Black holes appear to get all the eye. But what about their mirror twins, white holes? Do they exist? And, in that case, where are they?
To know the character of white holes, first we’ve got to look at the way more familiar black holes. Black holes are regions of complete gravitational collapse, where gravity has overwhelmed all other forces within the universe and compressed a clump of fabric all the way in which all the way down to an infinitely tiny point often known as a singularity. Surrounding that singularity is an event horizon, which just isn’t a physical, solid boundary, but simply the border around a singularity where the gravity is so strong that nothing, not even light, can escape.
We all know how the universe forms black holes. When a large star dies, its immense weight crushes onto its core, triggering the creation of a black hole. Any matter or radiation that wanders too near the black hole gets trapped by the strong gravity and pulled beneath the event horizon to its ultimate doom.
Related: What happens at the middle of a black hole?
We understand this strategy of black hole formation, and the way black holes interact with their environments, through Einstein’s theory of general relativity. To reach on the concept of a white hole, we’ve got to acknowledge that general relativity doesn’t care concerning the flow of time. The equations are time-symmetric, meaning the mathematics works perfectly wonderful running forward or backward in time.
So if we were to take a movie of the formation of a black hole and run it in reverse, we’d find an object streaming radiation and particles. Eventually, it might explode, forsaking a large star. This can be a white hole, and based on general relativity, this scenario is perfectly wonderful.
White holes can be even stranger than black holes. They might still have singularities at their centers and event horizons at their borders. They might still be massive, gravitating objects. But any material that entered a white hole would immediately get ejected at a speed greater than that of sunshine, causing the white glow to shine ferociously. Anything on the skin of a white hole would never have the opportunity to get inside it, because it might need to travel faster than the speed of sunshine to cross inward through the event horizon.
But when white holes are allowed by the mathematics of general relativity, then why don’t we suspect that they exist in the actual universe? The reply is that general relativity just isn’t the one word on the cosmos. There are other branches of physics that tell us concerning the inner workings of the universe, like our theories of electromagnetism and thermodynamics.
Inside thermodynamics, there’s the concept of entropy, which is, very roughly speaking, a measure of the disorder in a system. The second law of thermodynamics tells us that the entropy of closed systems can only go up. In other words, disorder at all times increases.
For instance, say you throw a piano right into a wood chipper. Out comes a bunch of pulverized debris. Disorder within the system has increased, and the second law of thermodynamics has been satisfied. But for those who throw a bunch of random pieces into that very same wood chipper, you will not get a totally formed piano out of it, because that will cause disorder to diminish. (Highly ordered systems, like life, can arise on Earth — but they arrive at the fee of increased entropy inside the sun. You are still not getting pianos out of wood chippers, irrespective of the way you construct your system.)
We won’t simply run the strategy of black hole formation in reverse and get a white hole, because that will cause entropy to diminish — stars don’t miraculously appear out of gigantic cosmic explosions. So, while general relativity is agnostic concerning the reality of white holes, thermodynamics gives the concept a tough no.
The one method to form a white hole can be to have some exotic process operating within the early universe that baked the existence of a white hole into the material of space-time itself. That way, the white hole formation process would bypass the difficulty with decreasing the entropy — the white hole would simply be there, existing, because the starting of time.
Unfortunately, white holes would even be fantastically unstable. They might still gravitate and pull material toward them, but nothing would have the opportunity to cross the event horizons. As soon as anything, even a single photon (particle of sunshine) approached a white hole, it might be doomed. If the particle approached the event horizon, it might not have the opportunity to cross it, sending the energy of the system skyrocketing. Eventually, the particle would have a lot energy that it might trigger the collapse of the white hole right into a black hole, ending its existence.
So, as fun and mind-bending as white holes seem like, they don’t appear to be features of the actual universe — just ghosts haunting the mathematics of general relativity.