# Synopsis: Pinpointing Planck’s Constant with GPS

Using the orbiting network of GPS satellites, researchers have placed new limits on how much Planck’s constant varies with respect to relativistic changes.

GPS is helping drivers find their way and parents track their kids and pets. But now a pair of researchers—reporting in Physical Review Letters—has used the same technology to put new limits on variations in Planck’s constant.

Certain theories allow physical constants, such as the speed of light or the gravitational constant, to vary, and some astronomical observations have been interpreted as suggesting the electromagnetic coupling was different in the past. Testing these hypotheses often requires sophisticated instruments. But James Kentosh and Makan Mohageg of California State University, Northridge, have found a way to use the ubiquitous global positioning system, or GPS, to evaluate the constancy of Planck’s constant, $h$.

GPS relies on atomic clocks, which are sensitive to Planck’s constant through their ticking frequency, $f=E/h$, where $E$ is the energy of a specific atomic transition. For a clock orbiting in one of the $32$ GPS satellites, this frequency can shift with respect to ground-based clocks because of well-known relativistic effects. The GPS system keeps track of this frequency drift and broadcasts a clock correction with its signal.

Kentosh and Mohageg looked through a year’s worth of GPS data and found that the corrections depended in an unexpected way on a satellite’s distance above the Earth. This small discrepancy could be due to atmospheric effects or random errors, but it could also arise from a position-dependent Planck’s constant. Assuming the latter, the authors derive an upper limit on Planck variation. – Michael Schirber

More Features »

### Announcements

More Announcements »

Gravitation

## Previous Synopsis

Atomic and Molecular Physics

Nanophysics

## Related Articles

Gravitation

### Synopsis: LIGO Picks Up on the Third Ring

The LIGO collaboration reports its third detection of gravitational waves coming from the merger of two black holes. Read More »

Gravitation

### Synopsis: Restricting the Fifth Force

Observations of the orbits of two stars at the center of the Milky Way constrain gravitational models involving a hypothetical fifth force. Read More »

Gravitation

### Viewpoint: Measuring the Tidal Force on a Particle’s Matter Wave

The effect of the tidal force, which is directly related to the curvature of spacetime, on an individual particle’s wave function has been measured with an atom interferometer. Read More »