Without gravity the night sky would look very empty. Stars, galaxies, moons, and planets — none of these could exist without gravity holding them together. Neither, for that matter, could the Sun or Earth. It is gravity that pulled together diffuse atoms and built the universe that we see around us today.
Gravity has also been at the heart of our own efforts to understand the nature of reality, from Newton’s formulation of the universal law of gravity to Einstein’s theory of relativity. Today gravity lies at the centre of new problems in science, and holds the key to uncovering what may be the ultimate theory of physics.
For something that has had such a profound effect on humanity, gravity is surprisingly weak. Of the four fundamental forces known to physics, gravity is by far the weakest. This weakness can easily be demonstrated — the magnetic force of a small fridge magnet can easily lift a pin into the air, thus defeating the gravitational force of an entire planet.
The weakness of the gravitational force means it can be almost completely ignored at the level of atoms and molecules. It is only when we look at very big objects — planets, stars and galaxies, that gravity starts to matter. While the other fundamental forces fade away over short distances, the force of gravity can be felt from one side of the galaxy to the other.
Humans have known about gravity, or at least the effects of gravity, since the most ancient times. The basic fact that things fall down when dropped is known to everyone, and would have been obvious even in the Stone Age. Ancient civilizations in Greece and India developed a basic understanding of the nature of gravity, and knew that falling objects accelerate, but were unable to apply these ideas to the wider universe.
It was not until the Renaissance, and the days of Galileo, Kepler and Newton, that a truly scientific analysis of gravity was made. Experiments by Galileo demonstrated the counter-intuitive fact that all objects, regardless of how heavy they are, accelerate at the same rate when falling. And Kepler, working with observations of the planets, developed laws describing the motion of the planets around the Sun.
In 1687 Isaac Newton published a book, Philosophiæ Naturalis Principia Mathematica, summarising his three laws of motion, and the universal law of gravitation. For the first time gravity was expressed mathematically, and with this Newton was able to show that Kepler’s laws arose from a simple equation describing the force of gravity. Gravity was revealed as the force that shaped the universe, governed the motion of the stars and planets, and gave us seasons, tides and falling apples.
Over the next two centuries scientists built on Newton’s laws to develop what is now known as Classical Physics. These scientists had tremendous success in describing and predicting the natural world. Astronomers, noticing discrepancies in the orbit of Uranus, were able to use classical physics to predict the position of an eighth planet further out in the Solar System, predictions that led to the discovery of Neptune in 1846.
Despite these successes, cracks were forming in our understanding of gravity. The orbit of Mercury also did not follow predictions, and astronomers searched fruitlessly for another planet to explain this strange behaviour. And although scientists could explain the effects of gravity, nobody could understand quite how stars and planets millions or billions of miles away could exert a force on the Earth.
Answering these questions required a revolution in physics, and the early twentieth century brought one. In just a handful of years Classical Physics was swept aside by two new theories — Quantum Physics and Relativity. While quantum theory was mostly associated with very small things, and could therefore largely ignore gravity, relativity became intimately linked with gravity.
Einstein’s famous special theory of relativity concerns the speed of light, and the behaviour of objects moving close to the speed of light. As first formulated by Einstein this had little to do with gravity. However, in the years following the publication of the special theory of relativity, Einstein developed a more general theory. This theory, appropriately known as the General Theory of Relativity, described the behaviour of gravity more accurately than Newton’s theory, and not only solved the problems with the motion of Mercury, but predicted a whole host of exotic objects — black holes, wormholes and even the possibility of time travel.
The general theory of relativity is an extraordinarily beautiful theory. In classical physics the ideas of matter, motion, space and time are all thought of separately. Space acts as a stage, upon which matter can act. Time is simply a clock, ticking away, allowing matter to move through the stage. But in relativity these ideas are united. Space and time combine into a single entity, spacetime. The presence of matter distorts spacetime, and as matter moves through both space and time, those distortions in turn affect the motion of both matter and light.
The predictions of general relativity were first confirmed by Arthur Eddington in 1919. Physicists studying the complex equations soon found solutions pointing to the existence of black holes — objects so massive that they distort space in such an extreme way that light itself cannot escape. Treated at first as just a mathematical curiosity, the idea of black holes gradually gained acceptance over the following decades. Other solutions to Einstein’s theories have also been proposed, suggesting that bizarre objects such as wormholes, linking distant regions of space through higher dimensional space, or even closed loops of time, are possible.
Despite the revolution in physics, problems with gravity still persisted. Our understanding of the size of the universe changed radically in the early twentieth century when other galaxies were identified for the first time. But observations of these galaxies revealed that they did not spin at the speed predicted by our theories of gravity. Physicists have tried to solve this problem by invoking “Dark Matter”, theorised to be some kind of almost invisible particle that adds additional mass to galaxies. Dark matter cannot be seen by our telescopes, and despite many years of searching we still don’t know what dark matter is. This has led some scientists to try looking for other solutions, including making modifications to the laws of gravity.
During the 20th Century, three of the four fundamental forces of nature were described in the language of quantum physics. In this theory each fundamental force has an associated particle — for electromagnetism this is the photon, for the two nuclear forces (known as the Strong and Weak Nuclear Forces) three particles are associated — gluons, and the W and Z bosons.
It seems reasonable to expect the fourth fundamental force, gravity, would have a quantum particle as well. This particle, known as the graviton, is actually quite well defined theoretically, but if it does exist it is extremely hard to detect. It is hard, even in theory, to design a detector that could find the graviton. Indeed, scientists believe that it is impossible to build such a detector on the Earth. Problems also arise when trying to fit the graviton in with the mathematics of the rest of quantum physics.
Building a quantum theory of gravity remains one of the key challenges of physics. The hunt for quantum gravity has led to a number of theories — string theory, M theory, quantum loop gravity, twister theory, M8 theory… Theoretical physicists have thrown out dozens of suggestions on how to incorporate gravity into the quantum world, but so far it has been impossible to determine which, if any, is correct.
The weakness of the gravitational force is the main issue. The force is so weak that its effects cannot normally be seen at the quantum scale. Only in some of the most extreme places in the universe — in the big bang or at the heart of a black hole — can quantum gravity be observed. Black holes, by their very nature, cannot be directly observed, and neither can we peer back to the very earliest moments of the universe when quantum gravity may have been present.
All this means that for now the final theory of gravity remains out of our reach. Physicists will no doubt continue to build theoretical models of quantum gravity, but until we find a way to experimentally test those theories it is unlikely we make any real progress. And without a theory of quantum gravity, some of the most bizarre and mysterious structures in the universe will remain unknowable to us.