What is gravity?

 Gravity is a phenomena of the physical world. We can observer this phenomena when an object falls to the ground, or in ourselves standing up on the surface of the Earth. Gravity is responsible for tides, keeping our satellite and natural satellites in orbit, planet formation, their orbits around the Sun, ... . Gravity can be observed at grater scales giving structure to galaxies and clusters in the universe. It gathers gas into nebulosas, where hydrogen compacts creating the conditions for nuclear fusion and the birth of stars. It gives us the most mysterious and exotic places and events in the universe: gravitational lensing, supernovas, neutron stars, and blackholes!.

Some ideas about gravity

Early in the 17th century Galileo Galilei discovered that every object will accelerate in the same amount when falling to the ground, and he proposed his basic principle of relativity. The idea was a constant for gravity that was explored during the second half of the 17th century [^1].

The classic definition of gravity was proposed by Isaac Newton in his 'Law of universal gravitation'. Newton defined gravity as the force that attracts bodies with mass. This force is proportional to the mass of the bodies, the more massive the bodies the greater the force that attracts each to one another. This force acts on distant objects but its influence reduces with distance, the famous inverse square [^2].

Newton's gravity unified gravity with the behaviour of astronomical objects known at that time, this is known as the 'first great unification' [^2]. Together with his second law, that relates force to acceleration and mas, we have that the force of gravity manifests as an acceleration in the bodies it is acting on.

Formulation of Newton's gravity

$$F=G\frac{m_1 m_2}{r^2}$$

Were F is the resultant force,  $m_1$ y $m_2$ the mass of the bodies, r is the distance between them, and G is a constant, the 'Gravitational constant' that is an empiric constant.

Einstein's Gravity

Although Newton's gravity was a very good approximation to the observations, there were some discrepancies with the measurements in astronomical orbits and objects. Another problem with his law is that it didn't actually explain the mechanism of the force of gravity. It has to be Einstein and his theory of relativity who proposed that boies with mass do curve spacetime, and that we experience this curvature as gravity, an acceleration. Since then observations of the effect of gravity have confirmed Einstein theories even in the most unexpected situations, gravitational lensing and blackholes are examples of predictions and laer discoveries based in his theories of relativity.

Foto de Pixabay en pexels.com

References

[^1] History of gravitational theory ttps://en.wikipedia.org/wiki/History_of_gravitational_theory

[^2] Newton's law of universal gravitation https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation