Gravity Slows Down The Flow Of Time

Close your eyes and envision a location suspended in the vast expanse of the universe. This is a place where the fundamental attributes of time exhibit behaviors that defy our conventional understanding. You might immediately conjure images of locales in proximity to black holes, where the very laws of physics and the fabric of space-time warp into unrecognizable and enigmatic phenomena. Or perhaps, your imagination leads you to places on the outermost fringes of the cosmos, where dark energy propels the boundaries of the universe to expand at speeds surpassing that of light. These are regions humans may never hope to explore, let alone comprehend what life might be like trillions of miles away from our Milky Way.

Yet, what if we were to reveal that such a place exists much closer than the remote recesses of space or a distant galaxy's black hole? What if we told you that this place is, in fact, our very own planet Earth? Here, at the Earth's core, time behaves in such a manner that it is younger by 2.5 years than it is on the surface. How could this be, you ask? The answer is deceptively simple: gravitational time dilation. However, beneath this simple explanation lies a complexity that is nothing short of awe-inspiring.

The fact that one part of a planet can age differently from another part of the same planet merely scratches the surface of the concept of gravitational time dilation. This phenomenon stands as one of the many enigmas waiting to be unraveled within our galaxy and beyond.

Gravitational time dilation, as defined by Britannica, refers to the slowing down of a clock, as perceived by an observer in relative motion with respect to that clock, due to changes in gravitational potential as the clock moves further from the source of gravity. In other words, as you approach a gravitational source like Earth, time passes more slowly, even if the difference is just a fraction of a microsecond.

This curious effect is the reason that if we were to place an atomic clock at the Earth's core and another atop Mount Everest, these two clocks would differ by two and a half years after several billion years of counting. Some may have heard of the similar phenomenon known as the twin paradox, a theoretical scenario in which a set of twins are equipped with clocks, one remaining on Earth in a non-inertial state and the other sent on a rocket ship on a journey through space. According to the principles of special relativity, the twin staying on Earth would age less than the one traveling, resulting in one twin being older than the other.

While the twin paradox is a subset of special and general relativity, gravitational time dilation emerges as a subset of both special and general relativity. In fact, our understanding of gravitational time dilation owes much to the insights of Albert Einstein, the father of general relativity. In 1907, two years after his groundbreaking theory of special relativity, Einstein realized that it was inadequate for explaining the effects of gravity and acceleration.

On Earth, everything is influenced by our planet's gravitational field, and specifically by the acceleration of Earth's gravitational force, which is approximately 9.8 meters per second. If you were placed in a room with no connection to Earth's gravity and it was accelerated at precisely 9.8 meters per second, you wouldn't be able to discern whether you were bound by gravity or subjected to acceleration. Einstein's musings on light's behavior in such scenarios were equally profound. He postulated that without gravity, light moving across an accelerating room would bend just as it does in a gravitational field, as long as both experiences involve constant, comparable speed.

Over a decade later, British scientists conducted experiments during a solar eclipse in South Africa, observing the bending of light by the sun's gravitational field. This provided substantial evidence supporting Einstein's theory of general relativity, leading to widespread acceptance and the acknowledgment of his genius. Einstein's theory revolutionized our understanding of gravity and its effects on time, both on Earth and throughout the cosmos.

In 1959, Canadian-American physicists Robert Pound and his graduate assistant Glenn Rebka Jr. conducted a groundbreaking experiment aimed at further confirming Einstein's theory of relativity and how gravity affects light. They designed a gravitational redshift experiment, a phenomenon where photons and electromagnetic waves lose energy as they move away from a gravitational well, such as stars, planets, and moons.

Pound and Rebka conducted their experiment from the top of the Jefferson Laboratory's Left Tower on Harvard University's campus. They emitted a series of gamma rays from the tower's peak to its base, and a receiver placed at the bottom measured the frequency. To prevent gamma ray scattering, the physicists passed the rays through a helium-filled mylar bag, which also contained iron-57, a stable isotope of elemental iron.

The gamma rays were directed through a loudspeaker cone and traveled 22.5 meters downward to the basement, where the iron-57 was located. The results of their experiment demonstrated a gravitational blue shift, where the wavelength decreased, and the energy and frequency of the electromagnetic waves increased. This experiment not only paved the way for new research on the specific aspects of general relativity but also provided further insights into gravitational time dilation. Their results, while more accurate than previous experiments, confirmed the existence of time dilation.

Approximately two decades after the Pound-Rebka experiment, NASA partnered with the Smithsonian Astrophysical Observatory to extend the study of general relativity beyond Earth's boundaries. In 1976, they launched Gravity Probe A from the NASA Wallops Flight Center on Wallops Island, Virginia, using a scout rocket. This probe reached an altitude of ten thousand kilometers and remained in space for nearly two hours.

While the primary objective of Gravity Probe A was to investigate the equivalence principle, the mission also shed light on gravitational time dilation. As the probe descended back to Earth, instruments measured the rate of atomic time while experiencing varying gravitational potentials. To achieve this, astronomers placed a maser (a device that stimulates radiation emissions from excited atoms to create coherent electromagnetic waves) in the probe and an identical maser in a stationary position on Earth. These specific hydrogen masers were renowned for their precision, emitting 1.42 billion cycles per second. They exhibited remarkable stability, with drifts of less than two seconds every 100 million years.

At its highest point in space, the maser on the probe should have ticked 4.5 parts in 10 billion times faster than the maser on Earth, equivalent to one second every 73 years. Researchers on Earth recorded the maser's oscillations as a clock's ticks and charted the frequency measurements as the probe ascended, offering real-time representation of gravitational time dilation. Although the experiment harnessed Earth's gravitational field on the edge of the planet's atmosphere, the results were only more precise than the original Pound-Rebka experiment by 0.01, reaffirming the reality of time dilation.

Following the early experiments that confirmed general relativity's role in the interplay between time and gravity, further tests have been devised to deepen our understanding of this mostly mysterious phenomenon. Among them was the Hafele-Keating experiment and its variants, where atomic clocks aboard airplanes were found to tick slightly faster than ground-based clocks. This discovery led to the adjustment of the internal clocks of global positioning systems (GPS) and other satellites every few years.

Even the Viking 1 lander on Mars conducted experiments involving time signals and atomic clocks on Earth. More recently, researchers have developed a laser system and an atomic clock of extraordinary accuracy and stability.