All motion is relative. This is called the reference system. For example for us the reference system can be fixed on the surface of the Earth, while a space traveler will find it much more simpler to use the Sun as the reference system. In an accelerating elevator, if you stand on a balance, what you read is the normal force. This is a fictitious force.
Coriolis force is a fictitious force.
Special Relativity Albert Einstein developed the Special Theory of Relativity in the early 1900's. The theory has two postulates. The laws of physics are the same in all inertial reference systems. The speed of light in vacuum is a constant value regardless of the speed of the source. Consequences of the theory of special relativity is that at high speed time, length and mass do not stay constant.