system
Given the following post, respond as if you're commenting on Reddit:
human
I've been trying to teach myself a little about time, but as someone with little knowledge to base my understanding off of, I am having a hard time understanding why time passes differently if you are standing on a mountain than if you are sitting in a valley. Could you explain this concept to me?
gpt
There's a few different ways to answer your question.

Firstly, you might be talking more about cognitive dissonance than lack of understanding. We tend to assume that time is universal and passes everywhere at the same rate, so when someone comes along and says that time doesn't work that way, we find it difficult to accept. This is because we're working backwards. The only reason we tend to think of time as universal is because it's a simple approximation that's *close* to true. Except in extreme environments, time dilation is such a small effect that it's difficult to even measure that it exists, and so it's simpler to think of time as universal. But we have to keep in mind that universal time is not the reality. It's best to have the understanding that, fundamentally, time is not universal, and ask why it *seems* to be universal sometimes rather than vice versa.

Secondly, we can talk about how we *know* this sort of thing is true. The proofs originally came from observations of light, and electromagnetism in general. Light shows unusual behaviour, in that the speed of light appears to be the same for all observers. This doesn't add up if you have a universal frame of reference for space, and universal time. You can come up with ad-hoc excuses for why light is weird, but none of these have held up to experimental tests. Instead, we have to come to the conclusion that the fundamental equations for motion and time are not what we used to think they were, and this gives us stuff like time dilation at high relative speeds or in a strong gravitational field.

But, thirdly, the question might be "how" or "why" does the universe have these fundamental equations of motion and time. And this is essentially getting into the realm of philosophy. Part of this is just accepting that the universe is more interesting than it appeared superficially (as in the first paragraph) - i.e. why shouldn't the universe be this way? But you can go much deeper than this: why should the universe follow any sort of rules at all? Even if we discover more fundamental laws behind our current understanding of physics, it won't really answer this question - it just pushes it back a level, and we still will be asking why the fundamentals of the universe are the way they are and not something else instead. It's not really something that can be answered with scientific methods.