 In the early 1900s, Albert Einstein published his theory of general relativity. In it, he laid out his revolutionary idea that space and time weren't two separate entities, but rather, they were linked, in that as you travel faster through space, time went slower for you. And on top of that, the presence of matter can distort this interwoven space-time fabric around it. And this distortion is what we perceive as gravity. This revolutionary concept shook our understanding of physics and is even incorporated into many of our modern technologies like satellites and the GPS system. But now, we face another challenge, explaining gravity on a subatomic level. In the previous installments in our Graviton series, we examined the idea of the force of gravity being carried by a hypothetical subatomic particle called a Graviton. But we still have so many unanswered questions about the mysterious Graviton, with the biggest one being, would the existence of the Graviton be compatible with our current understanding of physics? Well, that's what we're about to find out. In order to understand the battle between classical and quantum physics on gravity, we first have to understand why the idea of Gravitons are so important. To do that, we have to take a look at what physicists call the Standard Model. The Standard Model of Physics is the currently accepted theory which classifies and explains all the fundamental particles and their interactions with each other. It also explains the particle interactions that govern the strong and weak nuclear force and the electromagnetic force. However, there is one gaping hole. The Standard Model cannot explain gravity and why it is so much weaker than the other three forces. That is where the Graviton hypothesis comes in. By offering a potential explanation of gravity on a subatomic level, we get a big missing piece of the Standard Model puzzle. The quest for finding Gravitons is part of humanity's greater pursuit for the theory of everything which is a hypothetical framework of physics that could explain every aspect of the universe from the smallest subatomic particle to the largest galaxy clusters. This is considered one of the biggest challenges in physics, but if complete, it would hold the key to truly understanding the world we live in. Of course, there is more to the theory of everything than just explaining gravity on both a macroscopic and subatomic scale. It also holds the key to potentially unlocking other mysteries of the universe like dark matter and dark energy, but that's another topic for another day. For now, let's try to examine whether the existence of a particle like the Graviton would even be compatible with general relativity. Well, one major roadblock in trying to reconcile quantum mechanics with classical physics is the uncertainty that comes with going into the quantum realm. In general relativity, the components of mass and energy are viewed in a classical light. This means that the strengths, directions, and velocities of various fields and objects have definite values. However, if we take a look at things on a subatomic scale, like when we explore the strong and weak nuclear forces, as well as the electromagnetic force, we start to see this fall apart. We may be able to locate the position of a subatomic particle with very high accuracy, but at the cost of knowing the particle's velocity with the same level of accuracy. Also, any physical property of the fields associated with this particle suffer from the same dilemma. In quantum mechanics, we have probability distributions for these properties, which gives us the likelihood of a physical property of a particle being a given value at any point in time. And unlike general relativity, where we use nice, lovely, real numbers to describe these values, we have to resort to imaginary numbers when talking about these same properties in the quantum realm. If Gravitons do exist, this means that they would be subjected to this uncertainty principle as well, which can make it quite a challenge to detect them, as the accuracy regarding the information of the physical properties of the Graviton that are available to us are limited. In the grand scheme of things, this uncertainty is one of the biggest reasons for the clash between general relativity and quantum mechanics. In general relativity, the laws of physics allow us to know everything in a macroscopic system with gravity at a very high accuracy. But when studying the same system in the subatomic world, we are limited to probability-based outcomes and can't know everything about everything the way we do in general relativity. So are there ways we can reconcile this difference? Well, this is the current roadblock physicists are still trying to find their way around. And while there are no definite answers, there are some routes that can be taken, such as quantum field theory, or even trying some exciting alternatives such as string theory. Now these ideas themselves have so much to cover that they deserve their own separate video, which is why you should stay tuned for the next edition in our Graviton series, where we examine the potential bridges between the macroscopic and microscopic view of gravity. If you enjoyed this video and wish to learn more about the secrets of our universe that science is unraveling, including gravity, then subscribe and stay tuned for more science videos.