Question

At what point does a black hole stop sucking in mass and explode creating a new time/space continuum?

Black holes are a prediction of general relativity. If you input the mass of a huge star (say one hundred times that of the Sun) into Einstein's theory, you see that when the star runs out of energy from nuclear fusion, it will start to fall in on itself, and the gravitational field surrounding this collapsing star will become so huge that it will be impossible for anything - even light - to escape from it.

The funny thing about Einstein's equations is that they suggest that everything falls into a point. This point has no volume and is therefore infinitely dense. Because it has infinite properties, we call it a singularity, and are unable to make any further predictions about the matter that has gone into it. Maybe we just don't have powerful enough imaginations to understand such singularities, but physicists commonly think that they are such an absurd idea that it demonstrates the theory we are working with isn't quite perfect.

What Einstein's' theory does not say, therefore, is that there is a point at which black holes will stop sucking in mass. Instead, they go on sucking in mass forever! (Hawking has suggested that matter can actually escape from black holes by quantum effects, and it would therefore be possible for a black hole to evaporate, but in an enormouss amount of time).

The opposite of a black hole is a white hole. White holes chuck out matter and light from a singularity. They are the time-reversal of a black hole. It is a curious feature of most physical theories that they are independent of the direction of time. So if you video-record one solution and play it backwards, you get another. Even though both black holes and white holes are plausible scenarios according to general relativity, there is still a crucial difference. To see this, let's make up a theory that says that mass is fifty times greater than height. This theory predicts that someone who is 1m tall will weigh 50kg (not a good theory, I agree!) This is like predicting the black hole, we have taken real measurements and ended up calculating the weight of a particular person. However, our pretend theory allows the possibility for someone to be 10m tall and 500kg tall, but does not actually make a prediction of this. This is like the white hole. It is allowed by the theory, but as yet, we have not found any situations in the universe where it could exist.

However, astrophysicistss have often postulated the existence of white holes in order to overcome various problems. The pure symmetry of the idea is attractive - that for every black hole that exists sucking up matter, there is a white one spewing it out. There is no evidence (theoretical or practical) for this. In fact, the existence of white holes in the universe would be difficult toincorporatee into one of our other physical theories - the second law of thermodynamics. The problem is with the amount of entropy in a white hole.

Entropy is a measure of the amount of disorder in a system. Intuitively we have a very good understanding of this idea. A jigsaw puzzle is much more disordered when it is in its box than when it is completed. A smashed beer glass is more disordered than when it was filled up and placed on the table. Scientists have been able to describe entropy mathematically (in a similar way that "energy" has a blurry meaning in everyday life but can be calculated by physicists!) A white hole emits lots of light photons - which are packets of highlyorganizedd energy and therefore have a low amount of disorder and therefore a low entropy value. If we calculate the entropy of a white hole this is indeed what we find - it is low.

The second law of thermodynamics states that entropy increases in time for a closed system. This doesn't mean that we only see increase of disorder all around us, I'm sure you can think of many instances where order increases (when you take the pieces of a jigsaw puzzle from the box and put them together you increase its order !) However, wherever there is an increase in order, the second law says that there must be a bigger increase in disorder somewhere else. So in a "closed system", one with no "entropy communication" with anything outside of itself, entropy always increases. If you think of an appropriate system, the second law seems intuitive - for example, gases disperse instead of lining up neatly. In the case of the jigsaw puzzle, heat is created by the person putting the pieces together in order to account for the increase in order that occurs. Heat is the most disorganized form of energy there is, and the person burns "organised" food molecules and creates heat in order to do the puzzle. The low entropy of the jigsaw actually therefore ultimately comes from the photons of the Sun ! Now if the second law proposes that the universe is always increasing in entropy, how can white holes be formed ?

So although a white hole may explain where the mass of the black hole has gone to, it does not explain where its high entropy has gone to. Many scientists suspect that white holes don't exist in the universe. Of course, there are some that are not convinced by the existence of black holes, and more that think we haven't got them quite right. It is still true that a black hole may in some way explode as a white hole, but we have no idea if it can, if it will, and if so, at what point this would happen.
Answered by: Sally Riordan, M.A., Strategy Consultant, London

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