Recent content by PeterDonis

This paper by Deser summarizes how the full nonlinear massless spin-2 QFT is derived, with classical GR as its classical limit: https://arxiv.org/pdf/gr-qc/0411023

This was thought to be the case when Feynman's book was published, but it turns out it's not. See below. If you're referring to the Feynman Lectures on Gravitation, while it's a good book, a lot of the material in it is out of date now, since it is based on lectures he gave at Caltech just at...

To be clear, we don't actually have evidence of an event horizon. We only have evidence of apparent horizons--very heuristically, regions where things fall in and nothing appears to come out. The only way to know for sure that these apparent horizons actually are (or more precisely are...

In the sense that we don't expect stress-energy to just disappear, that's true. However, it's also true that we do expect a black hole to be "made of" vacuum, not stress-energy--or at least that's one very common expectation. There are strong heuristic arguments indicating that, while the...

I suspect the enhancement algorithm made use of outside knowledge about what Earth looks like from space--for example, what clouds look like, what Africa looks like, etc--to help in making the enhanced image look realistic, even if not all of that information was in the original image.

And your clarifications showed that your usage was not correct, in the way I pointed out. In that spacetime diagram, an object "at rest" is not what you seem to be thinking. An object "at rest" in Kruskal coordinates has a worldline that's a vertical line in the diagram. In ordinary terms, such...

Yes. That's because that version of the photo was put through an enhancement algorithm to brighten the night side. Here's what it looked like before that was done: https://www.nasa.gov/image-detail/amf-art002e000193/ You can see them in the image I linked to above.

No. It's not even part of the spacetime. It's not. No, that's not correct. Look at a Kruskal diagram, as seen, for example, in this Insights article: https://www.physicsforums.com/insights/schwarzschild-geometry-part-3/ No flipping anywhere. No, that's not correct either. A particular...

Similar to the goal of Apollo 8, as compared to Apollo 11. (And to date myself, I watched Apollo 11 on TV as a preschooler--a small black and white TV. I still remember seeing how the astronauts hopped around on the Moon.)

@mfb summarized this in the OP of the thread.

Yes, it does.

I think you mean damping here, and yes, that is important, for the reason you give (and it's the reason why real accelerometers have damped springs).

But it's not what your description said. You said: That says that the accelerometer is measuring the acceleration of particle 2. But now you appear to be agreeing with me that it's measuring the acceleration of particle 1. That means the math you should be showing should be for ##\ddot{q}_1##.

And it looks like the TLI burn was successfully completed, so the astronauts are on their way to the Moon.

Moderator's note: Several off topic posts have been deleted. Please keep discussion on topic. Thanks.