Gravitation and effects from a distance

[Rjhere]

I am 90% sure that it's been asked before, but what the heck.

Let's say you have a object orbiting a black hole a light year away. Somehow the black hole disappears into nothingness, and the gravitational pull along with it. The object orbiting the black hole, would it continue orbiting for another year, or is it instant? I think that it will orbit for another year, but I'm not quite sure.

The reason for that is the theory of gravitational waves, just like the waves of sound or light(?) If gravitaiton is some kind of wave, it must be like light, wich cannot travel faster than light itself.

Am I correct on this, or is there something more to it. I know gravitation is a thing we don't understand yet, but I think this would be something scientist's knew.

Cheers!

[deadshot462]

Last I remember reading about this, there would be a 1-year delay before the orbiting body felt anything different.

[K^2]

Yes, but it's not quite so simple. You can't make a body like a black hole just disappear. It violates continuity equations. And anything consistent with continuity equations, like trying to yoink the black hole out really fast, results in a lot of other gravitational effects.

Edit: I can understand wanting to ask "Yeah, but what IF" questions, but in this case, the premise is, "Lets do the impossible. What will happen?" Well, that's impossible to predict. A false assumption results in entire chain of logic being broken.

[DaveofDefeat]

Gravity propagates at the speed of light or something near it, therefore if the sun disappeared you wouldn't know for 8 more minutes, and if a black hole disappeared a light year away, you wouldn't know for a year at which point you'll go flying off into space again.

Yes, but it's not quite so simple. You can't make a body like a black hole just disappear.

According to quantum mechanics there's a probability that you don't exist. Science is fun.

[K^2]

According to quantum mechanics there's a probability that you don't exist. Science is fun.

Yes, but QM also has continuity equations, which also say that things don't just disappear. There is also a bit where we don't have a theory of quantum gravity, so if you describe BH as a QM object, you're up a creek without a paddle. There is no model that predicts what would happen.

speed of light or something near it

Just to clarify - precisely the speed of light.

[SunJumper]

Apart from using negative mass, the only way to destroy a black hole is to wait it out. By the time the black hole is about to pop, the orbital velocity at any point will be negligible.

But yes, if you teleported a black hole elsewhere, it would take one year for a body orbiting one lightyear out to know (ie, be flung out)

[lajoswinkler]

It's not instant. Gravity propagates at c.

[Monkeh]

For some reason I always thought that it was instant. Spacetime bouncing back to flat once the mass is removed. But then, gravitons?

Head hurts. Lying down for a bit.

[Rjhere]

So my little "theory" was correct then, just wanted a bit of clarification, which i got. It all makes sense!

This forum is simply awesome. I love it!

[Drunken Hobo]

Am fairly sure it'd be the same for electric charge or magnetism, and somewhat easier to test. Have a large electromagnet or charged sphere and a detector a set distance away. Cut the power/ground the charge & check the time delay.

I suppose you could test it for gravity by using antimatter to annihilate the object, destroying its gravitational field. Complicated though.

As an aside, is gravity slowed down by anything? Light is fairly easily slowed down by air/glass etc, what about gravity?

[Monkeh]

Good question!

I suppose any wave that has to propagate through a substance as opposed to through a vacuum will slow down. Could we then, in fact, use some kind of shielding to create anti gravity?

We can create darkness by stopping photons in their tracks, next stop gravitynotness!

[Excalibur]

Slightly off-topic but still a gravitational question and probably a more plausible scenario;

You have a rogue rocky planet (say 1-5 Earth masses) in interstellar space, well away from any other significant gravitational influences. Orbiting a sufficient distance away (say 5 million km) is our spacecraft.

What would happen to the spacecraft's orbit if a relatively low-mass bolide struck the planet at such a high velocity that the rogue planet broke apart into a debris-cloud of mountain-sized fragments?

Would the spacecraft continue to orbit the CoM of the debris cloud? Or would the presumably now 'bumpy' gravitational field perturb the craft's orbit or even eject it away from the debris cloud? I imagine that such an impact would probably disperse the fragments considerably, thus complicating the scenario...

[Scotius]

For some reason I always thought that it was instant. Spacetime bouncing back to flat once the mass is removed. But then, gravitons?

Head hurts. Lying down for a bit.

Why not? Spacetime is bouncing back, but flattening propagates at the speed of light. Think about it like a tsunami spreading from the epicenter - you will feel it minutes/hours after everything settled down at the site of origin.

[Rjhere]

Slightly off-topic but still a gravitational question and probably a more plausible scenario;

You have a rogue rocky planet (say 1-5 Earth masses) in interstellar space, well away from any other significant gravitational influences. Orbiting a sufficient distance away (say 5 million km) is our spacecraft.

What would happen to the spacecraft's orbit if a relatively low-mass bolide struck the planet at such a high velocity that the rogue planet broke apart into a debris-cloud of mountain-sized fragments?

Would the spacecraft continue to orbit the CoM of the debris cloud? Or would the presumably now 'bumpy' gravitational field perturb the craft's orbit or even eject it away from the debris cloud? I imagine that such an impact would probably disperse the fragments considerably, thus complicating the scenario...

Probably would orbit the CoM for a bit, but later be struck by some debris. Or if lucky, the debris would just change it's course back and fourth because of the uneven gravity fields, eventually becoming so distant the debris will have minimal effect on it.

Could a good way of testing the gravity effect thing would be having an electric coil in vaccum, in a micro-g environment, put voltage over it, make something greatly affected by electromagnetism orbit the coil (seems ridiculous), and taking one of those light-speed cameras to film it when the coil shuts off?

My problems with my idea is that the magnetic field will variate, or distribute the field uneven maybe. Also the coil takes time to shut off too...

[K^2]

I suppose you could test it for gravity by using antimatter to annihilate the object, destroying its gravitational field. Complicated though.

No, you cannot. Annihilation does not reduce gravitational mass. Gravity is not caused by the invariant mass. It is caused by the stress-energy tensor, and energy is still there.

As an aside, is gravity slowed down by anything? Light is fairly easily slowed down by air/glass etc, what about gravity?

Speed of light isn't actually reduced by material. Propagation of light waves is. Similarly, you might be able to slow down propagation of gravitational waves, but not slow down gravity itself.

[hugix]

The black hole would disappear really slow. The orbit of the planet would slowly grow since there is less mass keeping the planet in it's current orbit. Eventually the planet will have such a big orbit the gravitational pull of the weaker getting black hole would be negligent and the planet would drift off.

It would not happen instantly or in a year time. It would happen during the evaporation of the hole.

and to all those talking about quantum theory, it has very little to do with this case. Yes it explains how the black hole is evaporating but the hole itself and the planet are wayyy to big to fit into the quantum world. So we can use normal physics here.

[magnemoe]

Has they been able to detect gravitation waves? know it has been plenty of tries.

One thought, as gravitation spreads with the speed of light the gravity from an planet is from where it look like it is, not from where it is?

Yes the effect will be very small, easier to measure how larger the sun is and how large and close the planet is, you want something who moves very fast and has a lot of gravity as it make the effect larger.

Two black holes in thigh orbit would be perfect.

[Ralathon]

Slightly off-topic but still a gravitational question and probably a more plausible scenario;

You have a rogue rocky planet (say 1-5 Earth masses) in interstellar space, well away from any other significant gravitational influences. Orbiting a sufficient distance away (say 5 million km) is our spacecraft.

What would happen to the spacecraft's orbit if a relatively low-mass bolide struck the planet at such a high velocity that the rogue planet broke apart into a debris-cloud of mountain-sized fragments?

Would the spacecraft continue to orbit the CoM of the debris cloud? Or would the presumably now 'bumpy' gravitational field perturb the craft's orbit or even eject it away from the debris cloud? I imagine that such an impact would probably disperse the fragments considerably, thus complicating the scenario...

It depends on how uniform the cloud is.

Gauss's theorem shows that the flux through a closed surface is equivalent to the divergence of the field enclosed by the surface.

In English, it means that a sphere with mass (or charge, or whatever) feels exactly the same as a point with the same mass (or charge). So if the debris cloud was uniform the gravity experienced by the spacecraft would be exactly the same as before.

What seems much more likely however, is that the impact event causes a plume of debris opposite of the impact side (exit wound style). This means the gravity field would start to deform considerably. It then depends on where the spacecraft was when the planet got hit. In most cases it ends up in a elliptic orbit that stabilizes itself as the planet starts to reform. It really depends on the mass of the planet, impact velocity, plume mass and all that fancy stuff.

[Ralathon]

Has they been able to detect gravitation waves? know it has been plenty of tries.

One thought, as gravitation spreads with the speed of light the gravity from an planet is from where it look like it is, not from where it is?

Yes the effect will be very small, easier to measure how larger the sun is and how large and close the planet is, you want something who moves very fast and has a lot of gravity as it make the effect larger.

Two black holes in thigh orbit would be perfect.

They haven't measured gravity waves directly yet (though there are experiments in that area ongoing). They did however observe 2 orbiting neutron stars and analyzed the orbit. It shows that the stars are spiraling towards each other, slowly shedding their orbital energy as gravitational waves. So we know they're there.

[-Velocity-]

They haven't measured gravity waves directly yet (though there are experiments in that area ongoing). They did however observe 2 orbiting neutron stars and analyzed the orbit. It shows that the stars are spiraling towards each other, slowly shedding their orbital energy as gravitational waves. So we know they're there.

The non-detection so far of gravity waves is actually predicted by theory. The reason we haven't detected gravity waves yet is almost certainly just chance luck. During the first run of LIGO, for example, theorists predicted only a 1 in 6 chance of detecting gravity waves, over a six year run of observations (at least, according to Wikipedia). The problem is, the gravity wave detectors we have constructed so far are only capable of observing only very rare, very "bright" transient gravity wave emission events, such as when two neutron stars or black holes merge, and at least in the case of neutron stars and stellar black holes, we can only detect relatively "nearby" events. So, given the short observation time, it would have actually been MORE surprising to have seen gravity waves already. We need to upgrade the sensitivity of gravity wave observatories (which is why LIGO is currently not making observations), or else keep them on for a longer period of time.

Edited October 2, 2013 by |Velocity|

[-Velocity-]

Am fairly sure it'd be the same for electric charge or magnetism, and somewhat easier to test. Have a large electromagnet or charged sphere and a detector a set distance away. Cut the power/ground the charge & check the time delay.

"Static" magnetic or electric fields are electromagnetic waves with a frequency of **nearly** 0 Hz (exactly 0 Hz in the case of the true static field), so of course they propagate at the speed of light. Of course, there is no such thing as truly static electric or magnetic fields, you have to create the field or put it in place, which adds non-zero frequency content. Something that is truly static is impossible, as the universe is finite in age.

When you "cut the power" to a static field, i.e., remove it, you will create a lot of high frequency content too- a true step function actually has infinitely high frequency content.

I suppose you could test it for gravity by using antimatter to annihilate the object, destroying its gravitational field. Complicated though.

That wouldn't destroy the gravitational field instantaneously. Instead, it would turn into an incredible amount of energy that would propagate outward at about the speed of light, probably (since it would probably be mostly gamma rays). But energy has mass, and thus gravity too, since Einstein showed us that energy and mass were one and the same (rest mass is just a very special form of super-dense energy). As an example, a fully charged, 150 AH, 12 V car battery should weigh like 72 ng more than an empty one (this tiny difference in mass will probably be swamped by the fact that car batteries are usually not sealed very well though).

As an aside, is gravity slowed down by anything? Light is fairly easily slowed down by air/glass etc, what about gravity?

If you could create a medium that responded to gravitational waves by shifting large, dense masses, maybe, but I'm not sure. Light slows down in many mediums because the electric charges in the medium respond and change their motion in response to the electric and magnetic fields of the light. So maybe the same kind of thing could be done for gravitation, but I'm not sure.

Edited October 2, 2013 by |Velocity|

[-Velocity-]

As an interesting aside, did you guys know that, viewed from a certain perspective, there really isn't such a thing as magnetism? It turns out you can derive all of Maxwell's equations just using Special Relativity (specifically, Lorentz length contraction) and Coulomb's Law. Wikipedia actually has a decent article on it now, http://en.wikipedia.org/wiki/Relativistic_electromagnetism, but there are a lot better websites out there that explain it better, with animations and that kind of stuff.

So really, magnetism is just an emergent phenomenon resulting from electric fields and the universe making sure that all frames of reference can agree that the same events are taking place in a reality where causality travels at the speed of light.

Edited October 2, 2013 by |Velocity|

[lajoswinkler]

More about that:

[Beowolf]

The last time I heard this explained, it was right on the edge of human knowledge and looked quite funky. The example I read in a peer-reviewed paper on the topic described the opposite of your example: A starship materializes in our solar system near Earth's orbit. Maybe it dropped out of hyperdrive? Anyway, gravity propagates at c as others have said, so it takes 8 minutes before the ship is affected by the sun's gravity.

And here's the funky part: Once this "connection" has been made, the sun is *NOT* pulling the ship towards where it was 8 minutes ago, but rather where it is right now. The point of this guy's paper was showing how, if that wasn't the case, Mercury wouldn't have a stable orbit.

[Starwaster]

The last time I heard this explained, it was right on the edge of human knowledge and looked quite funky. The example I read in a peer-reviewed paper on the topic described the opposite of your example: A starship materializes in our solar system near Earth's orbit. Maybe it dropped out of hyperdrive? Anyway, gravity propagates at c as others have said, so it takes 8 minutes before the ship is affected by the sun's gravity.

And here's the funky part: Once this "connection" has been made, the sun is *NOT* pulling the ship towards where it was 8 minutes ago, but rather where it is right now. The point of this guy's paper was showing how, if that wasn't the case, Mercury wouldn't have a stable orbit.

His paper is incorrect. It works the other way around. Gravity is present and affecting the ship when it 'materializes' because it propagated there long before the ship appeared on the scene. Propagation becomes relevant for determining what happens when the movement of a source of gravity carries it into range of what it's going to affect.