## Geodesics

### The Straightest Lines in Curved Space and Time

Traveler, there is no path.

One of the most important duties of Physics is to predict how things move. Newton wrote his First Law of Motion (First Law of Motion: The first of Newton's Laws of Motion, which says that moving objects move in a straight line. Specifically, the Law says, "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed (Speed: For a wave, the speed of a particular point (such as its crest).) and in the same direction unless acted upon by an unbalanced force."), saying that things move in straight lines, unless some force is pushing or pulling on them. In curved space, it's hard to tell just what a straight line is. If the curvature is fairly smooth, however, there is a trick.

Many people through history have thought that the Earth must be flat. Some still do! Of course, most of us accept that the Earth is one huge sphere (approximately). The flat Earth idea worked reasonably well, though, because it is nearly flat on a human scale. And there's the trick. Looking closely enough, a curved space will look flat, so we still know what a straight line is. We can move just a little bit along a straight line, then stop. Look again, and we have another little straight line to follow in the same direction. Taking many of these tiny steps builds up into one long line. This type of line is called a geodesic (Geodesic: Essentially the "straightest path" in a curved space or curved spacetime (Spacetime: A concept in physics which merges our usual notion of space with our usual notion of time.). This is the path followed by an object with no forces acting on it. In the curved spacetime of General Relativity (General Theory of Relativity: Einstein's version of the laws of physics, when there is gravity. Building on the Special Theory of Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity becomes very nearly the same as Newton's theory, which is much easier to use. ) becomes very nearly the same as Newton's theory, which is much easier to use. ), this theory generalizes Einstein's work so that the laws of physics must be the same for all observers (Observer: A person or piece of equipment that measures something in physics. Frequently, we speak of an observer measuring time or a distance in a particular place. ), even in gravity. Einstein showed that gravity is best understood as a warping of the geometry of spacetime, rather than as a pulling of objects on each other. The crucial idea is that objects move along geodesics — which are determined by the warping of spacetime — while spacetime is warped by massive objects according to the formula \(G = 8 π T\). ), these paths may seem to be very curved — even appearing as circles or ellipses, for example. A geodesic is easily understood by looking at a very small region around the object. Even in highly curved spacetime, a small enough region will seem flat, so there is a natural idea of a "straight path". By following short segments, the whole geodesic is built up into one long path. )}), and it's the closest thing to a straight line we can find in warped space.

For example, imagine going between two points on the Earth's surface. Suppose you want to go from New York City to Rome, Italy, and you can't tunnel. The quickest way to go is to fly around the surface of the Earth. These two cities are at nearly the same latitude. However, when the plane takes off from New York, it won't be headed due East. If the pilot chose this route, the plane would end up in Africa, or would have to be turning left for the whole trip. (See for yourself with a little toy car and a globe.) Instead, the pilot heads a little North of East — by 33 degrees, if we ignore winds. This way, the plane can go in a straight line, and end up in Rome. That is, the plane can follow a geodesic to make the flying easier.

Now we can visualize the trip. The whole time, the plane can just keep flying straight and level. The pilot doesn't need to be turning either left or right, but will end up in Rome nonetheless. It turns out that this is also the fastest route; a "straight line" is still the shortest distance between two points, even if it's a straight line that curves. This is, basically, the path that flights from New York to Rome actually follow.

We've just described geodesics in space; the plane's path follows a geodesic along the two-dimensional surface of the Earth. Geodesics can also be paths through time. In our charts of the movements of the astronauts above, each of the lines was a geodesic — straight lines from one point in spacetime to another. But we were imagining our astronauts to be in a nice, flat spacetime. Interesting things happen when we take our geodesics through warped spacetime.

Just as the geodesic "straight line" from New York to Rome looks curved to us, the geodesic of an astronaut moving through warped spacetime would look curved, too. We can see what this looks like. For a particular type of spacetime curvature, we get the chart shown to the right.

This looks like an acceleration (Acceleration: Any change in the speed with which an object moves, or the direction in which it moves.). In his General Theory of Relativity (General Theory of Relativity: Einstein's version of the laws of physics, when there is gravity. Building on the Special Theory of Relativity, this theory generalizes Einstein's work so that the laws of physics must be the same for all observers, even in gravity. Einstein showed that gravity is best understood as a warping of the geometry of spacetime, rather than as a pulling of objects on each other. The crucial idea is that objects move along geodesics — which are determined by the warping of spacetime — while spacetime is warped by massive objects according to the formula \(G = 8 π T\). ), Einstein kept Newton's First Law of Motion (Newton's First Law of Motion: The first of Newton's Laws of Motion, which says that moving objects move in a straight line. Specifically, the Law says, "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.") — objects still move along straight lines (geodesics) unless something is pushing or pulling on them. In Newton's world, this meant that the astronaut above would have to be pulled by some force, like gravity. In Einstein's world, it may just happen that spacetime is curved, so the astronaut only appears to accelerate, while she is simply following a geodesic. Einstein accomplished that most important duty of Physics — predicting how objects move — by understanding how and why spacetime warps.