Speed of gravity?

[asbjos]

Hello, I have something I have wondered about, lately:

Theoretically, imagine that the Sun suddenly disappeared. Then, we wouldn't observe it before the light which should have arrived at Earth doesn't arrive, it would be total black. Of course, the speed of light would do so that we wouldn't observe the lack of sunlight it until 8 minutes and 20 seconds after the Sun disappeared.

But the lack of the gravitational pull from the Sun would in some sort affect us. I guess we would notice if the force keeping the Earth in orbit suddenly disappeared.

So, when would we feel the lack of attraction? Immediately, breaking the speed of light, would we feel it at the same time as the light disappeared (500 seconds after the happening) or would we feel it at any other time?

 

[garyw]

Gravity is information so it propagates at the speed of light.

http://www.astrowatch.net/2012/12/chinese-scientists-find-evidence-for.html

 

[Urwumpe]

Yes, but we needed some decades to prove that it really travels at the speed of light. It is not really the most common knowledge about our universe.

If the sun would suddenly disappear, the spacetime in the solar system would change - and that not only from the sun outward. We would get gravity waves, when the spacetime normalizes for the new situation.

 

[Quick_Nick]

This has basically been asked before.
Gravity propagates at the speed of light.
I'm not sure exactly what would happen at that moment, but I'm guessing we wouldn't feel anything force-wise since we already do not feel the centripital force of the Earth revolving around the sun. But of course we would freeze to death and such. Maybe one day an "xkcd what-if" will address that fate.

nnnnnnninja'd

 

[PeterRoss]

I'm reading 'A Briefer History Of Time' by Stephen Hawking and he tells about the same situation exactly that we will notice the absence of sun gravity immediately when the sun disappears, while its light will be reaching us for 8 minutes 20 seconds more.
Is this book little outdated? It's written in 2005.

 

[Urwumpe]

I'm reading 'A Briefer History Of Time' by Stephen Hawking and he tells about the same situation exactly that we will notice the absence of sun gravity immediately when the sun disappears, while its light will be reaching us for 8 minutes 20 seconds more.
Is this book little outdated? It's written in 2005.

Yes, seems so. The study "The detection of gravity waves" was written in 2000 and documented how the speed of gravity and the existence of gravity waves explains the observed trajectory of the two neutron stars.

Before Einsteins general theory of relativity, the speed of gravity was considered instantious and gravity waves don't exist. Einstein claimed they exist, now we have more and more evidence that they exist.

But Einsteins general theory of relativity also explains, why the trajectories of Earth wouldn't change, when the sun disappears, and also it predicts how it would change - not like Newtons theory of gravity at all.

If you simply assume Newtonian gravity with a limited speed of gravity, all planetary orbits as we observe them would be instable. So, this can't be as we observe something different. But it needed Einstein to find a solution that works with all the observations.

Maybe Hawking referred to Newtonian gravity there.

 

[PeterRoss]

:facepalm: Sorry, I found this part and it was said exactly about Newtonian model of gravity. This is what happens when you read books about science in a bus...

 

[n122vu]

So, this is my limited understanding, but this has to do with the 'elasticity' of space-time, for lack of a better term? It would take 8 minutes and 20 seconds Earth time for the space-time between here and the sun's current location to 'rebound' from the impression made by the sun?

 

[statickid]

They addressed this same idea in an astronomy class I took many years ago. I have limited interest in the debate because it has so little practical use. It's SO hypothetical that it begins to loose all meaning.

 

[Ghostrider]

Gravity is information so it propagates at the speed of light.

In a vacuum, yes. Information in suburbia or high school travels much faster than that.

 

[Eccentrus]

well it may have little to no practical use, but the hypothesis and its confirmation proves that information will never exceed the speed of light along the curves of spacetime itself, which can be useful for many other things.

 

[Urwumpe]

In a vacuum, yes. Information in suburbia or high school travels much faster than that.

Wrong. The proper theory is:

"Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws."

The information, that you badly need right now for example, travels at geological speed.

 

[Ghostrider]

"Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws."

I propose we rename warp drive and hyperdrive as "bad news drive".:thumbup:

 

[Urwumpe]

I propose we rename warp drive and hyperdrive as "bad news drive".:thumbup:

Since I am pretty sure, that activating a warp drive would mean really bad news to the universe around it, a proper name. :lol:

 

[4throck]

...I have limited interest in the debate because it has so little practical use. It's SO hypothetical that it begins to loose all meaning.

Not really.

The Earth orbits the Sun and that is not hypothetical.
That means that the gravitational interaction between both bodies is permanent, and subject to a 8 min delay.

I'd say that it has some meaning :thumbup:

 

[statickid]

There's no way to confirm the hypothesis. The very wording of the statement claims that we will receive no information until its too late to make the observation. Then the sun will be gone and it won't matter anyway.

@n122vu: in my also limited understanding I think it would be better to think of the gravity like an EM field. One difference I can think of is that elasticity implies a gradual fade in gravity. Wouldn't the gravity simply stop just as the light does? Otherwise even fading gravity would be a type of information.

 

[Urwumpe]

@n122vu: in my also limited understanding I think it would be better to think of the gravity like an EM field. One difference I can think of is that elasticity implies a gradual fade in gravity. Wouldn't the gravity simply stop just as the light does? Otherwise even fading gravity would be a type of information.

Well, you have to include there, that the gravity field is a potential. It doesn't simply change or stop abrupt.

The first change of the gravity field in the solar system would propagate at c. But the gravity force of the sun would not be suddenly off. Warping space takes a lot of energy and binds a lot of energy, which you can't suddenly remove - you need to redistribute this energy, which is what the gravity waves do.

Imagine you put a round empty tub into a lake, so that it displaces all water down to the ground.

When you remove the tub rapidly, the hole is not suddenly filled and the water everywhere suddenly lower. You get waves in the lake, that propagate the change. Of course, gravity isn't water - but the events have many parallels in them.

It could even happen, that suddenly removing the sun from the gravity field of the solar system could force the planets for a short instance closer to the sun. Such sudden changes are a pretty stress test to any theory.

In reality, you would have to remove the mass of the sun somehow. If you would assume, it is all turned into gamma ray* radiation, the photons of the radiation would still have effect on the gravity field, while propagating outward at the same speed as the changes of the gravity field propagate. The one star radius thick shell of photons would reach you with the first changes in the gravity field.

That event would be actually VERY close to the described event of the OP and... it does happen in reality. Some massive stars completely explode into hard radiation, instead of becoming a black hole.

* for old Heavy Metal fanatics: they released new single today BTW...

 

[statickid]

Yes remove the mass somehow. That is the part that is not included. I've seen the OP discussion before in both school and books and it always bothered me because I believe it over simplifies the question. The discussion asks what would happen if the sun simply disappeared without a trace. Converting the mass of the sun into a gamma burst makes sense to me. Having the sun disappear like the Heart of Gold makes no sense to me.

 

[Fizyk]

Theoretically, imagine that the Sun suddenly disappeared.

Fun fact: such a situation can't even be considered in general relativity, because it contradicts Einstein's equations. Those equations have conservation of energy built-in in themselves in a way, and sudden disappearing of the Sun would violate conservation of energy quite a bit.

So, if we want to know what would happen according to the best theory we have and try to apply general relativity, it throws an exception

 

[Keatah]

Even if a star exploded into gamma radiation, all that radiation, for the short time it was present near the former star's location would have its own gravity field. Of course this would dissipate.

Another way to put it, 2 sufficiently bright beams of radiation can be attracted to each other thus each altering the other's direction.