An impossible reality?
We have all heard of black holes, but have you ever heard of white holes? They are hypothetical features of the universe and are considered to be the opposite of a black hole — a black hole sucks in material where nothing can escape, while a white hole spits out matter and nothing can get in.
White holes are just a mathematical solution to the laws of general relativity which imply that if black holes exist in the universe, then a white hole should also exist since they are time-reversals of a black hole. However, time can’t go backwards can it? It turns out the theory of relativity does not care if time goes forward or backwards — if it can go one way, it can easily go the other — but as we know, time tends to only move forward. So if a white hole did in fact exist, it would be very unstable and likely only last a few seconds before turning back into a black hole.
An important constraint to white holes is that they violate the second law of thermodynamics, which states that disorder (“entropy”) in the universe can either remain the same or increase but it can never decrease. However, white holes decrease entropy.
Since white holes are reverse black holes, they would also be formed by a gravitational singularity — a point in spacetime where the gravitational field becomes infinite — however a white hole is a “naked” singularity, meaning it does not have an event horizon. According to the principles of general relativity, the universe can not support these types of singularities because event horizons were primarily introduced to reconcile the issue of spacetime breakdown near black holes. However, some new theories have pointed to the existence of these naked singularities.
Astrophysicists have always wondered whether black holes destroy information when material falls into them, and a model performed in a study published in Nature in 2014 suggests that at the end of their lives, black holes may in fact turn into white holes — spitting out all the material they have ever consumed back into space.
According to the model developed by Carlo Rovelli and Hal Haggard from Aix-Marseille University in France, the transformation from a black hole to a white hole would occur right after the initial formation of a black hole by a process called loop quantum gravity. What this means is that as a star collapses under its own gravity, it will continue to shrink until it cannot shrink anymore. At this point, it will exert a pressure called quantum bounce that turns a black hole into a white hole. This process can take only thousandths of a second to occur, however according to the researchers, they can continue to appear as black holes lasting billions or trillions of years because of a phenomenon known as gravitational time dilation.
Scientists may have caught a glimpse of a white hole back in 2006, when they witnessed an explosion of white light that came from nowhere and then vanished. However no similar events have happened since to provide any new information about the phenomenon. Until astronomers see another event like the one that occurred almost 10 years ago, we can only wait and wonder. But think of it this way — at one point black holes were considered just theoretical, so maybe it is not so crazy to think that white holes can also exist.