system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is the 4th dimension something that we know actually exists? Or is it just a concept?
gpt
We have three spatial dimensions.

Length, Width, Height.

Using those three dimensions you can plot a spot in any space.

We do not know if there are more spatial dimensions, if there are, they are likely to be things we're not capable of observing.

What does it even mean to have a 4th spatial dimension, when you can plot any location in space using only three?

That's beyond the scope of eli5.

We have a temporal dimension as well.

We don't know exactly what this temporal dimension *is*.

We know what it feels like.  It feels like the past moving through the present to the future.

But Einstein turned that concept on its head when he declared the speed of light is constant.

That's also beyond the scope of an eli5.

Einstein called this Space-Time and it's the mathematical representation of both space and time as a singular entity, when you pull on one, the other is pushed.  When you push on one the other is pulled.  Space and Time are connected.  

But we don't know if time is an actual dimension, or if it's just the emergent property of having the ability to move through space while being constrained by the constant nature of the speed of light, which dictates that Space and Time *must* change, and that they change in ways that are correlated to one another.