Question

Black holes

Q: what is escape speed?

Q: what is a black hole? Size? What is the event horizon?

Q: How to “see” black holes if they are black? Evidence for stellar mass black holes? Supermassive black holes?

Answer 1

Escape velocity is the speed that an object needs to be traveling to break free of a planet or moon's gravity well and leave it without further propulsion.

A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it.

Black holes are singularities: points of infinitely small volume with infinite density. Such incredibly compact objects cause infinite curvature in the fabric of spacetime. Everything that falls into a black hole is sucked toward the singularity. At some distance away from the singularity, the escape velocity exceeds the speed of light, sometimes dramatically dubbed “the point of no return,” although the technical term is Schwarzschild radius or event horizon.

Generally, they would have been so tiny (the minimum mass would be the Planck mass) that they can only be properly described using quantum mechanics. But black holes evaporate through a process called Hawking Radiation. How quickly a black hole evaporates depends on its mass: the less massive a black hole, the more quickly it evaporates. For a primordial black hole to have survived to the present day, it would have to contain a few billion tons of mass, with a radius comparable to that of an atomic nucleus.

Scientists produced the first real image of a black hole, in a galaxy called Messier 87. The image is not a photograph but an image created by the Event Horizon Telescope (EHT) project. Using a network of eight ground-based telescopes across the world, the EHT collected data to produce the image. The black hole itself is unseeable, as it’s impossible for light to escape from it; what we can see is its event horizon. The EHT was also observing a black hole located at the centre of the Milky Way, but was unable to produce an image. While Messier 87 is further away, it was easier to observe, due to its larger size.