Theory · Intermediate · 1:46

White hole

The mirror image of a black hole: how one could be made, and what it would look like

Theory, not observation. Nothing here has ever been seen. Each idea comes from real physics and says how it could be made, and what it would look like. Light is bent as the equations say it would be; the things themselves are drawn.

Below are the 8 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the What if tab

1. A black hole, run backwards

The equations of general relativity work the same whichever way time runs. Play a black hole's story backwards and you get a white hole: something that nothing can enter and that throws matter out.

A black hole is a place from which nothing, not even light, can get out. Einstein's equations do not care which way time runs, so they have a mirror-image solution: a white hole, out of which things come and into which nothing can go. Igor Novikov proposed in 1964 that such objects might really exist.

From far away the two look alike. Gravity outside a white hole is the same as outside a black hole of the same mass, so light bends round it the same way and planets would orbit it the same way. The difference is at the edge: at a black hole's horizon everything goes in, at a white hole's everything goes out.

No white hole has ever been found. Everything here is theory: the picture is a black hole of ten Suns, 59 km across, and what follows is what the equations say about its mirror image.

2. One map, four regions

The full solution of Einstein's equations for a lone black hole holds four regions: our universe, a black hole, a white hole and another universe. The white hole is in the past, the black hole in the future.

In 1960 Kruskal and Szekeres found the complete map of the space round a black hole. Drawn as a diagram in which light always travels at 45 degrees, it is a square cut by its two diagonals into four triangles. Our universe is on the right and time runs upwards.

Anything that falls in from our universe crosses into the region above, the black hole. Everything that has ever come out of the white hole did so from the region below, in the past. Nothing from our universe can go into the white hole, because that would mean going back in time.

A black hole made by a collapsing star has no white-hole region: the star's surface cuts the map, and the white hole and the second universe are never there. That is why a white hole needs its own story of how it might be made.

3. How one could be made

Nothing known makes one. One idea: a collapsing star reaches the densest state quantum gravity allows, bounces, and the black hole turns into a white hole. The bounce is drawn.

Einstein's theory says the matter in a black hole collapses to a point of infinite density, which hardly anyone believes. Loop quantum gravity, one attempt to join gravity with quantum physics, says space cannot be squeezed below a smallest size, so the collapse stops and rebounds: a Planck star.

In 2014 Haggard and Rovelli showed how such a bounce could turn a black hole's horizon into a white hole's. The time it takes grows as the square of the mass, far less than the cube of the mass that Hawking's evaporation takes: for ten Suns, 2.1 × 10⁷⁰ years.

The other idea is older. Novikov suggested in 1964 that some regions of the early universe lagged behind the expansion and burst out late as white holes. Rovelli and Vidotto argued in 2018 that tiny leftovers of evaporated black holes could be white holes making up part of dark matter. Neither has been seen.

4. The last thousandth of a second

Play a star's collapse backwards. Out of the black shadow the surface climbs into view, brightening by a factor of e every 256 millionths of a second. A white hole of ten Suns would show itself in under a millisecond.

When a star collapses into a black hole, its surface, seen from far away, creeps towards the horizon, reddens and fades by a factor of e every quarter of a millisecond, for ten Suns (Ames and Thorne, 1968). Run backwards, that is a white hole opening: a dark disc, then a dim red glow that brightens and turns blue-white.

This is an exact solution, the collapse of a ball of dust run backwards (Oppenheimer and Snyder, 1939). The star's surface would come out of the shadow, at first exactly at the horizon and then higher and higher, until the whole shadow is filled with light.

So the shadow is not black for long: light that would be trapped can come out of a white hole. Every part of this step is drawn from the equations. No one has seen anything like it.

5. It cannot last

Nothing can fall into a white hole, but everything near it tries to. In 1974 Eardley showed that light falling towards one would pile up at its edge, gain unlimited energy and turn it into a black hole.

A white hole's horizon can only be left, never crossed the other way. But matter and light from outside still move towards it, drawn by its gravity, and cannot get in. So they gather at the horizon, like traffic at a shut gate.

Seen from the matter coming out of the white hole, this crowd of late arrivals is squeezed together and blueshifted without limit. Douglas Eardley showed in 1974 that even the faintest infall carries enough energy to collapse the white hole into a black hole. That same year Zel'dovich, Novikov and Starobinskii found a second instability, from quantum effects.

That is why an eternal white hole is not expected: if one formed, it would be short-lived. The pale blue shell drawn here stands for the light falling in; it cannot be seen.

6. A white hole you can build

You can make a white hole for waves. In 2011 Weinfurtner and colleagues sent waves against a fast current in a water channel: where the current outran them, they were stopped, as light is at a white hole's edge.

Waves on water travel at a speed that depends on their length. Make the water flow faster over a smooth bump, and long waves heading upstream slow down until the current matches their speed. There they cannot go on: an edge that waves cannot cross, like a white hole's horizon for light.

At that edge the long waves are turned into short ones, the analogue of what happens to light at a white hole. Silke Weinfurtner's team measured it in 2011 (Physical Review Letters) and found the conversion had the thermal character that Hawking radiation is predicted to have. Rousseaux and colleagues had seen a similar conversion in a tank in 2008.

It is not a real white hole: no gravity, no light, only water. But the mathematics is the same, which is why such tanks are used to test ideas about black and white holes. The waves and the current here are drawn.

7. Was the Big Bang a white hole?

On the grandest scale the Big Bang looks like a white hole: a burst of matter from a point. Some physicists think our universe is the inside of a black hole that bounced. The bounce is drawn.

The inside of a collapsing ball of dust is a closed universe run backwards, as Oppenheimer and Snyder found in 1939. Run it forwards instead and the ball is an expanding universe that starts from a point: a Big Bang. The two are the same mathematics, each the mirror of the other.

In 1972 Raj Pathria suggested our universe might be the inside of a black hole in a larger one. Nikodem Popławski proposed in 2010 a version in which spin and torsion stop the collapse inside every black hole and make it bounce into a new expanding universe.

These are ideas, not results. Nothing we can measure tells them apart from the standard Big Bang, and the real universe expands on all sides at once, not in a burst from one place.

8. Has anyone seen one?

No. Some ideas say tiny white holes could be flashing now as fast radio bursts, or hiding as dark matter, but no signal has been tied to one. A white hole stays in the equations.

Barrau, Rovelli and Vidotto suggested in 2014 that a black hole formed in the early universe could bounce into a white hole today and give off a burst of radio waves. Fast radio bursts, millisecond flashes of unknown origin, have wavelengths not far from what they estimated. That is a guess about their cause, with no evidence behind it.

Rovelli and Vidotto argued in 2018 that Planck-sized white holes, left when tiny black holes finish evaporating, could make up part of dark matter. Such objects would be far too small to see directly.

So a white hole is where it started: a mirror image in the equations, with no observation that needs it and a strong reason why it might not last. The tanks show the mathematics is right; whether nature ever makes one is still open.

Objects in this story