The beginnings of relativity

The Origins of Relativity

Relativity was first proposed by Albert Einstein in 1905 in a theory known as Special Relativity.
The second theory of Relativity, known as the theory of General Relativity, was published by Einstein a decade later in 1915.
Prior to these, there was classical physics, likewise known as Newtonian physics, which was based on the principles outlined by Isaac Newton.
The theory of Relativity proposed a different understanding of time and space, unlike the absolute time and space in Newtonian physics.

Einstein asserted that the laws of physics are the same in all inertial frames, and that the speed of light is the same for all observers, regardless of their state of motion or of the source of the light.
Special Relativity uses these postulates to describe the physical world at high velocities.
It led to the iconic equation (E = mc^2), which states that energy (E) and mass (m) are interchangeable; they are different forms of the same thing.
The theory of General Relativity, on the other hand, includes the effects of gravity. It suggests that gravity is a curvature of spacetime caused by mass and energy, not a force as Newton proposed.
General Relativity has been confirmed by several tests, such as the bending of light in gravitational fields and the slowing down of clocks in strong gravitational fields (gravitational time dilation).

The theories of Relativity have completely transformed our understanding of space, time, gravity, and the universe.
They led to novel predictions, like black holes and gravitational waves, which have since been observed and confirmed.
In physics, special relativity has brought about the understanding of relativistic mechanics, significantly affecting modern physics and technology, which includes GPS systems where both special relativity (due to the fast-moving satellites) and general relativity (because they are in a weaker gravitational field) corrections are necessary.
General relativity is used in cosmology to describe the expansion of the universe, the Big Bang, the formation of galaxies, and started the exploration of the possibility of time travel.
While both these theories are proven accurate over time and testing, they are currently incompatible with quantum mechanics, another pillar of modern physics. The union of these theories remains a great challenge in theoretical physics, the so-called theory of everything.