No abstract

Pressure-induced fractional changes of 10(-7) in the geometry of a large He-Ne ring laser gyroscope induce backscatter phase changes and thus a fractional pulling of the Sagnac frequency of ~5 x 10(-3). To counter this, the optical frequency was stabilized against an iodine-stabilized laser with a high-finesse Fabry-Perot interferometer and piezoelectric control of the ring perimeter. This scheme, although limited in principle by residual geometric asymmetry and in practice by low beam powers (10 pW), stabilized the perimeter to 2.4 nm (6 x 10(-10) or 300 kHz for the optical frequency) and the Sagnac frequency to 100 parts per million over several days.

The design and initial operation of a vertical square He-Ne ring laser G0 with a perimeter of 14 m is discussed. This builds on earlier demonstrations of the feasibility of large ring lasers (perimeter approximately 4 m) for single-mode gyroscope operation and with lesser pulling than navigation gyroscopes. With servoing of the rf excitation to yield single-mode operation, G0 gave a quality factor 1 x 10(12) and a Sagnac line with a frequency of 287.8 +/- 1.0 Hz induced by Earth rotation Omega(E). This has confirmed some vital questions over the feasibility of very large gyroscopes for geodetic measurements at the level of 10(-9) Omega(E).

No abstract

An historic and simple experiment has been revitalised through the availability of supercavity mirrors and also through a heightened interest in interferometry as a test of physical theory. We describe our helium-neon ring laser, and present results demonstrating a fractional frequency resolution of 2�1x10-18 (1�0 mHz in 474 THz). The rotation of the earth unlocks the counterrotating beams. A new field of spectroscopy becomes possible, with possible applications to geophysical measurements such as seismic events and earth tides, improved measurements of Fresnel drag, detection of ultraweak nonlinear optical propert~es of matter, and also searches for preferred frame effects in gravitation and for pseudoscalar particles.

A He-Ne ring laser has given the Earth-rotation-induced Sagnac frequency to a precision of 1 microHz, or 2 parts in 10(8), 2 x 10(-21) of the laser frequency. Because the beat frequency in a ring laser originates in either timereversal or (with a more complicated polarization geometry) parity-violating effects, such systems have the potential of detecting ultrasmall symmetry violations.

No abstract

Evidence for the structure of double-stranded DNA and related duplexes is considered in relation to the existing double helix model and a possible alternative to it. It is shown by mathematical analysis that an alternative side-by-side (SBS) model is capable of producing the gross features of the X-ray diffraction pattern of moist DNA fibres. This is complemented by model-independent calculations based on X-ray data for paracrystalline B-DNA that suggest the duplex molecule may not have tne circular (axial projection) symmetry expected for a regular double helix structure. In addition, the application of the SBS model to other areas of DNA structure and function are considered. Long-standing problems assocIated with the highly intertwined feature of the helix model, esrecially the problem of unwinding, do not arise for the SBS model. In particular, electron micrograph evidence for the formation of "bubble" regions during the melting of DNA raises serious questions about earlier attempts to rationalise kinetic data for strand separation. INTRODUCTIONThe Watson-Crick double helix model for DNA and related structures appears to be very well established from a wide range of physical studies. As far as we know the basic right-handed helical character of the model has never been seriously challenged, although doubts have been expressed about the reliability of the model as deduced from X-ray studies (Donohue 1969(Donohue , 1970.