Would Earth fly off into space if the Sun disappeared?

[Julius Ceasar]

This is a question about GR. If the sun hypothetically disappeared in an instant, would our planet Earth immediately fly off into space or, would it continue to orbit for the 8 or so minutes that the light continues to travel to the surface?

 

[DaveC426913]

This is a question about GR. If the sun hypothetically disappeared in an instant, would our planet Earth immediately fly off into space or, would it continue to orbit for the 8 or so minutes that the light continues to travel to the surface?

Caveat: In an Einsteinian universe, things can't disappear in an instant. Even if the Sun exploded, sending all its components into deep space, all those components still have their mass, and would continue to affect Earth until they're effectively out of range.

However, any changes to the local curvature of spacetime that occur because of the movement of mass will only propagate at the speed of light, so the Earth would experience a smooth decrease in the pull of the Sun - and those effects would indeed be eight minutes (or so) delayed from the event.

 

[Orodruin]

This is a question about GR. If the sun hypothetically disappeared in an instant, would our planet Earth immediately fly off into space or, would it continue to orbit for the 8 or so minutes that the light continues to travel to the surface?

It cannot just disappear. That would violate GR. Your question is therefore effectively asking what a theory predicts for a situation it predicts it is impossible.

 

[Julius Ceasar]

the Earth would experience a smooth decrease in the pull of the Sun

So if i imagine the Earth is on a string being swung around it would be 8 minutes of letting the string slip through my fingers?

 

[DaveC426913]

So if i imagine the Earth is on a string being swung around it would be 8 minutes of letting the string slip through my fingers?

No.
It would take eight minutes for your slackening of the string to propagate along the string. Earth would be oblivious during this time.

After eight minutes, the Earth would start curving away from its orbit - as the Sun's mass grew more remote - to eventually attain a straight line trajectory.

 

[pervect]

This is a question about GR. If the sun hypothetically disappeared in an instant, would our planet Earth immediately fly off into space or, would it continue to orbit for the 8 or so minutes that the light continues to travel to the surface?

General relativity makes no prediction as to what happens if the sun disappears, because it would be mathematically inconsistent with the theory to assume that it could.

 

[DaveC426913]

General relativity makes no prediction as to what happens if the sun disappears, because it would be mathematically inconsistent with the theory to assume that it could.

It cannot just disappear. That would violate GR. Your question is therefore effectively asking what a theory predicts for a situation it predicts it is impossible.

True, but the spirit of the OP's question remains intact. To-wit: as the Sun's local mass changes, its gravitational effect on the Earth is delayed by eight minutes (ish).

I've put the caveat about instant disappearance in post #2, In essence, altering the OP's question to conform to GR.

 

[Julius Ceasar]

Thanks Dave, the hypothetical is not the point and i wasn't looking for opinion either, I was asking what the rule of law is. Does GR say the string is released and the planet flys off or does it say the curved space decays from the source for 8min?

 

[DaveC426913]

Does GR say the string is released and the planet flys off or does it say the curved space decays from the source for 8min?

Neither. See post 5.

 

[Nugatory]

True, but the spirit of the OP's question remains intact. To-wit: as the Sun's local mass changes, its gravitational effect on the Earth is delayed by eight minutes (ish).

To preserve the spirit of the question, we have to agree to use the Newtonian theory in which gravity is a $1/r^2$ force.

Using Newtonian gravity, it is possible to string the words “sun”, “local mass”, and “changes” together as you want to produce the sensible question “as the sun’s local mass changes, how does its gravitational effect on Earth change?”. The answer to this sensible question is “immediately”, no eight-minute delay.

Using general relativity, we cannot string these words together to get a sensible question, so no way of recovering the spirit of the OP’s question.

 

[Orodruin]

True, but the spirit of the OP's question remains intact. To-wit: as the Sun's local mass changes, its gravitational effect on the Earth is delayed by eight minutes (ish).

I've put the caveat about instant disappearance in post #2, In essence, altering the OP's question to conform to GR.

The spirit of the OP’s question is debatable. I am afraid starting to put caveats and conditions om top will just serve to confuse the OP, as evidenced by OP’s replies and the likening of gravity to a string.

 

[Dale]

I was asking what the rule of law is.

The rules of GR forbid the scenario posed.

Unfortunately, to get a sensible answer you need to specify how the sun’s energy moves. It cannot disappear, so shall it move spherically outward at c or at some v<c, or shall it split into two halves and go in opposite directions, or what?

 

[Julius Ceasar]

Neither. See post 5.

slackening of the string to propagate along the string

so its not curved space, its a string

 

[Orodruin]

so its not curved space, its a string

If you are talking about gravity, no. Please read the answers from everybody else in this thread.

 

[Julius Ceasar]

The rules of GR forbid the scenario posed.

maybe the question should be why does GR forbid the question?

 

[Dale]

True, but the spirit of the OP's question remains intact. To-wit: as the Sun's local mass changes, its gravitational effect on the Earth is delayed by eight minutes (ish).

I am not sure what you mean by “local mass changes”, but the amount of mass cannot change locally either. I suspect that you mean “mass distribution” but that the OP will assume you mean “mass amount” and your well intentioned reading of the spirit of the question will lead to a misunderstanding.

 

[DaveC426913]

Using Newtonian gravity, it is possible to string the words “sun”, “local mass”, and “changes” together as you want to produce the sensible question “as the sun’s local mass changes, how does its gravitational effect on Earth change?”. The answer to this sensible question is “immediately”, no eight-minute delay.

No, the phrase still works in GR. "The Sun's mass changes" does not necessarily mean it changes in magnitude (say, by magically disappearing) - the mass can change in location (say, by exploding), altering its effect on Earth.I get what you guys are saying, I just don't see why the OP can't be guided toward the resolution to his question, which is do changes to the Sun affect Earth instantly, or are they delayed by 8 minutes? They are dleayed by eight minutes.

 

[Orodruin]

maybe the question should be why does GR forbid the question?

That one is easy. There is a local conservation law (in essence conservation of energy and momentum) in GR that the question violates.

 

[Julius Ceasar]

If you are talking about gravity, no. Please read the answers from everybody else in this thread.

I didnt say it was a string i was quoting Dave

 

[DaveC426913]

I am not sure what you mean by “local mass changes”, but the amount of mass cannot change locally either.

Currently, its mass is localized to a volume 800,000 miles in diameter. If it explodes, the mass in that volume decreases. Though the mass's effect is still within range of the Earth, the effect will diminish as time passes. Earth will follow a more linear path.

 

[Orodruin]

I get what you guys are saying, I just don't see why the OP can't be guided toward the resolution to his question, which is do changes to the Sun affect Earth instantly, or are they delayed by 8 minutes? They are dleayed by eight minutes.

This is incorrect. It depends on how the changes occur as eluded to by @Dale in #16. Apart from that these ”8 minutes” are highly dependent on simultaneity and observer definitions.

The OP’s responses in this thread should be sufficient to demonstrate why it is not a fruitful venture - it leads to more misconceptions than it resolves.

 

[DaveC426913]

I didnt say it was a string i was quoting Dave

What? No.

You introduced the string in post 4.

 

[Orodruin]

Earth will follow a more linear path.

The Earth is already on a geodesic. You cannot get more linear than that in curved spacetime.

 

[Orodruin]

Also note that the string is fundamentally different from gravity in GR.

 

[DaveC426913]

The Earth is already on a geodesic. You cannot get more linear than that in curved spacetime.

Fer cry'n' out loud - doesn't this thread have a B/I/A rating? OK, apparently correcting one misconception of the OP while preserving the gist doesn't seem to fly with y'all.
I concede.

OP: a suggestion: ask what would happen to Earth - and after how long - if the Sun exploded.

 

[Julius Ceasar]

no, when i asked if it was like a decay of curved space you said see post #5

No.
It would take eight minutes for your slackening of the string to propagate along the string

in post #5 you said it is a string

 

[Julius Ceasar]

I appreciate you seeing the spirit of the question and i think you're correct that there would be no effect on the Earth for 8mins, ty.

 

[DaveC426913]

in post #5 you said it is a string

See your post 4:

So if i imagine the Earth is on a string being swung around it would be 8 minutes of letting the string slip through my fingers?

And you're welcome.

 

[Orodruin]

OP: a suggestion: ask what would happen to Earth - and after how long - if the Sun exploded.

This will not help at all. As @Dale has already mentioned, he would have to specify how it explodes.

 

[Orodruin]

Fer cry'n' out loud - doesn't this thread have a B/I/A rating?

A B rating is no excuse for misleading OP.

 

[Julius Ceasar]

It all good Oro i am now aware the question is impossible in GR and will work on it. It was a high school level question so perhaps tone it down sunshine.

 

[Orodruin]

It all good Oro i am now aware the question is impossible in GR and will work on it. It was a high school level question so perhaps tone it down sunshine.

Regardless of level, do you want to learn something or just be told a story? This is the difference between science and popular science. Whereas learning science will tell you how the theories actually work and generally requires you to make more effort outside your comfort zone, popular science will tell you a story about science that comes with a million caveats you have no chance to catch as a layman. This is all good if you understand that and it is what you want. However, there are countless examples of people coming to these forums with the impression that they know science based on popularised accounts.

 

[Julius Ceasar]

I can appreciate that.

 

[fresh_42]

An object like the sun cannot go out of existence. But if it does (If the sun converted all its mass into energy in a second and all the energy is ejected from its poles perpendicular to Earth's orbit plane), it'll take 8:34 min to recognize, optical as well as gravitational since nothing is faster than the speed of light.

I do not see why there have been so many posts, and even less why this thread became personal.

Thread closed.