Black hole For large enough masses, as far as we know, nothing is stiff enough to stop the collapse. It continues down to a singularity: a defect in space and time.

Singularities Susan Scott Singularity theory Position Time Light rays emitted by star singularity Event horizon Singularities are points of infinite gravity, or more accurately, infinite space-time curvature, where space and time end. They are a kind of boundary of the universe. Particle paths in a collapsing star

Quantum Gravity & String Theory Understanding singularities will require new theories, probably combining General Relativity with Quantum Mechanics. One candidate is string theory. It assumes that the basic structure of matter is string-like, not point-like. Although still speculative, it allows the direct calculation of black hole entropy by counting micro-states.

The event horizon The singularity is surrounded by an event horizon. This is the boundary at which the escape velocity is the speed of light. Consequently, nothing can come out of the event horizon, not even light - hence the name "black hole".

Gravity deflects light Gravitational lensing by the Abell galaxy cluster Distant galaxies being imaged by the Abell cluster

The effect of gravity on light Relativity implies nothing can go faster than light.

Evidence for black holes A disk of dust fuelling a massive black hole in the centre of a galaxy, located 100 million light-years away. The speed of the gas swirling around the black hole indicates that the object at the centre of the disk is 1.2 billion times the mass of our Sun, yet concentrated into a region of space not much larger than our solar system. 800 light years

Black Hole at the centre of a galaxy

Gravity as curved space

Space-time “ The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality”. H. Minkowski, Speech, 1908.

Space-time Spatial separation and time separation are not individually fundamental: they depend on the observer. The geometric union of space and time into 4 dimensional SPACE-TIME is the fundamental, observer independent, reality.

Falling into a black hole: observing For a distant observer, time slows down for an object approaching the event horizon. Objects approach it without ever passing through it!

Falling into a super-massive black hole For an observer falling into a big enough black hole nothing much happens while falling through the event horizon. However the "irresistible" flow of time is accompanied by an "irresistible" falling towards the singularity - due to the mixing of space and time.

Falling into a stellar mass black hole An observer falling into a stellar mass black hole experiences huge gravitational gradients (tides) which stretch vertically and compress horizontally.

Black hole evaporation Quantum mechanics predicts that space is filled with virtual particle pairs. If one falls into the black hole the other gains its energy and becomes real: this is the Hawking radiation.

Black hole explosions? Hence Hawking radiation becomes more intense as the black hole shrinks. A black hole with the mass of an asteroid is as bright as the Sun. The title of a 1974 paper by Stephen Hawking. The gravity at the event horizon is inversely proportional to the hole's mass years