# Bodies of mass near the Schwarzschild Radius

1. May 10, 2012

### Locke H

What happens to bodies of mass as they approach and get near this value?

If they don't actually reach the criteria, will their properties be vastly different from bodies that do reach the criteria? Will it expand instead of maintain it's radius?

I'm also wondering how much energy it takes to compress a body down to this radius. I would imagine that at some point during the compression, the body loses all of it's material properties and needs to be treated more like a soup of atoms, so the actual material used doesn't really matter.

2. May 10, 2012

### Locke H

I noticed this thread was moved to special and general relativity, but I was actually particularly interested in what the condensed matter guys would have to say about extreme compression of matter.

I'm working on elements for a novel and was looking to make an educated representation of what would happen should we use compressed matter as a form of storing massive amounts of mechanical energy (like compressed air, but using other states of matter compressed down to a soup of extremely high density).

Last edited: May 10, 2012
3. May 10, 2012

### pervect

Staff Emeritus
The only local things that someone approaching the event horizon notices are tidal forces. So, the way the question is worded, there isn't anything for the condensed matter people to answer.

Possibly you meant what happens when you approach the central sigularity, or possibly you are confused about the nature of the event horizon. It's hard to tell at this point.

4. May 10, 2012

### yuiop

When a star runs out of fuel it collapses in a supernova explosion often leaving a dense central body. If the remnant body has insufficient mass to form a black hole (less than 2 or 3 Solar masses) one of the types of bodies formed is a neutron star. Normal atoms are made up of neutrons, protons and electrons and the ratios of these particles determine the identity and properties of the atoms. With the intense pressures (16×10^34 Pa) in a neutron star protons and electrons condense into neutrons and it is impossible to identify the constituent atoms that collapsed to form the neutron star. Further collapse is prevented by degeneracy pressure that in loose terms prevents individual neutrons occupying the same place. The density of these stars are tremendous but they are not necessarily the most dense objects in the universe. It is thought that slightly more massive stars may form more exotic bodies such as quark stars or strange stars but observational evidence for these are sketchy. See http://en.wikipedia.org/wiki/Neutron_star If the remnant mass is more than about 3 Solar masses it is thought that degeneracy pressure is insufficient to prevent total collapse and a black hole forms.

Normally pressure is a good way to store energy and the energy can be recovered by releasing the pressure. The pressure in massive bodies such as neutron stars is not so useful as a store, because we cannot recover the pressure by switching gravity off. A huge amount of energy is however released during the collapse. If you had sufficient matter to make a neutron star you would be better off making a normal star which will kindly produce nuclear fusion energy for you for a long time. If the massive body has angular momentum that can be used to store energy and rotational energy can in principle be recovered. A certain amount of energy can be obtained from these super dense bodies by dropping waste material onto them as the falling matter releases large quantities of electromagnetic radiation as it falls.

Last edited: May 10, 2012
5. May 10, 2012

### Locke H

Ah, I see. My picture of what happens near the event horizon wasn't very complete to begin with so I wasn't quite aware of how to approach the question. But you're right, at these pressures, since only the constituent atoms have any relevance to the body's behavior, I would imagine it to be very difficult to think of it as condensed matter.

This points me in the right direction, thanks. I was originally imagining what would happen if you took a relatively minuscule amount of matter (say several cubic meters of water, so long the compressed radius is greater than a Planck length), and compressed it down to it's Schwarzchild radius. I would imagine micro black holes to not be of much use towards energy generation, even in a sci-fi setting.

But here's the part I figured the compressed matter guys could answer, where I didn't properly present the question: What if a cubic meter of water were compressed to something extremely small (around 99% of it's original size), nowhere near the radius where atoms start fusing and the physics begin to change.

Sorry if I come off as a bit uninformed, I'm still reading through the material and really appreciate your help pointing me in the right direction :)

As a Mech E, some of this material isn't very native to our coursework.