Ring Laser Gyroscope

The Ring Laser Gyroscope is a type of gyroscope that is based on laser beams which move around a cavity. The principle of operation of a RLG is based on the Sagnac Effect, which was famed for showing that light changes velocity on a rotating platform. The changing velocity of light as seen in the Ring Laser Gyroscope and Sagnac Experiments is said to be contrary to the consistency of light as proposed by Special Relativity (See the Michelson-Morley Experiment and Sagnac Experiment).

Some have alleged that Ring Laser Gyroscopes have detected the rotation of the earth. When assessing these claims, it is found that modern Ring Laser Gyroscopes are very sensitive devices which are often double-purposed as seismometers. These sensitive devices are detecting patterns in the background microseismic noise, and certain features are interpreted to be caused by the earth's rotation. The feature of the background noise assumed to be an effect of the earth's rotation is called the "Earth line," and is admitted to be of unknown origin and cause.

The Earth Line
Strapdown Inertial Navigation Technology By David Titterton, John L. Weston, John Weston

From Chapter 15.6 (Archive) on p.497 we see that the rotation of the earth is seen with "the Earth line":

Analysis of the Earth line is used to monitor seismic events. On p.498 we read that the Earth line is around the 0.2-1Hz region:

We also read that seismic events around that frequency region is assumed to indicate the presence of rotational components associated with seismic events.

University of Canterbury
In Ring Laser Dynamics (Archive) by B Tom King it is admitted that the nature of the 'Earth line' is unknown and unverified. From p.152 we read:

We are told that the nature of the Earth line is unknown and that the signal may be traced back to some oscillation in the gain medium. The reader may ponder how it would be at all possible for a directly observed rotation rate of 15 degrees per hour beneath a device could be traced back to an "oscillation in the gain medium", and why it is called a "signal". This confirms that that the matter is actually a measurement of anthropogenic and microseismic noises, and not a direct measurement of rotation beneath the device as some have alleged.

The author goes on to deduce that because the earth induced line agrees with an equation, it is assumed that the rate is indeed from the earth.

"Excellent agreement" with an equation which was likely made to match observation, after the fact, and after the invention of the device.

On p.152 where the paper discusses the theory of the equations, it is shown that the rotation rate of the earth involves the period of 86164s applied to the interpetation, which is the Sierael Day. The process is using a predefined period to assess a daily variation or feature of the noise to get a "rotation rate".



The 'Sidreal Day' happens to be the time it takes for the sun, stars, and celestial bodies to return to their spots above the earth. This is opposed to the Solar Day, which is supposedly the true rotation of the earth, and is in regards to the sun.

Seismic Wave Propagation
Q: If the Ring Laser Gyroscope is assessing seismic noise in the attempt to observe the earth's rotation, how is it that Ring Laser Gyroscopes on airplanes can see the rotation of the earth? A. Not all types and sizes of Ring Laser Gyros claim to be capable of observing the rotation of the earth. It is unclear whether those types of RLGs can see the earth's rotation. However, seismic and anthropomorphic noise can propagate through the air and does not necessarily only originate from within the earth. Other possible sources for the noise are the daily changes of the atmosphere or influence from the celestial bodies. In the atmosphere seismic waves take the form of sound waves.

Background Seismic Noise
From http://microglacoste.com/gPhoneNoise/gPhoneSeismicNoise.pdf (Archive)we read:

Airborne Transmission
https://en.wikipedia.org/wiki/Seismic_wave (Archive)

https://amp.livescience.com/24209-earthquakes-infrasound.html (Archive)