We may not know exactly what gravity is (read the "What is Gravity?" article) at the most fundamental level, but that doesn't mean we can't put it to work for us. Here are 10 uses for gravity that may surprise you.
1. Track Earth's water and ice.
Of course we have to begin with this one. Gravity is not the same all over the planet. Locations with more mass (a big mountain, for example) have stronger gravitational pull. So if we monitor gravitational changes over time, we can see where mass is on the move. On Earth, that moving mass is largely water, both in its liquid and icy forms. GRACE and its successor, GRACE-FO, provide a fresh map of Earth's gravity field every 30 days. Comparing these maps enables us to track the rise in sea level and the changes in polar ice and major water reservoirs across the planet.
2. Provide energy.
From old-fashioned watermills to modern hydroelectric plants, putting something in the way of falling water is a time-honored way to turn gravitational energy into mechanical or electrical energy. Also, when we harness the wind, we indirectly harness gravitational as well as solar energy since both the Sun's heat and the Earth's gravity contribute to the atmosphere's motion.
3. Give a boost to spacecraft.
Virtually all space missions beyond Mars or Venus depend on "gravity assists" to add speed and change direction. For example, the New Horizons spacecraft, which flew by Pluto and its moons in 2015 and is now on its way to a Kuiper Belt object a billion miles beyond, flew by Jupiter at just the right distance and angle so that Jupiter's gravity pulled the spacecraft toward the planet but didn't capture it. The extra momentum the spacecraft gained in this maneuver shaved about three years off its travel time to Pluto. Voyager 1 famously used gravity assists to visit all four giant planets. Cassini repeatedly uses gravity assists at Titan to enable it to tour the Saturn system without using much chemical propulsion.
4. Weigh the unweighable.
How can you weigh the Sun? You start by realizing that at any given distance, the stronger the gravity of the object you're orbiting — and therefore the more mass it has — the faster you have to travel to remain in orbit. So by observing how fast a planet is moving in its orbit and knowing how far it is from the Sun, you can calculate the Sun's mass. Similarly, astronomers deduced that a black hole 4 million times as massive as the Sun sits at the center of our galaxy by observing the motions of stars orbiting it.
You can weigh entire galaxies and clusters of galaxies by yet another gravity-based method called "gravitational lensing." As Einstein predicted, massive objects bend the space around them, and rays of light follow those curves. The space around galaxies and galactic clusters is curved so much that it can focus light like a telescope's lens, warping and magnifying the images of much more distant galaxies which happen to be along the same line of sight. When astronomers find one of these gravitational lenses, they can calculate the mass of the galaxy or cluster causing the lensing phenomenon by seeing how much it bends the light coming from a galaxy far behind it.
5. Use gravity as a telescope.
Gravitational lensing can also help our telescopes to see objects whose light is dimmed by extraordinary distances. Astronomers were able to observe a galaxy 13.2 billion light years away — from the time when the universe was only 500 million years old — thanks to an intervening galactic cluster which magnified the light 15 times. You can read more about this discovery.
6. Hunt for planets around other stars.
In a version of gravitational lensing called "microlensing," the light of a distant star is focused and magnified by the gravity of a closer star that passes between Earth and the more distant star. If the closer star has an orbiting planet which happens to be on one side or the other of the star as seen from Earth, the image we see of the more distant star will brighten as the intervening star moves into the region where it can act as a lens, and brighten even more when both the star and its planet are in that region, thereby revealing the planet's existence. You can read about the first time this technique was used.
7. Investigate unseen planets.
There are several ways besides microlensing to infer the presence of unseen planets around other stars. But however the planets are detected, Newton's laws of gravitation can tell us something about them. If we know the mass of the star and how long it takes the planet to go once around the star, we can calculate the planet's distance from the star. That tells us whether the planet receives the right amount of heat from its star to allow liquid water on the planet's surface, which would make it a candidate for life.
In our own solar system more than a century ago, French astronomer Urbain-Jean-Joseph Le Verrier observed discrepancies between Uranus' actual orbit and that which Newton's laws predicted, and reasoned that Uranus was being affected by the gravity of some planet yet to be discovered. He calculated the position of that planet — which we now know as Neptune — so accurately that German astronomer Johann Galle was able to find it after only one hour of searching.
8. Probe the interiors of planets and moons.
How can you learn what lies beneath the surface of a planet or moon without even landing on it, let alone drilling into it? Track a spacecraft's response to its gravity as the vehicle orbits or flies by. The GRAIL mission produced a lunar gravity map which enables scientists to deduce information about the Moon's structure and composition all the way down to its core. Tracking the Cassini spacecraft provided information about the gravity of Saturn's largest moon, Titan, which scientists were able to interpret as evidence that it has an ocean of liquid water beneath its frozen crust.
Gravity even helps us find minerals and oil here on Earth. Geologists use portable gravimeters to detect the gravitational signposts of underground deposits.
9. Save the Earth.
One notion for deflecting an asteroid headed for collision with our planet is to use the gravity of a spacecraft as a "tractor beam." If deployed at a great enough distance from Earth, even the tiny gravity of a spacecraft flying close to an asteroid might alter the asteroid's trajectory enough to make it miss hitting us.
10. Probe black holes and the Big Bang.
According to Einstein's General Theory of Relativity, certain kinds of motion can create undulations in the fabric of space — known as "gravitational waves" — that spread outward like the ripples from a paddle stirring up the water in a pond. A Nobel prize was awarded to the discoverers of indirect evidence for the existence of gravitational waves, and experimenters at two LIGO (Laser Interferometer Gravitational wave Observatory) facilities in the U.S. and at the Virgo interferometer in Italy anticipate that by 2017, they will achieve the sensitivity needed to detect gravitational waves directly. That will open an entirely new branch of astronomy.
While we have amassed an astounding amount of knowledge by studying the light we receive from the cosmos, light has its limits. Gravitational waves carry information unavailable to electromagnetic waves, such as the inner workings of black holes and even the first moments of the Big Bang.
Gravitational wave astronomy will get a major boost if an ambitious spaceborne experiment called LISA (Laser Interferometer Space Antenna) is deployed. LISA's proposed detection system consists of three spacecraft flying in exquisitely tight formation, separated from each other by 5 million kilometers (3 million miles), which is more than 12 times the distance from Earth to the Moon. NASA and the European Space Agency (ESA) have both worked on developing technology for LISA, but ESA is currently pursuing the project alone.