One probably didn’t really fall on the head of Sir Isaac Newton in 1665, knocking loose enlightenment about the nature of falling bodies. And while you’re at it, forget what you learned about gravity in school. That’s not how it really works. But don’t take our word for it. Let the main contenders in the history of gravitational theory duke it out themselves.
Round 1: Newton
“Gravity really does exist,” Newton stated in 1687. “[It] acts according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies.” Before Newton, no one had heard of gravity, let alone the concept of a universal law.
Cambridge University, where Newton studied, was closed due to plague in 1665. Finding respite at his childhood home, the 23-year-old plunged into months of feverish mathematical brainstorming. This, plus a dubious apple descent in the back orchard, laid the foundation for his masterwork Philosophiae Naturalis Principia Mathematica. In Principia, Newton described gravity as an ever-present force, a tug that all objects exert on nearby objects. The more mass an object has, the stronger its tug. Increasing the distance between two objects weakens the attraction.
Principia’s mathematical explanations of these relationships were simple and extremely handy. With his equations, Newton was able to explain for the first time why the Moon stays in orbit around Earth. To this day, we use Newton’s math to predict the trajectory of a softball toss or of astronauts landing on the Moon. In fact, all everyday observations of gravity on Earth and in the heavens can be explained quite precisely with Newton’s theory.
Okay, we buy it. But how does it work?
Silence from Newton’s corner of the ring.
The truth is, Newton could describe gravity, but he didn’t know how it worked. “Gravity must be caused by an agent acting constantly according to certain laws,” he admitted. “But whether this agent be material or immaterial, I have left to the consideration of my readers.”
For 300 years, nobody truly considered what that agent might be. Maybe any possible contenders were intimidated by Newton’s genius. The man invented calculus, for Pete’s sake.
Ding. Round 2: Einstein
Apparently Albert Einstein wasn’t intimidated. He even apologized. “Newton, forgive me,” he wrote in his memoirs. “You found the only way which, in your age, was just about possible for a man of highest thought and creative power.”
In 1915, after eight years of sorting his thoughts, Einstein had dreamed up (literally—he had no experimental precursors) an agent that caused gravity. And it wasn’t simply a force. According to his theory of General Relativity, gravity is much weirder: a natural consequence of a mass’s influence on space.
Einstein agreed with Newton that space had dimension: width, length, and height. Space might be filled with matter, or it might not. But Newton didn’t believe that space was affected by the objects in it. Einstein did. He theorized that a mass can prod space plenty. It can warp it, bend it, push it, or pull it. Gravity was just a natural outcome of a mass’s existence in space.*
You can visualize Einstein’s gravity warp by stepping on a trampoline. Your mass causes a depression in the stretchy fabric of space. Roll a ball past the warp at your feet and it’ll curve toward your mass. The heavier you are, the more you bend space. Look at the edges of the trampoline—the warp lessens farther away from your mass. Thus, the same Newtonian relationships are explained (and predicted mathematically with better precision), yet through a different lens of warped space. Take that, Newton, says Einstein. With regrets.
Einstein’s theory also triumphantly punched a hole in Newton’s logic. If, as Newton claimed, gravity was a constant, instantaneous force, the information about a sudden change of mass would have to be somehow communicated across the entire universe at once. This made little sense to Einstein. By his reasoning, if the Sun disappeared suddenly, the signal for the planets to stop orbiting would logically have to take some travel time. And it would definitely take longer to arrive at Pluto than it would Mars. Nothing universally instant about that at all.
What did Einstein propose as the missing agent of communication? Enter, again, his very useful space warp. Much like a stone thrown into a pond, a change in mass will cause a ripple in space that travels out from its source in all directions at light speed. As it moves along, the ripple squeezes and stretches space. We call such a disturbance a gravitational wave.
With this final blow, Einstein’s General Relativity explained everything Newton’s theory did (and some things it didn’t), and better. “I am fully satisfied,” Einstein said in 1919. “I do not doubt anymore the correctness of the whole system.”
In this round, victory for Einstein.
Ding. Round 3: The Next Wave
Einstein may have predicted gravitational waves, but he had little faith scientists would ever detect them. Gravitational waves squeeze and stretch space only a small amount. In fact, it’s ridiculously, horribly, almost impossibly small: a distance hundreds of millions of times smaller than that of an atom.
So far, Einstein has been right. It’s been eight decades since he introduced General Relativity, and a gravitational wave has not yet been detected. It wasn’t until 1974 that scientists even got close. That year two radio astronomers, Joseph Taylor and Russell Hulse, were analyzing a pair of neutron stars (superdense collapsed stars) that orbit each other. Hulse and Taylor realized that the orbits were speeding up at a rate Einstein predicted would occur if gravitational waves were indeed being generated by the system. The first indirect evidence of gravitational waves was in, but the waves themselves were not directly measured.
Although any object can generate gravitational waves, only extremely massive ones produce warps of space big enough to measure. Such gargantuan changes in mass are found only in space, such as orbiting neutron stars, colliding black holes, or supernovas. Researchers are now searching for waves emanating from these sources with one of the most precise scientific instruments ever made: LIGO, the Laser Interferometer Gravitational-wave Observatory. LIGO is gigantic, clever, and odd-looking, and it took more than $365 million and 30 years to develop. Its ability to measure infinitesimal distances could help put the “discovery” of gravitational waves on the front page of every newspaper at any moment—and herald the next big round in our understanding of gravity.
*Einstein had, with his 1905 Special Theory of Relativity, added time as a fourth dimension to space, calling the result space-time. Large masses can also warp time by speeding it up or slowing it down.
NOVAís The Elegant Universe
Einstein at AMNH
Timeline of gravitational history
View Einsteinís General Relativity paper and an image of space-warping