What is "Time"?

            In the modern globalized world every person keeps thinking about time. Time is very important for all of us now. A famous saying in English "time and tide waits for no one" proves its importance. So is it for physics lovers. Time is a very interesting concept in physics and is also a very beautiful concept. Let’s start with the simplest thing about time, its unit.

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            SI unit of time is second. What is one second? One second can be defined as the time taken by light to cover 299,792,458 meters. As speed of light is 299,792,458 m/s, the time taken to cover that distance will be obviously, 1 second. Will one second be same for everyone? A common person will say yes, one second is one second for all how can it vary from person to person. If you think the same way sadly it’s wrong. Time varies with your velocity. As you approach the speed of light, you will feel the time to have slowed down. Why?

            Before understanding it let us understand few other concepts.

• 1.     Speed of light in a vacuum is same for all (3 x 10^8 m/s), irrespective of the velocity the observer is travelling. In the sense if a person is travelling at velocity of let’s say 1.5 x 10^8 m/s (for example), he would still measure the speed of light to be 3 x 10^8 m/s and a person at rest will measure the velocity of light to be 3 x 10^8 m/s.
• 2.     Law of physics remains the same in all inertial frame of reference.
• 3.     Speed of light is a cosmic speed limit; it is not possible to go faster than speed of light.

          
           Now let us look at a situation from two different frames of reference. Let’s say a person is travelling at velocity of 1.5 x 10^8 m/s, with a photon clock. In a photon clock a ray of light keeps getting reflected perpendicular between to reflecting surfaces, let's say mirrors. 


            Case (i)
            Let's say that the person who is travelling sees the clock. He will find the clock to be stationary and the ray of light will be getting reflected perpendicular to the mirrors, as per him.

            Case (ii)

            Now let's say another person is standing and watches this person with the photon clock moving. How will he see the ray of light in the clock? Is the clock stationary as per him? The answer is no. The clock is moving therefore by the time the ray reaches from one mirror the other mirror would have moved so the diagram will be like this:

            It is obvious that the distance covered by ray in Case (ii) is greater than that it Case (i). We know that distance covered is directly proportional to time taken. Therefore the observer without the clock will feel that the moving person is aging slowly. This phenomena is called time dilation. This phenomena is negligible at small scale however the when the velocity is huge this phenomenon is observed clearly. This gave rise to an interesting paradox called twin paradox.

            Therefore we can say that one second is not the same for all. So our speed makes a huge difference at large scale. What are the other affects it has? We will discuss about it later.