Yes, they do.
A new experiment done jointly by Stanford University and the University of Edinburgh has finally proven that when beer is poured into a glass, the bubbles sometimes go down.
"Bubbles are lighter than beer, so they're supposed to rise upward,"
– Richard N. Zare, the Professor of Natural Sciences at Stanford.
"But countless drinkers have claimed that the bubbles actually go down the side of the glass. Could they be right, or would that defy the laws of physics?"
In 1999, Australian researchers announced that they had created a computer model showing it was possible for beer bubbles to flow downward.
But Zare and former Stanford post doctoral fellow Andrew J. Alexander were unconvinced by the virtual Guinness model and decided to put it to the test by analysing it physically.
"Indeed, Andy and I first disbelieved this and wondered if the people had had maybe too much Guinness to drink," Zare recalled. "We tried our own experiments, which were fun but inconclusive. So Andy got hold of a camera that takes 750 frames a second and recorded some rather gorgeous video clips of what was happening."
A careful analysis of the video confirmed findings: Beer bubbles do sink.
"The answer turns out to be really very simple. It's based on the idea of what goes up has to come down. In this case, the bubbles go up more easily in the centre of the beer glass than on the sides because of drag from the walls. As they go up, they raise the beer, and the beer has to spill back, and it does. It runs down the sides of the glass carrying the bubbles -- particularly little bubbles -- with it, downward. After a while it stops, but it's really quite dramatic and it's easy to demonstrate."
The phenomenon also occurred in other beers, said Alexander
"The bubbles are small enough to be pushed down by the liquid. We've shown you can do this with any liquid, really -- water with a fizzing tablet in it, for example."
"It's just paying attention to the world around you and trying to figure out why things happen the way they do..."