# Why do stars collapse in ot itself? Part 2

1. Apr 10, 2013

### PhyHunter

I think space-time exerts a pressure on substance and it makes star to transorm blackhole.I think this can be possible becuse we know that space-time is something which it can be tiltled and it can creates virtual matters.Space-time is real thing.So Can be space-time exert pressure on substance ?

2. Apr 10, 2013

### Staff: Mentor

Gravity, described as curved spacetime in General Relativity, can lead to pressure in the material.
For sizes significantly larger than the Schwarzschild radius, you can use the easier model of Newtonian gravity, however.

3. Apr 10, 2013

### PhyHunter

I understand it thank you.Where can I find these equations ?

4. Apr 10, 2013

### Staff: Mentor

General Relativity or Newtonian gravity? In any textbook, the corresponding Wikipedia pages and various websites in the internet.

5. Apr 10, 2013

### PhyHunter

You mean gravity exerts pressure on matter

6. Apr 10, 2013

### PhyHunter

Becuse I think gravity is force not pressure

7. Apr 10, 2013

### Crazymechanic

In purely Newtonian way gravity indeed counts as a force because otherwise the apple would not fall down but stay up.
In General relativity gravity is a space time curvature , what that means is that you don't have gravity as a separate force indeed you have mass/energy in different forms like gasses , liquids or solids around which the space time dimensions which would otherwise be uniform bends just like the coach when you sit on it.

In other words it depends on the framework you look upon one is a older simpler one the other is a younger more complex describing large scale phenomenon o which the older Newtonian one would not be enough.

But to an everyday man walking down a street gravity indeed is more of a force than a mass/energy exerted property .

8. Apr 11, 2013

### mathman

Pressure is force per unit area.

9. Apr 12, 2013

### Naty1

I think that is an oblique way to word things; empty space-time will not make a star into a black hole. If anything it may have a bit of dark energy which is a negative repulsive pressure leading to cosmological expansion over vast distances. You can think of space-time as carrying the gravitational field analogous to how an electromagnetic field carries the EM force.

What makes a star become a black hole are the conditions described below and we model these via the Einstein Stress Energy [SET] tensor [and vacuum solutions of the Einstein Field Equation]: The SET is the source of gravity which curves space-time.

There is a decent overview here.

http://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit

PRESSURE can be a component of gravity, but with regard to formation of black holes 'pressure' usually is an opposing force....a repulsive force called 'degeneracy pressure' which takes various forms. Electron degeneracy pressure means that electrons don't like being pushed into atomic nuclei and oppose gravity in such compression...and neutron degeneracy pressure results after electrons have been forced into protons by gravity....this is an even more dense state [called a neutron star]. The neutrons are really,really compact.
Above about 1.44 solar masses, the famous
Chandrasekhar limit, nothing can stop such a mass from collapsing to a Black Hole.

10. Apr 13, 2013

### Staff: Mentor

Just to avoid misunderstandings for other readers: this limit applies to "cold" matter only.
Stars can easily exceed this limit. As long as they can fuse material, radiation pressure prevents them from collapsing.

11. Apr 13, 2013

### PhyHunter

neutrons are fermion and also quarks are fermion so how can matter collapse one single point ?

12. Apr 13, 2013

### PhyHunter

Where is going this matter ?

13. Apr 13, 2013

### Staff: Mentor

There is no quantum theory of gravity yet, therefore it is unknown if the "center" of a black hole is really a singularity or something else. That does not matter for the description outside.

14. Apr 13, 2013

### mathman

What happens inside a black hole cannot be described by current theory. General relativity and quantum theory conflict. The collapse to a single point results from using G.R. only.

15. Apr 14, 2013

### Naty1

Code (Text):
..neutrons are fermion and also quarks are fermion so how can matter collapse one single point ?
Nobody really knows the entire story. Both QM and GR show divergences....results approaching infinity....not so good; But the math is believed pretty good inside the horizon until the singularity is approached...until one gets very close.

[It's crudely analogous to Coulomb's law
F = k q1 q2/r2.....which also approaches infinity as the radial distance approaches a point. It suggests 'something is wrong' because so far nothing 'infinite' has been experimentally observed.]

At Planck length, about 10-33 cm, a prominent marker in QM, it appears so far that entities we recognize everyday like charge, gravity, space, and time, may cease to exist when compressed so far. [What we observe from the outside of a black hole is representative of the energy-matter at the time of collapse, say the dead star that collapses, not necessarily the final state at the singularity .]

At Planck scale matter-energy gets very energetic due to small distances and times...its loosely referred to a quantum 'foam' at shorter scales....this is way past when degeneracy pressure become significant. The way I think about that is via E =hf = h/λ....so when wavelength λ becomes compressed by gravity [matter is forced into tiny spaces] and approaches a 'point' the wavelength gets really,really small...and energy E gets really really large....

Really funny stuff happens at such high energies which is clearly not well understood.

PS: The same issue of 'collapsing matter' exists with regard to the BIG BANG! Except there we ask how energy-matter can EXPAND from some singularity. So the Big Bang is a singularity in our past from which everything emerged while a BH singularity is a 'point' our future [it's actually a point in time!] where everything 'disappears'. How are these singularities, their associated horizons and our universe related?? So far, nobody knows. Stay tuned!!

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