Progress in science is sometimes triggered by surprises. Data collection resembles gathering of new pieces in a jigsaw puzzle and placing them together. Sometimes one of the pieces does not quite fit. It is natural for scientists to instinctively argue that such a piece does not belong; perhaps it is an artifact driven by uncertainties in the data or a misinterpretation of the experiment. This might indeed be the case in most instances. But every now and then, an anomaly of this type signals a real discrepancy from expectations, either a violation of a highly respected but incomplete law of nature—namely an exception to the rule, or an unexpected surprise—signaling the possibility of “new physics.”

One of the well-known historical examples involves the discovery of the Planck spectrum of blackbody radiation, which could not have been explained by classical physics and which ushered in quantum mechanics. The anomaly was declared by the British physicist Lord Kelvin in 1900 as one of the two remaining dark clouds obscuring “the beauty and clearness of the dynamical theory” before its revolutionary role in the development of modern physics was recognized. A more recent example involves quasicrystals, which represent a state of solids that violates translational symmetry. Their accidental discovery by Dan Shechtman in 1982 was discredited for decades since it violated textbook assumptions, but its significance was eventually recognized by the Nobel Prize Committee in 2011.

An example for a current unresolved anomaly involves the reported discrepancy between the measured values of the Hubble constant H 0 (the expansion rate of the universe) in the local universe (based on observations of supernovae) and in the universe just 400,000 years after the big bang (as measured from the brightness anisotropies of the cosmic background radiation). If real, this anomaly might signal the existence of a sterile neutrino; a form of decaying dark matter; a growing dark energy or something else. Another current example involves the anomalously strong absorption of electromagnetic radiation by hydrogen atoms during the cosmic dawn, as measured by the EDGES experiment, which might potentially indicate some form of interaction between ordinary matter and dark matter.

Most anomalies are found to be associated with faulty interpretations or systematic errors in the experiments. Recent examples for such outcomes involve the experimental claims for faster-than-light neutrinos and unusually strong gravitational waves from cosmic inflation. However, some anomalies appear resilient to scrutiny and flag new discoveries.

Daring scientists who pursue an anomalous perspective that deviates from the mainstream dogma serve as agents of progress. In response to their claims, highly reputable but conservative leaders of the scientific community are irritated and attempt to prove them wrong, a process by which a new truth may be revealed. For example, when Cecilia Payne-Gaposchkin suggested in her Radcliffe-Harvard PhD defense in 1925 that the sun is made mostly of hydrogen rather having the same composition as the Earth, the highly respected director of the Princeton University Observatory, Henry Norris Russell, argued that she must be wrong and dissuaded her from including this conclusion in her published thesis.

As he attempted to prove her wrong in subsequent years, he realized instead that she was right. In another case, when Jacob Bekenstein suggested in 1973 that black holes may have an entropy proportional to the area of their horizons, his PhD adviser, John Wheeler, told him that his idea is “crazy enough that it might be right.” Stephen Hawking tried to prove Bekenstein wrong, but he ultimately fulfilled Wheeler’s prophecy and discovered Hawking radiation, his most important scientific result. The moral of these examples is that scientists should not be so hasty to dismiss frogs based on a first impression, since what is intended to be a “kiss of death” can turn one of them into Prince Charming.