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Gravity is the oldest known that the strength of the many ways the youngest. It was enough to understand the explanation of stars, black holes and the Big Bang, but at some point, it does not understand anything. To explain the gravity of the request, the two largest research staff advice and suggestions are contemporary history, Newton and Einstein, and now, the brilliant theoretical physicist, inventor of hundreds of people to try to please. Gravity has been reinvented every time, it caused a revolution. Newton's gravitational theory is stimulated great progress in mathematics and astronomy; actually, this is the start of modern theoretical physics. Einstein's theory of relativity, gravity, the general theory of relativity, and he called the unexpected, and completely open for the survey: the black hole's gravitational waves, the Big Bang. In the future, changes in gravity, quantum gravity and the possibility of only one of many facing the unified theory of all the forces of physics is a more far-reaching revolution might attendant.           Gravity, Gravity, Everywhere     Gravity, the oldest force known to mankind, is in many ways also the youngest. It is understood well enough to explain stars, black holes and the Big Bang, and yet in some ways it is not understood at all. Explaining gravity required the two greatest scientific minds of modern history, Isaac Newton and Albert Einstein; and now hundreds of the brightest theoretical physicists are working to invent it once again. Each time gravity has been re-invented, it has sparked a revolution. Newton's theory of gravity stimulated huge advances in mathematics and astronomy; indeed, it was the beginning of modern theoretical physics. Einstein's theory of gravity, which he called general relativity, opened up completely unexpected phenomena to investigation: black holes, gravitational waves, the Big Bang. When, sometime in the future, gravity changes into quantum gravity, possibly becoming just one of many faces of a unified theory of all the physical forces, the ensuing revolution may be even more far-reaching.   ... Gravity is everywhere. No matter where you go, you can't seem to escape it. Pick up a stone and feel its weight. Then carry it inside a building and feel its weight again: there won't be any difference. Take the stone into a car and speed along at 100 miles per hour on a smooth road: again there won't be any noticeable change in the stone's weight. Take the stone into the gondola of a hot-air balloon that is hovering above the Earth. The balloon may be lighter than air, but the stone weighs just as much as before.   This inescapability of gravity makes it different from all other forces of nature. Try taking a portable radio into a metal enclosure, like a car, and see what happens to its ability to pick up radio stations: it gets seriously worse. Radio waves are one aspect of the electromagnetic force, which in other guises gives us static electricity and magnetic fields. This force does not penetrate everywhere. It can be excluded from regions if we choose the right material for the walls. Not so for gravity. We could build a room with walls as thick as an Egyptian pyramid and made of any exotic material we choose, and yet the Earth’s gravity would be right there inside, as strong as ever. Gravity acts on everything the same way.   Every body falls toward the ground, regardless of its composition. We know of no substance that accelerates upwards because of Earth's gravity. Again this distinguishes gravity from all the other fundamental forces of Nature. Electric charges come in two different signs, the "+" and "-" signs on a battery. A negative electron attracts a positive proton but repels other electrons.   The existence of two signs of electric charge is responsible for the shape of our everyday world. For example, the balance between attraction and repulsion among the different charges that make up, say, a piece of wood gives it rigidity: try to stretch it and the electrons resist being pulled away from the protons; try to compress it and the electrons resist squashed up against other electrons. Gravity allows no such fine balances... this means that bodies in which gravity plays a dominant role cannot be rigid. Instead of achieving equilibrium, they have a strong tendency to collapse, sometimes even to black holes.   These two facts about gravity, that it is ever-present and always attractive, might make it easy to take it for granted. It seems to be just part of the background, a constant and rather boring feature of our world. But nothing could be further from the truth. Precisely because it penetrates everywhere and cannot be cancelled out, it is the engine of the universe. All the unexpected and exciting discoveries of modern astronomy — quasars, pulsars, neutron stars, black holes — owe their existence to gravity. It binds together gases of a star, the stars of a galaxy, and even galaxies into galaxy clusters. It has governed the formation of stars and it regulates the way stars create chemical elements of which we are made. On a grand scale, it controls the expansion of the Universe. Nearer to home, it holds planets in orbit about the Sun and satellites about the Earth.   The study of gravity, therefore, is in a very real sense the study of practically everything from the surface of the Earth out to the edge of the Universe. But it is even more: it is the study of our own history and evolution right back to the Big Bang.          — Bernard Schutz, Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity                             Indexes/21