What is it made of? Is it physical or just a concept? Why does mass bend it?
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> What is it made of?
It’s not made out of everything.
> Is it physical or just a concept?
The difference between these two is just a philosophical question.
> Why does mass bend it?
Because that’s how our universe works.
If you ignore the time part, it’s just space. You have some position, I have some position, everyone else has their own position. That is something we can measure. We can measure distances between positions, angles, and so on. We can also specify when something happens. If we want to meet, we’ll need to say where and when. That defines one “place” in space and time together.
We learned that treating space and time completely separately doesn’t work. We describe both together as spacetime. The meeting point is now one point in spacetime. We can measure distances between points in spacetime, too.
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.
I don’t see this in the other answers, so I will provide some extra context on spacetime. The concept of spacetime had been around previously, but it was formalized with Einstein theory of General Relativity. In essence Einstein said that the two are inexorably related. Einstein realized that massive object bend space and therefore objects traveling through that space. The gravity of massive objects will also bend light. Because the speed of light is constant, it must mean that time has to be bent as well, and equally so.
There have since been proof that extremely massive objects do bend light as the light travels around it, we call that gravitational lensing.
Space-time is an operating system on which the universe is running, the system is not rigid but malleable, denser in some places in others – relaxed/flat, still it has laws, physical constants and hard limits that allow matter and fundamental forces to interact with one another in an extremely specific way.
To answer what space-time is made of you’ve got to wrap your mind around the concept of probabilities and most importantly how they are not subject to relativistic effects i.e. probabilities work within the universe but they don’t need it to exist, this leads one to think they must be more fundamental than space-time. Another important concept to consider is how probabilities can form interdependent variable systems and by doing so become self-contained objects that appear to have qualities different than the sum of their parts when in relation to other similar objects – these would be particles.
Matter, energy, information – these are all interchangeable, where you have high concentrations thereof you naturally have denser space-time as a direct consequence because everything is made of the same stuff really.
Space-time is not physical in the usual sense in that in order to understand what it is you’ve got to measure it from outside of space-time, nor is it just a concept people use to collectively refer to effects they measure, it’s like the inside of a bubble.
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