Sadly, there isn't a quick answer to this. The shortest version I can think of is that there's a fixed limit on how fast something can travel. However, gravity

Sadly, there isn't a quick answer to this. The shortest version I can think of is that there's a fixed limit on how fast something can travel. However, gravity is a distortion of space and time, and the more gravity you have, the faster you have to go to leave what you're starting from. Think of it like running on a treadmill - the faster the treadmill goes, the faster you have to run if you want to run off the end of it. At a certain point, the treadmill goes faster than you can run, and you cannot run off the end. Past a certain point, the distortion of spacetime becomes so great that no speed attainable can overcome the effect of it dragging back. So even travelling at the speed of light, you couldn't go faster than the gravity treadmill speed pulling you back in. For clarification - gravity is nothing like a treadmill. It just happens to work nicely as an analogy for this.