It's even crazier because you don't need to reach the speed of light. It'll happen in a smaller degree for any speed. Even in mundane conditions.

For example, if your twin spent four days in a 300km/h bullet train, for you it would be four days plus a second.

Usually this difference is negligible, but for satellites (that run at rather high speeds, for a lot of time, and require precision), if you don't take time dilation into account they misbehave.

(For anyone wanting to mess with the maths, the formula is Δt' = Δt / √[1 - v²/c²]. Δt = variation of time for the observer (you), Δt' = variation of time for the moving entity (your twin), v = the moving entity's speed, c = speed of light. Just make sure that "v" and "c" use the same units.)

Here's something I just ran into looking stuff up for my comment: GN-z11 is one of the farthest galaxies we've ever seen. Thanks to the expansion of the universe, at a distance of over 30 billion light-years, it has to be moving away from us at over twice the speed of light.

What the fuck does that mean, temporally? Like, forget the speed of light, time dilation has to do with space and relative speeds. If I'm moving at near the speed of light relative to you, then my clock will physically tick more slowly. What happens if I'm moving over twice the speed of light? Is the real life GN-z11 in our reference frame moving backwards in time at over twice the rate we're moving forward?

The part that I understand in the intellectual sense, because I know or at least used to know how it follows from the math, but which just doesn't feel like it should be the case, is the whole "relativity of simultaneity" aspect of it. That there isn't an objectively true order in which events happen in, if the events in question aren't linked by cause and effect. That is to say, it is possible for one person to see an event A happen before another event B, a second person to see the two happen at exactly the same time, and a third to see event B happen first and then event A, and for all three of them to be equally right. It just feels like, on some level, there ought to be one objectively true order to time, a single valid timeline that all events can be placed in relative to eachother, and for time not to work that way feels so absurd as to not even be able to articulate why the idea feels wrong.

From what I understand, you are always travelling at the speed of light through space/time, but when you move at high speeds through space that shifts the proportion of your speed out of the time dimension. And a photon travels only through space, experiencing no time between the time it was emitted and the time it was absorbed. What I just can't wrap my head around is the concept of travelling at some speed without involving the time dimension at all.

Probably one of the most memorable and pivotal moments in my life was when my college professor showed us the origins of relativity and how Einstein came to the conclusion that E = mc^2

It's a proof that only took about 10 minutes to explain, and the mathematics really aren't that difficult to understand by most people. The geniuses in the fact that Einstein started by explaining how calculating relative motion meant that time had to be a variable that could be different depending on who the observer was. This in itself is an incredible observation, but you can take this to the extent to literally prove that mass and energy are directly related to each other. It's absolutely wild and one of the most sublime equations ever made.

I wish we could test this out with only simple apparatus. Unfortunately the common people do not have access to satellites or nonstop bullet trains.

Please dont do that