Gravity

Throughout this pathway, we've talked a lot about the force of gravity. That's because it's right at the heart of astronomy. It causes stars to form, and planets to orbit, and galaxies to keep their shape.

Now, it's time to look at gravity in more detail.

In simple terms, gravity is a force that pulls two objects towards each other. In slightly more complicated terms, it's a mutual attraction between any two things that have mass.

We're 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 gravity pulls you (one object with mass) back to Earth (another object with mass).

Although people had noted the existence of gravity before Isaac Newton, he was the first person to establish a formal law to describe the force.

Newton’s law of universal gravitation states that an object has gravity in proportion to its mass. In other words, if an object is heavier, its gravitational pull will be stronger. For example, the sun has more gravity than the Earth, but the Earth has more gravity than the moon.

Newton's 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 two objects, like a star and a planet, the gravity will be weaker by a factor of 2 squared (that's 4 times weaker).

If you triple the distance, the gravity will be weaker by a factor of 3 squared (that's 9 times weaker). The further the distance, the steeper the drop off in gravity.

Newton’s law is a great general rule for the force of gravity in most places. But it breaks down in a few important ways: for example, why can light be affected by gravity, when light doesn't have any mass?

That's where Einstein’s theory of general relativity and gravity comes in.

Einstein conceived of the universe as a four-dimensional fabric called space-time. He proposed that objects with a high enough mass could bend the space-time. Where the space-time bent, other objects were pulled in – this was his theory of gravity.

It's like placing a heavy ball on a taut piece of cloth. The cloth bends, creating a sink-hole in the fabric, and any smaller balls resting on the fabric would slide in.

As for light, Einstein argued that it's also affected by these gravitational sink-holes, bending slightly as it crosses the fabric of space-time.

The curving of space-time can produce some very strange effects. One that can actually be useful to astronomers is something called 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.

As we've already talked about, though photons (particles of light) have no mass of their own, they are influenced by gravity and bend their path 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 would otherwise be able to see. The Hubble Space Telescope, for example, took advantage of this effect, allowing us to see the most distant galaxies we've ever been able to observe.

Gravitational waves are another strange effect of the gravitational curving of space-time.

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, or two black holes that orbit or merge with each other.

Gravitational waves move invisibly through space at the speed of light, squeezing and stretching matter as they pass. Einstein predicted them, but didn't expect us to detect them – he thought they would be too small to find by the time they reached Earth.

But Einstein was wrong. In 2015, scientists successfully detected gravitational waves.

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The first ever detection of gravitational waves was achieved by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

It used two detectors, placed some distance from each other in the USA: one stationed in Louisiana and the other in Washington State. Each detector used mirrors to send incredibly precise lasers between two arms – each 4 kilometers long.

From this, scientists were able to measure tiny discrepancies caused by gravitational waves, as they stretched and squeezed the fabric of space-time between the two distant detectors.

The original LIGO mission, known as Initial LIGO, ran between 2002 and 2010. No gravitational waves were detected during this time. But one redesign later, LIGO was switched on again in September 2015. Within a few days, LIGO detected its first gravitational waves, changing the field of astronomy forever.

Black holes are another unusual aspect of the gravitational curving of space-time. These are essentially points with so much mass that space-time bends into a deep, inescapable well.

These wells have a rim – the event horizon. And if anything crosses the edge of that rim, it will sink into the well, and never be able to escape. Not even light can get out again.

If a human being was unlucky enough to cross, feet-first, over an event horizon, the gravity at their feet would be millions of times stronger than the gravity at their head. They would instantly stretch into a long, thin noodle – a process called spaghettification.

Some experiences are better in theory than in practice. Spaghettification is one of them.