system
Given the following post, respond as if you're commenting on Reddit:
human
Could I have a bag of neutrons? And if so, what would it look like, would they be reactive?
gpt
Depends on the density of the neutrons in question. If you're thinking standard every day densities regularly encountered in everyday life, then the answer is no.

Neutrons cannot exist by themselves for extended periods of time. Outside the extreme gravity fields of a neutron star (we'll get to that in a minute), your average neutron will beta decay with a half life of about 10 minutes, spitting out an electron and a proton, with the leftover mass energy being released as an electron antineutrino. In somewhat rare cases, the decay event will also spit out a gamma ray. In extremely rare cases, the electron does not gain enough energy to escape the proton, and the decay event forms a hydrogen atom, with the antineutrino taking away much more energy than it normally would.

In short, you'd have a fiercely radioactive (and potentially flammable due to the forming hydrogen) and glowing bag of neutrons in your possession.

When a high mass star dies, depending on its composition and how much mass ends up in the core at T-0, it can form a stellar remnant known as a neutron star during the ensuing supernova. The inward force of gravity is no longer balanced by the outward force of thermonuclear fusion (fusion above nickel/iron consumes instead of produces energy), and the core is made of what is known as degenerate matter, where for each atom no 2 electrons can share an identical state (See: Pauli Exclusion Principle). Eventually, the weight of the iron "ash" as well as the rest of the star builds up to the point where electron degeneracy pressure can no longer withstand the pressure, and the electrons take the only way out left to them: by merging with the protons in the core.

In the space of about a second, all of the electrons and protons in the core are consumed, and the resulting lump of matter screams inwards at a very significant fraction of the speed of light. Neutron stars run into another degenerate "barrier" and bounce off, rebounding outwards... right into the collapsing upper layers of star plasma crashing in. Unstoppable force (collapsing upper layers of star) meets immovable object (neutron star core), and you get a supernova as everything above the ball of neutrons goes boom.

After the star has died and supernova faded, we have ourselves a neutron star, a couple-solar mass ball of neutrons and more exotic matter crammed into a space roughly the size of Manhattan Island. This ball of neutrons is *extremely* hot (600,000 kelvin is the typical surface temperature), very highly magnetized (many many millions of times stronger than Earth's field), is rotating at several hundred revolutions per second (with linear surface velocities pushing up to .24c), and dense enough to where a teaspoon of the stuff weighs more than Mt Everest. The star itself is so dense it generates its own gravitational lensing effect, allowing a viewer to potentially see more than half of the surface at any given time. But the neutrons would be stable.

You still wouldn't want it though. A thimble-full of the stuff would still kill you, whether it be from the heat and radiation cooking you alive, the magnetism ripping all of the iron out of your blood, the attempt at extracting it, the tidal forces it would exert on you, or it exploding when it realizes its no longer being crushed together by several billion Gs of gravity.

**tl;dr**: Free neutrons can and likely will kill you and will leave an expensive mess for the rest of us to clean up.

Edit: Thanks for the gold, kind stranger!