Black holes are, in many ways, the simplest objects in the Universe. According to general relativity, black holes are described by only two parameters: their mass and their spin. Yet this apparent simplicity hides a profound mystery. A black hole is thought to be surrounded by an event horizon — a boundary beyond which nothing, not even light, can escape. At this surface, space and time effectively swap roles, and any information crossing it is forever hidden from the outside world.
The black hole paradigm leads to deep puzzles. One of the most famous ones is the information-loss paradox, which questions whether information that falls into a black hole is truly lost forever. If so, this would clash with the fundamental laws of quantum physics, which state that information must be preserved. This raises a fascinating possibility: is the event horizon a physical surface, or is it a mathematical prediction of Einstein’s theory that might need revision?
Gravitational waves offer a unique way to investigate this question. When two compact objects — such as black holes — orbit around each other, they gradually spiral inward and eventually merge. The newly formed object "rings" as it settles down, emitting gravitational waves like a bell produces sound. By measuring the frequency and how quickly these vibrations fade, we can infer the nature of the final object. If it has an event horizon, its ringing follows the predictions of general relativity. In the absence of a horizon, subtle differences may appear in the gravitational signal.
The ThorGW project set out to explore exactly this possibility. First, it developed a general description of how compact objects without an event horizon would vibrate, and connected these predictions to observable gravitational-wave signals. Second, it used current data to place the first constraints on whether astrophysical black holes truly possess an event horizon. Finally, it looked ahead, forecasting how next-generation gravitational-wave detectors will sharpen these tests and bring us closer to answering one of the most fundamental questions about the nature of space and time.
