A brief introduction to gravity, and some of its consequences, including why massive bodies attract, and why you should watch your step around black holes.
Newton’s law of universal gravitation
We are so used to the action of gravity in our everyday life that it hardly seems remarkable. If you jump in the air, it’s no surprise when you land back on Earth.
Although people had noted the existence of gravity before Isaac Newton, he was the first to realize gravity universally applies to all objects, and all objects attract each other with the force of gravity.
Newton’s law of universal gravitation states that an object has gravity in proportion to its mass. The mass of an object is a measure of how much matter is packed into it – it’s not just a question of size, but also of density.
The law also states that gravity is inversely proportional to the square of distance between objects. Essentially, this means that if you double the distance between yourself and an object, gravity will be weaker by a factor of 2 squared – 4 times weaker.
Strong gravity, Einstein, and the curvature of space-time
Newton’s law is great for predicting the force of gravity in most places. However, Einstein’s theory of general relativity and gravity is more useful when making precise measurements or dealing with very strong gravitational fields.
Einstein developed his general theory of relativity in the early 1900s, and the theory of gravity was part of that. He conceived of the fabric of the universe in 4 dimensions – the fabric of space-time.
He proposed massive objects could bend space-time.
Think of placing a heavy weight on a taut piece of fabric. The fabric would bend, changing the paths of objects moving over its surface. The objects move in a straight line, but when the fabric of space-time is bent, this straight-line changes. This is why we see objects with mass exert a gravitational pull.
Scientists have also speculated on the existence of the graviton – elementary particles responsible for gravity – but no evidence has yet been found for them.
Black holes
Black holes are points in space with such intense gravity that nothing can escape – not even light. At the center of a black hole is a singularity, a point at which gravity is effectively infinite. The edge of the black hole is called the event horizon.
We think four types of black holes exist:
Stellar black holes are formed when a massive star collapses in on itself.
Supermassive black holes are millions to billions of times as massive as our sun, and can be found at the heart of many major galaxies.
Intermediate black holes lie somewhere between the two in terms of mass, and we’re just starting to find evidence of them. They may be stellar black holes that sucked in matter and grew – or a relic from the early days of the universe.
Theoretical miniature black holes may have developed and disappeared shortly after the universe was formed too.
If you were unlucky enough to cross the event horizon of a stellar black hole, the gravity at your feet would be millions of times stronger than that at your head. You would stretch into a kilometers-long, thin noodle – a process called spaghettification. Some experiences are better in theory than in practice – spaghettification is probably one.
Gravitational lensing
The curving of space-time can produce some very strange effects. One that can be particularly useful to astronomers is gravitational lensing.
Gravitational lensing occurs when a hugely massive body – on the scale of a galaxy cluster, for example – bends space-time enough to change the path of light. Although photons, particles of light, have no mass, they are influenced by gravity and bend their light to follow the curvature of space-time.
This bending of light can act as a magnifying glass for more distant objects, allowing us to see farther and fainter objects than we otherwise could see. The Hubble Space Telescope, for example, took advantage of this effect to increase how far it could look, allowing us to see the most distant galaxies ever observed.
Gravitational waves
Gravitational waves are ripples in space-time caused by extremely energetic and violent cosmic events. When massive objects move, they cause disturbances in space-time, which ripple outward just like the ripples caused by a stone thrown into a pond. Some of the events that generate gravitational waves include supernovae and black holes orbiting or merging with each other.
They move invisibly through space at the speed of light, squeezing and stretching matter as they pass.
Einstein’s theory of general relativity predicted gravitational waves, but Einstein himself thought that we would never detect them – figuring that they would be too small to find once they reached Earth. We have only detected gravitational waves directly very recently – more than 100 years after Einstein first predicted their existence.
The Laser Interferometer Gravitational-Wave Observatory – LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) was designed to detect gravitational waves directly for the first time – breaking new ground in astronomy and opening up a completely new way of studying the universe.
It uses 2 detectors, placed some distance from each other in the USA: one stationed in Louisiana and the other in Washington. Each detector uses mirrors to send incredibly precise lasers between 2 arms – each 2.5 miles (4 kilometers) long. From this, we can measure tiny discrepancies caused by gravitational waves stretching and squeezing space.
Both detectors operate at once, and comparing their observations allows us to both confirm the observation and pinpoint the source of the gravitational wave in the sky.
The original LIGO mission, known as Initial LIGO, ran between 2002 and 2010. No gravitational waves were detected during this time. One complete redesign later, LIGO was switched on in September 2015. Within a few days, LIGO detected its first gravitational waves, changing the field of astronomy forever.
The results from LIGO and what this means
LIGO first detected signals from gravitational waves within days of being switched on. Since then, it has detected 90 waves and counting.
LIGO makes its observations in stages, getting switched off in between for maintenance and upgrades. It has made 3 runs of observations so far: from September 2015 to January 2016, from November 2016 to August 2017, and April 2019 to March 2020. It has detected the violent collisions of black holes and of neutron stars.
Predicted more than 100 years before they were detected, the existence of gravitational waves strengthens the case for general relativity – a case that was already pretty strong.
Detecting gravitational waves changes how we explore the universe. It will give us new insights into massive objects in space, and potentially help us learn more about how the universe was formed. Our understanding of the universe today comes in large part from many years of work using the electromagnetic spectrum. LIGO and the discovery of gravitational waves pushes us beyond that, giving us a totally new way to watch space.
