Special relativity builds upon Galileo Galilei's work that describes how all uniform, or steady, motion is viewed as being relative to a specific frame of reference. In other words, an object that appears to be moving from one observer's perspective may also appear to be motionless to another observer situated on that very same object. There is no absolute, or fixed, reference frame from which to define objects that are in motion. Special relativity adds to this that while many of the basic properties of motion may vary relatively between any two observers, the speed of light will always be invariant. This is because it is the universal speed limit for matter and energy in motion. Additionally, the physical laws by which these systems undergo any changes are experienced the same in every frame of reference, and the arena in which they happen is referred to as space-time: a unified version of space and time with three dimensions for space and one dimension for time.
Einstein's most widely recognized equation sets forth the mass-energy equivalence (Image: Me).
One of the most important revelations of the special theory of relativity is the mass-energy equivalence. It allows the mass of an object to be seen as a measure of its energy content. Einstein used this now famous relation to describe how matter is able to transform into radiated energy.
Peculiar differences in measurements regarding the passage of time, when an event happens, or even the apparent length of an object may arise if two observers are traveling through space-time with different frames of reference relative to each other. For example, a relative increase in an object's apparent velocity and/or its proximity to a gravitational source causes it to experience time more slowly. This phenomenon is known as as time dilation, and in everyday conditions its influence is very small. However, it can be verified to exist experimentally through the use of separate atomic clocks, one placed on the ground and the other traveling in orbit around the Earth. Varying frames of reference for different observers can also make it difficult to know whether two spatially separated events happen at exactly the same time. Relativity of simultaneity allows for one such event to appear to precede the other from one frame of reference, while in the opposite order from a different perspective, except when one event is the direct cause of the other. Lastly, an observer watching an object approaching an extremely high relative velocity may see the object's apparent length decreasing in the direction of its motion. Length contraction is an effect only noticeable for objects moving at velocities close to the speed of light and is not apparent at everyday speeds.
With the arrival of the special theory of relativity, the constancy of the speed of light was established and the need to account for space and time as being absolute and independent of one another had disappeared. Albert Einstein was able to show that perceived time, along with distances, varied between observers in different circumstances. Matter and energy also emerged as two versions of the same substance, linked together by a relation which is now considered fundamental for explaining processes in nuclear and particle physics. Special relativity has created an impression in modern day science that few other theories have been able to match by quickly gaining widespread acceptance and exciting the imagination of many through its extraordinary implications.