As the tittle says, I am constantly hearing about spacetime, which I sort of get (it’s a 4D space, with 3 spatial and 1 temporal axis) and curvature, which I do not get. What is curved in spacetime? When we say geodesics, what are they representing? I am getting the feeling that it is something like the spatiotemporal distance between two events that is being modified, but what does it mean in physical terms? Is it even physical, since two observers can disagree in almost everything, except the order of casually linked events?
Or I am thinking it too much, and it’s only a model of interpreting observation that only approximates complex reality up to a point?
In: 5
You, as a being capable of visualizing 3 dimensions, can look at a curved sheet and see where how it curves downwards within a 3-dimensional space. However if you were a 2-dimensional being and you lived within that sheet, then you would only perceive it as “flat” as you moved “straight” across it. You would perceive yourself as continuing to move straight even as you moved down the curved section – because your perception of reality simply wouldn’t include any such concept as “up” or “down”.
The *concept* is that our 3-dimensional space can likewise be curved when viewed from a hypothetical outside view by a being capable of visualizing 4 physical dimensions. The only possible way for *us* to visualize it is to liken it to a two-dimensional example, though. We experience those curves as gravity, and we visualize them as being “pulled” and going “straight” towards the other object, but that hypothetical 4-dimensional observer would see us moving along a curve.
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