Naked singularity
A hypothetical singularity without an event horizon, potentially observable.
A naked singularity is a hypothetical gravitational singularity that lacks an event horizon, making it observable from the outside. In general relativity, such an object would allow direct observation of the collapse of matter to infinite density, but its existence would cause foundational problems because the theory cannot predict the evolution of spacetime near a singularity. Naked singularities have not been observed in nature, though some astronomical objects hint at their possibility.
- Field
- General relativity, astrophysics
- Known for
- Hypothetical observable singularity without an event horizon
- Key concept
- Cosmic censorship hypothesis
- Related objects
- Black holes, Kerr metric, Reissner–Nordström geometry
Lore & Background
In general relativity, a naked singularity is a gravitational singularity not enclosed by an event horizon. When a causal geodesic extends in the future to an observer and in the past terminates at the singularity, that singularity is naked. Unlike black holes, where the singularity is hidden, a naked singularity would be directly observable. The theoretical existence of naked singularities is important because it would allow observation of collapse to infinite density and would cause foundational problems for general relativity, as the theory cannot make predictions near a singularity.
Reader's Guide
Naked singularities have not been observed, but astronomical observations of black holes indicate their rotation rates fall below the threshold to produce one (spin parameter 1). GRS 1915+105 comes closest with a spin parameter of 0.82–1.00, and GRO J1655−40 is hinted to be a naked singularity. According to the cosmic censorship hypothesis, gravitational singularities may not be observable. If loop quantum gravity is correct, naked singularities may be possible. The collapse of a massive star can result in either a black hole or a naked singularity depending on initial conditions, such as inhomogeneous density or non-zero pressures. Rotating singularities can become ring-shaped, and if spin is high enough, event horizons disappear, exposing the singularity. Studies of pulsars and computer simulations have been performed, and spinning white dwarfs or neutron stars could transmute into naked singularities by capturing asymmetric dark matter. Mathematician Demetrios Christodoulou showed that such naked singularities are unstable.
Did You Know?
- GRS 1915+105 has a spin parameter of 0.82–1.00, closest to the threshold for a naked singularity.
- In the Kerr metric, if angular momentum is high enough, event horizons disappear, exposing the singularity.
- Demetrios Christodoulou has shown that contrary to what had been expected, singularities which are not hidden in a black hole could also occur.
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