Science fiction fans love the possibility of other universes, even more so contemplating the possibility of being able to travel between them through exotic configurations of spacetime, notably wormholes, which are pretty much just black holes with an opening poking through the singularity.
Less well known is the equally exotic (and purely hypothetical) possibility of "white holes:" the opposite of black holes. Whereas matter and light can fall into a black hole and never escape, white holes would emit light and matter but wouldn't take anything in, for example.
But while we see evidence for black holes in space, thus far there hasn't been any observational evidence of white holes. Now a physicist at the University of Oregon in Eugene thinks he might be able to explain why.
Here's the standard analogy for the formation of a wormhole: Picture a bed sheet stretched taut. Place a large bowling ball in the center of the sheet, and the sheet will bend inward in response, creating a gravitational pull.
Now imagine that the bowling ball is being squeezed, so that the same amount of mass must fit into a smaller and smaller space. The ball will become denser and denser as it becomes smaller and smaller. This causes the sheet to dip lower and lower, until finally the ball has been squeezed down to the size of a pinhead.
At that point, its density becomes so great and the gravitational force so strong that it pokes a small hole in the center of the sheet. That's what would happen if a wormhole formed at the center of a black hole.
But what lies on the other side?
Always a stickler for symmetry in his equations, Einstein hypothesized that a "mirror universe" must exist on the other side: a "white hole."
If you think of a black hole as a large funnel with a long throat and then "cut" the throat and merge it with a second black hole that has been flipped over (a "white hole"), you end up with something that looks like an hourglass or a funnel, with the two ends connected by a thin filament. This so-called Einstein-Rosen bridge (named for Einstein and his collaborator, Nathan Rosen) is an early theoretical incarnation of a wormhole.
Back in 1971, an astrophysicist named Robert Hjellming of the National Radio Astronomy Observatory published a paper in Nature proposing that white holes could be more than mirror images of their black counterparts. Matter could actually fall into a black hole and re-emerge elsewhere in space - or even in a completely different universe, a notion proposed by British physicist Roger Penrose a few years earlier - via a white hole.
Hjellming even speculated that white holes might account for the huge amount of energy being emitted from distant quasars and the centers of galaxies - far more than scientists could account for at the time by known physical processes.
That was 1971; this is now. Scientists know quite a bit more about our vast universe than they did 40 years ago. That excess energy coming from quasars? It's probably coming from supermassive black holes as matter falls in and emits telltale radiation in the process.
That doesn't mean the properties and characteristics of white holes aren't mathematically interesting, and thus worth contemplating, Hsu argues. After all, for decades black holes were mostly hypothetical, too.
Is it possible that white holes are hiding in plain sight? In the 1970s, Stephen Hawking demonstrated that when a white hole and a black hole are in thermal equilibrium with their surroundings, they absorb and emit the same amount of radiation - and thus it's impossible to tell them apart. So maybe some of the objects we think are black holes could be white holes in disguise.
But that's a very specific circumstance. Hsu decided to investigate how white holes might behave in a vacuum (i.e., in isolation), when they would not be in thermal equilibrium with their black hole counterparts.
Prevailing theory is that black holes evaporate slowly over time, gradually shedding matter in the form of "Hawking radiation," but the white hole would neither absorb or emit radiation when isolated in space.
What happens to a white hole that is thus, well, constipated? Hsu postulates that it must explode, thereby releasing huge amounts of energy: "quasithermal radiation." He concludes that stable white holes simply can't exist in empty space, and that's why we see no evidence for them. They most likely exploded into quasithermal radiation long before we had the tools with which to observe them.