eli5 : can somebody explain the idea of “spacetime” for me?

Question

Hi
I’ve been studying pharmacy/medical sciences for all my adult life when i stumbled upon “a brief history of time” by stephen hawking
Anyway,for a person who hasn’t studied physics much..how does “spacetime” exist? I mean einstein abandoned the idea of absolute time
But what is spacetime? And how does time exist as a physical dimension? I mean it’s just something we measure as it passes
How does all this exist?

In: 3

Answer 1

So you got motion, gravity influences motion etc. For many years we considered motion as dynamics. Things move so as time progresses an object changes its position. So you have to think about functions that describe the motion. The path of the object the graph of the function is however a static thing. So many scientists had the idea that maybe motion can be expressed with something static like a path. We know an object starts at A and ends at B but what path it took. This is a more static view of motion rather than the dynamical view of Newtonian mechanics.

With Newton’s laws we in the most general case track how vectors of forces change over time, we might not be able to express the motion as a function so in general we dynamically track the vectors. Thats what we do when we have like 3 gravitationally attracted bodies. A 3 body problem, we simulate the thing and see how the vectors for forces and velocity change each second.

We use a 3D coordinate system for describing dynamics. The objects move around in our coordinate system. We track it as time progresses. Here comes Einstein and says what if instead of doing 3D dynamics we give our coordinate system a time axis. Its now a 4D coordinate system but we now just turned dynamics into static geometry. Yes static, nothing moves every motion is just some kind of curve or a straight line. We can now calculate speed and do coordinate transformations just by messing with angles.

Of course this geometry is not the good old friend euclidean geometry its hyperbolic. When we want to put gravity into this we even give it some curvature and trace our lines through that curved, hyperbolic spacetime. This is what spacetime is, its a coordinate system that allows us to treat motion as static geometry.