Stephen Hawking by John Gribbin Read Free Book Online

a speed, and speed is a measure that relates space and time. Speeds are always in the form of miles per hour, or centimeters per second, or any other unit of length per unit of time. You cannot have one without the other when you are talking about speed. So the fact that the fundamental constant is a velocity must be telling us something significant about the Universe. But what?
    If you multiply a speed by a time, you get a length. And if you do this in the right way (by multiplying intervals of time by the speed of light, c ), you can combine measures of length (space) with measures of time in the same set of equations. The set of equations that combine space and time in this way consists of the equations of the special theory of relativity that describe time dilation and length contraction † and lead to the prediction that a mass m is equivalent to an energy E as described by the formula E = mc 2 . Instead of thinking about space and time as two separate entities, as long ago as 1905 Einstein was telling physicists that they should be thinking about them as different aspects of a single, unified whole—spacetime. But this spacetime, the special theory also said, was not fixed and permanent like the absolute space or absolute time of Newtonian physics—it could be stretched or squeezed. And therein lay the clue to the next great step forward.
    Einstein used to say that the inspiration for his general theory of relativity (which is, above all, a theory of gravity)came from the realization that a person inside a falling elevator whose cable had snapped would not feel gravity at all. We can picture exactly what he meant because we have seen film of astronauts orbiting the Earth in spacecraft. Such an orbiting spacecraft is not “outside” the influence of the Earth’s gravity; indeed, it is held in orbit by gravity. But the spacecraft and everything in it is falling around the Earth with the same acceleration, so the astronauts have no weight and float within their capsule. For them, it is as if gravity does not exist, a phenomenon known as free fall. But Einstein had never seen any of this and had to picture the situation in a freely falling elevator in his imagination. It is as if the acceleration of the falling elevator, speeding up with every second that passes, precisely cancels out the influence of gravity. For that to be possible, gravity and acceleration must be exactly equivalent to one another.
    The way this led Einstein to develop a theory of gravity was through considering the implications for a beam of light, the universal measuring tool of special relativity. Imagine shining a flashlight horizontally across the elevator from one side to the other. In the freely falling elevator, objects obey Newton’s laws: they move in straight lines, from the point of view of an observer in the elevator, bounce off each other with action and reaction equal and opposite, and so on. And, crucially, from the point of view of the observer in the elevator, light travels in straight lines.
    But how do things look to an observer standing on the ground watching the elevator fall? The light would appear to follow a track that always stays exactly the same distance below the roof of the elevator. But in the time it takes the lightto cross the elevator, the elevator has accelerated downward, and the light in the beam must have done the same. In order for the light to stay the same distance below the roof all the way across, the light pulse must follow a curved path as seen from outside the elevator. In other words, a light beam must be bent by the effect of gravity.
    Einstein explained this in terms of bent spacetime. He suggested that the presence of matter in space distorts the spacetime around it, so that objects moving through the distorted spacetime are deflected, just as if they were being tugged in ordinary “flat” space by a force inversely proportional to the square of the distance. Having