The relative frequency stability of two beamtype maser oscillators is used to test the dependence of the velocity of light on velocity of the frame of reference with considerably more precision than has been obtained from experiments of the Michelson-Morley 1 type. Expressed in terms of an ether, the maximum ether drift is shown to be less than 1/1000 of the earth's orbital velocity.The experiment, which was performed at the Watson Laboratory, involves comparison of the frequencies of two masers 2 having their beams of NH 3 molecules traveling in opposite directions. Miller 3 has analyzed this case and given the change in frequency of a beam-type maser due to ether drift, assuming the molecules in the beam to have a velocity u with respect to the cavity through which they pass, and the cavity to have a velocity v with respect to the ether. The shift may be simply discussed by assuming that, if v is zero, radiation is emitted perpendicularly to the molecular velocity so that there is no Doppler shift. If the cavity and beam are then transported at velocity v through the ether in a direction parallel to u, radiation must be emitted by the molecules slightly forward at an angle 6 =7r/2 -v/c with respect to u. The fractional change in frequency due to the Doppler effect is then e = u/c cos0 or uv/c 2 due to motion through the ether, assuming that the proper molecular frequencies are unchanged by such motion 0 For a thermal molecular velocity of 0.6 km/sec and for the earth's' orbital velocity (30 km/sec), € -2 x 10 ~1 0 . The difference in frequency due to the above effect between two masers with oppositely directed beams would be 2ev, or about 10 cps for v equal to 23 870 Mc/sec, the NH 3 inversion frequency.Although uv/c 2 is of second order in the velocities, it is of first order in the velocity of the cavity, or of the laboratory, with respect to the ether Q The present experiment measures the entire effect with a rather small fractional error, which affords a particularly small upper limit to v since this quantity enters in first order, rather than in second order as in the Michelson-Morley experiment. A somewhat similar term would occur in the latter experiment if the interferometer used were transported by a plane of speed u, and interference fringes were compared for two opposite directions of flight.Two maser oscillators with oppositely directed beams were mounted with necessary auxiliary equipment on a rack which could be rotated about a vertical axis. The beat frequency between the two oscillators was adjusted to about 20 cps and recorded continuously. After approximately one minute of recording with the maser axes oriented in an east-west direction, the apparatus was rotated 180° and the beat frequency recorded in the new position.The change in beat frequency, on the basis of an ether drift, should be 4ev, or about 20 cps. Sixteen such comparisons were made during a period of about 20 minutes. These were repeated about once per hour during a time somewhat longer than 12 hours, so that the earth's rotation...

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Early scientists believed that a medium, which they called ether, was necessary for the propagation of light, just a s sound can be propagated only through matter, The velocity of light had to be m e a s u r e d relative to this ether and any motion of an observer with respect to the e t h e r would change the apparent velocity of light.Since the velocity of light is high, an observer must move at a high velocity to detect any difference in the velocity of light. If i t is assumed that the ether is fixed with respect to the solar system, then an o b s e r v e r on the e a r t h is moving at the earth's orbital velocity of 30 k m per sec. through the ether.In 1887 Michelson and Morley tried to measure a difference in the velocity of propagation of light traveling parallel to the e a r t h ' s o r b i t a l motion and at 90° to i t . They could detect no such difference, even though they felt that the experiment was precise enough to show a difference. This was not the expected result, and more accurate experiments which have been performed since then have f a i l e d to show a definite difference. Many more or less satisfactory explanations have been advanced. The m o s t s a t i s f a c t o r y is that of Einstein, who, in 1905, developed his revolutionary new special theory of relativity based on two postulates. One i s that physical laws are the same in any inertial frame of reference. Thus r e f e r e n c e of the velocity of light to. a n e t h e r is meaningless. In other words, Einstein postulates that no difference in the velocity of light should have been observed by Michelson and Morley. Since no positive effect is to be expected i t is especially important to test the postulate in many ways.C. H. Townes and C. Mdller proposed an experiment involving gas m a s e r o s c i l l a t o r s to provide an independent check on the postulate. This experiment, which is being performed at the Watson Laboratory with the help of Professor Townes, is not exactly analogous to the MichelsonMorley experiment, and i t s e t s a much lower limit to the dependence of the velocity of light upon the frame of reference.In o r d e r to gain some physical insight into the experiment we m u s t f i r s t e x a m i n e t h e m a s e r . T h e h e a r t of t h e g a s m a s e r o s c i l l a t o r i s a high Q microwave cavity tuned to a transition of the ammonia molecule. Molecules in the excited state are sent in a beam through the cavity and are stimulated to radiate in phase with the field in the cavity. For the TM010 mode this radiation must travel in a direction perpendicular to the cavity wall i f there is to be negligible loss of energy from the cavity.An e t h e r would have the effect of carrying the wave train with it; s o the radiation must be emitted at a slight angle in the direction opposite to the relative motion of the ether i f the energy propagation is to be perpendicular to the walls. This would give rise to a Doppler shift. 543