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Special Relativity Theory | Definition and Explanation

Special relativity theory

Special Relativity Theory | Definition and Explanation

Special Relativity Theory

The special relativity theory explains how space and time are connected for objects that move at consistent speeds in straight lines. One of the most famous aspects is that objects move at the speed of light.

How to explain special relativity simply?

Special relativity also makes the speed of light (in vacuum) an invariant quantity, which remains unchanged whatever the position of the observer. From 1907, Albert Einstein attempted to describe gravitation, based on the simple idea that a person in free fall no longer feels his weight.

Einstein’s special theory of relativity is based on two principles:

1. The principle of relativity

The laws of physics do not change for all objects moving in an inertial frame of reference (fixed speed).

2. The principle of the speed of light

The speed of light is the same for all observers, does not depend on the speed of motion of the observer relative to the light source.

Einstein’s special theory of relativity creates a fundamental relationship between space and time.
The universe can be seen as having three dimensions of space and one dimension of time. These four dimensions are called the space-time continuum.

Simply put, when an object approaches the speed of light, its mass becomes infinity and it cannot travel faster than light travels. This cosmic speed limit has been the subject of much discussion in physics, and even in science fiction, when people think about how to travel across great distances.

Proof of special relativity

The two postulates (thesis or dissertation) of special relativity are as follows: The laws of physics have the same form in all Galilean frames of reference. The speed of light in a vacuum is the same in all Galilean frames of reference.

The special theory of relativity was developed by Albert Einstein in 1905 and forms part of the foundations of modern physics

After completing his work in special relativity, Einstein spent a decade pondering what would happen if one introduced acceleration. It formed the basis of general relativity, which was published in 1915.


To understand the consequences of Einstein’s postulates, let’s embark on an imaginary train traveling at a speed close to that of light in a rectilinear and uniform movement. A passenger in the train notices that two light beams, emitted simultaneously in the center of the car, reach the opposite walls simultaneously. On the other hand, this is not what the station master observes from the platform.

As the speed of light is the same for all observers, the light beams reach the opposite walls of the wagon at different times because one of the beams must “catch up” with the train. The first consequence of Einstein’s relativity is that the simultaneity of two events is relative to the observer. Another consequence is that the duration separating two events depends on the frame of reference in which it is measured.

These consequences stem from an important conceptual reversal. Until then, time and space formed the stage on which events unfolded. They were considered fundamental notions and speed was a notion that derived from them. If time and space must adapt at an invariant speed, they then become relative to the observer’s frame of reference and are therefore no longer independent: they form a new unified entity, space-time.


The variation of durations with the movement of the observer has been verified experimentally with great precision, thanks to the decay of atmospheric muons or to particle accelerators.

Today, the principle of relativity of time is commonly used in fundamental physics. It is also essential to take this into account to synchronize the clocks of satellite geolocation systems.

Sources: PinterPandai, Space, Live Science

Photo credit: Author: MissMJ via Wikimedia Commons (CC BY-SA 3.0)
Photo description: an example of a light cone, the three-dimensional surface of all possible light rays arriving at and departing from a point in spacetime. Here, it is depicted with one spatial dimension suppressed.

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