Gravity and black holes....is this right ?

[buzzc150]

Gravity is purely dependant on the mass of the object that creates it. The more massive an object, the greater the gravity well it creates. Correct ? Does then the mass of an object increase if you compact it down ? For instance, if I took a planet, squashed it down to half it's original size without losing any of its mass, will it produce a greater gravitational pull ? To my mind, (and I'm no expert) the answer is no.

The reason I ask this is because I was thinking about black holes the other days whilst on the loo. As we know a black hole is a collapsed star. Why would it be that the black hole, which has no more mass than the star that created it (at least initially), have such a stronger gravitational pull ?

[Starlight 1]

When the Star collapse and shrunk to a critical size, Know as a Schwarzschild. Not all black holes are rotating. not all black holes are not entirely black "Hawking radiation.

Something to think about next time you go.!!!!!

[Alien 13]

I allways thought the size was important too as to whether the gravity well was shallow and broad or deep and narrow.

Alan

[Starlight 1]

Black holes may have any mass.

Supermassive  black holes (10s solar masses) may exist at the centers of active galaxies.

[Knight of Clear Skies]

At a large distance from a body its size makes no difference to its gravitational pull, and it can be approximated as a point source at its centre of gravity. However, its a very different matter at short distances, particularly when standing on its surface. if you crushed the Earth down to half its size the surface gravity would be far stronger as the average distance to the attracting mass is smaller.

It's a similar story with a star becoming a black hole, a distant planet would continue orbiting as before but the surface gravity becomes so high that light cannot escape. 

[Cath]

Wot Mr Knight said !

Squash the same amount of matter into a smaller area, the gravity gradient becomes somewhat strong near the surface. The overall amount of gravity is exactly the same though, just that it becomes more concentrated at/near the surface.

[Major]

Another question about black holes... Is it possible that one black hole kicks out another from the galaxy? I heard that on one so-called 'documentary' but can't see any info on that.

[Starlight 1]

Think I can rember seeing this in A brief history of time, (Stephen Hawking) ok from the book(Suddenly I realized that the paths of there light rays could never approach

one another, It would be like meeting tually run into one another.)

[symesie04]

This is my take on the relationship between mass and gravity. Im sure ive read somewhere that mass bending space time is about our best current understanding of gravity. So all things with mass bend or curve space time. The more mass the greater the curvature but the shape of the curve is dependent on the area the mass is packed into. Mass by the way (at least my understanding) is a measurement of how tightly stuff is packed into an area, hence neutron stars are considered more massive than giant blue stars, they contain more or even less stuff but packed into a far smaller area.So if you imagine a stretched out sheet and then you place say a rugby ball on it it will curve the sheet because of its mass. However if you then get a cricket ball with the same mass and place it on the sheet its curvature of the sheet will be different to that of the rugby ball. The cricket ball will create a smaller curve in terms of radius but the curve will be deeper and steeper. The Sun creates a curve in space time and that keeps the Earth in orbit and we observe it as gravity. There is of course a relationship between distance from the Sun and the speed of our orbit, any faster and we would climb up and out of the curvature and break orbit, any slower and we would fall in the curve and meet a hot death. Very massive objects bend or curve space steeply enough that even massless light is affected by it, its path is bent, and we observe this as gravitational lensing. With Black holes the curve they create due to huge mass in a very small area is so deep and steep that even light does not have enough momentum to climb out. Interesting enough despite the name i wonder if you could venture into deep well of a black hole if it would be incredibly bright with all that trapped light. So in answer to the original question yes mass does increase if the same amount of stuff is crushed down as mass is simply a measurement of how much stuff is packed in. 

With regards to black holes being thrown clear of merging galaxies, no idea. Im sure i did see or read something suggesting that may happen however all the galaxy collision models ive seen it seems that despite the initial and subsequent lesser sling shots the galaxies eventually come together and the black holes merge. Would be interesting to know what merging black holes would look like. Wonder if they do it quietly or with the mother of Quasars. 

[acey]

Gravity is purely dependant on the mass of the object that creates it. The more massive an object, the greater the gravity well it creates. Correct ? Does then the mass of an object increase if you compact it down ? For instance, if I took a planet, squashed it down to half it's original size without losing any of its mass, will it produce a greater gravitational pull ? To my mind, (and I'm no expert) the answer is no.

The reason I ask this is because I was thinking about black holes the other days whilst on the loo. As we know a black hole is a collapsed star. Why would it be that the black hole, which has no more mass than the star that created it (at least initially), have such a stronger gravitational pull ?

To answer the last question first, if you turn a star into a black hole then it has no greater gravitational pull: if our Sun were a black hole we would still orbit it at the same rate of one revolution every 365.25 days. Where you notice the difference is if you get close, i.e. within the "event horizon". Then you find that you can't get away.

To answer the first question: in general relativity the sources of gravity are mass, energy and pressure. There can also be background gravitational fields without source.

An object will become a black hole if its physical radius is smaller than a critical size (the Schwarzschild radius) which is dependent on mass. If the mass is small then this implies very high density, but for extremely large mass the density can be as low as you like. You could make a black hole out of air if you could just put enough air in one place (filling a volume much larger than our solar system). Supermassive black holes like the one at the centre of our galaxy have an overall density comparable with water. But this refers to their mass averaged over the volume within the event horizon; whereas all the mass is really assumed to be at the centre of the black hole.

[DRT]

I started to read this thread with a great deal of trepidation but symesie04's description seems very logical and relatively easy to grasp. I particularly like the rugby v cricket ball analogy.

Can I now claim to understand quantum physics?

[Tiki]

Gravity is purely dependant on the mass of the object that creates it. The more massive an object, the greater the gravity well it creates. Correct ? Does then the mass of an object increase if you compact it down ? For instance, if I took a planet, squashed it down to half it's original size without losing any of its mass, will it produce a greater gravitational pull ? To my mind, (and I'm no expert) the answer is no.

Correct, you'd feel always feel the same gravitational pull from your crushed down star (provided you were external to the star when you took your measurements).

Imagine what it would be like to be on the stars surface as it starts to crush down. When the star has crushed to half radius we would feel four times as heavy (inverse square force Law of Gravity). Most importantly though, we would notice an eightfold increase in the difference between the pull on our head and our feet (inverse cube law of tidal acceleration). As tidal accelerations account for the curvature of space it is easy to see that as the star crushes down space becomes increasingly more curved close to the surface.

Consequently, it is possible to fall into a supermassive black hole, not realize it and die of old age, whereas falling into a stellar sized black hole would lead to being torn to pieces by tidal forces.

Spacetime curvature and tidal accelerations are one and the same thing only expressed in different languages.

[Physicist13]

Gravity acts as a point source. Forces depend on the distance from that point source.

[Frenchgow]

Quantum theory is changing many common held beliefs about 'black holes' and to throw a spanner into this discusion ,Stephan Hawking latest thought that black holes don't exist has many theoretical physicists scrambling for the valium,have a look at 'nature international weekly journal of science'

[Ptarmigan]

Stephan Hawking latest thought that black holes don't exist

Interesting but,

not quite ! , ,  that black holes "dont exist as_we_know_them Jim ! "

He seems to be saying that event horizons ( and the recently proposed firewalls) of black holes dont exist, not that black holes themselves dont exist.

I think he is also doing away with the singularity, instead the collapsed information is is being held at a (diminishing) 'apparent horizon'.

" The absence of event horizons mean that there are no black holes - in the sense of regimes

from which light can’t escape to infinity. There are however apparent horizons which persist

for a period of time. This suggests that black holes should be redefined as metastable bound

states of the gravitational field. "

from http://arxiv.org/pdf/1401.5761v1.pdf

" Information Preservation and Weather Forecasting for Black Holes "

S. W. Hawking

[Frenchgow]

Nice explanation of what Hawking is saying,however my point was that in this particular context the theory of what is "really " happening is an evolving theory,and one shouldn't take statements like nothing can escape as fact,it seems to me that from the grave Einstien and Bohr are still arguing,nice to know that were still at times groping in the dark,regards gow

[Ptarmigan]

 "really " happening is an evolving theory,and one shouldn't take statements like nothing can escape as fact,

Nope, already conceeded "Hawking Radiation"

[Frenchgow]

“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. "firewall" "apparent horizon" the argument continues,but will still follow with interest

[Dan Leach]

If you replaced the sun with a black hole of equal mass the planets would carry on orbiting the same. Mass is independent of size. But like someone above has said, this would make a difference in short-range as you would be closer to the centre on a smaller object.