First of all, sorry in advance for calling you a dummy for not understanding the ‘Interstellar’ movie but honestly isn’t that what we all feel when we immediately google ‘Interstellar ending explained’ right after the movie ended?
In this article, we are going to answer the concept of time dilation in the movie – why time was experienced differently by the characters? Why did Cooper’s children get old so fast when Cooper aged normally? Or was it that Cooper aged too slowly as his children aged normally?
To properly teach you how time dilation works, imagine that you’re in a rocket. The rocket booster is underneath you. So, from your perspective, the rocket is moving upwards. You are moving in space at the acceleration of 9.8 ms-2. If you had a scale under your feet, it would show your weight as if you were on Earth. This makes sense because, on Earth, the gravity force that applies to you is also 9.8 ms-2. If your rocket didn’t have a window to show the beauty of space, you might be thinking that you’re still on Earth.
What if you had a water gun and sprayed some water outside of your rocket? The water will bend downwards because the rocket is moving upwards. But what if you turn on a torchlight? Will the light bend downwards? Yes, it will. Torchlight works just like a water gun. Instead of shooting droplets, it constantly shoots photons, a massless particle that moves in waves – just like water. The bend might not be noticeable, but the light does bend.
Now let’s take you back to Earth. Switch on the torchlight. Will it bend? Yes, it will. Because if it doesn’t, it will break the principle of equivalence. If the force from the rocket acceleration bends the light, then why won’t the same amount of force of gravity do the same? So, now, point number one: we are establishing that gravity bends light.
Now I want you to draw two dots. Draw a straight line to connect those dots using a ruler. Quick, isn’t it? Now draw another two dots. Draw a curved line to connect those dots. This takes longer, doesn’t it? That’s how things move. It moves quicker from point A to point B in a straight line than a curved line because it goes a shorter amount of distance.
However, this rule doesn’t apply to light. Because, unlike other things, light is made of photons and photons are massless. The speed of light, when moving in a vacuum space, is constant no matter what.
I want you to imagine two pairs of dots: point A and point B and point X and point Y. The distance between A and B is the same as the distance from X and Y. Let’s say that light is moving from A to B. There are no objects between them so the light moves in a straight line from A to get to B. Easy peasy. Now, between X and Y, there is a huge planet between the points. So the strong gravity is bending the light. And the light has to take a long detour from X to get to Y.
Here’s the crazy part, the amount of time to get from A to B is SAME as the amount of time to get from X to Y! Light is crazy! So, point number two, we established that gravitational force doesn’t change the speed of light (in a vacuum space)!
Why? Because according to Einstein, gravity bends space as well! Due to the curving of space, for the speed of light to remain constant, time itself must pass slower in the gravitational field relative to time in space! In other words, to obey the constant speed of light, time has to pass faster so that X can reach Y at the same moment as A reaches B! Isn’t that crazy? Light is so powerful that even time itself has to obey! So, point number three, gravity bends space, which makes time move faster.
Let’s say that you’re on a planet that is orbiting a black hole. A black hole is a collapsed star with an immense gravitational pull. The gravitational force upon the planet and you makes you feel that time appears to be short to you. But to the people on Earth, with a weaker gravitational pull, time appears to be long to them. This is time dilation. This is the whole point of Interstellar. So, point number four: the stronger the gravity, the faster time passes.
Remember what I said about the principle of equivalence just now? If gravity can make time pass faster, why won’t acceleration do the same? Let’s say that you have a twin. You decided to explore space in a spaceship that can move at the speed of light. You explored space for five years. Five years, according to you, feels like a nanosecond. You were in space for just a brief moment. It’s too short if you would call that an ‘exploration’. But when you get back to Earth, your twin is already five years older than you. Creepy, huh? That’s the horror that Christopher Nolan is trying to highlight in Interstellar. So, point number five: acceleration can also make time move faster.
As a closure, let me give a disclaimer; this entry explains physics in the simplest way possible. The aim of this essay is not to make you an expert physicist overnight. It’s just to make sure you understand how the Interstellar movie works. Therefore, there is a lot of oversimplification in explaining time dilation. We are not physicists here, we all want to watch Interstellar and geek out about the awesomeness of the sci-fi element in this movie. This movie shows the great price you need to pay to explore the infinite vastness of the universe, which is the time with your loved ones on Earth. Because no matter how long your journey is, how fast you think you’re going, how many planets you discovered, your home is with your family, on Earth. This movie makes you feel humble. It makes you feel tiny, compared to the greatness of the universe. And just like how Nolan wrote it, love transcends time.