system
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human
ELI5: What is "space time" and how does it "bend"?

I've heard the term "space time" thrown around, and that gravity "bends space-time" but how does that work?
gpt
Spacetime is exactly what is sounds like. You probably have an intuitive grasp of “space”. You also probably have an intuitive grasp of “time”. 

When you combine the two and look at *where* and object is at any given *moment*, you’re looking at the object’s path through spacetime. It’s pretty much just that. Spacetime is just the “position” (both in space and time) of events. 

So why is it important, and why do we combine them together? Well, it turns out that your movement through *space* affects time, and your movement through *time* affects space. 

For instance, when you stand still, you’re travelling directly through time, but not through space at all. 

When you start moving, you’re essentially “rotating” in spacetime, so part of your motion is through space now. 

This “rotation” so that part of your movement is through space actually means you move through time more slowly! (It’s not noticeable to you because you’re moving through space reallllly slowly, relative to light)

Things tend to follow straight paths in spacetime. You either keep moving forward through time, or forward through space, but you don’t usually turn unless you put effort into it. 

We’ve noticed that those straight paths “bend” around massive objects though. And every object “bends” the exact same way. It’s almost like they’re not bending at all, and instead they’re all following a straight path, just like you’d expect, and for some reason we see that straight path bending. 

It turns out the “bend” around massive objects can be perfectly described if you think of *spacetime itself* bending around massive objects, and everything just following a straight path in that bent spacetime. 

Why does spacetime bend around massive objects? We… don’t know. We’ve observed some effects that can be perfectly described and predicted by the math that you get if you assume spacetime bends a certain way. Black holes, gravitational lensing, gravitational waves, all stuff that we can predict perfectly by assuming spacetime bends. We know it *does* bend because of these perfect predictions. Why it bends? No clue. 

It’s possible we’ll come up with a deeper theory of reality that explains spacetime bending. As of now though, the best we can really do is say that it does.