 In our previous video, we begin to examine the clash between classical and quantum mechanics on gravity. This conflict has been intriguing us for decades and is one of the biggest barriers to completing the standard model. Now for those of you who watched the video, it's important to understand the uncertainty principle is just one of the many barriers between the unity of general relativity with quantum mechanics. So today, we're going to be covering some of the other roadblocks in developing the theory of everything along with the ways physicists are trying to circumvent them. Without further ado, let's dive right in. Let's start off with what we know. Gravitons if they existed would be subjected to the same uncertainty that predominates all particles in the quantum realm in that we can't know their position and location simultaneously, which means that the existence of gravitons would seem like a direct contradiction to the smooth continuous fixed entity that is the curve of space-time as described by general relativity's definition. But that's just a tip of the iceberg because when we start trying to merge quantum and classical mechanics, we hit another problem, non-renormalizability, but what exactly does that mean? Remember in the previous video where we talked about how probability distributions were used to explain certain parameters of quantum particles since the uncertainty principle makes it hard for us to know the exact values of these parameters? Well, in order for the probability distribution to be valid, one of the conditions it must meet is that it has to be renormalizable, meaning you can augment the distribution to where if you sum up all the probabilities on the distribution, it adds up to 100%. Unfortunately, gravity is non-linear, which means its effects don't directly scale with changes in mass or energy, and because of this non-linearity, when we try to make calculations to predict parameters of the graviton under certain conditions, like the high-energy interactions in the collision of a black hole, we get calculations that result in infinite possibilities, which is just a fancy way of saying, we can't really predict these interactions mathematically. Along with non-renormalizability, the graviton runs into issues with background independence. In physics, background independent theories is a term used to refer to theories where the structure of space-time itself can be changed, like how in general relativity, the presence of matter and energy can change the curvature of space-time. However, the concept of gravitons is based on the idea that the gravitons are propagating in a flat, fixed, and unchanging space-time, thereby contradicting the background independence of general relativity. So how exactly are physicists trying to get around this? Well string theory may be one of the ways to find light at the end of this tunnel. For those who are unfamiliar, string theory is a theory that matter consists of tiny one-dimensional vibrating particles called strings that come in an open-ended or closed-loop form, in that the vibrations of these strings can correspond to different properties of matter. The beauty of string theory is that it naturally includes gravitons as a closed string that has virtually all the properties that gravitons are theorized to have. Even better is that the calculations for string theory get rid of all these potentially infinite possibilities for how gravitons could interact, which helps us out with the pesky little problem of non-renormalizability. However, string theory still doesn't patch up the issues with background independence, so for that, we have to turn to string field theory. String field theory basically takes string theory, which usually explains the properties of one string at a time, and recasts it into a sort of field theory framework where it can explain all types of strings in the way it can be arranged. Because of this property, it may be able to lead us to find solutions where we have configurations of strings that are compatible with background independence. While there may be hope on the horizon, it's important to understand that all of these developments are theoretical in nature, as string theory has not been confirmed to be true as of the making of this video. However, physicists are still making efforts to navigate the ocean of complicated equations and possibilities to try and make this theory the key to understanding our universe. And it's definitely not an easy task given the mathematical complexity of the process. Not to mention, when we get into string theory, we open doors to other complexities, like the possibility of our universe having more than just three dimensions we're used to, but that's another topic for a future video. At the end of the day, what you need to take away from watching this is that trying to unite general relativity with quantum mechanics is not going to be an easy feat. The issues we listed in this video are just scratching the surface of the mountains we have to climb to reach the theory of everything. However, we hope this video gives you a deeper appreciation and understanding of the mysteries of the cosmos that captivate us, whether it's the search for the elusive graviton, or just trying to understand the forces that hold everything together. There are still a lot of mysteries of gravity waiting to be solved, and you can rest assured that we will be covering more of them in the future. In the meantime, if you want to stick along for that journey, then hit that subscribe button and notification bell, and let us know in the comments below what your thoughts on gravitons are. As always, stay tuned for more science videos.