White holes: The black hole's mysterious twin
In the infinite vastness of the universe, it is likely that many, if not most, objects escape the realm of human notice completely. Theoretical physicists, even Einstein, can only do so much in telling us what might and what might not be out there. To put it simply, there are some things that theory suggests should not exist, but which may in fact exist anyway. One such object is the black hole’s theoretical opposite — the white hole.
What is a white hole?
The simplest way to visualize a white hole is basically as a black hole in reverse.
White holes aren’t just black holes with a new paint job. In fact, despite their name, they’re thought to look very similar to black holes. Almost identical, even — a cosmic twin. A crew of astronauts approaching one would hardly be able to tell the difference.
However, astronauts would have an immediate ‘tell’ for a white hole: there wouldn’t be a massive gravity well threatening to suck them in upon passing too close to one in the far reaches of space. That’s because black holes and white holes can be thought of as functional opposites of each other.
While a black hole might suck in all nearby matter and crush it with enough force to pull it apart at the atomic level, white holes don’t have any ‘pull’ to speak of.
So in what way is a white hole the ‘opposite’ of a black hole, then? It’s all in the key characteristic of a black hole. Despite how black holes are portrayed as sucking nearby matter into a cosmic abyss, their actual defining characteristic is having a gravitational field so strong that no matter or radiation, not even light, can escape. Still grim, just in a different way.
In the case of a white hole, the reverse would be true — nothing could enter. It’s a cosmic gate that nothing can pass — not light, not matter. In contrast, a white hole would constantly emit matter and light, but while material inside a white hole can leave, once it exits there’s no way back in.
How does a white hole form?
As white holes are hypothesized to be closely related to black holes, there are several theories as to how they might form.
The theoretical origins of white holes can be traced back to Russian cosmologist Igor Novikov, in 1964. Novikov came up with the idea of white holes as a sort of cosmic twin to black holes as part of a solution to Einstein's field equations, building on the work of German physicist Karl Schwarzchild, who described the spacetime geometry of empty space surrounding any spherical mass.
Schwarzchild’s solutions to Einstein’s field equations included the prediction that if a mass were compressed inside of a critical radius (now called the Schwarzschild radius), then its gravity would become so strong that not even light could escape — in other words, it would become a black hole.
But Schwartzchild’s description also included the possibility of a theoretical ‘twin’ for the black hole, as well as what we now call wormholes — folds in spacetime that objects in space can theoretically pass through to near-instantaneously cross great distances — in between the event horizons of a black hole and a theoretical ‘negative’ version of the singularity.
In 1960, Mathematician Martin David Kruskal extended Schwarzchild’s work to include a reflection of the black hole singularity, but it was Novikov who developed this into the notion of a white hole.
Until more recently, physicists treated the possibility of white holes as a mathematical exercise — they could be shown to be mathematically feasible, but were seen as being impossible in “real life.” One reason for this was that no one could come up with a mechanism for how they would actually form — a black hole is formed when a star collapses, but the reverse of this — a black hole erupting into a star, would seem to violate the laws of entropy.
A different theory suggests that white holes aren’t twins of black holes, but what happens to a black hole upon its death, albeit for a very very brief moment.
However, the work of physicist Stephen Hawking demonstrated that black holes can in fact emit thermal radiation (Hawking radiation) due to the steady conversion of quantum vacuum fluctuations near the black hole into pairs of particles and anti-particle. The positive particle escapes, while the negative anti-particle falls in, causing the black hole to lose mass. Over time, Hawking radiation reduces the mass and rotational energy of black holes and could theoretically cause a black hole to evaporate.
This brings up a number of questions, however. One of which is that, if a black hole can evaporate away, what then happens to the information that it swallowed? According to general relativity, this information can't escape, and according to quantum mechanics, it can't be deleted. The answer, for some theoretical physicists, is that it disappears down a wormhole and emerges from a white hole.
Some physicists suggest that once a black hole grows small enough, it could transform into a white hole. This white hole would, Tardis-like, be minuscule on the outside but the inside would contain much of the information swallowed by the black hole, which would then emerge over time. Still, others have suggested that the explosion of the Big Bang might in fact have been the emergence of information from a white hole.
Do white holes exist?
Currently, there’s no evidence pointing to the existence of white holes in the universe. As of now, the white hole is a purely theoretical concept.
The closest thing we’ve seen to a potential sighting of a white hole in space came from a paper published in 2011. Scientists speculated that the known gamma-ray burst GRB 060614 may be the remnants of a white hole.
Aside from that, everything we’ve seen written on white holes is purely theoretical. Despite this, there’s hope by some in the scientific community that the existence of white holes will be proven eventually. After all, Einstein published his General Theory of Relativity in 1915, which predicted the existence of black holes, but it was 1971 before the first black hole was actually identified.
While many scientists view white holes as a purely mathematical exercise, others are hopeful that we’ll be able to spot this markedly rare astrological event eventually. Although, we may not recognize it when we do. Stephen Hawking pointed out that white holes and black holes could behave in an identical manner, making them virtually indistinguishable.
Much of the uncertainty with white holes comes from our current understanding of astrophysics. White holes are, by nature, thought to be incredibly unstable. There’s no way a cosmic event expelling that much matter could sustain itself long enough to be caught in an astronomer’s telescope.
Some speculate that when white holes begin to expel matter, once the expelled matter collides with any matter in orbit, the system would immediately collapse into a black hole, possibly creating an infinite loop of white holes turning into black holes and vice versa.
White hole gravity
Much like how what goes on at the center of a black hole’s singularity requires one to stretch their understanding of classical gravity, white holes may also need to be looked at through a special theoretical lens in order to be proven.
The closest thing to this we’ve seen is the idea of loop quantum gravity – currently a far-out theory on the fringes of mainstream physics.
According to this theory, space-time — the fundamental concept of Einstein’s groundbreaking work on relativity – is made up of a series of ‘loops’ at their fundamental level tying everything together in a neverending network of nodes. These loops tie together space, understood as blocks under this theory, and could prevent dying stars from collapsing into points of infinite density, instead recoiling and turning into white holes.
Should the approach of loop quantum gravity to white holes be demonstrated as possible, then many of the supernovae astronomers have observed over the years could turn out to be markers of a white hole’s formation and death, much like some of the theories around GRB 060614.
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