system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is space time, how can dimensions be fused, and how is time a dimension?
gpt
The dimensions aren't fused. Think about it in terms of degrees of freedom to move - generally there is an intuition that there are 3 dimensions. There is no "preferential" direction in physical terms - so all 3 are treated equally, which doesn't mean they are fused but simply recognizing that anything that happens in one dimension can also happen in the other identically.

The idea of spacetime is something that comes from Einstein's theory which has now been experimentally verified. Before Einstein, time was considered invariant - one second to you means one second to me or to anything else in the universe. Time passes the same for everyone and everything. With relativity proven, it is now clear that time is NOT an absolute or invariant and that how time passes varies depending on the relative speed of the measurer. This ties together motion in space with motion in time - no longer were these two independent - moving quickly in space means moving less quickly in time. 

From a mathematical viewpoint, time is no longer "invariant" and enters into the equations governing motion, therefore it is, conceptually, another dimension. Objects "move" through time as well as space.