I wrote the below for a friend of a friend's 7 year old child, who had come across white holes on a youtube science video and become fascinated by them. They sent me these questions, which I did my best to answer, only assuming the knowledge of science that a (very curious) 7 year old might have! I'm not sure how successful I was at this, but I found it a very interesting challenge, and I felt like my own understanding of white holes, and the connection between gravity and thermodynamics, improved in the process.
If you watch a video played backwards, you can normally tell that it is being played backwards, because it looks wrong. For example, if you film a glass smashing, and then play the video backwards, you know that what you are seeing can't happen in the real world (see here) . Glasses smash all the time, but little pieces of broken glass never leap into the air by themselves and stick together again! Also, you can burn toast if you heat it up too much, but it's much harder to unburn it!
But some things don't look wrong if you play them backwards. If you film a ball flying through the air in a curve, and then play it backwards, it still looks like something that could happen. If you showed it to someone backwards, they might not even be able to tell that the video was going backwards.
Gravity determines how the ball moves. As far as gravity is concerned, both directions of time work in the same way. So if you reverse time and look at things backwards, then what you see is still possible. The ball might be moving left instead of right, or planets might be going round the sun clockwise instead of anti-clockwise, but what you see is still something that could happen in the real world (not like a glass fixing itself!)
This is how gravity works in our solar system anyway. Amazingly, this is also how gravity works around black holes! This means that if you film a black hole, and play the video backwards, what you see should still be something that could happen. That's what a white hole is. It's like a video of a black hole played in reverse. Things will still get pulled towards white holes (just like things are still pulled towards the Earth when you play a video backwards). And objects could orbit white holes, just like they can orbit black holes. But there is one big difference between white holes and black holes. In a black hole, things can fall in but nothing can come out. In a white hole, things can come out but they can't fall in.
So how could a white hole get formed in the real world? We know that black holes can form when large stars run out of fuel and collapse. But we don't know of any way that a white hole could form. I think most physicists would think that there is no way to form a white hole in our universe. They exist in maths. They are one of the solutions to the maths equations which describe gravity. They must be, because black holes are solutions too, and the equations treat both directions of time in the same way. But we don't think you can make white holes in the real world.
One reason is something called the "Cosmic Censorship Conjecture". It's a fancy name, but it basically says that because of the way our universe began, we should never be able to see a singularity [concept was explained in a previous answer]. We can't see the singularity in a black hole, because no light can escape from a black hole, so that's ok. But a white hole has a singularity inside it too, and we can see that one. So if the Cosmic Censorship Conjecture is true, it should be impossible to create a white hole. White holes are also unstable. This means if one formed, it would not stick around for very long. It's also much harder to form an unstable thing than a stable thing (getting a ball to balance on top of a hill is harder than getting it to stay at the bottom of a ditch).
This could be wrong though. There's another physics theory called "quantum mechanics", and when you take quantum mechanics into account, the Cosmic Censorship Conjecture can go a bit wrong. We know that black holes eventually shrink and disappear when quantum mechanics kicks in (although it takes a very very long time). When they disappear, their singularity is visible very briefly, so Cosmic Censorship is broken. Some physicists have wondered whether a black hole might turn into a white hole just before this happens. But this isn't accepted by many people yet!
So where do white holes spawn? They probably don't form anywhere in our universe, although we can't be sure. How big can they be? Well just like black holes, in theory they can be any size. You could have a black hole (or a white hole) which weighed the same amount as you. In practice, we don't know of a way that a black hole can form unless it weighs much more than our sun. And we don't know of any way that a white hole can form, of any size.
There's one final thing I should tell you. I said that gravity treats both directions of time in the same way. It turns out this is true of all the basic forces we know about. That's very odd. Why do any videos look wrong when we play them backwards? Why is the future different to the past? Why do glasses smash but not fix themselves? Why can you remember the past but not the future? The best answer we have is that actually, what you see in a reversed video is not impossible. It is just extremely unlikely to occur in the real world, because of the way our universe began. There's many more ways to break a glass than there are to fix it. This is a bit like what we said about the white hole. So actually, a white hole and a self-repairing glass are not that different. They are both possible in theory, but probably not something you will ever see in practice!
Great question! I had to google this one! It is not known for sure. But the closest one we know about right now is 1000 light years away in a constellation called Telescopium. It was only discovered last year.
1000 light years means that it takes light 1000 years to travel that distance. If you tried to do it in an aeroplane (which can somehow fly through space and doesn't need to refuel) it would take over a billion years to get there! A billion years ago on Earth, no animals had evolved yet, only very primitive plants!
There are two stars orbiting the black hole, and we can tell from the way they are moving that the black hole is there. These stars are visible with the naked eye in the southern hemisphere!
How do we know so much about black holes when they are so far away and we can't see them? Einstein, a famous physicist, came up with a new theory for how gravity works in 1915, called "General Relativity". Einstein wrote down some maths equations for working out how objects will move when gravity pulls on them. His equations were better at describing the world than what we had before. They correctly said how the planet Mercury will move. The previous theory, from Newton, had got it a bit wrong. They also predicted how light will bend when it goes past the Sun. This was tested during a solar eclipse (where the moon moves in front of the sun). The stars behind the sun appeared in slightly different places because the starlight was being bent by the Sun's gravity. The amount of bending was just what Einstein said it would be.
So the equations had good evidence backing them up. And then people realised they also predicted black holes. The equations say that if you have enough stuff in a small enough space, you form an event horizon, that nothing can escape from. At first, people were not sure that black holes could actually exist in the real world (a bit like how I told you I was not sure white holes could exist!). But as they got better at doing the maths, they got more convinced that black holes might be real. People worked out that black holes were stable, that they can form in lots of situations, and that if stars get too heavy there should be no way they can hold themselves up any more once they run out of fuel.
But all the theory still relies on Einstein's equations being right. And to start with they had only been tested in our solar system, where gravity is a bit weak (by black hole standards!). How do we know they still work when gravity gets really strong? Astronomers have now brought together lots of evidence for black holes existing in the real world. They have seen stars moving in ways which are very hard to explain unless they are orbiting a black hole. Some real big proof came in 2016. Physicists were able to detect something called a "gravitational wave". These were predicted by Einstein's equations too, but had not been seen before. In 2016 we finally saw them, and their shape matched exactly what we expected to see from a black hole collision. So not only do black holes exist, but they collide sometimes as well, and we've seen the shockwaves coming out from one of those collisions, in the form of gravitational waves.