You can read a lot more about how we think about general relativity in my Centennial of General Relativity series of blog posts ( this is the first post) or in my series of short 3-minute Centennial of General Relativity vides ( this YouTube playlist). So why does the ping pong ball curve toward the rock? Because the shape of the table (the shape of spacetime) is curved, and it moves in response to the shape of spacetime. light from point 1 in space to point 2, in a straight. There is no (measurable) force between the rock and your ping pong ball (a fact you can prove to yourself by putting them both on your dining room table and seeing if they roll together). This phenomenon is a natural generalization of rotations in Euclidean geometry, to which we now turn. If you roll a pingpong ball across the table, its path curves toward the rock in the center of the table! If your large rock is in the center of the table, the fabric of the table (the fabric of the Universe is spacetime) stretches and curves in response. The two-line mantra of the modern gravitational astrophysicist is this: Matter tells spacetime how to curve, spacetime tells matter how to move. The spacetime table is a two dimensional model showing how the shape of space (the table surface) changes when masses are put in the space, and how the shape of space affects the motion of other masses. Particles travel on straight lines when spacetime is not warped, when it is " flat." If the spacetime has a shape, if it is warped, then a particle moves along the shape of spacetime, which is " curved." So what warps spacetime? Mass tells spacetime how to warp. The way you move through space is a consequence of the shape of spacetime. Then draw a straight line (red) across the plane, in a random direction. General relativity is the modern understanding of gravity, where astronomers think of gravity not as a force that pulls you, but rather as a curvature of space and time. Note that if you roll two ping pong balls at the same time, the inner one goes around faster than the outer one, just as Kepler's Laws of Planetary Motion predict. Depending on how you roll the ping pong ball, you can get circular or elliptical orbits (like planets, moons and asteroids have), or highly elongated parabolic or hyperbolic orbits that do not close (like many comets have). (3) CURVED SPACE ORBITS: With practice, you can make your ping pong balls "orbit" the mass in the center of the table. Now if you roll a ping pong ball across the table, it does NOT travel in a straight line! Instead, its path is curved, bending toward the mass in the center of the table! Place your heavy mass or rock in the middle of the table. This demonstrates motion in the presence of masses ("under the influence of gravitational forces"). ![]() If you roll a ping pong ball straight across the table it will go across without changing direction (until it falls off the edge of the Universe on the far side). A particle that is acted on by no forces travels in a straight line. This demonstrates motion in the absence of masses ("in the absence of gravitational forces"). There are two modes of demonstration with the table.
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