What is Gravity?

Even today, gravity is still a mysterious force, if it is a force at all. We all know the effects of gravity and how difficult it is to work against it. Think about climbing a long staircase. What is it that tries to pull you down? Obviously it is the mass of Earth, but how does mass accomplish this pulling down? This article gives an overview of the mainstream development of the theory of gravity over the last few centuries.

Kepler's Gravity (1605)

Johannes Kepler's discovered his three laws of planetary motion in 1605, by studying the precise measurement of the orbits of the planets, done previously by Tycho Brahe. Kepler found that these observations followed three relatively simple mathematical laws, i.e.

·         The orbit of every planet is an ellipse with the Sun at one of the two focus points.

·         A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

·         The squares of the orbital periods of planets are directly proportional to the cubes of the major axis (the "length" of the ellipse) of the orbits.

However, the physical explanation of this behavior of the planets came almost a century later, when Sir Isaac Newton was able to deduce Kepler's laws from his laws of motion and his law of universal gravity.

Newton's Gravity (1687)

In 1687 Isaac Newton published his 'Principia', including the famous three laws of motion and his law of universal gravitation, which can be briefly stated as:

·         An object in motion will remain in motion unless acted upon by a net force.

·         Force equals mass multiplied by acceleration.

·         To every action there is an equal and opposite reaction.

·         The force of gravity is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses.

Newton was uncomfortable with his own theory of gravity and in his words, 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.

It is now known that Newton's universal gravitation does not fully describe the effects of gravity when the gravitational field is very strong, or when objects move at very high speed in the field. This is where Einstein's general theory of relativity rules.

Einstein's Gravity (1916)

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 just called space-time curvature.

It is reasonably easy to accept that space can be curved – after all, we all know that a sphere has a curved surface, but how can time be 'curved'? The secret lurks in the combination of space and time into space-time. Normally, a space-time diagram is drawn with a straight horizontal spatial axis and a straight vertical time axis. Just bend the two straight axes a little and we have curved space-time.

Just like there are space geodesics (great circles) on Earth's curved surface, representing the shortest possible path between two points, there are spacetime geodesics through the curved spacetime of the universe. All material objects are always on the move through spacetime (time never stands still) and those movements are along spacetime geodesics.

As you are sitting in front of your computer, the gravitational fields of the Sun, Earth, Moon and planets, actually the matter of the whole universe, define your natural spacetime geodesic. The chair you are sitting on is pushing you ever so slightly out of your natural spacetime geodesic by applying a force to your body. Gravity is not the 'force' – it is the chair and Earth's surface that are applying forces onto you.

Remove all the forces and you enter into free-fall, which means you are following your natural spacetime geodesic until you hit the floor. This is what is happening in orbits – the International Space Station (ISS) follows its natural spacetime geodesic closely and hence suffers negligible forces.

Einstein's general relativity and its geodesics give us 'handles' on all but the most extreme gravitational situations. When gravity gets extreme, like is theorized for inside black holes and just after the big bang, even Einstein's theory of gravitation is thought to break down. This is where quantum gravity should take over.

Quantum Gravity

The latest developments attempt to unify general relativity, a theory coping well at macroscopic level, with quantum mechanics, a theory of the microscopic level and smaller. The two most promising directions seem to be 'string theory' and 'loop quantum gravity' (LQG for short).

String theory postulates string-like objects that vibrate in different modes and give rise to the elementary particles and the basic forces of nature, including the graviton, which is a virtual particle that can describe the effects of gravity. One of the problems with string theory is that it assumes a fixed background spacetime, which is somewhat in conflict with relativity theory.

Loop quantum gravity is an effort to formulate background-independent quantum gravity. It preserves many of the important features of general relativity, while at the same time employing quantization of both space and time at the Planck scale. Quantization basically means that there is a fundamental 'packet' of something that cannot be broken down further. Planck time and Planck length are the two fundamental packets of time and of space respectively.

There have been difficulties with LQG, amongst others that it has one crucial free parameter that has to be chosen in order to give result compatible with both general relativity and quantum physics. It would be better if the theory predicts the value. LQG does however give rise to gravitons, and allows gravitons to interact as expected, reproducing Newton's law of gravity.

Both string theory and LQG are incomplete as to date and it remains to be seen if one of them will come out on top and explain physics (including gravity) completely, all the way from fundamental particles up the Universe as a whole, i.e., become a 'theory of everything'