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

Scientists have long known that time passes faster at higher
elevations—a curious aspect of Einstein’s theories of relativity that
previously has been measured by comparing clocks on the Earth’s surface
and a high-flying rocket. Now, physicists at the National Institute of
Standards and Technology (NIST) have measured this effect at a more
down-to-earth scale of 33 centimeters, or about 1 foot, demonstrating,
for instance, that you age faster when you stand a couple of steps
higher on a staircase.

Credit: Loel Barr for NIST

Described in the Sept. 24 issue of Science,* the difference
is much too small for humans to perceive directly—adding up to
approximately 90 billionths of a second over a 79-year lifetime—but may
provide practical applications in geophysics and other fields. The NIST
researchers also observed another aspect of relativity—that time passes
more slowly when you move faster—at speeds comparable to a car
travelling about 20 miles per hour, a more comprehensible scale than
previous measurements made using jet aircraft.

NIST scientists performed the new “time dilation” experiments by
comparing operations of a pair of the world’s best experimental atomic
clocks. The nearly identical clocks are each based on the “ticking” of a
single aluminum ion as it vibrates between two energy levels over a
million billion times per second. One clock keeps time to within 1
second in about 3.7 billion years (see NIST announcement from Feb. 4,
2010, “NIST’s Second ‘Quantum Logic Clock’ Based on Aluminum Ion is Now World’s Most Precise Clock”
at http://www.nist.gov/physlab/div847/logicclock_020410.cfm) and the
other is close behind in performance. The clocks are precise and stable
enough to reveal slight differences that could not be seen until now.

The NIST experiments test two predictions of Einstein’s theories of
relativity. First, when two clocks are subjected to unequal
gravitational forces due to their different elevations above the surface
of the Earth, the higher clock—experiencing a smaller gravitational
force—runs faster. Second, when an observer is moving, a stationary
clock’s tick appears to last longer, so the clock appears to run slow.
Scientists refer to this as the “twin paradox,” in which a twin sibling
who travels on a fast-moving rocket ship would return home younger than
the other twin.

In one set of experiments, scientists raised one of the clocks by
jacking up the laser table to a height one-third of a meter (about a
foot) above the second clock. Sure enough, the higher clock ran at a
slightly faster rate than the lower clock, exactly as predicted.

The second set of experiments examined the effects of altering the
physical motion of the ion in one clock. The ions are almost completely
motionless during normal clock operations. NIST scientists tweaked the
one ion so that it gyrated back and forth at speeds equivalent to
several meters per second. That clock ticked at a slightly slower rate
than the second clock, as predicted by relativity.

Such comparisons of super-precise clocks eventually may be useful in
geodesy, the science of measuring the Earth and its gravitational field,
with applications in geophysics and hydrology, and possibly in
space-based tests of fundamental physics theories, suggests physicist
Till Rosenband, leader of NIST’s aluminum ion clock team.

The research was supported in part by the Office of Naval Research.
For more details, see the NIST Sept. 23, 2010, announcement, “NIST Pair of Aluminum Atomic Clocks Reveal Einstein’s Relativity at a Personal Scale” at http://www.nist.gov/public_affairs/releases/aluminum-atomic-clock_092310.cfm.

Media Contact: Laura Ost, laura.ost@nist.gov, 303-497-4880