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The same scientific theory that predicted the existence of black holes also predicts the existence of white holes, the opposite of black holes in almost every respect. Whereas black holes are endless takers of matter and energy, white holes (hypothetically) ceaselessly blast energy out into the universe. And since nothing can escape a black hole, nothing should be able to enter a white hole.

While black holes are tough to spot due to their lack of emissions, white holes should be bright fountains of radiation and, theoretically at least, should be difficult to miss. Yet, so far, astronomers havent been able to find any.

But that hasnt deterred many prominent physicists, such as Italian theoretical physicist and science communicator Carlo Rovelli, from positing their existence. This shouldnt be too surprising. After all, general relativity has a good track record of theoretically predicting aspects of the universe well before they are discovered including black holes, gravitational waves, and the deviation of light known as gravitational lensing (which is used by instruments like the James Webb Space Telescope to see objects in the early universe).

Yet, white holes stubbornly remain the unfulfilled prediction of general relativity.

You cant get into white holesif youll excuse the punwithout first thinking about Albert Einsteins magnum opus theory of gravity, general relativity.

General relativity was first introduced to the physics community in 1915 as Einsteins geometric theory of gravity, and it caused quite a stir. Up until then, the best description of gravity was that by Isaac Newton, which still works just fine on small scales but always had considerable failings when it came to explaining physics on massive scales.

The major difference between Einsteins formulation of gravity and that of Newton was whereas the latter saw space and time as the stages upon which the events of the universe played out, general relativity posited that the united four-dimensional entity of spacetime is an active player in this cosmic production.

That is because general relativity suggests that when an object of mass sits in spacetime, it causes its very fabric to warp. The more massive the object, the greater the warp in spacetime it causes, and gravity arises from this warping. That explains why the sun has a bigger gravitational influence than Earth: its warping of spacetime is more extreme. This warping then tells energy and matter how it should move through space.

An illustration of a black hole warping spacetime.

As theoretical physicist John Wheeler astutely put it: Matter tells space how to curve, and space tells matter how to move.

Just a year after the introduction of general relativity, to the surprise of Einstein, physicist and astronomer Karl Schwarzschild found a solution to the complex field equations that define it. Within this solution was the singularity that represents the heart of a black hole, making this development the theoretical birth of the concept of black holes.

In 1960, mathematician Martin David Kruskal expanded on the Shwartzschild solution to consider a version of spacetime that lacks edges, creating what has become known as the Maximaly Extended Version of Schwarzschild Metric. This included creating a reflection of the singularity at the heart of a black holethe interior of a white holethough it would be Soviet cosmologist Ivor Novikov who realized the significance of this four years later in 1964.

Very simply, a white hole could be considered a black hole that runs backward in time. White holes would have some things in common with black holes: they would possess the characteristics of mass, angular momentum or spin, and electric charge.

Like black holes, because they have mass, white holes would attract matter toward them, at least at first. The difference is that when matter and light pass the event horizonthe point at which the gravity is so strong, the escape velocity exceeds the speed of lightof a black hole, it would never actually be able to reach the anti-event horizon of the white hole. It is possible that matter that approaches the anti-event horizon of a white hole could be whipped away with an incredible amount of force.

Space, light, and time are warped by the strong gravity of a black hole, forming an accretion disk of matter on the event horizon.

The major difference between black holes and white holes is their formation. We know, thanks to the work of J. Robert Oppenheimer and collaborators, that when a massive star undergoes a complete gravitational collapse at the end of its nuclear fuel-burning life, its outer layers are blasted away in a supernova explosion while its core collapses to birth a black hole.

Yet if these death throes could somehow be rewound like a cosmic VCRbreaking all the laws of cause and effect in the processthat would not result in a white hole as the mathematics of Kruskal or Novikov surmise. Instead, this cosmic rewind button would just give us back a star on the brink of death.

That means there is actually no physical process in the universe that we know of that could create a white hole.

While considering the possibility of black holes leaking radiation, Stephen Hawking postulated that they would come to a thermal equilibrium, which has an interesting consequence for white holes.

A state of thermal equilibrium is time-reversal invariant, which means that the same laws of physics should apply to a body in thermal equilibrium whether time is running backward or forward. If white holes are just time reversals of black holes, that meant to Hawking that black holes and white holes are reciprocal in structure.

The leaking of this radiation, which would later be termed Hawking radiation, would cause black holes to gradually evaporate, ironically throwing a lifeline to the white hole concept. This is because there is a rule in quantum mechanics that says information cant be destroyed. That means all the information that is carried into a black hole must be preserved.

If black holes live forever, no problem; that information sits in the singularity in perpetuity. But if black holes leak and shrivel like a cosmic paddling pool before finally exploding as Hawking thought, then what happens to the information they harbor? It cant be carried away by Hawking radiation, as thermal radiation cant be encoded with information. It isnt allowed to get back out past the event horizon, since no signal is permitted to do this. So where does it go?

One possibility is that black holes are connected through spacetime to white holes. Matter entering a black hole could come gushing out of the white hole at the other side somewhere else in the universepossibly in an entirely different galaxy. The connective tissue between the black holes entrance and the white holes exit would be an Einstein Rosen Bridge, more commonly known as a wormhole, a tunnel through spacetime connecting two seemingly disparate points possibly millions of light years apart.

An illustration of a wormhole.

However, the lack of white holes detected in the universe could suggest that wormholes are actually something much more profound than simply a tunnel from one distant galaxy to another.

The seeming lack of white holes in our universe could mean that if a multiverse of universes exists, there could be a universe populated entirely by white holes with a complete absence of black holes.

That could be because time is a one-way system in each universe in the multiverse. In our universe, time can only run forwardthe future is infiniteand that forbids the creation of a white hole. Meanwhile, in our multiverse counterpart, time can only play backwardthe past stretches into infinityand that forbids the existence of black holes while allowing white holes.

An illustration of the multiverse, with entire universes existing alongside each other.

Scientists like Roger Penrose suggest this could mean that there is a universe in the multiverse all the cosmic junk from our universes black holes spills into. Think of it like a multiverse equivalent of the trash compactor scene from Star Wars: A New Hope minus Han, Luke, Chewy, Leia, and a tentacled monster (probably).

Some theoretical physicists also consider that maybe our universe had just one white hole, a tremendous one, right at its very beginning.

The Big Bang certainly sounds a little like a time-reversed black hole, if you consider the rapid expansion of space as being like a sudden eruption of matter at the beginning of the universe. And the forward-flow-of-time rule wouldnt apply if that white hole was here at the very start, before time started rolling forward. Perhaps every universe in the multiverse starts with matter that flows out of a parent universe through a black hole connected to a white hole via a wormhole.

Whatever the case, since black holes have an event horizon that prevents accessing the information sealed behind it, well likely never be able to see into another universe through a white hole even if the two are linked.

Whether they are doorways to other regions of space or other universes entirely, or sealed exits, scientists are not set to stop speculating about white holes any time soon, meaning they will always be a doorway to the imagination.

Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Robs articles have been published in Newsweek, Space, Live Science, Astronomy magazine and New Scientist. He lives in the North West of England with too many cats and comic books.

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What Is a White Hole, and Do White Holes Really Exist? - Popular Mechanics