Gravitational difference between a black hole and a star

[Ranku]

A star or a planet is a material object, while a black hole is an 'immaterial' spacetime object. Does the material or 'immaterial' nature of an object make any difference in how it curves or travels through spacetime as it manifests gravitation (apart from the powerful gravitation near a black hole)?

For instance, would there be any difference in the dynamics of a star - star binary, a star - black hole binary, and black hole - black hole binary?

 

[.Scott]

The gravity generated by an object is due only to its mass - and to an extent, its rotation. No other characteristics (such as the chemistry or electrical conductivity) will affect the gravitation.

I'm not sure that "immaterial" is a good way to describe a black hole. But I understand what you're getting at.

 

[snorkack]

A black hole has no shape, does not deform and undergoes no friction.

 

[Ranku]

A black hole has no shape, does not deform and undergoes no friction.

So would having no friction make any difference in how a black hole travels in spacetime compared to a material object?

 

[unusually_wrong]

So would having no friction make any difference in how a black hole travels in spacetime compared to a material object?

What would be causing friction in a vacuum?

 

[stefan r]

A planet orbiting a stellar mass black hole at 1 au which has Earth's mass and a moon with lunar mass will experience the same tidal and gravitational forces that the Earth and Moon experience around the Sun.

What would be causing friction in a vacuum?

Earth and the moon are traveling through near vacuum. Ocean water has friction. Also Earth's mantle and atmosphere. Tidal acceleration is slowly pushing the moon out of orbit. Wikipedia:

friction is an essential part of tidal acceleration, and leads to permanent loss of energy from the dynamic system in the form of heat.

 

[snorkack]

A planet orbiting a stellar mass black hole at 1 au which has Earth's mass and a moon with lunar mass will experience the same tidal and gravitational forces that the Earth and Moon experience around the Sun.

Earth and the moon are traveling through near vacuum. Ocean water has friction. Also Earth's mantle and atmosphere. Tidal acceleration is slowly pushing the moon out of orbit. Wikipedia:

No, it will not experience the same tidal forces.
The black hole would deform Earth atmosphere and rock exactly the same as Sun does, and cause the same tidal friction there.
However, Sun is also deformed by gravity of Earth, causing tidal friction forces in Sun.
And a black hole would not be deformed by gravity of Earth the way Sun is. That part would be different.

 

[Droidriven]

No, it will not experience the same tidal forces.
The black hole would deform Earth atmosphere and rock exactly the same as Sun does, and cause the same tidal friction there.
However, Sun is also deformed by gravity of Earth, causing tidal friction forces in Sun.
And a black hole would not be deformed by gravity of Earth the way Sun is. That part would be different.

I see this as saying that the tidal force that the Earth exerts on the mass that it is orbiting would differ(the star or BH would feel a different effect) more than it says that the Earth would feel a difference.

This much mass is this much mass and this much gravity is this much gravity(with some difference due to surface gravity accelertion). Objects occupying distant orbits such as Earth compared to the Sun wouldn't notice a difference if the Sun were suddenly replaced by a BH of equal mass.

 

[Grinkle]

Objects occupying distant orbits such as Earth compared to the Sun wouldn't notice a difference if the Sun were suddenly replaced by a BH of equal mass.

I think that is true. I have seen that comparison used in lay-person-level discussions to explain why black holes don't just suck the whole universe into them. One might as well ask why our sun doesn't suck the whole universe into it.

As for whether hypothetical BH notice the Earth's gravitational pull more or less than the sun does, I don't think we have any theory to predict, but I may be wrong about that.

 

[stefan r]

I see this as saying that the tidal force that the Earth exerts on the mass that it is orbiting would differ(the star or BH would feel a different effect) more than it says that the Earth would feel a difference.

This much mass is this much mass and this much gravity is this much gravity(with some difference due to surface gravity accelertion). Objects occupying distant orbits such as Earth compared to the Sun wouldn't notice a difference if the Sun were suddenly replaced by a BH of equal mass.

Assuming that I understood you correctly then: 1) Earth's rotation would change because of the effect of the Moon's gravity creating tides and 2) the moon orbit and rotation would not be effected by Earth's gravity because it is tidally locked to Earth and only has solar tides.

#2 is not correct. The Sun is bulging slightly because of Earth's gravity and the sun rotates in less than a year. The bulge is leading and pulls Earth ahead into a higher orbit (some trivial amount greater than 0). Tides on Earth would fling the moon out of orbit eventually if other things did not happen first (oceans boil off and Sun expands to red giant etc).

 

[stefan r]

As for whether hypothetical BH notice the Earth's gravitational pull more or less than the sun does, I don't think we have any theory to predict, but I may be wrong about that.

Gravity is reciprocally proportional to distance squared. The surface of the Sun closest to Earth "notices" more gravity than the far side. A singularity would be all the same distance.

 

[Droidriven]

Assuming that I understood you correctly then: 1) Earth's rotation would change because of the effect of the Moon's gravity creating tides and 2) the moon orbit and rotation would not be effected by Earth's gravity because it is tidally locked to Earth and only has solar tides.

#2 is not correct. The Sun is bulging slightly because of Earth's gravity and the sun rotates in less than a year. The bulge is leading and pulls Earth ahead into a higher orbit (some trivial amount greater than 0). Tides on Earth would fling the moon out of orbit eventually if other things did not happen first (oceans boil off and Sun expands to red giant etc).

I said nothing about the Earth's effect on the Moon or its rotation, that isn't relavent to what I said.

 

[stefan r]

I said nothing about the Earth's effect on the Moon or its rotation, that isn't relavent to what I said.

The earth-moon system is relevant because we have a tool that we can use to measure it.

Objects occupying distant orbits such as Earth compared to the Sun wouldn't notice a difference if the Sun were suddenly replaced by a BH of equal mass.

What is different? Why would the tide caused by Earth in the Sun not accelerate the Earth? Granted it must be an extremely small number. Would be interesting to check whether Poynting-Robertson drag is greater.

 

[snorkack]

Gravity is reciprocally proportional to distance squared. The surface of the Sun closest to Earth "notices" more gravity than the far side. A singularity would be all the same distance.

Not precisely. The ring singularity of a rotating black hole would be at different distances from Earth.
But still, the ring singularity would not be subject to mechanical friction while rotating through Earth´s field of gravity, the way Sun is.

 

[T Y Thomas Jr]

A star or a planet is a material object, while a black hole is an 'immaterial' spacetime object. Does the material or 'immaterial' nature of an object make any difference in how it curves or travels through spacetime as it manifests gravitation (apart from the powerful gravitation near a black hole)?

For instance, would there be any difference in the dynamics of a star - star binary, a star - black hole binary, and black hole - black hole binary?

Do you know who decided that a black hole is not a material object?

 

[JMz]

Do you know who decided that a black hole is not a material object?

Decide is probably not the right word. I believe by material, @Ranku meant "composed of many bits of matter".

The point there isn't so much the "matter", it's the "many bits": That allows degrees of freedom for the components, which allows rearrangement of the components and the generation of heat, i.e., dissipation of energy. BH's have no components to rearrange and very little capability of dissipating energy -- basically, just gravitational waves and Hawking radiation. In contrast, material objects like the Sun can easily rearrange their components (atoms, free electrons) and quickly produce photons to carry away the heat.

 

[Grinkle]

gravitational waves and Hawking radiation

I didn't know that gravitational waves coming from a BH reduce the mass of the BH. If two BH's collide, the resulting mass I guess is smaller than the sum of the two? How does this reduction happen?

 

[nikkkom]

I didn't know that gravitational waves coming from a BH reduce the mass of the BH. If two BH's collide, the resulting mass I guess is smaller than the sum of the two? How does this reduction happen?

It happens very fast ;)

 

[JMz]

I didn't know that gravitational waves coming from a BH reduce the mass of the BH. If two BH's collide, the resulting mass I guess is smaller than the sum of the two? How does this reduction happen?

Example: When the 2 collided in the first LIGO detection, the combined ~64 M_S (Solar masses) radiated ~3 M_S away in ~ 0.1 sec. (As I recall, that's a significant fraction of the all the energy emitted in the entire observable universe during that 0.1 sec. :)

I'm not sure what you mean by, "How does this happen?" The laws of physics say that when 2 masses (not an isolated BH, notice) affect each other gravitationally, they generate gravitational waves that propagate away. These waves carry energy, so that leaves the system. I would make a similar statement about 2 interacting charges generating light waves: It's what the laws of physics say about this situation.

 

[stefan r]

...These waves carry energy, so that leaves the system...

You left out E = mc². Energy carried away means the black hole has less mass.

 

[JMz]

You left out E = mc². Energy carried away means the black hole has less mass.

Yes, I took it for granted that readers of PF would recognize that.

 

[Grinkle]

Yes, I took it for granted that readers of PF would recognize that.

I get that - anything else would, I suppose, be a free lunch.

I'll hazard re-phrasing. Almost always on PF, if I have to re-phrase even once, it means my initial question just makes no sense. Anyway ...

How can colliding BH's generate gravity waves if none of their mass / energy can't leave their event horizons? Hawking radiation has an elaborate (to me at least) explanation describing how the BH can lose mass. Is there a similar explanation for how colliding BH's lose mass/energy in order to generate the gravity wave?

 

[JMz]

Well, ... Chandrasekhar took many years to solve that very hard problem. So I wouldn't want to understate the subtleties. But you can get a feel for it just by watching water waves: Provide some disturbance in the middle of a pool, and the water will do many things. One thing in particular is, it will organize itself into coherent waves that propagate outward, and, in the far field, they will be pretty much independent of the details of the disturbance.

Notice that the water in the disturbed region doesn't leave that region. It's water in the neighborhood that responds and propagates the waves. So also, the part of spacetime that propagates the waves is what's in the neighborhood, not what's inside the BHs.

One vocabulary point, BTW: "Gravity waves" often refers to something else. (Those water waves in the pool, in fact.) Alas, we are saddled with the much more cumbersome term.

 

[skanskan]

The gravity generated by an object is due only to its mass - and to an extent, its rotation. No other characteristics (such as the chemistry or electrical conductivity) will affect the gravitation.

I'm not sure that "immaterial" is a good way to describe a black hole. But I understand what you're getting at.

Where can I read the basics about how the "rotation" affects the gravity?

 

[JMz]

Where can I read the basics about how the "rotation" affects the gravity?

See https://en.wikipedia.org/wiki/Lense–Thirring_precession (aka "the dragging of inertial frames", which may be more evocative). Not my favorite intro, but it will get you started. Paragraph #2 gives the connection to your question.

 

[JohnnyGui]

You left out E = mc². Energy carried away means the black hole has less mass.

Sorry if I'm not making any sense here but isn't it possible for a black hole to give its energy from its rotational kinetic energy to the gravitational waves instead of sacrificing its mass?

 

[JMz]

Sorry if I'm not making any sense here but isn't it possible for a black hole to give its energy from its rotational kinetic energy to the gravitational waves instead of sacrificing its mass?

I don't know the mechanism to radiate rotational energy, though it might be possible. But still, E = mc^2. Always. If any energy leaves the BH, it will have less mass afterward. Always.

 

[JohnnyGui]

I don't know the mechanism to radiate rotational energy, though it might be possible. But still, E = mc^2. Always. If any energy leaves the BH, it will have less mass afterward. Always.

What I meant is that the rotational kinetic energy will turn into the energy that creates the gravitational waves and makes the black hole rotate less fast. Just a ball dipping up and down in a pool of water. So it's basically energy turning into another kind of energy. How can losing this kinetic energy then reduce the mass? Or are you takling about the relativistic mass in this case?

 

[JMz]

What I meant is that the rotational kinetic energy will turn into the energy that creates the gravitational waves and makes the black hole rotate less fast. Just a ball dipping up and down in a pool of water. So it's basically energy turning into another kind of energy. How can losing this kinetic energy then reduce the mass? Or are you takling about the relativistic mass in this case?

Well, remember that there is no matter in a BH. If there ever was any, it has been crushed out of existence, long ago. When I refer to mass, I mean precisely E/c^2.

On your question about rotation, I am not aware of any mechanism to extract any particular fraction of the rotational energy (more properly, spin). There isn't much about a BH to serve as a "handle". My guess is that, in a BH merger, the resulting gravitational waves carry off some fraction of the spin energy, and that fraction depends on the orientation and speed of the merger but on nothing else. The pre-merger orbit will also add some spin to the merged BH that results.

 

[JohnnyGui]

Well, remember that there is no matter in a BH. If there ever was any, it has been crushed out of existence, long ago. When I refer to mass, I mean precisely E/c^2.

On your question about rotation, I am not aware of any mechanism to extract any particular fraction of the rotational energy (more properly, spin). There isn't much about a BH to serve as a "handle". My guess is that, in a BH merger, the resulting gravitational waves carry off some fraction of the spin energy, and that fraction depends on the orientation and speed of the merger but on nothing else. The pre-merger orbit will also add some spin to the merged BH that results.

Thanks a lot for your answer. From what I understand regarding matter, planets and stars lose some of their kinetic energy of their orbits and spin to the gravitational waves that they're making, which is for example the cause for binary stars to eventualy merge together. Is this correct?

 

[JMz]

Generally, stars are sufficiently extended that, even when touching, their centers are not close enough, so their orbits are not fast enough, for these waves to carry any significant energy away: When/if they merge, it is because of classical physics, especially friction.

The exception is neutron stars. But even there, a very close pair can take 100 MYr or more for the gravitational waves to carry away enough orbital energy for a merger.