## From COSMOS: “Explainer: What is gravity?”

04 January 2018

Lauren Fuge

Two black holes spiralling together, emitting gravitational waves. Mark Garlick / Science Photo Library

We barely think about it, but gravity defines how we interact with our world. We grow up within its constraints, and our muscles, balance system, heart and blood vessels all depend on it. Gravity literally grounds us — but what exactly is it?

**The basic idea**

Physicists see gravity as one of the four fundamental forces that govern the universe, alongside electromagnetism and the strong and weak nuclear forces. A force is defined as an interaction that changes an object’s motion, and so these four forces underpin all of physics and define how everything in the universe interacts – from the vast cosmic interplay of galaxies to the tight bonds that bind quarks inside a proton or neutron.

Gravity is the weakest of these forces, but it’s the one we’ve been aware of for longest. For centuries, we knew that our feet are kept on the ground and the planets are kept in orbit around the Sun. Even before gravity was described mathematically, seventeenth-century astronomer and mathematician Johannes Kepler had formulated accurate laws to predict the motions of the planets.

Unfortunately, no one had any clue why the planets are orbiting in the first place.

**Newton’s law of universal gravitation**

Enter Isaac Newton, who realised there must be a force acting between the planets and the Sun. (He also defined what a force is.) Whether or not an falling apple really prompted his eureka moment, the equation he came up with to describe the behaviour of this force was revolutionary.

F = Gm1m2 / r2

This equation says that gravity is a force that two objects with mass exert on each other simply because they have mass. The strength of the force (F) is proportional the masses of the two objects (m1 and m2) divided by the square of the distance between them (r). The G is a constant that measures the basic strength of the force.

It boils down to this: the more massive objects are, the greater the force of attraction between them, but the further they are apart, the weaker the attraction.

The famous story goes that Newton first contemplated gravity after seeing an apple fall in his orchard, circa 1665. Hulton Archive / Getty

Consider the legend associated with Newton’s revelation about gravity: an apple falling from a tree. Newton’s law of universal gravitation tells us that not only does the Earth tug on the apple, the apple also tugs on the Earth. But the Earth’s enormous mass means it takes a lot more force to move it an appreciable amount, so the apple comes toppling down while the Earth remain practically motionless. The same is true in a broader context. Every object in the Universe is attracting every other object, but the closer and more massive it is, the greater its gravitational power.

By plugging a few numbers into this equation, we can describe and predict almost all gravitational phenomena on Earth plus the motions of planets, comets and moons. It explains why stars congregate into galaxies, and why galaxies bunch up to form clusters.

But the equation doesn’t perfectly describe everything we see – for example, the size of certain gradual changes in the orbit of Mercury around the Sun. And as Newton himself even wondered, how could a force work instantaneously at a distance even through the vacuum of space?

**Einstein’s theory of general relativity**

In Newton’s universe, space is a flat and empty place that objects like stars and planets move through, but Einstein took a different approach.

In one of the towering scientific achievements of the twentieth century, Einstein and his former professor Hermann Minkowski showed that space and time are not separate entities but rather a single four-dimensional continuum. They imagined it stretching through the universe like a fabric.

Any object with mass, Einstein reasoned, would interact with the fabric of spacetime and cause distortions. As a classic analogy goes, imagine spacetime as something like a trampoline. A bowling ball placed in the middle would bend the fabric and create a well. If you then set down a marble near the bowling ball, it would spiral around the well as being pulled inwards, much like how the Earth orbits around the Sun.

Einstein’s theory of general relativity therefore describes gravity as not just a force, but geometry – it is a consequence of how matter warps spacetime.

Unlike Newton’s theory, general relativity correctly describes the changing orbit of Mercury by taking into account the disturbances of spacetime caused by the Sun’s mass.

Since Einstein’s published his theory in 1915, it has acquired a hefty body of observational evidence to support it. It correctly describes the bending of light around the Sun and distant galaxy clusters – since masses curve spacetime, as light zooms past it follows a path along the contours of spacetime. General relativity is also verified by the measurement of gravitational redshift, correctly predicting how the gravitational pull of stars stretches the frequency of their light.

This Hubble Space Telescope image was created using a gravitational lens created by a cluster of galaxies. NASA/ESA/STScI/Advanced Camera for Surveys [ACS], Getty Images

Most recently and spectacularly, gravitational wave detections provided solid evidence that massive objects not only distort spacetime, but can also create ripples in it when two objects such as black holes violently collide.

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

**Gravity as a fundamental force**

Still, mysteries remain. Gravity may be one of the fundamental forces of the universe, but it currently seems fundamentally incompatible with the others.

Even before Einstein, physicists strove to formulate a single theory linking every physical aspect of the Universe, including all fundamental forces and particles. A ‘theory of everything’ would be the ultimate achievement of physics. While quantum field theory (QFT) succeeds in bringing together electromagnetism and the strong and weak nuclear forces, it struggles with including general relativity. Einstein spent the second half of his career trying to puzzle out how gravity fits in, but didn’t get very far. Linking these two theoretical frameworks is an open area of research, spanning the field of quantum gravity, string theory, M theory, and more.

These attempts seem a world away from Newton and his apple, but it’s a microcosm for science as a whole: sometimes the Universe is a whole lot more complicated – and a whole lot more interesting – than first thought.

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