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What does it mean for spacetime to have density? I could say that a small sphere is denser than a large sphere because the distances between points are smaller
by MichaelBurge 6y ago
What does it mean for spacetime to have density?
I could say that a small sphere is denser than a large sphere because the distances between points are smaller on average, but that requires the spheres to be embedded in something with a distance metric(Euclidean 3-dimensional space).
What is the thing that spacetime is embedded in that provides a pointwise distance metric?
- Koshkin 6y agoThis can be (kinda) explained via the notion of space-time expansion. (This is an intrinsic phenomenon, having nothing to do with objects moving through space.) As the space-time expands, its density becomes lower. The (relative) change in density can be measured indirectly by measuring (the effects of) the expansion. A technical explanation of the expansion involves the inflaton field. (There's also energy density of the gravitational field.)
- jessriedel 6y agoThis may help: Given a spacetime manifold with metric and no matter that is flat an infinity, you can mathematically define the amount of curvature in it, which is directly related to how much energy in conventional forms (say, kinetic) that could be transferred to matter. Just as two merging black holes can emit gravitational waves, which have energy in this sense and escape to infinity, gravitational waves coming in from infinity can collapse into a black hole where no black hole previously existed.