Simple Nature - Light and Matter

at rest relative to the water. But light isn’t a vibration of a physicalmedium; it can propagate through the near-perfect vacuum of outerspace, as when rays of sunlight travel to earth. This seems like aparadox: light is supposed to have a specific speed, but there is noway to decide what frame of reference to measure it in. The wayout of the paradox is that light must travel at a velocity equal to c.Since all observers agree on a velocity of c, regardless of their frameof reference, everything is consistent.u / A ring laser gyroscope.The Michelson-Morley experimentThe constancy of the speed of light had in fact already beenobserved when Einstein was an 8-year-old boy, but because nobodycould figure out how to interpret it, the result was largely ignored.In 1887 Michelson and Morley set up a clever apparatus to measureany difference in the speed of light beams traveling east-west andnorth-south. The motion of the earth around the sun at 110,000km/hour (about 0.01% of the speed of light) is to our west during theday. Michelson and Morley believed that light was a vibration of amysterious medium called the ether, so they expected that the speedof light would be a fixed value relative to the ether. As the earthmoved through the ether, they thought they would observe an effecton the velocity of light along an east-west line. For instance, if theyreleased a beam of light in a westward direction during the day, theyexpected that it would move away from them at less than the normalspeed because the earth was chasing it through the ether. They weresurprised when they found that the expected 0.01% change in thespeed of light did not occur.The ring laser gyroscope example 5If you’ve flown in a jet plane, you can thank relativity for helpingyou to avoid crashing into a mountain or an ocean. Figure ushows a standard piece of navigational equipment called a ringlaser gyroscope. A beam of light is split into two parts, sentaround the perimeter of the device, and reunited. Since the speedof light is constant, we expect the two parts to come back togetherat the same time. If they don’t, it’s evidence that the device hasbeen rotating. The plane’s computer senses this and notes howmuch rotation has accumulated.No frequency-dependence example 6Relativity has only one universal speed, so it requires that all lightwaves travel at the same speed, regardless of their frequencyand wavelength. Presently the best experimental tests of the invarianceof the speed of light with respect to wavelength comefrom astronomical observations of gamma-ray bursts, which aresudden outpourings of high-frequency light, believed to originatefrom a supernova explosion in another galaxy. One such observation,in 2009, 3 found that the times of arrival of all the different3 http://arxiv.org/abs/0908.1832396 Chapter 7 Relativity