Timeline for Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch?
Current License: CC BY-SA 4.0
20 events
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| Jul 29, 2021 at 16:11 | comment | added | David Hammen | Newtonian gravity implies that gravity propagates at an infinite speed. We now know that that is not the case, but it is a good model in the case of smallish masses (the Sun is a smallish mass) and largish distances (the distance between the Sun and Mercury is huge compared to the Sun's Schwarzschild radius). | |
| Jul 29, 2021 at 16:06 | comment | added | David Hammen | @Peter-ReinstateMonica General relativity has a concept of a static gravitational field. The Sun's static gravitation field has been there since the Sun first formed. It's also important to keep in mind that a scientific hypothesis that overturns a previous concept has to be consistent with that previous concept in areas where that previous concept was tested and validated. Newtonian gravity is a very good model. It misses the precession of Mercury by only 43 arc seconds per century. This is a very tiny amount. General relativity fully explains that 43 arc seconds per century precession. | |
| Jul 29, 2021 at 13:39 | comment | added | Peter - Reinstate Monica | @DavidHammen But allow me to request a follow-up clarification: Wouldn't then gravitation transmit certain information (namely about an object's position) faster than light? I'd be happy to ask a question over on physics SE but I'm afraid -- no, sure -- that I'm not understanding enough to even ask the question. | |
| Jul 29, 2021 at 13:29 | comment | added | Peter - Reinstate Monica | @DavidHammen That was a request for clarification rather than an expression of doubt ;-). Thanks. | |
| Jul 29, 2021 at 13:25 | comment | added | David Hammen | @Peter-ReinstateMonica Modeling gravity as having a speed of light lag delay is absolutely the worst thing one should do. | |
| Jul 29, 2021 at 12:43 | comment | added | Peter - Reinstate Monica | Are you saying there is a difference, for an object in relative motion, between the direction of "gravitational pull" and the direction from which an object's photons arrive at any given moment? Or in other words, between where an object appears to be gravitationally and where it appears to radiate from? (Assuming a perfect vacuum so that the photon speed is c.) | |
| Jan 5, 2019 at 11:14 | comment | added | Heopps | Wery interesting! @David Hammen could you tell the lags cancel each other because of closed trajectories (=orbits)? | |
| Jan 5, 2019 at 11:13 | comment | added | uhoh | Your edit caused me to re-read your answer, and to be thankful that I mostly "publish" in SE question posts rather than Phys. Lett. A ;-) However now I am again worried. I use this equation that you'd shown me earlier (the first-order post-Newtonian model of gravitation), but without giving any thought to "a relativistically-correct time scale". For Mercury it matches Horizons to 100 meters over one year, which is a good sign at least. Perhaps I should read more about Teph though. | |
| Jan 5, 2019 at 10:20 | history | edited | David Hammen | CC BY-SA 4.0 |
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| Jul 2, 2018 at 8:14 | comment | added | uhoh | I've cited you, hopefully not out of context. | |
| May 22, 2017 at 11:41 | comment | added | uhoh | The Physics SE question has two equations, the first has $\beta$ and $\gamma$ which are said to both be unity in GR. I don't really understand the implications, so I thought it safer to say "resembles". Anyway I'll give it a go sometime soon, thanks for the encouragement! | |
| May 22, 2017 at 11:35 | comment | added | David Hammen | Your previous reference used the Gaussian gravitational constant $k$ rather than $GM_E$ in the above. Otherwise, they're identical. | |
| May 22, 2017 at 10:41 | comment | added | uhoh | I just happened to see this recent question on GR and orbits. I don't know what it means, but that certainly resembles the approximation in Equation 1 in my previous link. Derivation of Post-Newtonian (PN) expression for acceleration in Schwarzschild geometry. | |
| May 12, 2017 at 3:58 | vote | accept | uhoh | ||
| May 12, 2017 at 3:58 | comment | added | uhoh | Thanks again for the comprehensive answer and history lesson to boot. As an aside, somewhat related though it's a different question. | |
| May 12, 2017 at 3:57 | comment | added | uhoh | OK it's morning now for me and I've given your answer a fresh read, and I understand just what you are saying, though I don't mean to suggest I understand much about GR. This reminds me of a read this some day paper I've been holding on to. After learning about modeling non-gravitational forces on comets for this answer I've been meaning to try out Equation 1 in lpi.usra.edu/books/CometsII/7009.pdf (linked in comments there). It seems the second line is such a correction. In the mean time, I wont use a lag. | |
| May 11, 2017 at 18:36 | history | edited | David Hammen | CC BY-SA 3.0 |
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| May 11, 2017 at 18:19 | comment | added | David Hammen | @uhoh - Adding a lag (and nothing else) will yield worse results (far worse results) than if you simply treat gravity as instantaneous. If you want to do GR right, you have to go whole hog. Well, almost whole hog. Except for very simple systems (e.g., a pair of neutron stars orbiting one another), nobody goes whole hog. They instead use some sort of post-Newtonian approximation or parameterized post-Newtonian formalism. | |
| May 11, 2017 at 17:43 | comment | added | uhoh | To double check, are you saying that if I calculate the force based on where the Sun and outer planets would have been, I'd get a worse result than if I treat gravity as instantaneous, or just that that's not the best, most correct way to do it? | |
| May 11, 2017 at 15:58 | history | answered | David Hammen | CC BY-SA 3.0 |