They are not fixed points in space but rather incredibly dense, moving masses.
Most black holes are not isolated but are part of a larger system, usually a galaxy. They are subject to the gravitational pull of billions of stars and dark matter, which causes them to move through space. This movement includes a general translational drift and, most commonly, an orbital path.
Stellar-mass black holes, which form from the collapse of a massive star, often orbit the center of their host galaxy, much like the Sun orbits the Milky Way’s center. If they are part of a binary system, they orbit a companion star or another black hole in a complex celestial dance. Their movement is a consequence of their gravitational mass and the forces exerted by neighboring cosmic objects.
Dynamic Behavior and Black Hole Mergers
The most dramatic examples of black hole movement and non-stationary behavior involve rotation and violent mergers. Most real black holes are of the rotating Kerr type, which introduces a phenomenon called frame-dragging. This effect causes the black hole to twist or “drag” the surrounding spacetime around with it, meaning that any matter or light near the event horizon is forced to co-rotate.
The movement of black holes in a tight binary system is especially dynamic, as they spiral toward one another. This extreme acceleration generates ripples in the fabric of spacetime, known as gravitational waves. These waves carry away energy and angular momentum from the system, causing the black holes to orbit closer and faster until they eventually collide.
When two black holes finally merge, they create a single, larger black hole in a highly energetic and non-stationary event. The newly formed black hole is often given a powerful “kick” or recoil due to the asymmetric emission of gravitational waves during the final moments of the merger. This phenomenon is like a rocket exhaust propelling the remnant in the opposite direction.
These merger recoils can launch the final black hole away from its birthplace at immense speeds. Calculations suggest that a merging black hole could be launched at up to 28,600 kilometers per second. Observed evidence from gravitational wave events confirms that recoil velocities of up to 5,000 kilometers per second are plausible, which is fast enough to eject the black hole entirely from its host galaxy or globular cluster.