5 ms·

It's a pseudoforce in the same way centrifugal acceleration is "fictitious" It's a consequence of massive objects following geodesic motion on a curved space;

by nimish 5y ago

It's a pseudoforce in the same way centrifugal acceleration is "fictitious" It's a consequence of massive objects following geodesic motion on a curved space; e.g. To keep an ant on a sphere you would need "something" to ensure the ant's velocity doesn't lead the ant's position "off" the sphere. That something is interpreted as gravity. Here's a more technical explanation: https://physics.stackexchange.com/questions/212167/what-is-a-christoffel-symbol https://physics.stackexchange.com/questions/212167/what-is-a...

DangitBobby 5y ago
Interesting, thank you.
lanstin 5y ago
And massless. Light follows a geodesic in GR. Also i don’t think intrinsic curvature is well explained by the ant analogy. The apparent force isn’t to stay in the manifold but to stay on the geodesic thru space time (which is free fall).
- nimish 5y ago
  Yep, though massless particles must follow null geodesics specifically. Yeah, the ant analogy isn't the best, though it does show off that movement on a curved surface needs "corrections" from the movement on a flat surface (and the degree to which that occurs happens to entirely characterize curvature itself). If you force the ant to always travel on a great circle, then it might be a little more precise :)
  - ars 5y ago
    It's neither massive nor massless object that follow geodesics, it's objects with energy (including the mc^2 kind).
    - nimish 5y ago
      Yes, and "massive" and "massless" exhaustively covers both, with the special highlighting of massless particles which are further constrained to follow null geodesics. Massive particles don't have that limitation but they only travel time-like geodesics (as far as we know).
      - ars 5y ago
        > with the special highlighting of massless particles which are further constrained to follow null geodesics. Only to an approximation. Photons generate gravitational fields, and the planets they travel near are attracted to the photons (not much obviously). (See: Kugelblitz) Gravity does not treat photons specially, the rules are the same as for other particles. For extra credit imagine a Kugelblitz traveling by at light speed - it's moving so fast, it doesn't have time to add any mass. Yet, mass transited inside the event horizon of this object.

by nimish 5y ago

DangitBobby 5y ago

Interesting, thank you.

lanstin 5y ago

And massless. Light follows a geodesic in GR. Also i don’t think intrinsic curvature is well explained by the ant analogy. The apparent force isn’t to stay in the manifold but to stay on the geodesic thru space time (which is free fall).

nimish 5y ago
Yep, though massless particles must follow null geodesics specifically. Yeah, the ant analogy isn't the best, though it does show off that movement on a curved surface needs "corrections" from the movement on a flat surface (and the degree to which that occurs happens to entirely characterize curvature itself). If you force the ant to always travel on a great circle, then it might be a little more precise :)
- ars 5y ago
  It's neither massive nor massless object that follow geodesics, it's objects with energy (including the mc^2 kind).
  - nimish 5y ago
    Yes, and "massive" and "massless" exhaustively covers both, with the special highlighting of massless particles which are further constrained to follow null geodesics. Massive particles don't have that limitation but they only travel time-like geodesics (as far as we know).
    - ars 5y ago
      > with the special highlighting of massless particles which are further constrained to follow null geodesics. Only to an approximation. Photons generate gravitational fields, and the planets they travel near are attracted to the photons (not much obviously). (See: Kugelblitz) Gravity does not treat photons specially, the rules are the same as for other particles. For extra credit imagine a Kugelblitz traveling by at light speed - it's moving so fast, it doesn't have time to add any mass. Yet, mass transited inside the event horizon of this object.