unimportant, and what we look for is a shift in the interference fringes when we rotate the apparatus. In carrying out the experiment, Michelson and Morley oriented the apparatus so that the line be was nearly parallel to the earth's motion in its orbit (at certain times of the day and night). This orbital speed is about 18 miles per second, and any"ether drift "should be at least that much at some time of the day or night and at some time during the year. The apparatus was amply sensitive to observe such an effect, but no time difference was found-the velocity of the earth through the ether could not be detected. The result of the experiment was null The result of the Michelson-Morley experiment was very puzzling and most disturbing. The first fruitful idea for finding a way out of the impasse came from Lorentz. He suggested that material bodies contract when they are moving, and hat this foreshortening is only in the direction of the motion, and also, that if the length is Lo when a body is at rest, then when it moves with speed u parallel D), is given by (156) When this modification is applied to the Michelson-Morley interferometer appara tus the distance from B to C does not change, but the distance from b to E is hortened to Lyl-u2/c2. Therefore Eq. (15.5)is not changed but the L of Eq(15. 4)must be changed in accordance with Eq. ( 15.6). When this is done we (2L/c)√1-u2/c2 r1+t2= (157) Comparing this result with Eq.(15.5), we see that t1+ 12= 2t3. So if the ap- paratus shrinks in the manner just described, we have a way of understanding why the Michelson-Morley experiment gives no effect at all. Although the contraction hypothesis successfully accounted for the negative result of the experiment, it wa open to the objection that it was invented for the express purpose of explaining way the difficulty, and was too artificial. However, in many other experiments to discover an ether wind, similar difficulties arose, until it appeared that nature was in a"conspiracy"to thwart man by introducing some new phenomenon to undo every phenomenon that he thought would permit a measurement of u It was ultimately recognized, as Poincare pointed out, that a complete conspiracy is itself a law of nature! Poincare then proposed that there is such a law of nature, that it is not possible to discover an ether wind by any experiment; that is, there is no way to determine an absolute velocity. 15-4 Transformation of time Othe i n checking out whether the contraction idea is in harmony with the facts experiments, it turns out that everything is correct provided that the times are also modified, in the manner expressed in the fourth equation of the set (15.3) That is because the time t3, calculated for the trip from b to C and back, is not the same when calculated by a man performing the experiment in a moving space ship as when calculated by a stationary observer who is watching the space ship To the man in the ship the time is simply 2L/c, but to the other observer it is (2L/c)/v1-u2c2(Eq. 15.5). In other words, when the outsider sees the man in the space ship lighting a cigar, all the actions appear to be slower than normal. while to the ma normal rate. So not only must the lengths shorten, but also the time-measuring instruments ("clocks")must appa ently slow down. That is, when the clock in the space ship records 1 second elapsed, as seen by the man in the ship, it shows 1/v1-u2/c2 second to the man outside This slowing of the clocks in a moving system is a very peculiar phenomenon and is worth an explanation. In order to understand this, we have to watch the machinery of the clock and see what happens when it is moving. Since that is 15-5