# Density physics problem

1. Aug 12, 2006

### fedorfan

Whats the facts about stuff that isnt dense(like aerogel) and stuff that is dense(like neutronium)? Does denser material pass through least dense material easier or what? I just want to know the basic nitty gritty of the property density.

2. Aug 13, 2006

### Andrew Mason

Normal matter (matter that is not subject to extremely strong gravitational fields) is mostly empty space. The distance between nuclei and the mass of the nuclei will determine the density of the material.

In a neutron star the nuclei are packed together so tightly that there is virtually no space between the nuclei. Such matter cannot exist, however, in the absence of an extremely strong gravitational field.

AM

3. Aug 13, 2006

### rbj

is that what the OP meant by "neutronium"?? i always thought that was just an old Star Trek plot device. there was one episode where Spock is holding a piece of it (saying it's the strongest material known to our science and then crumbling it - the big Romulan battle episode) and another with a Doomsday machine with an outer hull of neutronium that was impenetrable by any weapons (if only they had that Romulan weapon that made it into crumbly stuff).

i never thought that it was the material of a neutron star since, if anyone looked into it, no one could pick it up or do anything with it, being so damn heavy.

sorry. i just had to laugh a little at the use of the word "neutronium", which i hadn't come across before in any non-fiction context.

4. Aug 13, 2006

### fedorfan

What do you mean by extremely strong gravitational force, black hole type gravity or what? Also, why couldnt it exist in that type of gravity?

5. Aug 13, 2006

### fedorfan

Also, can you boil and/or melt a neutron star? Is there anything in the universe that you couldnt boil or melt? Im just saying theoretically, is there anything at all that you couldnt boil or melt with an unlimited amount of heat?

Last edited: Aug 13, 2006
6. Aug 13, 2006

### DaveC426913

A neutron star has collapsed under its own gravitational pull, crushing the matter until the atoms themselves collapse, the electrons and protons are squashed together into neutrons, and the neutrons are touching.

The neutrons are very repulsive at this subatomic distance, and will fly apart if not held together by extremely powerful gravity. Thus, neutronium cannot exist except in a neutron star.

A neutron star is as dense as matter can get before becoming a black hole.

7. Aug 13, 2006

### Andrew Mason

Do the neutrons in a neutron star really repel each other?

As I understand the process, the atoms repel each other until they are so close that the probability of electron capture by the protons becomes high and proton/electron pairs start turning into neutrons. When that occurs, the coulomb force disappears so there is no repulsion force left.

AM

8. Aug 13, 2006

### Danger

When you get to the point of an appropriately sized star collapsing into a neutron star (usually during a nova event), the electron orbitals are compressed to below the Planck distance for minumum, and thus combine with the protons to become neutrons (charge cancelation). I'm not quite sure where Dave got the impression of the neutrons being mutually repulsive. Protons would be, because they're positively charged. Neutrons are, by definition, neutral and thus don't either repel or attract one another. Density wise, a teaspoon full of neutronium would weight the same (in a gravity field) as half a dozen mid-size cars.

9. Aug 13, 2006

### rbj

wow. i woulda thunk that the trouble would have begun when the electron orbital is compressed to about 10-15 meter (the electron radius and, surprisingly, about the same as the proton radius). this is about 1020 times bigger than the Planck length. i dunno of any known physical structure of elementary particles that's as tiny as the Planck length.

supposedly, at one time, the universe was squeezed into less than a Planck length. musta been hot that day.

10. Aug 13, 2006

### Danger

Rbj, thank you for correcting that. My mis-use of the term was based upon my lack of education and a dozen beer. What I meant, and used the Planck length erroniously to express, was that the compression exceeds the lowest allowable orbital for an electron.

11. Aug 13, 2006

### rbj

i ain't no astrophysicist, it just was that the Planck scale is a whole 'nother thing. the smallest orbit for the electron is, i s'pose, about the Bohr radius which i thought was something like half an Angstrom. about 10-10 meter. so the size of these subatomic particles are about 100,000 times smaller than the size of atoms, i guess.

i haven't studied any of this since college about 30 years ago (i'm an electrical engineer, not a physicist), so i use the web to look things up. often i first go to Wikipedia, but you have to be careful there. sometimes non-facts get in there and go unchecked. but usually, the techincal/science articles are pretty good.

L8r, Danger,

12. Aug 13, 2006

### Gokul43201

Staff Emeritus
There is no nitty gritty. The only thing that you can say, based ONLY upon density (and some assumptions about the nature of the materials), is how different materials respondond to the same force (eg: flotation).

There are a whole host of other properties and phenomena that are related to density in addition to other things (eg: dielectric properties, interaction strengths, mean free paths, extinction coefficients). But there really isn't anything "'deep" to be gained from the definition of a density. It is just a very useful property that finds use in several places.

13. Aug 13, 2006

### DaveC426913

I oversimplified. I didn't mean to suggest literal electical charge repulsion. The point is, you could not hold a lump of neutronium in your hand. It can only exist under extremely high pressures.

Actually, I guess I don't really know why it would fly apart (except due to pressure from heat, like in a regular star.)

Last edited: Aug 13, 2006
14. Aug 14, 2006

### Danger

Ah. That would be the degenerate neutron pressure, based upon the exclusion principle. Now I'm starting to wonder if the electrons might escape from the neutrons when pressure is removed, and re-establish their orbitals with the attendant electronic repulsion.