What We Really Know About Black Holes

Black holes are among the most fascinating and often misunderstood objects in astronomy. This article separates what is well established from what is still being investigated.

What is well established

• Black holes exist. Astronomers detect them by their gravitational effects on nearby stars, by the intense radiation from material falling into them, and by gravitational waves from mergers.
• Around every black hole is an event horizon — a boundary beyond which nothing, including light, can escape.
• Every large galaxy studied in detail so far appears to contain a supermassive black hole at its center.

What is still being studied

• The physics inside the event horizon is not fully understood. General relativity predicts a singularity, but this likely signals that a deeper theory is needed.
• The relationship between supermassive black holes and their host galaxies is an active research area.
• Hawking radiation, a theoretical process by which black holes could very slowly lose mass, has not been directly observed.

Common myths

• Black holes are not cosmic vacuum cleaners. From far away, a black hole's gravity behaves like any other massive object. A star that formed at the same distance would keep its planets.
• A black hole with the same mass as the Sun would not swallow Earth's orbit — Earth would orbit it just as it orbits the Sun today.

Black holes are strange, but not magical. They are natural consequences of the same physics that shapes everything from planetary orbits to the flow of gas in galaxies.