One of the most ground-breaking theories in physics which led us to the understanding of universe that we know of now is Time dilation. But, before we move on to that, lets brush up with some of our basic concepts. So, we all have studied in physics about Newtonian relativity which states that the speed of any body is measured relative to the observer’s frame of reference. In simple words, If you are sitting in a moving car with your friend, your friend appears to be stationary to you as you both are moving with same speed in same direction viz, you both are in same frame of reference. But, if a guy who is standing on the road sees you he will record your speed as the speed of the car. Also, if you throw a ball from this moving car, at a speed , say 20 and car is at speed say 40. Then in ideal case, the ball will have total speed of 60 when thrown.
Now, lets apply similar logic with a slightly different scenario. You are in a stationery car, and you start its headlights and the exact speed of light is measured and recorded. Now, the car is moved at a speed of say, 80 km/hr and the headlights are blinked and again the speed is measured. Now, Newton’s theory says that the speed of light that would be measured must be addition of speed of light and speed of car (Which is of course very minute as compared to speed of light) But, the results say otherwise. The speed of light remains constant and not even minute increase in speed is recorded. This concludes that speed of light is constant irrespective of frame of reference in which it moves.
The question is, does Time depend on the frame of reference of the observer? Is time different for people in different frames of references? And surprisingly enough, the answer is Yes. The Time passes differently depending upon your frame of reference. If you are sitting at rest. The time passes normally as your speed is zero. But, if you are moving at a certain speed, the time will pass slower than it will at rest. This thing that time passing differently for each person depending on their frame of reference is called Time Dilation. So, why don’t we observe it? Because it is minute when compared to the human speeds. To observe measurable time dilation, one would have to go close to the speed of light. This theory was put forth by Albert Einstein and then was worked upon by many other physicist all of them leading to prove it right.
But,why should we believe on the theory? Is there any proof? Thanks to Hafele–Keating experiment there is one now. So, the pair of scientists did one experiment to prove or rather check the validity of the theory. They took two highly accurate atomic clocks. One of the clocks was taken on jet plane and other was kept on ground. The Jet plane flew around with its maximum speed and then landed to ground after some time. Now, the time in both the clocks were recorded. And they were not the same. Even if the difference was very minute as the jet speed is nowhere near the speed of light, yet the difference was not zero. Which proved that Time Dilation occurs. The recordings said that the clock on the ground which was at rest recorded more time than the clock which was in the Jet.Proving that the time passed slower to the clock and the people in plane as compared to those on ground. Simply put, time will move slower if you are moving considered to when you are at rest.
Okay, so the theory was proved. But, how the hell does it happen? Is still a big question. Let’s get into it then. Consider a light beam constantly reflecting between two mirrors. Let’s consider distance between two mirrors d. so beam will pass 2d distance for each loop. Now let’s move this apparatus with speed say s. Now, as the whole apparatus is in motion in same direction, it is in same frame of reference so, the light beam will still continue to reflect and the observer which is in this frame of reference for ex. in the same car as the apparatus will see no change. But, according to a stationary observer, now the beam has to cover a slant path before getting reflected back by other mirror.Thus, the light beam covers an extra distance d’ before reflecting from other surface. So, total distance covered will be 2d’. Thus covering an extra distance d’-d during each reflection. Now, as we know that speed of light is constant irrespective of frame of reference or relative motion, but, the distance has increased from d to d’.So to keep the ratio distance/time i.e. speed constant, time must move slow for the apparatus with respect to the stationary observer seeing the apparatus.
So, basically the extra distance that has to be covered by the light beam will be accommodated by elongating the duration of a second. This is Time Dilation. The Time dilation will occur only as a reference to stationary observer and the people in same frame of reference as that of the apparatus will experience time as usual. Just like motion. All the people in the train appear to be at rest if you are in the train but, not so when you are out of it. When you are observing passengers from train station they appear to be moving faster than you. Similar case is for time just that time appears to move slower as it is in inverse proportion. This can also conclude that as we move closer to the speed of light, the time will get slower and slower and at the speed of light time will stop. And if we were able to travel faster than speed of light, we might reverse the time but, that is practically impossible.
This theory proved to be ground breaking and led Albert Einstein to his most popular theory of General relativity and then to Special relativity. The concept is now used in various applications, and is even considered while satellite launches and Space missions. We also now a days see some of its references in movies or Sci fi TV shows. Thus If an astronaut travels in a space ship close to speed of light, time will move slower to him as compared to his twin brother on ground. When the astronaut will return home, his twin brother would have been aged much more than him as his 10 years might have been 25 years of time on earth. Well, at least it explains Interstellar movie.