Cosmic amnesia
A black hole rings like a struck bell, then settles into silence, forgetting almost everything about its history. Why?- by Richard DyerRead on Aeon.
Black holes emit a distinctive sound after merging, similar to a bell ringing. This phenomenon is called ringdown. When two black holes collide, they create ripples in spacetime called gravitational waves. These waves travel through space at the speed of light and can be detected on Earth. For example, in September 2015, the LIGO instruments in Washington and Louisiana detected gravitational waves from two colliding black holes 1.3 billion light years away. The remnant black hole from such a merger rings briefly before settling into silence, much like a bell that stops vibrating after being struck. This ringing follows predictable frequencies that help physicists test theories of gravity, including Albert Einstein’s general relativity.
After a black hole merges and rings down, it becomes a simple, featureless object with no trace of its violent history. This phenomenon is described by the no-hair theorem, which states that a black hole can only be fully described by three properties: its mass, spin, and electric charge. All other details—such as how it formed or what it consumed—are lost. For instance, an entire planet could fall into a black hole, and the black hole would show no evidence of this event. This forgetfulness is unlike everyday objects, which retain traces of their history. The black hole’s simplicity arises from its unique properties, particularly its event horizon, a one-way boundary where nothing can escape once it passes through.
Entropy is a measure of disorder or the loss of information in a system. It explains why some processes cannot be reversed, such as perfume diffusing through a room. Initially, the perfume is concentrated near the doorway, but over time, it spreads evenly, making it impossible to determine its original location. This increase in entropy is tied to the direction of time: isolated systems always move from lower to higher entropy. In the 1970s, physicists like Jacob Bekenstein noticed that black holes also exhibit increasing entropy. The surface area of a black hole’s event horizon can only stay the same or grow, mirroring the behavior of entropy. This connection suggests that black holes, like the room with perfume, forget details of their past.
The no-hair theorem, proposed by physicist John Wheeler, states that black holes are remarkably simple objects. Outside a black hole, only three properties can be observed: mass, spin, and electric charge. All other information about the black hole’s history—such as how it formed or what it consumed—is lost. This simplicity contrasts with everyday objects, which retain many traces of their past. For example, a coffee mug reveals its history through its temperature, material, and shape, while a black hole reveals almost nothing. The no-hair theorem implies that black holes are governed by the same physical principles as other systems where entropy increases, leading to the loss of information.
Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape. This is due to their *event horizon*, a spherical boundary beyond which escape is impossible. The first solution describing a black hole was found by Karl Schwarzschild in 1916, during World War I. Later, physicist Roy Kerr expanded this idea to include rotating black holes, known as *Kerr black holes*. These objects are stable and can be disturbed, much like a bell, causing them to ring briefly. The ringing black hole emits *gravitational waves*, which are ripples in spacetime that travel at the speed of light. Detecting these waves, such as those observed by LIGO in 2015, provides insights into the nature of black holes and gravity.
