Einstein's theory of special relativity predicts that the speed of light is independent of the velocity of the laboratory. Evidence of this velocity-independence gets closer to the truth all the time, as shown by physicists at the University of Konstanz (Konstanz, Germany) and the Heinrich-Heine University Düsseldorf (Düsseldorf, Germany). Using a frequency-stabilized laser, Achim Peters and colleagues demonstrated the smallest, most accurate measurement yet in proof of this theory.
To show that light travels at a constant velocity, c, even in a moving lab, the scientists used Earth's orbital motion around the sun, as well as the motion of the Sun through the cosmic microwave background to provide the varying laboratory velocity. The low accuracy of such a test, called a Kennedy-Thorndike (KT) experiment, is the limiting factor in verifying special relativity.
A standing wave in a cryogenic optical resonator (CORE) was shown to have no discernable frequency drift over long periods. Combined with an extremely low thermal expansion and no observable aging effects, COREs are the ideal tool for frequency stabilizing a laser to detect shifts of the speed of light due to the Earth's rotation and orbit. The team compared the frequencies of a 1064-nm Nd:YAG laser frequency stabilized to the CORE to that of an iodine frequency standard over a period of 190 days (see figure). Operated at the temperature of liquid helium at 4.3 K, the sapphire-cavity CORE has a finesse of about 100,000 and a linewidth of 50 kHz at 1064 nm. The iodine "clock" is a standard time reference due to an electronic transition between two states of the iodine molecule. The iodine transition clock is frequency stabilized to a second Nd:YAG laser via its second harmonic wave.
In a KT test, a vanishing velocity-dependence coefficient, A, strongly validates special relativity by restricting the dependence of the speed of light on the velocity of the laboratory. Results from the CORE/iodine experiment in Konstanz find that |A| = 1.9 ± 2.1 x 10-5, a value three times lower than the best previous test. Such a small velocity dependence is evidence in support of Einstein's equivalence principle. The researchers say that the sensitivity of the test could improve by an order of magnitude by replacing the iodine standard with a femtosecond optical comb generator directly locked to a primary cesium clock or hydrogen maser.
REFERENCE
1. C. Braxmaier, H. Müller, O. Pradl, J. Mlynek, A. Peters, and S. Schiller Phys. Rev. Lett., 88, 010401-1 (Jan. 2002).
