In our Universe, the mathematical forms that the laws of physics take on give us all of the possibilities for what’s allowed to conceivably exist. However, only by actually observing, measuring, and experimenting with our Universe itself can we determine which of the mathematical possibilities describes our physical reality. In Einstein’s general relativity, one of the very first theoretical solutions that was ever uncovered was the solution for a black hole: a region of space with so much matter-and-energy in one place that from within that volume, no matter or energy, not even something that moved at the speed of light, could ever escape.
The flip side of a black hole is an equally plausible mathematical solution that’s the reverse of a black hole: a white hole, from which matter and energy will spontaneously emerge. Black holes have been demonstrated, through many different types of observations, to not only be physically real, but to be quite abundant all throughout the Universe. What about their time-reversed counterpart: the white hole?
What are white holes, and might they be physically relevant for our Universe, too? That’s what Kristin Houser wants to know, as she asks: “[I] came across a random blog post about white holes and wondered if you’d ever written about them? […] I bet anything you wrote would be far better than what’s showing up on page 1 of Google.” It’s one of the most fascinating possibilities ever conceived of. Let’s take a deep look at all we know.
When matter collapses, it can inevitably form a black hole. Roger Penrose was the first to work out the physics of spacetime, applicable to all observers at all points in space and at all instants in time, that governs a system such as this. His conception has been the gold standard in general relativity ever since.
However, while it robustly applies to non-rotating black holes, there may be a flaw with the reasoning that predicts it for realistic, rotating black holes. The idea of white holes makes a lot more sense if you begin with its much more familiar counterpart: the black hole. First thought up in the 18th century by John Michell who referred to them as “dark stars,” it was realized that just as all masses in the Universe have an “escape velocity” from their surface — i.e., there’s a certain speed that one must reach to escape completely from its gravitational pull — that if enough mass is gathered in a small enough volume, that escape velocity would reach or exceed the speed of light.
Since nothing can move faster than that speed, these objects would only absorb light-and-matter, but would never emit any from within a certain distance: its event horizon. The original idea was put forth within the context of Newtonian gravity, but in 1915, Einstein’s general theory of relativity was released, superseding Newton’s and replacing it with a more comprehensive law of gravity. Nevertheless, black holes persisted: they were shown to arise within Einstein’s theory as early as 1916, and versions of black holes with electric charges and angular momentum (i.e., spin) as well as mass were also discovered.
Extract — continue reading at the source.
