In the early 2000s, physicists at a particle accelerator in France saw something that simply couldn't be. And it "couldn't be," two different ways. Now the world is attempting to figure out which way is more impossible.
In 2001, Francisco-Miguel Marqués and his team were having a typical day firing one tiny thing at another tiny thing. The first tiny thing was beryllium-14, and the tiny thing they were firing it at was a carbon target. Beryllium-14 had a cluster of four neutrons on it, and they were attempting to blow it apart and log the appearance of the neutrons. They should have gotten four little flashes. They got one big flash.
Instead of the neutrons splitting apart, the neutrons broke off in a cluster, traveled in a cluster, and were detected in a cluster. So what's the big deal? Well, the Pauli exclusion principle specifies that particles in the same system cannot be in the same quantum state. As a consequence of this even two neutrons shouldn't be able to stick together, let alone four. However, four neutrons smashing at high speed into a carbon atom, and then reaching a detector at exactly the same place and exactly the same time is nearly as impossible as the idea that a basic tenet of physics needs to be modified.
Over the years, the urgency in figuring out the tetraneutron has faded. So far, no one has been able to reproduce the results obtained by the Marqués team. The possibility that it was an anomaly or an error is becoming more likely. However, one group of physicists has come to believe that four neutrons, or rather two groups of two neutrons, might be a possibility within the confines of the exclusion principle. So who knows?
Top Image: ATLAS Detector