 In 1887, Michelson teamed up with Edward Morley and published the results of their experiment that used an interferometer to measure the differences in the speed of light from platforms moving in motion with respect to each other. We'll spend a little time here going over how they did it. Since 1801, when Young proved that light traveled as a wave and throughout most of the 19th century, it had been assumed that space was filled with a substance called the ether to support light propagation, just like air supports sound wave propagation. The ether represented the universal frame of reference against which all other motion could be measured. The question at the time was how fast is the ether moving, or more precisely how fast is the earth moving through the ether? Michelson and Morley were trying to answer this question with their experiment. A good way to see what's happening is to picture a river that measures D across and is flowing to the right with a speed capital V. Now we put two boats in the river, each moving with a speed lowercase v. One boat will move across the river to a point on the other bank directly opposite the starting point and then return. The other boat will travel downstream the distance D and then return to its starting point. We'll calculate the time required for each round trip. Let's take a look at the boat going across the river. If the boat headed directly to the destination point, the current would take it downstream and it would miss its target. To compensate, the upstream component of its velocity would have to match the flow velocity of the river. This would give us a right triangle, where V' would be the net speed across the river. We can calculate V' by using the Pythagorean theorem. The same analysis works for the trip back, so the time for the round trip can be calculated as twice the time for one way. It's two times the distance divided by V'. Substituting the value for V', we get the final equation. Now let's take a look at the boat traveling down the river and back. The time it takes to go the distance D is simply D divided by the speed of the boat plus the speed of the river. The trip back takes D divided by the speed of the boat minus the speed of the river. Taking the common denominator to add these two times gives us the time it takes to make this round trip. Let me take a quick aside here, because this is a good equation for illustrating how we use math in physics. Notice that if the speed of the river is greater than the speed of the boat, time goes negative. If we took the equation to be a general statement about time, one would conclude that time can flow backwards. But if we stick with the situation that we use to develop the equation, we see that a negative time simply means that the poor slow boat can never get back to its starting point. The river will simply continue to carry it downstream. Now back to our example. If we take a look at the ratio of the cross river time t sub a to the down river time t sub b, we see that it creates an equation that can be solved for the velocity of the river. For example, if the boat speeds are 25 kilometers per hour and we carefully measure the time of the two round trips to be 10 minutes for the cross river round trip and 15 minutes for the down river round trip, then we can find the river flow. In this example, it's 8.68 kilometers per hour. Michelson and Morley understood that the earth is moving through the ether in different directions at different seasons. In our segment on the solar system, we found that the earth is revolving around the sun at 30 kilometers per second. What Michelson and Morley did was to measure the ratios for light traveling with the ether and across the ether to determine the speed of the ether, just like we did for the boats in the river. Here's the apparatus they used. It worked like the one from MIT. Only it's mounted on a stone slab and floating in a pool of mercury to allow for slowly rotating the interferometer. Here's the actual interference pattern they saw. As the interferometer is rotated, the light flowing perpendicular to the direction of the ether would take time t sub a, and the light flowing with and against the ether would take time t sub b. Rotating the interferometer would change the ratio from t sub a over t sub b to t sub b over t sub a, and the interference pattern would shift. Using the speed of the earth through the ether, they estimated that the shift in the pattern would be just under one half of a fringe, but there was no shift. When the experiment was performed at different seasons and at different locations, the results were the same. No shift. Initially, the fact that there was no shift was viewed as a failure by Michelson and Morley to measure the velocity of the ether. But on reflection, scientists started asking some very fundamental questions. Is there an ether? How can we add the velocity of light and the velocity of the platform and come out with the velocity of light? Are the Galilean transformations wrong? And for us, in this video book, a big question was, does the fact that the speed of light is a constant mean that it is also a speed limit and nothing can go faster than that? These are the questions we'll address with Einstein's theory of special relativity in the next segment.