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Tetraneutron Discovered
Caid Raspa writes "According to this
Press Release the French have (accidentally) produced six nuclei of tetraneutron (nucleus with four neutrons and no protons). Theoreticians have previously thought that tetraneutron does not exist. As there is no electric charge in these nuclei, they allow better studies of the nuclear forces. The scientific article is also available at
arXiv.org."
Actually, the article says they may have produced some, not that they did produce some...
The trouble with high-level physics is that theoritical models are actually built on clay... nothing is ever sure, there are always things you need to adjust, and such...
More to the point, nothing is ever certain in any science. Science can only disprove hypotheses - it can never prove anything. The language is pretty standard for researchers talking about an unconfirmed result. They're pretty sure that they got it, but until it's been checked by other independent teams, no one will consider this a done deal.
It's just like Einstein saying he "may" have had new gravitational laws, or Pasteur saying he "may" have found a way to prevent disease. Both were sure, but the results were yet to be confirmed.
Give 'er a year and we'll have a definitive answer.
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This was a big concern when the Large Hadron Collider was about to go into action. Some feared that the energies would be high enough to create mini black holes, which would promptly fall out of the chamber and begin eating the Earth. Eventually someone realized that higher energy collisions from cosmic rays take place above the Earth every day, and we haven't gotten eaten, yet.
In other words, whatever we can do is already being done in that great laboratory in the sky. Literally in the sky - a few hundred miles over our heads.
The living have better things to do than to continue hating the dead.
One recurring theme is where you cross the line between quantum and classical behavior. How many Fe atoms to you need before it behaves like the Iron we all are familiar with.
This appears to be another case. At some point of glomming neutrons together you get a neutron star, though that's still an odd beast. Where do you cross the line between Tetra/Penta/Hexa-neutrons and a teeny-tiny neutron star? (I suspect this one's easy to figure, in terms balancing gravity against residual strong and weak forces, but I don't know how to do it.)
The living have better things to do than to continue hating the dead.
A single neutron, not bound to a proton, is not stable against decaying into a proton. (It's oh-so-slighlty more massive than the proton.) Half-life is on the order of minutes (it's been years since I did nuclear research in undergrad and the exact #'s are escaping me). In a nucleus it's stablizied because if it decayed, the electrical repulsion between the old protons and new proton would be to great, so it's more stable if it remains a neutron. Presumably, these neutrons would also decay, and you might expect (if the 4 nucleons remained together) to see it decay to 4He (ie 2 protons and 2 neutrons)--I don't know if Hydrogen 4 has any stability, but I don't think so.
We've got two lives, one we're given, and the other one we make. --Mary Chapin Carpenter
- Does a tetraneutron spontaneously fly apart?
- Does a tetraneutron undergo beta decay?
The second question doesn't even make sense to ask unless the answer to the first question is no. Until this experiment, nuclear physicists were pretty much convinced that the first answer was yes, which makes the second question nonsensical. Process #1 works via the strong nuclear force, so the time-scale for it to happen is simply the size of the nucleus divided by the typical speed of the neutrons, which is about (10^-15 m)/(10^6 m/s)=10^-21 s. Process #2 works via the weak nuclear force, so the time-scale is much longer --- probably on the same order of magnitude as the beta-decay lifetime of nuclei like 6He, which is maybe 10^-3 s.Since the paper appears to establish that process #1 does not happen, process #2 is what must happen. There is no doubt at all about its being beta-stable --- it's not.
So to answer the original poster's question, here's why people were expecting that the tetraneutron would fly apart. The reason is the Heisenberg uncertainty principle plus the Pauli exclusion principle. If you try to corrall 4 neutrons into a nucleus, their small delta-x requires a large delta-p. That's why they're moving at ~1% of the speed of light. Since they're moving so fast, their attraction might not be enough to hold them together.
So far, this reasoning applies to 4He just as much as it applies to a tetraneutron. So why would 4He be so much more stable? Well, the Pauli exclusion principle says that in a tetraneutron, the first two neutrons can both go in the lowest energy level, with their spins in opposite direction, but the third and fourth have to go in a higher energy level.
The real question is whether the experiment is right or not. Neutron detection is notoriously difficult. In their paper, they go to great lengths to try to show that it wasn't just four neutrons from unrelated events that happened to hit the same detector --- a random coincidence. Their arguments appear convincing, but it's the kind of thing that you could easily get wrong. I'd like to see it reproduced at another lab. If it is correct, then the next step is to start measuring the properties of element zero (zeronium?). What's its lifetime? Its binding energy? Its rms radius? Does it have any bound excited states?
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