- #1
Karl Coryat
- 104
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- TL;DR Summary
- Asking how four-acceleration and four-momentum play into general relativity
I had a discussion with an engineer about forces and accelerations in the context of general relativity. My contention (which I confirmed in these forums some time ago) was that in GR, the real force and real acceleration at the Earth’s surface is upward, not downward. He contended that in physics, the direction of a force is defined by which system is changing momentum, and that if something is in free fall, its momentum is changing but the surface of the Earth's momentum is not — and therefore the force must be considered downward.
To me that sounds like an engineering definition, not a general definition. I suggested that the surface of the Earth's momentum is changing in a falling object's inertial reference frame, but that it’s four-momentum. Is that correct? In the inertial frame of, say, the center of the Earth, is the four-momentum of the surface perpetually changing, all around the planet’s perimeter?
Also, if the acceleration is considered upward from the surface in GR — it's obviously not a linear acceleration, so is that considered a four-acceleration, with the vector pointed away from the Earth's center?
Thank you!
To me that sounds like an engineering definition, not a general definition. I suggested that the surface of the Earth's momentum is changing in a falling object's inertial reference frame, but that it’s four-momentum. Is that correct? In the inertial frame of, say, the center of the Earth, is the four-momentum of the surface perpetually changing, all around the planet’s perimeter?
Also, if the acceleration is considered upward from the surface in GR — it's obviously not a linear acceleration, so is that considered a four-acceleration, with the vector pointed away from the Earth's center?
Thank you!