# Celebrating Einstein’s century

Note4Students

From UPSC perspective, the following things are important :

Prelims level: Special theory of relativty

Mains level: Paper 3- Theories of relativity

#### Context

In 1921, the Nobel Prize Committee concluded that Einstein would have to wait and the Committee decided not to award the Prize to anyone in 1921. Opinions changed in a year and when Einstein did receive the 1921 Prize in 1922.

#### Background

• Noble Prize was not awarded for his theories of relativity but for “his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”.
• The citation harked back to the revolutionary theories that Einstein had established in 1905. ‘Annus Mirabilis’, or the Year of Miracles, is how 1905 is remembered by physicists because Einstein, only 26 then, published four remarkable papers that year.
• One of them explained that light was made of photons and when the light shone on metal, each photon’s energy correlated to the electron’s speed on the metal’s surface.
• This theory redefined the composition of light and Einstein himself dubbed it revolutionary.
• It was for this that he received the Nobel Prize.

#### Special theory of relativity

• The special theory of relativity was published in 1905.
• James Maxwell had established that light was an electromagnetic wave and the value of its speed was calculated. Building on this,
• Speed of light remains constant for all observers: Einstein understood that while moving from one frame of reference to another, which is moving at a different speed, the speed of light remains a constant.
• He gave a physical interpretation to the equations governing the transformation from one frame to another based on this fact.
• Time slows down when measured from the rest: Einstein’s theory establishes that time moves slower within a moving body when measured from a point at rest (but moves normally within the moving body itself).
• Length reduces: The length of the moving body contracts when measured from an outside point at rest.
• When a moving body emits light, the length contraction and time slowdown of the moving body are just exactly what are needed to restore the speed of light to its constant value.
• Einstein’s insight was that there was no absolute time because time was measured by the simultaneity of two events and this simultaneity would be observed differently.
• As lagniappe to the scientific community, Einstein published his famous mass-energy equivalence E=mc2 in late 1905.
• A mundane example of the application of the special theory of relativity is the use of GPS on our phones.

#### General theory of relativity

• The theory is general enough to apply to all forms of motion, including those where gravity does not appear.
• Einstein worked out equations using tensors, the mathematical implement to describe the transformation of different dimensions.
• In November 1915, Einstein completed the general theory of relativity.
• As per this theory, space and time form a continuum, like a fabric, and every object in the universe distorts this fabric, much like how dropping a large ball distorts a taut trampoline sheet.
• This distortion is gravity. It produces two effects.
• One, the fabric causes any other object in the vicinity to move towards the heavier object and this is why gravity causes an object to pull things towards it.
• Two, it bends light in the process of attracting it.

#### Conclusion

In just two decades, Einstein led physics out of its traditional moorings, laid the entablature of modern physics on Newtonian and Maxwellian pillars of classical physics and opened it up to newer questions.

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