Say we were able to produce a macroscopic quantity of neutronium (i.e., free neutrons) and keep it around long enough to observe it. (That should be possible as the half-life is 15 minutes.) What physical properties would this substance have? What would be its melting and boiling points? What would it look and feel like? What colour would it be?
There are good questions and then there are great questions. Great questions are the one’s for which I have an answer. Therefore, your’s is a good question.
The half-life of free neutrons is 15 minutes. That doesn’t mean the half-life of neutronium is 15 minutes - it’s not just free neutrons, essentially a single gigantic atomic nucleus. I have read that neutronium may well be violently unstable outside of the conditions which create it.
Atomic nuclei are tightly bound. Is there any reason that a bunch of free neutrons would aggregate into a nucleus rather than remaining separate?
Well, at the very least, I imagine it wouldn’t have any color. Color is caused by how light excites the electrons around an atom, isn’t it? So a structure of only neutrons should be pure black.
No. Which is why neutronium is unstable - the neutrons don’t want to be together in that configuration. Atomic nuclei get more unstable the larger thay get, and a piece of neutronium is effectively an atomic nucleus with a very, very large number of neutrons.
I dunno, you could probably make an equivalently good argument that neutronium would be a (near) perfect reflector.
Then again, it wouldn’t surprise me if the surface of a chunk of neutronium wasn’t particularly stable on a quantum level, so it might be radiating.
Not sure the normal physical properties like melting and boiling points make any sense under the conditions in which neutronium forms (inside massive stars). Those terms usually refer to breaking molecular bounds. From my non-extensive knowledge (the odd science fiction book), neutronium is just a neutron soup, essentially a fluid and not stable except under vast pressures. So it doesn't have a melting point, and if you can add enough energy to make it change state, the end result is probably less "boiling" and more "kaboom!" on a solar scale.
For pretty much the same reason, talking about what neutronium "feels" like probably doesn't make sense, I'd guess it would be completely smooth, but anyone trying to verify that guess would probably end up composing an infinitely thin slick on top of the neutronium.Or neutronium might be completely clear since it wouldn’t react with visible light at all.
Neutronium might be a fluid (a more generic term than “liquid”), but possibly under some conditions it might have a solid phase (I define solid here to mean “resists shearing”).
Wouldn’t this be essentially a neutron star? What does that look like?
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General Questions Moderator
This is not a misconception. Thet term “neutronium” was coined in 1926 by Andreas von Antropoff to mean a conjectured form of matter made up of neutrons with no protons—that is, free neutrons. This is the sense in which I am using the term.
No. While the word “neutronium” is sometimes used in science fiction and popular literature to refer to the substance of neutron stars, scientists do not really know what this substance is (and therefore avoid using the term themselves). It could be free neutrons, but it could just as easily be something else.
Well, if you’re going to define postulate a “neutronium” isotype that consists only of free neutrons that can exist in a macroscopic form and NOT as the result of a hellacious gravity field, then you’re probably going to have to be explicit about the conditions under which it exists before your questions can be answered.
You seem to be contradicting yourself here. First you say that the neutrons would be a giant nucleus, then you agree with me that there is no reason that they would be a giant nucleus, and then two sentences later you go back to saying that they would form a giant nucleus. In any case, calling a collection of free neutrons an atomic nucleus is a contradiction in terms, since “free neutron” means “a neutron existing outside an atomic nucleus”.
It is true that free neutrons are unstable, but the half life (886 seconds) is long enough that observation should not be a problem. Producing them is also not a problem, as they’re deliberately produced all the time by nuclear reactors. I expect the only problems would be collecting a macroscopic quantity for study.
News to me… We don’t know all of the properties of neutronium, but it is by definition the substance of which neutron stars are made. And I’ve heard plenty of physicists use the term.
To address the original questions, neutrons are electrically neutral, but that doesn’t mean they can’t interact with light. They do have an electric quadrupole moment and a magnetic dipole moment, and so can interact with electromagnetic fields (i.e., light) in those ways. Contrary to the popular conception, neutronium also contains a fair number of protons and electrons (the estimates I’ve seen put protons at about 10% of the mass, with an equal number of electrons to balance the charge), and they can also interact electromagnetically. In fact, the pressure which supports a neutron star is actually provided mostly by the electrons, not the neutrons. It wouldn’t be a bad guess to suppose that a neutron star’s surface would be mirrorlike (it’s certainly smooth enough), but that guess would seem to be incorrect: We’ve seen neutron stars radiating, and they appear to be very black.
For its other physical properties, neutronium is a superfluid, meaning that it can’t support any shear forces or viscosity at all. So it’s definitely not a solid. Of the three familiar phases of matter, it’s probably most like a liquid, but this is only an approximation, since a true liquid has a constant density, while the density of neutronium is believed to vary significantly with pressure. Exactly how it varies with pressure would be expressed in something called the equation of state, but the precise equation of state of a neutron star is one of the things which is currently unknown about them (though there’s plenty of theories). Temperature is essentially irrelevant to a neutron star: Although they can have a temperature of millions of degrees at the surface, and possibly much higher yet in their interiors, they can be excellently approximated as having zero temperature: The quantum mechanical processes that lead to degeneracy are much more significant in a neutron star than even those high temperatures.
Yes, but the term can mean different things in different contexts. It can refer either to a substance composed solely of free neutrons, or it can refer to the substance of a neutron star. The two are not necessarily the same, though. In fact, according to your use of the term, they are certainly not the same, since your definition supposes that neutronium contains a significant number of protons.
Unfortunately, your answer did not address the original question, as you were speaking about the second definition of neutronium, whereas my question was about the first. That’s why I referred to free neutrons in the question.
I did a search on every APS journal for ‘neutronium’ in full text from 1893 to 2006 and got no hits. On the other hand there were 628 hits for ‘neutron matter’. Dunno what the significance of this is - maybe ‘neutronium’ is a handy shorthand in conversation, but currently lacks a firm scientific definition? Besides, neutron stars aren’t made of just one type of neutronium. The material of the outer shell and the material of the interior are quite different, but both are ‘neutronium’. Kinda. Sorta. That’s the problem - if you refer to something as ‘positronium’ (1933 hits on APS), you know exactly what you’re talking about. When you’re talking about neutronium, it’s more along the lines of “this stuff is made of mostly neutrons”, rather than a hard definition of their interrelationships and their gross properties.
A handful of free neutrons are not neutronium.
Neutronium isn’t so much a substance as a phenomena; what happens to normal matter when it’s compressed not quite enough to become a singularity. The electrons and protons collapse into neutrons, and you’re left with a mass of nothing but neutrons held together under tremendous pressure. These are not free neutrons; they’re close enough to each other for the nuclear forces to affect their behavior.
If you release the pressure holding it, the neutronium ceases to be neutronium with remarkable violence. So you can’t so much synthesize neutronium; even if you gather a lot of free neutrons and put them near each other they won’t congeal into neutronium. If you could somehow make a container strong enough you could put matter in it and it would become neutronium when enough pressure was applied, but as soon as you released it that matter would cease to be neutronium.