Imagine that gravity stopped. This is a thought experiment, and it cannot really happen, because no technology we have can switch gravity off or block it. Still, the question is a good way to learn what gravity does, since whatever gravity does is something that would not happen without it. Gravity is the invisible pull that makes things fall down and keeps you on the ground. Everything that has mass has gravity, and things with more mass pull harder. Each time you throw something up, gravity pulls it back down.
Start with the air. Gravity holds down the air we breathe, so Earth keeps its blanket of air; without gravity's pull, nothing would hold that blanket in place. The ocean depends on gravity too. The Moon's gravity pulls on the seas, and that makes the ocean tides go up and down. Tides are very long waves that move through the ocean in response to the forces from the Moon and Sun, and they rise and fall regularly along the shore. High tide is when the sea is at its highest on a shore and low tide is when it is at its lowest. The gap between them is the tidal range.
Next, the planet itself. Gravity keeps the Moon going around Earth, and Earth going around the Sun. Everything in orbit is falling around its planet: it moves sideways so fast that it keeps missing the ground. The Moon takes about 27 Earth days to circle Earth, while Earth takes about 365.25 days to go once around the Sun. The International Space Station is in low Earth orbit and circles the planet once in about 90 minutes. Without gravity's pull, none of these orbits would happen, and the Moon, the Earth and the station would have nothing keeping them in their paths.
Gravity also keeps Earth at the right distance from the Sun, and the Sun itself is held together by it. The Sun's gravity keeps Earth at a comfortable distance for its light and warmth. The Sun is a giant ball of hydrogen gas, and gravity holds all of that gas together. It would take almost 1,000 Jupiters to fill up the Sun. Gravity pulls together the material that makes stars and planets; without that pull, nothing would gather the gas. A star is also a balance. The Sun's core is about 15 million degrees Celsius, hot enough for fusion, and the outward pressure this creates keeps the Sun's huge mass from collapsing.
How strong is gravity? Gravity gets weaker the farther apart two things are. Gravity is still strong where the space station flies: at about 250 miles up it is 88.8 percent as strong as on the ground. You may have heard that gravity is the strongest force because it holds planets and stars together. In fact, gravity is the weakest of the four forces of nature: a child's magnet can lift a steel paper clip against the pull of the whole Earth. It is slightly stronger over places with more mass underground, and NASA's GRACE spacecraft map those differences.
Gravity can be described with a law. Newton's helper wrote that the idea came to him in a garden as he watched an apple fall: gravity might reach as far as the Moon. Newton's law says the pull between two objects grows with the product of their masses and shrinks with the square of the distance between their centers. At the far end of the scale is the black hole: a black hole packs so much mass into so little space that its gravity does not even let light escape.
Last, the floating that people picture when they imagine gravity gone. Anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it. Astronauts on the space station look like they float because everything inside is falling together. On a roller coaster, free fall gives you a brief feeling of weightlessness at the top of a drop. You may have heard that astronauts float because there is no gravity in space. In fact, gravity does not stop in space; astronauts float because they are in free fall, not because there is no gravity.
Words to know
- orbit: curved path around
- fusion: atoms joining
- tidal: of the tides
- mass: amount of matter
