Season 17, Episode 30: What is a White Hole?

Hey StarTalkians! Season 17, episode 30 was a Cosmic Queries edition with Neil and Paul, with a grab-bag of questions on all things cosmic. Early in the show, they tackled a question about the difference between a black hole and a wormhole, which veered into some interesting territory:

Cosmic Queries – Scars in Spacetime - StarTalk Radio

(from 5:45)

A white hole is something you may not have heard of – outside of creative sci-fi like the excellent game Outer Wilds – and Neil’s description covered the basics. But would such a thing really exist? Could it exist based on known physics?

The White Hole as the Opposite of a Black Hole

The easiest way to understand what a white hole is follows Neil’s explanation: it’s the opposite of a black hole.

Theoretical physics often deals with symmetries in nature. You don’t need to tell me if you’re in Chicago or Rome for me to calculate the gravity acting on you, because gravity is symmetric.

One such symmetry is time-reversal symmetry. As an analogy, if I drop a ball and it bounces back to my hand, playing a video of this backwards would make the original “drop” look like the “catch” and vice-versa, but it would still make sense. The video would have time reversal symmetry.

This symmetry applies to general relativity, so what would a time-reversed black hole look like? It would be something which lets matter out but which nothing ever entered. This is a white hole, and ultimately the idea comes from pure theory – we’ve never found one.

Modern White Hole Theory

There are also more modern formulations of the idea. The simplest way to conceptualize these is to think of the white hole as a kind of “exit” to the black hole . This would basically be a solution to the black hole information paradox.  

This kind of works like it does in Outer Wilds (just on a much smaller scale), where falling in the black hole sends you flying out of the white hole, way across the solar system.

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A particularly compelling formulation sees white holes as “remnants” of black holes. The black hole doesn’t simply vanish, it quantum mechanically tunnels through into a white hole. This gets more likely the smaller the black hole is, so the resulting white hole would be low mass, and long-lasting.

This formulation appears consistent with modern theories of quantum gravity, so they could exist. But they still probably don’t.