Special Relativity
While Albert Einstein didn't win a Nobel Prize for his incredible discovery/invention (your choice) of relativity, it is truly an astounding concept that changed the way we perceive our universe. Relativity can be broken down in to two categories: Special Relativity (SR) and General Relativity (GR). For now, let's focus on SR, the simpler one.
SR describes the way objects experience space and time when moving at a constant velocity. Einstein first laid the foundation with two postulates, truths on which the rest of the theory can be created.
His first postulate says that the laws of physics are the same in all inertial frames of reference. This is a fancy way of saying that no matter how fast two objects are moving, as long as they are not accelerating, they will both experience the same physics. For example, if you are sitting in slow traffic going 5 miles per hour and you throw an apple up in the car, you will watch the apple fall in the exact same way as if you had thrown it while cruising down the highway at 200 miles per hour. This is the reason why you can enjoy your plane's fizzy water at 35,000 feet high the same way you would at the dinner table. Even though the plane is moving at 500 miles per hour in the air and you are moving 0 miles per hour at the table, you experience the same things. This also means that there is no way to tell who is actually moving in a situation (if you are not moving close to the speed of light). So, someone sitting in a train would be completely correct in saying that the train is stationary and the ground is moving past it. On the other hand, someone on the ground would be completely correct in saying that the ground is stationary and the train is moving. Because the physics are the same in both situations, both perspectives are perfectly correct.
His second postulate says that within these inertial (non-accelerating) reference frames, no matter how fast you are moving, you will view the speed of light as the same, approximately 300,000,000 meters per second. The mathematical derivation of this fact is intense and complicated. It stems from Maxwell's Equations, which describe how electromagnetic waves and fields work. For now, let's just assume it's true.
This is just a quick description of the foundation on which special relativity is built. Come back soon for a deeper dive into some of its effects.
