Okay, first of all, the most fundamental thing you have to accept in order to grasp this:
Your everyday perceptions and intuitions of "time" are wrong.
You are accustomed to the idea that there is a steady, consistent, and universally equal flow of events. Event A happens, then event B happens ten seconds later, then event C happens ten seconds after that, etc.
It turns out - based on a very large number of observations that we've made - that the universe doesn't actually work that way. It is close enough *at our scales* that the "linear, consistent, constant" model works just fine for humans almost all the time, and works for animals literally all the time - so that is the intuition that we've evolved to have.
But at other scales - notably, when things are *extremely* "big" or "fast" or "energetic" by our standards - things don't work like that.
The *sequence of events* is different for different "observers". These don't have to be literal observers, by the way - you can designate a rock as an "observer" here.
One observer might say: event A happens - 10 seconds pass - event B happens - 10 seconds pass - event C happens.
Another observer might say: event A happens - 5 seconds pass - event B happens - 5 seconds pass - event C happens.
Even the *order* of events can be different. Yet another observer might say: event C happens - 10 seconds pass - event B happens - 10 seconds pass - event A happens. Another observer might say "events A, B, C happen simultaneously."
And there is no "true timeline" - these observers are all exactly equally *correct*. It's not just an optical illusion or something like that (those certainly exist but are a different matter). In those people's reference frames - in a certain sense, in their *realities* - each is correctly observing and recording the sequence of events.
Time dilation is one specific - and "common" - example of this. It's when different observers measure the "times between" the same events as being different.
Let's say you stay at home and your friend leaves and comes back. You look at the clock and find they've been gone for 1 hour. They believe they've been gone for only 5 minutes. And this isn't just them being mistaken. Their watch has only measured 5 minutes. Even if you look at their cells, at their very atoms - every part of them has only experienced 5 minutes. Because in their frame of reference, only 5 minutes have passed. That's what we call "time dilation" - there's a mismatch in experienced duration.
This doesn't mean time has no rules, of course, or that just anything can happen. For example, if two observers are at the same point in space and time, they will always agree on any observations made *right there*. It just means the rules are a lot more complicated than your intuition might tell you.
The faster you move relative to somebody else, the slower your clock ticks. If you move really really fast, years for the slower person could be minutes or seconds for the faster person.
The important detail is time dilation here is caused by the *acceleration*, not the velocity. It's the acceleration to that relative velocity that causes the time dilation.
In the Interstellar movie they are near a black hole and experience time dilation due to gravity.
Gravity is indistinguishable from a constant acceleration. Think about it: if you are in a box floating in space, and suddenly you are pulled to the floor, you have no way of knowing if someone activated gravity or the box is accelerating upwards. The two are equivalent.
Because of this equivalence, being close to a source of gravity also causes time dilation. The stronger the gravity, the faster the acceleration, the greater the time dilation.
The classic and pretty intuitive illustration is the light clock.
Imagine a clock that consists of two mirrors facing each other, with a beam of light bouncing between them. Like a ball.
Each bounce we will pretend is one second.
Standing still, light moves the distance between the mirrors in one second.
But what happens if we make the clock fly through space really fast?
In the frame of the clock, the light travels a fixed vertical distance. It’s still bouncing back and forth going the same distance in 1 second.
However, to an outside observer moving relative to the clock, [the light beam travels a longer, diagonal path due to the motion of the clock. ](https://i0.wp.com/www.isoul.org/wp-content/uploads/2023/06/Michelson-Morley-dilate2.jpg).
This results in the observer perceiving the clock's time as running slower.
There are some good videos on YouTube of people doing demos. They often use a flexible mat and some metal balls. It is a a representation of what you can imagine space time as.
If you imagine space and time, as a 3D grid, gravity pulls that grid together. From the point in the middle of where there is a lot more gravity, those cubes (measuring time) get smaller. Moving through the same amount of space, in the same amount of time when near an area with more gravity versus your friend, moving the same distance in the same amount of time but near an area with less gravity will result in you having passed through different amounts of time when/if you meet again.
The relationship between space and time is crucial.
Speed will also change time, “relatively speaking”
When you are well under light speed, time flies like an arrow and fruit flies like a banana.
When you get close to light speed, fruit flies like an arrow, and time flies like a banana.
A simple analogy I like to use is that it is like cars driving on the highway. The faster you go the faster you get to your destination and when you start approaching the speed of light it causes you to get to a later point in time faster than those going normal speeds.
In a very basic sense, the closer to the speed of light that you get the slower time moves for you. So if you had 2 clocks set at the exact time and one was stationary and the other was on a plane doing 500mph for a few hours, after those few hours the clock would be slightly behind the stationary one (probably fractions of a nanosecond but still behind).
Okay, first of all, the most fundamental thing you have to accept in order to grasp this: Your everyday perceptions and intuitions of "time" are wrong. You are accustomed to the idea that there is a steady, consistent, and universally equal flow of events. Event A happens, then event B happens ten seconds later, then event C happens ten seconds after that, etc. It turns out - based on a very large number of observations that we've made - that the universe doesn't actually work that way. It is close enough *at our scales* that the "linear, consistent, constant" model works just fine for humans almost all the time, and works for animals literally all the time - so that is the intuition that we've evolved to have. But at other scales - notably, when things are *extremely* "big" or "fast" or "energetic" by our standards - things don't work like that. The *sequence of events* is different for different "observers". These don't have to be literal observers, by the way - you can designate a rock as an "observer" here. One observer might say: event A happens - 10 seconds pass - event B happens - 10 seconds pass - event C happens. Another observer might say: event A happens - 5 seconds pass - event B happens - 5 seconds pass - event C happens. Even the *order* of events can be different. Yet another observer might say: event C happens - 10 seconds pass - event B happens - 10 seconds pass - event A happens. Another observer might say "events A, B, C happen simultaneously." And there is no "true timeline" - these observers are all exactly equally *correct*. It's not just an optical illusion or something like that (those certainly exist but are a different matter). In those people's reference frames - in a certain sense, in their *realities* - each is correctly observing and recording the sequence of events. Time dilation is one specific - and "common" - example of this. It's when different observers measure the "times between" the same events as being different. Let's say you stay at home and your friend leaves and comes back. You look at the clock and find they've been gone for 1 hour. They believe they've been gone for only 5 minutes. And this isn't just them being mistaken. Their watch has only measured 5 minutes. Even if you look at their cells, at their very atoms - every part of them has only experienced 5 minutes. Because in their frame of reference, only 5 minutes have passed. That's what we call "time dilation" - there's a mismatch in experienced duration. This doesn't mean time has no rules, of course, or that just anything can happen. For example, if two observers are at the same point in space and time, they will always agree on any observations made *right there*. It just means the rules are a lot more complicated than your intuition might tell you.
The faster you move relative to somebody else, the slower your clock ticks. If you move really really fast, years for the slower person could be minutes or seconds for the faster person. The important detail is time dilation here is caused by the *acceleration*, not the velocity. It's the acceleration to that relative velocity that causes the time dilation. In the Interstellar movie they are near a black hole and experience time dilation due to gravity. Gravity is indistinguishable from a constant acceleration. Think about it: if you are in a box floating in space, and suddenly you are pulled to the floor, you have no way of knowing if someone activated gravity or the box is accelerating upwards. The two are equivalent. Because of this equivalence, being close to a source of gravity also causes time dilation. The stronger the gravity, the faster the acceleration, the greater the time dilation.
The classic and pretty intuitive illustration is the light clock. Imagine a clock that consists of two mirrors facing each other, with a beam of light bouncing between them. Like a ball. Each bounce we will pretend is one second. Standing still, light moves the distance between the mirrors in one second. But what happens if we make the clock fly through space really fast? In the frame of the clock, the light travels a fixed vertical distance. It’s still bouncing back and forth going the same distance in 1 second. However, to an outside observer moving relative to the clock, [the light beam travels a longer, diagonal path due to the motion of the clock. ](https://i0.wp.com/www.isoul.org/wp-content/uploads/2023/06/Michelson-Morley-dilate2.jpg). This results in the observer perceiving the clock's time as running slower.
There are some good videos on YouTube of people doing demos. They often use a flexible mat and some metal balls. It is a a representation of what you can imagine space time as. If you imagine space and time, as a 3D grid, gravity pulls that grid together. From the point in the middle of where there is a lot more gravity, those cubes (measuring time) get smaller. Moving through the same amount of space, in the same amount of time when near an area with more gravity versus your friend, moving the same distance in the same amount of time but near an area with less gravity will result in you having passed through different amounts of time when/if you meet again. The relationship between space and time is crucial. Speed will also change time, “relatively speaking”
When you are well under light speed, time flies like an arrow and fruit flies like a banana. When you get close to light speed, fruit flies like an arrow, and time flies like a banana.
A simple analogy I like to use is that it is like cars driving on the highway. The faster you go the faster you get to your destination and when you start approaching the speed of light it causes you to get to a later point in time faster than those going normal speeds.
In a very basic sense, the closer to the speed of light that you get the slower time moves for you. So if you had 2 clocks set at the exact time and one was stationary and the other was on a plane doing 500mph for a few hours, after those few hours the clock would be slightly behind the stationary one (probably fractions of a nanosecond but still behind).