NPR logo Joe Palca on the Scientific Principles of Guinness

Joe Palca on the Scientific Principles of Guinness

Bubbles in a pint of Guinness: Do they go up or down? The Zarelab/Stanford University hide caption

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The Zarelab/Stanford University

NPR Science Correspondent Joe Palca weighs in on the Guinness story.

Seeing your item on Guinness reminded me of an interesting fact I learned about the famous dark brew a few years ago. If you look at a freshly pulled pint of Guinness and watch as the foamy head subsides, you'll notice that the bubbles appear to be going down the side of the glass. If you think about the bubbles you're used to seeing in liquids, they typically go up. So is this an optical illusion, or do the bubbles really go down?

The answer comes from Stanford University chemist Richard Zare. In addition to being one of the world's foremost authorities on laser chemistry and a former chairman of the National Science Board, Zare is also fascinated by bubbles. He set out to find the answer to the Guinness bubble paradox. Here's what he found:

The bubbles do go up to start with, but because of friction along the sides of the glass, the bubbles in the middle go up faster than the bubbles you can actually see on the side of the glass. These fast rising bubbles create a circular current of liquid beer, which rises in the middle of the glass and sinks at the edge of the glass. The sinking beer pushes the bubbles down with it, and voila, bubbles that go down.

Check out these videos of Zare's demonstration.

Joe Palca