We all know that black holes are dangerous, if it swallows you, you are a goner. You will be stretched, ripped in half, folded and squeezed, again and again, until not a single particle of your body remains intact, before meeting an unfortunate demise of being absorbed by the singularity that eats all that falls in. You have no chance to survive… and, unlike in the old meme, you cannot make your time. The only safe option is to study the beast from afar. But, as usual, this coin, too, has a flipside. The mighty theory of relativity predicts that the time-reversed version of a black hole is just as valid an object. Enter the white hole.
If you thought black holes are weird, white holes are even weirder, trust me. Everything you learn about a black hole applies to the white hole, only with time running backwards. And this is where the weirdness starts. You cannot fall into a white hole, but something (or someone) might fall out of it. Yet, white holes have mass, just like black holes, so they attract things. Wait, what? They attract you but you cannot fall into them? How does that work? Can you get close to one? I mean, it attracts you, so you better be able to. From afar, you cannot tell without looking if an object that attracts you is a black hole, a white hole, or a garden-variety star, all you know is that something massive pulls you in. So, what magic kicks in when you get too close? Do you suddenly feel an overwhelming repulsion (this is a physical term, not a emotional one) and bounce right back? Do you try to fall in and somehow miss it? Does something even stranger happen? Let’s have a look.
Suppose you point your flashlight toward a black hole. What happens to the light beam? Here I have traced the rays and got the following picture (click to enlarge):
On this picture the light rays enter stage right, woosh around and disappear. The ones which do not get close to the black hole are barely deflected and exit stage left. As the light rays come closer to a black hole, they get deflected more and more. Actually, this happens with every massive object. Our own Sun deflects light, if only a little bit, and that was how the Einstein’s celebrated General Relativity Theory was first tested. But black holes bend light like there is no tomorrow. Which there won’t be for any daring ray of light which comes too close. You can see how the few really close ones hit the black hole and never come back out. If you were to shine light directly at the black hole, it would, of course, disappear without any deflection. There is one circle on that picture which shows that, if done just right, you can almost make a light ray spin around the black hole forever. “Almost” because you can’t, this circular orbit is unstable and before you can say “unstable equilibrium” the circling light ray will either fall in or spiral away and out to infinity.
This picture might remind you of the comets coming from the edges of the Solar system, circling the Sun and disappearing back to the edge, to not be seen again for a long long time, often forever. Black holes have strong enough gravity to do to light what the Sun does to comets.
Now, remember that white hole is black hole backwards in time? Well, this means that on that picture, you can run every light ray in reverse like a video run backwards, and you get the light deflection by white holes out of it for free, no need to do more calculations. So, for every light ray which comes from far away, spins around a bit and go back to the infinite void, there is another light ray which comes on the exact opposite trajectory.
So far, so good. Just like with black holes, light rays come in, light rays come out. There are also some light rays which come out of the white hole and disappear into infinity. That’s the “white” part, of course, you can never tell what will come out next, if anything. And now riddle me this: what happens if I shoot a ray of light straight into a white hole? Will it fall into it? Well, it cannot… nothing can fall into a white hole, just like nothing can escape a black hole. So… what happens?
Let’s think about it for a moment. Every possible light ray trajectory for the white hole has its counterpart trajectory for the black hole. What would be the black hole counterpart of “shooting a ray of light straight into a white hole”? Well, let’s time reverse it: it would be a ray of light coming “straight out of a black hole toward you”. And we know this is impossible. So, what goes wrong? Why can’t you shine light at a white hole? Does some magic cosmic force subvert your aim? After all, you can shoot what you can see, right? So, if you were to see a white hole somewhere in the sky, you just aim with your flashlight and voila! Only, apparently, not. Yes, relativity is weird, and general relativity is generally more so. Stay tuned.