Neutron matter is matter comprised entirely of neutrons, as it exists in neutron stars.

Most optical phenomena encountered in everyday life, such as light reflection and spectral absorption (i.e. color appearance) are the result of mechanisms involving electrons.

My simple question: How would a macroscopic sample of matter consisting entirely of neutrons (without electrons) appear to the naked eye? Assume the matter is degenerate and stable.

Let me add here that I'm not specifically asking about the appearance of a neutron star, as Wikipedia states that it would radiate so much that it appears white.

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    $\begingroup$ Note that even in neutron stars the equilibrium between neutrons and protons/electrons still consists of plenty of the latter, especially when considering the absolute density. $\endgroup$ – user10851 Dec 7 '13 at 8:35
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    $\begingroup$ Related: Are Neutron stars transparent? $\endgroup$ – Brandon Enright Dec 8 '13 at 4:03
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    $\begingroup$ Possible duplicate of physics.stackexchange.com/questions/22722/… $\endgroup$ – Brandon Enright Dec 8 '13 at 4:05
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    $\begingroup$ If the specific heat of neutronium per gram is anything comparable to the specific per gram of ordinary matter, it is going to take an enormous amount of time for neutron stars to cool down. So this question can't be answered by observation, and has to be answered by theory, which I suspect is not up to the task. $\endgroup$ – Peter Shor Dec 12 '13 at 22:14
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    $\begingroup$ Pure neutron matter cannot be "degenerate and stable". To be stable it has to have a population of degenerate protons and electrons. This question is way too hypothetical. It has to be a question about the appearance of neutron stars because that is actually the equilibrium state of a big ball of neutrons. A much "smaller" neutron star (below about 0.2 solar masses) cannot be stable either. Any attempt to "make" macroscopic degenerate neutron matter would result in atom bomb type explosions. $\endgroup$ – Rob Jeffries Feb 14 '15 at 14:46

This is mostly conjecture, based on physics and common sense.

We know that photons couple to other (charged) particles via the electromagnetic force. Whilst neutrons themselves have zero charge, they are comprised of bound (u,d,d) quarks, which are charged, and with which, the photons could interact.

The density of pure neutron matter would be extremely high, so even a small amount of it would contain a lot of neutrons, and thus many opportunities for photons to interact with quarks. Photons will either scatter directly off the neutrons or briefly induce an excited state, which would decay of the order of $10^{-24}$s, emitting a photon of equivalent energy.

As a simple model, this is not too conceptually different from why clouds appear the way they do (white when thin, black when dense - if the light source is behind the cloud). Therefore, I would conjecture that a lump of neutronic matter would appear black if in front of a light source and white if behind it.


Although Neutronium is used in SF a lot, I got a different take on it from a video of a researcher who put his iPhone in a neutron beam to see what happens (but that's another story).

He pointed out that a marble sized ball of neutronium would not only need a SF "force field" to keep it under pressure, but also be in time stasis because it would give off as much energy as some rediculous number of atomic weapons per second.

A time stasis field would block anything inside it from view (in Larry Nivin's stories they are perfect mirrors) and can't be effectivly transparent because the stopped time inside would stop the light from coming or going.

If it were contained and stabilized in some manner that did not cloak it, what might it look like? Bulk objects reflect light selectively to give color, due to the electrons blending into a net of sorts that have many ways to stash energy so "take" light of any frequency. A pure element gas would show spectral lines, coming or going as the case may be, and ignores light of different frequencies because there are only specific energy levels present. So, for example, it would be perfectly transparent except for a narrow bit of yellow taken out. Hey, sounds like air, nice and clear to our range of frequencies.

So are the electrons in our sample a mesh like a mineral grain, finiky like a gas, or liquidy like a metal? Uh... what electrons? Actually any charged particle will do. Nope, none of any kind, just neutrons.

So, it would be perfectly transparent. Not considering gravity lensing, and the effects of whatever is stabilizing it. Or maybe not... if it's really "like a neutron star" you would have other stuff in it at equilibrium, plus a crust of other materials covers it so you can't see it anyway so let's suppose pure contained neutrons somehow. But, in a story setting that would give you licence to make it clear, black, white, or mirror, "depending".

Actually, neutrons have a concentric separation of charges, so from the outside the concentric charges cancel out, but might that give some degrees of freedom for light to interact with? If the skin's charges can somehow blend together and work like a mineral grain, I think it would not react to anything less than hard x-rays, so still invisible to our eyes.

Effects of a very small electric dipole moment (and I do mean very) beyond the Standard Model would be correspondingly small, so don't expect anything visible from unknown effects yet to be discovered.

Does magnetism affect light, if very strong? That's a question for another topic. If you could get all the neutrons to line up their fields...

  • $\begingroup$ Why should neutron star matter "give off as much energy as some rediculous number of atomic weapons per second"? Clearly the matter of actual neutron stars is very hot, and I can imagine that when neutron fluid is formed there would be lots of high energy nuclear reactions (exothermic? no idea.) but why would this be continuous? $\endgroup$ – ikrase Sep 9 '19 at 7:07
  • $\begingroup$ Because neutrons decay. $\endgroup$ – JDługosz Oct 2 '19 at 18:14
  • $\begingroup$ Doesn't the pressure holding it in degeneracy prevent that from happening (or hold it in equilibrium)? $\endgroup$ – ikrase Oct 3 '19 at 5:20
  • $\begingroup$ If it’s a star, held together by gravity. If you had a marble-sized piece, there is nothing stopping it. $\endgroup$ – JDługosz Oct 3 '19 at 14:46
  • $\begingroup$ But if it's held in an imaginary force-field, that will hold it in degeneracy pressure with no need for time stasis. $\endgroup$ – ikrase Jan 9 at 5:46

The hypothetical matter you are referring to is called neutronium. It is basically a huge nucleus of pure neutrons. The problem is that current physics predicts that it would instantly decay via radioactive decay and explode much like an atomic bomb. thats how it would look like.

Edit: My answer concerns the only qualitatively different case from the neutron star case, because the only force that could keep your nucleus together is gravity.

the decay of individual neutrons can be neglected here.

Edit: And now i have the cooling down neutron star:

After several billion years they are still at many thousands of degrees, and it would take a large multiple of the age of the universe to get to room temperature (exact values depend on unknowns about the neutron star; for example, it matters how much energy they have lost in neutrinos). The surface of such a star would probably be made of hydrogen in long chains of atoms. At room temperature, the emission would be almost all in the infrared, so the star would appear completely dark. If you were to shine a light on the neutron star, its appearance would depend on whether it had maintained its strong magnetic field. If so, the light would not be absorbed until fairly deep in the atmosphere, and when it was radiated it would again be in the infrared, so it would appear black. If the magnetic field had decayed away, the star might have a metallic glint; I'm not sure, because it's difficut to project the optical look of such an object.


note that you most likely cannot get a qualitatively different phenomenon even if you replace the gravity with a fictional force of your choice.

  • $\begingroup$ I have edited the question to assume that the matter is degenerate and therefore stable. The question is not whether neutronium can exist under normal conditions, but rather, assuming it exists, how would it interact with light. $\endgroup$ – Brandon Enright Dec 13 '13 at 6:29
  • $\begingroup$ Is this so? Could you explain some more: yhis is not my field, but neutrons decay into protons, electrons and antineutrinos and the process would only be "explosive" if one decay provokes more than one other, which is not clear to me. Moreover, there has to be a way whereby gravity forestalls this decay when you've got enough of this stuff in one place to form a neutron star, which is stable. $\endgroup$ – WetSavannaAnimal Dec 13 '13 at 6:31
  • $\begingroup$ Yes, it would be a spectacular explosion, because anything it would decay into would also be extremely unstable already. unlike regular atomic bombs where you would need high energy neutrons to induce nuclear fission of the relatively stable nucleons. $\endgroup$ – Jani Kovacs Dec 13 '13 at 6:41
  • $\begingroup$ Free neutrons are unstable but that does not mean the tiniest bump sends them off exploding in a chain reaction. If they did, the sun would have blown up long ago. The Hiroshima bomb would have consumed the Earth. $\endgroup$ – Brandon Enright Dec 13 '13 at 6:51
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    $\begingroup$ @Wet Actually, all gravity does is shift the equilibrium concentration. Neutrons still decay in neutron stars, but they are being reformed by combining protons and electrons. Because both reactions release high-energy neutrinos, neutron stars cool by maintaining equilibrium, paradoxically enough. $\endgroup$ – user10851 Dec 13 '13 at 7:47

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