Idea on the edgeSpeculative · Universe

White holes

The time-reversed twin of a black hole. Nothing can enter a white hole; matter and light only pour out.

Speculative · never observed

Idea: The time reversal of a black hole
Property: Nothing can enter; everything exits
Status: Mathematically possible, never observed
Candidates: Some gamma-ray bursts, speculative

The one-way exit

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The reverse of a black hole

Run a black hole's equations backward in time and the horizon flips. Where a black hole lets everything in and nothing out, a white hole lets everything out and nothing in. It is a valid solution of general relativity, not just a science-fiction device.

No one has seen one, and it is not clear whether nature makes them.

Where one might appear

A white hole could be the endpoint of a black hole's evaporation, or the bounce that some quantum-gravity models put in place of the singularity. In either case the object would be short-lived or unstable, which may be why none are seen.

Why none are seen

A white hole's horizon is one-way outward, so incoming material cannot fall in and is instead pushed away. Any such object would be bright and brief, and the claimed candidates so far have not held up.

Key publications

Year	Paper	Venue	Link
1974	Eardley, Death of White Holes in the Early Universe	PRL 33, 442	DOI
2015	Haggard & Rovelli, Quantum-gravity effects outside the horizon spark black to white hole tunneling	PRD 92, 104020	DOI
2018	Bianchi et al., White holes as remnants: a surprising scenario for the end of a black hole	Class. Quantum Grav. 35, 225003	DOI

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

A white hole is the mathematical mirror of a black hole.
Falling in is impossible because the horizon is one-way outward.
No telescope has caught one.