Shapiro’s Bathtub Experiment


Over forty years ago, in the 1960s, the world briefly became captivated with how a bathtub drains. Did something called the Coriolis effect influence the twirling water?

The Earth’s rotation influences how fluids swirl on the planet’s surface. It’s why low-pressure systems in the northern hemisphere twist counterclockwise. This phenomenon, known as the Coriolis effect, is the appearance of an object to deflect to one side in a rotating reference frame. Since it is such a tiny effect on small scales, no one had yet proven that this inertial force actually affects how water leaves a bathtub, despite many previous efforts.

In 1962, the same year that Watson and Crick received their Nobel Prize for the discovery of the double helix, MIT professor Ascher Shapiro, an expert in fluid mechanics, set up an elaborate test to try to change that. Shapiro’s elementary experiment, which started with a bathtub, quickly turned into a complicated and ambitious undertaking that involved a tank six feet wide and six inches deep.

The Coriolis effect at MIT’s latitude, 42°, was just “thirty-millionths that of gravity, which is so small that it will be overcome by filling and even temperature differences and water impurities,” reported one of many newspapers and periodicals that covered the results of Shapiro’s experiment. After much tinkering to cancel out these interferences, and presumably a hefty water bill, Shapiro found the answer: the Coriolis effect does indeed cause a bathtub vortex in the northern hemisphere to swirl counterclockwise.

But even after his results were published in a letter to Nature, Shapiro’s confirmation drew the skepticism of readers. In correspondence with one reader, Shapiro noted: “Many results contradictory to this have been reported in the literature but all of them have involved faulty experiments due to a lack of realization of how sensitive the experiment is.” He was supported, however, by colleagues in the Northern hemisphere who confirmed the counterclockwise bathtub drainage, while those in the Southern hemisphere demonstrated the same effect in the opposite direction—a clockwise flow—just as anticipated.

In a world without electronic communication, where author correspondence was a more prolonged affair, a sort of chivalry existed between a scientist and a popular audience who took an interest in academics. Scrawled with a pencil on back-and-forth correspondences between Shapiro and his fans and housed today within a dusty and faded folder in the MIT archives are the records of reprints being sent, of questions being answered, and of careful and nuanced responses that understated Shapiro’s high standing at MIT. A Ford Professor at the time, and later elevated to Institute professor, Shapiro took time to send article reprints for those who asked for it and to answer mail from inquisitive readers, some of whom promoted dubious questions and claims.

For more than a decade, letters and local newspaper clippings from all over the world about the “bathtub vortex controversy” were sent to Shapiro:

–An eighth grader in St. Louis, a junior high schooler in Ontario, and a high schooler in Georgia looked to verify the bathtub experiment for a class project.

–A navy sailor recalled crossing the equator during a World War II Pacific tour, not noticing the Coriolis effect in the drainage system while cleaning the decks.

–A technical director of a chemical company heading to Quito proclaimed, “Ever since the R.A.F. taught me to turn 90° starboard with a tailwind to escape the horrors of low pressure areas I have been intrigued with swirls in tubs.” (He had been there before and promised to double-check the absence of a swirl direction.)

–A school teacher in California who had visited 87 countries remarked, “…pulling bathtub plugs has become a hobby. I’ve pulled from Tierra del Fuego to Barrow Alaska, and Narvik, Norway to Capetown and Tasmania.”

–A Pennsylvania publisher tried to somehow apply Shapiro’s results to plants: “I am trying to duplicate results now in my greenhouse, with geraniums. I have a terrific amount of data in this field and hate to let go with it, because the world is so ready to call anybody a crack-pot, who works in this particular field.” (Details were not provided.)

–A West Virginian reader began in a handwritten letter, “I have tried to get several people with ways and means to run this experiment for several months. Since I have no money, prestige, or a Ph.D. after my name no one would listen. You have done something that many Archbishops Scientists [sic] knew was impossible. I would like very much to know how this was conducted.”

–A man from Lausanne, Switzerland, insisted that bathtub drainage rotation was linked to barometric pressure. He, perhaps captured the mood of the time best with the end of his third reply; “I hope you don’t mind this; I’m enjoying it.”

Who would have thought the swirl of a bathtub would have been a matter of great interest? For a seemingly insignificant problem, the bathtub controversy loomed large in Shapiro’s career until his death in 2004. The first line of his obituary in the Boston Globe read: “Dr. Ascher Shapiro wanted to get a handle on how fluids move whether they were swirling down the bathtub drain, or flowing through the human body.”