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Gravity is the weakest of all the forces of nature. The gravitational force of attraction between two protons is 1036 times smaller than the electrostatic force of repulsion between them. It is a common experience that a fridge magnet will easily pick up a piece of iron or steel of its own weight off the floor, overcoming the gravitational attraction of the whole Earth in the process. Unlike magnetic and electrostatic forces, the force of gravity between two masses is always attractive. This means that as objects get bigger and bigger and like ourselves and our Earth remain electrically neutral, the force of gravity finally overcomes all other forces and even has the power to turn the largest masses into 'black holes'.
A black hole is the ultimate triumph of gravity over matter. It is a region of space where the gravitational field is so strong that not even light can escape. Black holes a few times the mass of the Sun are probably created when massive stars explode at the end of their lives, while all galaxies are now thought to have black holes up to a thousand million times the mass of the Sun at their centres. Black holes can never be seen but reveal themselves through the release of energy from matter falling into them.
The force of gravity starts to control and modify the fate of matter in the lowest-mass stars, which are only about 15 times the mass of Jupiter. In these stars the pressure at their centres, caused by gravity, is just enough to overcome the electrostatic force of repulsion between their protons (hydrogen nuclei) causing them to fuse and become helium atoms.
Gravity is the dominant force in shaping our universe on the largest scales. It makes planets and stars spherical. It binds the planets to the Sun and the Sun and all the stars to our galaxy. It binds stars in clusters and holds clusters of galaxies together. It even controls the expansion of the universe.
If gravity is so important, what stops the planets falling into the Sun and all the stars in our galaxy disappearing into a central black hole? The answer is the conservation of energy and momentum and in this case, specifically angular momentum. It is angular momentum that prevents the planets from falling into the Sun, confining them all to orbit in the same plane. It is angular momentum that creates and maintains the disk shapes of galaxies and prevents clusters of stars and galaxies collapsing down to a central point.
Conservation laws are the most fundamental laws in physics. The most familiar example of the conservation of angular momentum is the ballet dancer, whose spin increases as she draws in her arms. Similarly a planet orbiting the Sun, or a galaxy orbiting the common centre of gravity of a cluster of galaxies, has a certain angular momentum and an energy which is the sum of its gravitational potential energy and its kinetic energy. The energy and momentum must both be conserved. The closer a planet's orbit is to the Sun the lower its total energy. The only way the planet or galaxy could fall into the centre of the system is by losing more energy. The only way that this can happen is in a collision in which kinetic energy is converted into heat, which is then radiated out of the system. Overall energy in the Universe is still conserved but it has changed its form. With planets and stars this never happens because the chances of collisions are so small. However, as we shall see, planets and stars are created from clouds of gas and dust. We can think of each atom and dust particle in the cloud being in its own orbit around the centre of gravity, with its own energy and momentum. The big difference between planets and atoms is that atoms frequently collide with each other, sharing momentum and energy, and in the process radiate energy out of the cloud, thereby reducing the total energy of the cloud and allowing it to shrink in size. This would allow the cloud to collapse to a point, but nuclear forces come into play. As the atoms aggregate into particles and ultimately planets, collisions become rarer and each fragment retains the average energy and momentum of the material that made it.
The origin of the angular momentum that dominates so many structures in the Universe comes from the fact that the clouds from which the objects were made have shrunk by a factor of a million. This means that even the slightest initial net tendency to rotate will result in rapid rotation, through the law of conservation of angular momentum. Or to put it another way; in order not to rotate, the sum of the random turbulent motions of every part of the cloud would have to add up to zero with incredible precision, which is highly improbable.
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