An anomaly is an experimental result or an observation that doesn’t fit current scientific theories.

Sometimes anomalies are resolved without modification of the theories that they appear to conflict with. For example, in 2011, an Italian experiment run by respected physicists, appeared to show that neutrinos, a type of subatomic particle, travel faster than the speed of light. This result was anomalous. It appeared to conflict with Einstein’s Theory of Special Relativity, one of the foundations of modern physics. This theory says that nothing can travel faster than the speed of light in a vacuum. Later, it was found that the equipment in the Italian experiment was flawed. In fact, the particles were not traveling faster than light speed and posed no challenge to Special Relativity.

However, sometimes an anomaly is a valid clue that current theories need revision or replacement. For example, for centuries, astronomers had made observations of Mercury that showed its orbit around the sun to be anomalous. Observations of Mercury’s orbit showed a discrepancy from the mathematical predictions of Newton’s Laws.

This anomaly didn’t bother many astronomers because the discrepancy was very small. Possibly, sooner or later the discrepancy would be resolved by finding that observations had been in error, that an undiscovered heavenly body was influencing Mercury’s orbit, or that some other unknown factor was at work. Despite this anomaly, most astronomers and physicists did not question Newton’s Laws.

In fact, Mercury’s anomalous orbit was a clue that all was not well with Newton’s Laws. It turned out that the laws were approximations that failed in the vicinity of large masses such as the sun. In 1915, Einstein’s Theory of General Relativity overthrew Newton’s Laws. It replaced the concept of gravity as a force inherent in matter with the concept of spacetime interacting with massive objects. Spacetime is curved by massive objects, which together with other principles, creates what we call “gravity.”

In the influential book, The Structure of Scientific Revolutions (1962), author, Thomas Kuhn, points out the important role that anomalies have played in spurring revisions of physics theories and the development of entire new theories.