Music Timeline comes with some important caveats.

One, the tool commits the historian's sin of anachronism. What we see here is not necessarily what was popular then, but what people now listen to from then. In some cases, that will probably match up, but in others, it will not. Of course, if you're not a historian, knowing what other people still think is relevant from the past might be quite useful.

Two, the data is drawn from Google Play only, which means that we're not talking about "people" broadly, but the specific subgroup of people who use Google Play. Their demographics are unknown, so it's hard to know if they're representative of the population, current or historical.

Three, the metadata flowing into the system is itself suspect. As Google freely admits and music fans would testify, reissues throw off the data.

I asked writer and record-label owner Douglas Wolk to take a look at the tool with me. He's been studying and writing about music for 20 years.

He summed up the problems with Music Timeline in one sentence. He said, "What it's reflecting is what Google play users have in their personal libraries, which is different from what people listen to, which in turn is differnet from what people historically listen to."

That doesn't mean that it's not fun to play with, but merely that you wouldn't want to rely on this data for a college research paper.

And you might not want to use it to settle a bet in a bar, either. At least for more granular questions.

Elvis, for example, is classified as "Pop." Michael Jackson is in "R&B/Soul. U2 sits with "Alternative/Indie."

Wolk pointed to the tool's set of U2 data. It shows Rattle and Hum as the very peak of U2's musical success. It sold 5 million units, according to the band.

Achtung Baby, however, sold 8 million. All That You Can't Leave Behind sold 12 million copies. Neither album's massive success appears on the timeline.

It's possible that this is an artifact of the normalization process that Google had to go through to account for the boom in the number of albums being produced and sold. Or Google Play users (rightly) think All That You Can't Leave Behind is one of U2's worst albums.

Despite the best efforts of Google's Music Intelligence team, the issues with U2's profile highlight that it's hard to tell what's signal (from the underlying musical trends) and what's noise (from the artifacts of processing the data about the trends).

Wolk called attention to The Echonest's dataset, which powers the recommendation engines at Spotify, Rdio, and other streaming music services. He thinks that might be more useful data because it can tell us what people are actually listening to, not just what's in their libraries.

In that way, the new tool is like Google's Ngram Viewer, which takes the corpus of Google Books and allows people to see trends in word usage through time. It's fun! But the visualizations are so smooth and precise that one could be fooled into thinking that they represent some ultimate truth about music. And I don't think even Google would want you to do that.

Google Music Timeline is a way of starting a discussion about music popularity, not ending it.