**STEPHEN HAWKING: BLACK HOLES ARE HAIRLESS!**

An illustrated companion to Prof Stephen Hawking’s first Reith lecture about black holes.

While Prof Hawking describes the history of scientific thinking about black holes,

the artist Andrew Park draws the key points of the lecture in chalk on a blackboard.

(Animation produced by Cognitive.)

This is a beautyfully illustrated
presentation:

http://bbc.in/1SgCmdU

The background:

The **NO-HAIR-THEOREM**

postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism

in general relativity can be completely characterized by only three externally observable classical parameters:

**mass, electric charge, and angular momentum**.

All other information (for which "hair" is a metaphor)
about the matter which formed a black hole or is falling into it,

"disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers.

The

is a puzzle resulting from the combination of quantum mechanics and general relativity.

Calculations suggest that physical information could permanently disappear in a black hole,

allowing many physical states to devolve into the same state.

This is controversial because it violates a commonly assumed tenet of science

— that in principle complete information about a physical system at one point in time should determine its state at any other time.

A fundamental postulate of quantum mechanics is that complete information about a system is encoded in its wave function up to when the wave function collapses.

The evolution of the wave function is determined by a unitary operator,

and unitarity implies that information is conserved in the quantum sense.

References:

https://en.wikipedia.org/wiki/No-hair_theorem

https://en.wikipedia.org/wiki/Black_hole_information_paradox

Image: By XMM-Newton, ESA, NASA [Public domain]