 So, you're here after watching the previous video, wondering about all these wacky wormholes. Naturally, being the curious being that you are, your head must be filled with all sorts of questions, perhaps the most important being what exactly are these white holes and how exactly do they work. More importantly, what are their implications for the creation of a Schwarzschild wormhole? Picture the following scenario. You just exited a Schwarzschild wormhole to visit a planet in the neighboring Andromeda galaxy. However, you realize that you accidentally traveled to the wrong planet and wished to return to earth. But as you attempt to go back in the wormhole to earth, you find something blocking you. Not realizing it, you have just encountered a white hole. Now fortunately, things such as white holes are purely theoretical in nature, so you won't find yourself in a scenario like this. But the idea is still a very interesting one and worth looking into. The concept of white holes was first put forward by Russian cosmologist Igor Novikov in 1964. In a nutshell, white holes are the theoretical opposites of a black hole. In order to understand what is meant by this, we need to look at a concept called time symmetry. Time symmetry simply states that the laws of physics governing the motion interaction and state of an object are the same whether you run time forwards or backwards. For example, if you throw a ball in a way that it makes a parabola like shape, then it doesn't matter whether you reverse time or not, the course of the motion for the ball will be the same. Another instance we can look at is this pool ball. If we roll the ball in a straight line to the end of this table, then even if we reverse time, the course of motion would still be the same straight line. Most of the macroscopic phenomena in our universe are governed by time symmetry. However, there are some wonky exceptions with one of the most notable being a black hole. An object can enter a black hole once it crosses the event horizon but cannot go back and exit. If we reverse time, then the opposite would happen. The object can exit the event horizon but cannot re-enter. In a nutshell, this is what a white hole is. A region of space-time that an object can exit but cannot re-enter or a time-reversed version of a black hole. Now, before you get excited about white holes, you need to understand the reality of their existence. That is, a white hole cannot exist in our universe without violating the laws of physics, particularly the second law of thermodynamics. The second law of thermodynamics states that entropy in the universe is always increasing. In a very, very simple way of putting it, the level of disorder in our universe is going up. For example, if this glass bowl hits the ground and shatters, all the pieces of the glass will be randomly scattered around the room and the shards will never come back together to form the glass bowl on their own. Or, when it comes to temperature, heat will naturally flow from a hotter region to a colder one but will never flow from a cold to a hot region on its own, thereby having a more even distribution. So, how does this apply to white holes? Well, let's put it this way. Here we have a black hole, a giant blob formed by the collapse of a massive star sucking in everything in its path with its massive gravitational pull. Now, this giant dark blob is an entropy monster. Beautiful ordered objects like planets and moons get sucked into it and then ripped into a bunch of random chaotic shreds increasing the black hole's mass in the process. A white hole would basically be all of this but time reversed. Essentially, we have this hole which is spitting out all this matter and spontaneously going back to being a star. Now, you can probably start to see why physicists aren't exactly lining up to jump on the idea of white holes. The thing is Schwarzschild wormholes exist as a solution to Einstein's field equations, which are a set of equations relating the curvature of spacetime to the matter present within it. However, like we emphasize in this video, its existence would essentially be a violation of the second law of thermodynamics. Something like a white hole would not exist for very long, even if it was possible to find them in reality. In a fairly disappointing conclusion, it seems like the Schwarzschild wormhole we talked about in the previous video just isn't possible. This doesn't mean we shouldn't stop exploring other ways to make interstellar travel possible. The amazing thing about science is that when one door shuts in our face, another door always opens up. Unfortunately, we are all out of time for today, but we will definitely be looking at more ways to journey to the heavens and beyond in future videos. So, if you haven't already, hit that notification bell and subscribe. And most importantly, don't forget to stay tuned for more science videos.