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 Sep 1st 2019, 04:49 PM #1 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 551 Eight minutes, an ordinary topic Long long ago, I read a report. I still remember that what it said is about as below: “Since light from the Sun needs about eight minutes to reach the Earth, so something happened in the Sun has nothing to do with the Earth within that eight minutes…” Vice versa. Assume the Earth emits a jet, this semi-tramp wonder if people will have to wait about eight minutes before they can see the Earth changes orbit? Thank you. Attached Thumbnails
 Sep 2nd 2019, 04:48 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 998 The eight minutes is due to the distance between the position of the event causing the change and the position of the object feeling the result of the change. So a jet emitted from the Earth will have an immediate effect at the Earth, since there is no distance difference. However, if I alter your scenario a bit: Consider a small asteroid orbiting the Sun The Sun burps out a huge mass ejection, at an angle to the orbit of the asteroid, The change in the distribution of mass will alter the orbit of the asteroid. However the orbit of the asteroid will not start to change until the "information" about the change in the distribution of mass reaches it. This information can not move faster than the speed of light, so for an asteroid orbiting at about the same distance from the sun as the Earth, its orbit will not start to change until about eight minutes after the mass ejection from the Sun. __________________ ~\o/~
 Sep 2nd 2019, 05:47 AM #3 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 443 Yes, gravity has a lagging effect. It's very important to understand this phenomenon for galaxy dynamics.
 Sep 5th 2019, 01:13 PM #4 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 551 Star Wars Episode 7 Scenario: If the Sun is suddenly eaten by the dark force (first order), will the Earth still turns in the orbit? If it does, Newton's law of gravity will not be applicable in eight minutes? Attached Thumbnails
 Sep 6th 2019, 02:12 AM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 998 Yes, If an observer, sitting high above the plane of the ecliptic, saw the Sun suddenly swallowed into "hyperspace" (such that its gravity no longer acts in this universe) then the Earth would appear to continue to orbit a non-existent mass, before suddenly flying off at a tangent about eight minutes after the loss of the Sun. __________________ ~\o/~
 Sep 6th 2019, 04:00 AM #6 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 443 You could probably model the shape of the gravity well using GR, but I wouldn't know how to do that.
 Sep 6th 2019, 04:58 AM #7 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 551 What's Newton's opinion? He is just in England...
 Sep 6th 2019, 08:59 AM #8 Senior Member     Join Date: Jun 2016 Location: England Posts: 998 Newton did not know about the universal speed limit of light. In his view gravity was instantaneous. It was exactly to address the gaps that had been found in Newton's theories (as more precise measurement techniques were developed) arising from such things as the cosmic speed limit that necessitated Einstein to re-formulate the problem. __________________ ~\o/~
 Sep 6th 2019, 03:40 PM #9 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 551 Thank you, bunnies. This semi-tramp even has no a decent text book of GR at hand... At the end of 2017 and the very beginning of 2018, I set out to learn GR myself, on the other hand, to read projection geometry. I felt that the philosophical thought of the latter is more extreme, then...
Sep 7th 2019, 10:53 AM   #10

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 Originally Posted by neila9876 This semi-tramp even has no a decent text book of GR at hand...
"Gravitation and Cosmology" by Weinberg is very good, but it's pretty heavy on the Math. I don't know about a Chinese version, but see here. Maybe you can find a used one.

-Dan
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