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I have always presumed that gravity was 100% curvature. But, after thinking a bit I found a contradiction of understanding.

If gravity is only curvature, then all particles should behave as light does, and flow along the lines of curvature. However, light is massless and the expectation is that even when light is slowed down in a medium, it would still beam along an apparently straight line.

In other words, light is both massless and high velocity. Gravity does not act differently on light when it is stopped or slowed than when it is at speed -- at least that is how I understand it.

Instead, gravity acts on light via a curvature component and on massive items by curvature and something else.

But I have never understood or even encountered the "something else" that gravity might be acting on things with. It seems like a force, and in fact I was taught at a very young age that gravity was not a Force but instead a Curvature as Newton's theory of gravity was swapped out for Einstein's.

Further, Einstein's theory was completely simplified, and in fact was taught to me as a PBS video. Any technical approach thereafter amounted to symbols on paper (math), which is fine. But it is also math: something that comes after physical intuition. E.g. I don't remember the math or equations. I remember the video. The video is wrong, and I need to re-base the intuition.

So the question is: what is, at a high, non-symbolic, conceptual level, Gravity in terms of components:

  • Curvature (applies to massive and massless objects)
  • ??? (Force) applies to massive objects

And, finally, why should the (Curvature) and the (??? Force) be considered the same thing and not something co-incident, like electricity and magnetism?

Here I am using "component" in the functional sense. A functional component of a force would be, in terms of this question, an aspect of the force that scales with the mass of an object, but is zero with respect to massless objects.

Or is it all curvature?

Chris
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2 Answers2

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The spatial part of the trajectory is sharply curved, but the trajectory in spacetime is much less.

If you throw a rock and it takes one second to rise and hit the ground, the parabola is sharply curved. The distance it travels is a few meters and the maximum height is a meter or so.

If you include time, the separation between the launch and landing is about $3 \cdot 10^8$ meters, and the deviation from a straight path is a meter or so. The radius of curvature is about a light year.

mmesser314
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So the question is: what is, at a high, non-symbolic, conceptual level, Gravity in terms of components:

Curvature (applies to massive and massless objects)

??? (Force) applies to massive objects

And, finally, why should the (Curvature) and the (??? Force) be considered the same thing and not something co-incident, like electricity and magnetism?

In general relativity there is only the curvature “component”. There is no additional “??? Force” that acts only on massive objects.

Of course, besides the curvature there are also things like fictitious forces. Those are artifacts of the coordinate system (the Christoffel symbols), not anything physical. And in any case, they apply to light as well as massive objects. So there is simply nothing in GR that fits the requirements of your “??? Force”.

Furthermore, since GR already predicts a wide range of gravitational phenomena for both massive objects and light, any such addition to GR would lead to a theory that would probably immediately be invalidated by existing experimental data.

Dale
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