 As we've discussed in earlier videos, Newton didn't understand why his law of universal gravitation worked, only that it matched observations. One of the amazing ways that science progresses is by the discovery of simple rules and relationships that can explain many seemingly disparate events. So for example, we can derive all three of Kepler's laws from Newton's law of universal gravitation, and so Newton's law of universal gravitation is more fundamental. If you've been keeping track of all the physics you've done, you probably realize that this is the part where I tell you that everything I've told you has been a lie. So let's get into it. So to explore gravity that is more fundamental than Newton's law of universal gravitation, there are two aspects we can look at. Firstly, can we find any deeper relationships in physics, such as can we find an equation that explains not only gravitation, but also relates space and time? Or secondly, are there any problems we can find with Newton's law of universal gravitation? At this stage, we're going to take a brief diversion to discuss Einstein's theory of special relativity. A consequence of this theory, which you'll learn about in further detail later, is that nothing can travel faster than the speed of light in a vacuum, c. At typical speeds, the classical mechanics that we usually apply to solve motion problems can accurately predict motion. However, if we were to jump into a super-fast car and start accelerating and continue accelerating, while in good physics fashion, neglecting air resistance, then as we go faster and faster and keep accelerating, we'll start to notice that it is taking more and more force to accelerate us the same amount. This contradicts Newton's second law, f equals ma, which doesn't depend on velocity. To explain this kind of behavior, we need Einstein's theory of special relativity. In fact, the speed of light in a vacuum is kind of like the speed limit of the universe. Physically, we do not expect that anything, including any information, can travel faster than the speed of light. So let's think about that for a second. Have you heard that if the sun were to explode, we wouldn't know for eight minutes? This is how long it would take the light to travel from the sun. We know that we couldn't know any faster, because any signal that we tried to send could not go faster than the sunlight is travelling. So hang on, if you've been keeping gravity on the brain, you may see the predicament this leaves us in. We can imagine a situation where we have a large mass a long distance away from another mass that can feel its effects, let's say the sun and the earth. If the sun were to suddenly vanish, or we were to pull it away quickly, then the gravitational field strength that the earth experiences due to the sun would immediately change. We know that Newton's law of universal gravitation has no time dependency, so in this case we would know information from the sun before we could see the sun suddenly begin moving in the sky, and this information would therefore have moved faster than the speed of light. This means that either or both of special relativity and Newton's law of universal gravitation must be wrong. In fact, resolving this was arguably Einstein's greatest achievement, marrying the concepts of gravity with special relativity led him to develop general relativity. Einstein's theory uncovered the nature of space and time, or space-time, which is the fabric the universe is made of. Mass deforms this fabric, and the curvature of this deformation gives rise to gravity. As an analogy, think of space-time as a trampoline. If you place masses onto the trampoline, the fabric will tend to deform around the objects. If you place the objects close to one another, then they'll tend to roll toward one another. And if there's a light mass and a heavy mass, then the trampoline will curve more around the heavier mass, and the lighter mass will tend to fall into the heavier mass's deformation. Obviously, the heavier mass also moves toward the lighter mass, but to a smaller extent. This matches the behavior we intuitively associate with gravity, and the effects propagate through space-time at the speed of light in a vacuum, as we might expect from a thought experiment previously. One area in which Einstein's theory of general relativity differs from Newton's law of universal gravitation is that light, like everything, travels along the curvature of space-time. We have actually measured light deflection around our sun as it travels through space-time. Since light has no mass, Newton's law of universal gravitation could never predict that light would actually experience a gravitational force. In most circumstances, instead of general relativity, we can rely on special relativity, which is valid for weak gravitational fields, but breaks down in strong gravitational fields. This is similar to how Newtonian or classical mechanics breaks down at very high speeds. So when would we need to understand general relativity? Well, one of the results from general relativity that couldn't be predicted by Newton's law of universal gravitation, or by special relativity, is gravitational waves. It was announced in 2016 that these had been directly detected by the advanced LIGO project. Gravitational waves are like ripples in space-time that are radiated from objects with particular motion characteristics. These waves travel at the speed of light and literally stretch and contract the fabric of space. However, they generate only tiny changes in space. Even extremely large masses giving rise to the strongest gravitational wave sources only produce gravitational waves that stretch and contract space-time by an amount far smaller than the width of an electron. Currently, we've only just proved that we can directly detect gravitational waves from Earth. In the future, we can build bigger and better detectors, including by improving the advanced LIGO detectors, and by building detectors such as ELISA in space. These detectors can give us information about our universe that is impossible to get from other sources. For example, if there was an exploding supernova behind a gas cloud, then the light from the supernova may get scattered in the cloud. In this case, we wouldn't be able to study the explosion very easily using light or electromagnetic radiation that was scattered. However, if we had an amazing gravitational wave detector, then we could still see the supernova exploding just using a different sense from our usual sight. This could be done using gravitational waves, which are an active area of research in the physics community.