If the sun disappeared Earth would still orbit for 8 mins?

[Alfred001]

I heard a physicist say that it takes 8 mins for light to travel from the Sun to the Earth and that "gravitational information" travels slower than light and that therefore, if the Sun were to disappear, the Earth would continue to orbit around nothing for another 8 minutes.

Now, what is the explanation of this?

Is it that it takes that long for the bending of space time which the Sun had created to unbend? Is that the proper way to understand this?

Often people describe bending of space time like a bowling ball (Sun) resting on an outstretched sheet, so this would be like the depression in the sheet caused by the ball taking 8 minutes to straighten itself out.

Is this the right way to think of this or what's the proper explanation?

[StringJunky]

c is the maximum speed limit for information to travel, so until the information from spacetime changes, it will continue as though it is in orbit

[Alfred001]

But is information really the proper way to understand it? Gravity is not really about delivering information, it's not about the Earth being "told" something. "Information" is just a way of putting it, isn't it? I assume the actual underlying mechanism is different.

[StringJunky]

6 minutes ago, Alfred001 said:

But is information really the proper way to understand it? Gravity is not really about delivering information, it's not about the Earth being "told" something. "Information" is just a way of putting it, isn't it? I assume the actual underlying mechanism is different.

Yes, it is information. Mass-energy tells space-time how to bend and space-time tells mass-energy how to move. It takes some finite time for that to happen, which is because of c being the limit.

Edited August 6, 2018 by StringJunky

[Alfred001]

11 minutes ago, StringJunky said:

Yes, it is information. Mass-energy tells space-time how to bend and space-time tells mass-energy how to move. It takes some finite time for that to happen, which is because of c being the limit.

Is there something in physics that shows information transfer is the underlying mechanism? I mean, when I punch a hole in a boxing bag and it caves, what tells us information transfer is the ultimate underlying mechanism rather than just force acting upon matter?

[StringJunky]

5 minutes ago, Alfred001 said:

Is there something in physics that shows information transfer is the underlying mechanism? I mean, when I punch a hole in a boxing bag and it caves, what tells us information transfer is the ultimate underlying mechanism rather than just force acting upon matter?

Any action is information... I think. Maybe one of the physics guys will give a proper definition as I'm a bit vague on it myself.  

[beecee]

1 hour ago, Alfred001 said:

I heard a physicist say that it takes 8 mins for light to travel from the Sun to the Earth and that "gravitational information" travels slower than light and that therefore, if the Sun were to disappear, the Earth would continue to orbit around nothing for another 8 minutes.

Now, what is the explanation of this?

Is it that it takes that long for the bending of space time which the Sun had created to unbend? Is that the proper way to understand this?

Often people describe bending of space time like a bowling ball (Sun) resting on an outstretched sheet, so this would be like the depression in the sheet caused by the ball taking 8 minutes to straighten itself out.

Is this the right way to think of this or what's the proper explanation?

The earth would continue to orbit for 8.25 minutes if the Sun should magically disappear.

Analogies can be and obviously I believe are useful.eg; the rubber sheet analogy describing gravity. The danger is that all analogies have limitations and one can become confused if it is taken too far.  Another analogy illustrating the expansion of the universe is a container of dough with raisins and put in the oven to bake. The dough [spacetime] expands and the raisins [galaxies] are taken along for the ride so to speak.

[mathematic]

The earth will be in orbit as long as we can see the sun.

[zapatos]

The way it was explained to me was that gravity travels at c, just as we've seen with gravity waves. If the sun were to suddenly disappear, the geometry of space would necessarily change, with that change propagating outward from the location of the sun at c. Meaning we would continue to orbit until the change in geometry reached us.

[Markus Hanke]

9 hours ago, Alfred001 said:

Is it that it takes that long for the bending of space time which the Sun had created to unbend? Is that the proper way to understand this?

Essentially yes.

But you need to also remember that this is not really a physically valid question in the first place, because energy-momentum is a locally conserved quantity, so the sun cannot just disappear in the real world. But if it could, then this change would propagate at c, like all other information.

A more meaningful scenario would be to ask what happens if the distribution of mass changed (in the right ways), without anything disappearing - how long would it take for us to notice that? The answer is 8+ minutes. This is just gravitational radiation.

Note that the observations of gravitational waves we have made are consistent with them moving at c.