# What is a white hole?

I have read a lot about Black holes and have gained wisdom of the subject. My question- are there such things as white holes- and if so- what do they do?

Before I answer this question I would like to point you to Kate Becker's discussion of why it is so hard to understand the expansion of the universe. The exact same argument applies here. White holes are not something that it is possible to understand using physical intuition. White holes pop up in general relativity (which also explains the expansion of the universe) and that theory as a whole is not easy to understand physically. The only way most people can understand general relativity is through mathematics, which, like Kate said, it not the way that most people are used to understanding things.

Hopefully that will give you some idea of why it is so hard to explain some of these concepts without resorting to mathematics. This does not mean that we shouldn't try, but it does mean that we might not succeed at the first try. Many smart people try very hard to get ideas from General Relativity across without using the Mathematics which the theory is based on, but that is something that is very hard to do. It probably requires a level of understanding of the theory which I would absolutely NOT claim to have.

Anyway I'll have a go.....

## What is a white hole?

The short answer is that a white hole is something which probably cannot exist in the real universe. A white hole will turn up in your mathematics if you explore the space-time around a black hole without including the star which made the black hole (ie. there is absolutely no matter in the solution). Once you add any matter to the space-time, the part which included a white hole disappears.

## How can you have a black hole with no mass?

1. Mathematically this is actually the simplest kind of black hole. Once the singularity is set up it will hold itself together, so the tricky part is setting up the singularity.
2. The only way to set up the singularity in the real universe is to start with it being there. Somehow the universe has to form with ready made singularities.

## Why can't white holes exist in nature?

1. There is no reason to suggest that the universe started out with ready made singularities. It would actually be quite odd if it did.
2. Once even the tiniest speck of dust enters the part of space-time which includes the black hole, the part which includes the white hole disappears. The universe has been around for a long time and so even if it did start with white holes, they would have all disappeared by now.

## Why bother with the solution if it isn't realistic?

1. It's easier than realistic solutions. (!)
2. Part of the solution is close to being realistic. This part does not include the white hole and describes the space-time outside of a normal black hole.

## What would a white hole look like if it did exist?

The people/person who came up with the term 'white hole' was actually being quite literal. A white hole is pretty much like an 'anti-black hole'. A black hole is a place where matter can be lost from the universe. A white hole is a place where (if it could exist with any matter in it - which it can't) matter would pop out into the universe. This has many similarities to the Big Bang singularity (although it's not quite the same, since there was nothing before the Big Bang).

## Where can I read more?

Try Exploring Black Holes by Taylor and Wheeler for a good undergraduate level text book on black holes and relativity. I also like Black Holes and Time Warps by Kip Thorne, which has a nice history of the subject.

Footnote: This is my second attempt at an explanation. In the interest of openness I leave the first attempt here also:

"In the full, and most simple General Relativistic solution for a space-time which has a Black Hole (in a vacuum), there are two singularities. One is in what we call the 'future-light cone' and this is the Black Hole. The other is in the 'past-light cone' and is called a White Hole. This solution is however completely unphysical in many ways and in a real Black Hole (formed from the collapse of a star for example) we cannot use the vacuum solution as there is matter present, and the White Hole singularity disappears.

So the answer to your question is that there is only such a thing as a White Hole in the theory of Black Holes and no such thing is possible physically."

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