There just might be a way for white holes to form in the Universe.
According to a new, highly theoretical paper, the hypothetical objects may result from a kind of quantum 'bounce' at the heart of a black hole, spewing not just matter but time itself from the depths.
Time is really the essence of the problem with black holes. At some point the stream stops, and with it any information being carried in its current. This breaks a pretty fundamental rule in quantum physics, prompting researchers behind this new study to reconsider what we might mean by time in the first place.
"While time is, in general, thought to be relative to the observer, in our research time is derived from the mysterious dark energy which permeates the entire Universe," says physicist Steffen Gielen of the University of Sheffield in the UK.
"We propose that time is measured by the dark energy that is everywhere in the Universe, and responsible for its current expansion. This is the pivotal new idea that allows us to grasp the phenomena occurring within a black hole."
Black holes are among the Universe's coolest oddities. They are regions of space-time where matter has reached densities so extreme, they can best be mathematically described as a singularity – a single, one-dimensional point of infinite density.
At least some of them form from the collapsed cores of massive stars as they die; no longer supported by the outward pressure of fusion, these cores succumb to gravity, creating a region of space so dense that not even light speed is sufficient to reach escape velocity. That means, once matter comes close enough, it gets gravitationally slurped beyond the black hole event horizon, never to escape.
Probably. Possibly. Unless white holes exist. As the name suggests, these hypothetical objects are the antithesis of the black hole, a singularity that doesn't pull matter in, but forcibly spews it out.
To date, we have no evidence that there are any white holes out there in the Universe; but Gielen and his colleague, theoretical physicist Lucía Menéndez-Pidal of the Complutense University of Madrid, have shown that a black hole can transition into a white hole, at least on paper.
They based their calculations on a mathematical model of a black hole known as a planar black hole, one in a bent version of our Universe called an anti-de Sitter space.
This is a very simplified version of the real thing, described only in negative-curved two dimensions – a saddle-shaped plane – rather than a three-dimensional sphere in an otherwise flat space.
Then, they applied quantum mechanics to explore the singularity within.
"It has long been a question as to whether quantum mechanics can change our understanding of black holes and give us insights into their true nature," Gielen says. "In quantum mechanics, time as we understand it cannot end as systems perpetually change and evolve."
Under general relativity, the gravitational field around the singularity is infinitely powerful, the consequences of which aren't understood in terms of either relativity or quantum physics.
Yet by challenging a few basic assumptions, it might be possible to find some loopholes that turn the impossible singularity into something easier to understand.
Gielen and Menéndez-Pidal found that, rather than a singularity, the heart of the black hole becomes a region in which large quantum fluctuations are occurring, changes in the energy of space-time.
Under this model, the black hole transitions into a white hole, with the hole's singularity and horizons defined as regions of extreme quantum fluctuations where time doesn't end or fade into forever, but 'bounces', bringing its information with it.
It's not a new concept, with researchers considering in the past whether some kind of quantum echoes lay on the other side of black holes. But rethinking time itself could provide a new twist on the absurdities of singularities.
There are many questions surrounding white holes, and solid reasons to believe that the Universe might not be able to make them. This research is very theoretical, and is not intended to make the case for the existence of white holes; rather, it could help us better understand how black holes work.
In addition, physicists are keen to figure out how two very different, apparently incompatible frameworks – general relativity and quantum mechanics – can coexist in the same physical Universe. Works such as this are steps along the journey to resolve that perplexing problem.
Plus, it's just really, really neat.
"Hypothetically you could have an observer – a hypothetical entity – go through the black hole, through what we think of as a singularity and emerge on the other side of the white hole," Gielen says. "It's a highly abstract notion of an observer but it could happen, in theory."
The work has been published in Physical Review Letters.