White holes explained

In the world of astronomy and science fiction, black holes have long dominated headlines as invisible behemoths whose gravity even traps light.

First predicted as ‘dark stars’ by John Michell in 1783, and mathematically defined by Einstein in 1915 in his theory of general relativity, black holes were once considered purely theoretical.

Observational evidence in the 1970s and then the first direct image, captured in 2019 by the Event Horizon Telescope, showed that they were theoretical no more.

However, tucked deep within the intricacies of general relativity lurks an even stranger prediction: the white hole.

Why we think white holes could exist

A theoretical ‘opposite’ to a black hole, a white hole is an object that spews matter and light outward and forbids anything from entering.

If black holes are cosmic vacuums, white holes can be thought of as cosmic fountains.

The idea of a white hole arises from the equations of general relativity, which describe how mass and energy warp spacetime.

These equations are time-symmetric, meaning that for every process that runs forward in time, there is, mathematically, a time-reversed version.

So a black hole should have a time-reversed counterpart from which matter and light inevitably emerge. This raises a troubling question...

How would a white hole form?

We know that black holes can form in several ways, almost all involving a star collapsing under its own gravity, but there are no known physical processes that would naturally create a white hole.

Similarly, white holes push theoretical physics to its limits. If a white hole existed, it would violate the second law of thermodynamics: that disorder (entropy) in the Universe tends to increase.

A helpful analogy is a messy room: left alone, things naturally become more disordered; only added effort (energy) can restore order.

A white hole would do the opposite, taking disordered matter and ejecting it in a more ordered state, without any apparent energy input.

This is not something we observe in nature.

Why the fascination with white holes?

So, if we have never observed one, if they lack a viable formation mechanism and their existence seemingly violates a fundamental law of physics, why do physicists keep talking about them?

Some researchers propose that quantum gravity (the still-incomplete theory that unifies Einstein’s gravity with quantum physics) could eliminate the classic singularity inside a black hole and replace it with a transition to a white hole.

In these models, a black hole does not end in an infinite-density point but instead transforms into a white hole that slowly emits energy. 

This offers a possible solution to the black hole information paradox: the puzzle of what happens to information when matter falls into a black hole.

A 2018 research paper from Pennsylvania State University, White Holes as Remnants: A Surprising Scenario for the End of a Black Hole, shows that this process would not violate any known physics.

The similarity of the Big Bang to a white-​hole-like event is also worth considering: a singular eruption of space, energy and matter that seeded our cosmos.

Could the Universe’s birth be viewed as the ultimate white hole? Could white holes represent the birthplaces of other universes?

Could they be wormholes?

White holes also intersect (and are often conflated) with concepts like wormholes.

A wormhole is a hypothetical tunnel connecting distant regions of space, with some solutions to Einstein’s equations suggesting a wormhole connects a black hole at one end to a white hole at the other.

However, without exotic matter to hold them open, these structures would collapse too quickly, so they remain highly speculative.

In the end, white holes are reminders of how far theoretical physics stretches beyond observation.

They are not established objects like black holes, but they occupy an important place in our mathematical descriptions of the cosmos – a testament to the intricacy and strangeness of Einstein’s legacy.

White holes, black holes and wormholes – how to tell the difference

Physicists often talk about white holes, black holes and wormholes in the same breath, but they’re not the same thing.

A useful way to think about them is in terms of doors and tunnels in spacetime.

Black holes are best imagined as one-way doors. Once you cross their boundary (the event horizon), gravity becomes so strong that nothing can escape, not even light.

These objects have been observed throughout the Universe as the remnants of collapsed massive stars.

White holes are the mathematical opposite. If a black hole is a one-way door that matter can only enter, a white hole is a one-way door out – matter and energy are forced to emerge from it. The problem is, we’ve never seen one and have no idea how they form.

Wormholes are hypothetical tunnels connecting faraway regions in spacetime. In some mathematical solutions, they’re imagined as linking a black hole at one end to a white hole at the other.

However, within the realm of our known physics, wormholes are unstable and would quickly collapse.

In short: black holes trap, white holes eject and wormholes connect (at least on paper).

This article appeared in the March 2026 issue of BBC Sky at Night Magazine