I have always been under the impression and also heard it stated numerous times that if we brought a teaspoon’s work of neutron star material to earth it would immediately violently expand because gravity is no longer keeping it compressed.
As far as I know the material will be composed entirely of neutrons which will not repel each other so what will cause it to explode?
At very short distances, the strong nuclear force is repulsive.
A simple way to think of it is that a neutron star must be in a state of equilibrium, or it would continue to collapse. Take away an attractive force (gravity) and it’s going to go wang.
Neutron stars are supported by degeneracy pressure, which is caused by Neutrons being unable to all inhabit the same energy state. Since they can’t all be in the same energy state, most end up in very high energy states. Without gravitation to bind them togeather, all the high energy neutrons would fly off into their surroundings, and the neutron matter would explode.
Are you sure that the strong nuclear force is repulsive?
According to Wikipedia “Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is minor residuum of the strong interaction which binds quarks together to form protons and neutrons”
If the strong force binds quarks together which are inside a neutron then it seems the strong force is attractive even at a very small distance
The way I tend to understand this is that neutrons don’t want to get too close to each other and the pressure inside the neutron star compresses them together bringing them closer than they want to be. I would imagine this is similar to having a bunch of soft rubber balls, if you squash them together they will take up less space but as soon as you let go of the pressure they will fly apart.
This leads me to another question how close can naked neutrons get to each naturally? For example, if I had a bucket of neutrons how dense would it be and is this even possible? I suppose the fact that neutrons decay into proton and electrons with a half-life of under 15 minutes would probably confuse matters because now the protons and electrons would now repel each other or would they combine with the neutrons to form elements?
And another question while I am on a roll, how close are the protons and neutrons in the nucleus of an atom? Do they actually touch or there space between them?
The strong force is attractive over very short distances, and repulsive over very, very short distances, which keeps protons and neutrons an average distance apart within a nucleus.
In any case, I think Simplitico’s answer is better than mine. I don’t understand why the strong nuclear force become repulsive, and if it has any relation to the exclusion princicple. I’d welcome a clarification.
Due to the above, they maintain an average seperation.
Reading further is seems that the strong nuclear force is repulsive, quoting wikipedia once again - “The nuclear force is only felt among hadrons. At much smaller separations between nucleons the force is very powerfully repulsive, which keeps the nucleons at a certain average separation. Beyond about 1.7 femtometer (fm) separation, the force drops to negligibly small values.”
I never knew that, I guess that you really do learn something new everyday, I just hope that the new something is not that wikipedia is wrong 
It’s important to keep in mind that there are two things called the “strong force”:
(1) the force between quarks which is attractive at large distances and goes to zero at short distances (“asymptotic freedom”)
(2) the force between nuclei (e.g. protons and neutrons), sometimes called the “nuclear force” or “residual force”, which is repulsive at very short distances but attractive at comparatively larger differences.
This repulsion is different from the degeneracy pressure. I think degeneracy pressure plays the dominant role in neutron stars (but I’m no expert).
I think you may be right. Treat my answer #2 with suspicion until someone with more knowledge can clear this up. I was assuming that the nucleons would be at a closer than normal distance due to the immense gravity, I notice that the wiki article states the core of a neutron star is more dense than an atomic nucleus. Even if this is the case, I don’t know if it would be significant campared to the degeneracy pressure.
I didn’t know it would expand. I always wondered what it would behave like - it would probably crack any surface it’s put on because it’s so out-of-this-world dense. Would it fall right through to the core of the Earth? How can we hold it? Is a diamond anvil strong enough?
A diamond anvil would just shatter. A neutron star is absurdly denser than anything you could think of. The commonly stated comparison is that a teaspoon of a neutron star’s matter (which is a bit of a simplicication as a neutron star is not all the same, but whatever) weighs as much as a thousand Great Pyramids.
A diamond anvil would shatter if you put one Great Pyramind on it, so it’s not going to hold up a thousand.
If you could prevent the neutron goo from exploding, which you couldn’t but we’ll pretend, I don’t see how it wouldn’t seep its way into the Earth’s core.
What I found fascinating, once again according to wikipedia that on a neutron star “the resulting force of gravity is so strong that if an object were to fall from a height of one meter it would only take one microsecond to hit the surface of the neutron star, and would do so at around 2000 kilometers per second, or 7.2 million kilometers per hour.”
I can understand that a microsecond is a millionth of a second and 2000km would take me about 20 hours of highway driving but I can’t truly understand or grasp how incredible these figures actually are. The whole of astronomy is like this with incredible distances, times and masses. I’m surprised the true astronomers don’t go mad when they think about the figures - maybe they’re in denial 
Well it’s not the “bring back to Earth” part that’s important. The important part is that it’s missing the rest of its star. All that matter in the star was compacting the neutrons together and now it’s gone, so the piece breaks apart. As for the diamond anvil, mere carbon is nothing compared to the almighty degenerate matter. That stuff is the sharpest knife in the universe- it’ll cut straight through anything!
If we could stop it from expanding using Magic[sup]TM[/sup], it would fall through to the centre of the Earth. Normal matter is mostly empty with a sprinkling of protons, neutrons and electrons. In a neutron star, the nuclei are packed right up against each other. If my back of the envelope calculation is right, if the Earth was as dense it would be a sphere with a radius of 173 metres. Your diamond anvil is not going to stand a chance.
(Incidentally, don’t build your anvil out of diamond. It is strong but brittle, and will chip and shatter if you hit it with a hammer).
By “expanding” we mean something more like “rapidly exploding”, given the amount of pressure we’re talking about. Presumably, you’d end up with a whole bunch of free neutrons (which are unstable and decay into protons and electrons via the weak interaction).
Regarding whether the “magically compressed lump of neutron star stuff” would fall to the center of the Earth, it’d presumably depend on its initial velocity… you’ve got to consider conservation of angular momentum and so forth. But suffice it to say you couldn’t just move it which ever way you wanted (without more of our aforementioned “magic”).
I’m not certain, but I can make an educated guess. The attractive strong force between protons and neutrons is predominantly mediated by the exchange of pi mesons (pions). Perhaps at shorter distances other mesons are exchanged (which aren’t long-lived enough to make the longer trip), and these produce a repulsive force.
I recall that A. Zee’s Quantum Field Theory in a Nutshell includes an argument that changing the spin of the particle exchanged can flip the sign of the coupling constant. Maybe these short-lived repulsive mesons are spin 1 instead of spin 0.
Quoth tim314:
The equation of state of neutron star matter (the relationship between pressure and density) depends both on the degeneracy pressure and on the strong nuclear force. Since we don’t know all the details of the strong force, we also don’t exactly know the equation of state. Within the past year, though, a neutron star with a mass of two solar masses was found, which implies that the equation of state must be very “stiff” (high pressure for a given density), meaning that strong force repulsion is fairly significant-- Still probably less significant than the degeneracy pressure, but something like a 60-40 or 70-30 split, where you definitely can’t ignore it.
And treating the strong force as being mediated by pions (or other mesons) is really just an approximation, and not one I would expect to be valid under neutron star conditions. To do an a priori calculation, you’d need to do something like a full quantum chromodynamics calculation. With the caveat that we don’t actually know how to do full quantum chromodynamics calculations, outside of a few special cases.
Good info, Chronos. Thanks.
A neutron star is so dense that a pound of it weighs more than a million pounds.
Thanks for the clarification Chronos.
So any quantity whatever has infinite mass? :dubious: 