1. Why did we spend so much time discussing swimming in a river? Answer: because this example explains the underlying concept Of the Michelson-Moreley experiment – one of the “milestone experiments” in the history of physics. Some facts: ● In 1805, Thomas Young makes his historic observation of interference of light rays emerging from two narrow slits. It proves that light has a wave-like nature. The Newtons theory according to which light is a beam of micro-particles, or “corpuscles”, is dead. ● In the 1860s, a Scottish physicist and mathematician, James Clark Maxwell, discovers a set of four differential equations that “unify electricity and magnetism” – i.e., they provide a complete description of electric and magnetic fields. Based on these equations, he predicts the existence of electromagnetic waves.

2. Continued from the preceding slide: ● Soon, in 1888, the existence of electromagnetic waves of radio frequency is confirmed experimentally by Heinrich Hertz. Also, physicist realize that LIGHT is also an electromagnetic wave, only of much higher frequency than the waves generated by Hertz. ● For XIX-century physicists it is clear that all types of waves need a medium for propagating. For instance, as you know, sound waves can propagate in media such as …. (please answer). ● However, light does propagate in vacuum – it propagates extremely well! Light rays travel large distances in vacuum (e.g., in interstellar space) without losing any intensity, whereas in the most transparent materials we know (such as, e.g., perfect diamond crystals) there is always some intensity loss. In view of that, XIX-century physicists postulated the existence of a LUMINOFEROUS ETHER, a hypothetical substance (weightless, colorless, odorless….) filling the entire Universe (including all material bodies), and acting as a medium for light propagation (luminoferous means “light carrying” in Latin). gases, fluids and solids. But not in vacuum – it’s not a “medium”!

3. LUMINOFEROUS ETHER, continued: ● Maxwell believed that the equations he discovered actually represented the elastic properties of ether (similarly, the equations describing sound waves in solids are derived from equations describing elastic distortions of solids). Maxwell argued that ether actually IS a solid! Why? Well, we know two types of waves – longitudinal waves and transverse waves (recall Ph212 course). Light is certainly a transverse wave. How do we know that? The answer is simple: there is an effect that only transverse waves exhibit, but not longitudinal waves! Question: What is the name of this effect? Yes, you are right – the name of this effect is POLARIZATION! Sound waves in air and other gases, and in fluids can only be of longitudinal nature. Therefore, there is no such thing as “voice polarization”. However, transverse sound waves do exist – but exclusively in solids. Then, they may be polarized. For instance, polarized sound waves are used in certain studies of solids by a technique known as “sonography”.

4. IMPLICATIONS OF THE ETHER HYPOTHESIS: The “ether hypothesis” seemed to be so well grounded that physicists overwhelmingly accepted it. In the closing decades of XIX-th Century a large number of papers discussing the properties of the hypothetical ether was published in scientific journals. One conclusion that emerged from those speculations was that light propagation in ether should obey the principles of the classical (Galilean) relativity. Accordingly, if an observer were sitting in the middle of “ether wind” – we can also think of it as of a “river of ether” – light would behave like the swimmer in the example we have discussed.

5. Ether wind If the speed of light in stationary ether were c, then the observer would see light propagating “downstream” with c+u speed, and light propagating “upstream” with c-u speed (where is the “current speed” of the “ether river”). Well, Michelson’s idea was to test the above predictions experimentally. But where to find a suitable “ether river”? “No problem!” -- Michelson said. “Earth is orbiting Sun with a velocity of 30 km/s, Which is one part in a thousand in the speed of light”.

6. So, the observers on Earth see this situation as if they were sitting In the middle of an “ether river” (or “ether wind”). Such an effect, he reasoned, should yield a difference of 60 km/s. In Michelson times, direct measurements of c with the necessary precision (error margin much less than 0.1 %) were not yet feasible. Various measurements performed by the best expert of that period, Fizeau and Fucault from France, and others, yielded values that differed by as much as 1000 m/s, or more (see the plot to the right). Michelson’s idea was very simple: let’s mea- sure the difference be- tween the speed of light propagating “down- stream” and propagating “upstream”.

7. http://www.setterfield.org/report/report.html This is an interesting link. The authors of this document present a highly controversial hypothesis, according to which the speed of light changes with time. I definitely do not want to “advertise” this theory. Actually, in the opinion of most experts it is “trash science”. However, there is one valuable element in the paper. In order to support their claims, the authors had made meticulous literature searches, and In the paper they presented an extensive historical review of speed of light measurements performed over a period of more that 300 years – including many details that one can hardly to find in books. If you skip the sections in which the authors try to “indoctrinate” the reader, and you focus only on the “historical” parts of the document, it may really be a very interesting reading!

8. Michelson’s idea was to use an ingenious interferometry technique Observer looks

9. To explain the idea of the historic 1887 Michelson-Morley experiment, let’s assume that there is such thing as an “ether wind”. Let’s now run an animation: http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/mmexpt6.htm After the show: For the “cross-stream” beam (blue) it takes less time to travel forth and back along the path L than for the “upstream- downstream” beam (red ). Suppose that the time difference corresponds to one-half wavelength. Then, when the beams are recombined, the interference is destructive – the observer sees a dark spot at the center of the interference pattern.

11. Now, it is the red beam which is the “cross-stream” beam. Now this beam arrives first, so again there is a phase shift of one-half wavelength, and the inter- ference again is destructive. Again there is a dark spot in the center of the interference image.

12. Michelson-Morley experiments – conclusions: When the apparatus is rotated by 90°, the “ring pattern” goes through a full cycle: dark central spot → bright central spot → dark central spot again Michelson-Morley experiment was repeated many times, with better and better precision. The experiments always yielded the same result: the apparatus rotation never caused an change in the interference image, clearly indicating: “There is no ether wind!” Michelson very reluctantly accepted the negative results of his experiments. He seemed never to fully accept that there is no such thing as “cosmic ether”.