- What makes a black hole black?
- Nothing that goes in comes back out, light included, because leaving would take more than the speed of light. There is no surface to reflect anything and nothing inside to shine, so the object itself is invisible at every wavelength.
- How big is one?
- Mass alone fixes it: the horizon radius is about three kilometres for every solar mass. A hole of the Sun's mass would be three kilometres in radius, and the 4.3-million-solar-mass one at the centre of this galaxy is about eighteen times the radius of the Sun.
- Where do they come from?
- The stellar-mass ones are what a star of more than roughly twenty solar masses leaves when its core runs out of fuel and collapses. Where the supermassive ones came from is unsettled: they were already in place when the universe was a few hundred million years old, which is early for anything to have grown that heavy.
- How do you find something invisible?
- By what it does to its surroundings. A star orbiting nothing visible gives away the mass at the focus, gas falling in heats until it emits X-rays, and two holes merging shake spacetime hard enough for detectors on Earth to register it.
- Has one been photographed?
- Two have — the hole in M 87 in 2019 and the one at the centre of the Milky Way in 2022, both by the Event Horizon Telescope. Neither image shows the hole; each shows the ring of hot gas around it and the dark patch its gravity cuts out of that ring.
- Could a black hole swallow the Earth?
- Not from a distance. Gravity does not care what an object is made of, only how much of it there is and how far away it sits — if the Sun were replaced by a black hole of the same mass, the Earth would keep the orbit it has now and freeze rather than fall. A black hole is dangerous because of how close you can get to that much mass, not because it pulls harder than anything else.
- What would happen at the event horizon?
- For a large black hole, nothing locally dramatic: the horizon is not a wall, and someone crossing it would notice no boundary. The stretching force comes from the difference in gravity between head and feet, and that difference is weaker for bigger holes — fatal well outside a stellar-mass horizon, and gentle at a supermassive one. Falling in remains one-way in every case.
- Do black holes last forever?
- In theory no. Hawking radiation lets one leak energy at a rate that rises as its mass falls, so the smallest evaporate fastest. The timescales are not comparable to anything: a stellar-mass hole would take on the order of 10⁶⁷ years, against a universe presently about 13.8 billion years old. Nothing of the sort has been observed.
- What is in the middle?
- General relativity says a singularity — a point of infinite density — and that answer is generally read as the theory reporting its own limit rather than describing a place. Predicting what is really there needs a theory of quantum gravity, which does not yet exist. Nothing about the outside of a black hole depends on the answer.