You've probably heard of black holes. It's believed there's one at the heart of every galaxy, and we've even imaged one – possibly even the one that sits in the center of our own galaxy – yet they are still mysterious objects. But have you ever heard of white holes, black holes' evil twins?
What is a white hole?
A white hole is a hypothetical feature of the universe, considered the theoretical opposite of a black hole.
Black holes are objects that generate a field of gravity so strong that nothing in their vicinity – including light – can escape. Right on the edge of that precipice is the event horizon, the threshold where to escape the pull of the black hole, you’d need to move faster than the speed of light. According to Einstein’s theory of special relativity, nothing in space can move faster than the speed of light.
They gobble up material that gets compressed down into an infinitely dense ball called a singularity. Black holes may have the mass of a million Suns compressed to the size of single Sun. If you looked at one, however, it would be invisible, because it doesn't let light escape. Instead, we know they are there from the way massive objects (ie. with huge gravity) warp space-time around them. This is Einstein's theory of general relativity.
White holes are thought to exist based on the same general relativity concepts as black holes. They are expected to have gravity, so they attract objects – but anything on a collision course with a white hole would never reach it.
This is because white holes don't let anything enter, including light and matter, functionally opposite to black holes. Instead, they constantly emit matter and energy, preventing anything from re-entering past their event horizon. If you looked at them, they would look just like black holes (if you could see them) but in reverse, with light and matter spewing out into the cosmos.
WHAT WOULD HAPPEN IF YOU APPROACHED A WHITE HOLE?
Theoretically, if you were to approach a white hole in a spacecraft, you would be inundated by a colossal amount of energy, which would most likely destroy your ship. Even if your spaceship could withstand gamma rays, light itself would start slowing you down like air resistance slowing down a moving vehicle on Earth.
And even if the spaceship is built to be unaffected by the energy emission, space-time would be weirdly warped around a white hole; approaching a white hole would be like going uphill. The acceleration required would get higher and higher while you move less and less. There isn't enough energy in the universe to get you close let alone inside.
Of course, this is fairly counterintuitive. How could energy in a white hole seemingly come from nowhere other than space-time itself? This is one reason why their existence is very unlikely. However, there are some theories in which white holes are possible, but perhaps not quite as described in general relativity.
How do white holes form?
There are a few hypotheses on how white holes may form. One traced back to cosmologist Igor Novikov in 1964 is that white holes are the theoretical evil twin of black holes. Novikov built on the work of physicist Karl Schwarzchild, who posited black holes plus what we now call wormholes, which objects could theoretically pass through to instantly jump huge distances, between event horizons and a theoretical opposite version of the singularity.
However, physicists have shown mathematically that white holes could exist, just not that they could in real life, because they couldn't work out how they could form if they were the opposite of a black hole. Black holes form when a star dies and collapses. The opposite or reverse would be a black hole exploding into a star, which violates the rules of entropy.
Another hypothesis suggests at the end of their lives, black holes become white holes, although only very briefly. In 2014, theoretical physicist Carlo Rovelli developed a model that suggests that at the end of their lives, black holes turn into white holes, explosively pouring all the material they have ever swallowed into space.
The transformation from black hole to white hole would occur right after the initial formation of the black hole. Their model is based on a theory called “loop quantum gravity” – where gravity and space-time are quantized, woven from tiny-individual loops that can’t be subdivided any further. As a dying star collapses under its own gravity, the star will continue to shrink – but eventually, it will reach a stage where it can’t get any smaller because the loops cannot compress anymore.
At this point, the loops exert an outward pressure called “quantum bounce,” which transforms the black hole into a white hole. According to Rovelli's rough estimates, it takes just a few thousandths of a second for a black hole to transform into a white hole. Yet even though the transformation is nearly instantaneous, black holes can still appear to us as lasting billions or trillions of years because their gravity stretches light waves and dilates time.
Do white holes exist?
As of right now, white holes are still considered a theoretical concept, although it may come down to our current understanding of physics.
White holes are thought to be very unstable. There's little chance a cosmic event spewing out that much matter and energy could sustain itself long enough to be caught by a telescope. Some have theorized that when it expels all that matter, if it collides with any other matter in the white hole's orbit the whole thing would collapse in on itself and become a black hole, creating an endless loop of black, then white, then black holes. Stephen Hawking himself pointed out they may behave so similarly we wouldn't be able to distinguish them anyway.
A lot of phenomena have been put forward as potential white holes. They are usually chosen because they are mysterious objects that we have not been able to explain in detail yet. Gamma ray bursts, fast-spinning pulsars, and black holes reaching the end of their lives have all been considered. Even the Big Bang has been described as a white hole.
But so far, no white holes have ever been directly observed, and even their theoretical existence raises some red flags. It seems that white holes are to be used as a placeholder until more observations or a better theory come along.
Although it's worth noting Einstein published his theory of general relativity, predicting black holes, in 1915 and it took until 1971 for the first one to be identified.