With this, relativity can be put to a crucial test: Two neutron stars orbiting each other should lose a precisely defined amount of energy as they radiate gravitational waves. This, in turn should lead to a characteristic orbital decay: a decrease in their orbital period. In this way,the first indirect evidence for the existence of gravitational waves was obtained in the 1970s, culminating in the 1993 Nobel prize for physics for Joseph Taylor and Russell Hulse. The figure shows the comparison of relativistic prediction (blue curve) and observational date (light-blue bullets). As the orbital period decreases, the neutron stars complete each orbit a bit earlier than expected. In the figure, the number of seconds that the neutron stars arrive too early is plotted against the observation date.