In such cases, different predictions made with each theory can be used to set up experiments that select one theory over another. In the 17th century there were competing theories about whether electromagnetic radiation, such as visible light, consisted of particles or waves. At the beginning of the 20th century Max Planck postulated that energy can only be emitted or absorbed in small, discrete packets called quanta. This seemed to favor the particle theory, particularly after Einstein demonstrated that light behaves like a stream of particles in photoelectric cells. However, diffraction experiments with electrons, which were considered particles because they had a measurable weight, showed all the characteristics of waves.
Some scientific facts are very hard to comprehend.
Yet, these are observable phenomena verified over and over again by many people all over the world. The behavior of the speed of light is another physical fact that is hard to understand. The speed of light in a vacuum is approximately 299,792 kilometers per second. A famous experiment conducted by Michelson and Morely at the end of the 19th century showed that the speed of light was the same perpendicular to the orbit of the earth and parallel to the orbit of the earth. The orbital speed of the earth of 29 kilometers per second could not be detected in the measurement of the speed of light.