First Black Holes, Now White Holes?

Backward Black Hole (Artist's Concept); Credit - NASA/JPL: https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA13168

You have probably heard of black holes: incredibly dense regions of space-time with such strong gravitational attraction that not even light can escape from them. They were first predicted by general relativity, but Einstein’s famous theory does not stop there. The same equations that revealed the presence of black holes point to an equally fascinating phenomenon – white holes. ^[1]

Black holes in reverse

While black holes are notoriously known for pulling matter inward, white holes do the opposite, ‘spewing [it] out.’ ^[2] Black holes are extraordinarily massive, with supermassive black holes ranging from ‘millions to billions of times the mass of the Sun.’ ^[3] White holes are born when you consider what would happen if a black hole singularity had no mass. They have not yet been discovered by astronomers and are currently just a mathematical prediction. However, what would happen if they did exist? Well, while the event horizon of a black hole marks the point of no return, its white hole counterpart would be a ‘boundary of no admission’ ^[1] – an impenetrable wall from the outside.

Do they exist?

Since white holes are essentially time reversed black holes, to form them, you need to effectively reverse time. This violates the second law of thermodynamics which states that entropy, a measure of disorder, always increases. Surprisingly, this is not impossible because ‘entropy is a statistical phenomenon’ ^[4] so although rare, reductions in entropy are technically possible as long as it increases overall on average. However, ultimately entropy wins and as soon as it and time resumes, the white hole will cease to exist and instead ‘explode in a burst of energy.’ ^[4]

Interesting developments

One of the greatest unanswered questions in science is: ‘what lies on the other side of a black hole?’ Some have proposed that the solution is a white hole. This would mean that energy and matter entering the black hole would emerge from the white hole. Furthermore, the description of white holes sounds very familiar: matter ejecting out in a burst of energy sounds remarkably similar to the Big Bang. For these reasons, physicist Lee Smolin suggested that the white whole on the other side of a black hole could be the big bang of another universe such as ours.  ^[5] If true, this would indicate the existence of a multiverse where our universe is just one of multiple others.

 

Sources:

1. Charlie Wood, Space.com: https://www.space.com/white-holes.html

2. Carlo Rovelli, New Scientist: https://www.newscientist.com/article/mg24032080-100-if-you-think-black-holes-are-strange-white-holes-will-blow-your-mind/

3. Caitlyn Buongiorno, Astronomy: https://astronomy.com/news/2020/04/how-much-space-does-a-black-hole-take-up

4. Matt O’Dowd, PBS Spacetime: https://www.youtube.com/watch?v=S4aqGI1mSqo

5. Ella Alderson, Medium: https://medium.com/predict/a-universe-from-a-black-hole-25b2f51decf0