Albert Einstein was famous for many things, but his greatest brainchild is the theory of relativity. It forever changed our understanding of space and time.
What is relativity? Succinctly put, it is the notion that the laws of physics are the same everywhere. We here on Earth obey the same laws of light and gravity as someone in a far off corner of the universe.
The universality of physics means that history is provincial. Different viewers will see the timing and spacing of events differently. What for us is a million years may just be a blink of an eye for someone flying in a high speed rocket or falling into a black hole.
It’s all relative.
Einstein’s theory is divided into special and general relativity.
Special relativity came first and is based on the speed of light being constant for everyone. That may seem simple enough, but it has far-reaching consequences.
Einstein came to this conclusion in 1905 after experimental evidence showed that the speed of light didn’t change as the Earth swung around the Sun.
This result was surprising to physicists because the speed of most other things does depend on what direction the observer is moving. If you drive your car alongside a railroad track, a train coming at you will seem to be moving much faster than if you turned around and followed it in the same direction.
Einstein said that all observers will measure the speed of light to be 186,000 miles per second, no matter how fast and what direction they are moving.