Gravity is something we rarely think about until we slip on ice or fall down the stairs. Gravity wasn’t even a force to many ancient thinkers—it was simply the natural tendency of items to gravitate toward the center of the Earth, whereas planets were subject to other, unconnected laws.
Of course, we now know that gravity is capable of far more than just causing things to fall. It controls the motion of planets around the Sun, keeps galaxies together, and shapes the structure of the universe as a whole. We also understand that, together with electromagnetism, the weak force, and the strong force, gravity is one of the four fundamental forces of existence.
Einstein’s general theory of relativity, the contemporary theory of gravity, is one of the most successful theories we have. At the same time, we don’t know everything there is to know about gravity, including how it interacts with the other fundamental forces. But there are six important truths regarding gravity that we do know.
1. Gravity is the weakest force we are aware of.
Gravity just attracts; there isn’t a counter-force to push objects apart. While gravity is strong enough to hold galaxies together, it is so weak that you have to fight it every day. When you take up a book, you are defying the gravitational pull of the entire planet.
The electromagnetic force between an electron and a proton within an atom is around one quintillion (that’s a one followed by 30 zeroes) times stronger than the gravitational pull between them. Gravity is so weak that we don’t even know how weak it is.
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2. The terms “gravity” and “weight” are not interchangeable.
Astronauts aboard the International Space Station float, and we casually refer to them as being in zero gravity. That, however, is not the case. An astronaut experiences around 90% of the force of gravity that they would experience on Earth. Astronauts, on the other hand, are weightless since weight refers to the force that the ground (or a chair, or a bed, or whatever) imposes on them on Earth.
Take a bathroom scale into a posh hotel elevator and stand on it while traveling up and down, disregarding any suspicious stares you might get. Even when your weight changes and the elevator accelerates and decelerates, the gravitational pull remains constant.
Astronauts, on the other hand, travel with the space station in orbit. There’s nothing to push them against the spaceship’s side to help them gain weight. General relativity was born from this idea and Einstein’s special theory of relativity.
3. Gravity creates waves that travel at the speed of light.
Gravitational waves are predicted by general relativity. When two stars, white dwarfs, or black holes are locked in mutual orbit, gravitational waves transfer energy away, bringing them closer. In truth, when Earth is around the sun, it emits gravitational waves, but the energy loss is insignificant.
Gravitational waves had been suspected for 40 years, but the Laser Interferometer Gravitational-wave Observatory (LIGO) only confirmed the phenomenon in 2016. A burst of gravitational waves created by the collision of two black holes more than a billion light-years apart was detected by the detectors.
Nothing can travel faster than the speed of light in a vacuum, according to relativity. That also applies to gravity: if something catastrophic happened to the sun, the gravitational influence would arrive at the same time as the sunset light.
4. Researchers have been stumped by the tiny behavior of gravity.
Quantum theories, notably the Standard Model, describe the other three fundamental forces of existence at the tiniest scales. Even though academics are working on it, we still don’t have a completely functional quantum theory of gravity.
One line of inquiry is loop quantum gravity, which uses quantum physics techniques to describe the structure of space-time. It suggests that on the tiniest scales, space-time is particle-like, just as matter is made up of particles.
On a flexible, mesh-like structure, the matter would be limited to hopping from one place to another. This enables loop quantum gravity to represent the action of gravity on a scale far smaller than an atom’s nucleus.
String theory is a more well-known concept, in which particles, including gravitons, are thought to represent vibrations of strings coiled up in dimensions too small for tests to reach. At the moment, neither loop quantum gravity nor string theory, nor any other theory, can give testable specifics regarding gravity’s microscopic behavior.
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5. Gravity might be transported by gravitons, which are massless particles.
Particles interact with one another in the Standard Model via other force-carrying particles. The photon, for example, is the electromagnetic force’s carrier.
Gravitons are the hypothetical particles for quantum gravity, and general relativity gives us some ideas about how they should work. Gravitons, like photons, are most likely massless. Experiments should have revealed something if they had mass—but that doesn’t rule out an extremely little mass.
6. Quantum gravity manifests itself at the shortest length possible.
Gravity is weak, but it becomes stronger as two objects move closer together. It eventually achieves the strength of the other forces at a minuscule distance known as the Planck length, which is many times smaller than an atom’s nucleus.
Quantum gravity’s effects will be powerful enough to quantify there, but it will be far too minuscule to be probed by any experiment. Some theories have been developed that would allow quantum gravity to appear at millimeter scales, however, these effects have yet to be observed.
Others have experimented with employing vibrations in a massive metal bar or collections of atoms stored at ultracold temperatures to intensify quantum gravity effects.
Gravity appears to be catching scientists’ attention at all scales, from the smallest to the greatest. Maybe that’ll give you some comfort the next time you fall and gravity grabs your attention as well.