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Good question; I think the answer from general relativity is that it doesn't, and that those changes are propagated out locally at the speed of light. So it's a

Good question; I think the answer from general relativity is that it doesn't, and that those changes are propagated out locally at the speed of light. So it's a Newtonian fudge to have variables like "distance between two bodies" in the equations. Mass changes spacetime curvature, and spacetime curvature pushes masses around, back and forth in a grand dance!

lmarcos 5y ago
I definitely don't know much about general relativity, but isn't it yet another model/theory? A better one I bet, but one that still relies on information, so when we talk about "mass changes spacetime...", well my question remains: "how does the universe know, for instance, the mass of the sun in order for the universe to allow the deformation of spacetime that the sun causes?" I know it's probably not a rational question, but I used to ask that question to myself when I was a student.
- mhh__ 5y ago
  Moving to a field theoretic model is precisely what allows you to abstract away at least some of those questions. Space-time is distorted by energy, rather than just mass, which reduces the number of things the universe has to be prescient to. We can further eliminate some more prescience, by thinking in terms of density rather than mass: The laws of physics stated locally require only (say) a number and a field, rather than a pesky integral. "Space tells matter how to move, Matter tells space how to curve" And asking these questions is a good thing. I've been sitting down and really thinking about special relativity recently, it's fun going through old papers and seeing about how to derive the algebra in the most smugly experiment-less way.
- evanb 5y ago
  On of the (self-admitted) flaws in Newton's conception of gravity is that it's in terms of forces (or potentials) that act across large distances; it's part of his "hypotheses non fingo". One of the philosophically more pleasing things about GR is that it is local. But, of course, Newton's conception is a small-mass / low-velocity limit, so how can that be? GR says that the effect of stress/energy at a place x changes the metric at that place. But the metric is something made of derivatives, so the space in some small neighborhood (this is the local part) nearby gets deformed. That deformation is itself a form of stress, and so places in the neighborhood of x effect places THEIR neighborhoods and so on. So there's nothing built-in that's long-distance. Big long-distance effects are built up out of everybody talking to their immediate neighbors.
voakbasda 5y ago
And all of that back and forth can be represented mathematically as an n-body problem: https://en.wikipedia.org/wiki/N-body_problem https://en.wikipedia.org/wiki/N-body_problem

lmarcos 5y ago

I definitely don't know much about general relativity, but isn't it yet another model/theory? A better one I bet, but one that still relies on information, so when we talk about "mass changes spacetime...", well my question remains: "how does the universe know, for instance, the mass of the sun in order for the universe to allow the deformation of spacetime that the sun causes?" I know it's probably not a rational question, but I used to ask that question to myself when I was a student.

mhh__ 5y ago
Moving to a field theoretic model is precisely what allows you to abstract away at least some of those questions. Space-time is distorted by energy, rather than just mass, which reduces the number of things the universe has to be prescient to. We can further eliminate some more prescience, by thinking in terms of density rather than mass: The laws of physics stated locally require only (say) a number and a field, rather than a pesky integral. "Space tells matter how to move, Matter tells space how to curve" And asking these questions is a good thing. I've been sitting down and really thinking about special relativity recently, it's fun going through old papers and seeing about how to derive the algebra in the most smugly experiment-less way.
evanb 5y ago
On of the (self-admitted) flaws in Newton's conception of gravity is that it's in terms of forces (or potentials) that act across large distances; it's part of his "hypotheses non fingo". One of the philosophically more pleasing things about GR is that it is local. But, of course, Newton's conception is a small-mass / low-velocity limit, so how can that be? GR says that the effect of stress/energy at a place x changes the metric at that place. But the metric is something made of derivatives, so the space in some small neighborhood (this is the local part) nearby gets deformed. That deformation is itself a form of stress, and so places in the neighborhood of x effect places THEIR neighborhoods and so on. So there's nothing built-in that's long-distance. Big long-distance effects are built up out of everybody talking to their immediate neighbors.

voakbasda 5y ago

And all of that back and forth can be represented mathematically as an n-body problem: https://en.wikipedia.org/wiki/N-body_problem https://en.wikipedia.org/wiki/N-body_problem