In that case, there is no answer to your question. Individual neutrons may have a half-life of minutes, but any sort of macroscopic object or substance made up solely of many neutrons would last for a far shorter time, short enough that questions like “what does it look like” would be meaningless.
Indeed. The conditions everyone else seems to be using (“Take large amount of protons, neutrons, and electrons. Crush until protons and electrons collapse into neutrons. Crush further. [strike]Serve with lemon wedges[/strike]”) is pretty much exactly what’s going on with a neutron star. The pressure is being provided by the gravitational field of the matter itself.
and
Hmm, no mention in this thread of the Strong Nuclear Force, which without the opposing electromagnetic repulsion of Protons in close proximity, should be very capable of holding a bunch of Neutrons together. Why do we think Neutronium should be so unstable again?
The Strong Force is a lot more complicated than just “neutrons and protons stick together”. It’s fairly easy to see why nuclei with not enough neutrons would be unstable, but nuclei with too many neutrons are also unstable. Two neutrons and a proton, for instance, will only last about 20 years or so, and I don’t think that you can get three neutrons and a single proton to stick together at all for any measureable length of time.
I’m in agreement with Chronos about how physicists use the term amongst themselves and - more importantly - the actual physics, so I’ll just comment on the historical anachronism here.
Antropoff (1878 - 1956 ) is a little remembered German chemist whose ideas had little, if any, influence on the nuclear physicists of the 1920s or later. His “neutronium” does indeed seem to have been a form of matter with Z=0 that he proposed placing at the start of the periodic table. (Pauling’s version of the table on this page is apparently based on Antropoff’s 1926 version; while Pauling evidently dropped “neutronium”, it’s obvious where it presumably went.) Without seeing the exact details of what Antropoff was suggesting, there seem only two possibilities:
[ul]Antropoff had something closely akin to Rutherford’s previously proposed “neutron” in mind. To be precise, in 1920 Rutherford had proposed a tightly bound electron-proton pairing that would behave as a neutral particle. Such ideas were then pretty common in the 1920s, though with no great stability over names for them. (For example, long after Rutherford used the term, Pauli tried to nick the name “neutron” for the rather different particle we now call the neutrino.)[/ul]
[ul]He was proposing something more exotic. Given that the periodic table was still only a largely empirical ordering of chemical and atomic properties, it wouldn’t surprise me if he was proposing something that would seem truly strange once quantum mechanics had acted as a brake on such speculations.[/ul]
If it’s the latter case, then his “neutronium” is probably an entirely meaningless concept outside of the context he proposed it in in the 1920s.
If it’s the former, then his terminology was overtaken by events. What he would call “neutronium” now maps to what physicists unambiguously call neutrons. Calling these “neutronium” because Antropoff did is just falling into the trap of using a (very) archaic name. Which is slightly bizarre, given how universal the current usage is.
It is plausible that his suggestion did influence the SF usage of the term and hence, indirectly, the usage that “neutronium” is a large bunch of neutrons in the form of a neutron star. But that history has been forgotten in the current usage.
Now certain people have periodically (pun intended) re-proposed the notion that, as a Z=0 particle, the neutron is the first entry in the Periodic Table. But that’s never ever quite caught on. It may in future, but it hasn’t to date. The most recent high-publicity example of this has been the Chemical Galaxy. It’s proposer does want to use “neutronium” in connection with Z=0 (since that site doesn’t quite seem to be working, here’s the Google cache). But he’s recognising that the only place where one might get the stuff in any collective sense of the term is in a neutron star.
From what I understand of the physics involved -
Neutrons tend to spontaneously break down into protons-electron pairs. In atomic nuclei this doesn’t happen, IIRC because of strong force interactions, unless the ratio of neutron to protons is too high. In neutron-star conditions, the tremendous pressure forces the electrons and protons to combine into neutrons, and then forces them to stay that way. When you release that pressure, the neutrons will start to decay back into protons via beta emission The half-life of an individual neutron may be 15 minutes, but you only need a tiny fraction of the neutrons to become protons for the mass to tear itself apart from electrostatic forces (for the same reason that nuclei with too many protons are unstable, the electrostatic repulsion overwhelms the strong nuclear force.). This will continue until the entire mass is normal matter, rapidly expanding and at very high temperature. You don’t want to be standing nearby.
I don’t know of any form of neutron-only matter other than what forms in neutron-star gravity conditions.
Well, neutronium is not entirely unknown to physicsts. They use it all the time in the form of neutron beams. Now I’m not the one to know much about them, but maybe if we ask “what are the properties of neutron beams,” we’ll get our answers. Also, please let us not mention neutron stars any longer. Their whole point is that their gravity is so massive that once a neutron decays, its byproducts can’t leave far enough and end up turning back into a neutron. Certainly that’s not what is going to be happening for our neutronium. Someone said that it would exist only very briefly before the radioactive byproducts pollute it beyond recognition. I agree.
Now the biggest problem with neutronium (neutron beams) is that it’d be very hard to contain them. You can’t use electromagnetic traps, obviously, and the neutrons would tend to just either pass through or bond with any vessel made of matter (since the thing that’s keeping my ass from going through my seat is the electromagnetic force).
I don’t know what color it would be, or how neutrons interact with light. Them being neutral also obviously makes them a bit weak when interacting with the EM force. Don’t know if they do not interact at all. At room temperature, they would certainly be a gas. There might be a temperature low enough that the weak nuclear force will get them to stick to one another. They would then presumably turn into one superdense nucleus, resembling a superfluid (or rather a bose-einstein condensate), but they would also probably decay much faster in that state.
That’s a bit tricky. Bose-Einstein condensates apply only to bosons, particles with an integer spin, while neutrons are fermions, particles with half-integer spins. The famous Pauli Exlusion Principle applies to fermions, but not to bosons (or, strictly speaking, it also applies to bosons, but in a more subtle way). Fermions cannot exist in the same quantum state as each other, but bosons can, and in fact that’s exactly what a Bose-Einstein condensate is.
It is possible, in some cases, for fermions to pair up into what are called Cooper pairs, with each Cooper pair acting like a single particle, and the Cooper pairs (which are bosons) can then form into a Bose-Einstein condensate. I don’t think, though, that this can occur with neutrons, since there’s no stable binding of two neutrons.
Androgynous
To maybe help put into perspective how unstable neutronium (meaning a substance made up of only neutrons) would be outside of a neutron star, people have been searching for tetraneutrons, a state analogous to a helium nucleus, but with four neutrons instead of two protons and two neutrons. Just four neutrons can’t hold together long enough to be easily enough detectable so that it’s unambiguous whether they even exist. With a macroscopic amount of neutrons, it would seem hopeless.
[aside]Four separate neutrons ought to exist with a half-life of about 3 or 4 minutes, before they became three neutrons and a proton. Putting them into a tetraneutron, I’d expect the strong force to help bind them together, making its half-life be, well, over 3 or 4 minutes, anyway. Since they’re not able to easily and unambiguously detect them, apparently this isn’t the case. Why the hell not?[/aside]
I apologize for picking this nit. And I’m not picking on Chronos, just using his statement as a starting point.
The binding holding a Cooper pair together is not stable. A Cooper pair is composed of two charge carriers (typically electrons) in a crystal lattice (not free electrons) held together by phonon (quantized lattice vibrations) interactions. That interaction is small compared to the repulsive force between the charge carriers. While a Cooper pair is a stable entity, the particles it’s composed of are not fixed. It’s meaningless to speak of the a particular pair of electrons forming a Cooper pair.
Neutronic matter is outside my speciality, but a knowledgeable guess is that neutrons do form superpairs akin to Cooper pairs. Superfluidity and superconductivity is a bulk property of bosonic matter. A weak, attractive force between two neutrons in the bulk matter (probably supplied by phonons) could be sufficient to create superpairs. The superpairs would form a superfluid. (Keep in mind that only a fraction of neutrons would form superpairs, the rest staying in a fermionic state.)
Which means it would look like kittens?
Yes, but half the time, one of them’s dead.
On thinking about this some more, it seems to me that there might be something approximating an answer to the OP’s question. You can’t have a “lump” of free neutrons, or in fact of free anything, because if they’re bound into a lump, they’re by definition not free (and you’d have to specify what’s binding them, which is rather tricky to do with neutrons). But you could have a free-neutron gas. I’m not sure how you could confine it, if at all, but that’s OK, we can just allow it to dissipate freely. It would remain a gas no matter how much you cooled it (even to absolute zero), because the degeneracy pressure would be enough to prevent it from changing state, even if the ideal gas pressure went to zero. You could force it into a non-gas state with extreme pressure, but the pressure needed would be that of a neutron star, which the OP is apparently not interested in. It would interact with light, but it would be essentially completely transparent to anything below the hard gamma range. Neutrons are fairly penetrating, so it’d be tough to get something you could “feel” this gas with, but a lead slab or the like might do the trick: It’d feel much like moving a sheet of material through any other gas, with a drag force proportional to the square of the speed at which you’re moving the sheet.
Is it still “conjectured,” or has real-life neutronium ever been identified/observed?
[ralph wiggum, ph.d.]
Neutronium looks like neutrons.
[/rwphd]
At what point does Mel get mad enough to chew it?
(pop culture trivia challenge)
Forget what it looks like, when is Qadgop going to tell us what it tastes like? (And “You know, impervious.” is not an acceptable answer.)
“Qadgop the Mercotan slithered flatly around the after-bulge of the tranship. One claw dug into the meters-thick armor of pure neutronium, then another. Its terrible xmex-like snout locked on. Its zymolosely polydactile tongue crunched out, crashed down, rasped across. Slurp! Slurp!”
Sorta like vegemite, but fizzy. With a hint of habanero pepper.
What are the contra-indications for ingesting neutronium? Just in case I come across some.
I believe that not being a Mercotan is one of the standard contra-indications. I could be mistaken, though :P.