Discover the secrets of the fundamental force that holds entire galaxies together
1. Your weight changes as you accelerate
Earth’s gravitational pull compels you toward the centre of the planet, but the ground is in the way; it pushes up against you, matching the force
exactly. It is this push that makes you feel heavy. As you accelerate to the top of a roller coaster, the chair pushes harder against your body, opposing the force of gravity and making you feel heavier. Once you reach the top and the coaster starts to accelerate downward, your body naturally wants to continue moving upwards. The chair pulls away from your body, and you feel lighter. As the coaster car continues to drop, the support of the chair is removed completely, and you feel momentarily weightless before descending. These indirect changes in weight are known as ‘g-force’.
2. Without gravity your body starts to go wrong
Our bodies evolved on the surface of the Earth and are optimised to function in Earth’s gravity, but in microgravity our systems cannot function normally. If there is no weight on our load-bearing bones, the body starts to remove calcium, and if we stop using the muscles that support our backs and legs, they become weaker.
3. You would feel weightless at the centre of the Earth
Hypothetically, if you dug a tunnel from one side of the Earth to the other and jump in, you would accelerate toward the centre, reaching a speed of around 7,900 metres (25,919 feet) per second. In the centre, you would be momentarily weightless, but inertia would continue to carry you through the tunnel. You would then decelerate as you exited, emerging from the other end 42 minutes later.
4. Fish have stones in their heads that tell them which way is up
Plants and animals have evolved amazing ways to sense Earth’s gravitational pull. In the sea, bony fish have floating calcium carbonate deposits in their heads called ‘earstones’, which are pulled down by gravity. On land, plants have starch grains in their root tips, which sink toward the ground, helping to guide their roots downward.
5. You can still feel 90 per cent of Earth’s gravity on the International Space Station
The International Space Station orbits Earth at an altitude of between 320 and 400 kilometres (200 and 250 miles), but although astronauts and cosmonauts living inside feel weightless, they are still under the influence of microgravity. The gravitational pull of the Earth keeps the space station in orbit, but it is travelling so fast that it never falls back down to the ground; the astronauts feel weightless because they are in constant freefall around Earth.
6. Gravity can bend light
If a large collection of matter lies between the Earth and a light source, the path of the light curves as it travels toward us, creating smears, multiple images, or even a complete Einstein Ring. This is known as gravitational lensing and is down to the effect mass has on the fabric of the universe.
Einstein explained that gravity is the result of massive objects distorting the fabric of space-time, a bit like bowling balls sitting on a sheet of rubber. As light travels past a massive object, it is forced to travel around the curve and its path becomes bent.
7. Quantum mechanics and gravity don’t mix
Einstein’s theory of general relativity tells us how the universe works on a large scale, while quantum mechanics explains how atoms, molecules and fundamental particles interact. The trouble is the two ideas don’t fit together. Reconciling the two theories is one of the biggest challenges facing modern physics.
8. Some bacteria grow better in microgravity
Colonies of bacteria grown on board a NASA Space Shuttle behaved very differently to identical bacteria grown on Earth. They stuck together in shapes never seen on the ground and managed to survive in much higher densities. Understanding these changes could help protect astronauts from dangerous bacterial biofilms during long-term space flight.
9. ￼The Moon’s gravity makes Earth’s oceans bulge
￼￼The gravitational pull of the Moon has a noticeable effect as it tugs at the Earth; it pulls on the water, causing ￼￼￼￼￼￼￼￼￼￼￼￼￼the oceans to bulge. As Earth and the Moon orbit together, water on the opposite side of the planet also bulges outward, thanks to centrifugal force. As the Earth spins on its axis, these bulges move, causing tides.
10. Dark energy works against gravity
Gravitational attraction is not powerful enough to hold the universe together indefinitely. The universe has been expanding since the Big Bang, and the pull of gravity is being opposed by so-called dark energy. Gravity is only strong over short distances, but dark energy is thought to be spread evenly, causing the expansion of the universe to accelerate, and preventing gravity from pulling everything back together.
11. Extreme gravity can tear whole star systems to pieces
When a massive star dies it leaves behind a remnant core with so much mass that it collapses under the force of its own gravity, creating a black hole. Black holes have such a strong gravitational pull that not even light can escape. They can tear nearby stars to pieces and completely shred their component atoms.
12. Newton wasn’t hit by an apple
Sir Isaac Newton’s big idea about gravity was not prompted by a knock on the head. Just seeing an apple fall to the ground was enough to get him thinking.
13. Gravity is not a force
According to Einstein, gravity is not really a force at all. The fabric of space-time is bent by massive objects, distorting the paths of other objects – a phenomenon we see and feel as gravity.
14. Gravity is one directional
Magnets can both attract and repel, but gravity only works in one direction. It compels massive objects to come together, but cannot be reversed to force them apart.
15. The range of gravity is infinite
Gravity might be the weakest of the four fundamental forces, but has unlimited range. Its strength decreases rapidly as objects move farther apart, but its reach is theoretically infinite.
16. Earth’s gravity is lumpy
It is clear by looking at mountains and valleys that Earth is not a perfect sphere, and beneath the surface, the distribution of rocks and minerals is uneven, creating pockets of varying density, and therefore varying gravity.
NASA has been probing Earth’s gravity using two GRACE satellites. As the first satellite approaches a dense region, it is pulled forward, racing ahead a bit, and as it moves past, it is pulled backward. By measuring the distance between the two, detailed gravity maps are produced.
17. Newton and Einstein aren’t the only people to have wondered about gravity
Gravity has been pondered by some of the world’s greatest minds. In Ancient Greek philosophy, Aristotle taught that all objects moved toward their ‘natural place’; some, like stones, would be drawn to the centre of the Earth, others, like steam, would be attracted upward toward the heavens.
18. Gravity travels at the speed of light
According to Newton’s theory of gravitation, the pull of gravity is ￼￼￼￼￼￼￼￼￼￼￼￼instantaneous, but according to Einstein the effects of gravity travel at the speed of light, meaning that if the Sun suddenly disappeared, we would continue to orbit an empty space for more than eight minutes. This idea was confirmed in 2002, when the speed of gravity was measured for the first time.
19. You weigh almost three times more on Earth than you would on Mars
Mass is a fixed property based on all of the atoms that make up your body; it is always the same, whether you are on Earth, inside the International Space Station, or standing on the surface of the Moon. Your weight is the force of gravity acting on your mass, and is defined as mass multiplied by the acceleration due to gravity. On other planets, the gravitational pull is different, so although your mass is exactly the same, your weight would change.
20. Gravity can be used to make a space slingshot
It takes an enormous amount of energy to send spacecraft to the far reaches of the Solar System, so space agencies use a technique called a ‘gravity assist’ to help. Instead of heading straight for the target, spacecraft loop around and fly past a planet like Earth, Mars or Jupiter, using their orbital momentum as a boost.
21. Earth’s gravitational pull is actually weaker than the pull of a fridge magnet
Gravity is the weakest of the four fundamental forces. The entire gravitational pull of the Earth cannot compete against the nuclear strong force, which holds the nuclei of atoms together. It cannot oppose the electromagnetic force that holds a magnet onto your fridge and it cannot stop the nuclear weak force, which is responsible for radioactive decay.
22. Astronauts practice weightlessness in aeroplanes
Aeroplanes can perform a swift up-and-down motion called a parabolic arc to induce a brief period of weightlessness, allowing people to train for space travel.
23. Scientists study gravity in cosmic laboratories
One of the best places to study gravity is in the ready-made laboratory of outer space. Observing closely orbiting pairs or trios of neutron stars and white dwarfs enables scientists to measure the effects of gravity.
24. Spinning creates artificial gravity
On a spinning fairground ride, the outer walls provide an inward centre-seeking force that keeps people travelling in a circle. This is called centripetal force, and the effect feels just like gravity.
25. Massive objects make gravitational waves when they move
According to Einstein’s theory of relativity, massive objects warp the fabric of space-time. When these objects move they create ripples, much like a water boatman skating across a pond.
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