How Does Gravity Affect Time? Gravitational Time Dilation

Gravity makes time pass more slowly. A clock sitting on the ground floor of a building ticks slightly slower than an identical clock on the roof, because the ground-floor clock is deeper in Earth’s gravitational field. This effect, called gravitational time dilation, is not a quirk of the measuring equipment or a metaphor for how time “feels.” It is a physical difference in the rate at which time itself flows, predicted by Einstein’s general theory of relativity in 1915 and confirmed by experiments ever since. The differences near Earth are tiny, on the order of billionths of a second over an ordinary human lifetime, but they become dramatic near extremely massive objects and are already large enough to matter for technologies you rely on every day.

Clocks Flown Around the World

The first direct test of gravitational time dilation using portable clocks came in 1971, when physicists Joseph Hafele and Richard Keating loaded four cesium atomic clocks onto commercial airliners and flew them around the globe, once eastward and once westward. Because the planes were at altitude, those clocks spent time in a weaker gravitational field than the reference clocks sitting at the U.S. Naval Observatory on the ground. General relativity predicted the airborne clocks should gain time relative to the ground clocks due to the gravitational difference, though the picture was complicated by the planes’ speed, which introduced a competing effect from special relativity that slows moving clocks. The net results matched Einstein’s predictions: the flying clocks lost about 59 nanoseconds on the eastward trip and gained about 273 nanoseconds on the westward trip, with both numbers falling within the predicted ranges once both gravitational and velocity effects were accounted for.¹

The asymmetry between the two directions is itself revealing. Eastward flights move with Earth’s rotation, so the clocks are moving faster relative to the non-rotating frame in which general relativity’s predictions are most naturally stated. That extra speed amplifies the special-relativistic slowdown, which partially cancels the gravitational speedup. Westward flights move against Earth’s rotation, reducing the clocks’ absolute speed and letting the gravitational effect dominate. The fact that both trips produced results consistent with theory, despite pulling in opposite directions, made this a convincing demonstration that both forms of time dilation are real and measurable with lab-grade equipment.

Measuring Time Dilation Across a Millimeter

Fast-forward five decades, and the precision has improved almost beyond comprehension. In 2022, a team at JILA (a joint institute of the National Institute of Standards and Technology and the University of Colorado) measured time dilation across a height difference of just one millimeter, roughly the width of a pencil tip. They used a cloud of about 100,000 ultracold strontium atoms arranged vertically in a lattice of laser light. By comparing the ticking rates of atoms at the top and bottom of this tiny column, the researchers detected a frequency shift consistent with general relativity’s prediction, at a level of precision about 50 times better than any previous clock comparison.²

At a millimeter scale, the time difference is fantastically small, on the order of one part in ten quintillion. No human would ever notice it. But detecting it matters enormously for physics, because it confirms that general relativity holds at scales that start to approach the quantum domain. It also hints at what optical atomic clocks could eventually do if their precision keeps improving: sense tiny changes in Earth’s gravitational field, map underground density variations, or detect the gravitational influence of nearby masses with extraordinary sensitivity.

Why GPS Depends on Gravitational Time Dilation

If you have used a navigation app on your phone this week, you have benefited from corrections for gravitational time dilation. GPS satellites orbit at about 20,200 kilometers above Earth’s surface, where gravity is weaker than at ground level. Their onboard atomic clocks tick faster than identical clocks on the ground by roughly 45 microseconds per day due to the gravitational difference. Meanwhile, the satellites’ orbital speed introduces a special-relativistic slowdown of about 7 microseconds per day, so the net effect is that the satellite clocks run about 38 microseconds per day fast relative to the ground.

Thirty-eight microseconds sounds negligible, but GPS works by measuring the travel time of radio signals to within nanoseconds. If the relativistic corrections were not applied, position errors would accumulate at a rate of roughly 10 kilometers per day, rendering the system useless for driving directions, let alone precision agriculture or aircraft landing. The relativistic effects are so large that the system would simply not function without accounting for them.³ This is often cited as the most tangible demonstration that general relativity is not an abstraction confined to astrophysics; it is a practical engineering constraint baked into consumer technology.

The Earth’s Core Is Younger Than the Surface

One of the more startling consequences of gravitational time dilation applies to the planet beneath your feet. Because the gravitational field is strongest at Earth’s center, time there runs slower than at the surface. Over the roughly 4.5 billion years since Earth formed, this difference has added up. Richard Feynman once estimated the age gap at “one or two days,” but a more careful calculation published in the European Journal of Physics found the actual figure to be about two and a half years.⁴

Two and a half years over four and a half billion is proportionally minuscule, but it is genuinely real. If you could somehow retrieve a clock that had been ticking at Earth’s center since the planet’s formation and compare it with a surface clock running for the same span, the center clock would be about two and a half years behind. The effect is a direct consequence of the same physics that shifts GPS clocks, just accumulated over geological time.

How Gravity Slows Light Itself

Gravitational time dilation does not only affect clocks. Because time runs slower in stronger gravitational fields, light signals passing near a massive object take longer to arrive at their destination than they would if the object were not there. This is known as the Shapiro time delay, after physicist Irwin Shapiro, who predicted it in 1964. The effect is distinct from gravitational lensing (the bending of light’s path). Even along a straight-line trajectory, the signal arrives late because it traverses a region where time itself is dilated.

The most precise solar-system test of this delay came from radio signals sent to and from NASA’s Cassini spacecraft as they passed near the Sun.⁵ The measured delay matched general relativity’s prediction to within a few parts per hundred thousand. More recent theoretical work has calculated the tiny discrepancies between exact solutions and the approximations typically used, finding that the errors are at the picosecond and sub-picosecond level for light grazed by the Sun’s gravity.⁶ For practical purposes, the standard approximations are spectacularly good, but these refinements could matter for the next generation of deep-space navigation and fundamental physics tests.

Black Holes and Infinite Time Dilation

If gravitational time dilation grows as gravity strengthens, what happens at the most extreme gravitational fields in the universe? Near a black hole, time dilation becomes enormous. An astronaut hovering just outside the event horizon would experience time passing at an almost normal rate from their own perspective, but a distant observer watching them would see their clock slow to a crawl. At the event horizon itself, the math of general relativity says the time dilation becomes infinite: the distant observer would need to wait forever to see the astronaut’s clock tick even once more.

This infinite time dilation at the event horizon is not just a mathematical curiosity. Theoretical work has shown that it is deeply connected to Hawking radiation, the faint thermal glow predicted to emerge from black holes. One analysis demonstrates that the thermal character of this radiation can be understood as a direct consequence of the infinite time dilation at the horizon, without needing to invoke the full machinery of quantum field theory in curved spacetime from scratch.⁷ The slowdown of time at the boundary of a black hole is, in a sense, the engine that powers one of the most famous predictions in theoretical physics.

White Dwarfs and Gravitational Redshift

You do not need a black hole to see gravitational time dilation written into starlight. White dwarfs, the dense remnants of stars like our Sun, have surface gravity tens of thousands of times stronger than Earth’s. Light climbing out of that gravitational well loses energy and shifts toward redder wavelengths, a phenomenon called gravitational redshift. This is the frequency-domain signature of time dilation: clocks on the white dwarf’s surface tick slower, so the light they emit arrives at lower frequencies than it was emitted.

Astronomers have measured this redshift across populations of white dwarfs, using it to independently verify the relationship between their mass and radius.⁸ By averaging the apparent velocities of many white dwarfs with similar properties, random motions cancel out and the underlying gravitational redshift emerges. In one particularly interesting case, researchers detected gravitational redshift in X-ray emissions from a magnetic white dwarf in the binary system RX J1712.6−2414, marking the first such detection from a magnetic white dwarf and providing a new method for estimating its mass.⁹

Gravitational Redshift Across Galaxy Clusters

Gravitational time dilation operates on scales far larger than individual stars. Galaxy clusters, the most massive gravitationally bound structures in the universe, produce measurable gravitational redshifts in the light of the galaxies sitting deep within them. In 2011, researchers reported detection of this cluster-scale gravitational redshift at a 99 percent confidence level, using archival data from thousands of galaxy clusters. The observed shift matched the prediction of general relativity and also agreed with a modified gravity theory designed to explain cosmic acceleration without dark energy, but it was inconsistent with some alternative models that try to eliminate dark matter entirely.¹⁰

This result matters beyond just confirming Einstein’s theory one more time. It provides an independent way to test competing models of gravity and dark matter on cosmological scales, using time dilation as a discriminator. When galaxies at the center of a cluster emit light that is slightly redder than expected from their recession alone, that extra redness is the gravitational time dilation of general relativity stamped directly onto the photons.

Time Delays as a Cosmic Ruler

Gravitational time dilation also gives cosmologists a tool for measuring the expansion rate of the universe. When a massive galaxy or cluster sits between us and a distant quasar, the quasar’s light can take multiple paths around the lens, creating multiple images. These images do not arrive simultaneously. The time delay between them depends on two things: the different path lengths and the different gravitational potentials along each path. That second factor is a direct manifestation of gravitational time dilation.

By monitoring the brightness fluctuations of a lensed quasar across its multiple images, astronomers can measure the time delay, sometimes ranging from days to months. From this, combined with a model of the lens’s mass distribution, they can infer absolute distances and independently estimate the Hubble constant, which describes how fast the universe is expanding.¹¹ This technique, known as time-delay cosmography, is particularly valuable because it does not depend on the same assumptions as other methods like the cosmic distance ladder or the cosmic microwave background. It offers an entirely independent check, and the fact that it rests on gravitational time dilation connects a local physical effect to the largest-scale structure of the cosmos.

Atomic Clocks as Tests of Fundamental Physics

The extreme precision of modern atomic clocks has turned gravitational time dilation into a tool for testing whether the fundamental constants of nature are truly constant. General relativity predicts a specific relationship between gravitational potential and clock rates. If some deeper theory required, say, the fine-structure constant or the proton-to-electron mass ratio to vary slightly with gravitational potential, that variation would show up as an anomalous frequency shift in clocks at different locations in Earth’s orbit around the Sun. Earth moves closer to and farther from the Sun over the course of a year, changing the solar gravitational potential at our location.

Researchers have exploited this annual variation to look for violations of “local position invariance,” the principle that the outcome of a non-gravitational experiment should not depend on where or when it is performed in a gravitational field. By comparing rubidium and cesium fountain clocks over extended periods, one team set the first limit on how the ratio of those two atoms’ frequencies changes with gravitational potential.¹² More recent work using optical clocks has pushed the bounds even further, improving limits on variations of the fine-structure constant by a factor of about 20.¹³ So far, no deviation has been found. Einstein’s version of gravitational time dilation, with strictly constant constants, holds up to the best precision we can achieve.

Beyond testing constants, these clocks could eventually reshape how we define altitude itself. Because clock frequency depends on gravitational potential, a sufficiently precise clock is essentially a gravity sensor. Comparing optical frequency standards at different locations could contribute to defining height reference systems at centimeter-level resolution, replacing or supplementing traditional surveying methods with direct measurements of gravitational time dilation.¹⁴

Where Gravity Meets Quantum Mechanics

Gravitational time dilation may play a surprising role in one of the deepest puzzles in physics: why large objects behave classically rather than quantum mechanically. A theoretical proposal published in Nature Physics showed that even the weak gravitational time dilation on Earth’s surface is sufficient, in principle, to destroy quantum superpositions of objects at the micrometer scale. The reasoning is that a composite object in a superposition of being at two different heights simultaneously would experience two different rates of internal time. Its internal degrees of freedom, the vibrations of its atoms, the ticking of any internal “clock,” evolve at slightly different rates in each branch of the superposition. Over time, this creates correlations between the object’s position and its internal state that effectively “decohere” the superposition, collapsing it without any interaction with an external environment.¹⁵

If this picture is correct, gravity would be partially responsible for the emergence of the everyday classical world we experience, a much more profound role than simply keeping our feet on the ground. The effect has not been observed directly yet, but future matter-wave interferometry experiments with increasingly massive particles could test it. Separate theoretical work has investigated what would happen if spacetime curvature is fundamentally classical rather than quantum, finding that such a scenario would necessarily produce decoherence and unpredictability in quantum fields interacting with gravity.¹⁶ Whether gravity is ultimately quantum or classical, gravitational time dilation appears to be entangled (perhaps literally) with the question of why quantum weirdness fades at human scales.

Researchers have even started exploring how gravitational time dilation would affect quantum computing hardware. A recent analysis of transmon qubits, one of the leading platforms for quantum computers, showed that gravitational redshift and related effects cause entangled quantum states to lose coherence at a rate that depends on their position in a gravitational field.¹⁷ Current quantum computers are far too noisy from other sources for gravitational dephasing to matter in practice, but the finding is a reminder that as quantum technologies push toward ever-greater precision, gravity will eventually become a factor that engineers need to reckon with rather than ignore.