## That Zero > "...it is absurd to imagine"As a graduate student in 1880 in Hermann von Helmholtz's famous Berlin laboratory, against everyone's advice, Michelson decided to follow an off-hand comment of Maxwell's in a letter written the year before, during his last year of life. Maxwell imagined a way to measure the speed of the Earth relative to the fixed ether, but lamented that the precision required was beyond any terrestrial technique. This captured Michelson's attention and the speed of light went on the back-burner as he embarked on his most important – and personally disappointing – experiment. ### What's Waving? What's more ridiculous than a wave that has nothing to wave in? ```{margin} Water is the medium in which water waves propagate. What could be more simple? ``` From the time of Aristotle, it was assumed that empty space was not empty, but consisted of a strange substance archaically called "aether" and in the 19th century, "ether." The persistent belief in this substance was reinforced in the 19th century when Newton's demand that light consisted of particles was dethroned by (the young) Thomas Young's (unwelcome – he was drummed out of British science for disputing Newton) demonstration that light consisted of waves. This was the passage of light through two slits, showing that the emerging waves diffracted – only waves do that. We considered that in detail in a previous lesson. Maxwell's subsequent unification of electricity, magnetism, and optics into a single model of waves of electric and magnetic field vectors and Hertz's demonstration of their existence made it clear: light, electricity, and magnetism are waves and so it has to propagate in a substance which supports that disturbance. The ether was to light as water is to a dropped pebble and air is to sound. And nobody imagined otherwise. Sir Oliver Lodge was passionate (and relentless) on the subject, even after it was clear he was wrong: > ... it is absurd to imagine one piece of matter acting mechanically on another at a distance, whether that distance be large or small, without some intervening mechanism or connecting link... ### But Maxwell Said *c*! There was a serious fundamental problem. The solution to Maxwell's equations is indisputably a wave and furthermore, the speed of that wave is a single number, $c=300,000$ km/s. *But with respect to what?* Maxwell's model didn't include the freedom for it to be $c$ plus whatever the speed of an emitter might be...like walking with a lantern. No. The speed of light is a fixed value and everyone, including, and especially, Maxwell believed that the ether provided a fixed reference and that the speed of light – and the light waves themselves – were fastened to that material. The ether was a very strange beast. If you were to do an experiment (that you should not do) involving a railroad track and a hammer, you would find that sound travels faster in a solid than in air. If your (former) friend bangs on a railroad track a 100 yards away, and if you put your ear to the track, you'll hear it through the metal before you hear it through the air. Now, get off the track. That's dangerous. In fact the speed of a wave in a medium is dependent on the square root of the "stiffness" (called the "bulk modulus") of the material – steel is a million times stiffer than air, so the speed of sound in steel would be 1000 times faster. Now light is the fastest thing there is! So turning that argument around, *the stiffness of the ether must be incredibly higher than even steel*. And yet, the ether needs to be easily plowed through by the planets that presumably swim around in their orbits through it while they delicately reflect their light back to our telescopes. How can a substance offer no resistance to a planet's motion (or even a car, or a person, or a bird) and yet be incredibly stiffer than steel? No matter. A detail. It's actual existence, while required, seems circumstantial. There had to be an ether in order for their to be a wave of light. It seemed to have two functions: 1. As I remarked above, Maxwell created his four equations for electricity and magnetism, out of which popped the mathematical representation of a transverse wave, the speed of that wave, "$c$" was required by him to be the speed of light relative to the ether. 2. Newton insisted that space was a "thing," a property of the universe and that there is an absolute coordinate system (Absolute Space) against which all motions – constant velocity and accelerated motions – could be measured. The ether seemed to be that perfect construct: it and only it would function as that absolutely at-rest structure that anchors space. The question is how do we detect the ether? Michelson set out to determine our speed as we move through it.