B How does GR explain increase/decrease in speed?

[Jakaha]

OK, so let's then talk about KE in non inertial frames. (Still in Newtonian physics)

Suppose that you are in a non inertial frame, specifically, one that is uniformly accelerating in a straight line. In that non inertial frame, an inertial object starting at rest will accelerate, continuously gaining KE. Where did that KE come from?

In Newtonian physics we talk about forces as the agents of energy transfer.
That is why I am seeking the agent of energy transfer in GR where gravity is no longer a force.

 

[Dale]

In Newtonian physics we talk about forces as the agents of energy transfer.
That is why I am seeking the agent of energy transfer in GR where gravity is no longer a force.

Forces transfer momentum, not energy. They are not completely disconnected, but they are not the same.

Going back to the non inertial frame I asked about previously. Where does the energy come from?

 

[Jakaha]

In fact, both GR and Newtonian gravity say that the feather and the hammer will 'fall' at different rates.
It's just that people forget the feather and the hammer are not 'falling' to the moon.
Rather, the feather and the moon, or the hammer and the moon, are 'falling' to their common center of gravity.

 

[PeterDonis]

Think of the feather and its curvature of space-time.

The spacetime curvature produced by the feather is negligible. So is the spacetime curvature produced by the hammer. That is the basis for the GR prediction that I gave in my last post, which is certainly valid to any accuracy of measurement we are capable of now or in the foreseeable future.

If you insist on including the spacetime curvature due to the feather and hammer, then, as I said in an earlier post, there is no known exact solution in GR that describes the spacetime; you would have to solve it numerically. But there's no point to that if you haven't even gotten a good understanding of the simpler cases that we can solve exactly, like the case where the curvature produced by all bodies except one (the Moon in this case) is negligible. You don't seem to grasp that case yet, so talking about much more complicated cases that we can't even solve exactly is pointless.

 

[Jakaha]

Forces transfer momentum, not energy. They are not completely disconnected, but they are not the same.

Going back to the non inertial frame I asked about previously. Where does the energy come from?

The usual trick would be to imagine a fictitious force acting on the other object and imparting it KE.

As I wrote, in any model of physics without an absolute frame of reference, all velocity-derived attributes will be frame-dependent.

 

[PeterDonis]

And where did this PE come from?
Answer: the KE that the Big Bang provided when it separated the ball and the quasar by sending them their separate ways billions of years ago.

No, this answer is incorrect. The PE of the ball relative to the quasar only depends on its position relative to the quasar. It does not depend on how it got there. I could construct a ball on the spot and drop it towards the quasar, and it would have the same PE as a ball that had existed since the Big Bang.

Also, the concept of potential energy only applies in a static situation anyway--"static" meaning that the body which is the source of gravity, the quasar in this case, is at rest. The universe as a whole is not static, so the concept of "potential energy" in the universe as a whole, looking at how things evolved since the Big Bang, is not even well-defined.

 

[Jakaha]

The spacetime curvature produced by the feather is negligible. So is the spacetime curvature produced by the hammer. That is the basis for the GR prediction that I gave in my last post, which is certainly valid to any accuracy of measurement we are capable of now or in the foreseeable future.

If you insist on including the spacetime curvature due to the feather and hammer, then, as I said in an earlier post, there is no known exact solution in GR that describes the spacetime; you would have to solve it numerically. But there's no point to that if you haven't even gotten a good understanding of the simpler cases that we can solve exactly, like the case where the curvature produced by all bodies except one (the Moon in this case) is negligible. You don't seem to grasp that case yet, so talking about much more complicated cases that we can't even solve exactly is pointless.

Never mind the ad hominems.

I explained why both GR and Newtonian gravity predict that the feather and hammer will 'fall' at different rates towards their common center of gravity.
If you don't understand it, that's fine.

 

[PeterDonis]

I explained why both GR and Newtonian gravity predict that the feather and hammer will 'fall' at different rates towards their common center of gravity.

You "explained" something that, while it is true in principle, is negligible in practical terms and in any case irrelevant to the discussion in this thread. As I said, if we haven't come to a common understanding of the simplest possible case, where there is only one gravitating body and nothing else produces any curvature, then having a productive discussion about more complicated cases is pointless. You have repeatedly failed to address valid questions about the simpler case, so there's no point in continuing the discussion. Thread closed.