Is Your Head is Older Than Your Feet?

Several years ago (too many actually) I was enthralled reading an too obscure paper by Bjerhammar, Arne. “On a relativistic geodesy.” Journal of Geodesy, Heidelberg:1985. He was a visiting geodesist at NGS for a year and proved the viability of using the relativistic phenomena of time dilation in gravity fields to measure Earth’s gravity field

Gravity is a property of mass and the mass of the earth is not evenly distributed. The uneven distribution results in slight deviations in the direction of “down” from place to place that differ from the uniformity one might expect.

These deviations can be critical to surveyors trying to determine or describe how high one place is relative to others. The somewhat wavy undulating invisible surface against which heights are usually expressed is called the geoid. Once upon a time we called it mean sea level, but sea level in one place did not equate to seal level in another. Additionally, we couldn’t measure the gravity field or elevations well enough to determine the geoid precisely yet.

The military were very interested in an improved model of the geoid. For instance, when a ballistic missile is fired it follows the path of a “ball” hence the name. The path depends on the earth’s gravity field. The more precisely it is modeled  the more precisely the missile can be targeted.

The Ohio State University geodesists, among others, led the research. By the mid 1980s the work progressed well enough that the US and USSR had enough confidence in long rang missiles that President Regan and General Secretary Gorbachev were able to sign the Intermediate Missile Treaty in 1986. Most folks are unaware that in order for an “everyday” GPS unit to work requires the application of the differences due to relativity.

Today I happened upon an article at the NIST (National Institute of Standards and Technology) with the following headline.

NIST Clock Experiment Demonstrates That Your Head is Older Than Your Feet

In it the NIST scienists describe using a super precise aluminum ion quantum logic clock to measure the difference in the speed of time over a change in height of only about a foot! If any of this sparks your interest, check out the site. I’d love to find practical ways to apply this to geoid measurement.