Relativity is a theorem, formulated by Albert Einstein, which states that space and time are relative and all the motion must be relative to frame of reference. It is a notion that states, laws of physics are same everywhere. This theory is simple but hard to understand.

It states:

- There is no absolute reference frame. One can measure velocity if object or momentum is only in relation to other object.
- The speed of light is constant irrespective of who measures it or how fast the person measuring it, is moving.

Albert Einstein’s theory of Relativity encompass two theories namely Special Relativity Theory and General Relativity Theory.

## Special Theory Of Relativity

Einstein first introduced this term in the year 1905. It is a theorem that deals with the structure of space-time. Einstein explained this theory based on two postulates –

- The laws of physics are same for all irrespective of the velocity of the observer.
- Speed of light is always constant regardless the motion of light source or the motion of the observer.

This is the theory which laid the foundation to time travel. According to Einstein, the rate at which time tics decreases with the increase the velocity of the person. But this is hard to notice as the decrease in the time is relatively very low when compared to the increase in time. So, it can be assumed that if you can equal the velocity of light, you will be in a situation where time is still. This phenomenon is called Time Dilation. There are other surprising consequences of this theory such as –

- Relativity of simultaneity – two actions, simultaneous for one person may not be simultaneous for another person in relative motion.
- Length Shrinking: Objects are measured and appeared to be shorter in the direction that they are moving with respect to the observer.
- Mass – Energy Equivalence: Study of relativity lead to a one of the greatest inventions i.e., E = mc
^{2 }where E is Energy, m stands for mass and c for velocity of light. Many scientists observed that the mass of the object is increased with the velocity but never knew how to calculate it. This equations is the answer to their problem which explains that the increased relativistic weight of the object is equal to the kinetic energy divided by the square of speed of light.

## General Theory Of Relativity

General Relativity theory developed by Einstein in the year 1907-1915 states that being at rest in the gravitational field and accelerating are identical physically. For example, an observer can see ball fall the same way on the rocket and on Earth. This is due to the acceleration of the rocket which is equal to 9.8 m/s^{2}. This theory relates Newton’s gravitational theory and special relativity.

Some consequences of General Relativity are

- Gravitational Time Dilation: Gravity influences the passage of time. Clocks in the deeper gravitational wells run slower than in general gravitational levels.
- Light rays will bend in the gravitational field.
- The universe is expanding and parts of it are moving away from Earth faster than the speed of the light.

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#### Universal Law Of Gravitation

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#### Special Theory of Relativity

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#### Elementary Idea Of Relative Density

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#### What Is Zero Gravity?

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