It is popularly witted that only a handful number of physicists around the world completely know and understand Einstein’s Special theory of Relativity !

Most physicists are famous for ‘thought experiments’, - do u know why ? It’s because they really cannot verify their hypotheses through experimental observation - something that cannot be reproduced in lab , say - travelling with the speed of light !

Lemme get you all through a very interesting thought experiment of Einstein ! And this experiment is popularly known as the Twin Paradox ! Einstein said, if two clocks are perfectly synchronized and one clock was in motion and the other relatively stationary, then after some time the clocks go out of sync ! i.e., the clock in motion would be slower than the one at rest. This phenomenon was later detailed by another physicist Paul Langevin by which this experiment got its name.

But I would like to explain this in simple words and with a little bit of Physics and mathematics to be understood by everyone. Here we go, tighten your seat belts !

Let us consider a situation where in one pair of twins being born on earth ( so simple logic says to your mind that they are alike and grow in similar fashion - Yes, but the most concerned similarity between them for us is their ‘Age’). At once they are born, they are separated and one twin (say ‘A’) is boarding a spaceship for a galactic voyage and the other [wat else ? 'B' :) ] remains on earth. ( Old bollywood style :-) )

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If we assume that the Spaceship travels with the speed of 99.9 percent velocity of light (100% is never achievable - If achieved , Einstein’s Theory of Relativity fails at that point ! ), then the flow of time or the ‘ageing’ of the twin A on the spaceship reduces by a factor called ‘Ageing Factor ‘ - This factor is the reciprocal of Lorentz’s factor (please recall intermediate class physics )

Lorentz factor = 1 / [ Sqrt ( 1 - ( V ^ 2 / C ^ 2 ) ] —– (1)
So, Ageing factor , E = Sqrt ( 1 - ( V ^ 2 / C ^ 2 ) ] ———- (2)

Since we are travelling at 99.9 % velocity of light,
V = 0.999 C ————- (3)
where C is the Velocity of light = 2.997925 x 10 ^ 8 m/s
Now substitute (3) in (2),
Ageing Factor E = Sqrt [ 1 - ( 0.999 C ^ 2 / C ^ 2 ) ]
= Sqrt [ 1 - ( 0.999 ^ 2 )
= Sqrt [ 1 - 0.998001 ]
= Sqrt [ 0.001999 ]
= 0.0447101778 = 0.05 ( approx )

now , if we consider that the spaceship returns after 25 earth years, ( i.e., twin B is 25 yrs old) then the twin travelled on the spaceship would have aged by a factor of just 0.05 , i.e, 25 * 0.05 = 1.25 yrs ! He is still a baby :) ! Or if we take 25 years in the frame of reference of the travelling twin ‘A’ ( i.e ‘A’ returns to earth when he is 25 years of age ) then the earth would have aged 25/0.05 = 500 years ! Gosh ! He can’t see his fellow twin brother - rather what he will see is he has simply gained an extra 475 years of age which is needed on earth to see 500 years into the future ! Very simply, he has got into a time machine (‘Back to the Future’ fame :) ) Fast forward !

Now was that simple mathematic enough to understand the complex theory of Relativity ?

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