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Gravity: From Apples to Planets

Trampolines need more than springs and stretchy fabric to be fun—they also need gravity. Read on to learn why this force isn’t as simple as bouncing up and falling down.

Except for amnesiac astronauts and cartoon characters who haven’t looked down yet, everyone knows that gravity works. But when it comes to how and why, gravity fits a little uneasily into the physical explanations of our universe. Perhaps the most important explanation comes from Albert Einstein. According to his general theory of relativity, anything that has mass warps space-time, causing a “dimple” that, if the mass is big enough, draws other objects into its orbit. (Consider how a child sitting on a trampoline warps the fabric; now imagine that that fabric is not a surface but a four-dimensional field surrounding the child on all sides.)

Einstein’s theory predicts the behavior of our universe’s bodies with great accuracy. It is even largely compatible with later developments in quantum mechanics, the laws of which account for forces on the subatomic scale: you simply need to posit the existence of the graviton, a theoretical particle that is the “substance” of gravity in the same way that photons are the stuff of electromagnetism. This convenient construction does, however, break down at distances smaller than the ultra-tiny unit known as the Planck length. Some have theorized that this may because quantum effects take over entirely at that scale, others that it’s because space-time itself is actually discrete: if distances smaller than the Planck length do not exist, it is nonsensical to consider how gravity works within them. This minute realm is one of physics’ major frontiers, which can only be settled with the help of gravity.

Things seemed much simpler in Isaac Newton’s day. According to the best-known story in science history, he was conked on the head by a falling apple and, after baking an apple pie in revenge, struck upon his most famous law. But as readers of Newton’s Principia will know, it wasn’t so much the falling apple that inspired the theory as the non-falling moon. After experimentally observing the acceleration of bodies on earth, Newton discovered that the same force controlling their fall could also account for the moon’s continual orbit. If in one sense Einstein’s later theory utterly transformed our concept of the universe, in another it was only a small improvement on Newton’s: the latter is still considered reliable enough to be used in planning the trajectories of spacecraft.