The Day the Universe Changed - James Burke [143]
Michelson’s equipment consisted of a ring trough filled with mercury on which floated a massive sandstone slab 16 feet square and a foot thick. At each corner of the slab were four metal plane speculum mirrors. A monochromatic sodium light from an Argand burner was passed through two slits and a lens, in order to produce the ‘point source’beam. When aimed at a half-silvered mirror, the beam split in two, one beam travelling at right angles to the other. The two beams then travelled back, each reflected by its two sets of mirrors, so that they had both covered the same distance when they returned to join again at the half-silvered beam-splitting mirror.
One of these beams was aimed in the same direction as the earth’s movement in space. In this way, one beam would go directly ‘against’the ether as the earth passed through it, while the other would go out at right angles to the same path. The beam moving ahead of the earth into the ether should therefore encounter more resistance than the other and return fractionally later, altering the interference ‘fringes’when its late arrival put it out of phase with its twin.
The route followed by each half-beam was 36 feet. The experiments were carried out as the slab was slowly rotated, so as to investigate the result of shooting the beams in all directions ‘in the ether’. Sixteen rotations were done over a period of three days. When the two Americans published their results in 1887, they had no results to show. At every point in the experiment no change in interference patterns occurred. It was as if the ether did not exist.
But the previous year the two men had looked at Fresnel’s evidence for the speed of light in water. Whether with or against the flow of water, the light moved at a speed relative to the water flow. On the evidence of the data, either there was no ether, which was unthinkable given the fact that light took time to travel, or else the earth was dragging an envelope of ether along with it, holding it stationary relative to the planet. Either way, the absolute ether which everybody was looking for seemed to be missing. It looked as if Newton had been wrong. Since such a possibility was unacceptable, in 1892 an Irishman came up with an ingenious way to save the day. He was G. F. Fitzgerald, Professor of Natural and Experimental Philosophy at Trinity College, Dublin.
George Fitzgerald’s attempt at flying. Students pulled on tow-ropes to give the eminent professor a start as he ran down the take-off platform in his glider, wearing the top hat considered de rigueur for Fellows of Trinity College, Dublin.
Fitzgerald knew that Lorentz had previously shown how an electrostatic charge in motion could set up a magnetic field as does a current. If such an electrostatic charge were at rest on earth, it would not set up a magnetic field but merely an electrostatic field. However, viewed from, say, the sun, the electrostatic field, though static on earth, would actually be in motion through space and so could create a magnetic field.
If, as Lorentz had said earlier, the force were a charge moving through the molecules of objects in the field, and given that the form of objects was, as everyone knew, determined by the position and state of their atoms, the form of an object could be altered as it moved through a field. If that change of form were to cause a shortening of the equipment used by Michelson and Morley by exactly the length by which the retarded returning half-beam of light had lagged behind, then the lag would be cancelled out, compensated for by the shrinkage of the equipment along the axis of movement of the earth. This change of form would not occur along the instrument arm at right angles. The ‘shrinkage’became known as the Fitzgerald-Lorentz contraction.
This theory saved the ether, though at the expense of having to take a relativistic view. The nineteenth century had begun with the identification of an entirely new phenomenon, only briefly investigated before then, and as first magnetism and