Gravity is not a force
Physicists and enthusiasts debate whether gravity should be seen as a traditional force or as the effect of curved spacetime, as described by general relativity. Many argue that free fall, accelerometer readings, and the universality of gravity point to a geometric, non-force interpretation, while others see this as largely semantic and prefer the intuitive Newtonian picture of a force acting on mass or energy. The exchange touches on implications for singularities, unification with other forces, pedagogy, and how different models remain experimentally equivalent in most everyday regimes.
Status of the “gravity is not a force” claim
- Several commenters note that “gravity as spacetime curvature, not a force” has been standard GR pedagogy and popularized for decades, not a new idea.
- Others stress that many working physicists still casually talk of a “gravitational force” in appropriate limits (Newtonian approximation), so the distinction is often treated as semantic in practice.
- There is disagreement on whether insisting “gravity is not a force” clarifies or confuses; some see it as deep insight, others as unhelpful dogma.
Accelerometers, free fall, and proper acceleration
- Core GR argument: a real force causes proper acceleration, which an accelerometer measures; free‑falling objects show zero proper acceleration, so no gravitational force acts on them.
- Standing on the ground, an accelerometer reads ~1g upward due to the normal (electromagnetic) force from the surface preventing free fall.
- Multiple subthreads dissect how phone accelerometers actually work and why they read ~g at rest and ~0 in free fall; confusion over sign conventions and “force vs acceleration” is common.
Geodesics, curvature, and orbits
- In GR, free particles follow geodesics—“straight lines” in curved spacetime. What looks like spatial acceleration (falling, orbiting) is just straight motion in a curved 4D geometry.
- A force, in this view, is what pushes you off a geodesic (rocket thrust, contact forces, EM forces, radiation reaction on charged particles).
- Some question why a particle “must move” along a geodesic rather than remain spatially at rest; answers appeal to least‑action principles and the fact that you always move forward in time, with curvature mixing time and space directions.
Forces, frames, and pseudoforces
- Relativity treats all inertial frames as equivalent; accelerated frames introduce pseudoforces (centrifugal, etc.).
- One side argues calling gravity a pseudoforce “privileges” a flat background and misleads; another side says we can equivalently model gravity as a real force in flat spacetime or as curvature, and both pictures are useful.
- There is extended debate over inertia, centrifugal force, and whether distinguishing “real” vs “pseudo” forces is meaningful or pedagogically harmful.
Singularities and limits of GR
- Singularities are widely cited as evidence GR is incomplete: where the math diverges, the model “bluescreens.”
- Some emphasize GR as a bulk/continuum theory, analogous to fluid mechanics: excellent at large scales, expected to fail at very small/strong‑field scales (inside black holes).
- Coordinate vs physical singularities (event horizon vs central singularity) and the role of horizons as mathematical vs physical entities are discussed.
Geometry vs force formulations and unification
- Commenters note that other interactions can also be cast geometrically (e.g., Kaluza–Klein–type constructions), so “being geometry” may not be unique to gravity.
- Others argue flat spacetime is mathematically special (unique zero curvature, measurable deviations), so calling “flat” privileged is not obviously wrong.
- A recurring theme: there are (at least) two equivalent descriptions—curved spacetime with no gravity force, or flat spacetime with a gravitational force field—and which is “fundamental” is unclear; many see the choice as largely semantic and pedagogical.