This concept is important to understand, to understand one simple fact, that is, time is not constant. Just like the other three dimensions, time can be stretched, compressed, or bent at the mercy of the universe. Christopher Nolan, the director of movies like Interstellar, has always been praised for his brilliance in addition to scientific accuracy. In Interstellar, we watch Cooper spend about 2 years in outer-space to come back to see his young daughter Murphy to be an old woman. If you dismissed this incident considering it to be fiction, we are here to tell you that it isn’t entirely fiction. That is exactly what gravitational time dilation means!
To put it into simple words, as you get closer to the gravitational centre, or the “big ball”, slower your time goes in reference to a distant observer who is not under a gravitational influence. The curiosity might arise again, how does this happen? To understand this, we need to look at another concept. We need to understand that if there is truly an universal constant, it is the speed of light. Everything else is relative. To understand this, let us consider another analogy. Consider that you are standing on the road, and you see a car pass by at the speed of 40 kmph. As you are standing, your speed is 0 kmph. Thus, for you, the car’s speed is 40 kmph. Consider another situation where instead of standing on the road, you are in a car going at 30 kmph. A car overtakes you, whose speed is 40 kmph. However, for you, this car’s speed will be 40-30 that is 10 kmph. This is what relativity means. This example explains the fact that speed is relative, that is, it depends on where you are observing it from.
However, the above analogy does not hold true when we consider a photon, a particle of light. A photon is unique, such that it has no mass. Its speed in vacuum, 3,00,000 kmph will stay the same, irrespective of your reference frame, irrespective of where you’re looking at it from. Your speed could be 99% the speed of light, or you could be stationary, but light’s speed stays the same. Moreover, as long as you have mass, you can never achieve the speed of light. This is what it means when we say that the speed of light is constant and it is the universal limit, as no object with mass can go faster than light.
But… that doesn’t seem correct, right? If the speed of light is constant, how does it fit into the General theory of relativity? Well this is because, in such cases, time becomes relative! Speed cannot change according to reference point, so something has to change. In such cases, it is time. This whole discussion has been in an attempt to prove that time is not constant, instead relative. To give you real life examples to discuss this better, I am going to give you two instances where we see time dilation. Firstly, let us consider the Earth to be the gravitational centre. Atomic clocks, known for their definite accuracy, have been experimented upon. Placed at different altitudes, they have shown a difference in the passage of time. By nanoseconds, the one at a lower altitude runs slower than the one at a higher altitude. This is solid proof of gravitational time dilation, as lower altitudes mean that the clock is closer to the Earth, and that is why it is slowing down.