The 1916 exact solution found on the Russian front by an astronomer who died months later (1916)
Karl Schwarzschild, who died of illness on active service in May 1916
Science · Physics
Black holes
1916 solution; imaged 2019
Physics — general relativity and astrophysics
A region of spacetime whose gravity is so strong that nothing, not even light, can escape from inside its boundary, the event horizon. Karl Schwarzschild found the first exact solution of Einstein's field equations describing one in 1916, months after general relativity was published and while he was serving on the Russian front. The name was popularised by John Archibald Wheeler in 1967. Stellar-mass examples form when massive stars collapse; supermassive ones, millions to billions of times the Sun's mass, sit at the centres of galaxies, including Sagittarius A* at the centre of ours. The first direct image, of the one in M87, was released in 2019.
How to spot them
- ▸A boundary called the event horizon, from inside which not even light escapes
- ▸The catchy name was popularised by John Wheeler in 1967
- ▸Sagittarius A* marks the four-million-solar-mass example at the centre of our galaxy
- ▸The first picture, an orange ring around a dark centre, was released in April 2019
Key facts
The radius at which the escape velocity reaches the speed of light
The Schwarzschild radius — about 3 km for the Sun, 9 mm for the Earth
The snappy name popularised at a 1967 New York conference (1967)
John Archibald Wheeler, who did not coin it but made it stick
The slow thermal leak predicted in 1974 that means these objects eventually evaporate (1974)
Hawking radiation — smaller means hotter and faster
The X-ray source in the constellation of the swan, subject of a bet Hawking lost to Kip Thorne (1964)
Cygnus X-1; Hawking conceded the bet in 1990 and paid with a magazine subscription
The orange ring released on 10 April 2019 by a network of radio telescopes (2019)
The Event Horizon Telescope's image of the giant in the galaxy M87
The 2015 LIGO chirp from two objects of about 36 and 29 solar masses merging (2015)
GW150914, the first gravitational-wave detection; Nobel Prize 2017
Key ideas
- General relativity says mass curves spacetime; pack enough mass into a small enough region and the curvature closes off a one-way surface, the event horizon, beyond which all paths lead inwards.
- Singularity theorems by Penrose and Hawking showed that such collapse is a robust prediction of the theory, not an artefact of assuming perfect symmetry.
Quick-fire facts
- •Karl Schwarzschild produced the first solution in 1916, while serving in the German army on the Russian front; he died that May
- •John Michell in 1783 and Pierre-Simon Laplace in 1796 anticipated 'dark stars' from which light could not escape
- •John Archibald Wheeler popularised the term in 1967, though he did not originate it
- •The Schwarzschild radius is proportional to mass: about 3 km per solar mass
- •Roger Penrose shared the 2020 Nobel Prize for proving that formation is a robust consequence of general relativity; Reinhard Genzel and Andrea Ghez shared it for tracking stars around the Milky Way's centre
- •Sagittarius A*, about 4 million solar masses, sits at the centre of the Milky Way and was imaged in 2022
- •The Event Horizon Telescope released the first image, of M87*, on 10 April 2019; the object masses about 6.5 billion Suns
- •Stephen Hawking predicted in 1974 that they radiate and slowly evaporate; Hawking radiation has never been observed, and he never won a Nobel Prize
- •The 'no-hair theorem' says only three properties matter: mass, electric charge and angular momentum
- •A common misconception is that they suck things in: replace the Sun with one of equal mass and the planets' orbits would be unchanged
Drill it — 10 cards
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Sources
- NASA Science — Black Holes
- European Southern Observatory — Astronomers Capture First Image of a Black Hole
- The Nobel Foundation — The Nobel Prize in Physics 2020