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i think the main scare here was that the bad doctor was now on his way to figuring out everyone's alt (through the transitive property, starting with sue and EM)




The problem with that theory is that, quite simply, Elongated Man NEVER had a Secret ID. In fact, it was established that he was a "celebrity superhero," who would go on talk shows and write books as "Ralph Dibny, the World Famous Elongated Man" Furthermore, Sue was a celebrity before they got married. She was the daughter of the one the DCU's richest men.

Basically, Ralph and Sue were, long before the Satellite Era JLA, one of the DCU's most famous couples, sort of the Bennifer or Brangelina of Earth One.

Therefore, Dr. Light, along with every villain around, not only knew Ralph's ID, they knew who he was married to.

As such, if there was any way to figure out the rest of the JLA's Secret IDs simply by knowing Ralph's, everyone in the DCU, not just a few supervillains and Batman, would require a mindwipe.