What Is Gravity?

Gravity is the invisible force that pulls every object with mass towards every other object with mass. It is the reason apples fall from trees, the reason the Moon orbits the Earth, and the reason the Earth orbits the Sun. Gravity is the weakest of the four basic forces of nature, but because it has unlimited reach and acts on absolutely everything that has mass, it shapes the entire universe. Without gravity, the universe would be nothing but thin gas spreading endlessly outwards.

• What it doesPulls masses togetherAlways attracts, never repels
• Discovered byIsaac Newton1687, with the apple story
• Updated byAlbert Einstein1915, general relativity
• Strength on Earth9.8 m/s2Acceleration of falling objects
• Strength compared to other forcesVery weakElectromagnetic force is 10^36 stronger
• RangeInfiniteActs across the whole universe

Newtons big idea

The English scientist Isaac Newton first explained gravity in 1687. According to legend, he watched an apple fall in his garden and realised that the same force pulling the apple down was also keeping the Moon in orbit. He worked out the famous law of universal gravitation:

• Every object with mass attracts every other object with mass.
• The bigger the mass, the stronger the pull.
• The further apart they are, the weaker the pull.

The exact formula is F = G x (m1 x m2) / r^2, where F is the force, G is the gravitational constant, m1 and m2 are the two masses, and r is the distance between them. The 1/r^2 part means doubling the distance makes the force four times weaker.

Why heavy things do not fall faster

It seems obvious that a heavy hammer would fall faster than a light feather. In reality, in the absence of air, all objects fall at exactly the same rate. Galileo is said to have proved this around 1600 by dropping objects of different mass from the Leaning Tower of Pisa.

The reason: yes, gravity pulls harder on a heavy object, but a heavy object also has more inertia (it resists being moved). The two effects cancel exactly. All objects fall at the same acceleration, regardless of their mass.

The reason a feather usually falls slower is air resistance. On the Moon, where there is no air, astronaut David Scott famously dropped a hammer and a feather together during Apollo 15. They hit the ground at exactly the same time.

Fact Earths gravity makes falling objects speed up by 9.8 metres per second every second. After 1 second of free fall, you are moving at about 35 km/h. After 5 seconds, you would be falling at 175 km/h. Air resistance eventually limits your speed (this is called terminal velocity), but in a vacuum you would keep speeding up forever.

Einsteins update

In 1915, Albert Einstein replaced Newtons picture with a deeper one called general relativity. In Einsteins theory, gravity is not really a force at all. Instead, every object with mass bends the shape of space and time around it. Other objects move along the curves like marbles rolling on a stretched rubber sheet.

Einsteins picture explains things Newtons cannot:

• How light bends as it passes massive objects (proven by photographing star positions during a solar eclipse in 1919).
• How time runs slightly slower in strong gravity (true for clocks on Earths surface compared to clocks on GPS satellites).
• How extreme objects like black holes can warp space so much that even light cannot escape.

Gravity around the universe

• On Mercury, gravity is about 38% of Earths.
• On Mars, about 38% (similar to Mercury).
• On the Moon, about 17% (astronauts bounce).
• On Jupiter, about 240% (you would feel super heavy).
• On the Sun, gravity is so strong you would weigh almost 30 times more than on Earth.
• On a neutron star, gravity is so extreme a teaspoon of its matter would weigh billions of tonnes.
• In a black hole, gravity is so strong that nothing, not even light, can escape.

Did you know? Your weight depends on the gravity wherever you are, but your mass stays the same. A child weighing 40 kg on Earth would weigh only about 6.7 kg on the Moon, but still has the same 40 kg of matter in their body. That is why astronauts in low gravity look heavier and stronger than they actually are: their muscles are not having to fight as hard against gravity.

What gravity does for life on Earth

• Keeps the air, water and us on the surface.
• Holds the oceans down and creates the tides.
• Pulls rain down from clouds.
• Lets rivers flow downhill.
• Holds the Moon in orbit, stabilising Earths spin.
• Keeps Earth orbiting the Sun, so we get sunlight and seasons.

Without gravity, the Earth and Moon would drift apart, the atmosphere would float into space, and life would be impossible.

Try this Drop a coin and a small piece of paper at the same time. The coin lands first because air resistance slows the paper. Now scrunch the paper into a tight ball and try again: the two land at almost the same time, because air resistance now affects the paper much less. You have just (almost) shown what Galileo proved: gravity pulls everything at the same rate.

Deeper dive: searching for gravitational waves

Einsteins theory of general relativity predicted that any sudden movement of mass should send ripples through space-time, called gravitational waves. The ripples spread out at the speed of light, gently stretching and squeezing space as they pass.

The ripples are tiny. The kind of event that could send a detectable wave (two black holes colliding) would still only stretch a 4 km long ruler on Earth by a fraction of the width of a proton. For decades, detecting them seemed impossible.

In September 2015, the LIGO observatory in the USA finally succeeded. Its giant L-shaped laser detectors picked up the tiny ripples from two black holes that collided 1.3 billion light years away. The signal was a faint chirp lasting a fraction of a second, but it perfectly matched Einsteins predictions. The discovery won the 2017 Nobel Prize in Physics.

Since then, LIGO and similar detectors in Italy and Japan have detected dozens of gravitational waves, from black hole mergers, neutron star collisions and other extreme events. Gravitational waves give us a whole new way to "see" the universe, complementing the light-based astronomy we have had for centuries. They confirm that gravity really is the bending of space-time, just as Einstein imagined.

For more, see why dont we float away and gravity in space.