Amid all the excitement about gravitational waves (check my video for more on that) another fascinating discovery may have just been made in Tokyo by Kisamori and Shimoura, whose latest findings are published in Physical Review Letters (3 February, 2016). This new discovery concerns a hypothetical particle that's been debated for over half a century: the tetraneutron.
Inside the nucleus of an atom the smallest particles are things called quarks. Quarks have charge (just like protons and neutrons do) but they have other properties as well which we need to consider, one of these properties is called "spin". The theory gets very weird, very quick but the key features of spin are as follows.
1) It doesn't mean the particle is literally rotating around an axis.
2) It is, however, related to the possible rotations a particle can have.
3) Only certain values of this property seem to be present in the Universe.
The same way there's no such thing as half an electron-charge, there's no such thing as "a quarter spin" for instance. It's a property the Universe uses and there are restrictions imposed on it. Precisely why there are restrictions on particle spin is unclear. It might just be a feature of the Universe (like the fact that matter exists at all) or there could be some underlying principle we don't know about yet. At the moment spin seems to obey certain rules and we sum up these rules rather neatly using what's called the Pauli Exclusion Principle. This principle states that no two Fermions (particles with half a "spin") can have all the same exact features...if they did, they would just be the same particle.
For instance, if there was a copy of me, it would have the exact same mass, height, appearance, charge, colour etc. etc. and for some reason the Universe refuses to let two identical things like this exist. If there is a deeper reason, we haven't got to it yet. We just know it's seemingly true. If your particles have a whole spin, they can be identical clones of one another, but if we're talking fermions, the Universe seems to obey Pauli.
In the core of a nucleus, quarks are lumped together in groups of three and groups of two. The groups of three are what we call protons and neutrons and they obey the Pauli Exclusion Principle. The way protons and neutrons bind to each other is via something called "The Strong Nuclear Force" which, rather confusingly, is not the same as "The Strong Force" - one of the four fundamental forces of nature. The Strong Nuclear Force is actually a pseudo-Force, it's the result of protons and neutrons exchanging quarks, so it should really be called the Strong Nuclear Interaction (and it sometimes is). As protons and neutrons hand quarks back and forth between each other, they are held together and, in this process, briefly have lots of properties in common. But never all of them, never violating Pauli.
There are a lot of fiddly rules called "Quantum Chromodynamics" which list what protons and neutrons can and can't do, but one of the important outcomes is that, due to the Pauli Exclusion Principle, a proton and a neutron can bind to each other because they're different enough. If you had two neutrons, say, and tried to bind them, they would have to have identical energies as well as having identical charge, spin etc. etc. In other words, they won't do it.
Two neutrons and a proton can bind together comfortably (it's called Tritium) because the two neutrons are interacting with the proton slightly differently at any one time, so they never violate Pauli, but two neutrons together, or three, or four or five doesn't seem to be possible to bind as one stable particle. You could attract lots of neutrons together via gravity (as is proposed in a neutron star) but they don't actually bind to one another.
So, if our understanding of the Pauli Exclusion Principle is right, and our understanding of how neutrons and protons bind to each other is also right, dineutrons, trineutrons, tetraneutrons and so on should be impossible. For them to bind, they'd have to be undergoing the Strong Nuclear Interaction/Force which would at some point, violate Pauli.
The recent discovery in Tokyo provides some scant evidence of what might, possibly, maybe be a tetraneutron briefly existing. More research is needed obviously, but if it's validated over the coming years then either Pauli is wrong or we need to change our model of how protons and neutrons bind together (Strong Nuclear Interaction). If I was a betting man I'd say this discovery will probably turn out to be something else. But, if tetraneutrons do turn out to be real, I'd bet on our model of particle-bindings being at fault rather than the Pauli Exclusion Principle.