So a guy in Brooklyn spotted Google founder Sergey Brin wearing Google's Project Glass glasses on the New York subway, and the photo went viral. But the not-so-incognito ride still begs the question: What was Sergey Brin doing underground in robot glasses that only work with Internet? Others have spotted Brin out and about with the glasses — on the streets of San Francisco, at fundraisers, even at a Diane Von Furstenberg fashion show — but that hasn't stopped the conspiracy theories from mounting. Here are the plans people think Brin may have been hatching ... and what they say about Google's vision for the future.

A Planned PR Stunt

The Theory: It's all a little too coincidental — Noah Zerkin, the man who snapped the shot pictured above, also happens to describe himself as a "Wearable Computing and Augmented Reality enthusiast" on his Twitter bio, and he works at The SuperTouch Group, where he plays with stuff similar to Google Glass. Questions have been asked: 

Zerkin, with his augmented-reality connections on social media, is just the right kind of person to get attention in the Google Glass arena. Adding to the curiosity over Zerkin is that his spotting occurred just days after Google sent out invites for its first Project Glass "hackathon."

The Likelihood: Low. In a followup tweet, Zerkin denied there was a planned meeting. And in a followup post on his personal site, he writes that he "ran into Sergey Brin on the subway ride home." Zerkin added: "But I have a funny way of running into people." Funny, eh? That's a good way to stoke the speculation.

A Not-So-Planned PR Stunt

The Theory: Brin was just riding around the subway, waiting for someone to recognize him and take a picture that would no doubt go viral. It's not likely Brin rides the subway often, notes The Verge. And he hates doing traditional publicity, notes ReadWriteWeb's Dan Lyons, with the Google founder tending to opt for stunts over press releases — the few Google Glass introductions involved skydiving and a fashion show.

The Likelihood: Medium. Brin likes the subway, sources told Lyons. And, as you can see from the otherwise disinterested people in the photo, he doesn't have the same celebrity status as, say, Jay-Z — he of the semi-planned conversation with that cool lady on the subway last year. So maybe Brin was just commuting, and he does like showing off the glasses in public, to normalize them. (His campaign to make them look cool is failing, by the way — The New York Times's Nick Bilton said Brin looked "like an assassin.") In any case, the chances of someone recognizing a man of Brin's status in a city like New York aren't really that low, and the chance to gain instant recognition would only sweeten the deal.

A Test Run

The Theory: Brin was simply just out and about, testing the glasses. The Independent's Martin Hickman suggests Brin was learning information about his fellow passengers. Or maybe he was testing some of the offline features, surmises TechCrunch's Darrell Ehterington

The Likelihood: Possible. Wi-Fi on the subway remains something of a pipe dream for commuters, but certain New York stations do have Internet — care of Google — including the 14th street station where Brin got off, according to Zerkin's post. So maybe while he was down there, Brin was checking out offline transit maps, as Etheringon suggests, then looking into updated schedules while on the Wi-Fi connected platform. 

Google hasn't commented on the spotting, and Zerkin's telling of his "run-in" with Brin doesn't add much clarity. We might never know the answer to the Great Subway Nerd Mystery of 2013.