Time slows down when you travel faster. How?

                                                                                           By Sudhir Rai 

Physics Department

What is time? For this question, we have numerous answers. And since we know that time plays a very significant role in the universe's structure. And despite so many years of investigation into the nature of time, there are still many unsolved issues, both philosophical and scientific.

There are many questions and theories that captivate a learner's mind and one of them is “time travel”, which is related to the special theory of relativity given by the world's renowned theoretical physicist Albert Einstein. He stated that time is variable and can be changed.

Before getting into the details, there are two important facts that are related to this theory:

1.    1. In 1636, Galileo Galilei's principle of relativity stated that it is impossible to determine by any mechanical experiment conducted in any inertial system whether the given system is at rest or moving uniformly and rectilinearly. OR, we can say that all steady motion is relative and cannot be detected without reference to an outside point.

EXAMPLE: Imagine you are on a train and you look outside the window and see trees, houses passing by. So, you know that you are moving in a particular direction. Galileo principle of relativity says that if all the windows were blocked and you couldn't see outside, you would have no way of knowing whether you are moving or are at a stationary position.

2.    2. The speed of light is constant. In 1887, two physicists Albert Michelson and Edward Morley conducted an experiment and concluded that it didn't matter whether the light travelled with or against the direction and rotation, its speed remained constant, i.e. (299,792,158m/sec).

Einstein's thought experiment:- In his vision that led to the theory of relativity, he visualized that if he travelled in a spaceship at the speed of light while looking in a mirror then, light bounces off you and travel to the mirror, where it is reflected back to you and you see your reflection. But he wondered that if the spaceship is travelling at the speed of light so the mirror too is travelling at the speed of light, then surely the light from you can't actually reach the mirror because it can't catch you. Then you will not be able to see your reflection and this would contradict Galileo’s principle of relativity, that if you could not see your reflection then you wouldn't know if you are travelling at the speed of light because you have no reference to anything outside the spaceship.

SO, WHAT HAPPENS???

Now, his second thought was maybe light leaves you at twice the speed so that it can catch up to mirror at the actual speed but, this was not possible because its speed is constant.

So, he thought something else must be changing.

Since,

Speed= distance ÷ time

Now, if speed is constant then it must be time and distance that are changing.

 

LIGHTCLOCK (an imaginative clock that works in a manner when light from the clock hits the mirror and bounces back to the clock and hits the sensor and let's assume one click is equivalent to 1 sec.)

NOW, imagine a light clock in a spaceship [notice: the direction of the light clock is perpendicular to the direction of the spaceship.] And now imagine the same light clock viewed by a stationary person outside the spaceship, watching the spaceship pass by.

The spaceship moves and the light clock emits light to the mirror. But when the spaceship moves along with the mirror, the light from the clock chases the mirror as the spaceship moves away from the light. When the spaceship was travelling as rapidly as the speed of light, the time observed on the clock stopped. The light could no longer reach the spaceship with the new time.

Therefore, time slows down as you travel faster because momentum bends the fabric of space-time causing time to pass slower.

Lastly, I want to conclude by saying that “what I love about science is that as you learn, you don't really get answers you just get better questions.” These words are written in the novel “Turtles all the way down" by an American author John Green. And personally, I got introduced to these inspiring words from an article written by Sai Prasanna Thapa.

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