The ‘Pioneer Anomaly’ That Threatened to Upend Physics
In 1972 and 1973, two unmanned spacecraft—called Pioneer I and II—were launched on missions to the outer reaches of the solar system. The probes sent beautiful images of Jupiter and Saturn back to Earth, along with exciting new data about their makeup. And then the scientists monitoring the crafts sat back and relaxed as the ships began their long, lonely journeys into deep space.
Until something weird turned up in the data.
Around 1980, John Anderson, a physicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., noticed that the spacecraft were not as far away from Earth as they should have been. After careful measurements, he and his colleagues determined that both probes were slowing down, as though some tiny force were pushing them toward the sun. NASA had extremely detailed models of the solar system and of its deep space probes. And yet none of these could account for the deviation. The discrepancy came to be known as the Pioneer anomaly.
The deceleration Anderson observed was tiny, less than a nanometer per second squared. But it was extremely nettlesome to physics, a field that relies on being able to generate very accurate predictions about the universe. Over the ensuing decades, physicists have proposed any number of explanations for the slowdown, and even radical potential revisions to basic cosmology—including, in some cases, rejecting Einstein’s theory of gravitation—in attempts to explain this minute disagreement between theory and observation.
It may seem strange that such a small discrepancy could prompt physicists to reconsider a century of established science. But in that sense, the Pioneer anomaly offers an unusually clear view of how big changes in science often work.
This summer marks the 50th anniversary of the publication of “The Structure of Scientific Revolutions,” in which the groundbreaking science historian Thomas Kuhn famously introduced the notion of a “paradigm shift” to describe the dramatic change in worldview that he claimed accompanied revolutionary periods in the history of science.
Before Kuhn, most people who wrote on such things took for granted that the history of science is essentially progressive, a steady march toward knowledge. Kuhn turned this picture on its head. He argued that actually the history of science is discontinuous, marked by periods when well established theories are rejected entirely, replaced by alternatives bearing so little resemblance to what came before that in some cases it would be impossible to even translate between new theories and old.