Scientists use many ways of choosing interesting and hard problems. Mine is to look for anomalies.
There are several in the Quartet. One is quantum entanglement, which figures in the fusion process – it’s anomalous because it implies faster-than-light travel. Currently physics says FTL is impossible, so physicists deal with entanglement by either saying that two separated items acting as one do not imply travel, or, more usually, by not thinking about it.
Future posts will flag up more anomalies.