One of the most familiar forces is also the most mysterious, and that is gravity. In general, gravity is the force that pulls together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly. The amount of matter in something is called its mass. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces. In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth. Most of us know the effects of the mysterious force called gravity. However, the question "what is gravity" is not very easy to answer, because we don't really understand what this force actually is. What we do know about gravity is mostly due to the work of three men, Johannes Kepler, Sir Isaac Newton and Albert Einstein. Kepler worked out the details of how the orbits of the moon and planets can be described mathematically. This is also known as the Kepler laws of planetary motion, but it does not answer the question "what is gravity".
Newton, reportedly while observing an apple falling from a tree, got an inspiration that allowed him to work out how the force of gravity can be described mathematically. It later became apparent that there are some scenarios where Newton's mathematical description does not quite hold, but it still the simplest way of describing gravity. It does however also not answer the "what is" question. Newton was uncomfortable with his own theory of gravity. He said that his theory never "assigned the cause of this power". He was unable to experimentally identify what produces the force of gravity and he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was not sound science. Einstein later worked out how the force of gravity is not quite a force, but rather an artifact of the natural movement of objects through curved four-dimensional space-time. Einstein reportedly got the inspiration for this imaginative leap in understanding of gravity by contemplating a man falling off a building. Such a falling man would not experience any force while he is falling, at least not before hitting the ground and suffering severe forces. In his monumental 1916 work "The Foundation of the General Theory of Relativity", Albert Einstein unified his own Special Relativity, Newton's law of universal gravitation, and the crucial insight that the effects of gravity can be described by the curvature of space and time, usually called 'space-time' curvature. The radius of curvature is modified by relativistic factors, by a gravitational time dilation and by a velocity time dilation.
This causes the acceleration of a falling object, as experienced by the free falling object to be larger than what Newton predicted. So, what's gravity? The truth is that at the most fundamental level, no one really knows. This blog only summarizes the basics of Newton's and Einstein's gravity in terms of the gravitational acceleration caused by curved space-time and velocity. We may have to wait for a theory of quantum gravity to be completed for a better answer. Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics with general relativity in a self-consistent manner, or more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than the speed of light squared) and which limits Einstein's general relativity to large actions (action much larger than the Planck's constant). The theory must be able to predict the outcome of situations where both quantum effects and strong-field gravity are important (at the Planck scale, unless large extra dimension conjectures are correct). Although some quantum gravity theories such as string theory and other so-called theories of everything attempt to unify gravity with the other fundamental forces, others such as loop quantum gravity make no such attempt, they simply quantize the gravitational field while keeping it separate from the other forces. So far, cosmologists, physicists and mathematicians have not arrived at a consistent theory of gravity that melds quantum mechanics with the theory of relativity.
Newton, reportedly while observing an apple falling from a tree, got an inspiration that allowed him to work out how the force of gravity can be described mathematically. It later became apparent that there are some scenarios where Newton's mathematical description does not quite hold, but it still the simplest way of describing gravity. It does however also not answer the "what is" question. Newton was uncomfortable with his own theory of gravity. He said that his theory never "assigned the cause of this power". He was unable to experimentally identify what produces the force of gravity and he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was not sound science. Einstein later worked out how the force of gravity is not quite a force, but rather an artifact of the natural movement of objects through curved four-dimensional space-time. Einstein reportedly got the inspiration for this imaginative leap in understanding of gravity by contemplating a man falling off a building. Such a falling man would not experience any force while he is falling, at least not before hitting the ground and suffering severe forces. In his monumental 1916 work "The Foundation of the General Theory of Relativity", Albert Einstein unified his own Special Relativity, Newton's law of universal gravitation, and the crucial insight that the effects of gravity can be described by the curvature of space and time, usually called 'space-time' curvature. The radius of curvature is modified by relativistic factors, by a gravitational time dilation and by a velocity time dilation.
This causes the acceleration of a falling object, as experienced by the free falling object to be larger than what Newton predicted. So, what's gravity? The truth is that at the most fundamental level, no one really knows. This blog only summarizes the basics of Newton's and Einstein's gravity in terms of the gravitational acceleration caused by curved space-time and velocity. We may have to wait for a theory of quantum gravity to be completed for a better answer. Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics with general relativity in a self-consistent manner, or more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than the speed of light squared) and which limits Einstein's general relativity to large actions (action much larger than the Planck's constant). The theory must be able to predict the outcome of situations where both quantum effects and strong-field gravity are important (at the Planck scale, unless large extra dimension conjectures are correct). Although some quantum gravity theories such as string theory and other so-called theories of everything attempt to unify gravity with the other fundamental forces, others such as loop quantum gravity make no such attempt, they simply quantize the gravitational field while keeping it separate from the other forces. So far, cosmologists, physicists and mathematicians have not arrived at a consistent theory of gravity that melds quantum mechanics with the theory of relativity.