Zero - Charles Seife [68]
Einstein’s equations treat time and space as different aspects of the same thing. We are already used to the idea that if you accelerate, you change the way you move through space; you can speed up or slow down. What Einstein’s equations showed was that just as acceleration changes the way you move through space, it changes the way you move through time. It can speed up the way time flows or slow it down. Thus, when you accelerate an object—when you subject it to any force, be it gravity or be it the push of a gigantic cosmic elephant—you change its motion through space and through time: through space-time.
It’s a difficult concept to grasp, but the easiest way to approach space-time is through an analogy: space and time are like a gigantic rubber sheet. Planets, stars, and everything else sit on that sheet, distorting it slightly. That distortion—the curvature caused by objects sitting on the sheet—is gravity. The more massive the object that is sitting on the sheet, the more the sheet gets distorted, and the larger the dimple around that object. The pull of gravity is just like the tendency of objects to roll into the dimple.
The curvature of the rubber sheet is not only a curvature of space, but a curvature of time as well. Just as space gets distorted close to a massive object, time does, too. It gets slower and slower as the curvature gets greater and greater. The same thing happens with mass. As you get into greatly curved regions of space, bodies’ masses effectively increase, a phenomenon known as mass inflation.
This analogy explains the orbits of the planets; Earth is simply rolling around in the dimple that the sun makes in the rubber sheet. Light doesn’t go in a straight line, but in a curved path around stars—an effect that the British astronomer Sir Arthur Eddington went on an expedition in 1919 to observe. Eddington measured the position of a star during a solar eclipse and spotted the curvature that Einstein had predicted (Figure 51).
Einstein’s equations also predicted something much more sinister: the black hole, a star so dense that nothing can escape its grasp, not even light.
Figure 51: Gravity bends light around the sun.
A black hole begins, like all stars, as a big ball of hot gas—mostly hydrogen. If left to its own devices, a sufficiently large ball of gas would collapse under the weight of its own gravity; it would crush itself into a tiny lump. Luckily for us, stars don’t collapse because there is another force at work: nuclear fusion. As a cloud of gas collapses, it gets hotter and denser, and hydrogen atoms slam into one another with increasing force. Eventually, the star gets so hot and dense that the hydrogen atoms stick to one another and fuse, creating helium and releasing large quantities of energy. This energy shoots out from the center of the star, causing it to expand a little bit. During most of its life, a star is in an uneasy equilibrium: the propensity to collapse under its own gravity is balanced by the energy that comes from the fusing hydrogen in its center.
This equilibrium cannot last forever; the star has only a limited amount of hydrogen fuel to burn. After a while, the fusion reaction dims, and the equilibrium is upset. (How long this process takes depends on how big the star is. Ironically, the bigger the star—the more hydrogen it has—the shorter its life, because it burns much more violently. The sun has about five billion years of fuel left, but don’t let that make you complacent. The sun’s temperature will increase gradually before that, boiling off the oceans and turning Earth into an uninhabitable desert like Venus. We should count ourselves lucky if we have a mere billion years left of life on Earth.) After a drawn-out series of death throes—the precise sequence of events depends, again, on the mass of the star—the star’s fusion engine fails, and the star begins to collapse under its own gravity.
A quantum-mechanical law called the Pauli exclusion