Do white holes exist in space: how they differ from black holes and what scientists say

Yulia PoteriankoLife
A model of a white hole that theoretically functions in the opposite way to a black hole. Source: Future/Adam Smith

Even schoolchildren know about the existence of black holes in space. These supermassive objects are one of the greatest mysteries of the Universe. But scientists suggest that their antipodes, white holes, may exist in interstellar space.

These theoretical space areas were described by Space.com. It is believed that they can work in the opposite way. If nothing that falls inside can leave a black hole, then nothing can enter a white hole.

What is a white hole?

White holes have long been considered a figment of general relativity, generated by the same equations as black holes. Recently, however, theoretical physicists have asked themselves whether these dual vortices of space-time could be two sides of the same coin.

To a crew watching from a spaceship, a white hole looks just like a black hole. It is massive and can rotate. A ring of dust and gas can gather around the event horizon. But only a white hole can be observed to reject an object. It is only at this point that it is theoretically possible to understand that it is a white hole and not a black hole.

Physicists describe a white hole as a "time reversal" of a black hole. Roughly, as a bouncing ball is a time reversal for a falling ball. Whereas the event horizon of a black hole is a boundary beyond which there is no return, the horizon of a white hole is the most impenetrable boundary in the universe.

Objects inside a white hole can go out and interact with the outside world. But since nothing can get inside, its interior is cut off from the past universe: no external event will ever affect the interior.

Einstein's equation that revolutionised the way we think about space

Einstein's field equation revolutionised physics back in 1915, and scientists are still trying to understand the realities of the universe that it helped to deduce. In addition to describing the force of gravity, the famous physicist's hypotheses also radically changed the paradigm of understanding the nature of reality. Space and time began to be thought of as a solid body that could bend and fold together with the mass of stars and planets. Scientists began to find out what properties such processes might have.

Within a year, physicist and astronomer Karl Schwarzschild found the first exact solution to Einstein's equations by calculating how space-time curves around a single ball of mass. His answer laid the foundation for what physicists today call a singularity - a spherical mass compressed to an infinitely dense point that envelops space so tightly that this region is separated from the rest of the Universe. This creates an object whose event horizon breaks the connection between cause and effect.

The most famous example of a singularity is black holes. These are such curved regions of space that there are no exits from them. The outer universe can influence the inner part of the black hole's horizon, but the inner part cannot influence the outer part. It took 40 years to finally understand the nature of such objects. Their research is still ongoing. And, obviously, they are still very far from being completed.

How a white hole works

As explained by astrophysicist and gravitational lensing specialist Geraint Lewis (University of Sydney), in a simplified explanation, a white hole is the reverse of a black hole. Schwarzschild's calculations showed that there is nothing in general relativity that dictates in which direction time flows. In the usual flow of time, we see things falling endlessly into a black hole. But if you choose the opposite direction of time flow, the effect is also reversed. So, in addition to a black hole, Schwarzschild's mathematics also gives us a white hole, simply by flipping the way time works.

How do white holes differ from black holes?

A black hole has a powerful gravitational field that attracts things. It is surrounded by a one-sided membrane called the event horizon. Anything that falls beyond it cannot escape - it is trapped. Gravity captures the object, and its future from that moment on is to be in the centre of a black hole, which cannot be changed no matter what you do.

A white hole is the flip side of this. It can be described as an antigravity that endlessly ejects matter. It also has an event horizon where things from inside it cross it and are ejected into the Universe. And it is impossible to get inside this object. So, in a black hole, you can go inside, but not outside, and in a white hole, you can go outside, but not inside.

Is there any evidence for the existence of white holes?

There is no actual evidence for the existence of such objects in space - they remain theoretical models. As Lewis explains, this can happen because we are able to perceive only one direction of time movement - from the past to the future. This means that we can only have solutions for black holes.

Some scientists suggest that the point is that the Universe is asymmetrical, we see the beginning with the Big Bang, and we have an infinite future ahead of us. Mathematically, the existence of a white hole is possible, but the asymmetry of the Universe, which moves in one direction on the timeline, means that they are not physically realised.

How to detect a white hole?

Such an object is imagined as a place where matter is ejected with high energy. Scientists such as Roger Penrose (Nobel Prize in Physics 2020) suggest that a white hole is an exit for a black hole to another universe. Thus, matter that falls into a black hole in one Universe will be ejected into another Universe through a white one.

However, Lewis is convinced that there are no such objects in the Universe. So far, physicists have no idea how they can form. For example, a black hole is created by the collapse of a star. But reproducing this process in the opposite direction of time makes no physical sense. An event horizon turning into a functioning star would be a bit like an egg becoming an egg again - a violation of the statistical law that requires the universe to become increasingly disorderly over time.

And even if white holes were to form, they probably wouldn't last long. Any matter escaping from such an object would collide with matter in its orbit, and the system would turn into a black hole.

Why white holes can exist

Ever since Stephen Hawking realised in the 1970s that black holes release energy, physicists have been debating how these entities can cease to exist. If a black hole "evaporates", what happens to the internal record of everything it has absorbed? General relativity does not lose information, and quantum mechanics forbids its deletion.

Carlo Rovelli, a theoretical physicist at the Centre for Theoretical Physics in France, explains it this way: "How does a black hole die? We don't know. How is a white hole born? Perhaps a white hole is the death of a black hole. These two questions are perfectly combined, but when you move from one to the other, you have to break the equations of general relativity."

Rovelli is the founder of the theory of quantum loop gravity, an incomplete attempt to go beyond general relativity by describing space itself as built from Lego-style particles. Guided by the tools of this scheme, he and others describe a scenario in which a black hole becomes so small that it no longer obeys the rules of stellar existence. At the particle level, quantum randomness comes into play, and the black hole can turn into a white hole.

According to Hal Haggard, a theoretical physicist at Bard College in New York, such a microgram-sized white hole, with a mass similar to a human hair, would have nothing to do with the gravitational drama of its ancestor, the black hole. But it may hide a cavernous interior containing information about everything it has absorbed in its former life. Too small to attract the matter revolving around it, a white hole can remain stable enough to eventually release all the information accumulated by its predecessor. In such a picture of the Universe, white holes will one day dominate spacetime, when all black holes "evaporate".

A model of a white hole

Physicists also suggest that the effects of a white hole can exist everywhere. Thus, the Big Bang, which started our Universe, looks like the potential behaviour of a white hole. "The geometry in both cases is very similar. So much so that sometimes they are mathematically identical," Haggard explained.

Cosmologists call this pattern the "Big Bounce", and some look for the characteristics of a white hole in the early light of the Universe that can still be observed. Rovelli also wonders whether the powerful radio flares are echoes of theoretical mini-black holes left over from the Big Bang as they make the early transition to white holes (although this explanation seems increasingly unlikely).

The universe may not take all the forms allowed by general relativity, but Gaggard believes physicists should explore this rabbit hole to the end. "Why don't you explore whether they [white holes] have interesting consequences. Maybe these consequences are not what you expected, but it would be foolhardy to ignore them," he says.

Earlier, OBOZREVATEL told about mysterious radio signals from distant galaxies that turned out to be the echo of powerful explosions.

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