Why Does E=mc2_ - Brian Cox [2]
The existence of such a special speed, a cosmic speed limit, is a strange concept. As we will discover later in this book, linking this special speed with the speed of light turns out to be something of a red herring. It has a much deeper role to play in Einstein’s universe, and there is a good reason why light travels at the speed it does. We will get to that later on. For now, suffice to say that when objects approach the special speed, strange things happen. How else could an object be prevented from accelerating beyond that speed? It’s as though there were a universal law of physics that prevented your car going faster than seventy miles per hour, no matter how large the engine. Unlike a speed restriction, however, this law is not something that needs to be enforced by some kind of ethereal police force. The very fabric of space and time is constructed in such a way that it is absolutely impossible to break the law, and this turns out to be extremely fortunate, for otherwise there would be unpleasant consequences. Later, we shall see that if it were possible to exceed the speed of light, we could construct time machines capable of transporting us backward through history to any point in the past. We could imagine journeying back to a time before we were born and, by accident or design, preventing our parents from ever meeting. This makes for excellent science fiction, but it is no way to build a universe, and indeed Einstein found that the universe is not built like this. Space and time are delicately interwoven in a way that prevents such paradoxes from occurring. However, there is a price to pay: We must jettison our deeply held notions of space and time. Einstein’s universe is one in which moving clocks tick slowly, moving objects shrink, and we can journey billions of years into the future. It is a universe in which a human lifetime can be stretched almost indefinitely. We could watch the sun die, the earth’s oceans boil away, and our solar system be plunged into perpetual night. We could watch the birth of stars from swirling dust clouds, the formation of planets and maybe the origins of life on new, as yet unformed worlds. Einstein’s universe allows us to journey into the far future, while keeping the doors to the past firmly locked behind us.
By the end of this book, we will see how Einstein was forced to such a fantastical picture of our universe, and how this picture has been shown to be correct in many scientific experiments and technological applications. The satellite navigation system in your car, for example, is designed to account for the fact that time ticks at a different rate on the orbiting satellites than it does on the ground. Einstein’s picture is radical: Space and time are not what they seem.
But we are getting ahead of ourselves. To understand and appreciate Einstein’s radical discovery, we must first think very carefully about the two concepts at the heart of relativity theory, space and time.
Imagine you are reading a book while riding on an aircraft. At 12:00 you glance at your watch, decide to put your book down, leave your seat, and walk down the aisle to chat with your friend ten rows in front of you. At 12:15 you return to your seat, sit down, and pick up your book. Common sense tells you that you have returned to the same place. You had to walk the same ten rows to