 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 realise 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 behaviour, 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 try 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. And in fact, resolving this was arguably Einstein's greatest achievement. Marrying the concepts of gravity with special relativity led him to develop general relativity.