Yet look a little closer, and the certainties start to float away, revealing gravity as the most puzzling and least understood of the four fundamental forces of nature.
Michael Brooks investigates its mysterious ways
What is gravity?
You jump up, and gravity brings you back down to Earth. You reach the brow of a hill and gravity accelerates you down the other side. All neat and tidy then: gravity behaves in the way Newton thought of it, as a force that affects and changes the motion of something else.
That, at least, was how it seemed until Einstein came along. His general theory of relativity tells us that gravity is not quite that simple.
General relativity provides a framework under which the laws of physics look the same for everyone at every moment, regardless of how they are moving. Einstein achieved this by making gravity a property of the universe, rather than of individual bodies.
General relativity describes gravity as geometry. The fabric of the universe - the four dimensions of space and time - is full of lumps and bumps created by the presence of mass and energy. This warping is unavoidable; whenever anything - be it you, me, a piece of space dust or a photon of light - tries to travel through the universe in a straight line, it actually follows a trajectory that is curved by any mass and energy in the vicinity. The result of this curvature is what we think of as gravity. To look at it a slightly different way, gravity is not what one body does directly to another, but what a body's mass does to the surrounding universe.
Nevertheless, treating gravity as if it were a straightforward force, as Newton did, has served us well. It allowed us to send rockets to the moon and chart the orbits of the planets with astonishing accuracy. "The Newtonian description works to high precision," says Bernard Carr of Queen Mary, University of London.
Einstein's description has stood up equally well to scrutiny in situations when high speeds and accelerations are involved. Yet useful as they are, neither the Newtonian model nor relativity is a fundamental explanation for gravity. We still don't know how the fundamental, quantum properties of mass, energy and space-time combine to create the phenomenon.
We do have an idea, though. At the most basic level, the other three fundamental forces of nature - the electromagnetic force, the weak nuclear force and the strong nuclear force - work through the action of particles. The electromagnetic force, for example, is delivered by photons. This picture is known as quantum field theory. In the same way, there ought to be particles that deliver the gravitational force.
There are still two problems with this, though. First, we have yet to find any proof of the existence of these hypothetical particles, which have been dubbed "gravitons". Secondly, when quantum field theory is applied to gravity, it is prone to give nonsensical answers to straightforward questions. "These are fundamental obstructions that need to be overcome," says Bruce Bassett of the University of Cape Town, South Africa.
Why does gravity only pull?
All the other forces in nature have opposites. In the case of the electromagnetic force, for example, it can attract or repel, depending on the charges of the bodies involved. So what makes gravity different?
The answer seems to lie with quantum field theory. The particles that transmit the strong, weak and electromagnetic forces have various types of charge, such as electric or colour charge. "Those charges can be either positive or negative, leading to different possibilities for the sign of the force," says Frank Wilczek of the Massachusetts Institute of Technology. This is not the case with gravitons, the hypothetical particles that quantum field theory says should transmit gravity. "Gravitons respond to energy density, which is always positive," says Wilczek.
Or are we assuming too much here? "We don't know that gravity is strictly an attractive force," cautions Paul Wesson of the University of Waterloo in Ontario, Canada. He points to the "dark energy" that seems to be accelerating the expansion of the universe, and suggests it may indicate that gravity can work both ways. Some physicists speculate that dark energy could be a repulsive gravitational force that only acts over large scales. "There is precedent for such behaviour in a fundamental force," Wesson says. "The strong nuclear force is attractive at some distances and repulsive at others."
Either way, the apparent difference between gravity and the other fundamental forces poses a problem for physicists wanting to create a "theory of everything" that provides a single explanation for them all. At the moment, most theorists expect our best route to such a theory to lie via a hidden symmetry of nature known as supersymmetry, which suggests that every particle has a much heavier twin waiting to be discovered. Wilczek warns, though, that this may not be the final answer. "It's also possible that some essentially new idea will be required," he says.
Why is gravity so weak?
Take a moment to try a jump into the air. Have you ever thought about how remarkable it is that so little effort is required to jump a few inches off the ground. Your puny muscles, weighing just a few kilograms, can overcome the gravitational force of the Earth, all 6 × 1024 kilograms off it. Gravity is a real weakling - 1040 times weaker than the electromagnetic force that holds atoms together.
Although the other forces act over different ranges, and between very different kinds of particles, they seem to have strengths that are roughly comparable with each other. Gravity is the misfit. Why should this be so?
So far, our best explanation comes from string theory, the leading candidate for a "theory of everything". String theory requires that the universe has more than the three spatial dimensions that we experience, and possibly as many as 10. According to string theorists' best ideas, gravity is so weak because, unlike the other forces, it leaks in and out of these extra dimensions. We only get to experience a dribble of the true strength of gravity.
The proof of this might come through experiments that probe the gravitational attraction between objects that are very small distances apart. String theory suggests that the unseen dimensions are hidden from view because they are rolled up small, or lie "end-on" to our dimensions, making it hard to detect their presence. These compactified dimensions could alter the gravitational attraction between two bodies if they are very small distances apart. Experiments have got down to about 0.06 millimetres, but have failed to see anything so far.
One of the big hopes for the Large Hadron Collider at CERN near Geneva, Switzerland, is that it will tell us why gravity is so weak. "The LHC's purpose, more or less, is to understand this question," says Lisa Randall of Harvard University.
Though it is not likely to provide a complete answer, the case for gravity residing in extra, hidden dimensions would be strengthened if the LHC finds evidence for particles called Kaluza-Klein states.
These were mooted as far back as the 1930s by theorists attempting to unite electromagnetism and gravity. Kaluza-Klein states arise when familiar particles slip into an extra dimension. As they rattle around there, they create an "echo" that would manifest itself as a heavier particle.
Why is gravity fine-tuned?
The feebleness of gravity is something we should be grateful for. If it were a tiny bit stronger, none of us would be here to scoff at its puny nature.
The moment of the universe's birth created both matter and an expanding space-time in which this matter could exist. While gravity pulled the matter together, the expansion of space drew particles of matter apart - and the further apart they drifted, the weaker their mutual attraction became.
It turns out that the struggle between these two was balanced on a knife-edge. If the expansion of space had overwhelmed the pull of gravity in the newborn universe, stars, galaxies and humans would never have been able to form. If, on the other hand, gravity had been much stronger, stars and galaxies might have formed, but they would have quickly collapsed in on themselves and each other. What's more, the gravitational distortion of space-time would have folded up the universe in a big crunch. Our cosmic history could have been over by now.
Only the middle ground, where the expansion and the gravitational strength balance to within 1 part in 1015 at 1 second after the big bang, allows life to form. That is down to the size of the gravitational constant G, also known as Big G.
G is the least well-defined of all the constants of nature. It has been pinned down to only 1 part in 10,000, which makes it look pretty rough and ready next to the fundamental number called the Planck constant, which is accurate to 2.5 parts in 100 million. It's gravity's weakness that makes G difficult to measure more accurately - but that's just a laboratory issue. The important question is, where does this value come from? Why does G have the value that allowed life to form in the cosmos?
The simple but unsatisfying answer is that we could not be here to observe it if it were any different. As to the deeper answer - no one knows. "We can make measurements that determine its size, but we have no idea where this value comes from," says John Barrow of the University of Cambridge. "We have never explained any basic constant of nature."
Does life need gravity?
Plants certainly do. Charles Darwin was the first western scientist to show that rooted plants have gravity sensors: plumb lines, effectively, that give them a sense of up or down. Turn a plant pot on its side and you'll find that the roots continue to grow towards the centre of the Earth.Grown in space, plants show disoriented roots that don't get the best access to nutrients and water. Poor starch production is one of a number of adverse effects of this.
Animals suffer a raft of problems if they are deprived of gravity - though we don't yet know the whole story. "We've had animal life in space for half a century, but we've yet to have a single mammal go through its life cycle," says biologist Richard Wassersug of Dalhousie University in Halifax, Nova Scotia, Canada.
We do know that there can be problems from the start. Experiments on the Russian space station Mir found that fewer quail eggs than normal hatched, and chicks from those that did hatch were unusually likely to suffer abnormalities.
Then there was an experiment on the US space shuttle Discovery, paid for by the KFC fast food corporation, that investigated the development of quail embryos. None of its 16 embryos hatched. In normal gravity, the yolk lies next to the shell, but in microgravity it floats in the middle of the albumen. This led to problems in the transfer of gas between the embryo and the shell, which proved fatal for the embryos. Wasser reckons these difficulties could be solved with suitable engineering, or by taking the embryos into space at a later stage.
Even bigger problems emerge if embryos survive to see the light of day. Chicks hatched in microgravity can't balance and orient themselves well enough to feed. Amphibians have problems with breathing: their instinct is to go "up" for air, but there is no up.
Humans have problems with breathing for a different reason. In space, astronauts' lung capacity is reduced because there is no gravity to pull the diaphragm downwards. To make matters worse, the liver sits higher in microgravity, further reducing lung size. For a short trip this isn't too much of a problem, but what would happen to babies born in space?
"We don't know what happens if you grow from a crawling baby to an adult with smaller lungs," Wassersug says. "There's every reason to believe serious problems will start to set in during youthful growth." It's the simple things that get you: you can't cough your lungs clear, for example. This and other seemingly minor complications "could be seriously dangerous by the time you get to sexual maturity", Wassersug says.
Then there's the issue of bone loss. To grow properly, our bones need to be stressed by out body's weight. Space station crews returning to Earth show that we also suffer severe muscle loss in microgravity - possibly enough to make it impossible for women to give birth naturally in space.
Who knows what else could affect us after long years in space? "It's hard to appreciate how little we've learned about development and growth in weightlessness," Wassersug says.
Can we counter gravity?
Though the notion of building a gravity shield has a long history, no one has yet managed to do it. Perhaps the most famous attempt was by Russian émigré scientist Evgeny Podkletnov.
In 1992, Podkletnov published a paper in which he claimed to have detected a 2 per cent weight reduction around a spinning disc made out of a ceramic superconductor.
Martin Tajmar, a researcher at the company Austrian Research Centers, published a similar claim in 2003 and was able to pursue the research further with funding from the European Space Agency. Three years later Tajmar and ESA announced they had measured an effect in a spinning superconductor that might, with further development, be harnessed to affect gravity. Others have tried and failed to replicate this effect.
Why does anyone think it might even be possible? Because relativity does not rule out the possibility that the bent space-time that gives rise to gravity's pull can be "unbent". "By appropriate arrangements, it should be possible to diminish - or enhance - the influence of gravity," says physicist Bahram Mashhoon at the University of Missouri.
Tajmar invokes an effect called "gravito-magnetism" as a way of doing this. According to general relativity, the mass of a rotating body will drag space-time around with it, putting a twist into it. Just as a spinning charge creates a magnetic field, a spinning mass creates a gravito-magnetic field.
This should have real-world effects - the Earth's spin, for instance, should cause satellite orbits to precess - but you won't be surprised to hear there are practical issues with using the idea to reduce gravity. "The relativistic effects are extremely small in practice," Mashhoon points out.
Though it's not even clear that a spinning superconductor has any gravito-magnetic influence, people should not be ridiculed for continuing to research in this area, Mashhoon says. It might just turn out to be the only way we can achieve interstellar travel. Some researchers have suggested that above a certain critical speed, relativity can give repulsive gravitational effects that could be used for propulsion as well as gravity shielding. "With present technology, it would take us about a million years or so to go to the nearest neighbouring star," Mashhoon says. "It is hard to blame people for looking into these things."
Will we ever have a quantum theory of gravity?
Quantum mechanics and relativity, our two most successful theories of how the world works, both seem strangely at odds with the everyday world as we experience it - and with each other.
Quantum theory, which describes how things behave at the subatomic level, is certainly weird. Quantum objects can simultaneously be in two locations, or move in different directions.
Relativity is, if anything, worse. We can use it to describe gently curving space-time, but the equations turn to nonsense in the extreme conditions found at the heart of a black hole or the beginning of the universe.
From a physicist's point of view, the big problem is that no one has figured out how quantum theory and relativity fit together to make a quantum theory. There has to be a better theory out there, one that describes how and why everything works, from subatomic to cosmological scales, but it is proving extraordinarily elusive.
Einstein was among the first scientists to try to marry gravity with other theories of physics, yet we may be no closer now than he was when he began his search. The most popular quantum gravity theories today seem to have fundamental problems that no one knows how to solve.
Does that mean we'll never get there? We shouldn't despair, says Lee Smolin at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "I am a big believer in our capacity to understand the universe we find ourselves in," he says.
According to Roger Penrose of the University of Oxford, the final quantum gravity theory won't look like anything that's around at the moment. He reckons today's theories are not powerful enough to even be considered as candidates, as they ignore important issues such as solving the mysteries of the quantum world's strange behaviour. "There ought to be such a theory, but I would expect it to represent a major revolution in our picture of the physical world," he says. "It will require a major breakthrough, and a fundamental departure from current thinking."
Will we get there? Penrose is optimistic. "We might destroy ourselves in a nuclear war or by overheating of the Earth," he says. "But barring that sort of thing, I don't see why not."
