As a result of the importance attached to the ether and the absolute frame, it became of considerable interest in physics to prove by experiment that they existed. Since it was considered likely that Earth was in motion through the ether, from the view of an experimenter on Earth, there was an “ether wind” blowing through his laboratory. A direct method for detecting the ether wind would use an apparatus fixed to Earth to measure the wind’s influence on the speed of light. If v is the speed of the ether relative to Earth, then the speed of light should have its maximum value, c v, when propagating downwind, as shown in Figure 26.3a. Likewise, the speed of light should have its minimum value, c v, when propagating upwind, as in Figure 26.3b, and an intermediate value, (c 2 v 2)1/2, in the direction perpendicular to the ether wind, as in Figure 26.3c. If the Sun were assumed to be at rest in the ether, then the velocity of the ether wind would be equal to the orbital velocity of Earth around the Sun, which has a magnitude of approximately 3 104 m/s. Because c 3 108 m/s, it should be possible to detect a change in speed of about 1 part in 104 for measurements in the upwind or downwind directions. However, as we will see in the next section, all attempts to detect such changes and establish the existence of the ether (and hence the absolute frame) proved futile.