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GRAVITY : The Mysterious Fundamental Force.

Every time you jump, you experience gravity. It pulls you back down to the ground. Without gravity, you'd float off into the atmosphere -- along with all of the other matter on Earth.

You see gravity at work any time you drop a book, step on a scale or toss a ball up into the air. It's such a constant presence in our lives, we seldom marvel at the mystery of it -- but even with several well-received theories out there attempting to explain why a book falls to the ground (and at the same rate as a pebble or a couch, at that), they're still just theories. The mystery of gravity's pull is pretty much intact.

So what do we know about gravity? We know that it causes any two objects in the universe to be drawn to one another. We know that gravity assisted in forming the universe, that it keeps the moon in orbit around the Earth, and that it can be harnessed for more mundane applications like gravity-powered motors or gravity-powered lamps.

As for the science behind the action, we know that Isaac Newton defined gravity as a force -- one that attracts all objects to all other objects. We know that Albert Einstein said gravity is a result of the curvature of space-time. These two theories are the most common and widely held (if somewhat incomplete) explanations of gravity.

Although many people had already noted that gravity exists, Newton was the first to develop a cohesive explanation for gravity,
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1: Gravity mysteries: 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

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2: Gravity mysteries: 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,"

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3: Gravity mysteries: 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 ...

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4: Gravity mysteries: 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

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