WHAT IS GRAVITY?
Gravity in orbit above Earth is still 90 per cent what it is at the surface the Sun rather than falling towards it and burning up: they are moving fast enough in their orbits.
That’s also how gravitational lenses work. While attempting to observe the most distant galaxies in the universe, astronomers make use of the gravitational force of clusters of galaxies, which are some of the most massive structures in the cosmos. Their huge mass warps space so much that light from more distant galaxies can become magnified, in exactly the same fashion as a microscope lens magnifies small objects. The galaxy clusters, in essence, become a great cosmic lens.
But Einstein wasn’t the first to think about gravity. Many others, from Galileo to Robert Hooke, had a hand in developing our early understanding of the force that keeps our feet on the ground, but it was Isaac Newton who made the biggest conceptual leap, his ‘eureka’ moment being the story of the apple falling on his head as he rested peacefully beneath an apple tree – though historians suspect this is probably not true. Using his new theory of gravitation, which described how every object in the universe is attracting every other object, he set about describing how gravity is the key to explaining how the planets orbit the Sun. Newton explained how the force of gravity was proportional to the masses of the gravitating bodies, which is also what Einstein showed: the more massive an object, the stronger its gravity. He also showed that the force of gravity was inversely proportional to the distance between objects. If you move twice as far away from an object, its gravity feels four times weaker.
But despite all the success of Newton’s universal law, there was a strange problem with it. It was known that the orbits of the planets are elliptical – not circular – and that the axes of these ellipses slowly move around, or ‘precess’, about the Sun over time. Newton’s law could account for 93 per cent of the precession of Mercury’s orbit, but not the other seven per cent. Although this may seem like a very small discrepancy, hardly worth mentioning, in fact, scientists have to account for such things. It wasn’t until Einstein came along with his general relativity some 230 years later that the mystery was solved.
Newton’s laws are great for describing gravity in everyday situations, where the gravitational field isn’t too strong. Relativity is needed, however, for describing much stronger gravitational fields, like the one close to the Sun where Mercury orbits, or even stronger gravitational fields such as those belonging to neutron stars or black holes. Indeed, one of the reasons astronomers like to observe black holes and learn all they can about them is that their extreme gravity is the perfect place in which to test Einstein’s theory of relativity. So far, relativity has passed all the tests it’s been placed into – except for one.
The first third of the 20th century was an amazing time for science. Not only was Einstein developing general relativity, there was another scientific revolution going on, but this time for objects on the smallest possible scales. This was quantum mechanics. The trouble with general relativity is that it doesn’t say anything about how gravity operates on the tiniest scales of atoms and particles. In order to do so, we must unify gravity and quantum mechanics so that they tumble out of the same equation – an equation describing ‘quantum gravity’.
There are four fundamental forces in nature: electromagnetism; the ‘strong’ force, which holds atoms together; the ‘weak’ force, responsible for radioactivity, and gravity. However, scientists suspect that in the first few fractions of a second after the Big Bang, when the universe was still just a tiny but very hot and dense volume of space, all four fundamental forces were unified as one single, symmetric force. As the universe expanded and the temperature cooled, this single force fragmented into the four forces we experience today. Unifying gravity and quantum mechanics wouldn’t just give us a better understanding of what happens inside black holes, it would help us on our way to understanding the Big Bang. After nearly a century attempting to bring gravity and quantum mechanics together, nobody has yet succeeded.
Even so, quantum mechanics is able to say some helpful things about gravity. It explains how the fundamental forces can be described as fields that cross the
Whenever you pick something up, you’re counteracting the planet’s entire gravity.
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It causes objects to gravitate towards each other
Although gravity is the weakest fundamental force, it’s also the longest ranging. Bodies with a significant amount of mass can influence one another across thousands of astronomical units, or AU – one AU being the distance between Earth and the Sun. It’s no surprise that planets close to the Sun should orbit it in a gravitational ‘dance’. What is surprising, though, is that so should a tiny dwarf planet like Sedna, which can get out to 936 AU away.
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Ripples in space-time
One of the most extraordinary discoveries of this century was the existence of gravitational waves, or ‘ripples’ in space-time. These were detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) in February 2016 and are thought to have come from two colliding black holes. Although it was thought that they could be detected from such an extreme event, no one had any idea when, or if, that would ever happen.
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It bends light
One of the strangest things about gravity is that it can bend the path of electromagnetic radiation, including light. The stronger a body’s gravitational field, the more pronounced the effect. Some of the most spectacular examples of this come from Hubble Space Telescope images of galaxy clusters. Light from more distant galaxies is distorted, as if by a lens.