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Herzy

I've been thinking...

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So, as the title says, I've been thinking about black holes and how they might react to each other if they were to encounter one another.

Gravity causes tidal force, which means if you were to fall into a black hole (feet-first) your feet would feel more gravity then your head. That effect would be amplified when working with such great gravity that is a black hole and you would be stretched apart. 

A black hole is a black hole NOT because of its mass, but because of its density. Density is defined as mass/unit volume, which means if you add mass without increasing the unit volume a black hole will eventually form. The same logic applies in reverse, correct? If you were to increase the unit volume without increasing the mass the black hole would stop being a black hole, right?

Well, if that is true, then if you were to have two black holes close to each other the tidal forces would act on one-another untill they cease to be a black hole. The tidal forces would increase the unit volume, which will cause the gravity to not be great enough to be prevent light from escaping. When the moment happens, if it's possible, would it just be a massive explosion? So much energy and light had been captured by the black hole, so when it is released it should be very very violent.

There is some of my thinking, let me know if I went wrong somewhere. 

Man, space is cool.

Edited by Herzy
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When they merge, as per the LIGO detections, the mass is reduced but the loss is released as a gravity wave. Takes a lot of energy to get those gravity waves and it seems that the BH mass is the source - no idea of the mechanism. Think the first detection lost about 12 solar masses.

They merge more or less like 2 stars would merge, circle each other, get closer and closer then in they go to form one body.

Look up LIGO and Black Hole mergers, there are a few reasonable video models of such, the best I saw in a talk I cannot locate on the net. Whether correct or not there was a fair amount of distortion of space-time and not as you might expect.

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2 minutes ago, ronin said:

When they merge, as per the LIGO detections, the mass is reduced but the loss is released as a gravity wave.

So not so much loss as it were, but conversion from BH contents into gravitational waves ?

Does that mean that that's another way of extracting BH contents rather than just Hawking radiation ?

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33 minutes ago, ronin said:

When they merge, as per the LIGO detections, the mass is reduced but the loss is released as a gravity wave. Takes a lot of energy to get those gravity waves and it seems that the BH mass is the source - no idea of the mechanism. Think the first detection lost about 12 solar masses.

They merge more or less like 2 stars would merge, circle each other, get closer and closer then in they go to form one body.

Look up LIGO and Black Hole mergers, there are a few reasonable video models of such, the best I saw in a talk I cannot locate on the net. Whether correct or not there was a fair amount of distortion of space-time and not as you might expect.

Would tidal effects not increase the volume? I understand he LIGO data, but why wouldn't it stop being a black hole if the volume was increased but not the mass? You mentioned how some mass is lost in the process - that's even better. If both mass AND volume are decreasing the black hole should stop being a black hole, right?

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A black hole is made when a body's mass fits entirely within its event horizon. Once that happens, nothing can stop it continuing to shrink.

So the two black holes only need to get closer together than the diameter of their theoretical combined event horizon before they're destined to become one black hole. If they are stretched, at least a portion of each BH will combine, and the mass of that would probably draw back any mass that didn't make it.

 

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3 hours ago, Herzy said:

 

Gravity causes tidal force, which means if you were to fall into a black hole (feet-first) your feet would feel more gravity then your head. That effect would be amplified when working with such great gravity that is a black hole and you would be stretched apart. 

Man, space is cool.

You could fall into a supermassive black hole without coming to any immediate harm whatsoever. If however you were to fall feet first into a stellar-remnant sized black hole then you also wouldn't feel a thing but for a very different reason. In the latter case, the tidal forces would increase at such a rate that you would be ripped apart so quickly that there wouldn't be time enough for the nerve impulses from your feet to reach your brain!

I agree, space is cool. Black holes are also pretty cool as they provide a way for the curious to get their heads around GR.

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On 9/15/2016 at 14:44, Herzy said:

A black hole is a black hole NOT because of its mass, but because of its density

This is not right. Once a black hole has formed all the mass entering it eventually (quick for a small black hole slower for a large one) reaches the singularity and GR predicts an infinite density there as it does for the initial mass collapsing to form the black hole. The black hole is black as all "events" within the event horizon have light like future world lines that all terminate on the singularity.

Incidentally if you found yourself inside an event horizon and fired your space rockets to try to escape the black hole you would just speed up your demise as the longest proper time to the singularity is on the unaccelerated geodesic. Strange but true.

Regards Andrew

Edited by andrew s

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On 9/15/2016 at 16:13, Pippy said:

So not so much loss as it were, but conversion from BH contents into gravitational waves ?

Does that mean that that's another way of extracting BH contents rather than just Hawking radiation ?

Yes as per the gravitational waves you mention. I would phrase it as the conversion of BH energy into gravitational waves as energy (or equivalently mass), angular momentum and charge are the only independent properties a BH has.

Regards Andrew

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1 hour ago, andrew s said:

Yes as per the gravitational waves you mention. I would phrase it as the conversion of BH energy into gravitational waves as energy (or equivalently mass), angular momentum and charge are the only independent properties a BH has.

I was thinking of momentum rather than 'contents' as it were after I wrote the message, and wondered if anyone would correct my thinking at the time. Good on you Andrew :)

I guess it's not just angular momentum a BH has, but also linear momentum (movement through space, inertia) ?

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4 minutes ago, Pippy said:

I guess it's not just angular momentum a BH has, but also linear momentum (movement through space, inertia) ?

Yes, however the linear momentum is not intrinsic to the BH. 

Regards Andrew s

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9 minutes ago, andrew s said:

Yes, however the linear momentum is not intrinsic to the BH.

You'll have to elaborate on what you mean exactly for me.

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In the rest frame of the BH it is fully characterised by its mass, angular momentum and electric charge. Obviously by picking an inertial frame with a boost (velocity) relative the the rest frame of the BH it can have any linear momentum you like (as could you or I but that does not change us intrinsically).

Hope this helps Pippy.

Regards Andrew  

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4 minutes ago, andrew s said:

In the rest frame of the BH it is fully characterised by its mass, angular momentum and electric charge. Obviously by picking an inertial frame with a boost (velocity) relative the the rest frame of the BH it can have any linear momentum you like (as could you or I but that does not change us intrinsically).

So when you say 'rest frame', you mean what it basically is, it's independent make up ?

And so linear momentum doesn't change what it is, it's still the same no matter what it's relative speed through space is.

But then, does not an objects mass change depending on it's relative speed ? .. as per the closer you get towards the speed of light, the more mass you have (something I've not yet comprehended) ?

A thread about thinking ! .. how dangerous is that in world where we're suppose to be nothing but pawns lol

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Pippy, 

in "the rest frame" means that the observations are made when the observer has no motion relative to the thing being measured.

Momentum does change but it depends on the relative velocity.  If my car crashes into a wall at 30mph you could look at if the the car had zero momentum and the wall rushed into it or the other way around. 

In old speak the mass of an object increases as it relative speed increases in more modern speak its inertia goes up. Only its rest mass is intrinsic its relativistic mass/inertia depends on the relative velocity of the observer and the object.

This is all standard Special Relativity and available via google as I type too slow for a full presentation!!!!!

Regards Andrew

Edited by andrew s

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