Gravity

Gravity 

     Gravity is the one force of Nature that operates everywhere; it controls the effects of all the other forces wherever they act; it regulates countless natural clocks, from the orbits of planets to the lifetimes of stars. Gravity rules the most violent places in the Universe – quasars, pulsars, gamma-ray bursters, supernovae – and the most quiet – black holes, molecular clouds, the cosmic microwave background radiation.

        Today gravity binds stars and galaxies and clusters of galaxies together, but much earlier it pushed the Universe violently apart. Gravity explains the uniformity of the Universe on very large scales and its incredible variety on small scales. Gravity even laid the path toward the evolution of life itself. If we understand how gravity works, then we begin to understand the Universe.

        Rich as our understanding of the workings of gravity in the Universe has become, it is far from complete. The gaps are not just hidden regions, phenomena yet to be discovered, although when such discoveries occur they are sure to bring more amazement and delight. The most exciting gaps are those in our understanding of the laws of Nature.

How Gravity Evolves?

      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.

         Each of these revolutions has built on the previous one, without undermining it. Newton’s gravity is just as important today for explaining the motions of the planets as in Newton’s time. It is used to predict the trajectories of spacecraft and to understand the structure of galaxies. Yet general relativity underpins all of this, because Newton’s gravity is only an approximation to the real thing. We need only Newton to help us understand how a star is born and evolves; but when the star’s evolution leads to gravitational collapse and a supernova, then we have to ask Einstein’s help to understand the neutron star or black hole that is left behind. When we have a theory of quantum gravity, it won’t stop us from using general relativity to explain how the Universe expanded after the Big Bang; but if we want to know where the Big Bang came from, and why (or whether) time itself started just then, we will need to ask the quantum theory.

        There is a deeper reason for this continuity from one revolution to the next. As an example, consider the fact that two of the fundamental ideas in Einstein’s general relativity, called the principle of relativity and the principle of equivalence, originated with Galileo. Einstein’s revolution brought a complete change in the mathematical form of the theory, added new ideas, and opened up new phenomena to investigation. But there was a profound continuity in physical ideas, and these were as important to Einstein as the mathematical form of the theory. The coming quantum revolution will surely likewise be grounded firmly in concepts that physicists today use to understand gravity to investigation. But there was a profound continuity in physical ideas, and these were as important to Einstein as the mathematical form of the theory. The coming quantum revolution will surely likewise be grounded firmly in concepts that physicists today use to understand gravity.