What is it made of? Is it physical or just a concept? Why does mass bend it?
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spacetime is a system for defining how things exist. Its a coordinate system, in which you describe both where something is and when it will be there / was there. We use it because if you just try to describe where something is, you’re automatically wrong because everything is also moving through time. On earth it doesn’t matter because everything is where it is to everyone, but on large scales this is necessary.
its a concept and physical. It describes a way to describe reality, and it does work. But like most things, there are more ways you could define the same system. This is just the best one we have.
Mass bends it because that’s how our universe works. Its a fundamental property of reality. If there is a “why” then we don’t know what it is. or how to figure it out.
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