This interactive activity adapted from NASA examines an important physics experiment from the late 1800s. The activity features an analogy of swimmers racing in a river to represent light traveling through the hypothetical ether, and then demonstrates how an interferometer, the instrument the scientists used to determine whether light needed a substance to travel through, works. It also explains the experiment’s shocking results, how the results influenced Einstein’s theory of special relativity, and how interferometers have been adapted for use in modern astronomy.