# Gravity and light speed

Wraithious
I have a question I have been wondering about lately ,somewhat abstract . Hopefully my description will be adequate . Here it is … The Earth revolves around the sun because of the suns gravitational pull and Earth's inertia . It takes about 8 minutes for sunlight to reach the Earth . What would happen if some cosmic force 'blinked' the sun out of existence .( the all powerful Q on Star trek made the sun disappear for some reason) Would the Earth continue its rotation for the 8 minutes with the sun no longer there ,or (because the sun was instantly gone ) would the Earth just continue in a straight line immediately ? Thanks for your responses

Gold Member
2022 Award
Summary: Gravitational pull and light speed ?

I have a question I have been wondering about lately ,somewhat abstract . Hopefully my description will be adequate . Here it is … The Earth revolves around the sun because of the suns gravitational pull and Earth's inertia . It takes about 8 minutes for sunlight to reach the Earth . What would happen if some cosmic force 'blinked' the sun out of existence .( the all powerful Q on Star trek made the sun disappear for some reason) Would the Earth continue its rotation for the 8 minutes with the sun no longer there ,or (because the sun was instantly gone ) would the Earth just continue in a straight line immediately ? Thanks for your responses
Given the caveat that such speculation is fairly useless because you are invoking magic, I think the answer to your real question is "yes, changes in gravity travel at the speed of light"

Homework Helper
A pithy answer would be to note that the laws of physics do not allow a mass to wink out of existence.

So instead, let us suppose that this supernatural being split the sun into two parts, sending one piece up "north" perpendicular to the plane of the ecliptic and the other piece "south" taking care to conserve both momentum and energy in the process.

Unless I am mistaken (entirely possible), the resulting change in the geometry of space-time would amount to a gravitational wave. It would propagate at the speed of light.

l0st and Dale
2022 Award
Summary: Gravitational pull and light speed ?

What would happen if some cosmic force 'blinked' the sun out of existence
It turns out not to be possible to describe this. It violates the conservation of energy, and the conservation of energy is baked into the equations of general relativity.
Would the Earth continue its rotation for the 8 minutes with the sun no longer there ,or (because the sun was instantly gone ) would the Earth just continue in a straight line immediately ?
As noted, you can't describe this situation. And it turns out to be really difficult to work out an experiment to measure (even in principle), the "speed of gravity". There's no way to neutralise mass the way you can neutralise electric charge, and the only way you can carry it away is at or below the speed of light.

That said, as phinds says, if you change the distribution of the mass in ways that change the gravitational field, those changes propagate as gravitational waves at light speed.

Wraithious
A pithy answer would be to note that the laws of physics do not allow a mass to wink out of existence.

So instead, let us suppose that this supernatural being split the sun into two parts, sending one piece up "north" perpendicular to the plane of the ecliptic and the other piece "south" taking care to conserve both momentum and energy in the process.

Unless I am mistaken (entirely possible), the resulting change in the geometry of space-time would amount to a gravitational wave. It would propagate at the speed of light.
I appreciate your response . I am aware that the sun could not magically disappear, it was theoretical . Clearly my question was not worded adequately

In my limited understanding, one thing Einstein's general relativity "fixed" compared to Newton's gravity was the propagation of gravitational effects at the speed of light. Interestingly, that (propagation speed) was already baked into Maxwell's equations for electrodynamics.

Wraithious
Thanks for your reply . And excuse my ignorance . That have been said ,I am fascinated with gravity and how it works . And since I did bring up 'Star trek' in my question ( yes ,I know it is fiction ) Is it even theoretically possible to have artificial gravity without spinning an object in zero gravity ? (the deck plating which creates Earth type gravity on that show )

2022 Award
I am aware that the sun could not magically disappear, it was theoretical .
I think you misunderstand. We aren't nitpicking for fun - the problem is deeper than that.

In Newtonian gravity, you can perfectly well describe the gravitational effects of the Sun just vanishing, even though there's no known way to make it happen. Gravity propagates instantly in this theory.

But we know that's wrong. So we need to go to general relativity. But we immediately run into a problem: GR simply cannot answer the question, even in a hypothetical way. You can write down a stress-energy tensor (the thing that acts as a source of gravity in GR, like mass does in Newtonian gravity) describing the Sun just vanishing. But the equations for the gravitational field will not solve given this source term - basically because they enforce local conservation of energy and the Sun vanishing violates that.

So the kind of half answer you are getting is the best we can do.

Dale
One way to look at the question you are asking is to rephrase it as, "If the laws of physics did not apply, what do the laws of physics say would happen?" I hope you see that there is no way to answer that question. Once you have decided that the laws of physics do not apply, you can make up anything you want.

2022 Award
Is it even theoretically possible to have artificial gravity without spinning an object in zero gravity ?
Taking this question literally, yes. For example a rocket under 1g thrust will feel exactly like being in the rocket sat on the launch pad. The key is acceleration - spinning provides this, but so does linear acceleration. This insight (Einstein's Equivalence Principle) was a key step in the development of general relativity.

But you asked about deck plating. The problem isn't generating gravity - any old lump of matter does that. The problem is generating it in sufficient strength, while not massing millions of tons. And stopping it from propagating downwards (so the deck below doesn't see the ceiling as the floor). I don't think we know any way to solve either of these problems.

Wraithious
thanks for your reply .So here is another question to wrap your mind around . 'Supposedly' people have seen alien aircraft . 99.9% of these people are probably nut jobs . Jerry Springer level nut jobs -but ,for the sake of argument ,that the low % actually have witnessed this ( Navy pilots- Walter Kronkite ,Jimmy Carter) people who are reputable. If this is true ( and I am not certain it is ) doesn't it just blow Einsteinian theory out of the water ? they likely did not come from Alpha Centauri ,and spend 4 light years traveling through space to visit us . If extraterrestrials indeed have visited (observed ) us -They did so defying all known physics. I want to know these physics

BWV
Reputable people claim to see Bigfoot as well. The fact that not every single claim of alien spacecraft sighting can be refuted does not provide evidence for aliens visiting earth. Physical evidence would be required - an artifact, body something like that.

russ_watters
Wraithious
who are the 'reputable ' people who have seen 'bigfoot '? I am not getting into an argument about this . I think that 99.9% of all people who have witnessed UFO sightings are complete bull. But there is that % that is not ,which leads me back to the physical nature of the universe

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