 ## Einstein’s special theory of relativity. A key to explore the cosmos.

### What is the special theory of relativity?

In 1905, A German-born theoretical physicist, Albert Einstein published the theory of special relativity (or special theory of relativity), which explains how to interpret the motion between different frames of reference i.e. inertial frame of reference and non-inertial frame of reference i.e. places that are moving at constant speeds relative to each other and places that are moving with variable speeds relative to each other respectively.

Einstein’s special theory of relativity was based on two key principles (postulates):

• The principle of relativity: The laws of physics don’t change, even for objects moving in inertial (constant speed) frames of reference.
• The principle of the speed of light: The speed of light (3,00,000 km/s) is the same for all observers, regardless of their motion relative to the light source.

Before Albert Einstein came into play, astronomers and astrophysicists understood the universe in terms of three laws of motion presented by Isaac Newton in 1686. These three laws are still present in our textbooks and we still use these laws to solve problems. These laws are:

1. Law of Inertia: Objects in motion (or at rest) remain in motion (or at rest) unless an external force imposes change. Also known as the first law of motion.
2. Law of momentum: Force is proportional to the rate of change of momentum. For a constant mass, force equals mass times acceleration i.e F=m×a. Also known as the second law of motion.
3. Law of action-reaction: For every action, there is an equal and opposite reaction. Also known as the third law of motion.

These laws suitable for the calculations in which speed of objects comparatively very lower than light but these laws fail to demonstrate the problems having objects moving very fast or with speed of light.

### Terms derived from this theory

Space-time continuum: Einstein’s theory of special relativity created a fundamental link between space and time. The universe can be viewed as having three space dimensions i.e up-down, left-right, forward-backward. And a 4-dimesional time dimension. This 4-dimensional space is referred to as the space-time continuum. Einstein further told that space and time both exist in a unified entity, there can be no significance for single space and time.

Energy-mass equation: One of the most famous equations in physics comes from the theory of special relativity. The equation i.e E = m×c² popularly known as the energy-mass equation. It shows that energy (E) and mass (m) are interchangeable, means “they are different forms of the same thing”. If the mass is somehow totally converted into energy, then quite a lot (infinite) energy liberated.

Note: This equation also demonstrates why atomic bombs and hydrogen bombs are so powerful. This equation also shows that mass increases with speed, which effectively puts a speed limit on how fast things can move in the universe. Simply put, the speed of light (c) is the fastest velocity at which an object can travel in a vacuum. As an object moves, its mass also increases. Near the speed of light, the mass is so high that it reaches infinity, and would require infinite energy to move it. The only reason light moves at the speed it does is because photons, the quantum particles that make up light, are massless.

Time dilation: It is one of the most important and interesting terms in the space-time continuum. If you move fast enough through space, the observations that you make about space and time differ somewhat from the observations of other people, who are moving at different speeds.

Time dilation, in the theory of special relativity i.e. slowing down of a clock as determined by an observer who is in relative motion with respect to that clock. In special relativity, an observer in inertial (non-accelerating) motion has a well-defined means of determining which events occur simultaneously with a given event. A second inertial observer, who is in relative motion with respect to the first, however, will disagree with the first observer regarding which events are simultaneous with that given event.

Note: Neither observer is wrong in this determination; rather, their disagreement merely reflects the fact that simultaneity is an observer-dependent notion in special relativity.

A notion of simultaneity is required in order to make a comparison of the rates of clocks carried by the two observers. If the first observer’s notion of simultaneity is used, it is found that the second observer’s clock runs slower than the first observer’s by a factor of √(1 − v²/c²), where v is the relative velocity of the observers and c is the speed of light.

Similarly, using the second observer’s notion of simultaneity, it is found that the first observer’s clock runs slower by the same factor. Thus, each inertial observer determines that all clocks in motion relative to that observer run slower than that observer’s own clock.

These are some key terms and principles derived from this theory. We will discuss these and some other terms briefly in the upcoming posts. If you have any suggestions and queries related to the post, let me know in the comment section below. Please do share this post to your circle to explore the knowledge you must know.

Thank you.

#### Anil Kashyap

I want to explore new things every single moment of life. Life motto: Learn and teach as much as you can until your last breath.