Gravitation or gravity is a force, interaction or mechanism that causes all things to attract one another, influencing everything from subatomic particles to entire galaxy clusters. Even massless particles like photons are affected by it, as massive stars are known to bend light that is made of photons.
Gravity is what makes things fall on the ground, it makes everything stick to the surface of Earth, it causes Earth to orbit the Sun and is responsible for tides. If there wasn’t for gravity, the Universe wouldn’t be organised into galaxies and stars within galaxies. It is then interesting that although we know its effects and have theories that explain it well, at its essence, we do not know how gravitation works.
Gravitation is one of four fundamental interactions in the Universe. It is best described by Einstein’s general theory of relativity where it is actually not seen as a force, but as a result of curvature of spacetime, caused by an uneven distribution of energy and mass in the Universe. Imagine the Sun not as a magnet that attracts Earth, but rather as a whole lot of mass that curves the spacetime through which Earth travels. This is often demonstrated by a bowling ball in the middle of trampoline and a smaller ball that circles the bowling ball as it moves on the curved trampoline. In four-dimensional space that we call spacetime, Earth moves in the straightest line possible, but in three dimensions we perceive, it seems like it is circling our star. If you imagine the surface of Earth as a two-dimensional space where you have only two dimensions available for your movement i.e. you can’t fly, you appear to move in a straight line from point A to point B, but because in three dimensions Earth is a sphere with a curved surface, even if you move along the straightest line possible, your path is still bent in a three-dimensional space.
In the framework of quantum field theory that describes other three fundamental forces, electromagnetic force, the weak interaction and the strong interaction, it is impossible to describe gravitation. There are two reasons for this. First, all other interactions or forces are propagated by their carrier bosons. For gravity, that would be yet-to-be-discovered graviton. To discover it, we would need to replicate an environment of extremely high energies, which is something we are far from being able to do. Second, all attempts to theoretically merge gravity with other three forces have failed. Theory of general relativity that describes gravity and Standard Model that describes other three forces are both tested time and time again and both seem to be correct, but they are not compatible with each other. At the moment, we still lack what is popularly called “the theory of everything”.
Gravitation is the weakest of all forces. When observed at a subatomic level, it is 10^38 times weaker than the strong force and its effect on particles is negligible. For large bodies, however, where those same particles are bulked together, gravity takes over and has more influence than any other force. It has infinite range and travels at the speed of light. If the Sun suddenly disappeared, Earth would continue to orbit what is now an empty space for another eight minutes.
Newton’s law of universal gravitation is well suited for day-to-day life. It states that two bodies attract each other with the force proportional to their masses and inversely proportional to the square of the distance between them. For explaining interactions of huge objects in space, we use Einstein’s equations which are more precise. One of these uses of Einstein’s equations is to explain time dilation that gravitation causes. A clock on Earth runs slower than an identical clock on the Moon in a similar way that really fast spaceship causes time to run slower than time on Earth.