Expert: Steve Nelson - 7/3/2008

Question
QUESTION: Hi:

What about white holes makes their existence impossible?

A white hole is the opposite of a black hole. If one can exist, then why not the other?


Thanks,

Green

ANSWER: It's nice to give it a name that sounds opposite, and ask why it can't exist, but you have to come up with a physical definition before you'll understand the answer.

Matter stretches space towards it.  As far as we know, anti-matter does the same (an experiment to test this for sure is planned, and it's surprisingly difficult to test).  So there's nothing that makes anti-gravity, bunching up space around it and repelling matter instead of attracting it.  A black hole attracts matter and energy so strongly that not even light can escape it.  A white hole would do what, exactly?  Repel matter and energy so strongly that not even light could reach it?  Anything that repulsive would, instead of bunching up in one place, repel itself to pieces instantly.  It would never form a "hole" of any kind because that would be a localized phenomenon.  And as far as science knows, no material exists that makes anti-gravity anyhow.  So when you have a physical definition of "white hole" you realize that it wouldn't make any sense.

---------- FOLLOW-UP ----------

QUESTION: Hi:

Thanks for your response.

I've heard one reason white holes can't exist is because they violet the second law of thermodynamics.

Just would while holes violate the 2nd law?


Thanks again,

Green

Answer
White holes are a mathematical solution to Einstein's field equations involving no matter, hence there is nothing to make a singularity in the first place.  http://curious.astro.cornell.edu/question.php?number=108  Hence you have to get the energy density to create a singularity without actually having the energy to do so present.  Another way to think of them is a time-reversal of a black hole, and in reversing time you can decrease entropy...a violation of the second law.  Entropy increases with time, or stays the same...not decreases.  For anything deeper than that you'll have to ask an expert on gravitational solutions to the Einstein field equations (not my field of expertise).