# Eight minutes, an ordinary topic

#### neila9876

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.

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#### Woody

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.

topsquark

#### benit13

Yes, gravity has a lagging effect. It's very important to understand this phenomenon for galaxy dynamics.

#### neila9876

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?

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#### Woody

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.

#### benit13

You could probably model the shape of the gravity well using GR, but I wouldn't know how to do that.

#### neila9876

What's Newton's opinion? He is just in England...

#### Woody

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.

#### neila9876

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...

#### topsquark

Forum Staff
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