Theories of the Universe: Curved Space

Curved Space

The second aspect of general relativity is based on the exotic notion of space being something that can bend, warp, and ripple. Einstein's theory of general relativity describes the force of gravity as the unseen geometry of space. In other words, gravity is not a force that makes things fall but is rather a property of space. Things fall because that is their normal straight-line path in a curved, four-dimensional space-time continuum. But curved space is really a weird concept to grasp. Is there anything called straight space? A straight line is the shortest distance between two points and usually the shortest distance means line of sight or the path of a light beam. But we assume that light travels in a straight line. If the light-beam curves, is it going straight in curved space? Or is it curving in straight space? On some occasions the shortest distance between two points is the path that takes the longest according to your watch, because time slows as you go faster.

Well, without confusing you any further, let's do another thought experiment to give you an idea of how and why space curves. Some of the most common ways to think of space is to see it as an elastic medium. Think of a waterbed or a sheet of rubber or a pool of Jell-O. If you place a heavy object like a bowling ball on it, the mass of the ball depresses into the material. Now if you roll another smaller ball, like a marble, out onto your bed sheet or Jell-O, the marble will fall into a curved path made by the depression of the bowling ball and circle around it. If the marble has just the right speed and direction, it will go into orbit around the bowling ball.

If you replace the Jell-O or the bed sheet with space, the bowling ball with the Sun, and the marble with the Earth, you can get an idea of how the Earth orbits around the Sun. If you apply this model to the planets and moons in our solar system you can see how its fundamental structure is laid out. The only other thing you have to realize is this warping of space happens in all directions, not just the two-dimensional surface of our Jell-O pool or rubber bed sheet. Think of the bowling ball as being covered by the Jell-O or rubber sheet and depressing it in every direction.

Here's a list of some of the features that are a result of this aspect of general relativity:

• The more massive an object is the more it will warp the surrounding space.
• The more massive an object is the greater the gravitational influence it will exert on other bodies.
• The amount of spatial warping decreases as the distance from the massive object increases.
• All objects warp space, but the degree to which they do depends on their mass.
• Because a warp distorts space, it also must distort time. This occurs because you can't separate space from time.
• The more massive an object is, the more time will slow down the closer you get to it.

Einstein was able to develop this theory of a mass warping space by combining what he learned from special relativity and the principle of equivalence. Out of this came an answer to the question that Newton had asked. Newton believed that “gravity must be caused by an agent,” although he had no idea as to what that agent was. Part of his alchemical quest “Cosmology in the Age of Enlightenment,” was devoted to trying to find out what this agent was. Einstein revealed that the agent of gravity was the very fabric of space itself.